PROJECT SUMMARY Infections can cause irreversible damage to the brain. In this proposal, we aim to understand how the immune system is mobilized to specific regions of the brain to eliminate pathogens. The parasite Toxoplasma gondii causes a chronic brain infection where the parasite persists in an intracellular cyst form. When cysts reactivate, vigorous immune responses occur in these regions that limit parasite replication. Our preliminary data and a recent publication highlight the necessity of recruiting inflammatory monocytes to the brain to control infection. The inflammatory networks that recruit monocyte-derived macrophages to the brain and specifically to regions of cyst reactivate have not been identified and are the focus of this proposal. In preliminary studies, we find that two host alarmins (molecules that are released from damaged and dying cells), IL-33 and IL-1α, are necessary to bring myeloid cells into the brain and control parasite burden. In this proposal, we will address the central hypothesis that IL-33 and IL-1α act through non-redundant inflammatory pathways to drive the recruitment of monocytes to the brain to control parasite replication. Specifically, in Aim 1 we will examine the key IL-33R- expressing cells in the CNS, which we hypothesize make the chemokine, CCL2, that drives monocyte chemotaxis. In Aim 2, we propose to investigate the key cells that release and respond to IL-1α during infection. We hypothesize that IL-1α acts on the endothelium to induce adhesion molecule expression necessary for monocyte trafficking into the brain. In Aim 3 we will examine how the inability to sense both alarmins affects the control of infection. Overall, these studies will define how tissue damage drives protective immune responses in the brain. The inflammatory pathways triggered by alarmins are of great interest in the context of multiple neuroinflammatory conditions, including Alzheimer’s disease, where SNPs in the receptor required for both IL- 1α and IL-33 signaling, IL-1RAP have been linked to disease progression. The results from these studies may lead to the development of therapeutics that promote or prevent focal immune responses in the brain.