Toxoplasmic encephalitis has become a major opportunistic infection in patients with AIDS. The decrease in immunocompetence in AIDS appears to result in reactivation of latent infection in the brain, either by allowing rupture of Toxoplasma gondii cysts in the brain or by reinfection from tachyzoites in the circulation. Specific factors involved in the pathogenesis of toxoplasmic encephalitis remain poorly understood. Mouse model systems have been used to gain information about the effects of alterations in levels of circulating cytokines on pathology due to T. gondii. The role of cytokines, which are secreted by immunocompetent cells such as astrocytes and microglial cells in the brain, has not been examined. These cells and their secreted products have been demonstrated to play a major role in neurotropic viral infections and in multiple sclerosis. The working hypothesis that forms the basis of this proposal is that production of cytokines in the brain during T. gondii infection plays an important role in pathogenesis and/or protection and that differences in cytokine production in different mouse strains contribute to differences in resistance to toxoplasmic encephalitis. We have demonstrated a correlation between levels of expression in the brain of the cytokine, tumor necrosis factor-alpha (TNFalpha), and susceptibility or resistance to toxoplasmic encephalitis. We will measure production of IFN-gamma, IL-1, TGF-beta, and IL-6 in these mice, so that patterns of cytokine production can be established and correlated with disease pathology or resistance to pathology. To determine which cell types produce specific cytokines and to begin to understand the immunological interactions that take place in brains from resistant versus susceptible mice, we will identify brain-infiltrating cells using cell surface markers and cell transfer experiments into SCID mice. Expression of class I and class II MHC molecules will be monitored on astrocytes as well as on infiltrating cells. Using B10.A recombinant congenic mice, we have identified restriction fragment length polymorphisms (RFLPs) in the TNF-alpha gene that show 100% linkage to resistance or susceptibility to encephalitis. The polymorphic sites are located within the regulatory regions of the gene, thus differences in regulation of TNF-alpha gene expression may contribute to susceptibility. We have established an in vitro astrocyte culture system in which to examine differences in regulation of cytokine production by these cells as well as interactions between T. gondii organisms, T cells, and astrocytes. This first step toward a detailed dissection of immunological events that occur in the brains of T. gondii-infected mice should provide valuable information about immunoreactivity in brains of susceptible and resistant mice during infection with T. gondii.
