In this study, we aimed to investigate the role of PRG4 in regulating macrophage phenotype polarization and accumulation in synovial tissues and the recruitment of pro-inflammatory macrophages in response to acute synovitis, and to evaluate whether chronic synovitis and synovial fibrosis in Prg4 null joints are mediated by an enhanced polarization towards a pro-inflammatory macrophage phenotype. The gene discussed is PRG4; the disease is acute synovitis.