Cellular immune responses in cattle experimentally infected with Neospora caninum

Neospora caninum has recently been identified as an important cause of infectious abortion in cattle. The parasite is closely related to Toxoplasma gondii, but the two species are antigenically distinct. To examine cell proliferative responses and the induction of IFN-gamma in experimentally infected cattle, four 2-4 months old calves were subcutaneously inoculated with N. caninum tachyzoites. Peripheral blood mononuclear cells were collected regularly and stimulated in vitro with a crude lysate of N. caninum or T. gondii tachyzoites. Significant proliferative responses to N. caninum antigen were recorded in all calves from days 4-6 postinoculation. This response was accompanied by production of high levels of IFN-gamma. Although the calves remained seronegative to T. gondii, while seroconverting to N. caninum, stimulation with T. gondii lysate resulted in cell proliferation of a similar magnitude as that obtained using the N. caninum lysate. However, the T. gondii lysate appeared less effective than the N. caninum lysate to stimulate IFN-gamma production. Cells taken from uninfected control animals did not show any significant proliferation to either N. caninum or T. gondii antigen and no IFN-gamma was produced. These results suggest that the two parasites may possess cross-reacting T-cell epitopes, but that the T cells specific for N. caninum may have a different functional capacity. This highlights the need to investigate the antigen specificity and cytokine profile of T cells from infected animals to help understand their role in immunity to N. caninum.