Defensins and Antimicrobial Peptides: Mechanisms, Barrier Immunity, Disease, and Peptidomics

1. Defensins and Antimicrobial Peptides: Core Families and Biological Features

1.1 Antimicrobial Peptides as Innate Host-Defense Effectors

Most well-studied human AMPs are short and positively charged, with hydrophilic and hydrophobic regions that favor membrane interactions. This pattern is common rather than universal. AMP length, net charge, folding, disulfide content, and post-translational processing vary considerably, as do their cellular sources. Epithelial cells, neutrophils, macrophages, Paneth cells, sweat glands, and salivary glands each contribute distinct peptide pools.

Many AMPs are not released in their final active form. They are translated as precursors and activated by proteolytic cleavage, so transcript abundance alone cannot show which mature peptides are present or active. The term “host-defense peptide” is often used when immune signaling, barrier maintenance, and tissue repair are central to the biology (Mookherjee et al., 2020; Hellinger et al., 2023).

LL-37 illustrates precursor-to-mature AMP processing.

Figure 1. CAMP/hCAP18 organization and proteolytic release of mature LL-37. Reproduced from Voronko et al. (2025), International Journal of Molecular Sciences, 26(16), 8103, under CC BY 4.0; resized without content changes.

1.2 Major Human Defensins and Other Antimicrobial Peptide Families

Human defensins are cysteine-rich peptides whose folds are stabilized by disulfide bonds. Alpha-defensins include neutrophil HNP1-4 and intestinal HD5/HD6; beta-defensins are produced mainly at epithelial surfaces. LL-37, dermcidin-derived peptides, and salivary histatins extend the human AMP repertoire beyond defensins and differ in tissue source and dominant function (Mookherjee et al., 2020; Johnstone & Herzberg, 2022).

Table 1. Major Human AMP Families, Sources, and Functions

2. How Antimicrobial Peptides Work: Microbial Selectivity, Membrane Disruption, and Non-Lytic Functions

Membrane disruption is the best-known AMP mechanism, but no single model fits every peptide. Activity shifts with peptide concentration and aggregation, microbial species and membrane composition, pH, ionic strength, and surrounding proteins or extracellular matrix.

2.1 Electrostatic Selectivity for Microbial Membranes

Negatively charged phospholipids, lipopolysaccharide, lipoteichoic acids, and wall teichoic acids make many microbial surfaces favorable targets for cationic AMPs. After electrostatic attraction brings a peptide to the surface, amphipathic regions can associate with the lipid-water interface and sometimes enter the bilayer. Fungi and enveloped viruses present different lipid and surface chemistries, which helps explain why activity spectra vary among peptides.

Mammalian membranes differ from microbial membranes in outer-leaflet lipids, cholesterol, and surface charge, creating relative rather than absolute selectivity. Greater hydrophobicity or high peptide concentrations may increase cytotoxicity and hemolysis. Salts, serum proteins, mucus, extracellular DNA, and local pH can also reduce the amount of free peptide available for membrane binding (Zhang et al., 2021; Oliveira Júnior et al., 2025).

2.2 Carpet, Barrel-Stave, and Toroidal-Pore Models

Three models are commonly used to describe membrane permeabilization. Surface-bound peptides destabilize the bilayer in the carpet model; barrel-stave assemblies form peptide-lined pores; and toroidal pores are lined by both peptides and curved lipid headgroups. Real membranes often show transient defects or mixed states, so these models are useful descriptions rather than strict categories (Zhang et al., 2021; Oliveira Júnior et al., 2025).

Figure 2. Carpet, barrel-stave, and toroidal-pore models of AMP-mediated membrane disruption. Reproduced from Huan et al. (2020), Frontiers in Microbiology, 11, 582779, under CC BY 4.0; resized without content changes.

2.3 Beyond Membrane Lysis: Intracellular Targets, Microbial Trapping, and Immune Modulation

Some AMPs enter or perturb microbial membranes without causing immediate lysis. Once inside, they may interfere with cell-wall synthesis, ribosomes, nucleic acids, protein folding, or enzymes; others reduce adhesion or biofilm formation. Human alpha-defensin 6 follows a different strategy: it self-assembles into nanonet-like structures that trap bacteria in the intestinal lumen (Chu et al., 2012).

