Dendritic cells (DCs) serve as a bridge between innate and adaptive immunity and help to maintain intestinal homeostasis. Inflammatory bowel disease (IBD) is associated with dysregulation of the mucosal immune response. The concomitant hypoxic inflammation in IBD will activate the transcription factor hypoxia-inducible factor-1 (HIF-1) to also drive gene expression in DCs. Recent studies have described a protective role for epithelial HIF-1 in mouse models of IBD. We investigated the role of HIF-1 in DC function in a dextran sodium sulfate (DSS)-induced model of murine colitis. Wild-type and dendritic cell-specific HIF-1α knockout mice were treated with 3% DSS for 7 days. Knockout of HIF-1α in DCs led to a significantly larger loss of body weight in mice with DSS-induced colitis than in control mice. Knockout mice exhibited more severe intestinal inflammation with increased levels of proinflammatory cytokines and enhanced production of mucin. Induction of regulatory T cells (Tregs) was impaired, and the number of forkhead box P3 (Foxp3) Tregs was diminished by dendritic HIF-1α knockout. Our findings demonstrate that in DCs HIF-1α is necessary for the induction of sufficient numbers of Tregs to control intestinal inflammation.

Hypoxia is a hallmark of chronically inflamed tissue. Hypoxia develops from vascular dysfunction and increased oxygen consumption by infiltrating leukocytes. With respect to inflammatory bowel disease (IBD), hypoxia is likely to be of particular importance: Impairment of the intestinal barrier during IBD allows anoxia from the lumen of the gut to spread to formerly normoxic tissue. In addition, disturbed perfusion of inflamed tissue and a higher oxygen demand of infiltrating immune cells lead to low oxygen levels in inflamed mucosal tissue. Here, cells become hypoxic and must now adapt to this condition. The hypoxia inducible factor (HIF)-1 complex is a key transcription factor for cellular adaption to low oxygen tension. HIF-1 is a heterodimer formed by two subunits: HIF-α (either HIF-1α or HIF-2α) and HIF-1β. Under normoxic conditions, hydroxylation of the HIF-α subunit by specific oxygen-dependent prolyl hydroxylases (PHDs) leads to ubiquitin proteasome-dependent degradation. Under hypoxic conditions, however, PHD activity is inhibited; thus, HIF-α can translocate into the nucleus, dimerize with HIF-1β, and bind to hypoxia-responsive elements of HIF-1 target genes. So far, most studies have addressed the function of HIF-1α in intestinal epithelial cells and the effect of HIF stabilization by PHD inhibitors in murine models of colitis. Furthermore, the role of HIF-1α in immune cells becomes more and more important as T cells or dendritic cells for which HIF-1 is of critical importance are highly involved in the pathogenesis of IBD. This review will summarize the function of HIF-1α and the therapeutic prospects for targeting the HIF pathway in intestinal mucosal inflammation.