Method of attenuating porcine pseudorabies virus, attenuated strains of porcine pseudorabies virus, vaccine composition and use thereof

Abstract

This invention provides a method of attenuating porcine pseudorabies virus, which can effectively and reproducibly attenuate porcine pseudorabies virus. The attenuated strain of porcine pseudorabies virus by use of said method can provide effective immunization for pigs.

Claims

1. A method of attenuating porcine pseudorabies virus, comprising: (1) a step of cultivating the pseudorabies virus adapted to cell culture, wherein the pseudorabies virus is inoculated into subcultured mammalian cells, and then subcultured for at least five passages so as to obtain the porcine pseudorabies virus strain adapted to the subcultured mammalian cells; and (2) a step of attenuating the porcine pseudorabies virus, wherein the porcine pseudorabies virus strain adapted to the subcultured mammalian cells is inoculated into subcultured avian cells and then subcultured for at least one passage so as to obtain the attenuated strain of the porcine pseudorabies virus, wherein the genes of the attenuated strain of porcine pseudorabies virus have a serial deletion of 3455 bp starting from the 890.sup.th nucleotide of the gI gene.

2. The method of attenuating the porcine pseudorabies virus as described in claim 1, wherein said porcine pseudorabies virus in the step (1) comprises PRV JS-2012 strain, PRV HeN1 strain, NVDC-PRV-BJ strain, NVDC-PRV-HEB strain and NVDC-PRV-SD strain, PRV TJ strain, PRV variant strain PRV-ZJ01, PRV variant strain HN1201, PRV variant strain HN1202 or PRV Fa strain.

3. The method of attenuating the porcine pseudorabies virus as described in claim 1, wherein said subcultured mammalian cells in the step (1) comprises monkey embryonic kidney epithelial cell line Marc-145.

4. The method of attenuating porcine pseudorabies virus as described in claim 1, wherein said subcultured avian cells in the step (2) are DF-1.

5. The method of attenuating the porcine pseudorabies virus as described in claim 1, wherein said step of cultivating the pseudorabies virus adapted to cell culture in the step (1) comprises: dispersing said subcultured mammalian cells and digesting the subcultured mammalian cells with trypsin, and continuing to culture the subcultured mammalian cells with a cell growth medium, forming a monolayer of the subcultured mammalian cells; inoculating the monolayer of the subcultured mammalian cells with said porcine pseudorabies virus and continuing to culture the subcultured mammalian cells with a cell maintenance medium; harvesting a virus seed comprising the cell maintenance medium containing the porcine pseudorabies virus after 40 h-48 h of culturing the inoculated monolayer of subcultured mammalian cells; and culturing subcultured mammalian cells with the virus seed for at least 5 continuous passages, so as to obtain a porcine pseudorabies virus strain adapted to the subcultured mammalian cells having a cytopathic effect of at least 80%.

6. The method of attenuating the porcine pseudorabies virus as described in claim 1, wherein said step of attenuating the porcine pseudorabies virus in the step (2) comprises: dispersing said subcultured avian cells are dispersed and digesting the subcultured avian cells with trypsin, and continuing to culture the subcultured avian cells with a cell growth medium, forming a monolayer of the subcultured avain cells; and inoculating the monolayer of the subcultured avian cells with said porcine pseudorabies virus strain adapted to the subcultured mammalian cells obtained in the step (1) and continuing to culture the subcultured avian cells with a cell maintenance medium; and harvesting the cell maintenance medium comprising an attenuated strain of the porcine pseudorabies virus adapted to the subcultured mammalian cells after 40 h-48 h or culturing the subcultured avian cells with the cell maintenance medium comprising the attenuated strain of the porcine pseudorabies virus for at least one passage to obtain the attenuated strain of said porcine pseudorabies virus.

7. The method of attenuating porcine pseudorabies virus as described in claim 1, wherein a passage number of the porcine pseudorabies virus strain in the subcultured mammalian cells in the step (1) is equal to or at least 18; and a passage number of the porcine pseudorabies virus strain adapted to the subcultured mammalian cells in the subcultured avian cells in the step (2) is equal to or at least 3.

8. The method of attenuating porcine pseudorabies virus as described in claim 5, wherein said cell growth medium comprises 90%-97% (V/V) cell culture medium and 3%-10% (V/V) bovine serum, and the pH value of the cell growth medium is in the range of 7.0-8.0; the cell maintenance medium comprises 95%-99% (V/V) cell culture medium and 1%-5% (V/V) bovine serum, and the pH value of the cell maintenance medium is in the range of 7.1-7.5; wherein the cell culture medium comprises DMEM medium; wherein said bovine serum comprises fetal calf serum; and a temperature for culturing said cells is within the range of 36° C.-38° C.

9. An attenuated strain of the porcine pseudorabies virus obtained by using the method of attenuating the porcine pseudorabies virus as described in claim 1, wherein said attenuated strain of the porcine pseudorabies virus does not express gI, gE, 11K or 28K proteins, and the genes of the attenuated strain of porcine pseudorabies virus have a serial deletion of 3455 bp starting from the 890.sup.th nucleotide of gI gene.

10. The attenuated strain of the porcine pseudorabies virus as described in claim 9, wherein said attenuated strain of porcine pseudorabies virus is PRV HN1201-R strain, wherein said PRV HN1201-R strain has been deposited in the China Center for Type Culture Collection on Mar. 17, 2015, of which the accession number is CCTCC NO. V201516 and the address of depositary is Wuhan University, Wuhan, China.

11. A vaccine composition comprising: a carrier; and an attenuated strain of the porcine pseudorabies virus as described in claim 9 or a culture thereof, wherein a content of said attenuated strain of the porcine pseudorabies virus is at least 10.sup.6.0 TCID.sub.50/piglet.

12. The vaccine composition as described in claim 11, further comprising antigen of classical swine fever virus.

13. The method of attenuating porcine pseudorabies virus as described in claim 7, wherein said passage number in the subcultured avian cells of the porcine pseudorabies virus strain adapted to the subcultured mammalian cells is in the range of 3-110.

14. The method of attenuating porcine pseudorabies virus as described in claim 8, wherein said cell culture medium is DMEM medium, said bovine serum is fetal calf serum.

15. An attenuated strain of the porcine pseudorabies virus by using the method of attenuating the porcine pseudorabies virus as described in claim 9, wherein the genes of said attenuated strain of porcine pseudorabies virus have a serial deletion of 3455 bp starting from the 890.sup.th nucleotide of gI gene.

16. The vaccine composition according to claim 11, wherein the content of said attenuated strain of the porcine pseudorabies virus is in the range of 10.sup.6.0TCID.sub.50/piglet -10.sup.7.0TCID.sub.50/piglet.

Description

DETAILED DESCRIPTION

(1) The description of the present invention is further provided as follows with reference to the specific embodiments, and features and advantages of the present invention will become more apparent from the following description. However, these embodiments are only exemplary, but not forming any limitation to the scope of the present invention. It should be understood by a person skilled in the art that modifications or alternatives to details and forms of the technical solution of the present invention without deviation from the spirit and scope of the present invention will be allowed, while those modification and alternatives should all fall within the scope of protection of the present invention.

(2) In the embodiments of the present invention, PRV HN1201 strain, HN1202 strain, Fa strain, PRV-ZJ01 strain, HeN1 strain and JS-2012 strain are used as examples to illustrate the present invention.

(3) In the invention, the term “per pig” refers to the amount of vaccine each pig injected.

(4) In the invention, the term “TCID.sub.50” refers to 50% tissue culture infective dose, a way to represent viral infectivity.

(5) Dulbecco's Modified Eagle's Medium (DMEM) in the present invention is prepared with DMEM dry powdered medium (Gibco) according to the instruction.

(6) In the present invention, the term “PBS” is the abbreviation for Phosphate Buffer Saline, and 0.01 mM pH 7.4 PBS as used in the present invention was prepared as described in Molecular cloning: Laboratory manuals, 3rd edition.

(7) Fetal bovine serum was purchased from PAA.

(8) The PRV HN1201 strain (Pseudorabies virus, strain HN1201) used in the present embodiments was deposited in the China Type Culture Collection Center on May 20, 2013, of which the accession number is CCTCC NO. V 201311 and the address of depositary is Wuhan University, Wuhan City, Hubei Province.

(9) The PRV HN1202 strain (Pseudorabies virus, strain HN1202) used in the present embodiments was deposited in the China Type Culture Collection Center on Aug. 26, 2013 of which the accession number is CCTCC NO. V 201335 and the address of depositary is Wuhan University, Wuhan City, Hubei Province.

(10) The PRV HN1201-R strain (Pseudorabies virus, strain HN1201-R) used in the present embodiments was deposited in the China Type Culture Collection Center on Mar. 17, 2015 of which the accession number is CCTCC NO. V 201516 and the address of depositary is Wuhan University, Wuhan City, Hubei Province.

(11) “PRV” is the abbreviation for the term “pseudorabies virus”.

(12) Marc-145 cells were purchased from ATCC.

(13) DF-1 cells were purchased from ATCC.

