Invited critical reviewThe immunophysiological impact of bacterial CpG DNA on the gut
Introduction
Colonization of the sterile neonatal intestine begins during passage through the birth canal and upon exposure to the ex utero environment, and within hours postpartum bacteria can be detected in infant fecal material. By adulthood, the human gut (primarily the proximal colon) has a resident population of ∼1014 bacteria composed of an estimated 500 different species [1]. Recent models purport that this complex microbiota exists within and adhered to the intestinal mucus as a biofilm-like community of symbiont and commensal organisms [2], [3]. While the relationship of the host with its normal microbiota is in general poorly understood, it is becoming apparent that the microbes confer both health benefits and, under certain circumstances, will contribute to disease. For example, the short-chain fatty acid butyrate is generated by bacterial metabolism of indigestible carbohydrates and acts as a growth factor for colonic epithelial cells [4]. Furthermore, it is generally accepted that the presence and activity of the microbiota limit colonization of the colon by potential pathogens through competitive exclusion, a process that can be indirect (i.e., consumption of available nutrients) or direct (elaboration of antimicrobial products, e.g., bacteriocins). Contrasted against these beneficial effects, a substantial amount of evidence supports the hypothesis that in concert with a dysregulated immune response the normal gut microbiota participates in driving the pathogenesis of idiopathic disorders such as the inflammatory bowel diseases (IBD), Crohn's disease (CD) and ulcerative colitis (UC) [5].
A voluminous literature exists pertaining to the interaction of host immune and non-immune cells with bacterial pathogens, bacterial products (e.g., fMLP, superantigen) and to a lesser extent the normal commensal microflora, in the context of the gastrointestinal tract (GIT). Pathogens and commensal bacteria are sources of a variety of molecules that are structurally very similar, if not identical (e.g., nucleic acids, lipoproteins); this realization led to a resurgence in interest for considering a role for commensal bacteria, and their products, in disorders of unknown etiology, of which IBD is a classic example. Unmethylated cytosine–guanine (CpG)-rich DNA is a relatively new addition to the assemblage of microbial products that are recognized by the vertebrate innate immune system. The wealth of information on CpG DNA precludes a comprehensive discussion, so instead we concentrate our comments on the impact of bacterial CpG DNA on the gut.
Section snippets
The gastrointestinal tract
The human intestine houses a microbial community that outnumbers host eukaryotic cells by an order of magnitude. There are two fundamental strategies for managing this immense antigenic load-restriction within the gut lumen 1) effectively separate them from the body proper, and 2) neutralization of organisms that penetrate into the mucosa. The former is accomplished primarily by the barrier function of the epithelium and its secreted products, and the latter is the responsibility of the mucosal
Innate immunity and recognition of bacterial components
Immune processes are generally classified into one of two broad categories. Innate immunity depends on a variety of classical (e.g., macrophages) and non-classical (e.g., epithelial) cells employing germ-line encoded (i.e., genetically inherited) receptors to recognize foreign antigen and organisms. In contrast, adaptive immunity is the domain of T and B lymphocytes that, through genome recombination events, express clonally unique antigen receptors (T cell receptor and immunoglobulin), the
CpG DNA
Krieg has recently reviewed the discovery of CpG-containing DNA and the initial experiments that began to define its immunostimulatory activity [17]. Recognition of CpG DNA as an immunostimulant resulted from analysis of synthetic single-stranded antisense oligodeoxynucleotides (AS-ODN) [17], which, ironically, were being employed to suppress cellular responses — not activate them. AS-ODN are designed as short cDNA-complementary for the target mRNA transcript. When transduced into cells
The CpG DNA receptor: Toll-like receptor 9
An important advance in our understanding of innate immunity has been the identification and continuing characterization of the Toll-like receptors (TLR), a family of conserved PRRs that specifically recognize a small number of relatively static microbial structures or products (MAMPs). The immunological role of Toll receptors was originally identified in Drosophila with the finding that Toll mutant flies quickly succumbed to overwhelming fungal infection [25]. This was followed by
CpG and the gastrointestinal tract
TLR9 mRNA expression has been demonstrated in several immune cell-types including human B cells, plasmacytoid DC, NK cells, and monocyte/macrophages [17], [19], [47], [48], [49]. More novel is the finding that TLR9 is expressed in numerous non-immune cell types, including epithelia. We preface the following discussion with the opinion that demonstration of TLR9 protein expression, at least as determined with currently available commercial antibodies, must be considered critically. In our hands,
CpG DNA and the modulation of gut disease
Studies utilizing various models of atopic disease, including experimental asthma, have demonstrated that administration of CpG DNA reduces the severity of pathology associated with these models [70]. Allergy and atopy are generally accepted as being initiated, propagated and/or exacerbated by ongoing T helper cell type-2 (Th2) responses. In accordance with the Th1/Th2 balance paradigm, delivery of Th1-stimulatory CpG DNA would neutralize a Th2-shift by promoting the production of Th1-type
Conclusion
The study of CpG-containing bacterial DNA and its immunophysiological impact is a burgeoning area of innate immunity; continuing this research is necessary to further define the important and sometimes deleterious relationship shared between host and indigenous microbiota. Of interest also will be determining what association, if any, exists between the virulence of microbial pathogens and the nature of their DNA, e.g., hyper/hypo-methylation, and CpG motif content and context. Widespread
Acknowledgements
We would like to acknowledge funding support through grants from the Canadian Institutes of Health Research (CIHR) and the Crohn's and Colitis Foundation of Canada (CCFC).
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2018, Molecular ImmunologyCitation Excerpt :The functional components in probiotics responsible for their anti-allergic activity are still not sure. Relevant studies demonstrated that genomic DNA (CpG motif) (Zhong et al., 2012; Watson and McKay, 2006; Ghadimi et al., 2010), extracellular vesicles (Kim et al., 2015) might contribute to the immunomodulatory effect of living probiotics. In our study, we have tried to use conditioned medium or cell lysates from BB to treat DCs, but the results showed that only the cell lysates was effective to reduce ROS production and the expression of TIM4, which have not been reported before.
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2007, VirologyCitation Excerpt :Nevertheless, we observed a significant effect of CpG DNA on the long-term immune response if administered in combination with VLPs to BALB/c. The observed adjuvant effect is probably due to interaction of the CpG ODN with Toll-like receptor 9 molecules (TLR9), which are expressed by several cell types in the gastrointestinal tract including not only immune cells but also gastric epithelial cells, villus enterocytes, and Paneth cells (Watson and McKay, 2006). However, it is remarkable that, in the prokaryotic environment of the gastrointestinal tract, CpG ODN still have immune-stimulating effects.
The intestinal epithelial barrier: How to distinguish between the microbial flora and pathogens
2007, Seminars in ImmunologyCitation Excerpt :The second possibility is that, upon epithelial damage (induced by chemicals or pathogens), commensal-derived TLR ligands may induce the production of protective factors [93]. Moreover, recent evidence indicates a role for CpG DNA, the bacterial agonist of TLR9, in mediating some of the beneficial effects of probiotics in the intestine, and more generally, to modulate the immuno-physiological status of the gut [94]. Finally, Rakoff-Nahoum et al have demonstrated the critical role played by TLRs in spontaneous commensal-induced colitis in IL-10-deficient mice [95].