Baculoviral DNA elements for the expression of recombinant proteins in a host insect

Abstract

Reagents and methods are provided that allow for an improved expression of a recombinant protein in an insect, More specifically, the introduction of recombinant DNA elements into an insect larva allows for the increased expression of a recombinant protein, an improvement of the correct folding of said protein and an increase in the survival rate after infection of the insect These recombinant DNA elements can be introduced, for example, into insect larvae via a recombinant baculovirus, which has incorporated said elements. The recombinant DNA elements include nucleic acids encoding transcriptional regulators, such as IE-0 and IE-1, transcriptional, enhancer elements, such as the homologous region (hr) and promoters.

Claims

1. An insect belonging to the species Trichoplusia ni or Spodoptera frugiperda, comprising a first nucleic acid sequence introduced into the insect by a recombinant baculovirus wherein the recombinant baculovirus is Autographa californica multinuclear polyhedrosis virus (AcMNPV) comprising one copy of an Ac-ie-01 gene in its genome and said first nucleic acid sequence; wherein said first nucleic acid sequence comprises an extra copy of the Ac-ie-01 gene under control of a promoter and directs expression of immediate early protein-1 (IE-1) or immediate early protein-0 (IE-0) in the insect at levels greater than levels of IE-1 or IE-0 achieved by infecting the insect with a corresponding control baculovirus that comprises one copy of the Ac-ie-01 gene in its genome and does not comprise said first nucleic acid sequence; wherein said first nucleic acid sequence comprises a nucleic acid sequence selected from the group consisting of: the nucleic acid sequence of any of SEQ ID NOS:2-5; and the nucleic acid sequence encoding any of the amino acid sequences of SEQ ID NOS: 6-9; wherein said first nucleic acid further comprises at least one recombinant homologous region (hr) from a baculovirus as enhancer region, wherein said recombinant homologous region (hr) is operably linked to a promoter that drives expression of a recombinant heterologous protein, wherein the promoter is a nucleic acid sequence comprising any of SEQ ID NO: 10-16.

2. The insect according to claim 1, wherein said recombinant homologous region is SEQ ID NO: 27.

3. The insect according to claim 1, wherein the first nucleic acid sequence is selected from the group consisting of SEQ ID NOS: 17, 18, 19, 20, 21, 22, 23, 24, 25 and 26.

4. The insect according to claim 1, further comprising a second nucleic acid sequence encoding a recombinant protein, wherein said second nucleic acid sequence is operably linked to said first nucleic acid sequence, said recombinant homologous region (hr) or said promoter.

5. A method for producing a recombinant protein comprising expressing said recombinant protein in an insect of claim 1 and extracting and purifying said recombinant protein.

6. The method according to claim 5, wherein said recombinant protein is selected from the group consisting of subunit monomeric vaccine, subunit multimeric vaccine, virus like particle, therapeutic protein, antibody, enzyme, cytokine, blood clotting factor, anticoagulant, receptor, hormone and diagnostic protein reagent.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1: Schematic representation of the baculovirus recombinant DNA elements of the invention, comprising four principal elements: a sequence encoding for transcriptional regulators (A; e.g. IE0 and IE1), which expression is driven by a promoter (B; e.g. polh or pB2.sub.9); an enhancer homologous region (hr) sequence (C; e.g. hr1), upstream of the promoters (D; e.g. p10, pB2.sub.9p10 or p6.9p10) driving the expression of the foreign gene coding for the recombinant protein. The scheme shows the theoretical mechanism of interaction between the recombinant DNA elements of the present invention that results in the unprecedented overexpression of the recombinant protein.

(2) FIG. 2: Different strategies that result in the generation of recombinant baculoviruses by the Bac-to-Bac cloning system (Invitrogen).

(3) FIG. 3: General scheme for the generation of cloning, donor and transfer vectors compatible with other commercial technologies used to generate recombinant baculoviruses.

(4) FIG. 4: A) Recombinant GFP production 96 hours post-infection in Trichoplusia ni insect larvae (fifth instar) inoculated with 500 or 50,000 PFUs of a conventional baculovirus expressing the protein under the control of the polh promoter or by a baculovirus containing the baculovirus cassette of the invention polh Ac-ie-01/hr1p6.9p10GFP. B) Percentage of larvae surviving 96 hours post-infection with the same baculoviruses as in panel A and at two different infectious doses, i.e. 500 and 50,000 PFUs.

(5) FIG. 5: A) Percentage of larvae surviving 96 hours post-infection with the baculovirus containing the expression cassette of the present invention polhAc-ie-01/hr1p6.9p10GFP with respect to a conventional baculovirus (polhGFP) at different infectious doses. B) Insect biomass at the moment of infection with a conventional baculovirus (polhGFP) or with a baculovirus containing the expression cassette of the present invention (polhAc-ie-01/hr1p6.9p10GFP) and the recovered biomass 96 hours post-infection. The infectious dose was 510.sup.4 PFUs.

(6) FIG. 6: A) Percentage of larvae surviving 96 hours post-infection using 510.sup.4 PFUs as the infectious dose of the baculovirus overexpressing the Ac-ie-01 cDNA under the polh promoter (polhAc-ie-01) or using a conventional baculovirus expressing GFP under the polh promoter (polhGFP) B) Insect biomass at the time of infection with the same baculoviruses as in panel A and the recovered biomass at 96 hours post-infection. The infectious dose was 510.sup.4 PFUs.

