Growing evidence suggests that STAT3 signaling in CD4+ T-cells plays a key role in the pathogenesis of autoimmunity. We tested the role of the same pathway in the induction of GVHD and GVL response in a murine model of alloSCT. Using the MHC-matched B10.D2→BALB/c model in which GVHD is CD4+ T-cell-mediated and has clinicopathologic features consistent with human chronic sclerodermatous GVHD, we examined the role of STAT3 signaling in CD4+ T-cells in the pathogenesis of GVHD and GVL response. After conditioning (775 cGy) recipient mice received B10.D2 T-cell depleted (TCD) bone marrow (BM) and equivalent of 12 × 106 splenocytes (9.3 × 106 TCD splenocytes, repleted with 106 wild-type (WT) CD8+ and 1.8 × 106 WT, or CD4-Cre × STAT3flox/flox (STAT3KOCD4+) T-cells), a dose shown to induce all signs of GVHD. We reproducibly induced all signs of chronic GVHD in chimeras receiving WT CD4+ T-cells, but not in chimeras injected with STAT3KOCD4+ T-cells (median score of 0.0 vs. 5.2; P < .001). In situ studies showed that cutaneous GVHD was accompanied by prominent dermal infiltration of donor-derived inflammatory monocytes and complete turnover to donor CD11c+ epidermal DC chimerism in chimeras receiving WT but not STAT3KOCD4+ T-cells (P < .001). Splenic CD11c+ DCs, CD4+ and CD8+ T-cell chimerism was nearly completely donor-derived and did not differ between the two sets of described chimeras. We also found that in this model, pathogenic CD4+ T-cells do not acquire TH17 phenotype and that STAT3 signaling disruption leads to expansion of Foxp3+CD4+ T-cells. To examine the role of STAT3 pathway in eliciting GVL response, we developed a model of preestablished disease in which 106 A20 lymphoma cells were administered to recipients 10 days prior to alloSCT. Addition of GVH inoculum containing STAT3KOCD4+ T-cells enabled potent GVL response when compared to animals receiving only TCD BM, or BM with added TCD SPL (P < .001). However, it was inferior to that observed in the animals receiving WT CD4+ T-cells (P < .001). Our findings indicate that: a) intact STAT3 signaling in CD4+ T-cells is required for clinical manifestations of GVHD in B10.D2→BALB/c model; b) commitment of STAT3KOCD4+ T-cells to regulatory Foxp3+ lineage is likely a result of their insensitivity to IL-6 in the abundance of TGF-β; c) STAT3 ablation enables preservation of GVL, while reducing clinical manifestations of GVHD. Further exploration of the role of STAT3 pathway in posttransplant events is required. Growing evidence suggests that STAT3 signaling in CD4+ T-cells plays a key role in the pathogenesis of autoimmunity. We tested the role of the same pathway in the induction of GVHD and GVL response in a murine model of alloSCT. Using the MHC-matched B10.D2→BALB/c model in which GVHD is CD4+ T-cell-mediated and has clinicopathologic features consistent with human chronic sclerodermatous GVHD, we examined the role of STAT3 signaling in CD4+ T-cells in the pathogenesis of GVHD and GVL response. After conditioning (775 cGy) recipient mice received B10.D2 T-cell depleted (TCD) bone marrow (BM) and equivalent of 12 × 106 splenocytes (9.3 × 106 TCD splenocytes, repleted with 106 wild-type (WT) CD8+ and 1.8 × 106 WT, or CD4-Cre × STAT3flox/flox (STAT3KOCD4+) T-cells), a dose shown to induce all signs of GVHD. We reproducibly induced all signs of chronic GVHD in chimeras receiving WT CD4+ T-cells, but not in chimeras injected with STAT3KOCD4+ T-cells (median score of 0.0 vs. 5.2; P < .001). In situ studies showed that cutaneous GVHD was accompanied by prominent dermal infiltration of donor-derived inflammatory monocytes and complete turnover to donor CD11c+ epidermal DC chimerism in chimeras receiving WT but not STAT3KOCD4+ T-cells (P < .001). Splenic CD11c+ DCs, CD4+ and CD8+ T-cell chimerism was nearly completely donor-derived and did not differ between the two sets of described chimeras. We also found that in this model, pathogenic CD4+ T-cells do not acquire TH17 phenotype and that STAT3 signaling disruption leads to expansion of Foxp3+CD4+ T-cells. To examine the role of STAT3 pathway in eliciting GVL response, we developed a model of preestablished disease in which 106 A20 lymphoma cells were administered to recipients 10 days prior to alloSCT. Addition of GVH inoculum containing STAT3KOCD4+ T-cells enabled potent GVL response when compared to animals receiving only TCD BM, or BM with added TCD SPL (P < .001). However, it was inferior to that observed in the animals receiving WT CD4+ T-cells (P < .001). Our findings indicate that: a) intact STAT3 signaling in CD4+ T-cells is required for clinical manifestations of GVHD in B10.D2→BALB/c model; b) commitment of STAT3KOCD4+ T-cells to regulatory Foxp3+ lineage is likely a result of their insensitivity to IL-6 in the abundance of TGF-β; c) STAT3 ablation enables preservation of GVL, while reducing clinical manifestations of GVHD. Further exploration of the role of STAT3 pathway in posttransplant events is required.