TPH1 and rheumatoid arthritis: Indeed, the ratio of IL-21-producing CD8+ T cells correlated well with that of IL-21 producing CD4+ T cells in RASF (Figure 5B), implying the possibility that memory PD-1hiCD8+ T cells, in concert with Tph cells, promoted plasmablast differentiation in RASF, which is in accord with our finding that the ratio of IL-21-producing CD8+ T cells was significantly higher in seropositive (RF+) than seronegative (RF-) patients with RA (Figure 5C).