(7) FIG. 7: Comparison of the recombinant GFP protein expressed by a conventional baculovirus under the control of the polh promoter (2) or by a baculovirus containing the expression cassette of the present invention polhAc-ie-01/hr1p6.9p10GFP (1). Extracts from larvae infected with 510.sup.5 PFUs of every baculovirus were obtained 96 hours post-infection. A) Coomassie blue staining of the recombinant GFP protein. B) Cell integrity after baculovirus infection determined by tubulin detection with a specific antiserum. C) Western blot detection of recombinant GFP protein by using a specific anti-GFP serum.

(8) FIG. 8: Schematic representation of the preferred elements contained in the baculovirus expression cassettes of the invention, comprising encoding sequences for transcriptional regulators, homologous regions (hr) enhancing the transcription induced by promoter(s) of a foreign gene encoding a recombinant protein.

DETAILED DESCRIPTION OF THE INVENTION

(9) The present invention improves the expression of recombinant proteins by means of the introduction of recombinant DNA elements into insects.

(10) These recombinant DNA elements of the present invention are sequences that cause the expression of baculovirus transcriptional regulators above endogenous levels and optionally enhancer homologous regions (hr) and promoters operably linked to these aforementioned elements.

(11) Furthermore, the recombinant DNA elements may form part of an expression cassette.

(12) Expression cassette refers to a nucleic acid sequence that contains recombinant DNA elements, which control (e.g. the promoter) and/or are required (e.g. the gene itself) for gene expression. The expression cassette can be introduced in a recombinant vector or baculovirus.

(13) The recombinant DNA elements may be incorporated in a single nucleic acid sequence, cloning vector, transfer vector, recombinant baculovirus or cell. However, they can also be present in different nucleic acid sequences, cloning vectors, transfer vectors or recombinant baculoviruses and be introduced into the same cell.

(14) The present invention surprisingly shows that introduction into insect larvae of sequences that cause the expression of baculovirus transcriptional regulators above endogenous levels and optionally the introduction of an enhancer homologous region (hr) sequence, a promoter or a combination of promoters is able to increase the production of a recombinant protein to unprecedented levels. This indicates the usefulness of this system for the expression of recombinant proteins in vivo.

(15) Additionally, the introduction of these recombinant DNA elements into insect larvae with baculoviruses increases the survival rate of the larvae late after infection, especially after using high virus doses for infection (maximizes the recovered amount of biomass, i.e. the productivity of the system), as compared to larvae infected with a conventional baculovirus. The insect biomass recovered at the end of the production process is significantly increased as well.

(16) Also, the integrity of the molecular cell machinery and cell morphology of said baculovirus-infected larvae is improved as compared to larvae infected with a conventional baculovirus. An improvement in the integrity of cell functions during baculovirus infection also contributes to the correct post-translational processing of the recombinant protein.

(17) Thus, one aspect of the invention relates to an insect that contains a nucleic acid sequence that allows for the expression above endogenous levels of transcriptional regulators. In a preferred embodiment, the transcriptional regulators are IE-1, IE-0 and/or fragments thereof. In a further preferred embodiment, the insect is a transgenic insect.

(18) Transcriptional regulator refers to a regulatory protein that has the ability to modulate the transcription of specific genes by, for example, binding to enhancer or repressor regions and/or recruiting further proteins that are involved in transcription.

(19) IE-1 and its splice variant IE-0 are transcriptional regulators that are endogenously expressed during baculovirus infection. According to the present invention, IE-1, IE-0 and/or fragments thereof are recombinantly expressed to increase the total level of these proteins above endogenous levels. This can be achieved through, for example, introducing further copies of the endogenous gene or manipulating the expression of the promoter of the endogenous gene. Further copies of the endogenous genes can be introduced as transgenes under the control of a suitable promoter such as polh or pB2.sub.9.

(20) The expression level of the proteins IE-1, IE-0 and/or fragments thereof can be determined at both the mRNA and at the protein level with methods conventionally known to the person skilled in the art, such as quantitative PCR and Western Blot analysis.

(21) According to the invention, IE-1, 1E-0 and fragments thereof are encoded by the nucleic acids of SEQ ID NO: 3 (also referred to as Ac-ie-01) to SEQ ID NO: 5. SEQ ID NO: 3 is the Ac-ie-01 cDNA that encodes both IE-1 and 1E-0, SEQ ID NO: 2 is the coding sequence (CDS) of IE-1 and SEQ ID NO: 3 is the CDS of IE-0. SEQ ID NO: 4 and 5 are the CDSs of the N-terminal domains of IE-1 and 1E-0 respectively that retain the catalytic transcriptional regulator activity. The proteins that are encoded by SEQ ID NO: 2-5 are represented by SEQ ID NO: 6-9 respectively.

(22) The present invention furthermore discloses variants of SEQ ID NO: 2-9 that are or encode amino acids that are able to function as a transcriptional regulator. These variants are nucleic or amino acids whose nucleotide or amino acid sequence differs in one or more positions from these parental nucleic or amino acids, whereby differences might be additions, deletions and/or substitutions of nucleotides or amino acid residues.

(23) Nucleic and amino acid sequences of the present invention shall be distinguished from other nucleic and amino acid sequences by their degree of sequence identity or similarity respectively as determined using EMBOSS Needle with the default parameters. Methods for the generation of such variants include random or site directed mutagenesis, site-saturation mutagenesis, PCR-based fragment assembly, DNA shuffling, homologous recombination in vitro or in vivo, and methods of gene-synthesis.

(24) The sequence of the variants of SEQ ID NO: 2-5 is at least 70%, preferably at least 75%, more preferably at least 80%, more preferably at least 85%, more preferably at least 90% and most preferably at least 95% identical to the sequences of SEQ ID NO: 2-5.

