ACE2 Links amino acid malnutrition to microbial ecology and intestinal inflammation. Nature. 2012;487:477-481. [PMID: 22837003 PMCID: PMC7095315 DOI: 10.1038/nature11228]

Intestinal commensal microbes as immune modulators

Commensal bacteria are necessary for the development and maintenance of a healthy immune system. Harnessing the ability of microbiota to affect host immunity is considered an important therapeutic strategy for many mucosal and nonmucosal immune-related conditions, such as inflammatory bowel diseases (IBDs), celiac disease, metabolic syndrome, diabetes, and microbial infections. In addition to well-established immunostimulatory effects of the microbiota, the presence of individual mutualistic commensal bacteria with immunomodulatory effects has been described. These organisms are permanent members of the commensal microbiota and affect host immune homeostasis in specific ways. Identification of individual examples of such immunomodulatory commensals and understanding their mechanisms of interaction with the host will be invaluable in designing therapeutic strategies to reverse intestinal dysbiosis and recover immunological homeostasis.

Stories of love and hate: innate immunity and host-microbe crosstalk in the intestine

PURPOSE OF REVIEW

Recent advances in molecular techniques have enabled a deep view into the structure and function of the host's immune system and the stably associated commensal intestinal flora. This review outlines selected aspects of the interplay of innate immune recognition and effectors that shape the ecological niches for the intestinal microbiota.

RECENT FINDINGS

Several studies have demonstrated a pivotal role of innate immune receptor pathways (NOD-like receptors and Toll-like receptors) for the maintenance of microbial communities in the gut. Genetic deficiencies in these pathways have been associated with increased susceptibility to inflammation that in animal models can be transmitted via direct contact or by stool transplantation in the absence of abundant pathogens.

SUMMARY

The genetic architecture of the human host shapes the diversity and function of its stably associated intestinal microflora. Innate immune receptors such as NOD2 or the inflammasome component NOD-like receptor, pyrin-domain containing 6 play a major role in licensing the microbiota under physiological conditions. Understanding the symbiotic interplay in the intestinal tract should help develop procedures and therapeutic interventions aiming at the identification and restoration of disturbed microbiota states. Indeed, these states may be the missing trigger factor for the manifestation of a multitude of civilization disorders including inflammatory bowel disease and gastrointestinal cancer.

Indoleamine 2,3 dioxygenase in intestinal disease

PURPOSE OF REVIEW

Several gastrointestinal diseases including the inflammatory bowel diseases (IBDs) and malignancy are associated with elevated expression of indoleamine 2,3 dioxygenase-1 (IDO1). IDO1 initiates tryptophan catabolism along a pathway that generates several bioactive kynurenine-based metabolites. Promotion of T-cell-mediated tolerance and antimicrobial effects are among the variety of functions attributed to IDO1 activity. Recent advances addressing the diverse implications of gut-associated IDO1 expression are herein reviewed.

RECENT FINDINGS

In active IBD, IDO1 is highly expressed both in the cells of the lamina propria and epithelium. Experimental models demonstrate that IDO1 promotes gut immune homeostasis by limiting inflammatory responses and protecting the epithelium. In human colon cancer, high expression of IDO1 by the neoplastic epithelium correlates with poor prognosis. The serum kynurenine : tryptophan ratio is elevated in both active Crohn's disease and in colon cancer, suggesting this measurement may prove useful as a disease biomarker. IDO1 inhibitors have moved to clinical trials providing new hope as immunotherapy for advanced malignancy.

SUMMARY

IDO1 activity significantly shapes gastrointestinal disease pathophysiology and severity. Measures of IDO1 activity may be useful as a disease biomarker. Manipulation of IDO1 activity has great potential as a treatment for both inflammatory and malignancy associated gastrointestinal disease.

Cellulose supplementation early in life ameliorates colitis in adult mice

Decreased consumption of dietary fibers, such as cellulose, has been proposed to promote the emergence of inflammatory bowel diseases (IBD: Crohn disease [CD] and ulcerative colitis [UC]) where intestinal microbes are recognized to play an etiologic role. However, it is not known if transient fiber consumption during critical developmental periods may prevent consecutive intestinal inflammation. The incidence of IBD peaks in young adulthood indicating that pediatric environmental exposures may be important in the etiology of this disease group. We studied the effects of transient dietary cellulose supplementation on dextran sulfate sodium (DSS) colitis susceptibility during the pediatric period in mice. Cellulose supplementation stimulated substantial shifts in the colonic mucosal microbiome. Several bacterial taxa decreased in relative abundance (e.g., Coriobacteriaceae [p = 0.001]), and other taxa increased in abundance (e.g., Peptostreptococcaceae [p = 0.008] and Clostridiaceae [p = 0.048]). Some of these shifts persisted for 10 days following the cessation of cellulose supplementation. The changes in the gut microbiome were associated with transient trophic and anticolitic effects 10 days following the cessation of a cellulose-enriched diet, but these changes diminished by 40 days following reversal to a low cellulose diet. These findings emphasize the transient protective effect of dietary cellulose in the mammalian large bowel and highlight the potential role of dietary fibers in amelioration of intestinal inflammation.

Interaction of nutrition and infections globally: an overview

The interaction of nutrition and infections is known by experience by generations of medical doctors. Before the era of antibiotics, diet was an integral part of the management of infections. Now, it is necessary to take a fresh look at this interaction as the understanding of immune response has expanded considerably. Comparatively little research has addressed the impact of nutrition interventions on the management of infectious diseases. Most observations of the interaction between nutrition and infections are epidemiological in character. This holds especially true for measles as well as for tuberculosis. In AIDS, the deterioration of the nutritional status is an indicator of disease progression. Infections in undernourished children are a common cause of death, and taking this finding into account helps to reduce the case fatality rate in severely malnourished patients. Regarding the immune response, cellular as well as soluble components are affected by deficiencies of single nutrients or general undernutrition. The immunosuppressive effect of undernutrition starts during intrauterine life already: maternal nutrition status has been shown to impact on immune function in adult animals. Recent research suggests that not only undernutrition but also caloric overnutrition impacts on immune response to infections and immunization. This is partly due to the chronic inflammatory activity of the adipose tissue and partly due to neuroendocrine alterations. Infectious diseases also impact on the nutritional status, either specifically or through unspecific mechanisms, such as anorexia, tachypnea, and vomiting.

