How AMPs Defend Your Lungs From Infection

Defensins Against Influenza and COVID

30–50% of neutrophil protein

Defensins make up 30 to 50% of the total protein content in human neutrophil granules, making them one of the most abundant antimicrobial weapons in the body.

Ganz et al., Journal of Clinical Investigation, 1985

Ganz et al., Journal of Clinical Investigation, 1985

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Every breath pulls roughly 10,000 liters of air through your lungs each day, along with bacteria, viruses, fungi, and particulate matter. Yet the airways remain sterile below the vocal cords in healthy individuals. The reason is not just mucus or coughing. Your respiratory epithelium produces a chemical arsenal of antimicrobial peptides (AMPs) that kill pathogens on contact, recruit immune cells to infection sites, and regulate inflammation after the threat is neutralized. For a broader look at how these peptides fight respiratory viruses specifically, see the pillar article on defensins against influenza and COVID.

The two dominant AMP families in the lungs are defensins and cathelicidins. Together, they form an overlapping defense system where multiple peptides target the same pathogen through different mechanisms, making resistance far harder to develop than resistance to conventional antibiotics.

The Two Families: Defensins and Cathelicidins

Human airway antimicrobial peptides belong primarily to two families that evolved independently but converge on similar functions.

Defensins

Defensins were first isolated from human neutrophils in 1985 by Ganz and colleagues, who identified three small peptides (HNP-1, HNP-2, and HNP-3) with molecular weights below 3,500 daltons that killed bacteria, fungi, and enveloped viruses in vitro.^[1] These alpha-defensins are stored in neutrophil granules and released when neutrophils encounter pathogens in the airways.

Beta-defensins are produced by the airway epithelial cells themselves. Singh and colleagues demonstrated in 1998 that human airway epithelia express HBD-1 constitutively (always on) and upregulate HBD-2 in response to bacterial infection or inflammatory signals.^[2] This means the lungs maintain a baseline defensin shield at all times and can rapidly amplify it when pathogens are detected.

Cole's 2002 review established the broader role of defensins in lung biology, documenting that they function not just as direct microbicides but also as signaling molecules that bridge innate and adaptive immunity.^[3] Defensins recruit dendritic cells to the airway, which then present pathogen fragments to T-cells, activating the adaptive immune response. This dual role, killing pathogens directly while summoning reinforcements, makes defensins far more than simple antibiotics.

For a comprehensive overview of the defensin family beyond the lungs, see the cross-cluster article on defensins as your body's first line of defense.

Cathelicidins

LL-37 is the only cathelicidin expressed in humans. It is a 37-amino-acid peptide cleaved from its precursor protein hCAP-18 by proteinase 3 in neutrophil granules or by serine proteases on epithelial surfaces. Kosciuczuk and colleagues reviewed the cathelicidin family across species in 2012, noting that while other animals express multiple cathelicidins, humans rely on this single peptide for an extraordinary range of functions.^[4]

Bucki and colleagues characterized LL-37 as a "multitask antimicrobial peptide" in their 2010 review, documenting its ability to kill Gram-positive and Gram-negative bacteria, fungi, and enveloped viruses while simultaneously binding and neutralizing bacterial lipopolysaccharide (LPS), recruiting immune cells through chemotaxis, promoting wound healing, and modulating inflammatory cytokine production.^[5]

Tjabringa and colleagues specifically examined LL-37's role in pulmonary immunity, demonstrating that it is expressed by airway epithelial cells and alveolar macrophages and secreted into the airway surface liquid where it directly contacts inhaled pathogens.^[6] The concentration of LL-37 in healthy airway fluid is sufficient to kill common respiratory bacteria, and it increases dramatically during infection. The sibling article on cathelicidins and respiratory viruses covers LL-37's antiviral mechanisms in depth.

How Airway AMPs Kill Pathogens

Antimicrobial peptides use fundamentally different killing mechanisms than conventional antibiotics, which is why bacteria struggle to develop resistance against them.

Brogden's 2005 review, one of the most cited papers in the AMP field, described two primary mechanisms.^[7] The first is membrane disruption: cationic (positively charged) AMPs are electrostatically attracted to the negatively charged bacterial membrane. Once bound, they insert into the lipid bilayer and form pores, causing the cell contents to leak out. Three models describe different pore geometries: the barrel-stave model (peptides line up like staves of a barrel), the toroidal pore model (peptides and lipids curve together to form a pore), and the carpet model (peptides coat the membrane surface until it disintegrates).