Direct killing is only part of the response. AMPs can recruit leukocytes, neutralize endotoxin, alter cytokine signaling, promote epithelial migration, and support wound repair. A peptide may therefore matter in vivo even when it shows modest killing in a simplified assay. Studies should report whether the observed endpoint is killing, growth inhibition, trapping, antibiofilm activity, or host-cell signaling rather than treating all of them as the same “antimicrobial” effect (Mookherjee et al., 2020).

3. Defensins and Antimicrobial Peptides in Epithelial Barrier Immunity

At healthy barriers, AMPs work alongside epithelial junctions, mucus, ciliary or fluid clearance, resident immune cells, and commensal organisms. Together, these defenses keep microbes away from vulnerable tissue without sterilizing the surface.

3.1 Skin Barrier Defense and Wound Repair

Keratinocytes, resident immune cells, hair follicles, and sweat glands produce overlapping sets of AMPs. Human beta-defensins, LL-37, and dermcidin-derived peptides help limit colonization in the acidic, lipid-rich environment of the skin. Some also influence keratinocyte migration, immune-cell recruitment, angiogenesis, and wound closure. Their effects vary with anatomical site, injury, sweat and sebum composition, and the resident microbiota (Mookherjee et al., 2020).

3.2 Intestinal Crypt Defense and Microbiota Compartmentalization

Paneth cells at the base of small-intestinal crypts release HD5, HD6, lysozyme, and other effectors close to the stem-cell niche. This concentrated secretion limits microbial access to regions of epithelial renewal. Alpha-defensins also help organize host-microbiota spatial relationships, supporting compartmentalization rather than indiscriminate clearance (Wallaeys et al., 2023). HD6 adds a physical layer of defense by forming nanonets that trap microbes (Chu et al., 2012).

Figure 3. Generalized schematic of direct antimicrobial, immune-regulatory, and epithelial-protective AMP pathways at the intestinal barrier. Reproduced from Gong et al. (2021), Frontiers in Nutrition, 8, 751010, under CC BY 4.0; resized without content changes.

3.3 Coordinated AMP Defense Across Respiratory, Oral, and Urogenital Mucosa

Respiratory epithelia secrete beta-defensins and LL-37 into a system that also relies on mucus, mucociliary clearance, surfactant-associated factors, and recruited leukocytes. In the oral cavity, histatins and defensins work with salivary flow and mucosal integrity, with activity against both bacteria and fungi. Urogenital epithelia likewise deploy defensins and cathelicidin as part of locally regulated mucosal defense. At each site, peptide activity is shaped by secretion, diffusion, proteolysis, binding partners, and physical clearance (Johnstone & Herzberg, 2022).

4. Antimicrobial Peptide Dysregulation in Infection and Inflammatory Disease

An AMP increase is not automatically protective, and a decrease does not always indicate weaker defense. Pathogens may evade peptide attack, the local matrix may inactivate the peptide, and host production may be too low, excessive, or misplaced. These processes can reinforce barrier damage, dysbiosis, and persistent inflammation.

4.1 Pathogen Evasion and Microenvironmental Inactivation of AMPs

Bacteria evade AMPs by changing surface charge or membrane lipids, producing capsules, increasing efflux, releasing proteases, or sequestering peptides in extracellular material. Biofilms add diffusion and binding barriers and can shift cells into more tolerant physiological states. Heritable resistance, reversible tolerance, and loss of activity caused by a salt-, protein-, or protease-rich matrix are biologically distinct and should be reported separately (Tajer et al., 2024).

In chronic wounds and persistent airway infections, detecting LL-37 or a defensin does not show how much intact peptide reaches the pathogen. Proteolysis, oxidation, matrix binding, ionic inhibition, and spatial separation can all lower effective exposure (Mookherjee et al., 2020; Tajer et al., 2024).

4.2 AMP Dysregulation: Deficiency, Excess, and Mislocalization

Low production or weak induction can compromise local control, but persistently high expression can also intensify chemotaxis, inflammatory signaling, or tissue injury. Processing, secretion, degradation, and tissue localization further separate total peptide abundance from effective activity.

