Rescue of a pathogenic Marek's disease virus with overlapping cosmid DNAs: use of a pp38 mutant to validate the technology for the study of gene function

Marek's disease virus-encoded vIL-8 gene is involved in early cytolytic infection but dispensable for establishment of latency

Marek's disease, a lymphoproliferative disease of chickens, is caused by an alphaherpesvirus, Marek's disease virus (MDV). This virus encodes a virokine, vIL-8, with general homology to cellular CXC chemokines such as interleukin-8 (IL-8) and Gro-alpha. To study the function of vIL-8 gene, we deleted both copies of vIL-8 residing in the terminal repeat long and internal repeat long region of the viral genome and generated a mutant virus with vIL-8 deleted, rMd5/DeltavIL-8. Growth kinetics study showed that vIL-8 gene is dispensable for virus replication in cell culture. In vivo, the vIL-8 gene is involved in early cytolytic infections in lymphoid organs, as evidenced by limited viral antigen expression of rMd5/DeltavIL-8. However, the rMd5/DeltavIL-8 virus is unimpaired in virus replication in the feather follicle epithelium. vIL-8 does not appear to be important for establishment of latency, since rMd5/DeltavIL-8 and the wild-type virus have similar viremia titers at 14 days postinfection, a period when the virus titer comes primarily from reactivated latent genomes. Nevertheless, because of the impaired cytolytic infections, the overall transformation efficiency of the virus with vIL-8 deleted is much lower, as reflected by the reduced number of transformed cells at 5 weeks postinoculation and the presence of fewer gross tumors. Importantly, the revertant virus that restored the expression of vIL-8 gene also restored the wild-type phenotype, indicating the deficient phenotypes are results of vIL-8 deletion. One of the interesting differences between the MDV vIL-8 gene and its cellular counterpart is the presence of a DKR (Asp-Lys-Arg) motif instead of ELR (Glu-Leu-Arg) preceding the invariable CXC motif. To study the significance of this variation, we generated recombinant MDV, rMd5/vIL-8-ELR, carrying the ELR motif. Both in vitro and in vivo studies revealed that the DKR motif is as competent as ELR in pathogenesis of MDV.

Marek's disease virus latent protein MEQ: delineation of an epitope in the BR1 domain involved in nuclear localization

Marek's disease virus latent protein MEQ (MDV Eco Q) is abundantly expressed and consistently detected in MDV-induced tumors and cell lines. Deletion mutants were constructed to study the domain structure of MEQ. Four deletion mutants were obtained in the basic regions of MEQ, namely basic region 1 (DeltaBR1), basic region 2 (DeltaBR2), basic regions 1 and 2 (DeltaBR1 and 2), and the C-terminal (bZIP) domain. The BR1 and BR2 are nuclear localization signals and either is sufficient to cause transport of MEQ into the nucleus. In addition, the BR2 is also responsible for MEQ's nucleolar localization. A monoclonal antibody (Mab 23B46) was produced using recombinant fowlpox virus (rFPV) expressing MEQ (rFPV/MEQ) as a source of protein. The isotype of Mab 23B46 is IgG1 and immunoprecipitated a band in rFPV/MEQ infected cells with molecular weight of 60 kDa specific to MEQ protein. We detected abundant expression of MEQ in (rFPV/MEQ), recombinant baculovirus (rBac) (rBac/MEQ), and lymphoid tumors induced by MDV. In order to delineate the epitope of MEQ reactive with Mab 23B46, we used four deletion mutants from the basic and bZIP domains. We found the deletions in the N-terminal region including BR1 (DeltaBR1), and (DeltaBR1 and 2) completely abolished the specific binding with Mab 23B46 as shown by Western blot analysis and immunofluoresence test. Deletion of BR2 (DeltaBR2) and the C-terminal (bZIP) domain had no effect on antibody binding. These data provide direct evidence that monoclonal antibody reactive epitope is localized in the BR1 domain of the molecule. Since both BR1 and BR2 domains contain sequences important for nuclear entry, we now have reagent to further study and elucidate the mechanism of MEQ's involvement in nuclear and nucleolar localization.

