TPH1 and rheumatoid arthritis: This is consistent with Rao et al. [2] demonstrating the local pathogenic role of Tph cells in RA, where synovial studies showed expansion of PD-1+CXCR5+CD4+ T cells inducing greater differentiation of memory B cells into plasma cells, via IL-21 and signalling lymphocytic activation molecule-5 (SLAM5).