IDO1 and rheumatoid arthritis: However, given the ubiquity of FLS in the synovium as one of the primary resident cell types and the high expression of IDO1 observed by FLS in vitro relative to macrophages or DCs, these findings suggest that FLS-mediated tryptophan depletion, driven by IDO1, may contribute to T cell CD69 expression and suppression within the RA synovium in vivo.