PROJECT SUMMARY Infections of the brain cause significant morbidity and mortality. During infections, the brain can swell in response to direct damage by a pathogen and as a consequence of the immune response. The brain has a limited ability to expand within the skull, leading to increases in intracranial pressure that can cause seizures, coma, or even death due to brain herniation. In many tissues, lymphatic vessels remove excess fluid and immune cells to control swelling and resolve inflammation. The presence of lymphatic vessels in the central nervous system was described in 2015 and suggests that fluid and immune cells may exit the CNS via this meningeal lymphatic network. In our studies examining the immune response to Toxoplasma gondii, a parasite that causes chronic encephalitis in C57BL/6 mice, we find increased intracranial pressure and edema in the brain coincident with a strong immune response to the parasite. Subsequently, we also observe that the meningeal lymphatics begin to expand in length and complexity, consistent with lymphangiogenesis. The robust meningeal lymphangiogenesis we observe is associated with a resolution of elevated intracranial pressure. We hypothesize that the expansion of the lymphatic vessels allows for more fluid to leave the meninges which ameliorates elevated intracranial pressure and for immune cells to exit the CNS reducing meningitis. We will test these hypotheses in two aims: Aim 1. How does the blockade or promotion of meningeal lymphangiogenesis affect fluid homeostasis in the infected brain? Aim 2. How is inflammation within the meninges affected by the expansion of the local lymphatic vasculature? We will utilize two key tools to prevent and promote meningeal lymphangiogenesis. We will prevent lymphatic growth by targeting the growth factors VEGF-C and VEGF-D using a soluble decoy receptor (sVEGRF3) delivered to the meninges using an AAV. Conversely, we will treat mice with and AAV that expresses VEGF-C to expand the CNS lymphatics to understand if enhanced lymphangiogenesis has a therapeutic effect. We will measure fluid dynamics using tracer studies and measure any impact on tissue edema or intracranial pressure. We will also examine how the immune response in the meninges and parasite burden are affected by local the promotion or block of CNS lymphangiogenesis. Together these studies will examine how the newly discovered meningeal lymphatic vessels function during CNS infection to maintain fluid homeostasis and control inflammation. The induction of lymphangiogenesis in the meninges may have immense therapeutic potential as there are limited interventions for chronic elevations in intracranial pressure due to persistent CNS infections.