CSF1R and rheumatoid arthritis: When we compared c-FMS expression between freshly isolated RA synovial CD14+ cells, M-CSF-cultured CD14+ cells from healthy donors, and OA synovial CD14+ cells, we found higher levels of small fragments in RA synovial CD14+ cells, while the levels of mature and immature c-FMS were comparable (Fig. 1c).