CELL-MEDIATED IMMUNE-RESPONSES TO THEILERIA-PARVA (EAST COAST FEVER DURING IMMUNIZATION AND LETHAL INFECTIONS IN CATTLE

Abstract

Leukocytes from cattle with lethal or sub-lethal infections with T. parva were examined for responses in autologous mixed lymphocyte reactions (MLR) and for specific and nonspecific cytotoxicity against cultured cell lines. During lethal infections, lymphoblasts isolated from central lymph from days 8-14 stimulated proliferation in autologous lymphatic lymphocytes and peripheral blood leukocytes (PBL), although cytotoxicity was not generated in such cultures. After day 14, nonspecific cytotoxicity was exhibited by both lymphocyte populations and was maximal in moribund calves. Nonspecific lytic activity was also noted in PBL from recovering calves, but disappeared after parasite elimination. Calves immunized against T. parva by the concurrent administration of stabilate and long-acting tetracycline showed macroschizont forms of the parasite in host lymphocytes from days 15-18 after infection. During this period, lymph node cells from infected calves stimulated MLR-type responses in autologous PBL. PBL from immunized calves lysed directly, autologous infected lymphoblasts in a genetically-restricted fashion from days 14-21, although additional cytotoxicity was not generated in the MLR. Apparently when the parasitosis of T. parva is curtailed, specific cell-mediated responses are mounted against parasite-induced antigens in combination with polymorphic host antigens on the leukocyte membrane. These reactions are probably the major immune responses conferring to recovered cattle, immunity against rechallenge with the homologous isolate of T. parva.