Key phosphorylation site of temperature sensitivity of influenza A virus and use thereof

Abstract

A recombinant virus is obtained by mutating a codon that encodes a tyrosine residue at position 385 of NP protein in the genome of influenza A virus to a codon of phenylalanine residue. The virus WSN-Y385F is a temperature-sensitive virus that can normally replicate and survive at 37 C., and cannot normally replicate and cannot survive at 33 C. Phosphorylation of a NP protein of influenza A virus can be inhibited by mutating an amino acid residue at position 385 from N terminal of the NP protein of influenza A virus, from a tyrosine to a phenylalanine. The recombinant virus can be used in analyzing mechanisms of infection by influenza virus, and in connection with methods of prevention and treatment of infection by influenza virus.

Claims

1. A recombinant influenza A virus, comprising a genome with a nucleoprotein (NP) in which a codon that encodes a tyrosine residue at a position corresponding to position 385 in SEQ ID NO: 1 is mutated to a codon of that encodes a phenylalanine residue.

2. An influenza A nucleoprotein protein having a phenylalanine residue at a position corresponding to position 385 in SEQ ID NO: 1.

3. An isolated gene encoding the influenza A nucleoprotein of claim 2.

4. A recombinant plasmid comprising the isolated gene of claim 3.

5. A method for preparing a recombinant influenza A virus comprising: co-transfecting in vitro mammalian cells with plasmid pHH21-PA, plasmid pHH21-PB1, plasmid pHH21-PB2, plasmid pHH21-HA, plasmid pHH21-NA, plasmid pHH21-M, plasmid pHH21-NS, plasmid pcDNA3.0-PA, plasmid pcDNA3.0-PB1, plasmid pcDNA3.0-PB2, recombinant plasmid pHH21-NP-Y385F and recombinant plasmid pcDNA3.0-NP-Y385F, and then culturing the cells to obtain the recombinant virus.

6. A method of removing a phosphorylation site of a NP protein of influenza A virus comprising mutating an amino acid residue at a position corresponding to position 385 of SEQ ID NO: 1 from a tyrosine to an phenylalanine.

7. A method of preparing an influenza A virus vaccine comprising obtaining the recombinant virus produced in the method of claim 5, formulated in a solution.

8. A method of inducing an immune response in a subject comprising administering the recombinant virus of claim 1 to the subject.

9. An influenza A virus vaccine comprising as an active ingredient the recombinant virus obtained from the method of claim 7.

10. A plasmid combination consisting of the plasmid pHH21-PA, plasmid pHH21-PB1, plasmid pHH21-PB2, plasmid pHH21-HA, plasmid pHH21-NA, plasmid pHH21-M, plasmid pHH21-NS, plasmid pcDNA3.0-PA, plasmid pcDNA3.0-PB1, plasmid pcDNA3.0-PB2, recombinant plasmid pHH21-NP-Y385F and recombinant plasmid pcDNA3.0-NP-Y385F.

11. A kit for preparing a recombinant influenza A virus, comprising the plasmid combination of claim 10.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the result of step 1 of Example 1.

(2) FIG. 2 shows the result of step 2 of Example 1.

(3) FIG. 3 shows the result of Example 2.

(4) FIG. 4 shows the result of Example 3.

(5) FIG. 5 shows the result of Example 4. (A) 33 C., (B) 37 C.

(6) FIG. 6 shows the result of Example 5. (A) lung (37 C.), (B) turbinal bones (33 C.).

THE BEST MODE FOR CARRYING OUT THE INVENTION

(7) The following Examples will provide a better understanding of the present invention, but do not limit the invention. The experimental methods in the following Examples, unless otherwise specified, are conventional methods. The test materials used in the following Examples, unless otherwise specified, all are purchased from conventional biochemical reagents stores. Quantitative tests in the following Examples are set three times to repeat the experiments and the result is the average.

(8) WSN virus A/WSN/1933 (H1N1) strain: Neumann, G. et al., Generation of influenza A viruses entirely from cloned cDNAs. P Natl Acad Sci Usa 96 (16), 9345 (1999). WSN virus is the influenza virus. In the Examples, the virus infection solution is used to adjust the virus concentration so as to achieve different doses of infection.

