Marek's disease virus type 1-specific phosphorylated proteins pp38 and pp24 with common amino acid termini are encoded from the opposite junction regions between the long unique and inverted repeat sequences of viral genome

CRISPR-Mediated Gene Activation (CRISPRa) of pp38/pp24 Orchestrates Events Triggering Lytic Infection in Marek's Disease Virus-Transformed Cell Lines

Marek’s disease (MD) is an immunosuppressive and highly contagious lymphoproliferative disease caused by Marek’s disease virus (MDV) in poultry. Lymphoblastoid cell lines (LCLs) generated ex vivo from MD lymphomas are considered excellent models to study virus-host molecular interactions. LCLs mostly have latently infected MDV genome, but many of them also have varying populations of lytically-infected cells, thus making them very suitable to examine the molecular events associated with the switch from latent to lytic infection. MDV-encoded phosphoprotein 38 (pp38) is readily detectable in lytically-infected LCLs and hence considered as a biomarker for lytic infection. Whilst previous studies have suggested that pp38 is essential for the early cytolytic infection of B-cells, its role in the switch from latent to lytic infection of LCLs is still unclear. pp24, another phosphorylated protein in the same protein complex, shares the same promoter and N-terminal 65 amino acids as pp38. In this study we employed CRISPR activation (CRISPRa) technology for targeted activation of pp38/pp24 in LCLs to investigate their role in inducing lytic infection. Our results show that enforced expression of pp38/pp24 through CRISPRa induces orchestrated upregulation of other MDV genes including ICP4, gB, Meq and pp14 as well as differential expression of host genes thereby facilitating lytic infection. Our results also show that pp38/pp24 expression induces the lytic switch through inhibiting apoptosis.

The Transcriptional Landscape of Marek's Disease Virus in Primary Chicken B Cells Reveals Novel Splice Variants and Genes

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that infects chickens and poses a serious threat to poultry health. In infected animals, MDV efficiently replicates in B cells in various lymphoid organs. Despite many years of research, the viral transcriptome in primary target cells of MDV remained unknown. In this study, we uncovered the transcriptional landscape of the very virulent RB1B strain and the attenuated CVI988/Rispens vaccine strain in primary chicken B cells using high-throughput RNA-sequencing. Our data confirmed the expression of known genes, but also identified a novel spliced MDV gene in the unique short region of the genome. Furthermore, de novo transcriptome assembly revealed extensive splicing of viral genes resulting in coding and non-coding RNA transcripts. A novel splicing isoform of MDV UL15 could also be confirmed by mass spectrometry and RT-PCR. In addition, we could demonstrate that the associated transcriptional motifs are highly conserved and closely resembled those of the host transcriptional machinery. Taken together, our data allow a comprehensive re-annotation of the MDV genome with novel genes and splice variants that could be targeted in further research on MDV replication and tumorigenesis.

Characterization of a very virulent Marek's disease virus mutant expressing the pp38 protein from the serotype 1 vaccine strain CVI988/Rispens

Marek's disease virus (MDV), a highly cell-associated oncogenic chicken herpesvirus, causes Marek's disease in domestic chickens. A unique phosphoprotein of MDV, pp38, has previously been associated with the maintenance of transformation in MDV-induced tumor cell lines. However, recently, the biological properties of a deletion mutant virus (rMd5Deltapp38) revealed that pp38 is involved in early cytolytic infection in lymphocytes but not in the induction of tumors. Thus, pp38 is important for early cytolytic infection and not for transformation. The pp38 protein of the MDV serotype 1 vaccine strain CVI988/Rispens differs by one amino acid when compared to the pathogenic strains of MDV. Monoclonal antibody, H19, recognizes all serotype 1 MDV strains except CVI988/Rispens. Previous studies have also shown that the unique pp38 epitope in CVI988/Rispens induced high antibody response. In order to study the role of this epitope in the protective properties of CVI988/Rispens, we generated a mutant rMd5 virus in which the wild type pp38 gene has been substituted with that of CVI988/Rispens (rMd5/pp38CVI). The replication properties of rMd5/pp38CVI, both in vitro and in vivo, and tumor induction were examined. We found that the biological properties of rMd5/pp38CVI were similar to the wild type rMd5 virus with regards to in vivo replication, antibody response and tumor induction. This shows that the pp38 derived from CVI988/Rispens is not involved in protective properties as was previously suggested.