EXAMPLE 1

Acquisition of PRV HN1201-R Strain

(14) 1. The well-grown Marc-145 cells digested with trypsin, were inoculated in cell culture flasks and then cultured at 36° C.˜38° C. with cell growth medium (pH was adjusted to 7.0˜8.0) containing 90%˜97% (V/V) DMEM culture medium and 3%˜10% (V/V) fetal bovine serum, to form a proper monolayer for inoculation with virus. 2. The PRV HN1201 strain was inoculated into the above well-grown monolayer of subcultured cells, and the cells continued to be cultured at 36° C.˜38° C. with cell maintenance medium (pH was adjusted to 7.1˜7.5) containing 95%˜99% (V/V) DMEM and 1%˜5% (V/V) fetal bovine serum. After 40 h˜48 h, the cell medium containing viruses, i.e. the pseudorabies virus strain adapted to mammalian cells, was harvested when the cytopathic effect of cells reached 80%, as the virus seed for continued passage. Different passages of virus harvested, P1, P2, P3, P4, P5, P6, P7, P8, P9, P10, P15, P20, P30, P50, P70, P90, P110, P130, P150 and P200, was sequenced respectively, with the results indicating no deficiency of genes for each passage. 3. The well-grown DF-1 cells digested with trypsin, were inoculated in cell culture flasks and then cultured at 36° C.˜38° C. with cell growth medium (pH was adjusted to 7.0˜8.0) containing 90%˜97% (V/V) DMEM and 3%˜10% (V/V) fetal bovine serum, to form a proper monolayer for inoculation with virus. 4. Different passages of the pseudorabies virus strain adapted to mammalian cells harvested in the step 2, was inoculated into the above well-grown monolayer of subcultured DF-1 cells obtained from the step 3, and continued to be cultured at 36° C.˜38° C. with cell maintenance medium (pH was adjusted to 7.1˜7.5) containing 95%˜99% (V/V) DMEM and 1%˜5% (V/V) fetal bovine serum. After 40 h˜48 h, the cell medium containing viruses was harvested respectively when the cytopathic effect of cells reached 80%, i.e P1-1, P2-1, P3-1, P4-1, P5-1, P6-1, P7-1, P8-1, P9-1, P10-1, P15-1, P20-1, P30-1, P50-1, P70-1, P90-1, P110-1, P130-1, P150-1, P200-1. Each passage of viruses harvested was sequenced respectively, with the results indicating that there was no deficiency of genes for P1-1, P2-1, P3-1, P4-1, while P5-1, P6-1, P7-1, P8-1, P9-1, P10-1, P15-1, P20-1, P30-1, P50-1, P70-1, P90-1, P110-1, P130-1, P150-1 and P200-1 all has the deficiency of genes, in which each has continuous deficiency of 3455 bp started from the site of the 890.sup.th nucleotide of gI gene.

(15) In order to verify if it is related to passage times in DF-1 that there was no deficiency of genes for P1-1, P2-1, P3-1 and P4-1, such serial passage continued and P1-2, P1-3, P1-4, P1-5, P1-6, P1-7, P1-8, P1-9, P1-10, P2-2, P2-3, P2-4, P2-5, P2-6, P2-7, P2-8, P2-9, P2-10, P3-2, P3-3, P3-4, P3-5, P3-6, P3-7, P3-8, P3-9, P3-10, P4-2, P4-3, P4-4, P4-5, P4-6, P4-7, P4-8, P4-9 and P4-10 was harvested respectively. Each virus harvested was sequenced respectively, with the results indicating no deficiency of genes for each passage. It showed that in the case where PRV was passaged four times or less in Marc-145 cells, the occurrence of deficiency of genes was not related to passage times in DF-1, but to passage times of adaption in Marc-145 cells.

(16) In order to verify the stability of continued passage in Df-1 for the viruses with deficiency of genes, P5-1, P6-1, P7-1, P8-1, P9-1, P10-1, P15-1, P20-1, P30-1, P50-1, P70-1, P90-1, P110-1, P130-1, P150-1, P200-1 was continued to be passaged, resulted to the harvest of P5-2, P5-3, P5-4, P5-5, P5-6, P5-7, P5-8, P5-9, P5-10, P6-2, P6-3, P6-4, P6-5, P6-6, P6-7, P6-8, P6-9, P6-10, P7-2, P7-3, P7-4, P7-5, P7-6, P7-7, P7-8, P7-9, P7-10, P8-2, P8-3, P8-4, P8-5, P8-6, P8-7, P8-8, P8-9, P8-10, P9-2, P9-3, P9-4, P9-5, P9-6, P9-7, P9-8, P9-9, P9-10, P10-2, P10-3, P10-4, P10-5, P10-6, P10-7, P10-8, P10-9, P10-10, P15-2, P15-3, P15-4, P15-5, P15- 6, P15-7, P15-8, P15-9, P15-10, P20-2, P20-3, P20-4, P20-5, P20-6, P20-7, P20-8, P20-9, P20- 10, P30-2, P30-3, P30-4, P30-5, P30-6, P30-7, P30-8, P30-9, P30-10, P50-2, P50-3, P50-4, P50- 5, P50-6, P50-7, P50-8, P50-9, P50-10, P70-2, P70-3, P70-4, P70-5, P70-6, P70-7, P70-8, P70- 9, P70-10, P90-2, P90-3, P90-4, P90-5, P90-6, P90-7, P90-8, P90-9, P90-10, P110-2, P110-3, P110-4, P110-5, P110-6, P110-7, P110-8, P110-9, P110-10, P130-2, P130-3, P130-4, P130-5, P130-6, P130-7, P130-8, P130-9, P130-10, P150-2, P150-3, P150-4, P150-5, P150-6, P150-7, P150-8, P150-9, P150-10, P200-2, P200-3, P200-4, P200-5, P200-6, P200-7, P200-8, P200-9 and P200-10. Each passage of viruses harvested was sequenced respectively, with the results indicating that there was no change for the occurrence of deficiency of genes for each passage, in which each always has continuous deficiency of 3455 bp started from the site of the 890.sup.th nucleotide of gI gene, indicating a stable passage in DF-1 for the PRV gene-deleted strain.

(17) The attenuated strain of PRV, P30-10 was named PRV HN1201-R strain.

EXAMPLE 2

Study of Biological Characteristic of PRV HN1201-R Strain

(18) 1. Pathogenicity Test of the Virus

(19) 15 piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were randomly divided into 3 groups (A, B and blank control groups), each with 5 piglets. The grouping and challenging conditions are shown in Table 1.

(20) TABLE-US-00001 TABLE 1 Grouping of animals in the pathogenicity test Strains used Group for inoculation Dose of inoculation A HN1201-R strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation B HN1201 strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation Blank DMEM medium inoculated with l ml/piglet by control intranasal instillation

(21) Piglets were observed for 28 days after inoculation of virus, while the temperature of piglets was determined daily, and clinical signs and death status were observed. The specific results are shown in Table 2.

(22) TABLE-US-00002 TABLE 2 Pathogenicity of HN1201-R strains in 7-day-old piglets Death Group Number Clinical signs status A A1 Normal body temperature, no clinical Survived signs A2 Normal body temperature, no clinical Survived signs A3 Body temperature increased for 1 day, Survived no other clinical signs A4 Body temperature increased for 1 day, Survived no other clinical signs A5 Normal body temperature, no clinical Survived signs B B1 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling, challenge convulsions; neurological signs such as turning around, and making strokes with their arms, etc. B2 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. B3 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. B4 Body temperature increased for 3 days, Died on depression, complete loss of appetite, day 3 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. B5 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. Blank K1 No abnormal clinical signs Survived control K2 No abnormal clinical signs Survived K3 No abnormal clinical signs Survived K4 No abnormal clinical signs Survived K5 No abnormal clinical signs Survived

(23) It showed in the results that inoculation with PRV HN1201 strain in 7-day-old piglets could lead to death with a morality rate of 100% (5/5) of inoculated piglets, while PRV HN1201-R strain displayed a significant reduction of virulence, only causing increased body temperature of two pigs, without any other clinical signs, or any change of tissues or organs obtained from the necropsy.

(24) Through the pathogenicity test it indicated that compared with the parent virulent strain, i.e. PRV HN1201 strain, PRV HN1201-R strain displayed a significant reduction of pathogenicity, and was an attenuated virus strain.

(25) Meanwhile, in order to verify the stability of pathogenicity of different passages of PRV HN1201-R strain, a group of piglets (5 piglets) at 7 days of age which were negative for pseudorabies antigens and antibodies were inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1201-R strain, respectively, and each was inoculated with 1 ml (10.sup.7.0 TCID.sub.50/ml) by intranasal instillation. Another five piglets were used as the control group. The clinical manifestations of piglets were observed and recorded daily until 28 days after inoculation of virus.

(26) It showed in the results that, from the observation of piglets for 28 days after inoculation with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1201-R strain, PRV HN1201-R strain displayed a significant reduction of virulence, only causing increased body temperature of 1˜2 pigs/group, without any other clinical signs, or any change of tissues or organs obtained from the necropsy.

