The differentiation of the PSGL1loCD4+ T helpers in chronic GVHD recipients depends on the IL-6R-Stat3-BCL6 pathway, and Stat3 or BCL6 deficiency in donor CD4+ T cells prevented expansion of the PSGL1loCD4+ T cells in GVHD target tissues (47, 50). Here, STAT3 is linked to chronic graft versus host disease.