IL37 and rheumatoid arthritis: In addition, studies by Rong et al. (2012) showing increased expression of IL-23 in synovial fibroblasts and plasma of RA patients, as well as Rasmussen et al. (2010) who found significantly higher plasma IL-23 levels in RA patients by performing a comparative study between RA patients and healthy controls, showed a correlation between IL-23 levels and RA activity and supported the hypothesis that IL-23 is a risk factor for RA.