Could the gut be priming the body for a more severe stroke?

by · News-Medical

A bacterial byproduct produced in the gut may alter immune cells before a stroke occurs, offering new clues about why brain injury can vary so widely in severity.

Recent studies demonstrated that gut microbes modulate immune function and the risk of neurological disorders. By modulating systemic immunity, the gut microbiome may influence stroke outcomes. Gut-brain axis (GBA) dysfunction may worsen neurological injury; however, the biological mechanisms underlying these changes are unclear.

A recent study published in the journal Cell suggests that intestinal microbes may influence ischemic stroke severity through aryl hydrocarbon receptor (AHR) signaling that regulates dendritic cell (DC) function.

Tracing the gut-to-brain immune pathway

In this study, researchers explored how gut microbial composition influences stroke outcomes through AHR-dependent regulation of intestinal DCs and downstream T-cell responses.

Using a transient middle cerebral artery occlusion (tMCAO) model, the team performed metagenomic investigations to evaluate microbial profiles associated with stroke severity. In mice, stroke severity was classified using a composite measure of infarct size, sensorimotor deficits, and weight loss after three days of stroke. Selective culture and whole-genome sequencing (WGS) confirmed bacterial enrichment.

To evaluate whether microbes from stroke patients influence DCs and stroke outcomes, the researchers performed fecal microbiota transplantation (FMT) experiments. They obtained samples from individuals with varying severity of neurological deficits and then transplanted them into mice following antibiotic treatment. The National Institutes of Health Stroke Scale (NIHSS) scores ≤3.0 represented mild neurological deficit, whereas NIHSS of 13 or greater indicated moderate to severe deficits for selection of the FMT donors. A separate analysis compared fecal indole between patients with NIHSS scores below 5 and those scoring 5 or higher.

Using Kovács assays, they measured indole levels in ileum, plasma, stool, and urine samples, with mass spectrometry for more specific quantification of ileal indole. This helped researchers investigate whether microbial tryptophan metabolism influenced stroke outcomes through AHR-based gut immune responses.

To map AHR expression across immune cells and organs, the researchers performed flow cytometry and uniform manifold approximation and projection (UMAP). They also performed single-cell RNA sequencing (scRNA-seq), differential gene expression (DEG), and principal component analysis (PCA) to assess changes in gene expression. Using CH-223191, the team investigated whether AHR blockade could reverse the findings with and without regulatory T cell (Treg) depletion.

Indole-producing gut bacteria prime more severe stroke

In mice, administration of indole before stroke increased infarct volume and neurological deficits. At the same time, colonization with an indole-deficient E. coli strain produced less severe injury than colonization with indole-producing E. coli. In a small patient cohort, fecal indole was higher in moderate-to-severe stroke and correlated with NIHSS score, supporting an association, rather than a causal relationship in humans. Indole also induced AHR-based responses in DCs in vitro, while its detrimental effects on experimental stroke depended on AHR signaling in CD11c+ cells.

Upon AHR loss, C-C chemokine receptor type 7 (CCR7)-based migratory activity increased in intestinal DCs. CH-223191 administration also increased the accumulation of gut-derived DCs in the meninges and mesenteric lymph nodes and was associated with higher meningeal Treg frequencies and less neuroinflammation.

Experimental depletion of Tregs abolished the early neuroprotective effect of AHR inhibition, although intestinal conventional type 1 dendritic cells (cDC1s) still increased. AHR-deficient animals showed attenuated detrimental effects from indole-producing E. coli, indole administration, and microbiota transferred from stroke patients. In mice colonized with E. coli strains that could not produce indole, sensorimotor deficits and infarct volume were lower than in mice colonized with indole-producing E. coli, suggesting that microbiome-AHR communication may program intestinal immune responses before ischemic brain injury occurs.

Stroke produced limited, time-dependent changes in canonical AHR-responsive genes. Cyp1a1 and Ahrr were transiently reduced throughout the intestine 16 hours after stroke and returned to baseline by 72 hours. Cyp1b1 selectively increased in the ileum at 72 hours, the same location and time point at which indole-producing E. coli expanded.

In mice lacking AHR in CD11c+ cells, conventional type 2 dendritic cells (cDC2s) and inflammatory macrophages showed reduced expression of several inflammation-associated genes. Systemic inflammatory cytokines also showed modest trends, although these differences were not statistically significant. These results do not establish a comparable anti-inflammatory cytokine response in patients.

AHR was highly expressed in immune cells within the ileum, especially natural killer (NK) cells, monocytes, macrophages, and monocyte-derived dendritic cells (moDCs). In the brain and meninges, cDC1s demonstrated the strongest AHR expression.

In human metagenomic analyses, E. coli and tnaA were enriched in patients with ischemic stroke compared with controls. Higher tnaA abundance was also associated with unfavorable functional outcomes 90 days after stroke in unadjusted analyses. After adjustment for clinical factors, however, tnaA showed a strong trend rather than a statistically significant independent association with poor outcomes, while E. coli was no longer associated with outcomes.

Could the gut microbiome shape stroke severity?

The findings suggest that host-microbiome interactions may influence T cell populations. Tregs restrain neuroinflammation and promote recovery, and the experiments showed that pharmacological AHR inhibition increased Treg responses. Deliberate Treg depletion removed the early neuroprotective effect of AHR inhibition, indicating that Tregs were required for this protection in mice. The study did not establish that microbiome-driven AHR signaling depletes Tregs.

Systemic CH-223191 treatment could affect cell types beyond DCs, whereas the human microbiota findings were correlative. The researchers state that larger prospective human cohorts, studies in female animals, and work in other intestinal compartments are needed to test how widely these mechanisms apply.

Exploring inter-individual differences in tryptophan metabolism could help determine whether microbial tryptophan metabolism could eventually inform assessments of stroke susceptibility and severity. The enrichment of tnaA-encoding bacteria in patients with stroke and in people with obesity or type 2 diabetes supports further study of this possibility. Still, it does not establish microbial tryptophan metabolism as a clinical biomarker or treatment target in humans.

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