3. Effects on lymphoid tissue The T lymphocyte progenitor cells in the thymus are particularly susceptible to CAV infection and appear to be a major target for the virus. B cells and their precursors are not susceptible to infection and no substantial depletion of B cell numbers comparable to the dramatic depletion in numbers of T cells is observed following infection. Any effects on the bursa of Fabricius appear to result from an indirect effect of CAV infection mediated by disruption of cytokine networks resulting from destruction of other effector populations, or enhanced pathogenicity of other agents which adversely affect the bursa by direct means. The fact that no substantial de-pletion of B cells, comparable to the destruction of T cell populations, is observed following infection indicates that the common lymphoid progenitor cell in the bone marrow, which providesprogenitor cells for seeding of the thymus and bursa, is probably not susceptible to CAV.
In the thymus, the cortical lymphocytes were among the first cells to be destroyed,whereas non-lymphoid leukocytes and stromal cells were una€ected. Virus antigen has been demonstrated in large numbers of thymocytes inthe cortical region of the thymus, whereas infected cells in the medulla were less frequent. This suggested selective infection of immature T cells which constitute the major thymocyte population in the cortex. Double labelling studies of the T cell populations which contained CAV antigen also suggested selective infection of precursor T cells [11]. These investigations showed that only a small proportion of infected thymocytes stained positive with monoclonal antibodies specific for the CD3/TCR complex. Since the presence of the CD3/TCR complex on the T cell surface is considered an early marker for T cell maturation, this confirmed CAV infection of precursor T cells. Also, a higher percentage of infected cells were CD8+ than CD4+ in these studies,possibly suggesting that CD8+ cells were more susceptible to the virus [11]. In addition, studies
in vivo have demonstrated a greater decrease in CD8+ cells compared to CD4+ following CAVinfection, while in vitro studies of the growth of the virus in transformed T cell lines suggested that cell lines which were CD8+ contained a higher frequency of infected cells than lines which were CD8. Furthermore, it was observed that the vast majority of infected T cells in the spleen also appeared to be CD8+ (68%), and this may lend support to the view that the virus preferentially infects T cells which are CD8+. However, other studies have indicated no appreciable difierence in the reduction of CD8+ cell numbers compared to CD4+ following infection, and therefore no firm conclusion regarding the apparent preference of the virus for one type of effector T cell can be drawn at this time. How the infection of T cells in the spleen is initiated is not known. The cells may become
infected as precursor T cells in the thymus, subsequently moving to the spleen when they migrate as mature cells, or infection of mature cells may be initiated in the spleen. Experimental evidence suggests simultaneous and independent CAV infection in thymus and spleen, since virusinfected cells appear in spleen and thymus at the same time following infection at 1 day of age.Also, it has been shown that mature T cells in spleens from uninfected chickens were susceptible to CAV infection following exposure to the virusin vitro.