Skin disease provides clear examples. Atopic dermatitis combines barrier dysfunction, type 2 inflammation, dysbiosis, and altered AMP responses that can favor Staphylococcus aureus colonization (Peng et al., 2026). Psoriasis often shows strong AMP induction in already inflamed tissue. In rosacea and chronic wounds, LL-37 processing, protease activity, or peptide persistence may affect both inflammation and microbial defense (Mookherjee et al., 2020).

4.3 Barrier Damage, Dysbiosis, and Interpreting AMP Changes

Barrier injury increases microbial access and exposes AMPs to altered pH, salts, proteases, plasma proteins, and extracellular DNA. If the peptide layer is weakened or compositionally altered, microbial encroachment can sustain epithelial stress and inflammation. Similar feedback is seen in Paneth-cell dysfunction, chronic wounds, and recurrent airway infections (Mookherjee et al., 2020; Johnstone & Herzberg, 2022; Wallaeys et al., 2023).

Interpreting a measured change requires its compartment and disease context. Tissue abundance, secreted concentration, and circulating levels are not interchangeable, and interpretation should consider the producing cell, anatomical site, disease stage, treatment, microbiome, pathogen burden, and barrier integrity. Higher abundance may reflect effective defense, compensation, or inflammatory amplification; lower abundance may arise from reduced production, loss of producing cells, degradation, or dilution. Mechanistic interpretation requires both identification of the mature peptide and evidence of its biological activity.

5. Therapeutic Development of Antimicrobial Peptides: Opportunities, Barriers, and Engineering

AMP-derived therapeutics are being developed both to kill microbes and to modify host responses such as inflammation, immune-cell recruitment, and repair. Most practical formats emphasize local exposure: topical agents, wound dressings, antimicrobial coatings, delivery systems, antibiotic adjuvants, and peptidomimetics. Local delivery avoids some of the demands of prolonged systemic circulation, and combination use may improve membrane penetration, weaken biofilms, or reduce the dose of a conventional antimicrobial (Mookherjee et al., 2020; Cesaro et al., 2023; Oliveira Júnior et al., 2025).

Development is difficult because peptides can be rapidly degraded, cleared, or bound by serum proteins; activity may fall in physiological salt, and cytotoxicity, hemolysis, manufacturing, delivery, and resistance also require attention. Sequence optimization, cyclization, D-amino-acid substitution, lipidation, peptidomimetics, nanoparticles, hydrogels, and functionalized surfaces are being used to improve stability, selectivity, and local exposure (Cesaro et al., 2023; Oliveira Júnior et al., 2025). For now, AMP-derived molecules are better viewed as complementary tools for topical, biomaterial-associated, host-directed, or combination therapy than as universal antibiotic replacements. Each candidate still needs pharmacology, toxicity, resistance, and efficacy testing in the intended biological setting.

6. Peptidomics and Multi-Omics for Defensin and Antimicrobial Peptide Research

AMP abundance is shaped by transcription, precursor synthesis, proteolytic maturation, secretion, chemical modification, tissue distribution, and degradation. Biological activity depends on the local tissue and microbial environment, while measured activity also varies with assay conditions. Peptidomics measures endogenous peptide forms directly; other omics layers help identify their cellular source, regulation, and biological setting.

6.1 Why Mature-Peptide Measurement Requires Peptidomics

RNA-seq and qPCR measure DEFA, DEFB, CAMP, and related transcripts, but they do not reveal which mature peptides are present. One precursor may yield several cleavage products, and oxidation, truncation, disulfide formation, and other modifications can change stability or activity. Conventional bottom-up proteomics digests proteins with trypsin, whereas endogenous peptidomics aims to recover peptides already present in the sample (Hellinger et al., 2023).

LC-MS/MS peptidomics can identify sequences and cleavage sites, compare peptide abundance, and reveal forms not predicted from gene-level data. Recovery and identification remain difficult for low-abundance peptides, closely related isoforms, highly cationic sequences, and disulfide-rich defensins. A non-detection result is therefore not proof of biological absence.

