TPH1 and immunoglobulin G4-related sclerosing disease: In IgG4-RD patients, there are increased levels of the circulating PD-1high CXCR5−CD4+ Tph and the percentage of these PD-1hiCXCR5−CD4+ Tph cells is positively correlated with serum levels of IgG4 and soluble IL-2 receptors and with the number of involved organs in IgG4-RD patients [20, 40, 83].