The Airway Remembers: How Microbial Exposure Can Build Resilience—or Lock the Lung into Disease

The lining of the respiratory tract is often described as a barrier. That description is incomplete.

The airway epithelium is a living, metabolically active tissue that detects microbes, communicates with immune cells, produces mucus and antimicrobial molecules, regulates inflammation, and repairs itself after injury. It also appears capable of remembering what it has encountered.

A recent review in Cell Reports, “Microbial imprinting of the airway epithelium,” proposes that viruses, bacteria, microbial products, and microbiome-derived metabolites can leave lasting metabolic and epigenetic marks on airway epithelial cells. These imprints may alter how the airway responds to later infections, allergens, and inflammatory challenges. The authors organize these effects into four broad patterns: altered differentiation, tolerance, priming, and trained immunity.

The review does not use the terms metabolic congestion, carbon rerouting, mitochondrial throughput limitation, or gridlock. However, many of the mechanisms it describes fit naturally within this wider bioenergetic model.

Together, they suggest a simple but powerful principle:

Health depends not only on whether the body responds to a challenge, but on whether it has enough time, energy, and resources to adapt and then complete recovery.

The full physiological sequence is:

Respond → Adapt → Recover

When the sequence is completed, the airway may retain a protective, well-calibrated memory. When recovery is delayed or energetically constrained, temporary adaptation may become chronic remodeling, excessive inflammation, or vulnerability to future disease.

The airway is a memory-bearing tissue

Airway epithelial cells are among the first cells to encounter inhaled viruses, bacteria, allergens, smoke, dust, and pollutants. They recognize these exposures through pattern-recognition receptors and respond by producing interferons, cytokines, chemokines, antimicrobial peptides, reactive oxygen species, and mucus.

But these responses may not disappear completely when the original challenge ends.

The review summarizes evidence that microbial exposure can produce durable changes in:

• DNA methylation and chromatin accessibility

• Glucose and mitochondrial metabolism

• Interferon and inflammatory responsiveness

• Basal-cell proliferation

• Differentiation into ciliated, secretory, or mucus-producing cells

• Barrier repair and mucociliary clearance

Some of these changes may improve future protection. Others may leave the airway excessively reactive, structurally altered, or less capable of handling the next challenge.

This is why epithelial memory cannot be classified as simply good or bad. Its meaning depends on the trajectory through which it was formed.

Respond: defence creates metabolic pressure

The response phase begins when the airway detects danger.

During infection or injury, epithelial cells must rapidly increase the production of interferons, cytokines, antimicrobial molecules, mucus, membrane components, and replacement cells. They must also coordinate with immune cells and maintain the physical barrier while damage is occurring.

All of this requires energy and material resources.

The tissue needs ATP, amino acids, glucose, fatty acids, oxygen, reducing equivalents, nucleotides, and membrane lipids. Mitochondria must process more electrons, regenerate oxidized cofactors such as NAD⁺, regulate reactive oxygen species, and support both defence and repair.

This creates what we may call metabolic pressure.

More substrate enters the system. More carbon must be processed. More reducing equivalents must be oxidized. More biosynthetic work must be completed. Yet mitochondrial oxidative throughput cannot increase without limit.

When substrate delivery and cellular demand temporarily exceed oxidative processing

capacity, the system begins to experience congestion.

Congestion does not initially mean failure. It means that more carbon, electrons, and metabolic demand are arriving than can be handled through the usual oxidative route at that moment.

The body must adapt.

Adapt: carbon is rerouted to sustain defence

During an acute challenge, carbon does not simply flow through mitochondria to produce ATP. It is redistributed according to immediate priorities.

The review describes increased glucose uptake, glycolytic activation, mTOR-dependent metabolic reprogramming, altered mitochondrial respiration, lactate-related signaling, and changes in fatty-acid oxidation. These mechanisms are consistent with adaptive carbon rerouting.

Glucose-derived carbon may be redirected toward:

• Rapid glycolytic ATP production

• Lactate formation and NAD⁺ regeneration

• Nucleotide synthesis

• Protein and cytokine production

• Membrane synthesis

• Mucus production

• Antioxidant and redox support

• Proliferation of epithelial progenitor cells

This rerouting is not inherently pathological. It is a form of metabolic triage.

During immediate danger, the system prioritizes containment and survival over long-term efficiency. Glycolysis can support rapid basal-cell expansion even when mitochondrial oxidation cannot immediately match demand. Carbon intermediates can be retained for biosynthesis rather than fully oxidized to carbon dioxide.

The problem arises when a temporary emergency program cannot be reversed.

The review’s four forms of epithelial adaptation

The authors describe four broad forms of microbial imprinting.

Altered differentiation

Microbial infection or inflammation may change how epithelial progenitor cells rebuild the airway.

