Aging Gut Microbes Weaken Flu Defenses, but Acetate Restores Protection in Old Lung Cells

Aging reshapes nearly every system in the body, and the trillions of microbes living in the gut are no exception. A new study published in Aging Cell now shows that this age-related microbial shift may do more than disturb digestion and fuel chronic inflammation: it may directly weaken the lungs’ ability to fight off influenza. By combining a mouse model of aging with mechanistic experiments in human cells, researchers have traced a pathway that runs from the gut microbiome, through microbial metabolism, all the way to the antiviral defenses of senescent lung cells. Their findings point to acetate, a short-chain fatty acid produced by gut bacteria, as a protective mediator whose decline with age may help explain why influenza hits older adults so much harder.

The research team began by characterizing what aging actually does to the body before any infection takes place. They compared young C57BL/6 mice, aged eight to twelve weeks, with old mice aged twenty-four months. Microscopic examination of lung tissue revealed disrupted cytoskeletal architecture in the old animals, a sign of structural deterioration. Blood analysis showed pronounced systemic inflammation, with significantly elevated levels of interferons, the pro-inflammatory cytokines IL-6 and IL-1β, and chemokines such as IL-8, MCP-1, and CXCL-1. The anti-inflammatory cytokine IL-10, itself a factor secreted by senescent cells, was also elevated. Transcriptomic profiling of lung tissue confirmed the picture, revealing enrichment of inflammation-related pathways including phagosome activity, antigen presentation, and cytokine-cytokine receptor interactions, all pointing to heightened immune activation in the aged lung even in the absence of any pathogen.

The gut told a parallel story. Shotgun metagenomic sequencing of fecal samples showed marked age-associated shifts in microbial composition, with an increased abundance of the genus Clostridium and a significant depletion of Faecalibaculum in the old mice. An analysis of similarities confirmed that the microbial communities of old and young mice differed significantly at the genus level. Taken together, these baseline findings established a coordinated profile of aging: systemic cytokine elevation, inflammatory transcriptional changes in the lung, and gut dysbiosis, providing the foundation for asking how these changes shape the response to viral infection.

When the researchers infected both age groups with a sublethal dose of influenza A virus, distinct age-dependent patterns emerged. Young mice mounted an early, transient response, showing higher clinical scores during the first days of infection but recovering fully by the end of the observation period. Old mice followed a different trajectory: their clinical deterioration appeared later, from day eight onward, and their body weight never fully returned to baseline by day twenty-one. Critically, the bronchoalveolar lavage fluid of old mice contained higher viral loads on day eight, along with elevated levels of IL-6, IP-10, and IFN-γ. In other words, the aged animals cleared the virus more slowly while simultaneously producing more inflammatory signaling molecules.

Transcriptomic analysis of lung tissue mirrored this delayed but exaggerated response. On day four post-infection, infected old mice showed relatively subdued inflammatory gene expression compared with infected young mice, including significantly decreased expression of several type I and type III interferons such as Ifnl2, Ifna4, and Ifnl3. By day eight, however, the old mice displayed a significant surge in inflammation-related genes, with the notable exception of the interferons, alongside enrichment of pattern recognition receptor pathways such as Toll-like receptor cascades. This pattern suggests that aging disrupts the coordination between early antiviral immunity and the later resolution of inflammation, allowing both the virus and the inflammatory response to persist longer.

The most novel part of the study concerned what was happening in the gut during infection. Shotgun metagenomics of samples taken at baseline, day four, and day twenty-one post-infection revealed that influenza infection transiently reshaped the microbiome in both age groups, with decreases in Bacillota and increases in Verrucomicrobiota and Pseudomonadota during the acute phase. Akkermansia increased in both groups, most prominently in old mice, while species-level analysis showed that the two most abundant species, Faecalibaculum rodentium and Akkermansia muciniphila, were significantly decreased in old mice even before infection and remained lower throughout. Linear discriminant analysis highlighted strong effect sizes for these beneficial species in young mice, and for Escherichia coli and Parabacteroides goldsteinii in old mice during the acute phase.

