Antigen-specific lymphoproliferative responses to tetanus toxoid: A ans for the evaluation of Marek's disease virus-induced immunosuppression in chickens

Antigen-specific lymphoproliferative responses were examined in chickens following immunization with tetanus toxoid (Ttx). The immune competence of chickens was assessed by mitogen assay utilizing phytohemagglutinin (PHA)-stimulation and Ttx-specific antigen proliferation assay (Ttx-APA). Immune spleen cells but not peripheral blood leucocytes demonstrated specific proliferation following stimulation in vitro in a Ttx-APA. In this study, we examined firstly the effects of Marek's disease (MD) associated immunosuppression on specific immune responses. The humoral and cell-mediated immune responses were monitored by enzyme-linked immunosorbent assay (ELISA) and Ttx-APA, respectively. Secondly, we examined if vaccination against MD using a conventional herpesvirus of turkeys (HVT) vaccine and two recombinant HVT (rHVT) vaccines would affect the development of Ttx-specific immune responses. The rHVT vaccines used in this study included two constructs: one expressing both Newcastle disease virus (NDV) and MD virus (MDV) genes (HVT/NDV/MDV), and another expressing only MDV genes (HVT/MDV). The mitogenic responses of spleen cells of the vaccinated chickens were inconsistent allowing no definitive conclusions about vaccinal immunosuppression. The results of the Ttx-APA indicated that Ttx-specific lymphoproliferative responses provide a meaningful measure of immunosuppression. The MDV-induced immunosuppression resulted in the inhibition of Ttx-specific lymphoproliferation in vitro. Both HVT and rHVT vaccines were not immunosuppressive as indicated by the development of normal Ttx-specific lymphoproliferative responses in chickens. These results indicate that vaccination against MD results not only in the prevention of tumor formation but also protection from possible virus-induced immunosuppression.