IL1B and rheumatoid arthritis: Of note, the synovium of leukocyte-rich RA exhibits an increased population of monocytes polarized towards IL-1β+ and interferon (IFN)-γ-activated secreted phosphoprotein (SPP1)+ pro-inflammatory M1-like macrophages compared to OA synovium, where the majority of macrophages are nuclear protein 1 (NUPR1)+ and Mer proto-oncogene tyrosine kinase (MerKT)+ M2-like polarized in response to the unique homeostatic requirements of the synovium 48, 52.