Chaki, T. | Search bioRxiv

bioRxiv2026

Dietary tryptophan mitigates lung ischemia-reperfusion injury via microbiota-derived indole-3-propionate and aryl hydrocarbon receptor signaling

BackgroundLung ischemia-reperfusion (IR) injury drives early morbidity after lung transplantation and cardiothoracic surgery, yet targeted preventive therapies are lacking. The gut-lung axis and microbiota-derived tryptophan metabolites, including indole-3-propionate (IPA), may regulate pulmonary immunity and inflammation. We investigated whether a tryptophan-rich (Trp-Rich) diet attenuates sterile lung IR injury by increasing microbiota-derived indole metabolites and reprogramming alveolar macrophage (AM) inflammatory responses. MethodsC57BL/6 mice received isocaloric tryptophan-standard (Trp-Std; 0.18%) or Trp-Rich (0.60%) diets for 14 days, then underwent unilateral left lung IR (60 min ischemia followed by 60 min reperfusion). Oxygen saturation, lung cytokines, and aryl hydrocarbon receptor (AhR) signaling readouts (Cyp1a1/Cyp1b1) were evaluated. Gut microbiota was profiled by 16S rRNA sequencing, and targeted metabolomics quantified tryptophan metabolites in feces, portal vein (PV) plasma, and lung tissue. To further assess inflammatory priming in vivo, mice were additionally challenged with intratracheal lipopolysaccharide (LPS). Mechanistic studies compared IPA with related indole metabolites in MH-S cells and primary human AMs, including ex vivo nutritional IR, LPS stimulation, and AhR stimulation and blockade using synthetic agonists and antagonists. ResultsTrp-Rich feeding improved post-IR oxygenation, reduced lung IL-1{beta}, and increased pulmonary Cyp1a1/Cyp1b1 gene expression. Trp-Rich diet remodeled the gut microbiota, including enrichment of Bifidobacterium and Lactobacillus, and increased IPA levels across feces, PV plasma, and lung tissue, with lower kynurenine/IPA ratios across matrices. In the LPS intratracheal challenge, Trp-Rich feeding reduced IL-6 levels in lung tissue and systemic plasma. Primary murine AMs isolated from Trp-Rich mice also showed reduced IL-1{beta} and IL-6 release in an ex vivo nutritional IR model. Among tested indole metabolites, IPA showed the strongest dose-dependent suppression of LPS-induced cytokines and chemokines in MH-S cells and primary human AMs, remained active in the ex vivo nutritional IR model, and its anti-inflammatory effect was abrogated by AhR blockade and enhanced by co-treatment with other indole metabolites. ConclusionsA Trp-Rich diet attenuated sterile lung IR injury, coinciding with gut microbiota remodeling, increased systemic and pulmonary IPA, reduced inflammatory priming, and reprogrammed AM responses. These data support diet- or microbiome-directed strategies targeting IPA-AhR signaling to mitigate perioperative lung IR injury. Caption for graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=190 SRC="FIGDIR/small/714281v1_ufig1.gif" ALT="Figure 1"> View larger version (67K): org.highwire.dtl.DTLVardef@1762c91org.highwire.dtl.DTLVardef@1681386org.highwire.dtl.DTLVardef@1da00aforg.highwire.dtl.DTLVardef@162f17f_HPS_FORMAT_FIGEXP M_FIG A tryptophan-rich diet remodels the gut microbiota and indole metabolite profiles, including IPA, enhances alveolar macrophage AhR signaling, and attenuates sterile lung ischemia-reperfusion injury. C_FIG

immunology↗

bioRxiv2025

Gut Microbiome-derived Propionate Reprograms Alveolar MacrophagesMetabolically and Regulates Lung Injury Responses

Responses to lung injury can vary between individuals and the diet and gut microbiome represent two underappreciated sources of inter-individual variability. The gut microbiome can influence lung injury outcomes through the gut-lung axis, but exactly how diet and its effects on the microbiota are involved remains unclear. We hypothesized that a dietary fiber intervention would enrich for the presence of short-chain fatty acid (SCFA)-producing fermentative bacteria in the gut microbiome, thereby influencing resting lung immunometabolic tone and influence downstream immune responses to lung injury and infection. To test this hypothesis, we fed mice fiber-rich (FR) and fiber-free (FF) diets and observed changes in the steady-state transcriptional programming of alveolar macrophages (AMs) in the absence of injury. Next, we examined the effects of FR and FF diets on the responses to sterile and infectious lung injury in vivo while simultaneously profiling gut microbiota and SCFA levels transmitted along the gut-lung axis. Finally, we validated our in vivo observations with mechanistic studies of the metabolic, signaling, and chromatin-modifying effects of specific SCFAs on lung AMs ex vivo and in vitro. Overall, a FR diet reprogrammed AMs and attenuated lung inflammation after sterile injury while exacerbating lung infection. This effect of FR diets could be transferred to germ-free (GF) mice by fecal microbiome transplantation (FMT) and depended on the ability of the microbiota to produce propionate. Mechanistically, SCFAs altered the metabolic programming of AMs and lung tissue ex vivo without a role for free fatty acid receptors (FFAR) or chromatin remodeling. These findings demonstrate that the gut-lung axis can regulate the resting metabolic tone through dietary fiber intake and the enrichment of SCFA-producing gut bacteria, as well as influence sterile and non-sterile lung injury responses through the activity of gut metabolites. These results provide further evidence to support the development of therapeutic dietary interventions or the use of specific gut metabolites to preserve or enhance specific aspects of host pulmonary immunity.

immunology↗