Plasma Tryptophan and the Kynurenine-Tryptophan Ratio are Associated with the Acquisition of Statural Growth Deficits and Oral Vaccine Underperformance in Populations with Environmental Enteropathy

High SMAD7 and p-SMAD2,3 expression is associated with environmental enteropathy in children

Enteropathies such as Crohn's disease are associated with enteric inflammation characterized by impaired TGF-β signaling, decreased expression of phosphorylated (p)-SMAD2,3 and increased expression of SMAD7 (an inhibitor of SMAD3 phosphorylation). Environmental enteropathy (EE) is an acquired inflammatory disease of the small intestine (SI), which is associated with linear growth disruption, cognitive deficits, and reduced oral vaccine responsiveness in children <5 y in resource-poor countries. We aimed to characterize EE inflammatory pathways by determining SMAD7 and p-SMAD2,3 levels (using Western blotting) in EE duodenal biopsies (N = 19 children, 7 from Pakistan, 12 from Zambia) and comparing these with healthy controls (Ctl) and celiac disease (CD) patients from Italy. Densitometric analysis of immunoblots showed that EE SI biopsies expressed higher levels of both SMAD7 (mean±SD in arbitrary units [a.u.], Ctl = 0.47±0.20 a.u., EE = 1.13±0.25 a.u., p-value = 0.03) and p-SMAD2,3 (mean±SD, Ctl = 0.38±0.14 a.u., EE = 0.60±0.10 a.u., p-value = 0.03). Immunohistochemistry showed that, in EE, SMAD7 is expressed in both the epithelium and in mononuclear cells of the lamina propria (LP). In contrast, p-SMAD3 in EE is expressed much more prominently in epithelial cells than in the LP. The high SMAD7 immunoreactivity and lack of p-SMAD3 expression in the LP suggests defective TGF-β signaling in the LP in EE similar to a previously reported SMAD7-mediated inflammatory pathway in refractory CD and Crohn's disease. However, Western blot densitometry showed elevated p-SMAD2,3 levels in EE, possibly suggesting a different inflammatory pathway than Crohn's disease but more likely reflecting cumulative protein expression from across all compartments of the mucosa as opposed to the LP alone. Further studies are needed to substantiate these preliminary results and to illustrate the relationship between SMAD proteins, TGF-β signaling, and inflammatory cytokine production, all of which may be potential therapeutic targets.

Environmental enteric dysfunction pathways and child stunting: A systematic review

BACKGROUND

Environmental enteric dysfunction (EED) is commonly defined as an acquired subclinical disorder of the small intestine, characterized by villous atrophy and crypt hyperplasia. EED has been proposed to underlie stunted growth among children in developing countries. A collection of biomarkers, organized into distinct domains, has been used to measure different aspects of EED. Here, we examine whether these hypothesized relationships, among EED domains and between each domain and stunting, are supported by data from recent studies.

METHODOLOGY

A systematic literature search was conducted using PubMed, MEDLINE, EMBASE, Web of Science, and CINAHL between January 1, 2010 and April 20, 2017. Information on study objective, design, population, location, biomarkers, and results were recorded, as well as qualitative and quantitative definitions of EED. Biomarkers were organized into five EED domains, and the number of studies that support or do not support relationships among domains and between each domain with stunting were summarized.

RESULTS

There was little evidence to support the pathway from intestinal permeability to microbial translocation and from microbial translocation to stunting, but stronger support existed for the link between intestinal inflammation and systemic inflammation and for intestinal inflammation and stunting. There was conflicting evidence for the pathways from intestinal damage to intestinal permeability and intestinal damage to stunting.

CONCLUSIONS

These results suggest that certain EED biomarkers may require reconsideration, particularly those most difficult to measure, such as microbial translocation and intestinal permeability. We discuss several issues with currently used biomarkers and recommend further analysis of pathogen-induced changes to the intestinal microbiota as a pathway leading to stunting.

Causes of impaired oral vaccine efficacy in developing countries

Oral vaccines are less immunogenic when given to infants in low-income compared with high-income countries, limiting their potential public health impact. Here, we review factors that might contribute to this phenomenon, including transplacental antibodies, breastfeeding, histo blood group antigens, enteric pathogens, malnutrition, microbiota dysbiosis and environmental enteropathy. We highlight several clear risk factors for vaccine failure, such as the inhibitory effect of enteroviruses on oral poliovirus vaccine. We also highlight the ambiguous and at times contradictory nature of the available evidence, which undoubtedly reflects the complex and interconnected nature of the factors involved. Mechanisms responsible for diminished immunogenicity may be specific to each oral vaccine. Interventions aiming to improve vaccine performance may need to reflect the diversity of these mechanisms.

