The apicomplexan blood parasite Theileria parva is the etiologic agent of East Coast fever (ECF), a highly lethal condition in cattle across Africa. The progress of vaccine design has been hampered by limited insight into the nature of protective immunity that arises after either natural infection or immunization using the infection and treatment method (ITM). This challenge mainly results from the absence of suitable techniques for assessing the memory T-cell populations after exposure. To resolve this issue, immune assays originally optimized for other intracellular organisms were modified to analyze T. parva, enabling detailed study of effector T-cell characteristics in immune animals and supporting vaccine improvement. In this investigation, peripheral blood mononuclear cells (PBMCs) from cattle immunized by ITM were stimulated with irradiated, autologous, T. parva-infected cells, provoking a proliferative recall reaction consisting of CD45R0+/CCR7− CD4+ and CD8+ subsets. When these cultures were later incubated with infected targets, cytotoxic T lymphocytes (CTLs) capable of destroying infected cells were detected. Removal of either CD4+ or CD8+ T cells showed that cytotoxic function was mainly due to the CD8+ fraction. Notably, PBMCs from all vaccinated steers exhibited strong proliferative responses in both T-cell types. This represents the first key outcome derived from the newly applied assay.
