CD274 and rheumatoid arthritis: While the high expression of co-inhibitory markers PD-L1 and BTLA on mature synovial DCs may appear counterintuitive, we and others previously reported the upregulation of PD-1 and PD-L1 within the RA synovium (32–34), with multiple studies suggesting this pathway itself may be dysfunctional, leading to the unabated accumulation of self-reactive T cells in the joint (32, 33).