Immunosuppressive agents in intracellular infection: besnoitiosis in hamsters.

When tested for their activity in suppressing the acquisition of immunity during acute Besnoitia infection of hamsters, antilymphocyte serum (ALS), aminopterin, cyclophosphamide, cortisol, and whole-body irradiation were the most active agents and effectively blocked the development of immunity during 4 to 12 days of immunization. Actinomycin D and chlorambucil were moderately active, and nitrogen mustard, 6-mercaptopurine, and vinblastine exhibited slight immunosuppressive activity. Established immunity was especially labile to cortisol and cortisone administration with generalized Besnoitia relapse occurring consistently in all hamsters; this occurred infrequently during cyclophosphamide treatment. Focal relapse was seen in chronically irradiated and ALS-treated hamsters, but the location of the lesions differed. Irradiated hamsters had lesions in the lungs and brain, whereas ALS-treated hamsters showed splenic relapse. Acquisition of immunity was more sensitive to suppression than established immunity and did not necessarily parallel antibody development and vice versa, emphasizing the importance of cellular over humoral factors in immunity to this intracellular parasite.