Aim: Graft-versus-host disease (GvHD) is a major complication arising in patients undergoing allogenic hematopoietic stem cell transplantation. Material & methods: We tested ruxolitinib (a selective JAK1/2 inhibitor) efficacy in three different preclinical models of GvHD. Results: Ruxolitinib, at doses that mimic clinically achievable human JAK/signal transducers and activators of transcription target inhibition, significantly reduced alloreactive T-cell activation and infiltration in the lung and skin, leading to improved outcomes in two experimental models of steroid-refractory acute and chronic GvHD. Additionally, we describe a novel humanized GvHD model in which immunodeficient NOG animals are engineered to produce human IL-15 to facilitate enhanced T- and NK cell engraftment, leading to severe GvHD. Conclusion: Ruxolitinib treatment ameliorated disease symptoms resulting from targeted immune modulation via JAK/signal transducers and activators of transcription signaling inhibition.

Background: Graft-versus-host disease (GvHD) is defined as a syndrome in which cells originating from a donor recognize and mount a deleterious immune response against antigens found in the immunocompromised recipient. Most humanized pre-clinical studies of models of GvHD rely on adoptive transfer of human peripheral blood mononuclear cells (PBMC) into severely immunodeficient recipient animals. Transferred PBMC can recognize murine xeno-antigens presented by major histocompatibility complex (MHC) and develop GvHD. Only limited numbers of transferred human T-cells survive, and tissue damage is generally observed in only a few tissues (i.e. gastrointestinal (GI) tract and liver). There is a need to develop refined humanized models that better recapitulate the clinical settings of allogeneic hematopoietic cell transplantation. Aims: a) To develop a refined model of pre-clinical GvHD where NOG animals transgenic for human IL-15 (hIL-15 NOG) are able to support long-term engraftment of T-cells as well as NK cells. b) To characterize the role of JAK1/2 inhibition on steroid refractory GvHD in the hIL-15 NOG model. Materials and Methods: Xenogeneic GvHD was induced in hIL-15 NOG receiving 3 different doses of human PBMCs, as well as in control NOG animals. Engraftment was weekly assessed by flow cytometry. Ruxolitinib, prednisolone or vehicle was orally administered twice daily. Efficacy was assessed by GvHD symptom scores, cachexia, and histological analysis of end organ damage. Results: hIL-15 NOG animals develop a rapid onset, highly aggressive, form of GvHD after adoptive transfer of 5x106 PBMC, including a significant engraftment of NK, NK-T and T-cells. Significant PBMC infiltration and inflammation was found in GI tract, lung, liver, skin, eyes and pancreas. Ruxolitinib treatment, but not prednisolone, ameliorated GvHD severity scores with concomitant reduction of GI tract infiltration and tissue damage. Overall GvHD mice survival was significantly improved with JAK1/2 inhibition. Therapeutic ruxolitinib treatment initiated after steroid failure significantly inhibited the chronic-progressive disease and reduced tissue markers of inflammation and fibrosis. Conclusion: hIL-15 NOG animals develop a rapid onset, highly reproducible, form of GvHD after PBMC transfer, with successful engraftment of NK, NK-T and T-cells. Ruxolitinib, a potent and selective JAK1/2 inhibitor, significantly ameliorated GvHD scores, reduced tissue-inflammation and improved survival. Ruxolitinib was also able to improve clinical parameters in mice unresponsive to prednisolone treatment, underscoring the relevance of the new model. Ruxolitinib has demonstrated statistically significant improvement in in patients with steroid-refractory GvHD (REACH3, NCT03112603). Disclosures Huarte: Incyte corporation: Current Employment. Peel:Incyte corporation: Current Employment. Dube:Taconic: Current Employment. Stephens:Incyte: Current Employment. Stewart:Incyte: Current Employment. Long:Incyte: Current Employment. Czerniak:Incyte: Current Employment. Oliver:Incyte: Current Employment. Smith:Incyte corporation: Current Employment.

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