CD4 and chronic graft versus host disease: Patients with active chronic GVHD have marked reduction of PD-1hiCXCR5+CD4+ follicular T helper cells (Tfh) among peripheral blood mononuclear cells (PBMC), but high serum concentrations of IgG autoantibodies and CXCL13, the ligand of CXCR5 (13, 74), suggesting intense T helper activity for B cells.