Toxoplasma gondii: effect of sublethal irradiation on host resistance in mice.

The effect of sublethal whole-body irradiation (500 rad) on the resistance of C57BL/6 mice to infection with Toxoplasma gondii was studied. Mice irradiated 1 day before or 4 days after infection via the intraperitoneal or peroral route with cysts of the mildly virulent ME49 strain of T. gondii died sooner than did unirradiated controls. The time to death of irradiated mice was suggestive of impaired acquired immunity. Irradiated mice infected intraperitoneally with T. gondii cysts exhibited reduced levels of Thy-1+CD4-CD8- cells, less natural killer cell activity against YAC-1 targets, and lower levels of IFN-gamma than controls. Numbers of CD4+ and CD8+ cells were also lower in infected, irradiated mice. Irradiated mice immunized with a vaccine strain of T. gondii, and later challenged with a highly virulent strain, were less well protected than unirradiated controls. Irradiation appears to impair early, CD4(+)- and CD(8+)-independent resistance to T. gondii, as well as acquisition of immunity.