Host-Derived IL-22 Limits Graft Versus Host Disease and Protects the Intestinal Stem Cell Niche

Abstract 309

Despite decades of intensive research, graft vs. host disease (GVHD) remains a major complication of allogeneic transplantation. Much progress has been made toward understanding GVHD pathophysiology and the mechanisms regulating the donor immune response, however no factors have been identified to regulate the response of transplant recipients to GVHD and its concomitant damage. Furthermore, virtually all strategies available to reduce clinical GVHD do so by limiting the donor immune system at the expense of therapeutic graft vs. leukemia/lymphoma (GVL) responses. IL-22 is a recently characterized cytokine that is produced by both helper T cells and innate lymphoid cells (ILC). Expression of its receptor (IL-22R) is generally limited to epithelial and other non-hematopoietic cells, and it has demonstrated a protective role for intestinal epithelium during experimental colitis, providing a signal for intestinal epithelial cell survival, proliferation, and wound healing.

We investigated the role of IL-22 in GVHD during MHC-mismatched C57BL/6 (B6) into BALB/c murine bone marrow transplantation (BMT). Elimination of IL-22 with anti-IL-22 neutralizing antibody lead to increased GVHD mortality post-BMT in comparison to isotype control (p<.01). Interestingly, use of IL-22 knockout (KO) donor marrow or T cells had no impact on survival or on the ability to clear A20 lymphoma post-BMT, but IL-22 KO BMT recipients demonstrated significantly increased mortality during GVHD (p<.01). The critical role of recipient-derived IL-22 was confirmed in a minor antigen-mismatched LP into B6 BMT model (Figure 1). IL-22 KO recipients again demonstrated significantly increased mortality during GVHD (p<.001) and histopathologic evidence of GVHD in the small (p<.01) and large (p<.001) intestine and liver (p<.001). Furthermore, BMT into hematopoietic IL-22 KO chimeras also demonstrated increased GVHD mortality (p<.05) and clinical scoring (p<.001), indicating that the source of protective IL-22 is residual host-derived radio-resistant hematopoietic cells.

IL-22 ELISA on tissue homogenates following BMT with wild type donors and recipients indicated that IL-22 levels are increased three weeks post-BMT in both small and large intestine (p<.05), as well as following sublethal radiation without transplantation (p<.05 small intestine; p<.01 large) in comparison to normal controls. However, IL-22 was significantly reduced in both tissues during GVHD (2 weeks post-BMT in small intestine, Figure 2A; 3 weeks post-BMT in large, p<.05), suggesting that IL-22 was produced by residual host hematopoietic cells that were being eliminated during GVHD. Consistent with this, we identified IL-22-producing host ILC (CD45⁺CD3⁻RORg⁺) in small intestine lamina propria two weeks post-BMT, whereas no IL-22 production could be identified in CD3⁺ or RORg⁻ subsets, and these IL-22-producing ILC were significantly reduced during GVHD (p<.05, Figure 2B). Interestingly, although host IL-22 deficiency was associated with increased GVHD morbidity, mortality, and pathology, no major differences were observed in donor lymphocyte infiltration of recipient intestines, frequencies of donor lymphoid subsets, or donor T cell inflammatory cytokines post-BMT. In contrast, IL-22R expression on recipient intestinal epithelium was increased post-BMT, and GVHD in IL-22 KO recipients led to increased damage of the intestinal epithelium as measured by FITC-dextran absorption (p<.05). Additionally, IL-22R expression was identified by immunohistochemistry in intestinal crypts where the stem/progenitor cell niche is located, and use of LGR5-LacZ reporter mice indicated that intestinal stem cells (ISC) were indeed targets of GVHD. Finally, histologic assessment of the ISC niche demonstrated exacerbated loss of both ISC (p<.001, Figure 2C) and Paneth cells (p<.05) in IL-22 KO mice during GVHD.

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In summary, IL-22 is produced post-BMT by host ILC that are eliminated during GVHD, and IL-22 deficiency increases GVHD morbidity and mortality. While IL-22 deficiency did not significantly alter the donor immune response, it did lead to increased GVHD pathology, loss of epithelial integrity, and damage to the ISC niche. IL-22 is thus critical for protection of host epithelium during GVHD but is not involved in GVL. This may be exploited in the future to reduce clinical GVHD without limiting the curative potential of the transplant.