PDCD1 and rheumatoid arthritis: Furthermore, upon examination of the function of PD-1+ T cells in RA, Raptopoulou et al. demonstrated a less pronounced PD-1 mediated reduction in T cell proliferation in RA synovial T cells compared to peripheral blood, suggestive that T cells within the synovium may be more resistant to PD-1 mediated suppression [64].