Cell-mediated cytolysis of lymphoblastoid cells expressing Marek's disease virus-specific phosphorylated polypeptides

Immune responses against Marek's disease virus

It is more than a century since Marek's disease (MD) was first reported in chickens and since then there have been concerted efforts to better understand this disease, its causative agent and various approaches for control of this disease. Recently, there have been several outbreaks of the disease in various regions, due to the evolving nature of MD virus (MDV), which necessitates the implementation of improved prophylactic approaches. It is therefore essential to better understand the interactions between chickens and the virus. The chicken immune system is directly involved in controlling the entry and the spread of the virus. It employs two distinct but interrelated mechanisms to tackle viral invasion. Innate defense mechanisms comprise secretion of soluble factors as well as cells such as macrophages and natural killer cells as the first line of defense. These innate responses provide the adaptive arm of the immune system including antibody- and cell-mediated immune responses to be tailored more specifically against MDV. In addition to the immune system, genetic and epigenetic mechanisms contribute to the outcome of MDV infection in chickens. This review discusses our current understanding of immune responses elicited against MDV and genetic factors that contribute to the nature of the response.

Herpesvirus telomerase RNA(vTR)-dependent lymphoma formation does not require interaction of vTR with telomerase reverse transcriptase (TERT)

Telomerase is a ribonucleoprotein complex involved in the maintenance of telomeres, a protective structure at the distal ends of chromosomes. The enzyme complex contains two main components, telomerase reverse transcriptase (TERT), the catalytic subunit, and telomerase RNA (TR), which serves as a template for the addition of telomeric repeats (TTAGGG)(n). Marek's disease virus (MDV), an oncogenic herpesvirus inducing fatal lymphoma in chickens, encodes a TR homologue, viral TR (vTR), which significantly contributes to MDV-induced lymphomagenesis. As recent studies have suggested that TRs possess functions independently of telomerase activity, we investigated if the tumor-promoting properties of MDV vTR are dependent on formation of a functional telomerase complex. The P6.1 stem-loop of TR is known to mediate TR-TERT complex formation and we show here that interaction of vTR with TERT and, consequently, telomerase activity was efficiently abrogated by the disruption of the vTR P6.1 stem-loop (P6.1mut). Recombinant MDV carrying the P6.1mut stem-loop mutation were generated and tested for their behavior in the natural host in vivo. In contrast to viruses lacking vTR, all animals infected with the P6.1mut viruses developed MDV-induced lymphomas, but onset of tumor formation was significantly delayed. P6.1mut viruses induced enhanced metastasis, indicating functionality of non-complexed vTR in tumor dissemination. We discovered that RPL22, a cellular factor involved in T-cell development and virus-induced transformation, directly interacts with wild-type and mutant vTR and is, consequently, relocalized to the nucleoplasm. Our study provides the first evidence that expression of TR, in this case encoded by a herpesvirus, is pro-oncogenic in the absence of telomerase activity.

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.

The role of pp38 in regulation of Marek's disease virus bi-directional promoter between pp38 and 1.8-kb mRNA

Marek's disease virus (MDV) contains a bi-directional promoters located between pp38 gene and 1.8-kb mRNA in the long inverted repeat region of the viral genome. The involvement of pp38 gene in up-regulating the activity of these promoters was analyzed by transient expression of chloramphenicol acetyltransferase (CAT) reporter gene. Two CAT reporter plasmids, pP(pp38)-CAT and pP(1.8-kb)-CAT, were constructed to express CAT under the control of the bi-directional promoter in both orientations. These plasmids were transfected into chicken embryonic fibroblast (CEF), infected with rMd5 and pp38 deleted rMd5 (rMd5/Deltapp38), respectively. No CAT activity was detected in uninfected CEF as expected. CAT activities in rMd5/Deltapp38 virus infected CEF (rMd5/Deltapp38-CEF) were 3.5-fold lower using pP(pp38)-CAT and 12-fold lower using pP(1.8-kb)-CAT than those of the parental rMd5 infected CEF (rMd5-CEF). The significantly lower promoter activity in the pp38 deletion virus suggests that pp38 can regulate the activity of the bi-directional promoters, especially in the direction of 1.8-kb mRNA family. Co-transfection of pp38-expressing plasmid (pcDNA-pp38) into rMd5/Deltapp38-CEF significantly increased the activity of the bi-directional promoters using either pP(pp38)-CAT or pP(1.8-kb)-CAT. DNA mobility shift assay showed a binding of the 73-bp sequence of the bi-directional promoter with rMd5-CEF but not with rMd5/Deltapp38-CEF or uninfected CEF lysates. However, rMd5/Deltapp38-CEF lysates could bind the same 73-bp promoter sequence when co-transfected with pp38-expressing plasmid (pcDNA-pp38). All these data taken together suggest pp38 plays an important role in regulating the transcriptional activity of the bi-directional promoter.

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.

