Neospora caninum-infected cattle develop parasite-specific CD4⁺ cytotoxic T lymphocytes

Cattle infected with Neospora caninum readily experience transplacental parasite transmission, presumably after maternal parasitemia, leading to abortion or birth of congenitally infected calves. Cytotoxic T lymphocytes (CTL) are important mediators of protective immunity against Toxoplasma gondii, an intracellular apicomplexan protozoan closely related to N. caninum. In this study, N. caninum-specific CTL expanded from peripheral blood mononuclear cells of two major histocompatibility complex-mismatched, experimentally infected cattle were identified by using a ⁵¹Cr release cytotoxicity assay. Enrichment and blocking of CD4⁺- and CD8⁺-T-lymphocyte effector subsets indicated that CD4⁺ CTL killed N. caninum-infected, autologous target cells and that killing was mediated through a perforin/granzyme pathway. Detection and characterization of CTL responses to N. caninum in the natural, outbred, bovine host will facilitate identification of immunogens and design of immunization strategies to induce parasite-specific CTL against transplacental N. caninum transmission in cattle.