TPH1 and rheumatoid arthritis: Observing that carriage of the A risk allele corelated with higher ICOS expression in CD4+ memory T cells in healthy donors, and that ICOS expression paralleled Tph abundance in both and RA patients as well as plasmablast frequency in RA, we show that ICOS ligation accelerates Tph cell development, replicated as a function of rs117701653 in healthy donors.