Viral infection can reduce ciliated-cell formation, expand mucus-producing goblet cells, or leave basal progenitor cells in abnormal transitional states. Influenza-associated inflammatory signaling may promote dysplastic basal-cell populations. RSV and post-viral inflammation can alter epithelial differentiation long after the acute infection has passed.

From a bioenergetic perspective, altered differentiation may reflect not only changes in signaling but also a failure to complete the metabolic transition from proliferation to maturation.

Tolerance

Repeated low-level microbial exposure may reduce unnecessary inflammatory responses to later stimulation.

The “farm effect” provides an important example. Chronic exposure to low levels of microbial endotoxin can induce the regulatory protein A20 in airway epithelial cells. A20 restrains excessive NF-κB signaling and may reduce later allergic inflammation.

This is not immune weakness. It is better regulation.

The tissue has responded, adapted, recovered, and raised the threshold for unnecessary inflammation. In this setting, tolerance resembles a form of protective hormesis.

Priming

A prior microbial signal may leave the epithelium more responsive to a later challenge.

Microbiome-derived acetate, butyrate, and 18-HEPE can strengthen epithelial interferon responses and improve antiviral defence in experimental models. Previous rhinovirus exposure may also temporarily enhance antiviral protection against another virus.

Priming can therefore support faster pathogen control. But excessive priming can also amplify oxidative stress, allergic inflammation, or tissue injury.

Trained immunity

The original response may return toward baseline while metabolic and epigenetic changes remain.

When a second challenge occurs, the airway responds differently—often more strongly—even when the second trigger is not identical to the first. The review describes trained-immunity-like responses after exposure to flagellin, β-glucan, or repeated rhinovirus infection.

Such training may involve mTOR-regulated glycolysis, altered chromatin organization, and changes in inflammatory gene accessibility.

These four states are best understood not as fixed categories, but as possible outcomes of adaptation whose value depends on context, timing, and recovery.

Throughput limitation: when mitochondrial processing cannot keep pace

A central question is what determines whether adaptive rerouting resolves or becomes persistent.

One answer may be mitochondrial throughput: the capacity of the mitochondrial system to accept substrate-derived electrons, regenerate NAD⁺, maintain redox balance, support ATP production, and preserve enough reserve for repair and differentiation.

The review highlights several mechanisms relevant to this capacity.

Mitochondrial pyruvate transport is required for normal basal-cell function. When pyruvate entry into mitochondria is disrupted, epithelial regeneration becomes impaired and intermediate cell states accumulate.

Complex I activity contributes to NAD⁺ regeneration. When mitochondrial NAD⁺ production is inadequate, integrated stress-response pathways can become activated and interfere with normal alveolar epithelial differentiation.

Mitochondrial dysfunction during bacterial infection may also reduce NAD⁺ availability and weaken epithelial defence.

These observations point toward a broader mechanism:

When carbon delivery, reducing-equivalent production, and biosynthetic demand exceed mitochondrial oxidative throughput, the epithelium must increasingly rely on rerouting. If throughput is restored, recovery can proceed. If it remains constrained, congestion persists.

Lactate: a valve, fuel, and signal

Lactate is often described simply as a waste product. That view is outdated.

When pyruvate is converted to lactate, NADH is oxidized back to NAD⁺. This allows glycolysis to continue when mitochondrial oxidation cannot process all available pyruvate and reducing equivalents.

Lactate can therefore function as a metabolic valve.

During an acute response, this may be protective. It prevents glycolytic arrest and allows epithelial cells to continue producing ATP and biosynthetic intermediates.

But lactate also circulates between tissues and acts as a signal. The review discusses evidence that gut bacterial lactate can influence lung epithelial mitochondrial activity and may aggravate acute lung injury under some conditions.

Thus, lactate may have several meanings at once:

• A sign of increased glycolytic flux

• A route for NAD⁺ regeneration

• A transported carbon substrate

• A signal between microbial and host systems

• A potential contributor to metabolic pressure in another tissue

Its effect depends on dose, duration, tissue context, and whether the receiving system has sufficient oxidative capacity.

Succinate and the consequences of accumulated carbon intermediates

The review also discusses gut microbiota-derived succinate.

Succinate is a normal TCA-cycle intermediate, but under particular conditions it can become a signaling molecule and a substrate for protein succinylation. In experimental allergic airway inflammation, succinate promoted succinylation of the antioxidant enzyme SOD2, reduced its activity, increased oxidative stress, and worsened inflammation.

This illustrates a key feature of metabolic congestion: intermediates are not biologically neutral simply because they belong to a normal pathway.