More revealing than the taxonomic shifts were the changes in metabolic potential. Genes annotated to the CAZy database showed that enzymes involved in glucose fermentation were transiently upregulated in both age groups during acute infection, with the effect particularly pronounced in old mice, consistent with the central role of carbohydrate fermentation in short-chain fatty acid production. However, KEGG-based functional annotation indicated a potential decline in pyruvate production via the Entner-Doudoroff pathway and, crucially, reduced acetate generation through pyruvate dehydrogenase (quinone) in the old animals. Because acetate is a major microbiota-derived metabolite with established immunomodulatory and antiviral properties, this age-related decline in acetate-associated metabolism suggested a concrete mechanistic link between gut dysfunction and impaired antiviral defense.

To test that link directly, the team turned to a human cell model. They induced senescence in primary human lung fibroblasts (IMR-90 cells) using doxorubicin, confirming the senescent state through upregulation of p21, downregulation of Lamin B1, and elevated senescence-associated secretory phenotype markers. Flow cytometry revealed that senescent cells significantly overexpressed FFAR2 and FFAR3, the G-protein-coupled receptors that mediate short-chain fatty acid signaling. Consistent with the group’s previous work, senescent fibroblasts proved markedly more permissive to influenza A virus than quiescent controls, replicating the virus to much higher levels. But when the cells were pre-treated with 260 micromolar acetate for twenty-four hours before infection, viral replication dropped significantly, and inflammatory cytokine output was dampened, all without compromising cell viability. Acetate, in effect, restored a measure of antiviral defense to the aged cells.

The mechanism appears to be both immunological and epigenetic. RNA sequencing showed that influenza-infected senescent cells upregulate numerous inflammation and pathogen recognition pathways, including NF-κB and NOD-like receptor signaling, whereas these pathways remained largely unchanged when acetate was present. Ingenuity Pathway Analysis confirmed that pathogen-induced cytokine storm signaling was strongly elevated in infected senescent cells but attenuated by acetate treatment, while pathways linked to cellular senescence itself were unaffected. Acetate also enhanced pathways related to oxidative phosphorylation and reactive oxygen species production, suggesting it pushes cells toward a metabolic state less permissive to viral replication. The enzyme ACSS2, which activates acetate for histone modification, was upregulated in senescent fibroblasts, and histone extracts revealed that influenza infection decreased H3 acetylation in senescent cells, a reduction that acetate supplementation restored. Histone deacetylase activity, reduced at baseline in senescent cells, was likewise normalized by acetate. This coordinated epigenetic remodeling likely underlies the broad transcriptional changes that suppress both viral replication and cytokine overproduction.

The authors are careful to note the limits of the current work. Direct quantification of short-chain fatty acids, metabolomic profiling of primary old human lung tissue, and validation in clinical cohorts will be needed to establish a firm causal relationship between microbiota-derived metabolites and antiviral resilience in aging humans, and to determine whether circulating acetate levels correlate with influenza severity in older patients. Even so, the study delivers a compelling translational framework: age-associated gut dysbiosis reduces the microbial capacity to produce acetate, and this decline in metabolite availability directly compromises antiviral responses in senescent lung cells through FFAR2/FFAR3 signaling and histone acetylation. Because individuals over sixty experience substantially higher morbidity, mortality, and prolonged hospitalization from influenza, the identification of a modifiable, microbiota-derived mediator opens a concrete therapeutic avenue. Host-directed interventions that boost acetate availability, whether through diet, probiotics, or targeted metabolite supplementation, could one day help restore antiviral resilience in the aging lung, complementing vaccines rather than replacing them.

Subject of Research: Age-associated gut microbiota dysbiosis, short-chain fatty acid metabolism, and impaired antiviral defense during influenza infection

Article Title: Age‐Associated Gut Microbiota Dysbiosis Impairs Antiviral Defense During Influenza Virus Infection and Identifies Acetate as a Protective Mediator in Senescent Human Lung Cells

Article References: Wernike, C., Häder, A., Reisser, Y., Garakani, K., Hornung, F., Le Saux, C. J., & Deinhardt‐Emmer, S. (2026). Age‐Associated Gut Microbiota Dysbiosis Impairs Antiviral Defense During Influenza Virus Infection and Identifies Acetate as a Protective Mediator in Senescent Human Lung Cells. Aging Cell, 25(10), Article e70750. https://doi.org/10.1111/acel.70750

Image Credits: AI Generated

DOI: 10.1111/acel.70750

Keywords: gut microbiota, aging, influenza A virus, acetate, short-chain fatty acids, gut-lung axis, cellular senescence, FFAR2, histone acetylation, antiviral immunity, dysbiosis, lung fibroblasts