(27) Through the pathogenicity test of different passages, it indicated that different passages of PRV HN1201-R strain all displayed lower virulence.

(28) 2. Immunogenicity Assay

(29) On the 21.sup.st day after immunization, all of the five piglets inoculated with PRV 1201-R strain and five piglets in the control group were challenged with 1×10.sup.7.0 TCID.sub.50/piglet of PRV HN1201. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The results are shown in Table 3.

(30) TABLE-US-00003 TABLE 3 Pathogenicity of HN1201-R strains in 7-day-old piglets Death Group Number Clinical signs status A A1 Normal body temperature, normal 100% (5/5) appetite, no abnormal clinical signs, survived A2 Normal body temperature, normal appetite, no abnormal clinical signs, survived A3 Normal body temperature, normal appetite, no abnormal clinical signs, survived A4 Normal body temperature, normal appetite, no abnormal clinical signs, survived A5 Normal body temperature, normal appetite, no abnormal clinical signs, survived Blank K1 Body temperature increased,  0% (0/5) control depression, loss of appetite, significant clinical signs, died on day 4 after challenge K2 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 4 after challenge K3 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 3 after challenge K4 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 4 after challenge K5 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 4 after challenge

(31) The result indicated that all the piglets inoculated with PRV HN1201-R strain were healthy and alive, while all from the control group died.

(32) According to the immunogenicity assay, the PRV HN1201-R strain can provide excellent protection against PRV HN1201 strain, showing excellent immunogenicity.

(33) Meanwhile, in order to verify the stability of immunogenicity of different passages of PRV HN1201-R strain, on the 21.sup.st day after immunization, all the immune groups inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1201-R strain as well as the control group were challenged with 1×10.sup.7.0 TCID.sub.50/piglet of PRV HN1201. After challenge, the body temperature of piglets was determined daily, and in the meanwhile clinical signs and death status were observed.

(34) The result indicated that all the piglets inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1201-R strain were alive, while all from the control group died.

(35) According to the immunogenicity assay of different passages, different passages of PRV HN1201-R strain all can provide excellent protection against PRV HN1201 strain, showing excellent immunogenicity.

(36) 3. Reversion of Virulence Test of the Virus

(37) 30 piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were randomly divided into 5 groups, each with 6 piglets. 6 piglets of Group 1 which were negative for pseudorabies antigens and antibodies were inoculated with 10.sup.7TCID.sub.50/piglet of the cultures of PRV HN1201-R strain (the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th, 110.sup.th passages and the 1.sup.st+30.sup.th+60.sup.th+85.sup.th+110.sup.th passages) respectively by intranasal instillation. On day 14, they were raised together with the 6 piglets of Group 2 which were negative for pseudorabies antigens and antibodies. 14 days later, the 6 piglets of Group 1 were drawn back, and again, the 6 piglets of Group 3, which were negative for pseudorabies antigens and antibodies, were raised together with the 6 piglets of Group 2 and so on. After 4 serial passages, all the drawn piglets were killed in order to observe if there were any pathological changes.

(38) It showed in the result that no abnormal changes were found during the clinical observation and gross anatomy of 30 experimental piglets by the 4th serial passage of the co-habitation infection experiment, indicating that there was no reversion of virulence of this attenuated strain. Therefore, the safety of the vaccines can be ensured since the virus, after being inoculated into piglets, would not evolve into a virulent virus which is able to cause disease.

(39) 4. Genes Sequences Analysis

(40) The genome amplification of the cultures of the 1.sup.st passage to 110.sup.th passage of PRV HN1201-R strain was accomplished by means of RT-PCR (The genomic DNA of culture of different passages was amplified respectively). The product acquired from amplification was recovered, purified, and linked to the plasmid vector for sequencing, so that the nucleotide sequence of the viral gene was determined and transformed through computer softwares into the amino acid sequence of the virus. The obtained amino acid sequence was compared with the amino acid sequence of the parent virulent strain, HN1201 strain via sequence analysis softwares, and the amino acids sequence of the virus was characterized.

(41) It showed in the results that for the cultures of the 1.sup.st passage to 110.sup.th passage of PRV HN1201-R strain, each amino acid sequences encoded by the viral genes commonly has continuous deficiency of gI/gE/11K/28K genes (3455 bp in total), ie. continuous deficiency of 3455 bp started from the site of the 890.sup.th nucleotide of gI gene. The deleted sequence is as shown in SEQ No. 1.

(42) It indicated that a common characteristic change of the amino acids sequences encoded by the viral genes of the culture of different passages of PRV HN1201-R strain might be the reason for the reduction of virulence of the parent virulent strain.

EXAMPLE 3

Preparation of the Attenuated Live Vaccine of PRV HN1201-R Strain

(43) 1. Proliferation of Virus

(44) The virus seed of PRV HN1201-R strain prepared in Example 1 was diluted at 5×10.sup.4 fold, and then inoculated into a monolayer of ST cell. After 1 h adhesion, 1000 ml of DMEM medium containing 2% fetal calf serum was added into ST cell, which was then placed at 37° C. in a roller bottle with a rotation speed of 6 rph. The cell medium containing viruses was harvested when the cytopathic effect of cells reached 80%; the viruses were harvested after 2 times of freezing-thawing the medium and the virus titer was assessed. The virus solution was preserved at low temperature.

(45) 2. Preparation of a Protective Agent

(46) 40 g of sucrose and 8 g of gelatin was added into every 100 ml of deionized water, and the solution was autoclaved (under 121° C. for 30 min) after fully melted.

(47) 3. Preparation of Vaccine

(48) The virus solution prepared and preserved from above procedure was mixed with the protective agent prepared and preserved from above procedure at a volume ratio of 1:1 and the mixed virus solution was freeze-dried. The specific ratio of content of the vaccine is shown in Table 4.

(49) TABLE-US-00004 TABLE 4 ratio of content of the attenuated live vaccine of PRV HN1201-R strain Vaccine 1 Vaccine 2 Group (TCID.sub.50) (TCID.sub.50) Antigen of 10.sup.6.0 10.sup.7.0 HN1201-R strain protective agent 50% 50% (V/V)

EXAMPLE 4

Immunogenicity Assay of the Attenuated Live Vaccine of HN1201-R Strain

(50) 15 9-day-old piglets which were negative for PRV antibodies and PRV antigens were randomly divided into 3 groups, each with 5 piglets, and the piglets were injected with the attenuated live PRV HN1201-R strain prepared in Example 3. The first group was inoculated with Vaccine 1, and the second group was inoculated with Vaccine 2, and the third group was the control group. The piglets were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HN1201 strain on day 21 after immunization. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The results are shown in Table 5.

(51) TABLE-US-00005 TABLE 5 Results of Immunogenicity assay of the attenuated live vaccine of HN1201-R strain Number Dose of Clinical signs and Rate of Group of piglets Challenge death status protection Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) 1 piglet of temperature, normal HN1201 appetite, no abnormal strain clinical signs, survived Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) 2 piglet of temperature, normal HN1201 appetite, no abnormal strain clinical signs, survived Control 5 10.sup.7.0TCID.sub.50/ All the pigs displayed  0% (0/5) group piglet of symptoms like HN1201 increased body strain temperature, depression, partial or complete loss of appetite, and significant clinical signs; two died on day 3 after challenge, and all died within 4 days after challenge.

(52) The result indicated that immunizing piglets with the attenuated live vaccine of PRV HN1201-R strain prepared in example 3 can block virus infection (i.e. prevent occurrence of clinic signs), and provide 100% (5/5) protection rate for piglets, while all the piglets in the blank control group died by day 4 after challenge.

(53) It has proven that the attenuated live vaccine of PRV HN1201-R strain in two experimental groups can provide excellent protection, showing excellent immune protection and safety; in the meanwhile it indicated that PRV strain which has a continuous deficiency of gI/gE/11K/28K genes (3455 bp in total) would still maintain excellent immunogenicity.

EXAMPLE 5

Immunogenicity Comparison Assay of the Attenuated Live Vaccine of HN1201-R Strain

(54) 15 9-day-old piglets which were negative for PRV antibodies and PRV antigens were randomly divided into 3 groups, each with 5 piglets. The first group was inoculated with Vaccine 1, the attenuated live vaccine of HN1201-R strain prepared in Example 3; the second group was immunized with the live PRV vaccine, Bartha K-61 strain purchased from HIPRA, Spain, with Batch No. 42RH; and the third group is the blank control group. The piglets were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HN1201 strain on day 28 after immunization. After challenge, the body temperature of piglets was determined daily, and clinical signs and death status were observed as well. The detailed results are shown in Table 6.

(55) TABLE-US-00006 TABLE 6 Results of Immunogenicity comparison assay of the attenuated live vaccine of HN1201-R strain Number Dose of Clinical signs and Rate of Group of piglets challenge death status protection Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) 1 piglet of temperature, normal HN1201 appetite, no abnormal strain clinical signs, survived Control 5 10.sup.7.0TCID.sub.50/ Three pigs displayed 80% (4/5) vaccine piglet of symptoms like HN1201 increased body strain temperature for 7-10 days, depression, and loss of appetite; one died. Blank 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) control piglet of symptoms like group HN1201 increased body strain temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 3 after challenge, and all died within 4 days after challenge.

