Characterization of immunopathological changes in the feather pulp of CVI988-vaccinated pullets challenged with a very virulent plus Marek's disease virus strain

High load of oncogenic Marek's disease virus (MDV) DNA in the feather pulp (FP) as early as 21 days of age is a powerful criterion to predict the outcome of Marek's disease (MD) in apparently healthy chickens. The objective of this study was to elucidate the immunopathological changes in the FP of 21-day-old chickens that had been vaccinated with CVI988 vaccine (healthy), vaccinated and challenged with a very virulent plus (vv+) MDV strain 648A (well protected), or were unvaccinated and challenged with 648A strain (not protected) when compared to uninfected na & iuml;ve chickens. Oncogenic MDV DNA load, histopathological and immunohistochemical evaluation of the lesions, and immunophenotypic characterization of the infiltrates by flow cytometry were examined. Our results demonstrate that 648A-infected unvaccinated chickens had a significant increase in the percentage of CD3+ T cells, mainly CD4+MHC-II+ cells and CD8+MHC-II+ cells, when compared to all other groups. They also had a significantly decreased number of CD8 beta+ T cells compared to all other groups. Infection with 648A reduced the percentage of macrophages, not only in the unvaccinated group but also in the CVI988/648A group. In addition, groups that were vaccinated with CVI988, regardless of the challenge status, had higher levels of CD8 beta+ T cells, suggesting that the vaccine has an enhancing effect on the CTL cells. Our results showed that the load of oncogenic MDV is highly correlated with the infiltration of CD4+MHC-II+ cells and provide further confirmation that FP is indeed an appropriate sample for the early diagnosis of MD.