(9) Vector pHH21: Neumann, G. et al., Generation of influenza A viruses entirely from cloned cDNAs. P Natl Acad Sci Usa 96 (16), 9345 (1999).

(10) HEK 293T/17 cells (abbreviated as 293T cell-derived line, human embryonic kidney cells): ATCC, CRL-11268. Vector pcDNA3.0: Shanghai CPG Biotech. Co., Ltd., catalog number CPC030. E. coli DH5: Shanghai Beinuo Biotech. Co., Ltd. A549 cells (human lung adenocarcinoma cells): Shanghai Bioleaf Biotech Co., Ltd. BALB/c mice: Beijing Vital River Laboratory Animal Technology Co., Ltd. MDCK cells: ATCC, CCL-34.

(11) Cell lysis solution (pH 7.4): comprising 150 mM sodium chloride, 20 mM HEPES, 10% (by volume) glycerol, 1 mM EDTA, 1 g/100 mL NP40, protease inhibitor (cocktail), and the balance water.

(12) Elution buffer: the concentration of sodium chloride is 300 mM, the other components are same as those in the cell lysis solution.

(13) Virus infection solution: comprising 2 g/ml TPCK-treated trypsin (trypsin is added in a manner of trypsin stock solution, and the trypsin stock solution is a solution with the trypsin concentration of 0.25 g/100 mL formulated in PBS buffer), 100 U/ml penicillin and 100 U/ml streptomycin in serum-free DMEM medium.

(14) Alkaline phosphatase: Takara, catalog number D2250. Protease inhibitor (cocktail) is purchased from Roche company. SDM enzyme: Beijing SBS Genetech Co., Ltd., catalog number: SDM-15. The gel used in phos-tag SDS-PAGE is Phos-tag Acrylamide that is purchased from Wako (Japan). Anti-phosphotyrosine antibody (murine monoclonal antibody, sc-508) is purchased from Santa Cruz. Pre-stained protein standard with known molecular weight is purchased from Thermo. Sodium chloride, N-(2-hydroxyethyl) piperazine-N-2-ethanesulfonic acid (abbreviated as HEPES), glycerin, ethylenediaminetetraacetic acid (abbreviated as EDTA), Nonidet P-40 (abbreviated as NP 40) and TPCK-treated trypsin are all purchased from Sigma. Bovine serum albumin (BSA) is purchased from Jiang Chen Bio. Penicillin and streptomycin are purchased from Beyotime company. Sodium dodecyl sulfate (abbreviated as SDS) and low melting point agarose are purchased from Amersco Company.

(15) Anti-NP protein monoclonal antibody (i.e. murine monoclonal antibody against influenza A virus NP protein): M-R Yu#, X-L Liu#, Sh Cao, Zh-D Zhao, K Zhang, Q Xie, C-W Chen, Sh-Y Gao, Y-H Bi, L Sun, X Ye, George F. Gao, W-J Liu*. 2012. Identification and Characterization of three novel nuclear export signals in influenza A virus nucleoprotein. Journal of Virology, 86(9):4970-80.

(16) Plasmid pHH21-PA, plasmid pHH21-PB1, plasmid pHH21-PB2, plasmid pHH21-HA, plasmid pHH21-NP, plasmid pHH21-NA, plasmid pHH21-M, plasmid pHH21-NS, plasmid pcDNA3.0-PA, plasmid pcDNA3.0-PB1, plasmid pcDNA3.0-PB2 and plasmid pcDNA3.0-NP are co-transfected into HEK 293T/17 cells, then the cells are cultured to obtain WSN virus A/WSN/1933 (H1N1) strain. WSN virus A/WSN/1933 (H1N1) strain is also known as WSN virus wild type.