Expansion of a unique region in the Marek's disease virus genome occurs concomitantly with attenuation but is not sufficient to cause attenuation

Pathogenic Marek's disease viruses (MDVs) have two head-to-tail copies of a 132-bp repeat. As MDV is serially passaged in cell culture, the virus becomes attenuated and the number of copies of the 132-bp repeat increases from 2 to often more than 20 copies. To determine the role of the repeats in attenuation, we used five overlapping cosmid clones that spanned the MDV genome to reconstitute infectious virus (rMd5). By mutating the appropriate cosmids, we generated clones of infectious MDVs that contained zero copies of the 132-bp repeats, rMd5(Delta132); nine copies of the 132-bp repeats, rMd5(9-132); and nine copies of the 132-bp repeats inserted in the reverse orientation, rMd5(rev9-132). After two passages in cell culture, wild-type Md5, rMd5, and rMd5(Delta132) were stable. However, rMd5(9-132) and rMd5(rev9-132) contained a population of viruses that contained from 3 to over 20 copies of the repeats. A major 1.8-kb mRNA, containing two copies of the 132-bp repeat, was present in wild-type Md5 and rMd5 but was not present in rMd5(Delta132), rMd5(9-132), rMd5(rev9-132), or an attenuated MDV. Instead, the RNAs transcribed from the 132-bp repeat region in rMd5(9-132) and rMd5(rev9-132) closely resembled the pattern of RNAs transcribed in attenuated MDVs. When inoculated into susceptible day-old chicks, all viruses produced various lesions. Thus, expansion of the number of copies of 132-bp repeats, which accompanies attenuation, is not sufficient in itself to attenuate pathogenic MDVs.

Biocharacteristics shared by highly protective vaccines against Marek's disease

Attenuated serotype 1 Marek's disease virus strains vary widely in their protection properties. This study was conducted to elucidate which biocharacteristics of serotype 1 MDV strains are related with protection. Three pairs of vaccines, each one including a higher protective (HP) vaccine and a lower protective (LP) vaccine originating from the same MDV strain, were studied. Two other highly protective vaccines (RM1 and CVI988/BP5) were also included in the study. Comparison within pairs of vaccines showed that marked differences existed between the HP and the LP vaccines. Compared with LP vaccines, HP vaccines replicated better in vivo. Also, they induced a significant expansion of total T cells and of the helper and cytotoxic T cell lineages (CD45(+)CD3(+), CD4(+)CD8(-), CD4(-)CD8(+)) as well as a marked increase in the expression of the antigens of MhcI and MhcII on T cells. Thus, our results show that in vivo replication and early stimulation of the T-cell lineage are two characteristics shared by HP vaccines. However, comparison among the four HP vaccines that provided protection equal to that of CVI988 (RM1, CVI988/BP5, CVI988 and 648A80) revealed variability, especially regarding in vivo replication. Strains RM1 and CVI988/BP5 showed much stronger replication in vivo than the other two vaccine strains (CVI988 and 648A80). Thus, no single set of characteristics could be used to identify the most protective Marek's disease vaccines, implying, perhaps, that multiple mechanisms may be involved.

Characterization of the chromosomal binding sites and dimerization partners of the viral oncoprotein Meq in Marek's disease virus-transformed T cells

