TPH1 and rheumatoid arthritis: Given their capacity to provide help to B cells and their location in the periphery, PD-1hi CXCR5−CD4+ T cells were Tph cells.15 Whereas both Tfh and Tph cells are found in lymphoid aggregates in RA joints, Tph cells are the dominant PD-1hi cells that interact with B cells outside lymphoid aggregates.15