(25) The sequence of the variants of SEQ ID NO: 6-9 is at least 70%, preferably at least 75%, more preferably at least 80%, more preferably at least 85%, more preferably at least 90% and most preferably at least 95% similar to the sequences of SEQ ID NO: 6-9.

(26) The insect is preferably a lepidopter and more preferably an insect selected from the group consisting of Trichoplusia ni, Spodoptera frugiperda, Spodoptera exigua, Ascalapha odorata, Bombyx mori, Rachiplusia ni and Stigmene acrea. In a preferred embodiment, the insect is a larva.

(27) In a preferred embodiment, the insect of the present invention further contains, in addition to the nucleic acid sequence that allows for the expression above endogenous levels of the proteins IE-1, IE-0 and/or fragments thereof, a recombinant homologous region (hr) that can enhance the expression of a recombinant protein by being operably linked to the respective promoter.

(28) Homologous regions, hr, are comprised of repeated units of about 70-bp with an imperfect 30-bp palindrome near their center. Homologous regions are repeated at eight locations in the AcMNPV genome with 2 to 8 repeats at each side. Homologous regions have been implicated as both transcriptional enhancers and origins of baculovirus DNA replication.

(29) Enhancer region refers to a control sequence, whose binding by transcriptional regulators increases the level of transcription of associated genes.

(30) Recombinant protein refers to a protein that originates from recombinant DNA. Such proteins can be used for the benefit of humans and animals and may have industrial, commercial or therapeutic application.

(31) Being operably linked refers to two nucleic acid sequences that are connected in a way that one influences the other in terms of, for example, transcriptional regulation.

(32) Promoter refers to a DNA sequence to which RNA polymerase can bind to initiate transcription. The sequence may further contain binding sites for various proteins that regulate transcription, such as transcription factors. The promoter sequence may be composed of different promoter fragments (either different or the same fragments) that are localized closely in the DNA sequence and may be separated by linkers or spacer. Such promoters are referred to as chimeric promoters.

(33) The enhancer homologous region sequence hr upstream of the promoter/s is preferably hr1 (SEQ ID NO: 27). The promoters that drive the expression of the recombinant protein are preferably selected from the group comprising SEQ ID NO: 10-16 or a sequence that is able to function as a promoter and has at least 70%, preferably at least 75%, more preferably at least 80%, more preferably at least 85%, more preferably at least 90% and most preferably at least 95% identity with the nucleotide sequence indicated in any of SEQ ID NO: 10-16.

(34) In a preferred embodiment, the nucleic acid sequence comprises combinations of recombinant promoters, sequences encoding transcriptional regulators and enhancer regions selected from the group comprising SEQ ID NO: 17-26.

(35) As indicated above, the recombinant promoters, sequences encoding transcriptional regulators and enhancer regions of the present invention do not need to form part of a single molecule, instead these sequences may form part of distinct molecules as long as they are operably linked, i.e. contained within the same cells.

(36) The insect of the present invention preferably comprises a nucleic acid sequence encoding a recombinant protein. This nucleic acid sequence is operably linked to the nucleic acid sequence that allows for the expression above endogenous levels of the proteins IE-1, IE-0 and/or fragments thereof and optionally to a homologous region (hr).

(37) The above described recombinant DNA elements are preferably introduced into the insect by a recombinant baculovirus. Preferably, the baculovirus is AcMNPV or BmNPV and the insect an insect larva. The baculovirus is administered to the larva by oral administration (per as) or more preferably by injection. In a further embodiment the insect is infected, transfected, transduced or transformed with the recombinant baculovirus, transfer vector, cloning vector or nucleic acid sequence of the present invention. Preferably, the insect larvae are reared in a rearing module, such as described in the patent application ES 2 232 308.

(38) In a further aspect, the invention discloses methods for producing a recombinant protein using the insect of the present invention. To this end, an insect can be infected, transfected, transduced or transformed with the recombinant baculovirus, transfer vector, cloning vector or nucleic acid sequence of the present invention. After expression of the recombinant protein, extraction and purification of the recombinant protein is done by conventional means.

(39) In a preferred embodiment for protein production, the larvae are infected by injecting a high virus dose (higher than 10 Plaque Forming Units) of the recombinant baculovirus of the invention. 3-4 days after infection, the infected larvae are processed and the whole soluble protein extract is obtained by the use of appropriate extraction buffers. Extracts are centrifuged and the lipid fraction eliminated. Then, the recombinant protein is purified by conventional means.

(40) The recombinant protein that is preferably produced by the methods of the present invention is a protein selected from the group comprising subunit monomeric vaccine, subunit multimeric vaccine, virus like particle, therapeutic protein, antibody, enzyme, cytokine, blood clotting factor, anticoagulant, receptor, hormone or diagnostic protein reagent.

(41) One aspect of the invention relates to the use of the recombinant baculovirus, transfer vector, cloning vector or nucleic acid sequence of the invention in a rearing, feeding or injection medium for an insect.

(42) The present invention discloses a baculovirus that can be used to produce the insect of the present invention and comprises said sequence for expression above endogenous levels of the proteins IE-0, IE-1 and/or fragments thereof.

(43) Baculovirus refers to a family of infectious viruses for invertebrates, mainly infecting insects and arthropods. A recombinant baculovirus has further introduced recombinant DNA through, for example, homologous recombination or transposition. The recombinant baculovirus preferably originates from AcMNPV (Autographa californica nuclear polyhedrosis virus) or BmNPV (Bombyx mori nucleopolyhedrovirus).

(44) Recombinant DNA refers to a form of artificial DNA that is engineered through the combination or insertion of one or more DNA strands, thereby combining DNA that would normally not occur together.