Example 1. Obtainment of Phosphorylated NP Protein and Identification of Phosphorylation Sites

(17) I. Obtainment of Phosphorylated NP Protein

(18) 1. HEK 293T/17 cells were infected with A/WSN/1933 (H1N1) strain at a dose of MOI=0.1 and harvested after being cultured at 37 C. for 12-16 hours.

(19) 2. The cells harvested in step 1 were treated with cell lysis solution at 4 C. for 30 minutes and centrifuged at 12000 rpm for 15 minutes, the supernatant was collected.

(20) 3. Anti-NP protein monoclonal antibody was added to the supernatant obtained in step 2 and incubated at 4 C. for 1 hour. Then protein G beads were added and incubated at 4 C. for 3 hours. The supernatant was discarded, and the beads were washed with elution buffer 3 times (10 minutes each time) at 4 C., the substance bound to the beads was the NP protein.

(21) 4. The NP protein-bound beads obtained in step 3 were treated with alkaline phosphatase at 37 C. for 2 hours (the function of the alkaline phosphatase was to dephosphorylate the phosphorylated protein).

(22) 5. The NP protein prior to alkaline phosphatase treatment (obtained in Step 3) and the NP protein after alkaline phosphatase treatment (obtained in Step 4) were respectively subjected to phos-tag SDS-PAGE and silver staining for coloration.

(23) The result was shown in FIG. 1. In FIG. 1, lane 1 was the NP protein after alkaline phosphatase treatment and lane 2 was the NP protein prior to alkaline phosphatase treatment. The NP protein after alkaline phosphatase treatment was used as a standard NP protein, a band in which the NP protein prior to alkaline phosphatase treatment on the gel presents slower migration rate than that of the standard NP protein and sensitive to alkaline phosphatase was the phosphorylated NP band. The results showed that the NP protein obtained in step 3 was a phosphorylated protein that can be dephosphorylated by alkaline phosphatase.

(24) II. Phosphorylation Sites were Identified by Mass Spectrometry

(25) The phosphorylated NP band was cut from the gel and sent to a large instrument platform of the Institute of Zoology, Chinese Academy of Sciences, for sample processing and mass spectrometry identification (Nano-LC MS/MS, LCQ DECA XP.sup.PLUS Thermo).

(26) The result was shown in FIG. 2. The difference in nuclear mass ratio between B2 and b3 indicated that Y385 was modified by phosphorylation. The identification result showed that the band was the NP protein of influenza A virus and there was a phosphorylation modification at tyrosine residue at position 385.

Example 2. Preparation and Phosphorylation Identification of the Mutant Protein

(27) I. Construction of Recombinant Plasmids

(28) 1. Construction of Plasmid pHH21-PA

(29) The double stranded DNA molecule shown in SEQ ID NO: 3 of the Sequence Listing was inserted into the BsmBI restriction site of vector pHH21 to obtain a plasmid pHH21-PA.

(30) 2. Construction of Plasmid pHH21-PB1

(31) The double stranded DNA molecule shown in SEQ ID NO: 4 of the Sequence Listing was inserted into the BsmBI restriction site of vector pHH21 to obtain a plasmid pHH21-PB1.

(32) 3. Construction of Plasmid pHH21-PB2

(33) The double-stranded DNA molecule shown in SEQ ID NO: 5 of the Sequence Listing was inserted into the BsmBI restriction site of vector pHH21 to obtain a plasmid pHH21-PB2.

(34) 4. Construction of Plasmid pHH21-HA

(35) The double stranded DNA molecule shown in SEQ ID NO: 6 of the Sequence Listing was inserted into the BsmBI restriction site of vector pHH21 to obtain a plasmid pHH21-HA.

(36) 5. Construction of Plasmid pHH21-NP

(37) The double stranded DNA molecule shown in SEQ ID NO: 7 of the Sequence Listing was inserted into the BsmBI restriction site of vector pHH21 to obtain a plasmid pHH21-NP.

(38) 6. Construction of Plasmid pHH21-NA

(39) The double stranded DNA molecule shown in SEQ ID NO: 8 of the Sequence Listing was inserted into the BsmBI restriction site of vector pHH21 to obtain a plasmid pHH21-NA.