Study on the structure of heteropolymer pp38/pp24 and its enhancement on the bi-directional promoter upstream of pp38 gene in Marek's disease virus

In the latest report, Chloramphenicol acetyltransferase (CAT) gene was used as a reporter to investigate the influence of pp38 on its upstream bi-directional promoter, and it was found that the co-expression of pp38 and pp24 can significantly enhance the transactivity of the bi-directional promoter between pp38 gene and 1.8-kb mRNA transcript in genome of Marek's disease virus (MDV). In this study, enhanced green fluorescence protein (EGFP) gene was used as another reporter to further investigate the promoter activity. The transfection shows the promoter has the complete activity under the condition of co-expression of pp38 and pp24 in the same cells. Immunoprecipitation test was used to verify the structure of pp38/pp24 heteropolymer. The pp38-specific monoclonal antibody H19 was used in this test, and pp38, pp24 or both were prepared from the pcDNA-pp38, pcDNA-pp24 or pBud-pp38-pp24 transfected chicken embryonic fibroblast (CEF), respectively. Immunoprecipitation indicates that pp24 could be co-precipitated with pp38 by MabH19, implying that pp24 and pp38 were able to form a heteropolymer in the natural condition. The two separated tests clarify that pp38 and pp24 form a heteropolymer, which enhances the activity of the promoter.

Marek's disease virus unique genes pp38 and pp24 are essential for transactivating the bi-directional promoters for the 1.8 kb mRNA transcripts

The pp38 and pp24 genes of Marek's diseases virus (MDV) share the same promoter, which controls the transcription of pp38 or pp24 and a 1.8-kb mRNA bi-directionally. To understand the trans-activating activity of pp38 and pp24 on the bi-directional promoter, both genes were cloned into pcDNA-3 or pBudCE4.1 vectors either singly or in combination. These plasmids were expressed in transfected chicken embryonic fibroblast (CEF) cells. Chloramphenicol acetyltransferase (CAT) activity expressed under the control of the promoter in CEF co-transfected with pP(1.8 kb)-CAT and pBud-pp38-pp24 was significantly higher than that following transfection with only pBud-pp38 or pBud-pp24. This indicates the combination of pp24 and pp38 together are essential for the activation of the promoter. In DNA mobility shift assays, the promoter binds to pp38 and pp24 together, but not to pp38 or pp24 alone. By competitive inhibition tests with a set of DNA fragments from the promoter region, the sequence 5'-CTGCTCATTT-3' was identified as the core sequence for binding by pp38-pp24 in up-regulation of the bi-directional promoter activity.

Marek's disease virus encodes MicroRNAs that map to meq and the latency-associated transcript

MicroRNAs (miRNAs) are a class of small (approximately 22-nucleotide) regulatory molecules that block translation or induce degradation of target mRNAs. These have been identified in a wide range of organisms, including viruses. In particular, the oncogenic gammaherpesviruses Kaposi's sarcoma herpesvirus and Epstein-Barr virus encode miRNAs that could potentially regulate either viral or host genes. To determine if Marek's disease virus (MDV), an oncogenic alphaherpesvirus of chickens, encodes miRNAs, we isolated small RNAs from MDV-infected chicken embryo fibroblasts (CEF) and used the 454 Life Sciences sequencing technology to obtain the sequences of 13,679 candidate host and viral small RNAs. Eight miRNAs were found, five of which flank the meq oncogene and three that map to the latency-associated transcript (LAT) region of the genome. The meq gene is unique to pathogenic serotypes of MDV and is transcriptionally active during latency and transformation, and the LAT region of the MDV genome is antisense to the immediate-early gene ICP4. Secondary structure analysis predicted that the regions flanking the miRNAs could form hairpin precursors. Northern blot analysis confirmed expression of all miRNAs in MDV-infected CEF, MDV-induced tumors, and MDV lymphoblastoid cell lines. We propose that the MDV miRNAs function to enable MDV pathogenesis and contribute to MDV-induced transformation of chicken T cells.

