A large genetic study has uncovered tentative evidence that specific gut bacteria could influence the risk of multiple sclerosis (MS) through the activation of particular immune cells, offering a fresh — though still highly preliminary — glimpse into the hidden communication channels that may connect the microbes living in our intestines with the destruction of myelin in the brain and spinal cord.

The new research, published in the journal Immunity, Inflammation and Disease, used a sophisticated statistical technique known as Mendelian randomization (MR) to probe whether the gut microbiota might exert causal effects on MS risk, and whether immune cell traits stand in the middle of that relationship. The team, led by Pingping Ning and Rui Li, drew on genetic data from tens of thousands of people to build what they describe as an exploratory “gut microbiota–immune traits–MS axis.” Their headline finding: two well-known gut bacterial genera, Roseburia and Ruminococcus2, appear to influence MS risk at least partly by modulating markers on the surface of CD4+ and CD8+ T cells — the very soldiers of the adaptive immune system that are widely suspected of turning against the nervous system in people with MS.

Multiple sclerosis is one of the most common causes of non-traumatic disability in young adults, affecting an estimated 2.8 million people worldwide. The disease is characterized by the immune system’s assault on the myelin sheaths that insulate nerve fibers in the central nervous system, producing symptoms ranging from motor weakness and sensory loss to vision impairment and cognitive decline. Its incidence has climbed sharply in recent years — with roughly 500,000 new cases added globally since 2013 — and women are about three times more likely than men to be diagnosed. The annual economic burden of MS in the United States alone is estimated at $85.4 billion. Yet despite decades of research, the precise triggers of the disease remain stubbornly unclear, with evidence pointing to an intricate interplay between genetics, environmental exposures, and lifestyle factors.

That is where the gut microbiome enters the picture. The intestine is far more than a digestive organ; it is a major hub of immune regulation, and the trillions of microbes it harbors are known to shape immune function in profound ways. When the microbial community becomes imbalanced — a state known as dysbiosis — the production of microbial metabolites can shift, altering the behavior of antigen-presenting cells, T cells, and B cells. This, in turn, may set the stage for autoimmune diseases. Previous studies have found altered gut bacterial composition and reduced levels of short-chain fatty acids in people with MS, but whether these microbial changes actually cause the disease — and through what biological intermediaries — has remained an open question.

Mendelian randomization offered the researchers a way to address that question with observational data. The technique exploits the random allocation of genetic variants at conception as a natural experiment: because single nucleotide polymorphisms (SNPs) are inherited randomly and are fixed at conception, they are largely immune to the confounding and reverse-causation biases that plague traditional observational studies. In essence, MR uses genetic variants as proxies — instrumental variables — to ask whether an exposure genuinely influences an outcome, in a manner loosely analogous to a randomized controlled trial.

The data underpinning the study were formidable. For gut microbiota, the team used summary statistics from the MiBioGen consortium, covering 18,340 participants from 24 cohorts, most of European ancestry, spanning 211 microbial taxa across phyla, classes, orders, families, and genera. For immune traits, they turned to a dataset of 731 immune cell phenotypes measured by flow cytometry in 3,757 individuals from a Sardinian founder population — a rich catalog capturing absolute cell counts, surface antigen expression levels, morphological parameters, and ratios between cell populations. For MS outcomes, they accessed genome-wide association data from the International Multiple Sclerosis Genetics Consortium comprising 47,429 MS cases and 68,374 controls of European ancestry.

The analysis proceeded in stages. First, the researchers screened all 211 microbial taxa for genetic associations with MS and identified six taxa with nominally significant associations — four apparently protective, two apparently risk-increasing. Notably, the genus Ruminiclostridium5 showed the strongest nominal protective signal (odds ratio 0.695, 95% confidence interval 0.554–0.871; p = 0.0016). Next, they tested the 731 immune traits for causal links to MS, finding 52 with nominal associations. Finally, they searched for the crucial middle links: microbial taxa and immune traits that formed a statistically coherent triangle, in which the microbe was associated with both the immune trait and MS, and the immune trait was associated with MS, with effect directions consistent with a genuine mediation pathway.

Only two such pathways survived this filtering. The genus Roseburia appeared to influence MS through the expression of CD28 on CD28+ CD4+ T cells, with an estimated 8.97% of the taxon’s nominal effect on MS mediated through this immune trait. The genus Ruminococcus2 appeared to act through CD45 expression on HLA-DR+ CD8+ T cells, mediating an estimated 12.18% of its nominal effect. Both mediation effects had confidence intervals that did not cross zero, lending them a degree of statistical credibility despite the overall exploratory nature of the analysis.