Infection with chicken anaemia virus impairs the generation of pathogen-specific cytotoxic T lymphocytes

Infection with chicken anaemia virus (CAV), a circovirus, can result in immunosuppression and subsequent increased susceptibility to secondary infections. This is the first report of impairment of pathogen-specific cytotoxic T lymphocytes (CTL) after natural and experimental infection of chickens with CAV and Marek's disease virus (MDV) or reticuloendotheliosis virus (REV). MDV- and REV-specific CTL were generated at 7 days post infection by 9-30-day-old-chickens that were positive for maternal antibodies to CAV at 9-17 days of age. Replication of CAV could not be demonstrated in these chickens using quantitative real-time polymerase chain reaction (PCR) and reverse transcriptase (RT)-PCR assays. In contrast, REV-specific CTL failed to develop when chickens negative for maternal antibodies at 9-17 days of age were infected. Infection with CAV at 45 days of age after CAV maternal antibodies had waned also caused a decreased REV-specific CTL response. In these chickens increased levels of CAV DNA of up to 107 copy numbers per micro g DNA and increased relative transcript levels of CAV by up to a factor of 106 were detected by quantitative real-time PCR and RT-PCR. Interleukin (IL)-1beta and IL-2 mRNA levels were not significantly affected by CAV infection at 7 or 14 days p.i. Similar assays for interferon-gamma (IFN-gamma) transcripts demonstrated a 10-fold increase in IFN-gamma mRNA levels at 7 days post infection following REV or REV + CAV infection, while CAV alone caused a two- to fourfold increase. These results show a strong link between CAV antibody status, CAV replication, and the ability to generate REV-specific CTL. It is likely that the immunosuppressive effects of subclinical infection have previously been underestimated.

Cytotoxic T lymphocyte responses to Marek's disease herpesvirus-encoded glycoproteins

Cell-mediated immune responses are important for protective immunity to Marek's disease (MD), especially because MD herpesvirus (MDV) infection is strictly cell-associated in chickens with the exception of the feather follicle epithelium. A system previously developed using reticuloendotheliosis (REV)-transformed cell lines stably expressing individual MDV genes allows the determination of relevant MDV proteins for the induction of cytotoxic T lymphocyte (CTL) responses. To examine the importance of glycoproteins for the induction of CTL, the MDV genes coding for glycoproteins (g) C, D, E, H, I, K, L, and M were stably transfected into the REV-transformed chicken cell lines RECC-CU205 (major histocompatibility complex (MHC): B(21)B(21)) and RECC-CU91 (MHC: B(19)B(19)). All transfected cell lines were lysed by REV-sensitized, syngeneic splenocytes obtained from MD-resistant N2a (MHC: B(21)B(21)) and MD-susceptible P2a (MHC: B(19)B(19)) chickens, indicating that the expression of individual MDV glycoproteins did not interfere with antigen processing pathways. Only cell lines expressing gI were recognized by CTL from both N2a and P2a MDV-infected chickens. Cell lines expressing glycoproteins gC and gK, and to a lesser extent, gH, gL, and gM were lysed by syngeneic MDV-sensitized splenocytes from N2a birds but not P2a birds. In contrast, gE was recognized by MDV-sensitized effector cells from the P2a line and not the N2a line. Glycoprotein D was not recognized by either line, with the exception of one marginally significant P2a assay. These results indicate that late viral glycoproteins are relevant for the induction of cell-mediated immunity during MDV infection.

Marek's disease virus down-regulates surface expression of MHC (B Complex) Class I (BF) glycoproteins during active but not latent infection of chicken cells

Infection of chicken cells with three Marek's disease virus (MDV) serotypes interferes with expression of the major histocompatibility complex (MHC or B complex) class I (BF) glycoproteins. BF surface expression is blocked after infection of OU2 cells with MDV serotypes 1, 2, and 3. MDV-induced T-cell tumors suffer a nearly complete loss of cell surface BF upon virus reactivation with 5-bromo-2'-deoxyuridine (BUdR). The recombinant virus (RB1BUS2gfpDelta) transforming the MDCC-UA04 cell line expresses green fluorescent protein (GFP) during the immediate early phase of viral gene expression. Of the UA04 cells induced to express the immediate early GFP, approximately 60% have reduced levels of BF expression. All of the reactivated UA04 and MSB1 tumor cells expressing the major early viral protein pp38 display reduced levels of BF. Thus, BF down-regulation begins in the immediate early phase and is complete by the early phase of viral gene expression. The intracellular pool of BF is not appreciably affected, indicating that the likely mechanism is a block in BF transport and not the result of transcriptional or translational regulation.

Specific and nonspecific immune responses to Marek's disease virus

Marek's disease (MD) virus (MDV) has provided an important model to study immune responses against a lymphoma-inducing herpesvirus in its natural host. Infection in chickens starts with a lytic infection in B cells, followed by a latent infection in T cells and, in susceptible birds, T cell lymphomas develop. Non-specific and specific immune responses are important for the control of virus infection and subsequent tumor development. Interferon-gamma and nitric oxide are important for the control of virus replication during the lytic phase of infection and are also important to prevent reactivation of MDV replication in latently infected and transformed cells. Cytotoxic T cells (CTLs) are the most important of the specific immune responses in MDV. In addition to antigen-specific CTL against MDV proteins pp38, glycoprotein B (gB), Meq, and ICP4, ICP27-specific CTL can also be detected as early as 6 to 7 days post infection. The epitope for gB recognized by CTLs from P2a (MHC: B(19)B(19)) chickens has been localized to the Eco47III-BamHI (nucleotides 1515-1800) fragment. A proposed model for the interactions of cytokines and immune responses as part of the pathogenesis of MD is discussed.