The second mechanism is intracellular targeting. Some AMPs cross the bacterial membrane without destroying it and instead inhibit DNA replication, RNA transcription, protein synthesis, or cell wall formation inside the bacterium. This dual-mechanism attack means a bacterium would need to simultaneously alter both its membrane composition and its intracellular targets to develop resistance, a much higher evolutionary bar than single-target antibiotics require.

In the airways specifically, defensins and LL-37 operate in the thin airway surface liquid (ASL) that coats the epithelium. The salt concentration, pH, and protein composition of this fluid affect AMP activity. This becomes clinically relevant in diseases like cystic fibrosis, where altered ASL composition may impair AMP function.

Antiviral Activity: Beyond Bacteria

Airway AMPs are not limited to killing bacteria. Both defensins and LL-37 show activity against respiratory viruses through distinct mechanisms.

Tripathi and colleagues demonstrated in 2013 that LL-37 inhibits influenza A virus through a mechanism that differs from its antibacterial action.^[8] Rather than forming pores in the viral envelope, LL-37 directly disrupts the viral membrane and interferes with the ability of the virus to bind and enter host cells. This antiviral activity was observed at concentrations achievable in the airway during infection.

Currie and colleagues showed that LL-37 also has antiviral activity against respiratory syncytial virus (RSV), a leading cause of lower respiratory tract infections in infants and the elderly.^[9] RSV currently lacks an effective vaccine or disease-modifying treatment, making endogenous antimicrobial peptides one of the few natural defense mechanisms against it.

The pillar article on defensins against influenza and COVID covers the broader evidence for AMP antiviral activity across respiratory virus families.

Immune Signaling: The Second Job

Killing pathogens directly is only half of what airway AMPs do. The other half is orchestrating the immune response.

Aarbiou and colleagues reviewed the role of defensins in inflammatory lung disease in 2002, documenting that defensins participate in every phase of the pulmonary immune response.^[10] In the initial phase, they kill invading pathogens. In the recruitment phase, they act as chemokines, attracting neutrophils, monocytes, and dendritic cells to the site of infection. In the resolution phase, they help shut down inflammation after the threat has been cleared.

Niyonsaba and colleagues showed in 2007 that beta-defensins also stimulate epithelial cell migration, proliferation, and wound healing, functions that help repair tissue damage caused by infection or inflammation.^[11] This regenerative function is particularly important in the lungs, where damage to the epithelial barrier can lead to secondary infections.

This immunomodulatory capacity is why AMPs are sometimes called "host defense peptides" rather than simply "antimicrobial peptides." Their influence on the immune system may be as important as their direct microbicidal effects. For context on how vitamin D regulates LL-37 production, see that dedicated article.

When Airway AMP Defense Fails: Cystic Fibrosis

Cystic fibrosis (CF) provides the clearest clinical evidence that airway AMP function matters. CF patients suffer from chronic, progressive lung infections that are the primary cause of morbidity and mortality in the disease.

Chen and colleagues measured beta-defensin and LL-37 concentrations in bronchoalveolar lavage fluid from CF patients with mild lung disease versus healthy controls.^[12] They found altered concentrations of these antimicrobial peptides in the CF airway, supporting the hypothesis that impaired AMP function contributes to the chronic infection cycle.

Cabak and colleagues tested the activity of airway antimicrobial peptides specifically against common CF pathogens, including Pseudomonas aeruginosa, one of the most difficult-to-treat bacteria in CF lungs.^[13] The results showed that airway AMPs have measurable activity against these pathogens, but the altered salt concentration and mucus composition of CF airways may reduce their effectiveness in vivo.

The CF connection is not just about reduced AMP levels. The disease alters the airway surface liquid in ways that directly impair peptide function: higher salt concentrations inhibit defensin activity, thickened mucus prevents AMPs from reaching bacteria, and chronic inflammation changes the protease environment that activates LL-37 from its precursor. This multi-layered impairment explains why simply adding more AMPs to CF airways has proven difficult as a therapeutic strategy.

Emerging Therapeutic Approaches

The observation that natural airway AMPs protect against infection has driven efforts to develop AMP-based therapies for respiratory diseases. Several approaches are under investigation:

Boosting endogenous production. Rather than administering AMPs directly, some strategies aim to increase the body's own production. Vitamin D supplementation upregulates LL-37 expression in airway epithelial cells, and observational studies have linked vitamin D deficiency with increased respiratory infection rates.

Inhaled synthetic AMPs. Engineered peptides based on defensin and LL-37 structures but modified for greater stability, broader activity, or reduced toxicity are being developed for nebulized delivery directly to the airways. The challenge is maintaining activity in the complex airway environment while avoiding damage to host epithelial cells.