The mechanistic story behind these findings is biologically plausible. Both Roseburia and Ruminococcus2 are short-chain fatty acid (SCFA)-producing bacteria — fermenters of dietary fiber that generate acetate, propionate, and butyrate. SCFAs nourish colonic epithelial cells and regulate inflammatory responses throughout the body, and they play a key role in gut-brain communication. In MS patients, gut bacterial composition and SCFA levels are often reduced, which is thought to impair regulatory T cell function. Intriguingly, oral propionate supplementation has been shown to increase propionate availability in cerebrospinal fluid and improve clinical outcomes in MS patients, while butyrate ester treatment in mouse models of MS alleviated symptoms, reduced pro-inflammatory Th1 and Th17 cells, and boosted regulatory T cell proportions and IL-10 secretion.

The immune mediators identified in the study are equally compelling. CD28 is a co-stimulatory receptor on T cells that binds CD80 and CD86 on antigen-presenting cells, amplifying T cell activation when the T cell receptor recognizes an antigen. In MS patients, CD28+ CD4+ T cell activation is typically enhanced, and over-activated CD28+ T cells secrete interferon-γ and tumor necrosis factor-α, recruit B cells and macrophages, and contribute to the demyelination process. Excessive CD28 signaling may drive T cells to mount responses against neuroantigens, making CD28-targeting inhibitors a potential future therapeutic strategy. On the CD8+ side, HLA-DR expression marks activated cytotoxic T cells, and HLA-DR+ CD8+ T cells have been implicated in myelin destruction and neurodegeneration — they can recognize central nervous system antigens, exert cytotoxic effects, and release pro-inflammatory cytokines that worsen neuronal damage. CD45, meanwhile, is a tyrosine phosphatase that fine-tunes T cell receptor signaling, making its expression level a meaningful indicator of T cell responsiveness.

The authors are emphatic that these results must be interpreted with caution. All associations reported in the study were nominally significant only — with uncorrected p-values below 0.05 — and none survived false discovery rate (FDR) correction. In other words, the findings are hypothesis-generating clues, not proven causal relationships. The team itself attributes this partly to the low heritability of gut microbial taxa, which limits the statistical power of MR analyses of the microbiome, and to the fact that gut microbiota are heavily shaped by diet and environment, factors that genetic data alone cannot fully capture.

Several other limitations temper the conclusions. The immune trait data came from a Sardinian founder population, which may not represent immune trait distributions elsewhere in the world. The researchers did not perform MR-PRESSO outlier detection due to the computational burden of 58 exposure factors, and reverse MR analysis was not possible because no valid instrumental variables existed for MS as an exposure — meaning reverse causality cannot be ruled out. Multivariable MR and genetic colocalization analyses, which could help disentangle confounding and verify shared genetic signals, were likewise beyond the scope of the study and are flagged as priorities for future well-powered investigations.

Still, the study represents a meaningful step toward mapping the causal architecture of the microbiota-immune-disease axis in MS. By combining two-stage MR with formal mediation analysis, the researchers constructed a coherent, testable framework: fiber-fermenting gut bacteria produce SCFAs, which modulate the activation state of CD4+ and CD8+ T cells, which in turn influence the risk and progression of multiple sclerosis. If confirmed in independent populations and validated in laboratory experiments, this framework could point toward novel biomarkers for predicting disease progression — and toward microbiome-targeted or immune-cell-targeted therapies that intervene early in the disease process. For now, the message from this study is one of cautious optimism: the gut may indeed be whispering to the immune system about MS, and scientists are finally beginning to hear what it says.

The gut microbiome—a vast ecosystem of trillions of bacteria, archaea, viruses, and fungi inhabiting the gastrointestinal tract—is increasingly recognized as a pivotal modulator of host physiology. It influences immunity, metabolism, and even neurocognitive function. Strikingly, as humans age, the microbiota undergoes compositional and functional shifts, often culminating in dysbiosis characterized by reduced diversity and imbalanced microbial populations. These changes correlate with heightened inflammation and deteriorating cardiometabolic health, hallmark features of aging-related diseases. However, establishing a direct causal link has long been hampered by confounding factors inherent in observational studies.

To navigate these complexities, the authors harnessed Mendelian Randomization, a sophisticated analytical technique that uses genetic variants as proxies (instrumental variables) to infer causality between an exposure—in this case, specific microbial traits—and outcomes related to aging biology. By analyzing over 55,000 potential causal connections spanning gut microbial taxa, metabolic pathways, and systemic biomarkers of inflammation and cardiovascular function, the study achieved unprecedented granularity. This approach minimizes reverse causation and confounding, thereby providing a stronger foundation for therapeutic exploration.

