Mast Cells Reduce Gvhd Severity In Allogeneic Transplantation by Reducing the Proliferation of Conventional T Cells

Abstract 243

Mast cells are important effector cells in many innate and adaptive immune responses, and can influence the function of several immune cells including granulocytes, monocytes/macrophages, dendritic cells, T cells, B cells, NK cells, and NKT cells. In contrast to their well-know pro-inflammatory roles, in certain models, mast cells have been shown to decrease inflammation and tissue injury. We explored the biological activity of mast cells in a well-established model of mouse graft-versus-host disease (GVHD) by comparing C57BL/6-Kit^W-sh/W-sh mice (which lack mast cells) to wild-type (WT) C57BL/6 (H-2^b) recipients in a major-MHC mismatched model of allogeneic transplantation. After myeloablative irradiation, recipients received 5×10⁶ T-cell depleted bone marrow cells from FVB/N donors (H-2^q) on day 0 followed by transfer of 2×10⁶ FVB/N CD4 and CD8 conventional T cells (Tcon) at a 2:1 ratio on day 4. A dramatic decrease in survival was observed in the animals lacking mast cells where 100% of Kit^W-sh/W-sh recipients died by day 15 following transplantation while over 50% of WT recipients were alive at day 60 (p < 0.0001). The exacerbated GVHD in Kit^W-sh/W-sh mice correlated with an increase in Tcon proliferation as indicated by bioluminescence imaging (BLI), particularly in lymph node, liver, and gastrointestinal tract tissue sites (p < 0.001). The percentage of Foxp3 positive Treg cells was similar before and after transplantation in Kit^W-sh/W-sh and WT recipients, and CD4⁺CD25^hi Treg from Kit^W-sh/W-sh mice were capable of suppressing T cell proliferation in a mixed leukocyte reaction. When Kit^W-sh/W-sh mice were given 5×10⁶ bone marrow-derived cultured mast cells (BMCMCs) i.p., mast cell re-population of the gut and peritoneal cavity was indicated by both BLI and traditional staining methods. Preliminary experiments with Kit^W-sh/W-sh recipients engrafted with BMCMCs i.p. 6 weeks before allogenic transplantation demonstrated improved survival in the above GVHD model, but not if the BMCMCs were derived from IL-10^-/- C57BL/6 mice. Together, these results support the hypothesis that mast cells can decrease Tcon proliferation and reduce GVHD severity by a mechanism that involves production of IL-10 by mast cells but which is independent of CD4⁺CD25⁺ Treg.