(40) 7. Construction of Plasmid pHH21-M

(41) The double stranded DNA molecule shown in SEQ ID NO: 2 of the Sequence Listing was inserted into the BsmBI restriction site of vector pHH21 to obtain a plasmid pHH21-M.

(42) 8. Construction of Plasmid pHH21-NS

(43) The double stranded DNA molecule shown in SEQ ID NO: 9 of the Sequence Listing was inserted into the BsmBI restriction site of vector pHH21 to obtain a plasmid pHH21-NS.

(44) 9. Construction of Plasmid pcDNA3.0-PA

(45) The double-stranded DNA molecule shown in SEQ ID NO: 3 of the Sequence Listing was inserted between the KpnI and XhoI restriction sites of vector pcDNA 3.0 to obtain a plasmid pcDNA3.0-PA.

(46) 10. Construction of Plasmid pcDNA3.0-PB1

(47) The double stranded DNA molecule shown in SEQ ID NO: 4 of the Sequence Listing was inserted between the KpnI and XhoI restriction sites of vector pcDNA3.0 to obtain a plasmid pcDNA3.0-PB1.

(48) 11. Construction of Plasmid pcDNA3.0-PB2

(49) The double stranded DNA molecule shown in SEQ ID NO: 5 of the Sequence Listing was inserted between the KpnI and XhoI restriction sites of vector pcDNA3.0 to obtain a plasmid pcDNA3.0-PB2.

(50) 12. Construction of Plasmid pcDNA3.0-NP

(51) The double stranded DNA molecule shown in SEQ ID NO: 7 of the Sequence Listing was inserted between the KpnI and XhoI restriction sites of vector pcDNA3.0 to obtain a plasmid pcDNA3.0-NP.

(52) 13. Construction of Recombinant Plasmids NP-Y385A-F: 5-ctgagaagcagaGCGtgggccataaggaccagaagtggag-3 (SEQ ID NO: 11); NP-Y385A-R: 5-ccttatggcccaCGCtctgcttctcagttcaagggtacttg-3 (SEQ ID NO: 12). NP-Y385F-F: 5-ctgagaagcagaTTCtgggccataaggaccagaagtggag-3 (SEQ ID NO: 13); NP-Y385F-R: 5-ccttatggcccaGAAtctgcttctcagttcaagggtacttg-3, (SEQ ID NO: 14). NP-Y385E-F: 5-ctgagaagcagaGAGtgggccataaggaccagaagtggag-3, (SEQ ID NO: 15); NP-Y385E-R: 5-ccttatggcccaCTCtctgcttctcagttcaagggtacttg-3, SEQ ID NO: 16).

(53) A variety of recombinant plasmids were constructed using Newpep point mutation kit (Cat. No. 80111-01, Beijing Newpep Biolotechn Co., Ltd.) according to the kit instructions.

(54) (1) PCR amplification was carried out using the plasmid pHH21-NP as a template, and a primer pair consisting of NP-Y385A-F and NP-Y385A-R to obtain a PCR amplification product (mutating a plasmid).

(55) (2) The PCR amplification product in step (1) was digested with SDM enzyme at 37 C. for 2 hours (digesting a template plasmid).

(56) (3) The product in step (2) was transformed into competent cells of E. coli DH5 to obtain a recombinant bacterium Y385A-I (i.e. E. coli comprising the recombinant plasmid pHH21-NP-Y385A). Based on the sequencing results, the recombinant plasmid pHH21-NP-Y385A was structurally described as follows: codon tac encoding tyrosine at position 385 from N-terminal of the NP protein in the plasmid pHH21-NP was mutated to codon GCG of alanine.

(57) (4) PCR amplification was carried out using pcDNA3.0-NP as a template, and a primer pair consisting of NP-Y385A-F and NP-Y385A-R to obtain a PCR amplification product (mutating a plasmid).

(58) (5) The PCR amplification product in step (4) was digested with SDM enzyme at 37 C. for 2 hours (digesting a template plasmid).