(56) The result indicated that immunizing piglets with the attenuated live vaccine of PRV HN1201-R strain prepared in Example 3 can block virus infection (i.e. prevent occurrence of clinic signs), and provide 100% (5/5) protection rate for piglets; all the piglets in the control group died by day 4 after challenge; whereas the commercial vaccines in the prior art cannot provide full protection for pigs.

(57) It has proven that the attenuated live vaccine of PRV HN1201-R strain can provide excellent protection, showing better immune protection and safety than the commercial vaccines in the prior art.

EXAMPLE 6

Acquisition of PRV HN1202-R Strain and Fa-R Strain

(58) PRV HN1202 strain and Fa strain were subcultured respectively according to the procedures in Example 1, so as to obtain their attenuated PRV strains, named PRV HN1202-R strain and Fa-R strain.

EXAMPLE 7

Study of Biological Characteristic of PRV HN1202-R Strain and Fa-R Strain

(59) 1. Pathogenicity Test of the Virus

(60) 20 piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were randomly divided into 4 groups (C, D, E and F), each with 5 piglets; another 10 piglets were used as the blank control group. The grouping and challenging conditions are shown in Table 7.

(61) TABLE-US-00007 TABLE 7 Grouping of the animals in the pathogenicity test for HN1202-R strain and Fa-R strain Strain used Group for inoculation Dose of inoculation C HN1202-R strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation D HN1202 strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation E Fa-R strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation F Fa strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation Blank DMEM medium inoculated with 1 ml/piglet by control intranasal instillation

(62) Piglets were observed for 28 days after inoculation of virus, while the temperature of piglets was measured daily, and clinical signs and death status were observed. The results are shown in Table 8.

(63) TABLE-US-00008 TABLE 8 Pathogenicity of HN1202-R strains and Fa-R strain to 7-day-old piglets Death Group Number Clinical signs status C C1 Body temperature increased for 1 day, Survived no clinical signs C2 Normal body temperature, , no clinical Survived signs C3 Body temperature increased for 1 day, Survived no other clinical signs C4 Normal body temperature, , no clinical Survived signs C5 Body temperature increased for 1 day, Survived no other clinical signs D D1 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. D2 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. D3 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. D4 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. D5 Body temperature increased for 3 days, Died on depression, complete loss of appetite, day 3 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. E E1 Body temperature increased for 1 day, Survived no clinical signs E2 Body temperature increased for 1 day, Survived no other clinical signs E3 Body temperature increased for 1 day, Survived no other clinical signs E4 Normal body temperature, , no clinical Survived signs E5 Body temperature increased for 1 day, Survived no other clinical signs F F1 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. F2 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. F3 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. F4 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. F5 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. Blank K6 no abnormal clinical signs Survived control K7 no abnormal clinical signs Survived K8 no abnormal clinical signs Survived K9 no abnormal clinical signs Survived K10 no abnormal clinical signs Survived K11 no abnormal clinical signs Survived K12 no abnormal clinical signs Survived K13 no abnormal clinical signs Survived K14 no abnormal clinical signs Survived K15 no abnormal clinical signs Survived

(64) It showed in the results that inoculation with PRV HN1202 strain in 7-day-old piglets could lead to death of 100% (5/5) of inoculated piglets, while PRV HN1202-R strain displayed a significant reduction of virulence, only causing increased body temperature of three pigs, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy; inoculation with PRV Fa strain in 7-day-old piglets could lead to death of 100% (5/5) of inoculated piglets, while PRV Fa-R strain displayed a significant reduction of virulence, only causing increased body temperature of four pigs, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy.

(65) Through the pathogenicity test it indicated that compared with the parent virulent strain, i.e. PRV HN1202 strain, PRV HN1202-R strain displayed a significant reduction of pathogenicity, and was an attenuated virus strain; compared with the parent virulent strain, i.e. PRV Fa strain, PRV Fa-R strain displayed a significant reduction of pathogenicity, and was an attenuated virus strain.

(66) Meanwhile, in order to verify the stability of pathogenicity of different passages of PRV HN1202-R strain and Fa-R strain, a group (5) of piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml) of the cultures of 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1202-R strain by intranasal instillation, respectively, and another five piglets were used as the blank control group; a group (5) of piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml) of the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th, and 110.sup.th passages of PRV Fa-R strain by intranasal instillation, respectively, and another five piglets were used as the control group. The clinical manifestations of piglets were observed and recorded daily until 28 days after inoculation of virus.

(67) It showed in the results that, from the observation of piglets for 28 days after inoculation with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1202-R strain, PRV HN1202-R strain displayed a significant reduction of virulence, only causing increased body temperature of 2˜3 pigs/group, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy; from the observation of piglets for 28 days after inoculation with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1202-R strain, PRV Fa-R strain displayed a significant reduction of virulence, only causing increased body temperature of 3˜4 pigs/group, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy.

(68) Through the pathogenicity test of different passages, it indicated that different passages of PRV HN1202-R strain and Fa-R strain all displayed lower virulence.

(69) 2. Immunogenicity Assay

(70) On the 21.sup.st day after immunization, all the five piglets inoculated with PRV 1202-R strain and five piglets in the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV 1202 strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The results are shown in Table 9.

(71) On the 21.sup.st day after immunization, all the five piglets inoculated with PRV Fa-R strain and five piglets in the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV Fa strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The results are shown in Table 9.

(72) TABLE-US-00009 TABLE 9 Pathogenicity of HN1202-R strain and Fa-R strain in 7-day-old piglets Protection Group Number Clinical signs and death status Rate C C1 Normal body temperature, normal 100% (5/5) appetite, no abnormal clinical signs, survived C2 Normal body temperature, normal appetite, no abnormal clinical signs, survived C3 Normal body temperature, normal appetite, no abnormal clinical signs, survived C4 Normal body temperature, normal appetite, no abnormal clinical signs, survived C5 Normal body temperature, normal appetite, no abnormal clinical signs, survived Blank K6 Body temperature increased, 0% (0/5) control depression, loss of appetite, significant clinical signs, died on day 4 after challenge K7 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 5 after challenge K8 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 5 after challenge K9 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 4 after challenge K10 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 4 after challenge E E1 Normal body temperature, normal 100% (5/5) appetite, no abnormal clinical signs, survived E2 Normal body temperature, normal appetite, no abnormal clinical signs, survived E3 Normal body temperature, normal appetite, no abnormal clinical signs, survived E4 Normal body temperature, normal appetite, no abnormal clinical signs, survived E5 Normal body temperature, normal appetite, no abnormal clinical signs, survived Blank K11 Body temperature increased, 0% (0/5) control depression, loss of appetite, significant clinical signs, died on day 5 after challenge K12 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 4 after challenge K13 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 4 after challenge K14 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 5 after challenge K15 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 5 after challenge

(73) The result indicated that all the piglets inoculated with PRV HN1202-R strain were healthy and alive, while all from the control group died; all the piglets inoculated with PRV Fa-R strain were healthy and alive, while all from the control group died.

(74) According to the immunogenicity assay, the PRV HN1202-R strain can provide excellent protection against PRV HN1202 strain, showing excellent immunogenicity; the PRV Fa-R strain can provide excellent protection against PRV Fa strain, showing excellent immunogenicity.

(75) Meanwhile, in order to verify the stability of immunogenicity of different passages of PRV HN1202-R strain and Fa-R strain, on the 21.sup.st day after immunization, all the immune groups inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1202-R strain as well as the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HN1202; after challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed; on the 21.sup.st day after immunization, all the immune groups inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV Fa-R strain as well as the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV Fa. After challenge, the body temperature of piglets was determined daily, and in the meanwhile clinical signs and death status were observed.

(76) The result indicated that all the piglets inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HN1202-R strain were healthy and alive, while all from the control group died; all the piglets inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV Fa-R strain were healthy and alive, while all from the control group died.

(77) According to the immunogenicity assay of different passages, the culture of different passages of PRV HN1202-R strain all can provide excellent protection against PRV HN1202 strain, showing excellent immunogenicity; the culture of different passages of PRV Fa-R strain all can provide excellent protection against PRV Fa strain, showing excellent immunogenicity.

(78) 3. Reversion of Virulence Test of the Virus

(79) 30 piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were randomly divided into 5 groups, each with 6 piglets. 6 piglets of Group 1 which were negative for pseudorabies antigens and antibodies were inoculated with 10.sup.7.0TCID.sub.50/piglet of the cultures of PRV HN1202-R (the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th, 110.sup.th passages and the 1.sup.st+30.sup.th+60.sup.th+85.sup.th+110.sup.th passages) respectively by intranasal instillation. On day 14, they were raised together with the 6 piglets of Group 2 which were negative for pseudorabies antigens and antibodies. 14 days later, the 6 piglets of Group 1 were drawn back, and again, the 6 piglets of Group 3, which were negative for pseudorabies antigens and antibodies, were raised together with the 6 piglets of Group 2 and so on. After 4 continuous passages, all the drawn piglets were killed in order to observe if there were any pathological changes.

