Subcutaneous challenge of cattle with Mycobacterium avium subspecies paratuberculosis: an experimental system to study CD4+ T cell and gamma-delta T cell responses to infection

The cellular immune response in cattle to Mycobacterium avium subsp. paratuberculosis (M. paratuberculosis ) infection is poorly understood. To examine these early cellular events, we developed an experimental system in the cow to evaluate lymphocyte function following subcutaneous inoculation. Sixty days after infection, there was increased antigen-specific proliferation of lymphocytes from lymph nodes draining the site of infection in M. paratuberculosis-inoculated calves compared to those in saline-inoculated calves, but there were no differences in interferon (IFN)-gamma expression by lymph node-derived lymphocytes between these two groups, suggesting a lack of local T helper type 1 (Th1) polarization. All M. paratuberculosis-inoculated calves developed a strong delayed-type hypersensitivity response, consistent with development of Th1-polarized systemic effector/memory cells. Compared to antigen-stimulated draining lymph node cells, there was increased lymphocyte proliferation and IFN-gamma production by antigen-stimulated lymphocytes derived from the spleen in subcutaneously-infected calves. To test the hypothesis that the immune response at the site of infection is relatively impaired in comparison to the central immune response, we vaccinated calves with the commercial paratuberculosis vaccine, which induces a strong Th1 response, and then challenged the calves with live M. paratuberculosis . Whereas there was marked antigen-specific CD4+ T cell proliferation and IFN-gamma production by lymphocytes from the lymph node draining the vaccination site, there was no CD4+ T cell proliferation or IFN-gamma production from lymphocytes from the lymph node draining the challenge site in the same calf. The data suggest that live M. paratuberculosis impairs antigen-presenting cell (APC) function at the site of infection, such that there is defective signaling by the APC or reduced trafficking of effector CD4+ T cells into the lesion site. Draining lymph node cells were evaluated for their ability to activate M. paratuberculosis-infected macrophages. gammadelta T cells exposed to infected macrophages did not produce significant IFN-gamma, and nitric oxide production did not increase in cultures of infected macrophages containing gammadelta T cells; as such, gammadelta T cells do not appear to classically activate infected macrophages. In addition, antigen-stimulated CD4+ T cells producing significant amounts of IFN-gamma from infected calves also fail to induce macrophage activation