Cross-modulation of pathogen-specific pathways enhances malnutrition during enteric co-infection with Giardia lamblia and enteroaggregative Escherichia coli

Diverse enteropathogen exposures associate with childhood malnutrition. To elucidate mechanistic pathways whereby enteric microbes interact during malnutrition, we used protein deficiency in mice to develop a new model of co-enteropathogen enteropathy. Focusing on common enteropathogens in malnourished children, Giardia lamblia and enteroaggregative Escherichia coli (EAEC), we provide new insights into intersecting pathogen-specific mechanisms that enhance malnutrition. We show for the first time that during protein malnutrition, the intestinal microbiota permits persistent Giardia colonization and simultaneously contributes to growth impairment. Despite signals of intestinal injury, such as IL1α, Giardia-infected mice lack pro-inflammatory intestinal responses, similar to endemic pediatric Giardia infections. Rather, Giardia perturbs microbial host co-metabolites of proteolysis during growth impairment, whereas host nicotinamide utilization adaptations that correspond with growth recovery increase. EAEC promotes intestinal inflammation and markers of myeloid cell activation. During co-infection, intestinal inflammatory signaling and cellular recruitment responses to EAEC are preserved together with a Giardia-mediated diminishment in myeloid cell activation. Conversely, EAEC extinguishes markers of host energy expenditure regulatory responses to Giardia, as host metabolic adaptations appear exhausted. Integrating immunologic and metabolic profiles during co-pathogen infection and malnutrition, we develop a working mechanistic model of how cumulative diet-induced and pathogen-triggered microbial perturbations result in an increasingly wasted host.

Malnourished children are exposed to multiple sequential, and oftentimes, persistent enteropathogens. Intestinal microbial disruption and inflammation are known to contribute to the pathogenesis of malnutrition, but how co-pathogens interact with each other, with the resident microbiota, or with the host to alter these pathways is unknown. Using a new model of enteric co-infection with Giardia lamblia and enteroaggregative Escherichia coli in mice fed a protein deficient diet, we identify host growth and intestinal immune responses that are differentially mediated by pathogen-microbe interactions, including parasite-mediated changes in intestinal microbial host co-metabolism, and altered immune responses during co-infection. Our data model how early life cumulative enteropathogen exposures progressively disrupt intestinal immunity and host metabolism during crucial developmental periods. Furthermore, studies in this co-infection model reveal new insights into environmental and microbial determinants of pathogenicity for presently common, but poorly understood enteropathogens like Giardia lamblia, that may not conform to existing paradigms of microbial pathogenesis based on single pathogen-designed models.

A methodologic framework for modeling and assessing biomarkers of environmental enteropathy as predictors of growth in infants: an example from a Peruvian birth cohort

Background: Environmental enteropathy (EE) impairs the gut's absorptive capacity and immune function and causes decelerations in statural growth that manifest gradually over time.Objective: To illustrate an approach for assessing emerging biomarkers of EE, we separately assessed the associations between 3 such markers and subsequent nutritional status.Design: Stool samples were routinely collected between January 2010 and November 2014 from a cohort of 303 Peruvian infants and analyzed for concentrations of the biomarkers α-1-antitrypsin (AAT), myeloperoxidase, and neopterin. For each marker, a mixed-effects linear regression model was fitted for length-for-age z scores (LAZs) obtained from anthropometric assessments that incorporated covariate predictors, polynomial terms for age, and product interaction terms to test associations over varying lag lengths. The biomarkers' contribution to the models was assessed with the use of the likelihood ratio test and partial R² statistics.Results: Test statistics for the combined inclusion of the 4-model terms that involved the biomarker were highly statistically significant for AAT (28.71; P < 0.0001) and myeloperoxidase (62.79; P < 0.0001) over a 3-mo lag and moderately so for neopterin (13.97; P = 0.0074). AAT and myeloperoxidase seemed to interact strongly with age, with the magnitude and direction of the effect varying considerably over the first 3 y of life. The largest proportion of the variance explained by any biomarker (2.8%) and the largest difference in LAZ predicted between the 5th and 95th percentile (0.25) was by myeloperoxidase over a 2-mo lag.Conclusions: Of the 3 fecal biomarkers studied, 2 that related to intestinal function-AAT and myeloperoxidase-were associated with small but highly statistically significant differences in future statural growth trajectories in infants in this cohort, lending further evidence to the EE hypothesis that increased gut permeability and inflammation adversely affects subsequent nutritional status. This association exhibited a complex interaction with age. This trial was registered at clinicaltrials.gov as NCT02441426.

Causal Pathways from Enteropathogens to Environmental Enteropathy: Findings from the MAL-ED Birth Cohort Study

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Children living in these settings had a high prevalence of enteropathogens, high levels of intestinal inflammation, abnormal intestinal permeability, high markers of systemic inflammation, and postnatal acquired linear growth deficits when compared to children living in the US or Europe

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This study contributes empiric evidence to demonstrate that enteric infection alters both fecal markers of inflammation and permeability

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Current markers of enteropathy fail to account for a large portion of the observed shortfalls in linear growth in these populations, and markers of systemic inflammation appear as the most promising predictive biomarkers for identifying linear growth failure in children

Environmental enteropathy (EE) is hypothesized as a mediator of growth faltering, but few prospective studies have evaluated pathways linking enteropathogen exposure, intestinal inflammation and permeability, and growth. The MAL-ED study represents a novel analytical framework and explicitly evaluates multiple putative EE pathways in combination and using an unprecedented quantity of data. Despite evidence that gut inflammation and altered gut permeability are frequently present and that associations between enteropathogen exposure and gut dysfunction exist, the observed attributable effects of EE on growth faltering in young children were small.

