IL34 and synovitis: Spearman’s rho correlation analysis demonstrated that there was a significantly direct link between IL-34 mRNA expression and the synovitis severity in knee OA synovium (r = 0.75, P < 0.001), concordant with the associations of IL-34 protein expression in both the synovial lining layer (r = 0.88, P < 0.001) and sub-lining layer (r = 0.92, P < 0.001) with the severity of synovitis.