With identification of synovitis as a driver of osteoarthritis (OA), recent studies have placed an increasing focus on the synovium and the role of intra-articular inflammation in OA pathogenesis. Mast cells (MCs) are long-lived tissue-resident immune cells that act as early responders to infection or injury. Here we are looking to evaluate the temporal role of MCs in post-traumatic osteoarthritis (PTOA) progression and a potential mechanism via IL-33 signaling to alter MC phenotype to mitigate disease severity following an injury. In PTOA patients, OA may be initiated by an isolated event; therefore, a window may exist for therapeutic intervention by immune modulation before the disease progresses. Therefore, the overall objective of this proposal is to further elucidate the phenotype and functions of MCs in the acute and chronic phases of PTOA. Our central hypothesis is that MCs are pro-inflammatory in the acute phase following injury, but prolonged stimulation of MCs by IL-33 results in an immunomodulatory phenotype, mitigating joint inflammation. Specific Aims: To test our hypothesis and achieve our long-term goal to enhance endogenous immunomodulatory activity of MCs following joint injury through manipulation of IL-33 signaling we aim to (i) identify the temporal role of MCs in PTOA and (ii) determine the role of IL-33 in mediating MC phenotypes during disease progression.