Molecular pathogenesis of Marek's disease—recent developments

Marek's Disease in a Flock of Japanese Quails (Coturnix japonica) in Germany

A case of increased mortality was investigated at a German farm housing Japanese quails (Coturnix japonica) for egg production. Different age groups were kept in aviaries in one barn. The quail chicks had diarrhea and the adults were emaciated, some also with diarrhea and conjunctivitis. Postmortem examination showed gross tumorlike lesions in 7 of 15 adult quails examined, with ocular lesions in 2 of them. On histopathological examination, infiltrates of monomorphic round cells were found in liver, spleen, lung, and proventriculus. In the eyes of two quails, similar cells were infiltrating the choroid layer, limbus, and adjacent parts of the cornea. Malignant lymphoma was diagnosed. Immunohistochemical examination identified tumor cells as T cells, and Mardivirus gallidalpha 2-specific PCR was positive for five quails with gross lesions. Additionally, Proteus mirabilis and Klebsiella pneumoniae were detected in the chicks and Clostridium spp. and coccidia in the adults. Marek's EcoQ protein (Meq) gene from Mardivirus gallidalpha 2 was sequenced and analyzed, confirming Marek's disease. The results of our examinations demonstrate that the ocular lesions were caused by Marek's disease and that the Meq gene from Mardivirus gallidalpha 2 was detected in the flock of quails. As a control strategy for Marek's disease in quails, an all-in/all-out system was introduced. Additional laying quails acquired from a breeder at 35 days of life were vaccinated at stabling with a combined turkey herpesvirus (HVT)-Rispens vaccine and 14 days later with a Rispens vaccine. Subsequently, the losses and laying rates returned to normal.

Analysis of protein expression profiles in the thymus of chickens infected with Marek's disease virus

BACKGROUND

Marek's disease virus (MDV) is a highly cell-associated oncogenic α-herpesvirus that causes a disease characterised by T-cell lymphomas. The pathogenesis, or the nature of the interaction of the virus and the host, in the thymus are still unclear.

RESULTS

In this study, we identified 119 differentially expressed proteins using two-dimensional electrophoresis and mass spectrometry from the thymuses of chickens infected with the RB1B strain of MDV. These differentially expressed proteins were found mainly at 21, 28 and 35 days post-infection. More than 20 of the differentially expressed proteins were directly associated with immunity, apoptosis, tumour development and viral infection and replication. Five of these proteins, ANXA1, MIF, NPM1, OP18 and VIM, were further confirmed using real-time PCR. The functional associations and roles in oncogenesis of these proteins are discussed.

CONCLUSIONS

This work provides a proteomic profiling of host responses to MDV in the thymus of chickens and further characterises proteins related to the mechanisms of MDV oncogenesis and pathogenesis.

Expression of SC1, a cell adhesion molecule, promotes the metastatic activities of the Gallus gallus lymphoblastoid cell line MDCC-MSB1 derived from Marek's disease

SC1 is an immunoglobulin superfamily cell adhesion molecule purified from the Gallus gallus spinal cord. SC1 is present in embryonic tissues and plays a role in chick development through its cell adhesive property. Interestingly, increased SC1 expression is observed in some sporadic tumours of the chicken, including Marek's disease-induced lymphomas and in nephroblastomas. To elucidate the possible functions of SC1 in tumour progression in the chicken, SC1 cDNA was introduced into the endogenous SC1-negative Marek's disease-derived chicken lymphoblastoid cell line MDCC-MSB1, and subsequently the metastatic potentials of these cell lines were analysed. The in vitro analyses revealed that the SC1-transfected MDCC-MSB1 cells were enhanced in their adhesive and migratory activities in the presence of the SC1 proteins. In addition, the metastatic potential of the SC1-transfected MDCC-MSB1 cells to the lung was enhanced after intravenous implantation into chickens. These findings suggest that the expression of SC1 contributes to the malignancy and metastatic properties of chicken Marek's disease-induced lymphomas.