(59) (6) The product in step (5) was transformed into competent cells of E. coli DH5 to obtain a recombinant bacterium Y385A-II (i.e. E. coli comprising the recombinant plasmid pcDNA3.0-NP-Y385A). Based on the sequencing results, the recombinant plasmid pcDNA3.0-NP-Y385A was structurally described as follows: codon tac encoding tyrosine at position 385 from N-terminal of the NP protein in the plasmid pcDNA3.0-NP was mutated to codon GCG of alanine.

(60) (7) PCR amplification was carried out using the plasmid pHH21-NP as a template, and a primer pair consisting of NP-Y385F-F and NP-Y385F-R to obtain a PCR amplification product (mutating a plasmid).

(61) (8) The PCR amplification product in step (7) was digested with SDM enzyme at 37 C. for 2 hours (digesting a template plasmid).

(62) (9) The product in step (8) was transformed into competent cells of E. coli DH5 to obtain a recombinant bacterium Y385F-I (i.e. E. coli comprising the recombinant plasmid pHH21-NP-Y385F). Based on the sequencing results, the recombinant plasmid pHH21-NP-Y385F was structurally described as follows: codon tac encoding tyrosine at position 385 from N-terminal of the NP protein in the plasmid pHH21-NP was mutated to codon TTC of phenylalanine; that was, the double stranded DNA molecule shown in SEQ ID NO: 10 of the Sequence Listing was inserted into the BsmBI restriction site of the vector pHH21.

(63) (10) PCR amplification was carried out using pcDNA3.0-NP as a template, and a primer pair consisting of NP-Y385F-F and NP-Y385F-R to obtain a PCR amplification product (mutating plasmid).

(64) (11) The PCR amplification product in step (10) was digested with SDM enzyme at 37 C. for 2 hours (digesting a template plasmid).

(65) (12) The product in step (11) was transformed into competent cells of E. coli DH5 to obtain a recombinant bacterium Y385F-II (i.e. E. coli comprising the recombinant plasmid pcDNA3.0-NP-Y385F). Based on the sequencing results, the recombinant plasmid pcDNA3.0-NP-Y385F was structurally described as follows: codon tac encoding tyrosine at position 385 from N-terminal of the NP protein in the plasmid pcDNA3.0-NP was mutated to codon TTC of phenylalanine; that was, the double stranded DNA molecule shown in SEQ ID NO: 10 of the Sequence Listing was inserted between the KpnI and XhoI restriction sites of the vector pcDNA3.0.

(66) (13) PCR amplification was carried out using the plasmid pHH21-NP as a template, and a primer pair consisting of NP-Y385E-F and NP-Y385E-R to obtain a PCR amplification product (mutating a plasmid).

(67) (14) The PCR amplification product in step (13) was digested with SDM enzyme at 37 C. for 2 hours (digesting a template plasmid).

(68) (15) The product in step (14) was transformed into competent cells of E. coli DH5a to obtain a recombinant bacterium Y385E-I (i.e. E. coli comprising the recombinant plasmid pHH21-NP-Y385E). Based on the sequencing results, the recombinant plasmid pHH21-NP-Y385E was structurally described as follows: codon tac encoding tyrosine at position 385 from N-terminal of the NP protein in the plasmid pHH21-NP was mutated to codon GAG of glutamic acid.

(69) (16) PCR amplification was carried out using the plasmid pcDNA3.0-NP as a template, and a primer pair consisting of NP-Y385E-F and NP-Y385E-R to obtain a PCR amplification product (mutating a plasmid).

(70) (17) The PCR amplification product in step (16) was digested with SDM enzyme at 37 C. for 2 hours (digesting a template plasmid).

(71) (18) The product in step (17) was transformed into competent cells of E. coli DH5a to obtain a recombinant bacterium Y385E-II (i.e. E. coli comprising the recombinant plasmid pcDNA3.0-NP-Y385E). Based on the sequencing results, the recombinant plasmid pcDNA3.0-NP-Y385E was structurally described as follows: codon tac encoding tyrosine at position 385 from N-terminal of the NP protein in the plasmid pcDNA3.0-NP was mutated to codon GAG of glutamic acid.

