Tissue-resident, extravascular CD64-Ly6C- population forms a critical barrier for inflammation in the synovium

Monocytes are one of the most abundant immune cells infiltrating the inflamed organs. However, the majority of studies on monocytes focus on circulating cells, rather than those in the tissue. Here, we identify and characterize an intravascular (i.v.) and extravascular (e.v.) synovial population (Syn Ly6C- cells) which lack cell surface markers of classical monocytes (Ly6C and CD62) or tissue macrophages (CD64 and Tim4), are transcriptionally distinct and conserved in RA patients. e.v. Syn Ly6C- cells are independent of NR4A1 and CCR2, long-lived and embryonically derived while the i.v. Syn Ly6C- cells are dependent on NR4A1, short lived and derived from circulating NCM. e.v. Syn Ly6C- cells undergo increased proliferation and reverse diapedesis dependent on LFA1 in response to arthrogenic stimuli and are required for the development of RA-like disease. These findings uncover a new facet of mononuclear cell biology and are imperative to understanding tissue-resident myeloid cell function in RA. ### Competing Interest Statement The authors have declared no competing interest.