6.2 Pre-Analytical and Analytical Considerations for AMP Peptidomics

Potential research matrices include skin or intestinal tissue, wound fluid, saliva, sputum or bronchoalveolar lavage fluid, culture supernatant, and organoid-conditioned media. Because endogenous peptide recovery is highly matrix-dependent, sample feasibility, stabilization strategy, extraction conditions, and expected peptide abundance should be confirmed during study design. Collection and processing must suppress ex vivo proteolysis and artificial modification. Rapid cooling or stabilization, validated protease control, low-binding consumables, standardized extraction, limited freeze-thaw cycles, randomized processing, pooled quality controls, and batch monitoring are key design elements (Hellinger et al., 2023).

Record pH, salt concentration, hemolysis or cellular contamination, extraction recovery, and whether disulfide bonds were preserved, reduced, or alkylated. Peptide abundance does not measure antimicrobial activity. Functional assays should specify the microorganism and growth state, medium, salt and protein content, peptide concentration, exposure time, and endpoint so that results can be interpreted alongside LC-MS/MS data.

Figure 4 shows a published wound-fluid workflow that combines peptide extraction, LC-MS/MS, peptide identification, protease analysis, AMP prediction, and data visualization (Hartman et al., 2021).

Figure 4. Representative wound-fluid peptidomics workflow combining peptide extraction, LC-MS/MS, peptide identification, protease analysis, AMP prediction, and visualization. Reproduced from Hartman et al. (2021), Frontiers in Immunology, 11, 620707, under CC BY 4.0; resized without content changes.

6.3 Integrating Peptidomics with Complementary Omics Layers

Transcriptomics measures the expression of defensin genes, CAMP, cytokines, epithelial differentiation programs, and proteases. Proteomics adds information on barrier proteins, immune effectors, processing enzymes, and tissue-damage markers, while peptidomics identifies mature AMP forms and cleavage products. Microbiome profiling describes community composition and pathogen burden, and metabolomics characterizes microbial metabolites, nutrient availability, and redox-associated metabolites in the local environment.

Table 2. Sample and Multi-Omics Strategies for AMP Research

Each omics layer should test a specific part of the proposed mechanism. Reduced HD5 abundance, for example, can be examined together with Paneth-cell markers, protease expression, microbiome encroachment, and intestinal phenotype. In wound fluid, LL-37 forms can be compared with biofilm signatures, host proteases, inflammatory proteins, and healing outcomes. These comparisons can distinguish changes in peptide production from altered processing, degradation, microbial escape, or downstream tissue responses.

7. Frequently Asked Questions About Defensins and Antimicrobial Peptides

Are defensins the same as antimicrobial peptides?

No. Defensins are one major AMP family, alongside cathelicidins, dermcidin-derived peptides, and histatins. They are defined by conserved cysteines and disulfide-stabilized folds, whereas other AMP families differ in structure, source, and function (Mookherjee et al., 2020).

How do antimicrobial peptides kill or control microbes?

Many cationic AMPs bind negatively charged microbial surfaces and disrupt membranes. Others act on intracellular targets, inhibit biofilms, trap microbes, or modulate host immunity. Which mechanism dominates depends on the peptide, microorganism, concentration, and local environment (Zhang et al., 2021; Oliveira Júnior et al., 2025).

Can antimicrobial peptides replace conventional antibiotics?

Not broadly. AMP-derived therapies may suit topical delivery, wound care, coatings, or combination treatment, but stability, toxicity, delivery, manufacturing, and resistance remain important barriers. They are better viewed as complements to conventional antibiotics than as universal replacements (Cesaro et al., 2023; Oliveira Júnior et al., 2025).

Why is peptidomics useful for defensin and AMP research?

Gene and conventional protein measurements may not reveal which mature endogenous peptides are present. Peptidomics can identify cleavage products and compare peptide forms and abundance, but functional assays are still needed because abundance alone does not establish antimicrobial activity (Hellinger et al., 2023).

What samples are suitable for antimicrobial peptide peptidomics?

Potential samples include tissue, wound fluid, saliva, sputum or BALF, culture supernatant, and organoid-conditioned media, depending on the study question. Because endogenous peptides are sensitive to proteolysis and sample handling, matrix feasibility, stabilization strategy, and extraction workflow should be discussed before study launch.

Can RNA-seq replace peptidomics for defensin research?

No. RNA-seq can measure defensin genes, CAMP, cytokines, proteases, and epithelial-response programs, but it cannot determine which mature peptide forms are present. Peptidomics is useful because many AMPs are produced as precursors and become active only after proteolytic processing.