Marek's disease virus (MDV) is an acute transforming alphaherpesvirus that causes T-cell lymphomas in chickens. We previously reported the identification of a putative oncogene, meq, that is encoded only by the oncogenic serotype of MDV. The gene product, Meq, is a latent protein that is consistently expressed in MDV-transformed lymphoblastoid cells and tumor cells. Meq has a bZIP (basic leucine zipper) structure resembling the family of Jun/Fos. The mechanism whereby Meq transforms T cells remains poorly understood. In this study, we explored the properties of Meq as a transcriptional factor. We analyzed Meq's dimerization partners and its target genes in MSB-1, an MDV-transformed T-cell line. By using in vitro assays, we first demonstrated Meq's potential to dimerize with a variety of bZIP proteins. We then identified c-Jun as the primary dimerization partner of Meq. Both are found to be colocalized in the nucleus and corecruited to promoters with AP-1 sequences. By using chromatin immunoprecipitation (ChIP), we scanned the entire MDV genome for Meq binding sites and found three regions that were enriched with Meq binding: the MDV lytic replication origin, the promoter for Meq, and the promoter for ICP4. Transactivation assays using the above promoters showed that Meq/Meq homodimers exhibited repression activity, whereas Meq/Jun heterodimers showed activation. Finally, we were able to show by ChIP that Meq is recruited to the interleukin-2 promoter in a region encompassing an AP-1 site. Thus, in addition to providing general knowledge about the transcriptional properties of Meq, our studies revealed for the first time the ability of Meq to interact with the latent MDV and host genomes. Our data suggest, therefore, a role for Meq in viral genome regulation during latency, in addition to its putative causal role in T-cell transformation.

Infection of macrophages by a lymphotropic herpesvirus: a new tropism for Marek's disease virus

Marek's disease virus (MDV) is classified as an oncogenic lymphotropic herpesvirus of chickens. MDV productively and cytolytically infects B, alphabetaT and gammadeltaT lymphocytes and latently infects T-helper lymphocytes. The aims of this study were to identify whether MDV infects macrophages in vivo and, if so, whether quantitative differences in macrophage infection are associated with MDV strain virulence. Chickens were infected with either virulent MDV (HPRS-16) or 'hypervirulent' MDV (C12/130). Flow cytometry with monoclonal antibodies recognizing MDV pp38 antigen and leukocyte antigens was used to identify MDV lytically infected cells. Macrophages from HPRS-16- and C12/130-infected chickens were pp38(+). It is demonstrated that macrophages are pp38(+) because they are infected and not because they have phagocytosed MDV antigens, as assessed by confocal microscopy using antibodies recognizing MDV antigens of the three herpesvirus kinetic classes: infected cell protein 4 (ICP4, immediate early), pp38 (early) and glycoprotein B (gB, late). Spleen macrophages from MDV-infected chickens were ICP4(+), pp38(+) and gB(+), and ICP4 had nuclear localization denoting infection. Finally, MDV pp38(+) macrophages had high inherent death rates, confirming cytolytic MDV infection, although production of virus particles has not been detected yet. These results have two fundamental implications for understanding MDV pathogenesis: (i) MDV evolved to perturb innate, in addition to acquired, immunity and (ii) macrophages are excellent candidates for transporting MDV to primary lymphoid organs during the earliest stages of pathogenesis.

Marek's disease virus reactivation from latency: changes in gene expression at the origin of replication

Marek's disease is a contagious lymphoma of chickens caused by Marek's disease virus (MDV). MDV replicates in chicken lymphocytes and establishes latency within and transforms chicken CD4+ T-cells. Transformed T-cells are seen as skin leukosis or as lymphomas in visceral organs. A major focus of our laboratory is the functional study of genes flanking the origin of replication. This origin (OriLyt) is contained within the repeats flanking the unique long (UL) region of the genome (IRL and TRL). To the left of this Ori are genes associated with MDV latent/transforming infection [1.8-kb RNA family, pp14, Meq), and to the right (UL) are genes associated with early stages of MDV lytic infection [BamHI-H-encoded protein (Hep), pp38/pp24, Mys]. During latency, MDV suppresses lytic gene expression and has evolved mechanisms for blocking the apoptosis of latently-infected CD4+ T-cells. Of the genes expressed during MDV latency and in the transformed cell, the Meq (Marek's EcoRI-Q-encoded protein) has been shown to block apoptosis and transactivate gene expression. Upon reactivation to lytic infection, we have found that splice variants of Meq predominate and that these forms lack several of the domains important to Meq trans-activation and trans-repression. We have found that rightward from the origin of replication, a family genes, including phosphoprotein 38 (pp38) are expressed during early stages of reactivation. Three separate open reading frames (Hep, Mys, and pp38) are encoded by distinct transcripts from this region. We are now determining the kinetics of expression of these transcripts and their relative abundance during reactivation.