Microbial‐derived indoles inhibit neutrophil myeloperoxidase to diminish bystander tissue damage

Abstract

During episodes of acute inflammation, polymorphonuclear leukocytes (PMNs) are actively recruited to sites of inflammation or injury where they provide anti-microbial and wound-healing functions. One enzyme crucial for fulfilling these functions is myeloperoxidase (MPO), which generates hypochlorous acid from Cl − and hydrogen peroxide. The potential exists, however, that uncontrolled the extracellular generation of hypochlorous acid by MPO can cause bystander tissue damage and inhibit the healing response. Previous work suggests that the microbiota-derived tryptophan metabolites 1H-indole and related molecules ("indoles") are protective during intestinal inflammation, although their precise mechanism of action is unclear. In the present work, we serendipitously discovered that indoles are potent and selective inhibitors of MPO. Using both primary human PMNs and recombinant human MPO in a cell-free system, we revealed that indoles inhibit MPO at physiologic concentrations. Particularly, indoles block the chlorinating activity of MPO, a reliable marker for MPO-associated tissue damage, as measured by coulometric-coupled HPLC. Further, we observed direct interaction between indoles and MPO using the established biochemical techniques microscale thermophoresis and STD-NMR. Utilizing a murine colitis model, we demonstrate that indoles inhibit bystander tissue damage, reflected in decreased colon 3-chlorotyrosine and pro-inflammatory chemokine expression in vivo. Taken together, these results identify microbiota-derived indoles that acts as endogenous immunomodulatory compounds through their actions on MPO, suggesting a symbiotic association between the gut microbiota and host innate immune system. Such findings offer exciting new targets for future pharmacological intervention.

Microbial Metabolite Regulation of Epithelial Cell-Cell Interactions and Barrier Function

“…Indoles have also been shown to be protective in the context of intestinal inflammation by acting on the innate immune system. For example, indoles, including indole, IPA, and IAA, have been demonstrated to inhibit neutrophil myeloperoxidase (MPO) in assays using both purified, recombinant human MPO and primary human neutrophils [151,152]. MPO, through its generation of hypochlorous acid from hydrogen peroxide, is essential for neutrophil anti-microbial activity but can cause damage to "bystander" tissues during uncontrolled inflammation [153].…”

Metabolic Host–Microbiota Interactions in Autophagy and the Pathogenesis of Inflammatory Bowel Disease (IBD)

“…Supplementation of mice during DSS colitis with these compounds revealed that tryptophan metabolites are protective in the context of acute intestinal inflammation, at least in part through AHR-dependent induction of IL-10R1 on epithelial cells. Subsequent studies using bacteria-derived indole compounds found that they specifically inhibit neutrophil myeloperoxidase, a key inflammatory mediator during acute colitis, and suggest that microbial-derived tryptophan metabolites can directly act on innate immune cells to attenuate the inflammatory response [ 137 ].…”

“…In mouse models of IBD, the metabolite indole-3-propionic acid (IPA) dramatically alleviated DSS-mediated intestinal inflammation resulting in restored intestinal tissue architecture and a reduction in inflammatory parameters, such as histologic score and colon shortening [ 136 ]. IPA, exclusively produced from tryptophan by distinct subsets of the gut microbiota, has been implicated in both regulation of innate immune activity and suppression of gut inflammation through synergistic activation of the pregnane X receptor with another microbiota-derived tryptophan metabolite, 1 H -indole [ 137 , 184 , 185 , 186 , 187 ]. IPA also demonstrates potent anti-oxidant activity and shows neuroprotective characteristics in models of neurodegenerative disorders such as Alzheimer’s disease and ischemic stroke [ 188 , 189 ].…”

Probiotics in Intestinal Mucosal Healing: A New Therapy or an Old Friend?

“…Indoles Induction of IL-10 signaling and inhibition of excessive neutrophil myeloperoxidase production [128,129] Tuberonic acid…”

“…Enhance epithelial proliferation and barrier integrity. [20,135] Alexeev et al [128,129] reported that microbial-derived indoles assist during epithelial damage and promote tissue healing via the induction of IL-10 signaling and the inhibition of the excessive neutrophil myeloperoxidase production. In the same vein, Sung et al [130] identified another microbial metabolite, the tuberonic acid, that exerts anti-inflammatory properties and has a beneficial role during epithelial tissue repair.…”