IL1B and synovitis: Synovitis is the principal pathological characteristic of RA, and inflammatory cytokines are primarily secreted by macrophages to exacerbate RA progression.37,38 In addition to the inflammatory cytokines secreted by macrophages, we found that the upregulated secretion of FABP4 by M1-polarized macrophages enhanced the production of inflammatory cytokines (such as IL-1β, IL-6, and IL-18) in HUVECs and FLSs partially by activating the NF-κB pathway.