Abstract In many inflammatory conditions such as inflammatory bowel disease (IBD) platelet counts rise, resulting in thrombocytosis, but what initiates this platelet up-regulation is not well understood. Thrombocytosis in IBD is a significant health concern, as it plays an active role in exacerbating disease morbidity. Increased platelet count and reactivity in patients with IBD correlate with disease severity, and platelets actively contribute to the mucosal inflammation and tissue-destructive inflammatory processes that are a hallmark of the disease. In addition, due to their elevated platelet counts, patients with IBD have an increased risk of venous thrombosis. As such, understanding the mechanisms driving this thrombocytosis and figuring out strategies to reverse it are important knowledge gaps to address. We have recently discovered a novel regulator of megakaryocyte (MK) differentiation and maturation known to be upregulated during inflammation, the inflammatory chemokine ligand 5 (CCL5, RANTES). Our preliminary data revealed that CCL5 administration to healthy mice leads to proliferation of hematopoietic stem cells (HSCs) and significantly increased platelet counts in the absence of increasing MK progenitors. These data suggest that CCL5 may act to increase platelet counts by mobilizing MK-biased HSCs. Consistent with this hypothesis, treatment of HSCs and MKs with CCL5 in vitro resulted in ehanced mitochondrial and cell cycle activity, respectively. Therefore, we hypothesize that CCL5 may induce thrombocytosis through activation of MK-biased HSCs, ultimately resulting in enhanced MK and platelet production. Furthermore, our previously published work established a role for CCL5 in driving platelet count in a murine IBD model. As such, our overall hypothesis is that CCL5 plays a role in inflammatory-mediated thrombocytosis in IBD. This proposal will elucidate the mechanism by which CCL5 affects HSC activation and differentiation in vitro (Aim 1) and in vivo (Aim 2). In addition, Aim 2 will examine the role of CCL5 on HSC activation in a murine IBD model.