Tryptophan Catabolism Restricts IFN-γ–Expressing Neutrophils and Clostridium difficile Immunopathology

Abstract

The interplay between Clostridium difficile and the host's metabolome is believed to influence the severity of infection. However, the mechanism for this phenomenon remains unclear. In this study we model one of these metabolic pathways by focussing on tryptophan metabolism in the host. We found that inhibition of tryptophan catabolism in indoleamine 2,3-dioxygenase (IDO1) knockout mice led to increased mucosal destruction and cecal haemorrhage, increased production of IFNγ in response to C. difficile infection, but no significant change in mucosal effector or regulatory T cell numbers or IL-10 mRNA expression. The increased immunopathology in infected IDO1 knockout mice was associated with a lower Clostridium difficile burden and an increased percentage of IFNγ-expressing neutrophils. We further demonstrated the ability of kynurenine to induce apoptosis in bone marrow-derived neutrophils while the presence of tryptophan reversed this effect, providing a possible mechanism for the increased neutrophil accumulation in IDO1-/- mice. We conclude that Clostridium difficile induces tryptophan catabolism in cecal lamina propria cells, which restricts Clostridium difficile-associated immunopathology and the accumulation of IFNγ-expressing neutrophils. This might represent a self-regulatory mechanism for neutrophils, via the IFNγ-IDO1 pathway, to restrict their own accumulation during infection. These findings have important clinical implications as IDO inhibitors are currently used to treat cancer in clinical trials (in patients particularly susceptible to getting Clostridium difficile infection) and treatment with IDO1 inhibitors may exacerbate the severity of Clostridium difficile colitis.

Role of indoleamine 2,3-dioxygenase in health and disease

“…For example, Romani et al [159] reported that impaired IDO1 activity in lung neutrophils is linked to the excessive immune response and ensuing chronic inflammation in a mouse model of CGD, whereas IDO1-expressing human eosinophils are capable of attenuating Th1 and supporting Th2 responses, although, in this study, the involvement of IDO1 was not directly investigated with an IDO1 inhibitor and/or L-Trp supplementation [50]. IDO1 expression, L-Trp depletion and production of Kyn have also been linked to increased neutrophil apoptosis [263], which may relate to reports of elevated neutrophil tissue infiltration and responses noted in Ido1 gene-silenced or -deficient mice subject to infection in vivo [263,264].…”

“…Recent data indicate that C. difficile-infected IDO1 −/− mice show increased intestinal tissue destruction and haemorrhage that correlates with elevated numbers of IFNγ -expressing neutrophils, reduced C. difficile burden, but no change in effector or regulatory T-cells [263]. These data indicate that, rather than regulating T-cells and despite an increase in bacterial burden, IDO1 plays a protective role in moderating neutrophil tissue infiltration during C. difficile infection aimed at avoiding excessive inflammatory tissue damage and immunopathology that can be mediated by these innate immune cells.…”

“…(K) Further emphasising IDO1's pro-inflammatory role in certain contexts, IDO1 can also promote the development of autoreactive B-cells [251,330]. (L) IDO1 expression, L-Trp depletion and production of Kyn have also been linked to increased neutrophil apoptosis [263]. (M) DC maturation, migration and immune-stimulatory capacity are also dependent on IDO1 activity under certain conditions [249,250].…”

Local Generation of Kynurenines Mediates Inhibition of Neutrophil Chemotaxis by Uropathogenic Escherichia coli

“…Here, we demonstrate that in the regulation of innate cellular responses, PMN transepithelial migration is inhibited via the action of kynurenines, rather than by tryptophan limitation. Of note, one recent study showed diminished PMN recruitment to infected cecum in Ido1-deficient mice (43). In that paper, L-kynurenine was found to promote apoptosis of murine bone marrow-derived neutrophils, though only in tryptophan-deficient media.…”

Amelioration of Clostridium difficile Infection in Mice by Dietary Supplementation With Indole-3-carbinol

“…Cecal cells were collected using a modified version of a previously published protocol 24 . IEL and lamina propria (LM) cells were analyzed by flow cytometry.…”