The microbiome is an exceptionally active community of microbes that influence nearly, if not all, aspects of human physiology. From the synthesis of vitamins, neurotransmitters, and key signaling metabolites such as short-chain fatty acids, to the modulation of brain and immune function, these various microbial niches are an imperative to long term health.
While the equilibrium of the microbiome is largely stable throughout life, two exceptions emerge during infancy and aging. The microbiome initially undergoes a prominent shift from birth to the first years of life, quickly assembling and maturing into a diverse, trained landscape capable of engaging in an immense number of roles that govern host metabolism. Aging produces the second, although far less adaptive, shift toward greater incidence of dysbiosis and dysregulation.
It has been suggested that this steady decline in microbial performance occurs in pairwise fashion with compromised immune surveillance. The host-microbiome interaction is not passive, but characterized by continuous and dynamic cross-talk. The microbiome and immune system are intricately linked with the microbiome controlling immunometabolism and immune activation, and the immune system feeding back to regulate microbial composition, proliferation, and activity.
The self-reinforcing layer of immunosurveillance filters and imposes a constraint that molds and trains microbial function, and maintains order in conjunction with ecological and environmental forces. As immunosenescence progressively sets in over the course of the later decades of life, it loses its hold on microbial populations. The microbiome is subsequently able to elude the guiding hand of the immune system, facilitating the development of pathologies such as tumor immunoevasion.
A number of distinct mucosal immune populations dictate microbial composition and proliferation and act as a surveillance network. These include mucosal-associated invariant T (MAIT) cells, naive T cells, innate effectors, innate lymphoid cells, and IgA-producing plasma cells. Epithelial cells detect damage- and pathogen-associated molecular patterns (DAMPs and PAMPs) as well as inflammasome activation, and secrete antimicrobial peptides and cytokines based on what they detect. The innate lymphoid cells of the lamina propria interpret these signals, translate them to guide immune response, and feed back on the epithelial cells to stimulate or restrain their activity. MAIT cells are also involved in cytokine release to graduate microbial growth. The secretory antibodies IgA, IgM, and IgD, create a secondary layer of calibration, that of mucosal antibody surveillance. IgA and IgG coordinate to identify immune-activating species and stimulate effector response.
Signals produced by the entirety of the microbiome inform the immune system of the microbial status. The cell wall components lipopolysaccharide and lipoteichoic acid are shed by bacteria and recognized by toll-like receptor 4 (TLR4) and 2 (TLR2), respectively. They serve as indicators to the immune system of bacterial load within a specific compartment. Metabolic and proliferative activity are elucidated via the help of peptidoglycan fragments released during bacterial cell wall remodeling. Bacterial motility is evidenced by flagellin, read by TLR5, and signals both load and proliferation. Finally, TLR8 identifies single-stranded RNA that correlates with living microbial content. Collectively, these signals relay information to the immune system about the total load, proliferation, and viability of the microbiome and enable ongoing immunosurveillance and curation.
The result of the aforementioned immune activities prohibit harmful, rapidly-dividing bacteria from dispersing and colonizing new niches. Pathobionts, commensal bacterial with pathogenic potential, almost always exist in small quantities. They are merely restrained by effective immunosurveillance and competitive pressure by other commensals. The orchestration of localized immune defense reduces immune burden while ensuring high levels of microbial diversity. Proliferation is held in check and no single species is allowed to monopolize resources in order to become dominant. As one taxon begins to disproportionately grow and prevail at the expense of others, the immune system imposes a constraint that serves to restore and sustain community diversity.
With age, the performance of mucosal immune cell populations deteriorates. The ability to quickly sense overabundance of one particular taxon and intervene accordingly erodes. Competitive exclusion is no longer restrained and opportunistic pathogens bully their way into dominance. Thymic involution and hematopoietic stem cell aging result in fewer naive lymphocytes, and both receptor diversity and IgA specificity decline. Cytokines predominate and a leaky intestinal lining permits passage of microbial products that stimulate innate immunity. The immune system becomes less discerning and discriminatory, immune cells display constitutive activation, and a chronic inflammatory tone consistent with immunosenescence is observed. As immune suppression falters, the conditionally stable state of a diverse microbiome is disrupted. In the end, expansion of opportunistic microbial species encroach upon commensal populations and disorder the microbiome structure.
When studying centenarians, what is notable is the relative enrichment of health-associated taxa and containment of opportunists and pathobionts. In age-associated dysbiosis, on the other hand, this evenness and distribution of core taxa is lost with concomitant opportunistic overgrowth that correlates with metabolic dysfunction and chronic disease. Constitutive immune activation and impaired immune cell function not only translate into lessened immune pressure on a rebellious microbiome, but also suggest a loss of precision and subsequent misapplied activity.
How do we restore immunosurveillance that has lost its direction and potency? IL-7 administration has been shown to restore naive T-cell expansion and receptor diversity. Thymic regeneration may also enable a more robust immune cell repertoir. Use of senolytics to dispose of senescent cells may act to reduce inflammatory burden and immune confusion. Pre-, pro-, and synbiotics are being explored as potential modulators of microbial diversity and proliferation. However, microbiome stabilization is not as easy as supplying copious probiotics. A deficient surveillance network overburdened by the introduction of diverse microbial taxa may exacerbate dysbiosis and gut infection. The most comprehensive approach must coordinately address immunosurveillance and community composition as opposed to one or the other independently, and apply interventions during the most opportunistic windows.
Liu S, Costa FS, Valenzano DR. Immune surveillance and microbial escape in the aging host: Why does the microbiome lose its balance?. PLoS Biol. 2026;24(5):e3003815. Published 2026 May 20. doi:10.1371/journal.pbio.3003815