Abstract Graft-versus-host disease (GVHD) is the major cause of non-relapse morbidity and mortality after allogeneic stem cell transplant (allo-SCT). Chronic cutaneous manifestations of GVHD are common late after allo-SCT with limited treatment options beyond protracted steroid therapy. Cytokines are critical mediators of inflammatory processes during GVHD and both IL-17A and IL-22 have been found to have dual pathogenic and protective roles, which are largely dependent upon their cellular source. We have demonstrated in mice that whilst recipient-derived IL-17A and IL-17R signaling suppresses inflammatory responses and prevents gut dysbiosis, donor T cell-derived IL-17A is pathogenic and drives GVHD in both lung and skin. Similarly, contrasting roles have also been described for IL-22 in mice, such that recipient-derived IL-22 helps maintain gut epithelial integrity after allo-SCT, but donor IL-22 can exacerbate acute GVHD pathology. To date, investigation of the effects of donor-derived IL-22 in allo-SCT has focused on acute GVHD, with no data regarding the role of IL-22 in chronic GVHD. Given the complexities and the potential competing risks of targeting IL-22 early post-transplant, we examined IL-22 deficiency in murine models of chronic GVHD following allo-SCT. In the absence of donor-derived IL-22, we observed a significant decrease in skin GVHD clinical scores and chronic GVHD skin pathology (histopathology scores in WT donors: 10.2±2.1, vs. IL-22-/- donors: 2.6±0.9, p <0.01), which is in line with the pathogenic role of IL-22 in autoimmune skin diseases. Intriguingly, lung manifestations of chronic GVHD (i.e. bronchiolitis obliterans) were not IL-22-dependent (histopathology scores in WT donors: 1.6±0.5, vs. IL-22-/- donors: 1.5±0.3, p >0.05), despite the fact we have recently defined their IL-17-dependency. These data demonstrate a relationship between IL-22 and IL-17 and the distribution of chronic GVHD. We identified CD4+ T cells as the major source of donor IL-22 by both direct protein detection and cytokine reporter systems, where we observed co-expression of IL-22 with a range of pro-inflammatory cytokines including IL-17A, IFNγ, GM-CSF and TNF. We identified IL-22+IL-17A+ and IL-22+IL-17A- CD4+ T cells as two distinct sources of donor-derived IL-22 post-transplant (2.2±0.1% and 6.5±0.3 % of LN CD4+ T respectively), both of which were highly dependent upon IL-6 for their development (IL-22+IL-17A+: IgG 1.3±0.2% vs. anti-IL-6Rα 0.08±0.01% of splenic CD4+ T, p < 0.001; IL-22+IL-17A-:IgG 3.4±0.2 vs. anti-IL-6Rα: 0.6±0. 1% of splenic CD4+ T, p < 0.001). Since we have previously identified significant cytokine plasticity within IL-17A+ T cells after allo-SCT, we performed lineage assessment to determine if these two populations arose independently. Using IL-17creRosa26YFP fate-mapping mice that permanent label any cell that has expressed IL-17A, we found that IL-22+IL-17A- CD4+ T cells did not have a history of IL-17A production and were therefore identified as a definitive Th22 population after allo-SCT. Since IL-22 and IL-17A have been reported to induce synergistic responses in the skin, we also explored the possibility of interdependence between Th17 and Th22 development. Using both IL-17RC-/- mice and an IL-17creRosa26YFP/iDTR reporter-deleter system we demonstrate that IL-17 signaling to donor Th22 directly promotes their development in allo-SCT (WT donors: 6.5±0.4 %, Th/Tc17 deleter donors: 3.6±0.2, IL-17RC-/- donors: 3.7±0.3 % Th22 in LN CD4+ T, p <0.001). Finally, we observed a similar pro-inflammatory cytokine gene expression signature (Il22, Il17, Ifng) in the skin of patients who developed cutaneous GVHD > 100 days following allo-SCT but not in those after autologous SCT. These data demonstrate a key role for donor-derived IL-22 in chronic skin GVHD and confirm parallel but symbiotic developmental pathways of Th22 and Th17 differentiation. Together these findings suggest that IL-22 intervention late post-transplant may reduce cutaneous chronic GVHD, whilst maintaining the protective effects of IL-22 in the gut early after transplant. Disclosures Serody: Merck: Research Funding.