PRG4 and rheumatoid arthritis: In vivo pre-clinical animal studies and ex vivo studies on human tissues have demonstrated that altered expression and function of PRG4 (i.e. diminished lubrication and increased inflammation) has been associated with joint damage and pain in osteoarthritis (OA) and RA (Ludwig et al., 2012; Iqbal et al., 2016; Das et al., 2019).