IL15 and rheumatoid arthritis: IL-7 protein levels were higher in the synovial fluid of some RA patients compared to OA patients and it was locally transcribed in the synovium.261,262 Levels of IL-15 protein in synovial fluid of RA patients were also higher than OA patients in some cohorts, and this was also transcribed in the synovium.262–265 Fibroblasts isolated from the synovial tissue of RA patients spontaneously secreted more IL-7 and IL-15 than OA synovial fibroblasts.