These results were confirmed using a genetic approach that demonstrated that, while animals reconstituted with CD4+ T cells from mice had a significant increase in survival relative to GVHD control mice, this effect was completely lost when mice were transplanted with CD4+ T cells from donors (Figure 7D). there is loss of tolerance and breakdown of mucosal barriers. Introduction Graft-versus-host disease (GVHD) is usually a proinflammatory syndrome that is initiated by donor T cells and is the major complication of allogeneic hematopoietic stem cell transplantation (1C3). The overproduction of inflammatory cytokines is usually a critical component of this process and is able to mediate pathological damage directly, or indirectly by activation and/or recruitment of other effector cell populations (4C6). During the acute phase, GVHD generally targets a restricted set of organs, which include the skin, liver, and gastrointestinal (GI) tract. Of these tissues sites, the GI tract is usually of particular relevance in the pathophysiology of this disorder, as damage to this organ plays a crucial role in the amplification of systemic GVHD severity (3, 7). This is attributable to breakdown of the mucosal barrier, which leads to increased systemic proinflammatory cytokine secretion arising from interactions between bacterial products (e.g., endotoxin) and donor-derived immune effector cells that are Gemifloxacin (mesylate) resident in the GI tract (8). Clinically, this damaged mucosal barrier predisposes patients to infectious complications that can be life-threatening. Within the GI tract in both GVHD and other inflammatory bowel diseases, interleukin 23 (IL-23) has emerged as a pivotal cytokine that sits at the apex of a proinflammatory cytokine cascade and is directly responsible for the ensuing tissue damage that occurs in these disorders (9, 10). Secretion of IL-23 by activated antigen-presenting cells results in widespread inflammatory cytokine production as well as activation and growth of immune effector cell populations. Signaling of IL-23 occurs by binding of the cytokine to an IL-23 receptor (IL-23R) complex that is composed of IL-12R1 and a unique IL-23R subunit and is expressed on CD4+ T cells, monocytes/macrophages, dendritic cells, and other members of the innate immune system (11). Thus, IL-23 is able to mediate proinflammatory effects in the GI tract through both the innate and adaptive arms of the immune system (10, 12), although the relative importance of Gemifloxacin (mesylate) each component is not completely comprehended. The severity of GVHD is also a function of the balance between effector and regulatory arms of the immune system (13, 14). The absence of regulatory cell populations has been shown to exacerbate GVHD severity (15, 16), indicating that counterregulatory mechanisms are operative during GVHD, although often insufficient to prevent or mitigate the disease. The precise pathways by Gemifloxacin (mesylate) which the proinflammatory effects of IL-23 are regulated within the colon microenvironment, however, have not been well delineated. Herein, we used multiple murine models of GVHD to delineate the specific immune cell populations that mediate the proinflammatory effects of IL-23 within the colon and to determine how inflammation mediated through IL-23/IL-23R signaling was regulated. During the course of these studies, Gemifloxacin (mesylate) we identified a novel subset of CD4+IL-23R+ T cells that constitutively expresses the 2 2 integrin CD11c and exhibited that these cells constitute a highly pathogenic CD4+ T cell populace that plays a critical role in colonic inflammation. Moreover, we show that this cell Gemifloxacin (mesylate) population has a biased IL2RA central memory T cell phenotype, a memory T cell transcriptional profile, and increased expression of gut-homing molecules, which poises them for early entry into the GI tract under inflammatory conditions. Additionally, we demonstrate that these cells are primarily regulated by IL-10 that is produced by CD4+ nonCFoxp3-expressing.