John M. Pollock

It is accepted that cell-mediated immune responses predominate in mycobacterial infections. Many studies have shown that CD4⁺T cells produce Th1 cytokines, such as gamma interferon (IFN-γ), in response to mycobacterial antigens and that the cytolytic activity of CD8⁺cells toward infected macrophages is important. However, the extent and manner in which γδ T cells participate in this response remain unclear. In ruminants, γδ T cells comprise a major proportion of the peripheral blood mononuclear cell population. We have previously shown that WC1⁺γδ T cells are involved early inMycobacterium bovisinfection of cattle, but their specific functions are not well understood. Here we describe an in vivo model of bovine tuberculosis in which the WC1⁺γδ T cells were depleted from the peripheral circulation and respiratory tract, by infusion of WC1⁺-specific monoclonal antibody, prior to infection. While no effects on disease pathology were observed in this experiment, results indicate that WC1⁺γδ T cells, which become significantly activated (CD25⁺) in the circulation of control calves from 21 days postinfection, may play a role in modulating the developing immune response toM. bovis. WC1⁺-depleted animals exhibited decreased antigen-specific lymphocyte proliferative response, an increased antigen-specific production of interleukin-4, and a lack of specific immunoglobulin G2 antibody. This suggests that WC1⁺γδ TCR⁺cells contribute, either directly or indirectly, toward the Th1 bias of the immune response in bovine tuberculosis—a hypothesis supported by the decreased innate production of IFN-γ, which was observed in WC1⁺-depleted calves.