(80) It showed in the result that no abnormal changes were found during the clinical observation and gross anatomy of 30 experimental piglets infected with HN1202-R strain and 30 experimental piglets infected with Fa-R strain, by the 4th serial passage of the cohabitation infection experiment, indicating that there was no reversion of virulence of the two attenuated strains. Therefore, the safety of the vaccines can be ensured since the viruses, after being inoculated into piglets, would not evolve into virulent viruses which are able to cause disease.

(81) 4. Gene Sequences Analysis

(82) The genome amplification of the cultures of the 1.sup.st passage to 110.sup.th passage of PRV HN1202-R strain was accomplished by means of RT-PCR (The genomic DNA of culture of different passages was amplified respectively). The product acquired from amplification was recovered, purified, and linked to the plasmid vector for sequencing, so that the nucleotide sequence of the viral gene was determined and transformed through computer softwares into the amino acid sequence of the virus. The obtained amino acid sequence was compared with the amino acid sequence of the parent virulent strain, i.e. PRV HN1202 strain via sequence analysis softwares, and the amino acids sequence of the virus was characterized.

(83) Meanwhile, the genome amplification of the cultures of the 1.sup.st passage to 110.sup.th passage of PRV Fa-R strain was accomplished by means of RT-PCR (The genomic DNA of culture of different passages was amplified respectively). The product acquired from amplification was recovered, purified, and linked to the plasmid vector for sequencing, so that the nucleotide sequence of the viral gene was determined and transformed through computer softwares into the amino acid sequence of the virus. The obtained amino acid sequence was compared with the amino acid sequence of the parent virulent strain, PRV Fa strain via softwares for sequence analysis, and the amino acids sequence of the virus was characterized.

(84) It showed in the results that for the cultures of the 1.sup.st passage to 110.sup.th passage of PRV HN1202-R strain, the amino acids sequences encoded by each viral genes commonly have continuous deficiency of gI/gE/11K/28K genes (3455 bp in total), in which the deficiency site and size are totally the same as those of PRV HN1201-R strain; for the cultures of the 1.sup.st passage to 110.sup.th passage of PRV Fa-R strain, the amino acids sequences encoded by each viral genes commonly have continuous deficiency of gI/gE/11K/28K genes (3455 bp in total), in which the deficiency site and size are totally the same as those of PRV HN1201-R strain; compared with their parent virulent strains, each of PRV HN1202-R strain and PRV Fa-R strain has continuous deficiency of gI/gE/11K/28K genes (3455 bp in total).

(85) It indicated that a common characteristic change of the amino acids sequences encoded by the viral genes of the cultures of the different passages of PRV HN1202-R strain as well as PRV Fa-R strain is consistent with that of the amino acids encoded by the viral genes of PRV HN1201-R strain, which is caused by the deficiency of genes encoding said amino acids sequences, showing the stability of the method of attenuating the PRV by passage according to the present invention, and in the meanwhile, further indicating that continuous deficiency of gI/gE/11K/28K genes (3455 bp in total) in PRV is the reason for the reduction of virulence of its parent virulent strain.

EXAMPLE 8

Preparation of the Attenuated Live Vaccines of PRV HN1202-R Strain and Fa-R Strain

(86) The attenuated live vaccines of PRV HN1202-R strain and Fa-R strain were prepared according to the procedure in Example 3. The specific ratios of contents of the vaccines are shown in Table 10.

(87) TABLE-US-00010 TABLE 10 ratios of contents of the attenuated live vaccines of PRV HN1202-R strain and Fa-R strain Antigen (TCID.sub.50) protective agent (V/V) Vaccine 3 10.sup.6.0 50% (HN1202-R strain) Vaccine 4 (Fa-R 10.sup.6.0 50% strain)

EXAMPLE 9

Immunogenicity Assay of the Attenuated Live Vaccine of PRV HN1202-R Strain and Fa-R Strain

(88) 20 9-day-old piglets which were negative for PRV antigens and antibodies and PRV antigens were randomly divided into 4 groups, each with 5 piglets, and the piglets were injected with the attenuated live vaccine of PRV HN1202-R strain and Fa-R strain prepared in Example 8. The first group was immunized with Vaccine 3, and the third group was immunized with Vaccine 4, and the second and fourth group was the control group. The piglets in the first and second groups were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HN1202 strain on day 21 after immunization, and those in the third and fourth groups were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV Fa strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The specific results are shown in Table 11.

(89) TABLE-US-00011 TABLE 11 Results of immunogenicity assay of the attenuated live vaccine of HN1202-R strain and Fa-R strain Number Dose of Clinical signs and Rate of Group of piglets Challenge death status protection Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) 3 piglet of temperature, normal HN1202 appetite, no abnormal strain clinical signs, survived Control 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 80% (4/5) vaccine piglet of symptoms like HN1202 increased body strain temperature, depression, complete or partial loss of appetite, and significant clinical signs; four died on day 4 after challenge, and all died within 5 days after challenge. Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) 4 piglet of temperature, normal Fa strain appetite, no abnormal clinical signs, survived Control 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) vaccine piglet of symptoms like Fa strain increased body temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 4 after challenge, and all died within 4 days after challenge.

(90) The result indicated that immunizing piglets with the attenuated live vaccine of PRV HN1202-R strain prepared in example 8 can block virus infection (i.e. prevent occurrence of clinical signs), and provide 100% (5/5) protection rate for piglets, while all the piglets in the blank control group died by day 5 after challenge; immunizing piglets with the attenuated live vaccine of PRV Fa-R strain prepared in example 8 can block virus infection (i.e. prevent occurrence of clinical signs), and provide 100% (5/5) protection rate for piglets, while all the piglets in the blank control group died by day 5 after challenge.

(91) It has proven that the attenuated live vaccines of PRV HN1202-R strain and Fa-R strain can provide excellent protection, showing excellent immune protection and safety; meanwhile it indicated again that a continuous deficiency of gI/gE/11K/28K genes (3455 bp in total) from the PRV virus would have no effect on its immunogenicity.

EXAMPLE 10

Construction of PRV HN1201-gI−/gE−/11K−/28K− Gene-Deleted Strain

(92) In order to obtain the PRV HN1201-gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.− gene-deleted strain, the gI/gE/11K/28K genes were knocked out through molecular cloning by means of homologous recombination. The detailed procedures are as follows:

(93) The sequence at the front end of US7 and the sequence at the back section of US2 were amplified respectively, as the left and right homologous recombinant arms US7L and US2R, with the genomic DNA of PRV HN1201 strain as the template, and US7-LP1/US7-LP2 and US2-RP1/US2-RP2 as the primers, in which there is a loxP site at each end of gIL and US2R. The transfer vector, pSKUS7-2-GFP with the loxP sites was constructed by use of pBluescript SK plasmid, with the green fluorescent protein GFP as the selectable marker.

(94) The total DNA of PK-15 cells infected by PRV HN1201 strain was extracted via the DNAZol method, and the total DNA and the transfer vector, pSKUS7-2-GFP was co-transfected at the ratio of 10 μg:1 μg, into the PK-15 cells via the lipofectin-mediated transfection. The viruses were harvested when the cytppathic effect of cells reached 80%. After serial dilution, the harvested viruses was inoculated into a monolayer of PK-15 cells, to obtain the recombinant virus rPRV-US7-2.sup.−/GFP.sup.+ with GFP by means of plaque purification. 10 μg of the DNA of rPRV-US7-2.sup.−/GFP.sup.+ was added with 2.5 units of Cre recombinase and reacted for 1 h at 37° C.; the DNA was extracted to prepare DNA of rPRV-US7-2.sup.−, which was then tansfected into PK-15 cells. After plaques purification, the gene-deleted PRV strain containing no GFP, PRV HN1201-gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.− gene-deleted strain was obtained.

EXAMPLE 11

Preparation of the Attenuated Live Vaccines of PRV HN1201-gI−/gE−/11K−/28K− Gene-Deleted Strain

(95) The attenuated live vaccines of PRV HN1201-gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.− gene-deleted strain was prepared according to the procedure in Example 3. The specific ratios of content of the vaccine are shown in Table 12.

(96) TABLE-US-00012 TABLE 12 ratios of contents of the attenuated live vaccines of PRV HN1201- gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.− gene-deleted strain Antigen (TCID.sub.50) protective agent (V/V) Vaccine 5 10.sup.6.0 50% (HN1201-gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.−)

(97) 10 9-day-old piglets which were negative for PRV antigens and antibodies were randomly divided into 2 groups, each with 5 piglets, and the piglets were immunized with the attenuated live vaccine of PRV HN1201-gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.− strain. The piglets were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HN1201 strain on day 21 after immunization. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The detailed results are shown in Table 13.

(98) TABLE-US-00013 TABLE 13 Results of Immunogenicity assay of the attenuated live vaccine of HN1201-gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.− strain Number Dose of Clinical signs and Rate of Group of piglets Challenge death status protection Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 80% (4/5) 5 piglet of temperature, normal HN1201 appetite, no abnormal strain clinical signs, survived Control 5 10.sup.7.0TCID.sub.50/ All the pigs displayed  0% (0/5) vaccine piglet of symptoms like HN1201 increased body strain temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 3 after challenge, and all died within 4 days after challenge.

