PDCD1 and rheumatoid arthritis: The results revealed that although PD-L1 protein was readily detectable in tonsil tissue sections and PD-L1 overexpressing cells lines, its expression was not observed in most of the early RA synovial tissue biopsies, thus indicating that in the synovial tissue, PD-L1 protein is not available for agonistic interaction with the widely expressed PD-1 (Fig 4 and Table 2).