Infection with Toxoplasma gondii (Tg) is an important cause of morbidity and mortality in persons infected with HIV or who have other defects in cell-mediated immunity. Treatment of toxoplasmosis in HIV-infected patients is complicated by the need for life-long therapy, the increased tendency to allergic reactions to drugs in this population, and by the overlapping toxicities of some antiretroviral and anti-Tg drugs. Clearly, better therapeutic agents are needed. A better understanding of the immune response to this important intracellular pathogen might someday lead to an immunotherapeutic to serve as an adjunct to anti-Tg chemotherapy. The principal investigator's laboratory has investigated the human cellular immune response to Tg over the past four years, as a considerable body of evidence suggests that protection against toxoplasmosis is mediated primarily by cellular defenses. Unlike in mouse models, where Tg- specific, class I-restricted CD8+ cytotoxic T lymphocytes (CTL) have been shown to be pivotal effectors, human Tg- specific CTL generated in the applicant's laboratory are primarily class II-restricted and CD4+. Although most CTL described to date are CD8+ and HLA class-I restricted, recent evidence has demonstrated that in many infections, CD4+, class-II restricted CTL are important mediators of cellular immunity. These studies are relevant not only from the standpoint of understanding immunity to an important HIV- associated opportunistic pathogen, but also from the perspective of furthering the understanding of the role of CD4+ CTL in immunity. Intracellular tachyzoites form a structure known as the parasitophorous vacuole (PV), which does not fuse with phagosomes, does not acidify normally, and which acts as a molecular sieve, controlling the entry and egress of various molecules. Recent work in the applicant's laboratory suggests that Tg may use its PV to evade host immune responses by decreasing Tg antigen availability for class I processing. Accordingly, it is hypothesized that human anti-Tg CD4+ CTL arise because the Tg PV plays a role in exclusion of Tg antigens from class I processing. It is also likely that endogenous Tg is processed by, and presented in the context of, the class II pathway. The specific aims of the application are: 1) to evaluate the role of the Tg PV in the processing and presentation of Tg antigens; 2) to look for cytoplasmic processing of Tg antigen in the class II pathway; and 3) to examine the biology of CD4+ compared to CD8+ Tg-specific CTL.