STAT3 and graft versus host disease: Taken together, these results indicate that deficiency of Stat3 in donor CD4+ T cells results in (1) marked reduction of expansion of extrafollicular PSGL-1loCD4+ T cells and reduction of their infiltration of thymus; (2) lower production of serum IgG autoantibodies and decreased IgG deposition in GVHD target tissues such as skin and thymus.