(72) II. Preparation of the Mutant Proteins

(73) 1. Preparation of Y385F Mutant Protein

(74) (1) Plasmid pHH21-PA, plasmid pHH21-PB1, plasmid pHH21-PB2, plasmid pHH21-HA, recombinant plasmid pHH21-NP-Y385F, plasmid pHH21-NA, plasmid pHH21-M, plasmid pHH21-NS, plasmid pcDNA3.0-PA, plasmid pcDNA3.0-PB1, plasmid pcDNA3.0-PB2 and recombinant plasmid pcDNA3.0-NP-Y385F were co-transfected into HEK 293T/17 cells by liposome Lipofectamine 2000 (Invitrogen) at a equal mass ratio, and the cells were cultured at 37 C. for 6 hours.

(75) (2) Culture medium of cells in step (1) was replaced by virus infection solution, and the cells were harvested after 72 hours of culture at 37 C.

(76) (3) The cells harvested in step (2) were treated with cell lysis solution at 4 C. for 30 minutes and centrifuged at 12000 rpm for 15 minutes, and the supernatant was collected.

(77) (4) Anti-NP protein monoclonal antibody was added to the supernatant obtained in step (3), incubated at 4 C. for 1 hour. Then protein G beads were added and incubated at 4 C. for 3 hours. The supernatant was discarded, and the beads were washed with elution buffer 3 times (10 minutes each time) at 4 C., the substance bound to the beads was the NP protein.

(78) 2. Preparation of the NP Protein

(79) Recombinant plasmid pHH21-NP-Y385F was replaced by plasmid pHH21-NP and the recombinant plasmid pcDNA3.0-NP-Y385F was replaced by plasmid pcDNA3.0-NP, and the others were the same as those in step 1, the substance bound to the beads was the NP protein.

(80) 3. Western Blot Detection

(81) The proteins obtained in step 1 and step 2 were subjected to western blot, respectively. The primary antibody used was anti-phosphorylated tyrosine antibody, and the secondary antibody used was HRP-labeled goat anti-mouse IgG.

(82) The result was shown in FIG. 3. The result showed that phosphorylation level of Y385F mutant protein was decreased significantly compared with a NP protein, that was, the tyrosine residue at position 385 of the NP protein was the main phosphorylation site.

Example 3. Virus Rescue

(83) 1. HEK 293T/17 cells were seeded in 60 mm dishes, 110.sup.6 cells per dish, and cultured for 12 hours.

(84) 2. After step 1 was completed, HEK 293T/17 cells were grouped and treated as follows:

(85) Group 1: plasmid pHH21-PA, plasmid pHH21-PB1, plasmid pHH21-PB2, plasmid pHH21-HA, recombinant plasmid pHH21-NP-Y385A, plasmid pHH21-NA, plasmid pHH21-M, plasmid pHH21-NS, plasmid pcDNA3.0-PA, plasmid pcDNA3.0-PB1, plasmid pcDNA3.0-PB2 and recombinant plasmid pcDNA3.0-NP-Y385A, each 0.5 g, were co-transfected into HEK 293T/17 cells by liposome Lipofectamine 2000 (Invitrogen), after the cells were cultured at 37 C. for 6 hours, the medium was replaced by virus infection solution, and the cells were cultured for a continuation of 72 hours and were harvested.

(86) Group 2: the difference between group 2 and group 1 only lies in that the recombinant plasmid pHH21-NP-Y385A was replaced by the recombinant plasmid pHH21-NP-Y385F and the recombinant plasmid pcDNA3.0-NP-Y385A was replaced by the recombinant plasmid pcDNA3.0-NP-Y385F.

(87) Group 3: the difference between group 3 and group 1 only lies in that the recombinant plasmid pHH21-NP-Y385A was replaced by the recombinant plasmid pHH21-NP-Y385E and the recombinant plasmid pcDNA3.0-NP-Y385A was replaced by the recombinant plasmid pcDNA3.0-NP-Y385E.

