During maturation, dendritic cells (DCs) regulate their capacity to process and present major histocompatibility complex (MHC) II–restricted antigens. Here we show that presentation of exogenous antigens by MHC I is also subject to developmental control, but in a fashion strikingly distinct from MHC II. Immature mouse bone marrow–derived DCs internalize soluble ovalbumin and sequester the antigen intracellularly until they receive an appropriate signal that induces cross presentation. At that time, peptides are generated in a proteasome-dependent fashion and used to form peptide–MHC I complexes that appear at the plasma membrane. Unlike MHC II, these events do not involve a marked redistribution of preexisting MHC I molecules from intracellular compartments to the DC surface. Moreover, out of nine stimuli well known to induce the phenotypic maturation of DCs and to promote MHC II presentation, only two (CD40 ligation, disruption of cell–cell contacts) activated cross presentation on MHC I. In contrast, formation of peptide–MHC I complexes from endogenous cytosolic antigens occurs even in unstimulated, immature DCs. Thus, the MHC I and MHC II pathways of antigen presentation are differentially regulated during DC maturation.

CD1 molecules are distantly related to the major histocompatibility complex (MHC) class I proteins. They are of unknown function. Screening random peptide phage display libraries with soluble empty mouse CD1 (mCD1) identified a peptide binding motif. It consists of three anchor positions occupied by aromatic or bulky hydrophobic amino acids. Equilibrium binding studies demonstrated that mCD1 binds peptides containing the appropriate motif with relatively high affinity. However, in contrast to classical MHC class I molecules, strong binding to mCD1 required relatively long peptides. Peptide-specific, mCD1-restricted T cell responses can be raised, which suggests that the findings are of immunological significance.

We have previously presented evidence demonstrating that mice deficient in NF-κB subunits are susceptible to colitis induced by the pathogenic enterohepatic Helicobacter species, H. hepaticus. However, it has not been determined whether NF-κB is required within inhibitory lymphocyte populations, within cells of the innate immune system, or both, to suppress inflammation. To examine these issues, we have performed a series of adoptive transfer experiments using recombination-activating gene (Rag)-2−/− or p50−/−p65+/−Rag-2−/− mice as hosts for wild-type (WT) and p50−/−p65+/− lymphocyte populations. We have shown that although the ability of H. hepaticus to induce colitis in Rag-2−/− mice is inhibited by the presence of either WT or p50−/−p65+/− splenocytes, these splenocyte populations are unable to suppress H. hepaticus-induced colitis in p50−/−p65+/−Rag-2−/− mice. Colitis in these animals is characterized by increased expression of inflammatory cytokines including IL-12 p40, and depletion of IL-12 p40 from p50−/−p65+/− mice ameliorates H. hepaticus-induced disease. Consistent with a primary defect in the regulation of IL-12 expression, H. hepaticus induced markedly higher levels of IL-12 p40 in p50−/−p65+/− macrophages than in WT macrophages. These results suggest that inhibition of H. hepaticus-induced IL-12 p40 expression by NF-κB subunits is critical to preventing colonic inflammation in response to inflammatory microflora.

The risk of colon cancer is increased in patients with Crohn's disease and ulcerative colitis. Inflammation-induced DNA damage could be an important link between inflammation and cancer, although the pathways that link inflammation and DNA damage are incompletely defined. RAG2-deficient mice infected with Helicobacter hepaticus (Hh) develop colitis that progresses to lower bowel cancer. This process depends on nitric oxide (NO), a molecule with known mutagenic potential. We have previously hypothesized that production of NO by macrophages could be essential for Hh-driven carcinogenesis, however, whether Hh-infection induces DNA damage in this model and whether this depends on NO has not been determined. Here, we demonstrate that Hh infection of RAG2-deficient mice rapidly induces expression of iNOS and the development of DNA double-stranded breaks (DSBs) specifically in proliferating crypt epithelial cells. Generation of DSBs depended on iNOS activity, and further, induction of iNOS, the generation of DSBs, and the subsequent development of dysplasia were inhibited by depletion of the Hh-induced cytokine IL-22. These results demonstrate a strong association between Hh-induced DNA damage and the development of dysplasia, and further suggest that IL-22 dependent induction of iNOS within crypt epithelial cells rather than macrophages is a driving force in this process.