Gut microbes affect immunotherapy: The unleashing of antitumor T cell responses has ushered in a new era of cancer treatment. Although these therapies can cause dramatic tumor regressions in some patients, many patients inexplicably see no benefit. Mice have been used in two studies to investigate what might be happening. Specific members of the gut microbiota influence the efficacy of this type of immunotherapy (see the Perspective by Snyder et al. ). Vétizou et al. found that optimal responses to anticytotoxic T lymphocyte antigen blockade required specific Bacteroides spp. Similarly, Sivan et al. discovered that Bifidobacterium spp. enhanced the efficacy of antiprogrammed cell death ligand 1 therapy. Science , this issue, p. 1079 and p. 1084 ; see also p. 1031

Nod2 activates the NF-κB pathway following intracellular stimulation by bacterial products. Recently, mutations inNod2 have been shown to be associated with Crohn's disease, suggesting a role for bacteria-host interactions in the etiology of this disorder. We show here that Nod2 is a general sensor of peptidoglycan through the recognition of muramyl dipeptide (MDP), the minimal bioactive peptidoglycan motif common to all bacteria. Moreover, the 3020insC frameshift mutation, the most frequent Nod2 variant associated with Crohn's disease patients, fully abrogates Nod2-dependent detection of peptidoglycan and MDP. Together, these results impact on the understanding of Crohn's disease development. Additionally, the characterization of Nod2 as the first pathogen-recognition molecule that detects MDP will help to unravel the well known biological activities of this immunomodulatory compound.

The gene encoding the Nod2 protein is frequently mutated in Crohn's disease (CD) patients, although the physiological function of Nod2 in the intestine remains elusive. Here we show that protective immunity mediated by Nod2 recognition of bacterial muramyl dipeptide is abolished in Nod2-deficient mice. These animals are susceptible to bacterial infection via the oral route but not through intravenous or peritoneal delivery. Nod2 is required for the expression of a subgroup of intestinal anti-microbial peptides, known as cryptdins. The Nod2 protein is thus a critical regulator of bacterial immunity within the intestine, providing a possible mechanism for Nod2 mutations in CD.

John Rioux and colleagues report results of a large genome-wide association meta-analysis and follow-up study of ulcerative colitis. They identify 29 new risk loci for this inflammatory disease and show that many risk loci are shared between ulcerative colitis and Crohn's disease. Genome-wide association studies and candidate gene studies in ulcerative colitis have identified 18 susceptibility loci. We conducted a meta-analysis of six ulcerative colitis genome-wide association study datasets, comprising 6,687 cases and 19,718 controls, and followed up the top association signals in 9,628 cases and 12,917 controls. We identified 29 additional risk loci (P < 5 × 10−8), increasing the number of ulcerative colitis–associated loci to 47. After annotating associated regions using GRAIL, expression quantitative trait loci data and correlations with non-synonymous SNPs, we identified many candidate genes that provide potentially important insights into disease pathogenesis, including IL1R2, IL8RA-IL8RB, IL7R, IL12B, DAP, PRDM1, JAK2, IRF5, GNA12 and LSP1. The total number of confirmed inflammatory bowel disease risk loci is now 99, including a minimum of 28 shared association signals between Crohn's disease and ulcerative colitis.