PDCD1 and rheumatoid arthritis: In rheumatoid arthritis (RA), T peripheral helper (Tph) and related helper states deliver sustained B-cell help in synovium when augmented co-stimulation overwhelms exhausted or numerically constrained inhibitory circuits, paralleling PD-1 high T-follicular helper/T-follicular regulatory (Tfh/Tfr) imbalance in follicles [1,3,8].