Microglial IFN-y production is a key defense system to coordinate both innate and T cell-mediated protective immunity in the brain for preventing reactivation of cerebral Toxoplasma infection

Toxoplasmic encephalitis (TE) caused by reactivation of chronic infection with Toxoplasma gondii is a life- threatening opportunistic infectious disease in AIDS patients. To improve prevention and treatment of TE, it is critical to define the mechanisms by which the immune system prevents cerebral T. gondii growth and TE. We recently discovered that IFN-γ production by brain-resident cells is crucial not only for activating the innate immunity to limit the parasite growth during the early stage of reactivation of the infection but also for facilitating T cell-mediated immunity in the brain to prevent TE. We also identified that microglia (one of brain-resident cell populations) produce IFN-γ in response to cerebral T. gondii growth. These evidences led us to hypothesize that IFN-γ production by microglia is a key first-line defense system of the brain to coordinate both the innate and T cell-mediated immunity to prevent TE. Aim 1 will employ the mice deficient in IFN-γ only in microglia among brain-resident cells and identify an importance of microglial IFN-γ for recruiting monocytes, NK cells, and ILC1 cells and activating their IFN-γ production to suppress the pathogen growth. Although the innate immunity is important for limiting cerebral tachyzoite growth during the early stage of reactivation of the infection, IFN-γ production by CD8+ T cells is ultimately required to prevent TE. CD8+ T cells recognize their target antigens presented by MHC class I molecules. Thus, Aim 2 is to reveal crucial coordinating roles of microglial IFN-γ in recruiting CD8+ T cells to areas of cerebral tachyzoite growth and augmenting the MHC class I antigen presentation in those areas for activating those T cells to prevent TE. For understanding the mechanisms of the protective immunity to prevent TE, it is also crucial to identify the T. gondii antigens that activate the protective CD8+ T cells. There are three most common MHC class I molecules, HLA-A2, -A11, and -B7, in humans. Thus, Aim 3 is to identify the T. gondii antigens that effectively activate IFN-γ-producing CD8+ T cells capable of preventing TE specifically for each of HLA-A2, -A11, and -B7 using transgenic mice expressing these human HLA molecules. The proposed studies all together will shed light on the crucial first- line defense roles of microglial IFN-γ production to coordinate both innate and CD8+ T cell-mediated immunity in the brain and markedly improve our understanding on the mechanisms of the protective immunity to prevent TE. Notably, we identified that once the protective CD8+ T cells are efficiently primed with their target antigens in the presence of CD4+ T cells, those primed CD8+ T cells can prevent TE without further depending on CD4+ T cells. Thus, priming the protective CD8+ T cells by a vaccination with the T. gondii antigens identified in Aim 3 in individuals co-infected with HIV and T. gondii specifically for the HLA molecules expressed in these individuals before their CD4+ T cell counts decrease could prevent their development of TE even when their CD4 counts become low at later time points. Thus, the proposed studies also have a potential to compose the vital basis for an immunological intervention for preventing TE in individuals co-infected with HIV and T. gondii.