(45) Recombinant DNA element refers to a functional element within recombinant DNA, such as a promoter, enhancer or a gene. As mentioned above, the recombinant DNA elements of the present invention are sequences that cause the expression of baculovirus transcriptional regulators above endogenous levels, enhancer homologous regions (hr) and promoters operably linked to these aforementioned elements. Preferably, the baculovirus transcriptional regulators IE-1, IE-0 or fragments thereof are expressed above endogenous levels.

(46) The recombinant baculovirus preferably contains in addition to (i) the sequence for expression above endogenous levels of the proteins IE-0, IE-1 and/or fragments thereof, (ii) a recombinant homologous region (hr) linked to (iii) a suitable promoter for driving the expression of a recombinant protein. The preferred combinations of these recombinant DNA elements are as described above for the insects. Furthermore, the recombinant baculovirus preferably contains a nucleic acid sequence encoding a recombinant protein.

(47) The present invention discloses a transfer vector that can be used to produce the insect and/or recombinant baculovirus of the present invention and comprises said sequence for expression above endogenous levels of the proteins IE-0, IE-1 and/or fragments thereof, in addition to a sequence suitable for integration or transposition in a baculovirus.

(48) Transfer vectors generally permit the insertion of genetic information into a baculovirus.

(49) The transfer vector preferably contains in addition to (i) the sequence for expression above endogenous levels of the proteins IE-0, IE-1 and/or fragments thereof, (ii) a recombinant homologous region (hr) linked to (iii) a suitable promoter for driving the expression of a recombinant protein. The preferred combinations of these recombinant DNA elements are as described above for the insects.

(50) In one preferred aspect, the transfer vector comprises a nucleic acid sequence encoding said recombinant protein, whereas in another preferred embodiment the transfer vector lacks such sequence.

(51) In a preferred embodiment, the transfer vector is a bacmid.

(52) Bacmid refers to a plasmid construct which contains the DNA sequence sufficient for generating a baculovirus when transfected into a cell.

(53) In a further preferred embodiment, the transfer vector is derived from any of the commercially available baculovirus expression systems Bac-to-Bac (Invitrogen), BacPAK (Clontech), FlashBAC (Oxford Expression Technologies), BacuVance (GenScript), Bac-N-Blue DNA (Invitrogen), BaculoDirect (Invitrogen), BacVector 1000, 2000, 3000 (Novagen), DiamondBac (Sigma-Aldrich) or BaculoGold (BD Biosciences).

(54) The present invention discloses a cloning vector that can be used to produce the insect, recombinant baculovirus and/or transfer vector of the present invention and comprises said sequence for expression above endogenous levels of the proteins IE-0, IE-1 and/or fragments thereof, which is further suitable for bacterial replication.

(55) Cloning vector refers to any vector that is suitable for cloning, which generally involves the presence of restriction sites, an origin of replication for bacterial propagation and a selectable marker.

(56) The cloning vector preferably contains in addition to (i) the sequence for expression above endogenous levels of the proteins IE-0, IE-1 and/or fragments thereof, (ii) a recombinant homologous region (hr) linked to (iii) a suitable promoter for driving the expression of a recombinant protein. The preferred combinations of these recombinant DNA elements are as described above for the insects.

(57) In one preferred aspect, the cloning vector comprises a nucleic acid sequence encoding said recombinant protein (also referred to as the donor vector), whereas in another preferred embodiment the cloning vector lacks such sequence.

(58) The present invention discloses a nucleic acid sequence that can be used to produce the insect, recombinant baculovirus, transfer vector and/or cloning vector of the present invention and comprises said sequence for expression above endogenous levels of the proteins IE-0, IE-1 and/or fragments thereof.

(59) The nucleic acid sequence preferably contains in addition to (1) the sequence for expression above endogenous levels of the proteins IE-0, IE4 and/or fragments thereof, (ii) a recombinant homologous region (hr) linked to (iii) a suitable promoter for driving the expression of a recombinant protein. The preferred combinations of these recombinant DNA elements are as described above for the insects.

(60) Its one preferred aspect, the nucleic acid sequence comprises a nucleic acid sequence encoding said recombinant protein, whereas in another preferred embodiment the nucleic acid sequence lacks such sequence.

(61) TABLE-US-00001 SUMMARY OF SEQUENCES SEQ ID NO: Name: 3 Complete Ac-ie-01 cDNA 2 Protein coding sequence (CDS) of IE-1 3 CDS of IE-0 4 CDS of the IE-1 N-terminal domain 5 CDS of the IE-0 N-terminal domain 6 IE-1 protein 7 IE-0 protein 8 IE-1 N-terminal domain protein 9 IE-0 N-terminal domain protein 10 polh (promoter) 11 p10 (promoter) 12 pB2.sub.9p10 (promoter) 13 p6.9p10 (promoter) 14 pB2.sub.9 (promoter) 15 pB2p10 (promoter) 16 polhp10 (promoter) 17 polhAc-ie-01/hr1p10 18 polhAc-ie-01/hr1pB2.sub.9p10 19 polhAc-ie-01/hr1p6.9p10 20 pB2.sub.9Ac-ie-01/hr1p10 21 pB2.sub.9Ac-ie-01/hr1pB2.sub.9p10 22 pB2.sub.9Ac-ie-01/hr1p6.9p10 23 polhAc-ie-01/hr1polh 24 pB2.sub.9Ac-ie-01/hr1polh 25 polhAc-ie-01/hr1polhp10 26 pB2.sub.9Ac-ie-01/hr1polhp10 27 Homologous region enhancer hr1 28 polhAc-ie-01 29 polhGFP
Deposition of Microorganisms According to the Budapest Treaty

(62) Plasmids were deposited in the Spanish Type Culture Collection (CECT) (www.cect.org); University of Valencia, Parc Cientfic Universitat de Valncia; Catedrtico Agustin Escardino, 9; 46980 Paterna (Valencia), Spain, with the accession number CECT 8031, on the date Oct. 4, 2011.

