INTESTINAL EPITHELIAL-DERIVED IL-34 REGULATES MACROPHAGE POLARIZATION AND GASTROINTESTINAL TRACT GRAFT VERSUS HOST DISEASE
Gastrointestinal (GI) tract graft versus host disease (GVHD) is a major complication after allogeneic hematopoietic stem cell transplantation (HSCT) and is attributable to dysregulation that occurs between the effector and regulatory arms of the immune system.  Whereas regulatory T cells have a primary role in counterbalancing GVHD-induced inflammation, identifying and harnessing other pathways that promote immune tolerance remains a major goal in this disease.  Herein, we have identified interleukin 34 (IL-34) as a tissue-intrinsic regulatory cytokine that is able to mitigate the severity of GVHD within the GI tract.  Specifically, we observed that absence of recipient IL-34 production exacerbated GVHD lethality, promoted intestinal epithelial cell death, and compromised barrier integrity.  Mechanistically, the absence of host IL-34 skewed donor macrophages towards a proinflammatory phenotype and augmented the accumulation of pathogenic CD4+ GM-CSF+ T cells within the colon.  Conversely, the administration of recombinant IL-34 significantly reduced GVHD mortality and inflammation in the GI tract which was dependent upon the expression of apolipoprotein E (ApoE) in donor macrophages.  Complementary genetic and imaging approaches demonstrated that intestinal epithelial cells were the biologically relevant source of IL-34.  Colonic biopsies from patients with GVHD also revealed expression of IL-34 in intestinal epithelial cells, as well as ApoE in lamina propria macrophages, validating similar cellular localization in humans.  Thus, these studies define IL-34 as an intestinal epithelial-derived cytokine that regulates macrophage polarization in an ApoE-dependent manner, positioning this cytokine as a key regulator of GVHD in the GI tract and a potential therapeutic target for amelioration of this disease. Overall design: Cells were isolated from the colons of mice that were transplanted with Bone marrow (BM) alone or BM plus adjunctive spleen cells (GVHD). To delineate macrophage populations within colon microenvironment, scRNAseq was performed on sorted donor derived CD11b+ CD64+ macrophages from the colons of BM and GVHD mice. To define the effect of IL-34 on the regulation of macrophages, scRNAseq was performed on sorted donor CD11b+ CD64+ macrophages from the colons of WT and IL-34-/- transplant recipients. To address the question of whether IL-34 binding to CSF-1R on macrophages affects the emergenece of proinflammatory T cells in GVHD target organs, scRNAseq was performed on donor derived sorted T cells.