S242: mast cells are mediators of inflammatory effector cell recruitment and fibrosis in dermal chronic graft-versus-host disease

Background: Allogeneic hematopoietic stem cell transplantation (allo-HCT) is a potentially curative treatment for patients with blood cancers. Benefits of allo-HCT are hampered by graft-versus-host-disease (GVHD), which can be debilitating and potentially lethal. In chronic GVHD (cGVHD), inflammation and aberrant wound healing lead to pathological fibrosis, most frequently in the skin, yet the exact pathophysiology is not well-understood. Mast cells are an important part of the immune system and involved in atopic disease. Recent studies have demonstrated new paradigms for mast cell activity, showing that they also play an important role in wound healing and fibrosis. Aims: Our study is aimed to understand the role of mast cells as effectors of inflammatory effector cell recruitment and fibrosis in dermal cGVHD after allo-HSCT Methods: In a murine model of cGVHD (LPj into C57BL/6), lethally irradiated wild type (wt) mice and mast cell-deficient (mcd) B6.Cg-KitW-sh recipients received bone marrow and T cells from either syngeneic wt B6 or allogeneic LP/J donors. Animals were monitored until day 50 after allo-HCT and analyzed for chronic GVHD related organ injury and associated inflammatory and fibrotic changes. Results: Allogeneic wt recipients had significantly more cGVHD symptoms than mast cell-mcd B6.Cg-KitW-sh recipients. Clinical GVHD scores correlated with a significant increase in skin pathology (dermal thickness and sclerosis), collagen deposition, and expression of pro-fibrotic genes in WT as compared to mcd mice. Dermal mast cell numbers were increased in wt recipient mice, but were nearly undetectable in mcd recipient mice, implying that the mast cells that are present were recipient-derived and had survived conditioning. Skin from allogeneic wt but not mcd recipients was enriched in T cells and cytokines (IL-33, IFNgamma, TNF alpha, IL-1b, IL-6, IL-21) as well as chemokines (CCL2, CCL4, CCL5, CXCL5, CXCL9, CXCL10, CXCL11, CXCL13, CXCL16). This increase associated with expression of ST2, IL-21R, CXCR3 and CXCR6, MMP12, ICOS. mRNA expression for CD80, CD86 lymphocyte activation score was also increased. Supernatant from stimulated mast cells induced fibroblast proliferation in vitro. Mast cells upon stimulation with IL33 were sources of many of these factors, production of which was blocked when treated with ibrutinib and ruxolitinib. Summary/Conclusion Recipient mast cells contribute to a proinflammatory environment in the skin after allogeneic HCT, leading to T cell recruitment and progressive fibrosis. Mast cell function or dependent pathways present novel targets to prevent and treat this devastating complication of allo HCT. Keywords: Chronic graft-versus-host, Mast cell, Allogeneic hematopoietic stem cell transplant, Mouse model