When production, delivery, or accumulation exceeds downstream processing, metabolites can:

• Alter enzyme activity

• Modify proteins

• Change chromatin regulation

• Influence inflammatory signaling

• Increase oxidative stress

• Reinforce the original metabolic disturbance

Carbon that cannot move efficiently through one route may be redirected into signaling, storage, export, or post-translational modification.

Redox pressure and NAD⁺ regeneration

Carbon flow and electron flow are inseparable.

As glucose and fatty acids are metabolized, electrons are transferred to NAD⁺ and FAD, producing NADH and FADH₂. These reducing equivalents must ultimately be oxidized by the mitochondrial electron-transport system.

When electron input exceeds respiratory processing, NADH may accumulate relative to NAD⁺. This creates reductive pressure and restricts reactions that require oxidized NAD⁺.

The consequences can include:

• Slower pyruvate oxidation

• Greater reliance on lactate formation

• Altered TCA-cycle activity

• Increased reactive oxygen species

• Reduced metabolic flexibility

• Impaired differentiation and repair

• Activation of cellular stress responses

The review’s discussion of complex I, NAD⁺ metabolism, mitochondrial dysfunction, and epithelial cell fate therefore connects directly to the concept of throughput limitation.

ROS: protective signal or evidence of unresolved pressure?

Reactive oxygen species are another example of a mechanism whose meaning depends on timing and magnitude.

Controlled mitochondrial ROS production can support antimicrobial defence and cellular signaling. The review describes epithelial immunometabolic programs in which mitochondrial ROS contribute to protection against bacterial infection.

But persistent or excessive ROS can damage proteins, membranes, DNA, and mitochondria. It can also amplify inflammatory signaling and worsen allergic airway disease.

The critical distinction is therefore not whether ROS are produced, but whether the system can regulate and resolve them.

A temporary increase in ROS may be part of the response.

Persistent ROS after the threat has passed may indicate that electron flow, antioxidant capacity, mitochondrial quality control, or metabolic recovery remains incomplete.

Recover: repair requires a metabolic handover

Recovery is not the passive disappearance of inflammation. It is a coordinated and energetically demanding biological program.

The epithelium must terminate unnecessary inflammatory signaling, clear damaged material, rebuild tight junctions, restore ciliated cells, normalize mucus production, repair mitochondria, replenish metabolic reserves, and complete cellular differentiation.

The review describes an important metabolic sequence during epithelial repair:

Early repair depends substantially on glycolysis to support basal-cell proliferation.

Later repair requires a shift toward fatty-acid oxidation and mitochondrial metabolism to support epithelial maturation and restoration of a fully functional barrier.

This is a metabolic handover.

The early glycolytic state is appropriate for rapid expansion. But the tissue cannot remain indefinitely in a proliferative, inflammatory, carbon-rerouting state. It must regain oxidative flexibility and move toward differentiation.

Successful recovery therefore requires:

Glycolytic mobilisation→ progenitor-cell expansion→ mitochondrial restoration→ oxidative maturation→ balanced epithelial differentiation

If this handover occurs, the airway restores function while retaining useful memory.

If it fails, the epithelium may remain caught between inflammation, proliferation, and incomplete differentiation.

Gridlock: when adaptation cannot transition into recovery

We may describe this unresolved state as metabolic and cellular gridlock.

Gridlock does not mean that metabolism has stopped. On the contrary, many pathways may remain highly active.

Glucose uptake may remain elevated. Glycolysis may continue. Cytokines and mucus

may still be produced. Basal cells may continue proliferating. Mitochondria may remain stressed. ROS may remain elevated. Carbon may continue to be rerouted.

Yet the system is no longer progressing toward restoration.

It is active but not resolving.

At the tissue level, this may appear as:

• Basal-cell hyperplasia

• Goblet-cell expansion

• Excess mucus production

• Reduced ciliated-cell formation

• Impaired mucociliary clearance

• Persistent inflammatory signaling

• Abnormal transitional cell states

• Increased susceptibility to later infection or allergy

The pathway may therefore shift from:

Challenge → Response → Adaptation → Recovery

to:

Challenge → Response → Persistent adaptation → Throughput limitation → Gridlock

In this state, a program that was initially protective becomes a source of chronic vulnerability.

Time determines whether adaptation remains useful

Biological responses have a time dimension.

Inflammation that is beneficial for hours can become harmful when maintained for weeks. Glycolysis that supports early repair can obstruct maturation if the tissue cannot transition back toward oxidative metabolism. Mucus that traps pathogens can impair airflow and clearance when overproduced chronically.

The timing of the next exposure is therefore crucial.

When challenges are adequately spaced, the airway may have time to:

• Resolve inflammation

• Restore mitochondrial capacity

• Regenerate NAD⁺

• Repair epithelial damage

• Complete differentiation

• Replenish metabolic reserves

• Consolidate protective memory

When challenges arrive too frequently, recovery may remain incomplete.