Out of this extensive analysis, 91 statistically significant causal relationships emerged, highlighting how particular bacterial populations and metabolic activities within the gut microbiome influence pivotal aging phenotypes. Remarkably, the study identified a link between elevated levels of certain commensal gut bacteria and an increased risk of age-related macular degeneration (AMD), a leading cause of vision loss among older adults. This finding aligns with emerging evidence implicating systemic inflammation and immune dysregulation in AMD pathogenesis, suggesting gut-derived microbial signals may contribute to ocular aging mechanisms.

Another salient discovery relates to the metabolic pathway “purine nucleotides degradation II,” which was associated with reduced plasma levels of apolipoprotein M (ApoM). ApoM is instrumental in lipid metabolism and possesses anti-inflammatory properties that confer cardioprotective effects. Reduced ApoM has been implicated in heightened cardiovascular risk, a major contributor to morbidity and mortality in the elderly. Crucially, these results were replicated using independent genome-wide association studies (GWAS), underscoring the robustness and reproducibility of the findings.

Beyond identifying static associations, the researchers delved into the nuanced interplay between gut microbiota and host genetics, revealing an interaction shaped by blood type. Individuals with blood type A, who express particular glycan structures on their cells, were found to harbor gut microbes capable of metabolizing GalNAc (N-acetylgalactosamine), a sugar moiety present in mucosal glycans. This microbial activity was linked to alterations in circulating protein levels implicated in inflammation and cardiovascular disease pathways. Such gene-microbe interactions underscore the potential for personalized microbiota-targeted interventions tailored to an individual’s genetic makeup.

The study’s meticulous design further strengthens its conclusions. The team implemented stringent statistical criteria to mitigate false discoveries, applied replication analyses with independent datasets—a crucial yet often neglected step in microbiome research—and rigorously tested for reverse causality. This comprehensive methodological framework enhances confidence that observed microbiome effects genuinely influence aging-related traits rather than reflecting downstream consequences.

While the precise molecular mechanisms underlying these microbiome-to-host protein interactions warrant further exploration, the implications are profound. The gut microbiota emerges not merely as a passive passenger but as an active agent capable of modulating inflammatory networks and cardiometabolic pathways that deteriorate with age. This causal evidence reshapes our understanding of aging biology and opens new therapeutic avenues. Targeting microbial composition or function through diet, prebiotics, probiotics, or microbial metabolites could potentially delay or mitigate chronic inflammation and age-related diseases.

Importantly, these findings catalyze a shift towards integrating microbiome science with precision medicine. Recognizing the modulatory role of host genetics—such as blood group antigens—on microbial influence invites the development of interventions customized to individual microbial and genetic profiles. This personalized strategy holds promise for enhancing efficacy and safety in preventing or slowing the progression of complex age-associated conditions.

The broader scientific community stands to benefit from these insights, as the study champions rigorous causal inference methodologies within microbiome research. By eschewing correlative paradigms and prioritizing replication, it sets a new standard for investigations into microbial contributions to human health and disease. Moreover, the multi-omics integration exemplified here—combining genetic, microbiome, proteomic, and clinical data—serves as a blueprint for future endeavors aiming to untangle multifactorial aging processes.

Nonetheless, this pioneering study acknowledges that further research is essential to elucidate the pathways through which gut microbes affect systemic physiology. Longitudinal studies, controlled clinical trials, and mechanistic experiments are needed to translate these causal associations into viable interventions. Additionally, expanding investigations into diverse populations will be critical to generalize and refine therapeutic approaches.

In summary, the study by Grosso, Zanetti, and Sanna firmly establishes a causal link between gut microbial traits and hundreds of age-related biological markers, including inflammatory and cardiovascular risk proteins. The evidence is compelling enough to inspire a reimagining of aging interventions, spotlighting the gut microbiome as a modifiable determinant of healthspan and lifespan. This research not only enhances our molecular understanding of aging but also charts a course towards microbiome-based precision medicine designed to combat the global burden of age-related diseases.

Subject of Research: Not applicable

Article Title: Causal relationships between gut microbiome and hundreds of age-related traits: evidence of a replicable effect on ApoM protein levels

News Publication Date: 1-Aug-2025

Web References:

• Study journal issue: https://www.aging-us.com/issue/v17i8
• Mendelian Randomization methodology (general): https://doi.org/10.1038/nrg.2017.13

References: Not provided in the source content

Image Credits: Copyright: © 2025 Grosso et al., Figure 1 created in BioRender by Sanna, S. (2025), https://biorender.com/a45o861

Keywords: aging, causal inference, gut microbiome, inflammatory proteins, age-related macular degeneration, apolipoprotein M, Mendelian Randomization, microbiota, cardiovascular health