PDCD1 and immunoglobulin G4-related sclerosing disease: Among the subsets, cTfh2 cells and activated cTfh2 cells with a high expression level of programmed cell death 1 (PD-1, i.e., PD-1hi cTfh2 cells) have been consistently associated with IgG4-RD (33), (34), (35).