Given that IL-34 and CSF-1 localized to distinct regions of the synovium, and macrophages in RA and PsA synovial sublining and intimal lining layers display distinct phenotypic characteristics [43], we examined the influence of these two cytokines in peripheral blood from HD and patients with RA, and their influence on synovial fluid-derived monocyte differentiation, proliferation and gene expression in RA. Here, CSF1 is linked to rheumatoid arthritis.