EXAMPLES

Example 1. Overexpression of Baculovirus Transcriptional Regulators Potentiates the Enhancer Function of a Homologous Region hr Functionally Linked to a Promoter Increasing Recombinant Protein Expression in a Baculovirus Vector Expression System (BEVS)

(63) Immediate early viral proteins encoded by the Ac-ie-01 cDNA, i.e. IE-1 and IE-0, from AcMNPV are potent transcriptional regulators in baculoviruses. Transactivation mediated by these proteins is enhanced by their binding as a homodimer to the baculovirus homologous region (hr) sequences, which act as transcriptional enhancers. AcMNPV IE-1/IE-0 are 67-72 kDa dimeric DNA-binding proteins that stimulate transcription in plasmid transfection assays through the activity of their N-terminal acidic domain (7, 8). Synthesized very early during infection, IE-1 and IE-0 accumulate within the nucleus, where they are maintained through late times. Using the dual plasmid pFastBac (Invitrogen), the Ac-ie-01 cDNA was cloned under the control of the polh promoter. In the same plasmid, but in another locus, the GFP encoding gene was cloned downstream of the hr1p6.9p10 chimeric promoter that was previously synthesized and contains the homologous region hr1 fused to the promoters p6.9 and p10. A schematic representation of the resulting baculovirus expression cassette of the present invention and the putative function of the recombinant DNA elements is shown in FIG. 1. The resulting plasmid was used to generate a recombinant baculovirus by the Bac-to-Bac system (Invitrogen). In parallel, a conventional baculovirus expressing the GFP protein under the control of polh promoter was generated by the same system.

(64) The expression of GFP protein mediated by the different baculoviruses was studied by fluorimetry at 96 hours post-infection in Trichoplusia ni larvae using a low or high infectious dose (510.sup.2 or 510.sup.4 respectively). The expression level of GFP was increased in larval extracts by the baculovirus containing above expression cassette of the present invention by about 13 to more than 40% depending on the virus dose used (FIG. 4A). Similar results were observed by using baculoviruses in which the Ac-ie-01 cDNA was expressed under the control of the insect-derived pB2.sub.9 promoter or when the GFP was expressed under the control of the hr1pB2.sub.9p10 promoter (data not shown).

Example 2. The Baculovirus Expression Cassettes of the Invention Increase the Baculovirus-Infected Insect Larvae Surviving Rates and Insect Biomass Recovered Using High Infectious Doses Through the Transcriptional Regulators Encoded by the Ac-ie-01 cDNA

(65) In the previous example an advantage of baculoviruses expressing the recombinant protein in the context of the baculovirus cassette of the present invention in terms of protein productivity was shown. However, the main difference observed was in respect to the percentage of surviving larvae at high infectious doses (maximum productivity). Under such infection conditions, the baculovirus containing the expression cassette of the present invention increased by about 70% the larvae survival rates (FIG. 4B). This means that by using a conventional baculovirus the optimal infection conditions were using an infectious dose of 510.sup.2 PFUs (maximum larvae surviving rate). In contrast, by using a baculovirus containing the expression cassette of the present invention, a dose of 510.sup.4 could be used, recovering the same number of larvae at the end of the production process (FIG. 4B). Under such optimal production conditions (infection with 510.sup.4 PFUs), the baculovirus containing the expression cassette of the present invention yields more than twice the amount of recombinant protein produced in insect larvae infected with the optimal dose for the conventional baculovirus (510.sup.2 PFUs) (FIGS. 4 A and B).

(66) Larvae infection studies using different infectious doses of a conventional baculovirus (polhGFP) or a baculovirus containing the expression cassette of the present invention polhAc-ie-01/hr1p6.9p10GFP, both expressing the GFP protein, revealed increasing surviving rates of infected larvae when the baculovirus containing the expression cassette of the present invention was used (FIG. 5A). At the highest infectious dose (510.sup.4) the survival rate of larvae infected with the expression cassette of the present invention increased by more than 70% as compared to larvae infected with a conventional baculovirus. This increase in infected larvae surviving rates had direct dramatic consequences in the insect biomass recovered at the end of the production process, with a 80% increase when the baculovirus containing the expression cassette of the present invention was used at the highest infectious dose (maximum productivity) (FIG. 5B).

(67) To determine the genetic element/s responsible for such interesting properties related to the increase of survival rates after infection with high doses of the baculovirus of the invention, a recombinant baculovirus expressing the transcriptional regulators encoded by the Ac-ie-01 cDNA under the control of the polh promoter was generated. Then, T. ni larvae were infected with a high infectious dose (510.sup.4 PFUs) of this baculovirus. As control, we used a baculovirus expressing the GFP protein under the control of the same promoter. Similarly to the larvae infected with the expression cassette of the present invention polhAc-ie-01/hr1p6.9p10GFP, larvae infected with the baculovirus overexpressing the Ac-ie-01 cDNA (polhAc-ie-01) also showed increased survival rates when compared with larvae infected with a conventional baculovirus expressing the GFP reporter protein under the control of the same promoter (polhGPF) (FIG. 6A). This strongly suggests that the overexpression of the transcriptional regulators used in the baculovirus expression cassette of the present invention protects the insect larvae from the baculovirus-induced mortality, allowing long-term expression (more recombinant protein production) and increasing the insect biomass recovery using high infectious doses (maximum productivity) (FIG. 6B).

