Regulation of follicular T-helper cell positioning and function by PD-1

Follicular T-helper (TFH) cells highly express the programmed cell death-1 (PD-1) molecule. Whereas inhibition of T cell receptor (TCR) signaling and CD28 co-stimulation is thought to be the primary mode of PD-1 functions, how PD-1 regulates TFH development and function remains unclear. Here we show that, when engaged by the ensemble of bystander B cells constitutively expressing PD-1 ligand 1 (PD-L1), PD-1 inhibits T-cell recruitment into follicles. This inhibition involves suppression of PI3K activities downstream of follicle-guidance receptor CXCR5, is independent from co-signaling with the TCR, and necessitates ICOS signaling to overcome. PD-1 further restricts CXCR3 upregulation on TFH cells, serving to concentrate these cells toward the GC territory, where PD-L1-PD-1 interactions between individual TFH and B cells optimize B-cell competition and affinity maturation. Therefore, operating in both costimulation-independent and -dependent manners, PD-1 plays an integral role in orchestrating tissue positioning and function of TFH cells. Differential expression analysis between WT and Pdcd1KI/KI CXCR5hi Tfh cells