(99) The results indicated that immunizing piglets with the attenuated live vaccine of PRV HN1201-gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.− strain prepared by means of genetic engineering can block virus infection (i.e. prevent occurrence of clinical signs), and provide 80% (4/5) protection rate for piglets, while all the piglets in the blank control group died by day 4 after challenge.

(100) It has proven that the attenuated live vaccine of PRV HN1201-gI.sup.−/gE.sup.−/11K.sup.−/28K.sup.− strain can provide excellent protection, showing excellent immune protection and safety; meanwhile it indicated again that a continuous deficiency of gI/gE/11K/28K genes (3455 bp in total) from the PRV virus would have no effect on its immunogenicity.

EXAMPLE 12

Acquisition of PRV PRV-ZJ01-R Strain, HeN1-R Strain and JS-2012-R Strain

(101) PRV PRV-ZJ01 strain, HeN1 strain and JS-2012 strain were subcultured to be attenuated respectively according to the procedures in Example 1, in order to obtain their attenuated PRV strains, named PRV-ZJ01-R strain, HeN1-R strain and JS-2012-R strain.

EXAMPLE 13

Study of Biological Characteristics of PRV-ZJ01-R Strain, PRV HeN1-R Strain and PRV JS-2012-R Strain

(102) 1. Pathogenicity Test of the Virus

(103) 30 piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were randomly divided into 6 groups (G, H, I, J, L and M), each with 5 piglets; another 15 piglets were used as the blank control group. The grouping and challenging conditions are shown in Table 14.

(104) TABLE-US-00014 TABLE 14 Grouping of the animals in the pathogenicity test for PRV-ZJ01-R strain, HeN1-R strain and JS-2012-R strain Strains used Group for inoculation Dose of inoculation G PRV-ZJ01-R strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation H PRV-ZJ01 strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation I HeN1-R strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation J HeN1 strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation L JS-2012-R strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation M JS-2012 strain inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml)/ piglet by intranasal instillation Blank DMEM medium inoculated with 1 ml/piglet by control intranasal instillation

(105) Piglets were observed for 28 days after inoculation of virus, while the temperature of piglets was measured daily, and clinical signs and death status were observed. The specific results are shown in Table 15.

(106) TABLE-US-00015 TABLE 15 Pathogenicity of PRV-ZJ01-R strain, HeN1-R strain and JS-2012-R strain in 7-day-old piglets Death Group Number Clinical signs status G G1 Body temperature increased for 1 day, Survived no other clinical signs G2 Body temperature increased for 1 day, Survived no other clinical signs G3 Body temperature increased for 1 day, Survived no other clinical signs G4 Normal body temperature, no clinical Survived signs G5 Body temperature increased for 1 day, Survived no other clinical signs H H1 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. H2 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. H3 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. H4 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. H5 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. I I1 Body temperature increased for 1 day, Survived no other clinical signs I2 Body temperature increased for 1 day, Survived no other clinical signs I3 Normal body temperature, , no clinical Survived signs I4 Body temperature increased for 1 day, Survived no other clinical signs I5 Body temperature increased for 1 day, Survived no other clinical signs J J1 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. J2 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. J3 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. J4 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. J5 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. L L1 Normal body temperature, no clinical Survived signs L2 Body temperature increased for 1 day, Survived no other clinical signs L3 Body temperature increased for 1 day, Survived no other clinical signs L4 Body temperature increased for 1 day, Survived no other clinical signs L5 Body temperature increased for 1 day, Survived no other clinical signs M M1 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. M2 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. M3 Body temperature increased for 5 days, Died on depression, complete loss of appetite, day 5 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. M4 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. M5 Body temperature increased for 4 days, Died on depression, complete loss of appetite, day 4 after staying lying, dyspnea, trembling; challenge neurological signs such as convulsions, turning around, and making strokes with their arms, etc. Blank K16 no abnormal clinical signs Survived control K17 no abnormal clinical signs Survived K18 no abnormal clinical signs Survived K19 no abnormal clinical signs Survived K20 no abnormal clinical signs Survived K21 no abnormal clinical signs Survived K22 no abnormal clinical signs Survived K23 no abnormal clinical signs Survived K24 no abnormal clinical signs Survived K25 no abnormal clinical signs Survived K26 no abnormal clinical signs Survived K27 no abnormal clinical signs Survived K28 no abnormal clinical signs Survived K29 no abnormal clinical signs Survived K30 no abnormal clinical signs Survived

(107) It showed in the results that inoculation with PRV PRV-ZJ01 strain into 7-day-old piglets could lead to death of 100% (5/5) of inoculated piglets, while PRV PRV-ZJ01-R strain displayed a significant reduction of virulence, only causing increased body temperature of four pigs, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy; inoculation with PRV HeN1 strain into 7-day-old piglets could lead to death of 100% (5/5) of inoculated piglets, while PRV HeN1-R strain displayed a significant reduction of virulence, only causing increased body temperature of four pigs, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy; inoculation with PRV JS-2012 strain into 7-day-old piglets could lead to death of 100% (5/5) of inoculated piglets, while PRV JS-2012-R strain displayed a significant reduction of virulence, only causing increased body temperature of four pigs, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy.

(108) Through the pathogenicity test it indicated that compared with the parent virulent strain, i.e. PRV PRV-ZJ01 strain, PRV-ZJ01-R strain displayed a significant reduction of pathogenicity, and was an attenuated virus strain; compared with the parent virulent strain, i.e. PRV HeN1 strain, PRV HeN1-R strain displayed a significant reduction of pathogenicity, and was an attenuated virus strain; compared with the parent virulent strain, i.e. PRV JS-2012 strain, PRV JS-2012-R strain displayed a significant reduction of pathogenicity, and was an attenuated virus strain.

(109) Meanwhile, in order to verify the stability of pathogenicity of different passages of PRV PRV-ZJ01-R strain, PRV HeN1-R strain and PRV JS-2012-R strain, a group of piglets (5 piglets) at 7 days of age which were negative for pseudorabies antigens and antibodies were inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml) of the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV PRV-ZJ01-R strain by intranasal instillation, respectively, and another five piglets were used as the blank control group; a group of piglets (5 piglets) at 7 days of age which were negative for pseudorabies antigens and antibodies were inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml) of the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HeN1-R strain by intranasal instillation, respectively, and another five piglets were used as the blank control group; a group of piglets (5 piglets) at 7 days of age which were negative for pseudorabies antigens and antibodies were inoculated with 1 ml (10.sup.7.0TCID.sub.50/ml) of the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV JS-2012-R strain by intranasal instillation, respectively, and another five piglets were used as the control group. The clinical manifestations of piglets were observed and recorded daily until 28 days after inoculation of virus.

(110) It showed in the results that, from the observation of piglets for 28 days after inoculation with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV PRV-ZJ01-R strain, PRV PRV-ZJ01-R strain displayed a significant reduction of virulence, only causing increased body temperature of 4˜5 pigs/group, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy; from the observation of piglets for 28 days after inoculation with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HeN1-R strain, PRV HeN1-R strain displayed a significant reduction of virulence, only causing increased body temperature of 4˜5 pigs/group, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy; from the observation of piglets for 28 days after inoculation with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV JS-2012-R strain, PRV JS-2012-R strain displayed a significant reduction of virulence, only causing increased body temperature of 4˜5 pigs/group, without any other clinical signs, or any changes of tissues or organs obtained from the necropsy.

(111) Through the pathogenicity test of different passages, it indicated that different passages of PRV PRV-ZJ01-R strain, HeN1-R strain and PRV JS-2012-R strain all displayed lower virulence.

(112) 2. Immunogenicity Assay

(113) On the 21.sup.st day after immunization, all the five piglets inoculated with PRV PRV-ZJ01-R strain and five piglets in the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV PRV-ZJ01 strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The results are shown in Table 16.

(114) On the 21.sup.st day after immunization, all the five piglets inoculated with PRV HeN1-R strain and five piglets in the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HeN1 strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The results are shown in Table 16.

(115) On the 21.sup.st day after immunization, all the five piglets inoculated with PRV JS-2012-R strain and five piglets in the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV JS-2012 strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The results are shown in Table 16.

