Haploidentical hematopoietic cell transplantation (haplo-HCT) is an increasingly used treatment for hematologic malignancies. Although post-transplant cyclophosphamide (PtCy) has improved graft vs. host disease (GvHD) prophylaxis in haplo-HCT, patients continue to experience life-threatening complications. IFN-γ and IL-6 are central in the pathophysiology of GvHD and cytokine release syndrome (CRS), and both cytokines signal through Janus kinase (JAK)-1. We tested the effect of adding the …