First International Symposium on Pediatric Inflammatory Bowel Disease: Session III. Intestinal Microbiota and Its Relevance to IBD Pathogenesis
Certain components of the complex commensal enteric bacterial population induce chronic, Th1-mediated immune responses in susceptible hosts leading to chronic intestinal inflammation, but protective (tolerogenic) immune responses in normal hosts. Genetic determinants of disease susceptibility include altered immunoregulation and/or barrier function.
The distal ileum and entire colon is colonized by an extraordinarily complex group of predominantly anaerobic bacteria (>400 individual species), many of which can not be cultured and can be identified only by molecular methods (16s ribosomal DNA composition). Studies in gnotobiotic (defined flora) rodents have convincingly demonstrated that normal enteric bacteria are essential for the development of chronic intestinal inflammation. We have shown that four separate genetically engineered rodents develop chronic intestinal inflammation in normal environments but fail to develop colitis in sterile (germ-free) conditions. These germ-free rodents exhibit colitis and bacterial-antigen specific T cell responses (Th1 profile) within 1 to 4 weeks after colonization with specific pathogen free enteric bacteria. These same bacteria do not cause disease in wild type (WT) rodents, showing that they are not pathogens. These studies indicate host and bacterial species specificity of responses. Gnotobiotic HLA B27 transgenic rats selectively develop colitis when monoassociated with Bacteroides vulgatus or B. thetaiotaomicron, but not B. distasonis or B. fragilis. The ability to induce disease appears to correlate with a 45 kDa band recognized by serum from B. vulgatus monoassociated HLA B27 TG rats. LPS from B. vulgatus activated NF-κB in intestinal epithelial cells in vitro and in vivo through TLR-4. Of potential importance to mucosal homeostasis, cocultured lymphocytes inhibited NF-κB activation in epithelial cells stimulated by B. vulgatus. In monoassociated IL-10−/− mice, E. coli and EnterococcuS. faecalis induced different phenotypes of disease. E. coli stimulated early onset, cecal-dominated lesions while E. faecalis induced slow onset distal colitis and eventually duodenitis with obstruction. 16s rDNA sequencing demonstrated an altered composition of enteric bacteria in SPF IL-10−/− mice, with preferential expansion of E. coli, Klebsiella and Clostridium ramosum, but decreased C. xylanolyticum and C. polysaccharolyticum, which accounted for 87% of identifiable species in WT mice.
In vitro studies mixing antigen presenting cells (APC) and CD4+ T cells from IL-10−/− and WT mice and HLA B27 transgenic and WT rats indicate that dysregulated APC responses can lead to loss of T cell regulation through physiologic enteric bacterial stimulation in these models. IL-10 produced by APC is a key determinant of appropriate downregulation of APC induction of pathogenic T cell responses to commensal enteric bacteria. In vivo transfer of dendritic cells show that DC from IL-10−/− mice, but not WT mice, can accelerate the onset of colitis in E. faecalis monoassociated mice.
These results indicate that dysregulated innate and cognate immune responses to selected commensal enteric bacteria induce chronic intestinal inflammation in genetically susceptible hosts.
IMPLICATIONS FOR TRANSLATIONAL RESEARCH
Identifying specific bacterial species that selectively induce disease in certain subsets of IBD patients has important potential diagnostic and therapeutic implications. Antibiotic, probiotic and prebiotic treatment could be customized for individual patients. Identification of dominant antigens could lead to serologic and T cell diagnostic tests to determine clinically relevant subsets of Crohn's disease and ulcerative colitis patients and, in principle, to immunization strategies to reverse loss of immunologic tolerance to commensal bacteria in these patients.
1) What mechanisms determine loss of tolerance to commensal enteric bacteria in genetically susceptible hosts?
2) Which bacterial species (and bacterial antigens/adjuvants) within this complex microbiota provide the dominant stimulus for chronic inflammation?
3) How can we translate these observations from rodent models and cell culture systems to improved patient care?
1. Rath HC, Herfarth HH, Ikeda JS, et al. Normal luminal bacteria, especially Bacteroides species,
mediate chronic colitis, gastritis, and arthritis in HLA-B27/human beta2 microglobulin transgenic rats. J Clin Invest
2. Kim SC, Tonkonogy S.L., Albright CA, et al. Regional and host specificity of colitis in mice monoassociated with different non-pathogenic bacteria. Gastroenterology
3. Albright C, Tonkonogy SL, Sartor RB. Endogenous IL-10 inhibits APC stimulation of T lymphocyte responses to luminal bacteria. Gastroenterology 2002;122:A270.
4. Albright C, Tonkonogy SL, Frelinger JA, et al. Adoptive transfer of E. faecalis-pulsed dendritic cells accelerates colitis in IL-10 deficient mice. Gastroenterology 2003;124:A73.