In a previous study including 51 RA patients, the level of PD-L1 expression on synovial cells in joints of RA patients was associated with factors of more severe disease, including rheumatoid factor positivity, higher C-reactive protein and higher synovitis score (p < 0.001) [36], which support a correlation between RA disease activity and PD-L1 expression. Here, CRP is linked to rheumatoid arthritis.