Stat3 Regulates Alloreactive T Cell Susceptibility Towards PD-L1-Mediated Tolerance Signaling, Gvhd and GVL Activity

Prevention of graft versus host disease (GVHD) while preserving graft-versus-leukemia/lymphoma (GVL) activity of alloreactive donor T cells remains an elusive and long-sought goal. We and others reported that Stat3 deficiency in donor T cells results in expansion of Foxp3⁺ Treg cells and reduction of pathogenic Th17 cells, leading to prevention of chronic GVHD. We have now tested whether Stat3 deficiency in donor T cells can prevent acute GVHD while preserving GVL activity.

We transplanted wild-type (WT) or Stat3^-/- T cells from C57BL/6 (H-2^b) donors into irradiated WT BALB/c recipients bearing B cell lymphoma line BCL1 or aggressive acute lymphocytic leukemia (ALL) cells. WT donor T cells caused acute GVHD, as expected. Stat3^-/- donor T cells did not cause acute GVHD but still had strong GVL activity. This separation of GVL activity from GVHD was associated with augmented expansion and impaired tolerance by Stat3^-/- T cells in lymphoid tissues such as the spleen. In contrast, tolerance by Stat3^-/- T cells was enhanced in GVHD target tissues by different mechanisms. The yield of Stat3^-/- T cells in gut tissues was lower than the yield of WT T cells due to the markedly higher apoptosis of Stat3^-/- T cells compared to WT T cells. The yield of Stat3^-/- T cells in the lung and liver was not lower than the yield of WT T cells, but expression of anergy/exhaustion markers such as PD-1 and KLRG1 was markedly higher in Stat3^-/- T cells than in WT T cells.

We recently reported that GVHD tissue expression of PD-L1 can tolerize donor CD8⁺ T cells in the absence of donor CD4⁺ T cells, but not when CD4⁺ and CD8⁺ T cells were given together. In contrast, PD-L1 expression in recipient GVHD target tissues tolerized Stat3^-/- donor T cells when both CD4⁺ T cells and CD8⁺ T cells were given together. The induction of tolerance in Stat3^-/- T cells by GVHD target tissue expression of PD-L1 depended on PD-L1/PD-1 interaction, which led to marked reduction of glycolysis in Stat3^-/- T cells but not in WT T cells. These observations fit with previous findings that enhanced glycolysis was associated with enhanced GVHD activity of alloreactive T cells, as reported by Xue-zhong Yu's group. Our results indicate that Stat3 deficiency in donor T cells can augment their susceptibility to tissue PD-L1-mediated tolerance mechanisms such as down-regulation of glycolysis. Current experiments are testing whether Stat3 knockdown in mature T cells in the graft can prevent GVHD while preserving GVL activity. (Grant support: R01AI066008 to Zeng)