The next response then begins not from a restored baseline but from a state of residual inflammation, altered metabolism, depleted reserve, or incomplete repair.

Repeated exposure can create a progressive sequence:

Response → partial recovery → new response → less complete recovery → chronic adaptation

This may be especially important during early life, when the airway epithelium, immune system, and microbiome are still developing.

Energy and resources determine whether recovery is possible

Time alone is not enough. Recovery also requires resources.

Repair requires ATP, oxygen, amino acids, fatty acids, glucose, micronutrients, redox support, mitochondrial quality control, vascular delivery, and functional progenitor cells.

A system facing repeated infection, nutrient limitation, sleep disruption, pollution, chronic inflammation, or poor mitochondrial reserve may possess the biological instructions for recovery without having enough capacity to execute them fully.

This helps explain why similar exposures can produce different outcomes in different people.

The decisive factors may include:

• Baseline mitochondrial reserve

• Availability of metabolic substrates

• Oxygen and nutrient delivery

• NAD⁺ regeneration

• Antioxidant and redox capacity

• Integrity of epithelial progenitor cells

• Frequency of repeated challenges

• Time available between challenges

• The energetic burden imposed by other organs and immune processes

Recovery is therefore conditional.

Hormesis requires completed recovery

The concept of hormesis is often summarized as “a little stress makes us stronger.” That is incomplete.

A limited challenge can build resilience only when the system has enough capacity to respond, adapt, and recover.

The farm-effect example illustrates this. Low-level microbial exposure activates epithelial sensing and induces A20-mediated regulation. The airway does not simply become less responsive. It becomes more appropriately calibrated.

The sequence is:

Low-level challenge→ controlled response→ regulatory adaptation→ successful recovery→ protective tolerance

But when exposure is excessive, persistent, or repeated before recovery, the same general pathway may lead instead to congestion, oxidative stress, abnormal differentiation, and disease.

Hormesis therefore depends on:

• Appropriate dose

• Limited duration

• Adequate spacing

• Sufficient metabolic reserve

• Successful resolution

Without recovery, repeated stress is not hormesis. It is cumulative burden.

Recovery does not mean returning to a naïve state

A fully recovered airway does not necessarily return to exactly the state it occupied before exposure.

Healthy recovery may retain useful biological information.

The ideal outcome is functional restoration with calibrated memory:

• Barrier integrity returns

• Metabolic flexibility is restored

• Mitochondrial throughput recovers

• Inflammation resolves

• Epithelial diversity is rebuilt

• Mucociliary function normalizes

• Future responses become faster or more proportionate

The tissue remembers, but it is not trapped by the memory.

This is the difference between resilience and chronic imprinting.

A unified model

The review’s epithelial-imprinting framework and the congestion–rerouting–throughput model can be integrated into two contrasting trajectories.

Protective trajectory

Microbial challenge→ increased energetic and biosynthetic demand→ temporary metabolic congestion→ controlled carbon rerouting→ effective defence and cell replacement→ restoration of mitochondrial throughput→ redox normalization→ completion of epithelial differentiation→ protective tolerance or calibrated trained responsiveness

Pathological trajectory

Microbial challenge→ excessive or repeated metabolic pressure→ persistent carbon rerouting→ inadequate NAD⁺ regeneration→ mitochondrial throughput limitation→ sustained ROS and stress signaling→ failure of the proliferation-to-maturation transition→ epithelial gridlock→ mucus hyperplasia, impaired clearance, chronic inflammation, or recurrent disease

The key question is not whether glycolysis, lactate production, ROS signaling, or epithelial remodeling occurs. These may all be necessary during an effective response.

The critical question is whether the system can later reverse the emergency program, clear the congestion, restore oxidative flexibility, and complete recovery.

The wider lesson

The airway epithelium provides a clear example of a broader biological principle.

Stress is unavoidable. Response is necessary. Adaptation preserves function during challenge.

But adaptation is not the final goal.

The response protects us during immediate danger. Adaptation keeps the system functioning while the challenge continues. Recovery determines whether that adaptation becomes resilience or disease.

The airway remembers what happened.

Whether that memory becomes protective tolerance, effective priming, or chronic vulnerability depends on the timing of exposure, the magnitude of metabolic pressure, mitochondrial throughput, and the availability of sufficient energy and resources to complete the cycle.

Respond. Adapt. Recover.

When recovery is completed, the system learns.

When recovery remains incomplete, adaptation can become congestion—and congestion can harden into gridlock.

Reference:

van de Ven, J. J. I., Amatngalim, G. D., Garssen, J., Netea, M. G., & Perdijk, O. (2026). Microbial imprinting of the airway epithelium. Cell Reports, 45, 117705. https://doi.org/10.1016/j.celrep.2026.117705