Cell-mediated cytotoxicity in Theileria annulata infection of cattle with evidence for BoLA restriction.

Recovery of calves from tropical theileriosis was accompanied by the disappearance of macroschizonts from lymph nodes and the appearance of cytotoxic cells in the blood and lymph nodes. Acute, fatal disease was associated with incremental parasitosis and parasitaemia and, in general, an absence of detectable cytotoxic cells in the blood or lymph nodes. After recovery from infection, calves were resistant to challenge. Challenge with sporozoites was followed sometimes by an immediate reappearance or by a later peak, or sometimes by twin peaks of cytotoxic cells but macroschizonts were not detected. Histocompatibility (BoLA) typing indicated that calves produced two sequential populations of cytotoxic cells during recovery from primary infection with Theileria annulata. The expression of lysis by the first appeared to be BoLA restricted. In contrast, both the peaks of lysis manifest after challenge appeared to be BoLA restricted. Results suggest that BoLA restricted cells are established in the immunological memory and are probably analogous to cytotoxic T cells, while non-BoLA restricted cytotoxic cells are natural killer like cells. The results suggest a role for cytotoxic cells in recovery from primary infection, in the inhibition of proliferation of macroschizonts which evade mechanisms of acquired resistance and in the lysis of macroschizont infected cells deriving from challenge sporozoites which have evaded serum-mediated inhibition.