Generation of autologous mixed leucocyte reactions during the course of infection with Theileria parva (East Coast Fever) in cattle.

During the course of East Coast Fever (ECF) in cattle, an examination was made of the evolution of autologous mixed leucocyte reactions (MLR) between normal responder lymphocytes collected prior to infection and stimulators cells prepared from infected lymphoid organs at intervals after challenge. Proliferative responses were induced by spleen cells, peripheral blood leucocytes (PBL) and cells (LNC) from both regional and distant lymph nodes. The proliferation by normal PBL in response to LNC occurred earlier in the course of infection and was of greater magnitude than the MLRs induced by spleen cells and PBL. The ability of LNC to stimulate an autologous MLR declined after 14 days post-challenge, whereas both spleen cells and PBL were stimulatory for normal PBL until the death of the host at 18--20 days after challenge. The results imply that the earliest stimulation to the lymphocyes involved in cell-mediated immunity is afforded in the absence of detectable macroschizonts (less than 0 . 1 %). The inductive signal(s) probably emanates from transformational or parasite-coded antigens on blast cells which first appear around 8--10 days after infection. The continual generation of infected lymphoblasts may initiate proliferation of additional leucocytes involved in cell-mediated immunity at later intervals during the course of infection. The capacity of the host to mount humoral and cell-mediated responses to the infected cells is late in relation to the pathogenesis and outcome of the infection.