Immunology of Digestive Disease

We are working on deciphering mechanisms of sensing of microbes by the immune system and how that is perturbed in diseases such as Crohn’s disease, a major inflammatory bowel disease resulting from breakdown in the normally friendly relationship between microbes present in the bowel and immune cells present in the gut wall. We use large-scale state of the art molecular approaches in primary human cells to decipher molecular pathways that are deranged in inflammatory disease in order to highlight molecules in these pathways that may be amenable to therapeutic manipulation.

Goals To define mechanisms of innate immune sensing, how defects in these processes lead to inflammation and how pathogens usurp detection by innate sensors. The HIV-1 accessory gene nef is a key HIV-1 pathogenicity factor. Our previous work showed that Nef increases the replicative capacity of HIV-1 in CD4+ T cells by triggering a signalling pathway mimicing CD4+ T cell activation with anti-CD3, and inducing key host cell factors required for viral replication (1). Proteomic analysis of CD4+ T cell signalling compartments showed that Nef positively regulates signalling by interfering with ubiquination and destruction of key CD4+ T cell signalling molecules and inhibiting Cbl activity (2). Mucosal DCs are the first cells encountered by HIV-1. They continuously sample environmental material and generate signals that determine either maintenance of immunological tolerance or activation of an adaptive immune response. Internalization of HIV-1 into DCs is mediated by dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN), important in dissemination of HIV-1 infection. To investigate the suggestion that pathogens can subvert DC-SIGN functions to avoid immune recognition we studied the signalling pathway activated by DC-SIGN triggering and showed that the signalling cascade involves activation of Rho via the guanine nucleotide exchange factor (GEF) leukaemia associated Rho GEF (LARG), important for enhancing infectious synapse formation (3). LARG has since been shown to be an essential factor for HIV-1 replication. The chronic gastrointestinal illness Crohn’s disease (CD) is thought to be due to a breakdown in immune tolerance to commensal bacteria in patients with a certain genetic background. The strongest associated CD susceptibility gene is NOD2, a cytosolic recognition receptor controlling immunity against intracellular bacteria and the inflammatory response, expressed exclusively in the monocyte lineage cells, intestinal epithelial cells and Paneth cells. We have shown that NOD2 senses intracellular bacteria and links with the antigen presentation machinery in DCs, can induce autophagy in these cells in concert with another CD susceptibility gene, ATG161L1, and is necessary for bacterial handling of MHC class II antigen presentation by human DCs. DC in CD patients are autophagy induction defective and have defects in MHC class II antigen presentation and bacterial destruction. These defects could result in abnormal clearance of bacterial components and trigger mucosal inflammation. For the first time we linked two of the strongest CD susceptibility genes within a single functional pathway (4). Most recently we have found that DCs in CD patients fail to induce miR-29 in response to NOD2 stimulation, and so do not downregulate IL-23 adequately at the end of an immune response (5). Future research plans Future work will concentrate on characterizing: (a) the function of NOD2 in relation to other PRRs and its role in intestinal epithelial cells and Paneth cells; (b) the mechanism of NOD2 mediated autophagy; (c) how post-translational modifications in microbes alter PRR sensing; (d) mechanisms of nucleic acid sensing in DCs and their implication for DC function; and on (e) genomic and chemical library screening of pathways, such as xenophagy, that are dysregulated in CD to define druggable targets. References: (1) Simmons et al. 2001 Immunity 14:763 (2) Simmons et al. 2005 Immunity 23:621 (3) Hodges et al. 2007 Nature Immunol 8: 569 (4) Cooney et al. 2010 Nature Med 16: 90 (5) Brain et al. 2010 Autophagy 3: 412.