TPH1 and rheumatoid arthritis: To confirm the identity of C5 cells as activated B cell-helper T cells, we established a tissue Tph/Tfh signature score based on the top 100 genes distinctively expressed by Tph/Tfh clusters from rheumatoid arthritis synovial tissue, where Tph cells were initially described (3, 25) (C1, C4 and C7 in Extended Data Figure 2).