Example 3. Overexpression in a Baculovirus Expression System of Transcriptional Regulators Encoded by the Ac-ie-01 cDNA Facilitates the Post-Translational Processing of Recombinant Proteins in Infected Insect Larvae Used as Biofactories

(68) Cellular integrity during baculovirus infection is of great importance to ascertain the correct folding or any other post-translational modification of foreign proteins expressed by this system. The baculovirus strong promoters commonly used for research and production, such as polh and p10, only express the foreign genes at late times post-infection when infected cells already show severe cytopathic effects and the cellular viability decreases. As described above, the overexpression of the transcriptional regulators used in the baculovirus expression cassette of the present invention protects cells from pathogenic effects of the baculovirus infection, allowing a wide temporal window for recombinant protein production in cells remaining fully viable.

(69) The relevance of the recombinant DNA elements incorporated into the expression cassette of the invention in relation to post-translational modifications of recombinant proteins in insect larvae as living biofactories was studied. For this purpose, a conventional baculovirus expressing the reporter protein GFP under the control of the polh promoter and a baculovirus incorporating the baculovirus cassette of the present invention and also expressing the GFP protein (polhAc-ie-01/hr1p6.9p10GFP) were used to infect insect larvae at a dose of 510.sup.4 PFUs.

(70) Infected larvae extracts were analysed at 96 hours post-infection by SDS-PAGE gels and Coomassie blue staining (FIG. 7A). Interestingly, GFP protein expressed by a conventional baculovirus showed a band with a reduced molecular weight (lower than predicted), suggesting degradation or misfolding of the recombinant protein. In contrast, when the GFP protein expression was mediated by the baculovirus expression cassette of the present invention, the GFP band presented the expected molecular weight of this protein, i.e. about 27 kDa.

(71) The infected larvae extracts were also analyzed by Western blot using anti-GFP monoclonal antibody (mab2515; Millipore) (FIG. 7C). The GFP protein was detected in larvae extracts infected by both baculoviruses, but showed a difference in electrophoretic mobility of the recombinant protein as observed by Coomassie blue staining.

(72) In parallel, the integrity of the cell machinery was measured at different times post-infection by Western blot analysis of the cellular tubulin protein using a specific antiserum (FIG. 7B). Infection with a conventional baculovirus impaired severely the cell integrity at 96 hours post-infection since the tubulin band detected decreased dramatically after this time point (degradation as a result of a complete loss of cell integrity). Consistent with the cellular protection induced by the recombinant DNA elements contained in the baculovirus expression cassette of the invention, cellular tubulin was not equally affected in cells infected by the recombinant baculovirus engineered with the expression cassette.

Example 4. Cell Culture and Viruses

(73) The Spodoptera frugiperda Sf21 or Sf9 cell lines were cultured in 6-well tissue culture plates (110.sup.6 cells/well) in TNM-FH insect medium (Pan Biotech, Germany) containing 10% heat-inactivated fetal bovine serum (Pan Biotech, Germany) at 27 C. AcMNPV recombinant baculoviruses were obtained by the Bac-to-Bac Baculovirus Expression System (Invitrogen, USA). Different transfer vectors containing the recombinant DNA elements of the invention were generated using the pFastBac-DUAL plasmid (Invitrogen). The promoters and regulatory elements incorporated into pFastBac-DUAL were synthesized (GenScript, USA) with the adequate flanking restriction sequences to facilitate the cloning. These transfer vectors were used to transfect Sf21 cells with Cellfectin (Invitrogen, USA). The resulting recombinant baculoviruses from the infection of Sf21 cells were then passaged twice in cells and titered by the plaque assay method. The obtained gene constructs of the baculovirus expression cassettes are schematically shown in FIG. 8, showing different potential combinations of promoters driving the expression of the Ac-ie-01 cDNA or the foreign gene (e.g. GFP). The different expression cassettes were used to generate the recombinant baculoviruses used in the examples shown in FIGS. 4 to 7.

Example 5. Generation of the Cloning Vector

(74) The cloning vector is a small piece of DNA containing the baculovirus expression cassette of the present invention into which a foreign DNA fragment can be inserted by treating the vehicle and the foreign DNA with a restriction enzyme that creates the same overhang, then ligating the fragments together. The essential characteristics of the cloning vector must include a synthetic multiple cloning site (MCS) to facilitate the insertion of foreign genes directed in a chosen orientation, a selectable marker, such as an antibiotic resistance to allow the selection of positively transformed cells and a functional origin of replication (ORI) for propagation in bacteria.

Example 6. Generation of the Donor Vector Containing the Baculovirus Expression Cassette of the Present Invention

(75) A donor vector consists of a cloning vector, for example a pUC57 plasmid, containing the baculovirus expression cassette, into which a foreign gene has been cloned using the appropriate restriction enzymes. The baculovirus expression cassette of the present invention was synthesized by ligating the following DNA sequences: (i) the baculovirus transcriptional regulator encoding sequence Ac-ie-01 downstream of a promoter sequence, such as the polh or the pB2.sub.9 promoter, and upstream of the HSV TK polyadenylation signal and (ii) in another locus an enhancer sequence, for example, the homologous region hr1, upstream of (iii) a promoter sequence, for example, pB2.sub.9p10, p10, p6.9p10 or polh, followed by a multiple cloning site (MCS) for cloning the gene of interest and the SV40 polyadenylation signal downstream of the MCS (FIG. 1). The baculovirus expression cassette is flanked by specific restriction sites (for example BglII and BstZ17I at the 5-terminal end and Bgl II and Sgf I at the 3-terminal end) to facilitate subcloning into a transfer vector of a commercial baculovirus generation system (based on transposition, for example the Bac-to-Bac system (Invitrogen), or based on homologous recombination, for example flashBACT (Oxford Expression Technologies), Baculogold (BD Biosciences), BacPAK6 (Clontech), Bac-N-Blue DNA (Invitrogen)) (FIGS. 2 and 3).

