Prevention of PSGL1loCD4+ T expansion by BCL6 or Stat3 deficiency and by blockade of ICOS or PD-1 interaction with ICOSL or PD-L2 on B cells markedly reduced serum concentrations of autoantibodies and decreased the severity of chronic GVHD (47, 50). Here, BCL6 is linked to chronic graft versus host disease.