Comparative Studies On The Vaccination And Revaccination Of Marek's Disease

Marek's disease (MD) is a lymphoproliferative disease of chickens caused by the highly cell-associated alphaherpesvirus, serotype 1 Marek's disease virus (MDV1). Since R L Witter and Okazaki developed the HVT vaccine in the 1970s, vaccination has been a routine to protect birds against MD. Although the vaccines of CVI988n SB1+HVT Z4+HVT afforded good protection against MD, vaccine breaks continue to occur along with variation in poultry industry and the evolution of the MDV. In this case, a novel vaccination method, revaccination, has been exploited.In the protective efficacy experiment, groups of Langshan chickens were inoculated with the following vaccines: CVI988/Rispens and HVT .The results showed that when the challenge strain was GA, the protection index (PI) for single vaccination and revaccination were 68.7-87.5 and 87.5-93.7; when the challenge strain was 584A the PIs were 44.4-77.8 and 88.9-100. PIs of the revaccinated groups were significantly higher than those of the single vaccinated groups (P<0.05). The result suggested that the revaccination could improve the protective efficacy.To explore the mechanism of increased antitumor by revaccination of MDV, we used the FCM to examine the antigen of the MDV which expressed on the PBMCs' membrane. Langshan chickens were vaccinated with HVT strain FC126 or CVI988/Rispens at day 1 or day 7 or day 1 and day 7. The results indicated that the numbers of MDV productive infection's PBMCs reached a peak at day 7 after Langshan chickens were inoculated at either 1 or 7 days old, and two peaks appeared at day 7 and day 14 in the revaccinated groups.The result indicated that the vaccine virus of revaccination also from the productive infection to multiplication. After a few days ,theproductive infection became the latency infection. Because the memory of the vaccine existed, the revaccination induced humoral and cell-mediated immunity to be stronger. These results suggested that productive replication of the vaccine virus given by revaccination could play an important role in anti-tumor immunity to MD.Experiments were carried out to evaluate cell-mediated immune responses induced by MD vaccines. Langshan chickens were immunized with CVI988 and HVT at either day 1 or day 7 or day 1 and day7. All vaccinated groups were held in isolators. Changes in T-cell subpopulations were determined by flow cytometry using monoclonal antibodies specific for chickens' CD3~+, CD4~+ and CD8~+ T cells. Subpopulations of CD3~+ and CD8~+ T lymphocytes from revaccinated groups were larger than those of isolated from the singly vaccinated groups. These results indicated that revaccination might stimulate stronger cell-mediated immune responses to MDV than single vaccination.