Combination with conventional antibiotics. AMPs and antibiotics can synergize when used together. AMPs that disrupt bacterial membranes can increase the intracellular concentration of antibiotics that target internal processes, potentially restoring efficacy against resistant strains. Cross-cluster context on antimicrobial peptides as antibiotic alternatives covers the broader resistance question.

None of these approaches has yet reached routine clinical use for respiratory infections. The gap between the robust in vitro and animal data and clinical application reflects the complexity of delivering active peptides to the right location in the lung at sufficient concentrations without causing inflammation or toxicity.

The broader challenge of antimicrobial peptides in wound care faces similar delivery hurdles, though topical application to skin wounds is technically simpler than aerosolized delivery to the deep lung.

The Redundancy Advantage

One of the striking features of airway AMP defense is its redundancy. The lungs do not rely on a single antimicrobial peptide. Multiple defensins and LL-37 are present simultaneously, each with overlapping but non-identical activity spectra. HBD-1 provides constitutive baseline protection. HBD-2 is induced during infection for reinforcement. Alpha-defensins arrive with recruited neutrophils. LL-37 adds antiviral and immunomodulatory functions that defensins alone do not provide.

This layered architecture means that losing any single peptide does not eliminate defense. A pathogen that evolves resistance to one AMP still faces several others with different mechanisms. This redundancy also explains why AMP resistance is rare in nature compared to antibiotic resistance: the evolutionary cost of simultaneously evading multiple membrane-disrupting and intracellular-targeting peptides is prohibitively high for most bacteria. For more on this resistance question, see can bacteria become resistant to antimicrobial peptides.

The Bottom Line

The human respiratory tract produces defensins and cathelicidins that provide continuous antimicrobial protection against inhaled pathogens. These peptides kill bacteria, viruses, and fungi through membrane disruption and intracellular targeting while simultaneously recruiting immune cells and promoting tissue repair. Impaired AMP function in conditions like cystic fibrosis demonstrates the clinical importance of this defense system. Therapeutic applications of airway AMPs remain in early development, with challenges around delivery, stability, and toxicity still unresolved.

Frequently Asked Questions

What antimicrobial peptides are found in the lungs?

The major antimicrobial peptides in human lungs are alpha-defensins (HNP-1, HNP-2, HNP-3) released from neutrophils, beta-defensins (HBD-1 through HBD-4) produced by airway epithelial cells, and the cathelicidin LL-37, which is made by both neutrophils and epithelial cells. Together, these peptides form an overlapping defense system that kills bacteria, viruses, and fungi in the airway surface fluid.

How do antimicrobial peptides kill bacteria in the airways?

Airway antimicrobial peptides kill bacteria primarily through two mechanisms. First, they form pores in bacterial membranes, causing cell contents to leak out and the bacterium to die. Second, some peptides cross the membrane and inhibit essential processes like DNA replication and protein synthesis inside the cell. This dual attack makes it very difficult for bacteria to develop resistance.

What is LL-37 and what does it do in the lungs?

LL-37 is the only cathelicidin antimicrobial peptide expressed in humans. In the lungs, it kills bacteria, fungi, and viruses on contact; neutralizes bacterial toxins like lipopolysaccharide; recruits immune cells including neutrophils and dendritic cells to infection sites; and promotes tissue repair. Bucki et al. (2010) described it as a "multitask" peptide because of this range of functions.

Why do cystic fibrosis patients get chronic lung infections?

Cystic fibrosis alters the airway surface liquid in ways that impair antimicrobial peptide function. Higher salt concentrations inhibit defensin activity, thickened mucus prevents peptides from reaching bacteria, and altered protease activity affects LL-37 processing. Chen et al. (2004) found altered defensin and LL-37 concentrations in CF patient airways, contributing to the cycle of chronic infection.

Can antimicrobial peptides fight respiratory viruses?

Yes. LL-37 has demonstrated activity against both influenza A virus and respiratory syncytial virus (RSV) in laboratory studies. Tripathi et al. (2013) showed that LL-37 inhibits influenza A through direct disruption of the viral membrane, a mechanism distinct from its antibacterial action. Currie et al. (2013) demonstrated similar antiviral effects against RSV.

Can you boost antimicrobial peptide levels in the lungs?

Vitamin D supplementation increases LL-37 production in airway epithelial cells, and vitamin D deficiency has been linked to higher respiratory infection rates. Other approaches under investigation include inhaled synthetic AMPs and combination therapies pairing AMPs with conventional antibiotics. None of these approaches has reached routine clinical use for respiratory infections yet.