Turkey herpesvirus infection in chickens: induction of lymphoproliferative lesions and characterization of vaccinal immunity against Marek's disease.

Chickens vaccinated at hatching with high doses of turkey herpesvirus (HVT) developed viremia that peaked in titer around the 12th day and gradually declined. HVT infection also induced mild microscopic lymphoproliferative lesions in the nerves and gonads. These lesions were most prominent around the 12th day and then regressed. The fact that such lesions were also induced by HVT in cyclophosphamide-treated chicks suggests that they were T-cell-dependent. Some of the cells in early HVT lesions appeared to have morphologic properties of neoplastic cells. HVT viremia and lesions were both dose-dependent and were less in chickens with maternal antibodies against Marek's disease virus (MDV). Sequential studies on chickens vaccinated with HVT and challenged with MDV showed that chickens were protected against the earliest detectable MD viremia and lymphoproliferative lesion response attributed to MD. Also, the transient necrobiotic lesions associated with productive infection of thymic lymphocytes by MDV were totally absent in vaccinated chickens. These data provide further insight on the mechanisms by which HVT protects against MD lymphoma induction. A limited oncogenic (transforming) potential of HVT as suggested by our data would provide the basis to assume that at least one component of HVT-induced immunity may be directed against tumor-specific antigens. On the other hand, our observations that HVT protects against productive MDV infection in the thymus and against cell-associated viremia are evidence for an anti-viral immune response. These hypotheses are not mutually exclusive.