Expression of Marek's disease virus phosphorylated polypeptide pp38 produces splice variants and enhances metabolic activity

The phosphorylated polypeptide (pp)38 of oncogenic Marek's disease (MD) herpesvirus (MDV) is expressed during lytic infections in vivo and in vitro, but its functions have not been fully elucidated. The quail cell line QT-35, latently infected with MDV, was used to generate QTP32 in which pp38 is expressed under control of a tetracycline controlled promoter to examine possible functions of pp38. Induction of pp38 did not influence late MDV genes expression, but it enhanced mitochondrial dehydrogenase activity significantly. Two new pp38 splice variants were found in induced QTP32 cells, in additional in vitro systems and MDV-infected chickens. Differential expression of full-length pp38 and splice variants suggests that the splice variants are important during latency and perhaps transformation. Polypeptides of 40 and 20kDa were detected by Western blot using monoclonal antibody H19. These polypeptides were also produced in DF-1 cells transfected with a pp38 construct in which the splice acceptor sites had been mutated. Our results add important new information to the role of pp38 in the pathogenesis of MD. The data suggest that pp38 and the two newly described splice variants may influence metabolic activity, which may have important consequences for the understanding of latency and tumor development.

The enhancement effect of pp38 gene product on the activity of its upstream bi-directional promoter in Marek's disease virus

There was a bi-directional promoter between gene 38 kd phosphorylated protein (pp38) gene and 1.8-kb mRNA transcript gene family in the genome of Marek's disease virus (MDV). In this study, enhanced green fluorescence protein (EGFP) reporter plamids, pP(pp38)-EGFP and pP(1.8-kb)-EGFP, were constructed under this bi-directional promoter in two directions. The two plasmids were transfected into uninfected chicken embryo fibroblast (CEF), MDV clone rMd5 infected CEF (rMd5-CEF) and pp38-deleted derivative rMd5deltapp38 infected CEF (rMd5deltapp38-CEF) respectively. Transfection analysis showed that EGFP was only expressed in rMd5-CEF, and no EGFP could be detected in uninfected CEF or rMd5deltapp38-CEF, implying that pp38 was a factor influencing the activity of the promoter. The pp38-expressing recombinant plasmid pcDNA-pp38 was constructed to co-transfect CEF or rMd5deltapp38-CEF with pP(pp38)-EGFP or pP(1.8-kb)-EGFP. In this case, EGFP could be detected only in rMd5deltapp38-CEF but still not in uninfected CEF, implying that pp38 needs other protein(s) to work together for the complete trans-acting activity. Another MDV gene, 24 kd phosphorylated protein pp24 gene was cloned into pcDNA3.1 as a pp24-expressing recombinant plasmid pcDNA-pp24. When uninfected CEF was co-transfected with pcDNA-pp38, pcDNA-pp24 and EGFP expressing plasmids pP(pp38)-EGFP or pP(1.8-kb)-EGFP, the EGFP could be detected. These results indicated that pp38 and pp24 could enhance the activity of the promoter when they worked together. DNA mobility shift assay showed that pp38 would bind to the bi-directional promoter with the co-existing of pp24, although neither of them alone influenced mobility of the promoter DNA. All the above suggested that MDV pp38 could transactivate the bi-directional promoter when combined with pp24. The results also indicated that the activity of the promoter in the direction of 1.8-kb mRNA was significantly stronger than that of pp38 direction.

Alterations of the MDV oncogenic regions in an MDV transformed lymphoblastoid cell line

AIMS

Lymphoblastoid cell lines derived from Marek's disease virus (MDV) induced tumours have served as models of MDV latency and transformation. They are stable and can be cultured with no detectable MDV genomic alterations upon repeated passaging. An MDV transformed lymphoblastoid T cell line (T9 cell line) has been reported to contain a disrupted MDV BamHI-H fragment and a Rous associated virus insertional activation of the c-myb protooncogene. In an attempt to define the respective participation of c-myb and MDV in the transformed phenotype of T9 cells, an analysis of MDV oncogenic sequences (BamHI-H, BamHI-A, and EcoQ fragments) was performed in these cells.

METHODS

Using two different passages of the T9 cell line (late and early passages), the organisation of the MDV oncogenic regions and their expression in these cells were analysed. In vivo assessment of the oncogenicity of the virus contained within these cells was assessed by injecting them into 1 day old chickens.

RESULTS

In T9 cells maintained in culture for up to six months (late T9), the MDV ICP4 gene was disrupted, whereas the meq gene was actively transcribed. The alterations of the MDV genome in these cells correlated with the inability of the virus to induce the classic signs of Marek's disease in 1 day old chickens. However, early T9 cells submitted to a limited number of passages induced classic MDV pathogenicity, as efficiently as the MDV control cell line (T5), and did not show gross structural changes in the oncogenic MDV sequences.

