HOXD13 and rheumatoid arthritis: Therefore, the findings of elevated PD-1+ T cells in the synovium along with elevated sPD-1 and undetectable PD-L1 suggest that the PD-1 pathway is dysregulated in RA, and this lack of PD-1-mediated inhibitory signalling may result in chronic T cell activation, thus contributing to pathogenesis (58, 60).