(116) TABLE-US-00016 TABLE 16 Pathogenicity of PRV-ZJ01-R strain, PRV HeN1-R strain and PRV JS-2012-R strain in 7-day-old piglets Protection Group Number Clinical signs and death status rate G G1 Normal body temperature, normal appetite, no 100% (5/5) abnormal clinical signs, survived G2 Normal body temperature, normal appetite, no abnormal clinical signs, survived G3 Normal body temperature, normal appetite, no abnormal clinical signs, survived G4 Normal body temperature, normal appetite, no abnormal clinical signs, survived G5 Normal body temperature, normal appetite, no abnormal clinical signs, survived Blank K16 Body temperature increased, depression, loss of 0% (0/5) control appetite, significant clinical signs, died on day 4 after challenge K17 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 5 after challenge K18 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 5 after challenge K19 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 5 after challenge K20 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 4 after challenge I I1 Normal body temperature, normal appetite, no 100% (5/5) abnormal clinical signs, survived I2 Normal body temperature, normal appetite, no abnormal clinical signs, survived I3 Normal body temperature, normal appetite, no abnormal clinical signs, survived I4 Normal body temperature, normal appetite, no abnormal clinical signs, survived I5 Normal body temperature, normal appetite, no abnormal clinical signs, survived Blank K21 Body temperature increased, depression, loss of 0% (0/5) control appetite, significant clinical signs, died on day 5 after challenge K22 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 4 after challenge K23 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 4 after challenge K24 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 5 after challenge K25 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 5 after challenge L L1 Normal body temperature, normal appetite, no 100% (5/5) abnormal clinical signs, survived L2 Normal body temperature, normal appetite, no abnormal clinical signs, survived L3 Normal body temperature, normal appetite, no abnormal clinical signs, survived L4 Normal body temperature, normal appetite, no abnormal clinical signs, survived L5 Normal body temperature, normal appetite, no abnormal clinical signs, survived Blank K26 Body temperature increased, depression, loss of 0% (0/5) control appetite, significant clinical signs, died on day 5 after challenge K27 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 4 after challenge K28 Body temperature increased, depression, complete loss of appetite, significant clinical signs, died on day 4 after challenge K29 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 5 after challenge K30 Body temperature increased, depression, loss of appetite, significant clinical signs, died on day 5 after challenge

(117) The results indicated that all the piglets inoculated with PRV PRV-ZJ01-R strain were healthy and alive, while all from the control group died; all the piglets inoculated with PRV HeN1-R strain were healthy and alive, while all from the control group died; all the piglets inoculated with PRV JS-2012-R strain were healthy and alive, while all from the control group died

(118) According to the immunogenicity assay, the PRV PRV-ZJ01-R strain can provide excellent protection against PRV PRV-ZJ01 strain, showing excellent immunogenicity; the PRV HeN1-R strain can provide excellent protection against PRV HeN1 strain, showing excellent immunogenicity; the PRV JS-2012-R strain can provide excellent protection against PRV JS-2012 strain, showing excellent immunogenicity.

(119) Meanwhile, in order to verify the stability of immunogenicity of different passages of PRV PRV-ZJ01-R strain, PRV HeN1-R strain and PRV JS-2012-R strain, on the 21.sup.st day after immunization, all the immune groups inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV PRV-ZJ01-R strain as well as the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV PRV-ZJ01 strain; after challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed; on the 21.sup.st day after immunization, all the immune groups inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HeN1-R strain as well as the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HeN1 strain; after challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed; on the 21.sup.st day after immunization, all the immune groups inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV JS-2012-R strain as well as the control group were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV JS-2012 strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed.

(120) The result indicated that all the piglets inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV PRV-ZJ01-R strain were healthy and alive, while all from the control group died; all the piglets inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV HeN1-R strain were healthy and alive, while all from the control group died; all the piglets inoculated with the cultures of the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th and 110.sup.th passages of PRV JS-2012-R strain were healthy and alive, while all from the control group died.

(121) According to the immunogenicity assay of different passages, the culture of different passages of PRV PRV-ZJ01-R strain all can provide excellent protection against PRV PRV-ZJ01 strain, showing excellent immunogenicity; the culture of different passages of PRV HeN1-R strain all can provide excellent protection against PRV HeN1 strain, showing excellent immunogenicity; the culture of different passages of PRV JS-2012-R strain all can provide excellent protection against PRV JS-2012 strain, showing excellent immunogenicity.

(122) 3. Reversion of Virulence Test of the Virus

(123) 30 piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were randomly divided into 5 groups, each with 6 piglets. 6 piglets of Group 1 which were negative for pseudorabies antigens and antibodies were inoculated with 10.sup.7.0 TCID.sub.50/piglet of the cultures of PRV PRV-ZJ01-R strain (the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th, 110.sup.th passages and the 1.sup.st+30.sup.th+60.sup.th+85.sup.th+110.sup.th passages) respectively by intranasal instillation. On day 14, they were raised together with the 6 piglets of Group 2 which were negative for pseudorabies antigens and antibodies. 14 days later, the 6 piglets of Group 1 were drawn back, and again, the 6 piglets of Group 3, which were negative for pseudorabies antigens and antibodies, were raised together with the 6 piglets of Group 2, and so on. After 4 serial passages, all the drawn piglets were killed in order to observe if there were any pathological changes.

(124) 30 piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were randomly divided into 5 groups, each with 6 piglets. 6 piglets of Group 1 which were negative for pseudorabies antigens and antibodies were inoculated with 10.sup.7.0TCID.sub.50/piglet of the cultures of PRV HeN1-R strain (the 1.sup.st, 30.sup.th 60.sup.th, 85.sup.th, 110.sup.th passages and the 1.sup.st+30.sup.th+60.sup.th+85.sup.th+110.sup.th passages) respectively by intranasal instillation. On day 14, they were raised together with the 6 piglets of Group 2 which were negative for pseudorabies antigens and antibodies. 14 days later, the 6 piglets of Group 1 were drawn back, and again, the 6 piglets of Group 3 which were negative for pseudorabies antigens and antibodies, were raised together with the 6 piglets of Group 2, and so on. After 4 serial passages, all the drawn piglets were killed in order to observe if there are any pathological changes.

(125) 30 piglets at 7 days of age which were negative for pseudorabies antigens and antibodies were randomly divided into 5 groups, each with 6 piglets. 6 piglets of Group 1 which were negative for pseudorabies antigens and antibodies were inoculated with 10.sup.7.0TCID.sub.50/piglet of the cultures of PRV JS-2012-R strain (the 1.sup.st, 30.sup.th, 60.sup.th, 85.sup.th, 110.sup.th passages and the 1.sup.st+30.sup.th+60.sup.th+85.sup.th+110.sup.th passages) respectively by intranasal instillation. On day 14, they were raised together with the 6 piglets of Group 2 which were negative for pseudorabies antigens and antibodies. 14 days later, the 6 piglets of Group 1 were drawn back, and again, the 6 piglets of Group 3, which were negative for pseudorabies antigens and antibodies, were raised together with the 6 piglets of Group 2, and so on. After 4 serial passages, all the drawn piglets were killed in order to observe if there are any pathological changes.

(126) It showed in the results that no abnormal changes were found during the clinical observation and gross anatomy of 30 experimental piglets infected with PRV-ZJ01-R strain, 30 experimental piglets infected with HeN1-R strain, and 30 experimental piglets infected with JS-2012-R strain by the 4th serial passage of the cohabitation infection experiment, indicating that there was no reversion of virulence of the three attenuated strains. Therefore, the safety of the vaccines can be ensured since the viruses, after being inoculated into piglets, would not evolve into virulent viruses which are able to cause disease.

(127) 4. Genes Sequences Analysis

(128) The genome amplification of the cultures of the 1.sup.st passage to 110.sup.th passage of PRV-ZJ01-R strain was accomplished by means of RT-PCR (The genomic DNA of the culture of different passages was amplified respectively). The product acquired from amplification was recovered, purified, and linked to the plasmid vector for sequencing, so that the nucleotide sequence of the viral gene was determined and transformed through computer softwares into the amino acid sequence of the virus. The obtained amino acids sequence was compared with the amino acids sequence of the parent virulent strain, i.e. PRV-ZJ01 strain via softwares for sequence analysis, and the amino acids sequence of the virus was characterized.

(129) The genome amplification of the cultures of the 1.sup.st passage to 110.sup.th passage of PRV HeN1-R strain was accomplished by means of RT-PCR (The genomic DNA of the cultures of different passages was amplified respectively). The product acquired from amplification was recovered, purified, and linked to the plasmid vector for sequencing, so that the nucleotide sequence of the viral gene was determined and transformed through computer softwares into the amino acid sequence of the virus. The obtained amino acids sequence was compared with the amino acids sequence of the parent virulent strain, i.e. PRV HeN1 strain via softwares for sequence analysis, and the amino acids sequence of the virus was characterized.

(130) The genome amplification of the cultures of the 1.sup.st passage to 110.sup.th passage of PRV JS-2012-R strain was accomplished by means of RT-PCR (The genomic DNA of the culture of different passages was amplified respectively). The product acquired from amplification was recovered, purified, and linked to the plasmid vector for sequencing, so that the nucleotide sequence of the viral gene was determined and transformed through computer softwares into the amino acids sequence of the virus. The obtained amino acids sequence was compared with the amino acids sequence of the parent virulent strain, i.e. PRV JS-2012 strain via softwares for sequence analysis, and the amino acids sequence of the virus was characterized.