(76) The encoding gene of the Green Fluorescence Protein (GFP) was cloned into the MCS of the cloning vector using the Nco I and Spe I restriction sites, generating the donor plasmid vector (FIG. 2).

Example 7. Generation of the Transfer Vector Containing the Baculovirus Expression Cassette of the Present Invention

(77) The transfer vector was generated by digesting a donor vector with BstZ17 I of the 5-flanking site and with Xha I and cloning it into the transfer vector pFastBac1 that was also digested with the same enzymes. In this case, as a result of the subcloning, the SV40 polyadenylation signal of the baculovirus expression cassette is exchanged by the SV40 polyadenlation signal from the transfer vector. Apart from this, all the elements of the expression cassette are included in the pFastBac transfer vector, substituting the polh promoter and MCS of the original commercial transfer vector (FIG. 2).

Example 8. Generation of the Baculovirus Expression Vector Containing the Baculovirus Expression Cassette of the Present Invention Using the Bac-to-Bac System

(78) The modified transfer vector pFastBac1 and the individual baculovirus expression cassette were used to generate a recombinant baculovirus by using the Bac-to-Bac Baculovirus Expression System. More specifically, the modified transfer vector was used to transform the E. coli host strain DH10Bac that contains a baculovirus shuttle vector (bacmid) and a helper plasmid, and allows the generation of a recombinant bacmid following transposition of the expression cassette. The DNA of the recombinant bacmid containing the baculovirus expression cassette of the present invention and the GFP encoding gene was then used to transfect insect cells, for example, Sf21 cells, using Cellfectin. 72 hours post-transfection, cells were harvested and the first recombinant baculovirus generation was obtained (FIG. 2). This recombinant baculovirus could then be further amplified and/or titered following conventional protocols. Similar procedures can be used to generate recombinant baculoviruses with other transfer vectors provided by commercial BEVSs (FIG. 3).

Example 9. Rearing and Infection of Insect Larvae

(79) Trichoplusia ni (cabbage looper) larvae were reared under level 2 biosafety conditions. Eggs were placed into specially designed larva developmental cages containing an artificial insect diet and were kept in growth chambers at 22 C. under controlled humidity (50%) and light period (8 h/day) conditions.

(80) Fifth-instar larvae (last instar larvae before pupation), were used for all infection experiments. The standard weight of each larva was approximately 120-130 mg and larvae were injected near the proleg (forward of the body cavity) with 5 l of recombinant baculoviruses diluted to reach the number of plaque forming units (PFU) per dose selected. Larvae were processed at 96 hpi. The larvae collected were frozen immediately to be stored at 20 C. until they were processed for recombinant protein quantification. Total soluble, non-denatured proteins (TSNDPs) from frozen T. ni larvae infected by the baculoviruses were obtained by homogenization using a Bag Mixer blender (Interscience, France) for 2 min. Extraction buffer was composed of PBS 1, Triton X-100 at 0.01%, Complete protease inhibitor cocktail (Roche, Germany), and DTT 25 mM.

Example 10. Fluorimetric Analysis

(81) About 20 g of total soluble proteins derived from infected cells, containing different amounts of recombinant GFP protein, were analyzed and quantified by a Tecan GENios (CA, USA) fluorescence plate reader (excitation [Ex], 485/emission [Em], 535).

Example 11. Western Blot Analysis

(82) Total soluble protein fractions (10 g) from larvae infected with the recombinant baculoviruses were resolved in 15% SDS-PAGE gels. Gels were stained by the Coomassie blue staining method or transferred to nitrocellulose membranes. Western blots were probed with the anti-GFP monoclonal antibody mab2515 (Millipore, USA) or tubulin antiserum (T5168; Sigma-Aldrich) at 1:1000 and the immunocomplexes were revealed with anti-mouse IgG-horseradish peroxidase (HRP)-labeled conjugate (KPL, UK), diluted 1:2,000 as a secondary antibody. Protein bands were detected using an ECL western blotting detection system and analyzed by the ChemiDoc XRS Gel Imaging System (BioRad, USA).

BIBLIOGRAPHY

(83) 1. Nettleship, J. E., Assenberg, R., Diprose, J. M., Rahman-Huq, N., Owens, R. J. Recent advances in the production of proteins in insect and mammalian cells for structural biology. J. Struct. Biol. 2010, 172, 55-65. 2. Gomez-Casado E, Gomez-Sebastian S, Nez MC, Lasa-Covarrubias R, Martnez-Pulgarin S, Escribano J M. Insect larvae biofactories as a platform for influenza vaccine production. Protein Expr Purif. 79: 35-43. 2011. 3. Smith, G. E., M. D. Summers, and M. J. Fraser. 1983. Production of human beta interferon in insect cells infected with a baculovirus expression vector. Mol. Cell. Biol. 3: 2156-21 65. 4. Tomita, M., Munetsuna, H., Sato, T., Adachi, T., Hino, R., Hayashi, M., Shimizu, K., Nakamura, N., Tamura, T., Yoshizato, K., 2003. Transgenic silkworms produce recombinant human type Ill procollagen in cocoons. Nat. Biotechnol. 21 (13, 52-56, 5. Perez-Filgueira, D. M, Resino-Talavan, P., Cubillos, C., Angulo, I., Barderas, M. G., Barcena, J., Escribano, J. M., 2007. Development of a low-cost, insect larvaederived recombinant subunit vaccine against RHDV. Virology 364 (2), 422-430. 6. Perez-Martin, E., Gomez-Sebastian, S., Argilaguet, J. M., Sibila, M., Fort, M., Nofrarias, M., Kurtz, S., Escribano, J. M., Segales, J., Rodriguez, F., 2010. Immunity conferred by an experimental vaccine based on the recombinant PCV2 Cap protein expressed in Trichoplusia ni-larvae. Vaccine 28 (11), 2340-2349. 7. Fernndez-San Milln A, Gmez-Sebastin S, Nuez MC, Veramendi J, Escribano J M. Human papillomavirus-like particles vaccine efficiently produced in a non-fermentative system based on insect larva. Protein Expr. Purif. 2010, 74: 1-8 8. Gomez-Casado E, Gomez-Sebastian S, Nez MC, Lasa-Covarrubias R, Martnez-Pulgarn S, Escribano J M. Insect larvae biofactories as a platform for influenza vaccine production Protein Expr. Purif. 2011, 79: 35-43 9. J. A. Medin, L. Hunt, K. Gathy, R. K. Evans, M. S. Coleman, Efficient, low-cost protein factories: expression of human adenosine deaminase in baculovirusinfected insect larvae, Proc. Nati Acad. Sci. USA 87 (1990) 2760-2764.

