Memory CD4 T cells, in particular synovial fluid–derived T cell subsets, express increased PD-1 levels. (A) CD4 T cells derived from synovial fluid (SF) of rheumatoid arthritis (RA) patients hardly contain any naïve T (Tn) cells and (B) express increased PD-1 levels. Representative histogram of PD-1 expression on PB and SF CD4 T cells of one donor is shown. Dashed line represents autofluorescence. (C) PD-1 expression is increased on central memory (Tcm) and effector memory (Tem) T cells in comparison with naïve T cells (Tn cells) from peripheral blood (PB; n = 9, both P < 0.01) and SF (n = 7, both P < 0.05). PD-1 expression was strongly upregulated on all CD4 T cell subsets derived from SF versus PB of RA patients. *P < 0.05, **P < 0.01 and ***P < 0.001 (statistically significant differences). MFI, mean fluorescence intensity; PD-1, Programmed death 1.