B cells do not play a role in vaccine-mediated immunity against Marek's disease

Background: Marek's disease virus (MDV), a highly oncogenic cell-associated lymphotrophic alpha-herpesvirus, is the etiological agent of Marek's disease (MD) in domestic chickens. The antiviral activity of vaccine-induced immunity against MD significantly reduces the level of early cytolytic infection, production of enveloped cell-free virus particles in the feather follicle epithelial cells (FFE), and lymphoma formation. Despite the success and availability of several vaccines that have greatly reduced the economic losses from MD, the molecular mechanism of vaccine-induced immunity is poorly understood. Methods: To provide insight into possible role of B cells in vaccine-mediated protection, we bursectomized birds on day of hatch and vaccinated them eight days later. The birds were challenged 10 days post vaccination with or without receiving adoptive lymphocytes from age-matched control birds prior to inoculation. The study also included vaccinated/challenged and non-vaccinated/challenged groups with intact bursae. Flowcytometric analysis of PBMN cells were conducted twice post bursectomy to confirm B cell depletion and to assess the effect of surgery on T cell population. Immunohistochemical analysis and viral genome copy number assessment in the skin samples at termination was performed to measure the replication rate of MDV in the FFE of the skin tissues of the challenged birds. Results: The non-vaccinated/challenged birds developed typical clinical signs of MD while the bursectomized/vaccinated and challenged groups with or without adoptive lymphocyte transfer, were fully protected with no sign of transient paralysis, immunosuppression or T cell lymphomas. Immunohistochemical analysis and viral genome number evaluation in the skin samples revealed that unlike the vaccinated/challenged birds a significant number of virus particles were produced in the FFE of the non-vaccinated/challenged birds at termination. In the bursectomized, vaccinated/challenged groups, only a few replicating virions were detected in the skin of birds that received adoptive lymphocytes prior to challenge. Conclusions: The study shows that B cells do not play a critical role in MD vaccine-mediated immunity.