(131) It showed in the results that for the cultures of the 1.sup.st passage to 110.sup.th passage of PRV PRV-ZJ01-R strain, the amino acids encoded by each viral genes commonly have continuous deficiency of gI/gE/11K/28K genes, 3455 bp in total, of which the deficiency site and size are totally the same as those of PRV HN1201-R strain; for the cultures of the 1.sup.st passage to 110.sup.th passage of PRV HeN1-R strain, the amino acids encoded by each viral genes commonly have continuous deficiency of gI/gE/11K/28K genes, 3455 bp in total, of which the deficiency site and size are totally the same as those of PRV HN1201-R strain; for the cultures of the 1.sup.st passage to 110.sup.th passage of PRV JS-2012-R strain, the amino acids encoded by each viral genes commonly have continuous deficiency of gI/gE/11K/28K genes, 3455 bp in total, of which the deficiency site and size are totally the same as those of PRV HN1201-R strain; compared with their parent virulent strains, each of PRV PRV-ZJ01-R strain, PRV HeN1-R strain and PRV JS-2012-R strain has continuous deficiency of gI/gE/11K/28K genes, 3455 bp in total.

(132) It indicated that a common characteristic change of the amino acids encoded by the viral genes of the different passages of cultures of PRV PRV-ZJ01-R strain, PRV HeN1-R strain and PRV JS-2012-R strain is consistent with that of the amino acids encoded by the viral genes of PRV HN1201-R strain, which is caused by the deficiency of genes encoding said amino acids sequences, showing the stability of the method of attenuating the PRV by passage according to the present invention, and in the meanwhile, further indicating that continuous deficiency of gI/gE/11K/28K genes, 3455 bp in total, in PRV is the reason for the reduction of virulence of its parent virulent strain.

EXAMPLE 14

Preparation of the Attenuated Live Vaccines of PRV PRV-ZJ01-R Strain, PRV HeN1-R Strain and PRV JS-2012-R Strain

(133) The attenuated live vaccines of PRV PRV-ZJ01-R strain, PRV HeN1-R strain and PRV JS-2012-R strain were prepared according to the procedure in Example 3. The specific ratios of contents of the vaccines are shown in Table 17.

(134) TABLE-US-00017 TABLE 17 ratios of contents of the attenuated live vaccines of PRV PRV-ZJ01-R strain, HeN1-R strain and JS-2012-R strain Antigen (TCID.sub.50) Cryoprotectant (V/V) Vaccine 6 10.sup.6.0 50% (PRV-ZJ01-R strain) Vaccine 7 (HeN1-R 10.sup.6.0 50% strain) Vaccine 8 (JS-2012-R 10.sup.6.0 50% strain)

EXAMPLE 15

Immunogenicity Assay of the Attenuated Live Vaccines of PRV PRV-ZJ01-R Strain, PRV HeN1-R Strain and PRV JS-2012-R Strain

(135) 30 9-day-old piglets which were negative for PRV antigens and PRV antibodies were randomly divided into 6 groups, each with 5 piglets, and the piglets were immunized with the attenuated live vaccines of PRV PRV-ZJ01-R strain, PRV HeN1-R strain and PRV JS-2012-R strain prepared in Example 3. Piglets in Group 1 were immunized with Vaccine 6, those in Group 3 were immunized with Vaccine 7 and those in Group 5 were immunized with Vaccine 8, and Groups 2, 4 and 6 were all the control groups. On day 21 after immunization, the piglets in Groups 1 and 2 were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV PRV-ZJ01 strain, those in Groups 3 and 4 were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HeN1 strain, and those in Groups 5 and 6 were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV JS-2012 strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The results are shown in Table 18.

(136) TABLE-US-00018 TABLE 18 Results of immunogenicity assay of three attenuated live vaccines Number Dose of Clinical signs and Rate of Group of piglets Challenge death status protection Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) 6 piglet of temperature, normal PRV-ZJ01 appetite, no abnormal strain clinical signs, survived Control 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) group piglet of symptoms like PRV-ZJ01 increased body strain temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 4 after challenge, and all died within 5 days after challenge. Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) 7 piglet of temperature, normal HeN1 strain appetite, no abnormal clinical signs, survived Control 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) group piglet of symptoms like HeN1 strain increased body temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 4after challenge, and all died within 5 days after challenge. Vaccine 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) 8 piglet of temperature, normal JS-2012 appetite, no abnormal strain clinical signs, survived Control 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) group piglet of symptoms like JS-2012 increased body strain temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 4 after challenge, and all died within 5 days after challenge.

(137) The results indicated that immunizing piglets with the attenuated live vaccines of PRV-ZJ01-R strain prepared in example 14 can block virus infection (i.e. prevent occurrence of clinical signs), and provide 100% (5/5) protection rate for piglets, while all the piglets in the blank control group died by day 5 after challenge; immunizing piglets with the attenuated live vaccine of PRV HeN1-R strain prepared in example 14 can block virus infection (i.e. prevent occurrence of clinical signs), and provide 100% (5/5) protection rate for piglets, while all the piglets in the blank control group died by day 5 after challenge; immunizing piglets with the attenuated live vaccine of PRV JS-2012-R strain prepared in example 14 can block virus infection (i.e. prevent occurrence of clinical signs), and provide 100% (5/5) protection rate for piglets, while all the piglets in the blank control group died by day 5 after challenge.

(138) It has proven that the attenuated live vaccines of PRV PRV-ZJ01-R strain, PRV HeN1-R strain and PRV JS-2012-R strain can provide excellent protection, showing excellent immune protection and safety; meanwhile it indicated again that continuous deficiency of gI/gE/11K/28K genes, 3455 bp in total, from the PRV virus would have no effect on its immunogenicity.

EXAMPLE 16

Broad-Spectrum Immunogenicity Assay of the Attenuated Live Vaccine of PRV HN1201-R Strain

(139) 50 9-day-old piglets which were negative for PRV antigens and PRV antibodies were randomly divided into 10 groups, each with 5 piglets, and the piglets were immunized with the attenuated live vaccine of PRV HN1201-R strain prepared in Example 3. Piglets in Groups 1, 3, 5, 7 and 9 was immunized with Vaccine 1, and Groups 2, 4, 6, 8 and 10 were the control groups. On day 21 after immunization, the piglets in Groups 1 and 2 were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HN1202 strain, those in Groups 3 and 4 were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV Fa strain, those in Groups 5 and 6 were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV PRV-ZJ01 strain, those in Groups 7 and 8 were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV HeN1 strain, and those in Groups 9 and 10 were challenged with 1×10.sup.7.0TCID.sub.50/piglet of PRV JS-2012 strain. After challenge, the body temperature of piglets was measured daily, and in the meanwhile clinical signs and death status were observed. The specific results are shown in Table 19.

(140) TABLE-US-00019 TABLE 19 Results of broad-spectrum immunogenicity assay of the attenuated live vaccines of PRV HN1201-R Number Dose of Clinical signs and Rate of Group of piglets Challenge death status protection  1 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) piglet of temperature, normal HN1202 appetite, no abnormal strain clinical signs, survived  2 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) piglet of symptoms like HN1202 increased body strain temperature, depression, complete or partial loss of appetite, and significant clinical signs; Four died on day 4 after challenge, and all died within 5 days after challenge.  3 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) piglet of temperature, normal Fa strain appetite, no abnormal clinical signs, survived  4 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) piglet of symptoms like Fa strain increased body temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 4 after challenge, and all died within 5 days after challenge.  5 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) piglet of temperature, normal PRV-ZJ01 appetite, no abnormal strain clinical signs, survived  6 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) piglet of symptoms like PRV-ZJ01 increased body strain temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 4 after challenge, and all died within 5 days after challenge.  7 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) piglet of temperature, normal HeN1 strain appetite, no abnormal clinical signs, survived  8 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) piglet of symptoms like HeN1 strain increased body temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 4 after challenge, and all died within 5 days after challenge.  9 5 10.sup.7.0TCID.sub.50/ Normal body 100% (5/5) piglet of temperature, normal JS-2012 appetite, no abnormal strain clinical signs, survived 10 5 10.sup.7.0TCID.sub.50/ All the pigs displayed 0% (0/5) piglet of symptoms like JS-2012 increased body strain temperature, depression, complete or partial loss of appetite, and significant clinical signs; two died on day 4 after challenge, and all died within 5 days after challenge.

(141) The results indicated that immunizing piglets with the attenuated live vaccine of PRV HN1201-R strain prepared in Example 3 can block virus infection (i.e. prevent occurrence of clinical signs), and provide 100% (5/5) protection rate for piglets, while all the piglets in the blank control group died by day 5 after challenge.

(142) It has proven that the attenuated live vaccine of PRV HN1201-R strain prepared according to the present invention can provide a fully protection against the epidemic PRV from different sources, showing excellent broad-spectrum immunogenicity.

(143) Those are only preferred embodiments of the present invention as described above, which cannot be used to limit the present invention in any forms. Although the present invention has been revealed as described above in the form of the preferred embodiments, they are not intended to limit the present invention. Any skilled in the art can make several changes to the above technical content or modify the above technical content as equivalent embodiments with equivalent substitution, without departing from the technical scope of the present invention. Any simple change, equivalent substitution or modification etc, which are made to the above embodiments, based on the technical nature of the present invention, without departing from the content of technical solution of the present invention, should fall within the scope of protection of the present invention.