(88) Group 4: the difference between group 4 and group 1 only lies in that the recombinant plasmid pHH21-NP-Y385A was replaced by the plasmid pHH21-NP and the recombinant plasmid pcDNA3.0-NP-Y385A was replaced by the plasmid pcDNA3.0-NP.

(89) 3. After step 2 was completed, the culture supernatant was harvested in each group. The culture supernatant obtained in the group 4 comprised wild-type WSN virus, so the culture supernatant was named as WSN-WT virus solution.

(90) The culture supernatant obtained in the group 1 comprised mutant WSN virus (the codon encoding tyrosine at position 385 from N-terminal of the NP protein in the mutant virus genome was mutated to codon of alanine, the mutant virus was named as WSN-Y385A virus), so the culture supernatant was named as WSN-Y385A virus solution.

(91) The culture supernatant obtained in the group 2 comprised mutant WSN virus (the codon encoding tyrosine at position 385 from N-terminal of the NP protein in the mutant virus genome was mutated to codon of phenylalanine, the mutant virus was named as WSN-Y385F virus), so the culture supernatant was named as WSN-Y385F virus solution.

(92) The culture supernatant obtained in the group 3 comprised mutant WSN virus (the codon encoding tyrosine at position 385 from N-terminal of the NP protein in the mutant virus genome was mutated to codon of glutamic acid, the mutant virus was named as WSN-Y385E virus), so the culture supernatant was named as WSN-Y385E virus solution.

(93) 4. Each virus solution obtained in step 3 was taken for virus titer detection by plaque identification.

(94) Plaque identification method: (1) MDCK cells were seeded in 12-well plate, about 110.sup.5 cells per well, and cultured in an incubator at 37 C., 5% CO.sub.2 overnight; (2) the cell medium on the surface of the cells was washed with PBS buffer, and the virus solution to be tested was diluted serially by virus infection solution and then added to each well, three replicate wells were set for each dilution, incubated at 37 C. for 1 hour; (3) the supernatant was discarded, and the cells were washed with PBS buffer, 1 ml of mixed solution (a method for preparing the mixed solution: 1 part by volume of 3% low melting point agarose melted and cooled to about 37 C. and 1 part by volume of phenol red-free DMEM medium preheated to 37 C. were mixed with equal volume, and TPCK-treated trypsin, penicillin and streptomycin were added to the mixture to make the concentration of trypsin be 2 g/ml, the concentration of penicillin and streptomycin each be 100 U/ml) was added to each well; (4) the 12-well plate was placed at 4 C. for more than 15 minutes, after the agar was solidified, the plate was turned over to place upside down and incubated in an incubator at 37 C., cytopathic condition was observed under a microscope, after the plate was incubated for 3 days (in actual application, 2-4 days), the 12-well plate was removed from the incubator, and the number of plaque was counted. The titer of WSN-WT virus solution was 6.512 log.sub.10 PFU/ml. The titer of WSN-Y385F virus solution was 7.179 log.sub.10 PFU/ml. The titer of WSN-Y385A virus solution was 0, that was, it could not make MDCK produce a plague. The titer of WSN-Y385E virus solution was 0, i.e., it could not make MDCK produce a plague.

(95) 5. After step 2 was completed, the cells were harvested in each group, and the cells were broken and subjected to western blot (detecting the expression of each of major viral proteins).

(96) In Western Blot: the primary antibody for detecting the NP protein was purchased from Thermo Scientific, catalog number: PAS-32242; the primary antibody for detecting the M1 protein was the monoclonal antibody against M1 protein.

(97) The result was shown in FIG. 4. Two important proteins (the NP protein and M1 protein) of influenza virus in the recombinant system in each group could be expressed normally.

Example 4. Difference in Virus Growth Curves at Different Temperatures Under Cellular Level

(98) 1. A549 cells were seeded in 10 mm dishes, 110.sup.8 cells per dish, and cultured for 12 hours.

