4. Discussion
The S1 protein is a critical immunogenic protein for inducing the neutralizing antibodies against IBV [3], [22] and [23]. Although other viral proteins such as nucleocapsid protein may affect chicken's recovery from viral infection, the S1 protein still represents the best antigen candidate for development of novel IBV vaccines. In this study we expressed the S1 gene of the IBV LX4 strain using a FPV vector and investigated co-expressed ChIFNγ’s effect on the recombinant viruses. The results in this study indicated that although ChIFNγ suppressed the production of specific anti-IBV antibody, clinical protection of rFPV-IBVS1-ChIFNγ in SPF chickens was not compromised but in fact enhanced, demonstrating that the absolute levels of antibody cannot be used alone to evaluate the immunoprotective effects of a vaccine, and that cellular immunity may play an important role in protection. In this study the number of the CD8+ T lymphocytes in peripheral blood of chickens in rFPV-IBVS1-ChIFNγ group was significantly higher than that of chickens in rFPV-IBVS1 group (data not shown), which may explain the better clinical protection of rFPV-IBVS1-ChIFNγ as CD8+ T cells are cytotoxic T lymphocytes that play a major role in clearing or controlling viral infections [24]. This is consistent with those found in rats by Leong et al. [17] where expression of IFNγ markedly suppressed antibody responses without altering the cell-mediated immunity. However, Rautenschlein et al. [23] reported that antibodies to Newcastle disease virus (NDV) induced by the rFPV-NDV-ChIFNγ were detected earlier than in other vaccinated groups, and the levels of NDV HI antibodies were higher in the rFPV-NDV-ChIFNγ vaccinated group than in other vaccinated groups on day 11 post-vaccination.