F2RL1 and synovitis: There is increasing recognition of the role of synovial inflammation in OA pathogenesis as it is linked to the severity of knee OA,21 and synovitis is detectable by MRI in 90% of patients with knee OA.22 In humans, PAR2 is associated with synovitis in OA with the degree of synovitis and PAR2 expression being strongly correlated.23 Thus, PAR2 is a likely contributor to synovitis.