CONCLUSIONS

Although the expression pattern of the MDV oncogenes in early T9 cells was identical to the one reported for other MDV transformed cells, longterm culture of an MDV transformed cell line containing a RAV insertional activation of the c-myb protooncogene led to the disruption of the MDV BamHI-H and BamHI-A oncogenic regions. In the late T9 cells MEQ was the only detected MDV oncoprotein. These results suggest that in the late T9 cells the truncated MYB protein compensates for the loss of MDV oncoproteins and reinforce the possibility that MEQ and MYB cooperate in the maintenance of the transformed state and the tumorigenic potential of these cells.

The genome of turkey herpesvirus

Here we present the first complete genomic sequence of Marek's disease virus serotype 3 (MDV3), also known as turkey herpesvirus (HVT). The 159,160-bp genome encodes an estimated 99 putative proteins and resembles alphaherpesviruses in genomic organization and gene content. HVT is very similar to MDV1 and MDV2 within the unique long (UL) and unique short (US) genomic regions, where homologous genes share a high degree of colinearity and their proteins share a high level of amino acid identity. Within the UL region, HVT contains 57 genes with homologues found in herpes simplex virus type 1 (HSV-1), six genes with homologues found only in MDV, and two genes (HVT068 and HVT070 genes) which are unique to HVT. The HVT US region is 2.2 kb shorter than that of MDV1 (Md5 strain) due to the absence of an MDV093 (SORF4) homologue and to differences at the UL/short repeat (RS) boundary. HVT lacks a homologue of MDV087, a protein encoded at the UL/RS boundary of MDV1 (Md5), and it contains two homologues of MDV096 (glycoprotein E) in the RS. HVT RS are 1,039 bp longer than those in MDV1, and with the exception of an ICP4 gene homologue, the gene content is different from that of MDV1. Six unique genes, including a homologue of the antiapoptotic gene Bcl-2, are found in the RS. This is the first reported Bcl-2 homologue in an alphaherpesvirus. HVT long repeats (RL) are 7,407 bp shorter than those in MDV1 and do not contain homologues of MDV1 genes with functions involving virulence, oncogenicity, and immune evasion. HVT lacks homologues of MDV1 oncoprotein MEQ, CxC chemokine, oncogenicity-associated phosphoprotein pp24, and conserved domains of phosphoprotein pp38. These significant genomic differences in and adjacent to RS and RL regions likely account for the differences in host range, virulence, and oncogenicity between nonpathogenic HVT and highly pathogenic MDV1.

The genome of a very virulent Marek's disease virus

Here we present the first complete genomic sequence, with analysis, of a very virulent strain of Marek's disease virus serotype 1 (MDV1), Md5. The genome is 177,874 bp and is predicted to encode 103 proteins. MDV1 is colinear with the prototypic alphaherpesvirus herpes simplex virus type 1 (HSV-1) within the unique long (UL) region, and it is most similar at the amino acid level to MDV2, herpesvirus of turkeys (HVT), and nonavian herpesviruses equine herpesviruses 1 and 4. MDV1 encodes 55 HSV-1 UL homologues together with 6 additional UL proteins that are absent in nonavian herpesviruses. The unique short (US) region is colinear with and has greater than 99% nucleotide identity to that of MDV1 strain GA; however, an extra nucleotide sequence at the Md5 US/short terminal repeat boundary results in a shorter US region and the presence of a second gene (encoding MDV097) similar to the SORF2 gene. MD5, like HVT, encodes an ICP4 homologue that contains a 900-amino-acid amino-terminal extension not found in other herpesviruses. Putative virulence and host range gene products include the oncoprotein MEQ, oncogenicity-associated phosphoproteins pp38 and pp24, a lipase homologue, a CxC chemokine, and unique proteins of unknown function MDV087 and MDV097 (SORF2 homologues) and MDV093 (SORF4). Consistent with its virulent phenotype, Md5 contains only two copies of the 132-bp repeat which has previously been associated with viral attenuation and loss of oncogenicity.