Background

Environmental enteropathy (EE), the adverse impact of frequent and numerous enteric infections on the gut resulting in a state of persistent immune activation and altered permeability, has been proposed as a key determinant of growth failure in children in low- and middle-income populations. A theory-driven systems model to critically evaluate pathways through which enteropathogens, gut permeability, and intestinal and systemic inflammation affect child growth was conducted within the framework of the Etiology, Risk Factors and Interactions of Enteric Infections and Malnutrition and the Consequences for Child Health and Development (MAL-ED) birth cohort study that included children from eight countries.

Methods

Non-diarrheal stool samples (N = 22,846) from 1253 children from multiple sites were evaluated for a panel of 40 enteropathogens and fecal concentrations of myeloperoxidase, alpha-1-antitrypsin, and neopterin. Among these same children, urinary lactulose:mannitol (L:M) (N = 6363) and plasma alpha-1-acid glycoprotein (AGP) (N = 2797) were also measured. The temporal sampling design was used to create a directed acyclic graph of proposed mechanistic pathways between enteropathogen detection in non-diarrheal stools, biomarkers of intestinal permeability and inflammation, systemic inflammation and change in length- and weight- for age in children 0–2 years of age.

Findings

Children in these populations had frequent enteric infections and high levels of both intestinal and systemic inflammation. Higher burdens of enteropathogens, especially those categorized as being enteroinvasive or causing mucosal disruption, were associated with elevated biomarker concentrations of gut and systemic inflammation and, via these associations, indirectly associated with both reduced linear and ponderal growth. Evidence for the association with reduced linear growth was stronger for systemic inflammation than for gut inflammation; the opposite was true of reduced ponderal growth. Although Giardia was associated with reduced growth, the association was not mediated by any of the biomarkers evaluated.

Interpretation

The large quantity of empirical evidence contributing to this analysis supports the conceptual model of EE. The effects of EE on growth faltering in young children were small, but multiple mechanistic pathways underlying the attribution of growth failure to asymptomatic enteric infections had statistical support in the analysis. The strongest evidence for EE was the association between enteropathogens and linear growth mediated through systemic inflammation.

Funding

Bill & Melinda Gates Foundation.

Environmental enteropathy is associated with cardiometabolic risk factors in Peruvian children

Environmental enteropathy (EE) is a syndrome of altered small intestine structure and function hypothesized to be common among individuals lacking access to improved water and sanitation. There are plausible biological mechanisms, both inflammatory and non-inflammatory, by which EE may alter the cardiometabolic profile. Here, we test the hypothesis that EE is associated with the cardiometabolic profile among young children living in an environment of intense enteropathogen exposure. In total, 156 children participating in the Peruvian cohort of a multicenter study on childhood infectious diseases, growth and development were contacted at 3-5 years of age. The urinary lactulose:mannitol ratio, and plasma antibody to endotoxin core were determined in order to assess intestinal permeability and bacterial translocation. Blood pressure, anthropometry, fasting plasma glucose, insulin, and cholesterol and apolipoprotein profiles were also assessed. Extant cohort data were also used to relate biomarkers of EE during the first 18 months of life to early child cardiometabolic profile. Lower intestinal surface area, as assessed by percent mannitol excretion, was associated with lower apolipoprotein-AI and lower high-density lipoprotein concentrations. Lower intestinal surface area was also associated with greater blood pressure. Inflammation at 7 months of age was associated with higher blood pressure in later childhood. This study supports the potential for a relationship between EE and the cardiometabolic profile.

Environmental Enteric Dysfunction is Associated with Carnitine Deficiency and Altered Fatty Acid Oxidation

BACKGROUND

Environmental enteric dysfunction (EED), a condition characterized by small intestine inflammation and abnormal gut permeability, is widespread in children in developing countries and a major cause of growth failure. The pathophysiology of EED remains poorly understood.

METHODS

We measured serum metabolites using liquid chromatography-tandem mass spectrometry in 400 children, aged 12-59months, from rural Malawi. Gut permeability was assessed by the dual-sugar absorption test.

FINDINGS

80.7% of children had EED. Of 677 serum metabolites measured, 21 were negatively associated and 56 were positively associated with gut permeability, using a false discovery rate approach (q<0.05, p<0.0095). Increased gut permeability was associated with elevated acylcarnitines, deoxycarnitine, fatty acid β-oxidation intermediates, fatty acid ω-oxidation products, odd-chain fatty acids, trimethylamine-N-oxide, cystathionine, and homocitrulline, and with lower citrulline, ornithine, polyphenol metabolites, hippurate, tryptophan, and indolelactate.

INTERPRETATION

EED is a syndrome characterized by secondary carnitine deficiency, abnormal fatty acid oxidation, alterations in polyphenol and amino acid metabolites, and metabolic dysregulation of sulfur amino acids, tryptophan, and the urea cycle. Future studies are needed to corroborate the presence of secondary carnitine deficiency among children with EED and to understand how these metabolic derangements may negatively affect the growth and development of young children.