Abstract
Mice vaccinated with X-ray irradiated tachyzoites of Toxoplasma gondii have developed complete resistance to a lethal challenge with a highly virulent Toxoplasma strain (RH). To determine the roles for T cell subpopulations responsible for this protection, CD4+ or CD8+ T cells were depleted in primed BALB/c mice or were transferred into naive mice, which were then subjected to challenge infection. We found that primed mice depleted of CD4+ T cells were protected while mice depleted of CD8+ T cells became susceptible. As expected, CD8+ T cells from primed mice could transfer protection to naive mice, even in the absence of host CD4+ T cells. Contrary to our expectation, CD4+ T cells were also competent in conferring protection so long as host CD8+ T cells existed. These results also suggest that primed CD8+ T cells may work to help unprimed CD8+ T cells to differentiate into effector cells upon challenge infection. X-ray-irradiated Toxoplasma gondii may serve as a useful tool for elucidating the mechanisms of the protective immunity for controlling toxoplasmosis.