PDCD1 and rheumatoid arthritis: Moreover, significant increases of both PD-1+ cell frequencies and PD-1 median expression were noted in PD-1+ memory B cells in seropositive RA; increased PD-1+ cell frequencies were also noted among both IgD+ and IgD- memory B cell subsets, as well as in ABCs (Figures 3A, B, Supplementary Table 3).