PDCD1 and rheumatoid arthritis: Strikingly, most of these correlations were inverse (5/11, 5/6, and 24/25, respectively). Conversely, ESR and/or serum CRP displayed a positive correlation with the PD-1+CD4+ Treg across both seropositive and seronegative RA, as well as in PsA (Table 2, Supplementary Table 7).