Stat3^-/- Alloreactive Donor T Cells Reduce FAO but Increase Leakage of ROS in Response to PD-1 Signaling Triggered By Host-Tissue PD-L1, Leading to Prevention of Gvhd While Preserving GVL Effect

The GVHD pathogenicity of alloreactive T cells is associated with enhanced glycolysis or enhanced fatty acid oxidation (FAO). PD-1 signaling in T cells inhibits glycolysis but augments FAO. Host tissue PD-L1 interaction with PD-1 on activated alloreactive T cells is not able to effectively prevent acute GVHD mediated by donor CD4⁺ and CD8⁺ T cells together, although it can reduce the severity of the disease. Using a murine HCT model of C57BL/6 donor to BALB/c recipient, we show that although WT T cells (CD4⁺ and CD8⁺ T cells together) induced lethal GVHD at 1-5 x10⁶/mouse, the same dose of Stat3^-/- donor T cells induced little signs of acute or chronic GVHD because of induction of T cell anergy, exhaustion and apoptosis, but with retention of a strong GVL effect. This induction of tolerance depends on donor T cell PD-1 interaction with host-tissue PD-L1, because Stat3^-/- T cells induce severe lethal GVHD in the PD-L1^-/- recipients and in WT recipients treated with blocking anti-PD-1 mAb. The tolerance of Stat3^-/- donor T cells induced by PD-1 signaling is associated with lower FAO and mitochondria membrane potential (MMP) but higher ROS leakage, with little difference in glycolysis, as compared to non-tolerant WT T cells. In the absence of PD-1 signaling, Stat3^-/- T cells manifest markedly enhanced glycolysis and loss of tolerance. These results indicate that activated donor T cell PD-1 interaction with host-tissue PD-L1 reduces glycolysis and reduces their GVHD capacity. In addition, absence of mitochondrial pStat3 in Stat3^-/- donor T cells leads to reduced FAO and MMP but increased ROS leakage, resulting in effective anergy, exhaustion, and apoptosis of tissue infiltrating donor T cells and prevention of GVHD. At the same time, the reduced expression of PD-L1 in recipient lymphoid tissues allows donor T cell expansion and strong GVL activity. Our studies have revealed novel roles of mitochondria pStat3 in regulating PD-1 signaling, T cell metabolism and tolerance. (Grant support: R01AI066008 to Zeng) .