Pathogenesis of Marek's disease (MD) and possible mechanisms of immunity induced by MD vaccine

Marek's disease (MD) is a lymphoproliferative disease of chicken, which is characterized by malignant T cell-lymphoma formation. This disease can be effectively prevented by vaccination with attenuated MD virus (MDV), apathogenic MDV or herpesvirus of turkey. MD vaccines are ones of a few vaccines which can prevent virus-induced tumor among mammalian and avian species. To determine the roles of T cell subsets in the protection mechanism, chickens vaccinated with an attenuated MDV (CVI988) were depleted of either CD4+ or CD8+ T cells by neonatal thymectomy and injections of monoclonal antibodies against chicken CD4 or CD8 molecules and then challenged with an oncogenic MDV. These birds were effectively protected from MDV-induced tumors. However, virus titers in CD4+ T cells, which are the main target cells for MDV-latent infection and subsequent transformation, were much higher in CD8-deficient vaccinated chickens than in untreated vaccinated chickens at the early stage of the latent phase. These results suggested that CD8+ T cell responses induced by the MD vaccine are essential for anti-virus but not anti-tumor effects. Here, we will discuss how the attenuated vaccine prevents chickens from lymphoma-formation by an oncogenic MDV.

Characterization of Marek's disease herpesvirus-specific cytotoxic T lymphocytes in chickens inoculated with a non-oncogenic vaccine strain of MDV

Previously we have reported that reticuloendotheliosis virus (REV)-transformed cell lines expressing Marek's disease virus (MDV) genes pp38, meq or gB were lysed by syngeneic MDV-specific splenocytes from major histocompatibility complex (MHC):B9B19 and MHC:B1B21 chickens. In contrast, REV-transformed cell lines expressing the MDV gene ICP4 were only lysed by syngeneic MDV-specific splenocytes from MHC:B21B21 chickens. In this study we report that this syngeneic cell-mediated immune response is induced by cytotoxic T lymphocytes (CTL). Splenocytes from MDV vaccine strain, SB-1 inoculated MHC:B19B19 and MHC:B21B21 chickens, depleted for CD4+, CD8+, TCR gamma delta +, TCR alpha beta 1+ and/or TCR alpha beta 2+ cells, were used as effector cells in chromium-release assays. Effector cells depleted of CD8+ or TCR alpha beta 1+, but not TCR gamma delta + or TCR alpha beta 2+, markedly reduced the MDV-specific release. Depletion of CD4+ effector cells did not influence the specific release significantly. This is the first report on identification of virus-specific CD8+ CTL in chickens inoculated with a non-oncogenic vaccine strain of MDV.

Syngeneic lysis of reticuloendotheliosis virus-transformed cell lines transfected with Marek's disease virus genes by virus-specific cytotoxic T cells

Cell-mediated immune responses against Marek's disease virus (MDV) antigens were examined using reticuloendotheliosis virus (REV)-transformed cell lines of two haplotypes (B19B19 and B13B13). These cell lines were stably transfected with cloned fragments of MDV DNA resulting in the expression of the MDV-specific phosphoprotein pp38. Effector cells were obtained from P2a (B19B19) and S13 (B13B13) chickens at 7 days post inoculation with REV, oncogenic or attenuated serotype 1 MDV (JM-16/O and JM-16/A, respectively), serotype 2 MDV (SB-1), or herpesvirus of turkeys (HVT). Transfection of MDV genes did not influence the expression of Class I major histocompatibility complex antigens. The optimal effector to target cell ratio was determined to be 100:1. REV-sensitized effector cells lysed REV cell lines and REV cell lines transfected with MDV DNA in a syngeneic fashion. Effector cells from chickens inoculated with JM-16/O, JM-16/A, SB-1 or HVT lysed only the syngeneic, transfected cell lines, but not the parent REV cell lines. The percentage specific release caused by the MDV-sensitized effector cells was low, but statistically significant.

Cell-mediated immune effector functions in chickens

Cell-mediated immune responses form an important part of the protection against intracellular pathogens. The MHC Class I and Class II antigens are important for the proper presentation of degraded proteins to cytotoxic T lymphocytes (CTL) and helper T cells, respectively. Recent developments in the knowledge of the molecular structure of the MHC in relation to antigen presentation are discussed. Although CTL are important, there is a paucity of information concerning their relevance for the control of viral diseases in poultry. A newly developed approach of stable transfection of viral genes into cell lines transformed by reticuloendotheliosis virus has shown promise as a method to define proteins, which are important for the induction of cell-mediated immunity.