(99) 2. After step 1 was completed, A549 cells were grouped and treated as follows:

(100) Group 1: the WSN-WT virus solution (virus dose was 10.sup.6 PFU) prepared in Example 3 was inoculated into the A549 cells, and the medium was changed to virus infection solution one hour after inoculation; the cells were cultured at 37 C., the supernatant was collected at 12, 24, 36, 48, 60 and 72 hours after inoculation, and the virus titer was detected by plaque identification.

(101) Group 2: the WSN-Y385F virus solution (virus dose was 10.sup.6 PFU) prepared in Example 3 was inoculated into the A549 cells, and the medium was changed to virus infection solution one hour after inoculation; the cells were cultured at 37 C., the supernatant was collected at 12, 24, 36, 48, 60 and 72 hours after inoculation, and the virus titer was detected by plaque identification.

(102) Group 3: the difference between group 3 and group 1 only lies in that the culture temperature was changed from 37 C. to 33 C.

(103) Group 4: the difference between group 4 and group 2 only lies in that the culture temperature was changed from 37 C. to 33 C.

(104) The method for identification of plaque was same as that in Example 3. 10 repetitions were set for each group, and the results were averaged.

(105) The result was shown in FIG. 5. In FIG. 5, A was the results of group 3 and group 4, and B was the results of group 1 and group 2.

(106) During the culture at 33 C., all the WSN-WT viruses could replicate normally and the virus titer was kept a relatively stable and slowly rising tendency. The virus titer of WSN-Y385F was 0 after 24 hours of culture at 33 C., i.e., WSN-Y385F virus could not replicate at 33 C. During the culture at 37 C., both WSN-WT virus and WSN-Y385F virus could replicate normally and the virus titers of both were kept a relatively stable and slowly rising tendency. The result indicated that WSN-Y385F virus is a temperature-sensitive virus.

Example 5. Difference in Virus Growth Curves at Different Temperatures Under Animal Level

(107) 36 6-8 weeks old BALB/c mice with body weight of about 17 g were subjected to ether anesthesia and then randomly divided into three groups, there were 12 mice in each group, and the mice were respectively treated as follows:

(108) Group 1: 50 l of the WSN-WT virus solution (virus titer was 10.sup.4 PFU/ml) prepared in Example 3 was inhaled by nasal inhalation;

(109) Group 2: 50 l of the WSN-Y385F virus solution (virus titer was 10.sup.4 PFU/ml) prepared in Example 3 was inhaled by nasal inhalation;

(110) Group 3: 50 l of sterilized PBS buffer was inhaled by nasal inhalation.

(111) After the above treatment was completed, timing was started. The mice were dissected (3 mice in each group at each time point), and lungs and turbinal bones were obtained (temperatures of the lungs and turbinal bones were different, the temperature of the nasal bones was lower, about 33 C., and the temperature of the lungs was higher, about 37 C.) at day 1, day 3, day 5, and day 7, respectively.

(112) 0.1 g fresh weight lung or turbinal bone was weighed, 1 ml of ice bathed PBS buffer with pH 7.2 was added, the tissue was homogenated using QIAGEN TissueLyser II (homogenization parameters: 30 cycles/s, in total 4 min), and then centrifuged at 5000 g for 10 min, the supernatant was collected.

(113) The virus titer in the supernatant was detected by plaque identification (the method for identification of plaque was same as that in Example 3).

(114) The result was shown in FIG. 6. In FIG. 6, A was the result from the lung, and B was the result from the turbinal bone.

(115) In the urbinal bones, all WSN-WT viruses could replicates normally and the virus titer was kept a relatively stable and slowly descending tendency. In the urbinal bones, the virus titer of WSN-Y385F was 0, that was, WSN-Y385F virus could not replicate at 33 C. In the lungs, both WSN-WT virus and WSN-Y385F virus could replicate normally and the virus titer was kept a relatively stable and slowly descending tendency. The result showed that WSN-Y385F virus is a temperature-sensitive virus.

INDUSTRY APPLICATION

(116) The present invention has great value for the mechanism analysis of infection of influenza virus, prevention and treatment of influenza virus and the like.