Cyclosporin A, but not bursectomy, abolishes the protective immunity of chickens against Leucocytozoon caulleryi.

Abstract Chickens that survive primary infection with Leucocytozoon caulleryi show strong resistance to reinfection. Using bursectomized (BX) or cyclosporin A (CsA)-treated chickens, we performed experiments to determine which type of immunity, humoral or cellular immunity, plays an important role in the resistance of chickens against reinfection with L. caulleryi . BX chickens were inoculated with 2, 20 or 200 sporozoites of L. caulleryi intravenously at 3 weeks of age. Some BX chickens which were inoculated with 2 or 20 sporozoites survived the primary infection. These birds had no parasites in their peripheral blood after challenge infection with 5 × 10 3 sporozoites, even though they had no antibody to L. caulleryi . In contrast, CsA-treated chickens had parasitemia, serum-soluble antigen and antibodies, as did untreated chickens during primary infection. After secondary infection, CsA-treated chickens had parasitemia and serum-soluble antigen, even though they had specific antibodies to L. caulleryi whereas untreated chickens showed no parasitemia. The number of CD4 + , CD8 + , T cell receptor (TCR) α s-bearing cells and TCRγ δ-bearing cells decreased markedly in the peripheral blood of CsA-treated chickens compared to those of untreated chickens. Lymphocyte proliferation in response to concanavalin A, and T cell growth factor production, were also markedly suppressed in CsA-treated chickens. These results suggest that cell-mediated immunity plays an important role in the development of resistance of chickens against reinfection with L. caulleryi .