Relationship between Marek's disease virus load in peripheral blood lymphocytes at various stages of infection and clinical Marek's disease in broiler chickens

Vaccination with herpesvirus of turkey (HVT) vaccine provides protection against clinical Marek's disease (MD) but does not preclude infection with wild-type MD virus (MDV). The quantity of MDV detected in circulating lymphocytes during the early period after infection may be a useful predictor of subsequent clinical MD later in the life. A study was designed to quantify MDV and HVT copy number in peripheral blood lymphocytes (PBL) using real-time polymerase chain reaction between days 5 and 35 post-challenge and to relate this to subsequent development of gross MD lesions. Female commercial broiler chickens were vaccinated with HVT or were sham-vaccinated at hatch, then challenged with MDV strain MPF-57 at day 2 post-vaccination and reared in positive-pressure isolators up to 56 days post-challenge, when all survivors were euthanized. All dead and euthanized chickens were examined post mortem for gross MD lesions. Birds were scored for MD lesions and mortality. MDV and HVT genome copy numbers were determined for each PBL sample. There was an increase in HVT load in PBL between days 7 and 37 post-vaccination, with marked increases between days 7 and 16 and again between days 30 and 37. There was a steady increase in MDV load to 35 days post-challenge. The mean MDV copy number (log(10)) was greater in chickens subsequently exhibiting gross MD lesions (5.05 +/- 0.21) than in those that did not (2.88 +/- 0.223), with the largest difference at 14 and 21 days post-challenge (P < 0.001). Quantification of MDV during early infection is therefore a potential tool for monitoring MD in broiler flocks.

Early cell-mediated immune responses to Marek's disease virus in two chicken lines with defined major histocompatibility complex antigens

N2a and P2a chickens, resistant and susceptible to Marek's disease (MD), respectively, were used to examine relationships between major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTL) and natural killer (NK)-like cell activity with resistance to infection with Marek's disease virus (MDV). Ten-day-old chickens were infected with MDV and euthanatized at selected times to evaluate for NK cell and MHC-restricted cytotoxicity. The N2a MDV-infected chickens had an early cell-mediated immune response characterized by a sustained NK-like cytotoxicity that coincided with a measurable MHC-cytotoxicity that was lower than controls. Although MHC-restricted and NK cell cytotoxicity was demonstrated in P2a MDV-infected chickens at 8 dpi, both abruptly decreased and remained low for the remainder of the 20-day experiment. The critical time point that may determine the resistance to MD appears to be within the first 2 weeks post-infection. Improvement of the chicken NK cell activity may be a good candidate for both selection and immunomodulation MD control programs.

Immunosuppressive effects of Marek's disease virus (MDV) and herpesvirus of turkeys (HVT) in broiler chickens and the protective effect of HVT vaccination against MDV challenge

Much of the impact of Marek's disease in broiler chickens is considered to be due to immunosuppression induced by Marek's disease virus (MDV). The present study evaluates the effects of an Australian isolate of pathogenic MDV (strain MPF 57) and a non-pathogenic vaccinal strain of herpesvirus of turkeys (HVT) (strain FC 126) on the immune system of commercial broiler chickens for 35 days following challenge at days 0 or 3 of age. It also investigates the extent of protection provided by HVT vaccine against MDV-induced immunosuppression. Immune system variables, including relative lymphoid organ weight, blood lymphocyte phenotype (CD45+/CD3+, putatively T, and CD45+/LC+, putatively B) and antibody production following vaccination against infectious bronchitis (IB) at hatch, were used to assess the immune status of chickens. Immunosuppression was also assessed by susceptibility to secondary challenge with pathogenic Escherichia coli on day 29 post-MDV challenge. MDV infection reduced the weight of the thymus and bursa of Fabricius, the numbers of circulating T lymphocytes and B lymphocytes, and IB antibody titre. The timing of these effects varied. MDV infection greatly increased susceptibility to E. coli infection. HVT alone caused mild depletion of T and B lymphocytes but no effect on immune organ weight or IB titre. Vaccination with HVT provided good protection against most of the immunosuppressive effects of MDV but not against MDV-induced growth impairment and reduced responsiveness to IB vaccination, suggesting that recent Australian strains of MDV may be evolving in virulence to overcome the protective effects of HVT.

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.