The complete unique long sequence and the overall genomic organization of the GA strain of Marek's disease virus

We have determined the DNA sequence of the unique long (UL) region and the repeat long (RL) region in the genome of serotype 1 GA strain of Marek's disease virus (MDV), a member of the alpha-herpesvirus family. With this information, the complete nucleotide sequence of GA-MDV is now known. The entire GA-MDV genome is predicted to be about 174 kbp in size, with an organization of TRL-UL-IRL-IRS-US-TRS, typical of a alpha-herpesvirus. The UL sequence contains 113,508 bp and has a base composition of 41.7% G + C. A total of 67 ORFs were identified completely within the UL region, among which 55 are homologous to genes encoded by herpes simplex virus-1. Twelve of them are unique with presently unknown functions. The sequence of RL reported here together with those published earlier reveal the major structural features of the RL. Virtually all of the ORFs encoded by RL are specific to serotype I of MDV. These ORFs are likely to contribute to some of the unique biological properties of MDV. Among the proteins encoded by MDV-specific ORFs are Meq, a jun/fos family of transcriptional factor implicated in transformation and latency, virus-encoded interleukin-8, a CXC chemokine, and pp38 and pp24, two phosphoproteins with undefined functions. There is also a putative lipase gene (LORF2) that has homologies in HPRS-24 (serotype II) strain of MDV and in various avian adenoviruses. An additional unique feature of MDV is the presence of long terminal repeat remnant sequences of avian retrovirus reticuloendotheliosis virus. These remnant sequences are derived from the U3-enhancer region through ancestral insertions by reticuloendotheliosis virus proviruses.

Difference in the meq gene between oncogenic and attenuated strains of Marek's disease virus serotype 1

Serotype 1 strains of Marek's disease virus (MDV1), except attenuated vaccine strains, are known to cause lymphomas in visceral organs of infected chickens. To know additional genetic differences between oncogenic and nononcogenic MDV1, polymerase chain reaction (PCR) was performed to amplify the meq gene of the viral genome. In addition to the 1,062-bp band including the native meq open reading frame (ORF), a 1.2-kb band was amplified from the DNA sample prepared from chick embryo fibroblast infected with an attenuated strain, CVI988, but not with oncogenic strains. Sequence analysis of the 1.2-kb band showed that a 178-bp sequence was inserted to the meq ORF of CVI988. This ORF could encode for the Meq protein with a different transactivator domain. Southern blot analysis also confirmed the insertion of the 178-bp sequence in the meq ORF of CVI988. This insertion of 178-bp sequence may explain the reason why CVI988 is not oncogenic.

Recombinant Marek's disease virus (MDV)-derived lymphoblastoid cell lines: regulation of a marker gene within the context of the MDV genome

Marek's disease is a herpesvirus (Marek's disease virus [MDV])-induced pathology of chickens characterized by paralysis and the rapid appearance of T-cell lymphomas. Lymphoblastoid cell lines (LBCLs) derived from MDV-induced tumors have served as models of MDV latency and transformation. We have recently reported the construction of mutant MDVs having a deletion (M. S. Parcells et al., J. Virol. 69:7888-7898, 1995) and an insertion (A. S. Anderson et al., J. Virol. 72:2548-2553, 1998) within the unique short region of the virus genome. These mutant MDVs retained oncogenicity, and LBCLs have been established from the mutant-induced tumors. We report the characterization of these cell lines with respect to (i) virus structure within and reactivated from the cell lines, (ii) surface antigen expression, (iii) kinetics of MDV and marker gene induction, (iv) localization and colocalization of induced MDV antigens and beta-galactosidase (beta-Gal), and (v) methylation status of the region of lacZ insertion in recombinant- and non-recombinant-derived cell lines. Our results indicate that (i) recombinant-derived cell lines contain no parental virus, (ii) the established cell lines are predominantly CD4(+) CD8(-), (iii) the percentage of Lac-expressing cells is low (1 to 3%) but increases dramatically upon 5'-iododeoxyuridine (IUdR) treatment, (iv) lacZ expression is induced with the same kinetics as several MDV lytic-phase genes (pp38, US1, gB, gI, and US10), and (v) the regulation of lacZ expression is not mediated by methylation. Furthermore, the MDV-encoded oncoprotein, Meq, could be detected in cells expressing beta-Gal and various lytic antigens but did not appear to be induced by IUdR treatment. Our results indicate that regulation of the lacZ marker gene can serve as sensitive measure of virus lytic-phase induction and the reactivation from latency.

