Inflammation, the kynurenines, and mucosal injury during human experimental enterotoxigenic Escherichia coli infection

Abstract

Enterotoxigenic Escherichia coli (ETEC) is an important cause of diarrhea in children and travelers, especially in low- and middle-income countries. ETEC is a non-invasive gut pathogen colonizing the small intestinal wall before secreting diarrhea-inducing enterotoxins. We sought to investigate the impact of ETEC infection on local and systemic host defenses by examining plasma markers of inflammation and mucosal injury as well as kynurenine pathway metabolites. Plasma samples from 21 volunteers experimentally infected with ETEC were collected before and 1, 2, 3, and 7 days after ingesting the ETEC dose, and grouped based on the level of intestinal ETEC proliferation: 14 volunteers experienced substantial proliferation (SP) and 7 had low proliferation (LP). Plasma markers of inflammation, kynurenine pathway metabolites, and related cofactors (vitamins B2 and B6) were quantified using targeted mass spectrometry, whereas ELISA was used to quantify the mucosal injury markers, regenerating islet-derived protein 3A (Reg3a), and intestinal fatty acid-binding protein 2 (iFABP). We observed increased concentrations of plasma C-reactive protein (CRP), serum amyloid A (SAA), neopterin, kynurenine/tryptophan ratio (KTR), and Reg3a in the SP group following dose ingestion. Vitamin B6 forms, pyridoxal 5'-phosphate and pyridoxal, decreased over time in the SP group. CRP, SAA, and pyridoxic acid ratio correlated with ETEC proliferation levels. The changes following experimental ETEC infection indicate that ETEC, despite causing a non-invasive infection, induces systemic inflammation and mucosal injury when proliferating substantially, even in cases without diarrhea. It is conceivable that ETEC infections, especially when repeated, contribute to negative health impacts on children in ETEC endemic areas.

Keywords: Calprotectin; Experimental infection; Indoleamine 2,3-dioxygenase; Longitudinal; Truncation; Tryptophan.

Fig. 1

A, B, C represent line graphs of median values of plasma levels of inflammation markers in the substantial proliferation group and low proliferation group on each day, with bands ranging from the 25th percentile to 75th percentile. For SAA isoforms, shown in B, values are represented in log values. D shows a radar plot illustrating the kinetics of the SAA1.1 isoform ratios and its truncated variant patterns over four time points. Values with asterisks represent the significant change from the baseline (day 0) to the corresponding day as results of post hoc tests, with the detailed values specified in the Supplementary Tables 1, 2–1 and 2–2

Fig. 2

Line graphs show the median values of plasma levels of metabolites of kynurenine and vitamin B6 together with mucosal injury markers in the SP group and LP group on each day, with bands ranging from the 25th percentile to 75th percentile. For the kynurenines, we took only significant markers for simplicity and values represented in log values. Values with asterisks represent the significant change from the baseline (day 0) to the corresponding day as results of post hoc tests, with the detailed values specified in the supplementary Table 1, 2–1 and 2–2. HK 3ʹ-hydroxycotinine, Pic picolinic acid, QA quinolinic acid, Trp tryptophan, PA 4-pyridoxic acid, PL pyridoxal, PLP pyridoxal 5ʹ-phosphate, PAr index, PA: (PLP + PL)