(84) 10. N. M. Tremblay, B. P. Kennedy, I. P. Street, W. J. Kaupp, F. Laliberte, P. K. Weech, Human group II phospholipase A2 expressed in Trichoplusia ni larvae-isolation and kinetic properties of the enzyme, Protein Expr. Purif. 4 (1993) 490-498. 11. U. Reis, B. Blum, B. U. von Specht, H. Domdey, J. Collins, Antibody production in silkworm cells and silkworm larvae infected with a dual recombinant Bombyx mori nuclear polyhedrosis virus, Biotechnology (NY) 10 (1992) 910-912. 12. F. Gil, M. Perez-Filgueira, M. G. Barderas, C. Pastor-Vargas, C. Alonso, F. Vivanco, J. M. Escribano, Targeting antigens to an invariant epitope of the MHC Class II DR molecule potentiates the immune response to subunit vaccines, Virus Res, (2010). 13. S. Mathavan, V. T. Gautvik, E. Rokkones, O. K. Olstad, B. N. Kareem, S. Maeda, K. M. Gautvik, High-level production of human parathyroid hormone in Bombyx mori larvae and BmN cells using recombinant baculovirus, Gene 167 (1995) 33-39. 14. S. Sumathy, V. B. Palhan, K. P. Gopinathan, Expression of human growth hormone in silkworm larvae through recombinant Bombyx mori nuclear polyhedrosis virus, Protein Expr. Purif. 7 (1996) 262-268. 15. X. Shi, J. Qin, J. Zhu, D. Zhu, Expression of biologically active human granulocyte-macrophage colony-stimulating factor in the silkworm (Bombyx mori), Biotechnol. Appl. Biochem. 24 (Pt 3) (1996) 245-249. 16. M. Q. Pham, S. Naggie, M. Wier, H. J. Cha, W. E. Bentley, Human interleukin-2 production in insect (Trichoplusia ni) larvae: effects and partial control of proteolysis, Biotechnol. Bioeng. 62 (1999) 175-182. 17. J. B. Katz, A. L. Shafer, K. A. Eernisse, Construction and insect larval expression of recombinant vesicular stomatitis nucleocapsid protein and its use in competitive ELISA, J. Virol. Methods 54 (1995) 145-157. 18. D. M. Perez-Filgueira, F. Gonzalez-Camacho, C. Gallardo, P. Resino-Talavan, E. Blanco, E. Gomez-Casado, C. Alonso, J. M. Escribano, Optimization and validation of recombinant serological tests for African swine fever diagnosis based on detection of the p30 protein produced in Trichoplusia ni larvae, J. Clin. Microbiol. 44 (2006) 3114-3121. 19. S. Gomez-Sebastian, D. M. Perez-Filgueira, E. Gomez-Casado, M. C. Nunez, I. Sanchez-Ramos, E. Tabares, J. M. Escribano, DIVA diagnostic of Aujeszky's disease using an insect-derived virus glycoprotein E, J. Virol, Methods 153 (2008) 29-35. 20. F. Todoli, M. Perez-Filgueira, I. Galindo, S. Gomez-Sebastian, J. M. Escribano, A. Rodriguez-Cortes, J. Alberola, Seroreactivity against raw insect-derived recombinant KMPII, TRYP, and LACK Leishmania infantum proteins in infected dogs, Vet. Parasitol. 164 (2009) 154-161. 21. Hill-Perkins M S, Possee R D. A baculovirus expression vector derived from the basic protein promoter of Autographa californica nuclear polyhedrosis virus. J Gen Virol. 1990, 71 (Pt 4):971-6. 22. Passareili, A. L., and L. K. Miller. Three baculovirus genes involved in late and very late gene expression: ie-1, ie-n, and lef-2. J. Virol. 1993, 67:2149-2158 23. Rodems, S. M., S. S. Pullen, and P. D. Friesen. DNA-dependent transregulation by IE1 of Autographa californica nuclear polyhedrosis virus: IE1 domains required for transactivation and DNA binding. J. Virol. 1997, 71: 9270-9277. 24. Lin X, Chen Y, Yi Y, Zhang Z: Baculovirus immediately early 1, a mediator for homologous regions enhancer function in trans. Virol J 2010, 7:32. 25. Okano K, Mikhailov V S, Maeda S: Colocalization of baculovirus IE-1 and two DNA-binding proteins, DBP and LEF-3, to viral replication factories. Journal of virology 1999, 73(1):110-119.