Effector Molecules for IFN-gamma-mediated Prevention of Toxoplasma Proliferation in the Brain

Reactivation of chronic infection with Toxoplasma gondii results in the development of life-threatening toxoplasmic encephalitis (TE) in AIDS and other immunocompromised patients. To improve prevention and management of TE, it is critical to gain a better understanding of the immunopathogenesis of the disease. IFN-ã is required for preventing cerebral tachyzoite proliferation and development of TE. However, the effector mechanisms by which the IFN-ã-mediated immunity prevents tachyzoite growth within brain-resident cells in vivo are poorly understood. In vitro studies showed that guanylate binding protein 1 (Gbp1) and indoleamine-2, 3-dioxygenase (IDO) are able to inhibit intracellular growth of T. gondii tachyzoites in IFN-ã-activated macrophages, astrocytes, fibroblasts, endothelial cells and/or epithelial cells. However, the roles of Gbp1 in prevention of tachyzoite proliferation in vivo have not been determined. It is important to define the protective activity of this molecule in vivo because other studies indicated the differences in the protective mechanisms against intracellular tachyzoite growth between in vivo and in vitro experimental systems. The protective roles of IDO (including both IDO1 and IDO2) in vivo have been controversial depending on the animal models used, suggesting that the effects of IDO possibly differ depending on the organs and organ-specific cell types involved in prevention of tachyzoite growth. In addition, the role of IDO1 in prevention of cerebral tachyzoite growth remains unknown. Our recent study revealed that an inhibition of cerebral tachyzoite growth by the IFN-ã-dependent protective immunity is associated with markedly increases in expression of Gbp1 and IDO1 in the brain. Therefore, it is possible that these two molecules play crucial roles in preventing tachyzoite growth within brain-resident cells and development of TE. It is also possible that the effector roles of these two molecules differ among different brain-resident cell populations. Thus, it is important to determine the roles of Gbp1 and IDO1 in prevention of tachyzoite growth in each of microglia and astrocytes in vivo. Whereas IFN-ã is required for prevention of cerebral tachyzoite growth within neurons in vivo, treatment of neurons with IFN-ã in vitro does not inhibit the parasite growth in these cells. It is likely that neurons become activated by a molecule(s) produced by IFN-ã-activated microglia and/or astrocytes and thereby inhibit tachyzoite growth in vivo. Neurons express CXCR3, the receptor for CXCL9, CXCL10 and CXCL11. Our studies revealed that cerebral IFN-ã production markedly upregulates CXCL9 and CXCL10 expression during reactivation of T. gondii infection. Thus, it is possible that CXCL9 and CXCL10 produced by IFN-ã-activated glial cells stimulate neurons through binding to CXCR3 expressed on their surface and inhibit the parasite growth within these cells. Thus, we will determine the roles of Gpb1 (Aim 1) and IDO1 (Aim 2) in microglia and astrocytes and the role of CXCR3 in neurons (Aim 3) in IFN-ã-mediated protective immunity to prevent tachyzoite proliferation in the brain in resistance against reactivation of cerebral T. gondii infection in vivo using murine models.