Abstract Malaria afflicted 247 million people in 2021, of which over 600,000 died. In the most severe cases, patients can develop organ-specific pathologies related to vascular leak, including cerebral malaria and malaria-associated acute respiratory distress syndrome (MA-ARDS). The mechanisms of pulmonary vascular leak are poorly understood, and the long-term goal of this work is to define the molecular mechanisms underlying pulmonary vascular leak in malaria to develop rationale therapeutics to treat this condition. It has been previously shown that CD8 T cells and platelets are activated by Plasmodium in humans and are required for vascular breakdown in mouse models, but specific mechanisms underlying how these cells incur damage is still unclear. The central hypothesis of this proposal is that platelets contribute both directly and indirectly to MA-ARDS by promoting CD8 cytotoxicity leading to barrier dysfunction. The rationale of this work is that identifying how platelets augment the pathogenicity of CD8 T cells will provide new avenues for chemotherapeutic targeting of MA-ARDS. Published work from the Lamb lab and preliminary data shown here demonstrate that mice deficient in platelet a-granules (Nbeal2-/-) survive Plasmodium berghei infection and fail to develop vascular leak in the lungs and brain. In intestinal colitis models, published work by the Petrey lab has demonstrated that platelet granule-derived hyaluronidase 2 (HYAL2) is responsible for driving degradation of hyaluronan (HA) in the endothelial glycocalyx (eGC), which serves as a key modulator of barrier integrity. Our preliminary data show that HA fragments in the plasma, an indicator of glycocalyx breakdown, are increased in infected wildtype mice but not in platelet a- granule-deficient mice. The literature shows that HA fragments can promote cellular activation and proliferation, and our preliminary data demonstrate that the Nbeal2-/-, which have low plasma HA, have low CD8 T cell accumulation in the lung during Plasmodium infection. The published literature and our preliminary data have led us to form the working hypothesis that platelet cleaves the endothelial glycocalyx, promoting CD8 T cell activation and recruitment via circulating HA fragments and exposing endothelial cells to CD8 effector molecules, resulting in barrier damage. We will test this hypothesis with the following specific aims: Aim 1: Demonstrate that platelet a-granule HYAL2 is required for eGC breakdown in MA-ARDS. Aim 2: Test the hypothesis that plasma HA promotes pathogenic CD8 trafficking via CD44 binding. We expect that the work proposed in Aim 1 will determine the mechanism by which platelet hyaluronidase causes pulmonary vascular leak during MA-ARDS, which has not previously been described in malaria, and demonstrate how the resulting plasma HA modulates CD8 T cell activation and trafficking. We anticipate that the findings from this work will provide a much-needed deeper understanding of the m...