NOD2-mediated dysbiosis predisposes mice to transmissible colitis and colorectal cancer

Instability in the composition of gut bacterial communities (dysbiosis) has been linked to common human intestinal disorders, such as Crohn's disease and colorectal cancer. Here, we show that dysbiosis caused by Nod2 deficiency gives rise to a reversible, communicable risk of colitis and colitis-associated carcinogenesis in mice. Loss of either Nod2 or RIP2 resulted in a proinflammatory microenvironment that enhanced epithelial dysplasia following chemically induced injury. The condition could be improved by treatment with antibiotics or an anti-interleukin-6 receptor-neutralizing antibody. Genotype-dependent disease risk was communicable via maternally transmitted microbiota in both Nod2-deficient and WT hosts. Furthermore, reciprocal microbiota transplantation reduced disease risk in Nod2-deficient mice and led to long-term changes in intestinal microbial communities. Conversely, disease risk was enhanced in WT hosts that were recolonized with dysbiotic fecal microbiota from Nod2-deficient mice. Thus, we demonstrated that licensing of dysbiotic microbiota is a critical component of disease risk. Our results demonstrate that NOD2 has an unexpected role in shaping a protective assembly of gut bacterial communities and suggest that manipulation of dysbiosis is a potential therapeutic approach in the treatment of human intestinal disorders.

The role of pattern recognition receptors in intestinal inflammation

Recognition of microorganisms by pattern-recognition receptors (PRRs) is the primary component of innate immunity that is responsible for the maintenance of host-microbial interactions in intestinal mucosa. Dysregulation in host-commensal interactions has been implicated as the central pathogenesis of inflammatory bowel disease (IBD), which predisposes to developing colorectal cancer. Recent animal studies have begun to outline some unique physiology and pathology involving each PRR signaling in the intestine. The major roles played by PRRs in the gut appear to be the regulation of the number and the composition of commensal bacteria, epithelial proliferation, and mucosal permeability in response to epithelial injury. In addition, PRR signaling in lamina propria immune cells may be involved in induction of inflammation in response to invasion of pathogens. Because some PRR-deficient mice have shown variable susceptibility to colitis, the outcome of intestinal inflammation may be modified depending on PRR signaling in epithelial cells, immune cells, and the composition of commensal flora. Through recent findings in animal models of IBD, this review will discuss how abnormal PRR signaling may contribute to the pathogenesis of inflammation and inflammation-associated tumorigenesis in the intestine.

Notable advances 2012

From the microbiome to the microenvironment, certain areas of biomedicine saw fast-paced discovery this year. Here's a rundown of the papers that helped these fields advance quickly in 2012.

Nutritional consequences of chronic diarrhoea

There is an undeniable link between gastrointestinal disorders and malnutrition. Chronic diarrhoea is one of the most common gastrointestinal conditions that can impact a patient's nutritional status. The nutritional consequences will depend on the cause of the diarrhoea as well as the location and extent of gastrointestinal involvement. In general, malabsorption plays a central role in the interaction between malnutrition and chronic diarrhoea. Malabsorption can result in both nutritional deficits and diarrhoea. With severe malnutrition, chronic diarrhoea can persist due to impaired immune function and poor mucosal recovery. Food intolerance and an inappropriate diet in the setting of malabsorption may also contribute to chronic diarrhoea. Patients may attribute their gastrointestinal symptoms to specific dietary intake, which can lead to self-imposed indiscriminate dietary restrictions. Therefore, disease-specific treatment in conjunction with appropriate nutritional counselling and intervention is recommended in the prevention and treatment of malnutrition in patients with chronic diarrhoea. Specialized nutritional support through enteral or parenteral administration may be required to treat severe caloric and micronutrient deficiencies. In this review, we aim to summarize the mechanism, diagnosis, and treatment of the nutritional consequences of chronic diarrhoea.

Therapeutic modulation of intestinal dysbiosis

The human gastrointestinal tract is home to an extremely numerous and diverse collection of microbes, collectively termed the "intestinal microbiota". This microbiota is considered to play a number of key roles in the maintenance of host health, including aiding digestion of otherwise indigestible dietary compounds, synthesis of vitamins and other beneficial metabolites, immune system regulation and enhanced resistance against colonisation by pathogenic microorganisms. Conversely, the intestinal microbiota is also a potent source of antigens and potentially harmful compounds. In health, humans can therefore be considered to exist in a state of natural balance with their microbial inhabitants. A shift in the balance of microbiota composition such that it may become deleterious to host health is termed "dysbiosis". Dysbiosis of the gut microbiota has been implicated in numerous disorders, ranging from intestinal maladies such as inflammatory bowel diseases and colorectal cancer to disorders with more systemic effects such as diabetes, metabolic syndrome and atopy. Given the far reaching influence of the intestinal microbiota on human health a clear future goal must be to develop reliable means to alter the composition of the microbiota and restore a healthy balance of microbial species. While it is clear that much fundamental research remains to be done, potentially important therapeutic options include narrow spectrum antibiotics, novel probiotics, dietary interventions and more radical techniques such as faecal transplantation, all of which aim to suppress clinical dysbiosis, restore intestinal microbiota diversity and improve host health.