Caspase-8 mediated control of CNS infection

PROJECT SUMMARY The brain is a unique environment in the context of infection. Many brain cells are long-lived and irreplaceable. To combat central nervous system (CNS) infection, the immune system must control the pathogen while preserving such cells. Thus, infection with intracellular pathogens, like the parasite Toxoplasma gondii that

infects nearly one-third of the world population, offers a complex challenge. Programmed cell death is a common mechanism for limiting intracellular pathogen replication. The cell death pathways that are operational in the

brain to limit CNS infection are just beginning to be defined. T. gondii is primarily controlled by T cells that produce the cytokine IFN-g, but parasites persist within neurons, raising questions regarding the presence of cell-intrinsic defense mechanisms of neurons and other CNS-resident cells. We find that caspase-8, an inducer of

programmed cell death, is essential for the control of T. gondii in the brain. We find high parasite burdens in the brains of mice lacking caspase-8 despite the presence of a strong immune response, suggesting that caspase- 8 functions in addition to defined immune mediators to protect the brain. To determine which cell types harbor

parasite in the absence caspase-8, we infected cre-reporter mice with cre-secreting parasites to label all cells that encounter parasite. As expected, we observe infected neurons in wildtype mice, but in mice lacking caspase- 8, we observed far more infected cells, including: neurons, astrocytes, and CD8+ T cells. We hypothesize that

caspase-8 in each of these cell types is required to restrict T. gondii, which we will test in two aims. In Specific Aim 1, we will test if CNS-resident neurons and astrocytes need caspase-8 to control T. gondii in the brain. To test this question, we have generated mice with floxed alleles of Casp8 on a Ripk3 knockout (to prevent

necroptosis) and cre-reporter background. Using AAVs, we will drive the expression of cre in specific cell types. We will measure parasite burden, visualize infected cells using confocal microscopy, and assess tissue pathology and neuronal health. Together, the experiments in this aim will address whether cell-intrinsic or

extrinsic caspase-8 activity is required to control T. gondii in CNS-resident cells. In Specific Aim 2, we will test the role of caspase-8 in CD8+ T cells. CD8+ T cells are the predominant immune cell type that encounters T. gondii in in the brains of Casp8Ripk3 knockout mice. Previous reports have indicated that CD8+ T cells are

infected by parasites in wildtype mice, raising questions about the fate of the parasites in these cells. We hypothesize that caspase-8 mediated cell death is an essential restriction mechanism in CD8+ T cells. To test this, we will infect mice that lack Casp8 specifically in T cells using a CD8a-cre and examine how cell death limits

parasite spread and replication. Then, we will culture CD8+ T cells to understand how caspase-8 contributes to parasite control using genetic knockouts and activators and inhibitors of caspase-8. Lastly, we will use in vivo experiments to test the importance of activators of caspase-8, TNF-a and FasL, in T cells during infection.

Together, this proposal will explore how specific cell types utilize caspase-8 to control brain infection.