Transcriptional analysis of Marek's disease virus (MDV) genes in MDV-transformed lymphoblastoid cell lines without MDV-activated cells

Spontaneously activated MDV is rarely included in MDV-transformed cells, while it may influence the results of transcriptional analysis. A population consisting of 10(3) MDV-transformed cells probably did not include spontaneously activated MDV, since the estimated frequency of MDV-transformed cells including activated MDV was below 0.01% according to limiting-dilution polymerase chain reaction (PCR) and the presence of the major early antigen pp38 in 6 transformed cell lines. Reverse transcriptase-PCR (RT-PCR) products corresponding to ICP27, pol, TK, US3, A41, gA, gB and UL50 genes were undetectable in 10(3) cells by Southern hybridization of the RT-PCR products. Transcripts of the VP16 and SORF2 genes were detected in the 10(3) cells of MSB-1, and the pp14 gene transcript was found in 10(3) cells of RPL-1 but not in 10(3) cells of HPRS-1, MOGA-2, MSB-1 or MTB-1. A transcript corresponding to the ICP4 sequence was detected as a 0.7 kbp RT-PCR product in 10(3) cells of these MDV cell lines but not in the retrovirus-transformed 1104B1 cell line. The transcript corresponding to the 0.7 kbp RT-PCR product suggested a splice by its size and sequence. Thus, transcriptional analysis of 10(3) MDV-transformed cells revealed that the transcript corresponding to the ICP4 sequence was a common transcript in latently infected MDV-transformed cells, while most of the genes did not transcribe in these cells.

Retention of oncogenicity by a Marek's disease virus mutant lacking six unique short region genes

We previously reported the construction of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique short (US) region of the viral genome (J.L. Cantello, A.S. Anderson, A. Francesconi, and R.W. Morgan, J. Virol. 65:1584-1588, 1991; M.S. Parcells, A.S. Anderson, and R.W. Morgan, Virus Genes 9:5-13, 1994; M.S. Parcells, A.S. Anderson, and R.W. Morgan, J. Virol. 68:8239-8253, 1994). These strains were constructed by using a high-passage-level serotype 1 MDV strain which grew well in chicken embryo fibroblasts. Despite the growth of the parent and mutant viruses in cell culture, in vivo studies were limited by poor growth of these strains in chickens. One of the mutants studied lacked 4.5 kbp of US region DNA and contained the lacZ gene of Escherichia coli inserted at the site of the deletion. The deletion removed MDV homologs to the US1, US2, and US10 genes of herpes simplex virus type 1 as well as three MDV-specific open reading frames. We now report the construction of a mutant MDV containing a similar deletion in the US region of the highly oncogenic RB1B strain. This mutant, RB1B delta 4.5lac, had a growth impairment in established chicken embryo fibroblasts similar to that described previously for MDVs lacking a functional US1 gene. In chickens, RB1B delta 4.5lac showed decreased early cytolytic infection, mortality, tumor incidence, and horizontal transmission. Several lymphoblastoid cell lines were established from RB1B delta 4.5lac-induced tumors, and virus reactivated from these cell lines was LacZ+. These results indicate that the deleted genes are nonessential for the transformation of chicken T cells or for the establishment and maintenance of latency. On the basis of the growth impairment observed for RB1B delta 4.5lac in cell culture and in vivo, we conclude that deletion of these genes affects the lytic replication of MDV. This is the first MDV mutant constructed in the RB1B oncogenic strain, and the methodology described herein provides for the direct examination of MDV-encoded determinants of oncogenicity.

Expression of Marek's disease virus (MDV) serotype 2 gene which has partial homology with MDV serotype 1 pp38 gene

We constructed the recombinant baculovirus expressing the gene of non-pathogenic Marek's disease virus (MDV) serotype 2 (MDV2) which encodes a polypeptide with partial homology to MDV serotype 1 (MDV1) pp38, an antigen associated with transformed cells. The recombinant MDV2 protein was detected as a band of 32 kDa in immunoblot analysis with MDV2-infected chicken serum. Mouse serum against insect Spodoptera frugiperda cells infected with the recombinant baculovirus immunoprecipitated a 38 kDa molecule from the lysate of MDV2-infected chicken embryo fibroblasts (CEFs) but did not immunoprecipitate the MDV1 pp38 from the lysate of MDV1-infected CEFs. This result indicates that the recombinant MDV2 protein has no epitopes shared with the MDV1 pp38.