To further evaluate the status of the PD-1 pathway during RA development, we next examined the protein expression of both PD-1 and PD-L1 in paraffin-fixed early RA synovial tissue biopsies by immunohistochemistry using an FDA-approved immunodiagnostic assay, which is routinely used for anti-PD-1 treatment decisions in clinical studies. Here, PDCD1 is linked to rheumatoid arthritis.