LL-37 — the body's own antibiotic

The World Health Organization has been calling antibiotic resistance one of the greatest threats to global health since at least 2014. By some projections, drug-resistant infections could kill more people annually than cancer does now by mid-century. The pipeline of new antibiotics is thin, because developing new small-molecule antibiotics that can outpace resistance development is structurally difficult — the target mechanisms bacteria use are similar across pathogens, and bacteria mutate their way around any fixed target given enough time and enough generations.

LL-37 is not a new antibiotic. It is older than antibiotics by several hundred million years. It is a 37-amino-acid cationic peptide encoded by the CAMP gene — cathelicidin antimicrobial peptide — and it is part of a family of immune defense molecules that animals, fungi, and plants have been producing since long before Fleming's petri dish. The LL in its name refers to the two leucine residues at its N-terminal end. The 37 is the number of amino acids. The name is functional rather than poetic, which is fitting for a molecule whose job has always been practical.

The human body produces LL-37 in neutrophils — the white blood cells that are among the first responders to bacterial infection — and in the epithelial cells that line the skin, respiratory tract, gastrointestinal mucosa, and urogenital tract. These barrier surfaces are the first point of contact with pathogens, and LL-37 is part of their standing defense infrastructure. It is not produced at baseline in large amounts in most tissues; its expression is upregulated in response to infection, injury, and certain vitamin D signaling, which is part of why vitamin D deficiency is associated with increased susceptibility to respiratory and other infections in epidemiological research.

The mechanism by which LL-37 kills bacteria is categorically different from how most antibiotics work, and this difference is the reason it has attracted intense research interest in the context of antibiotic resistance. Most antibiotics work by targeting a specific bacterial molecular process: penicillins and cephalosporins inhibit cell wall synthesis, fluoroquinolones inhibit DNA replication enzymes, macrolides inhibit the ribosome. Bacteria can develop resistance to these specific molecular targets through mutation or acquired genes, and many have. LL-37 works by a different principle: it is a cationic — positively charged — peptide that is drawn by electrostatic forces to the negatively charged surface of bacterial membranes, inserts itself into the membrane, and disrupts its integrity. The bacterial cell bleeds its contents and dies.

Resistance to membrane disruption is fundamentally harder to develop than resistance to a specific enzyme target. The bacterial membrane is not a single protein that can be mutated — it is the cell's basic boundary, the distinction between inside and outside, and it can only be changed so much before the bacteria loses functions it cannot survive without. This is why cationic antimicrobial peptides like LL-37 have persisted in biological immune systems across hundreds of millions of years of coevolution with bacteria: the fundamental mechanism is difficult to escape. Some bacteria have developed partial resistance mechanisms — modifying their surface charge to reduce electrostatic attraction, producing enzymes that degrade AMPs — but these are limited workarounds, not the kind of full resistance that has rendered entire classes of conventional antibiotics ineffective in some clinical contexts.

LL-37 is not only antimicrobial. It is also antifungal, antiviral, and immunomodulatory in ways that go beyond direct killing. Against fungi, it disrupts fungal membrane integrity in a mechanism similar to its antibacterial action. Against viruses, including enveloped viruses, it disrupts lipid envelopes and interferes with viral cell entry in ways that have been documented in vitro for respiratory viruses including influenza and respiratory syncytial virus. The antiviral research has become more prominent since the COVID-19 pandemic, when several research groups published data suggesting that LL-37 showed activity against SARS-CoV-2 in cell culture models, and that vitamin D's known ability to upregulate LL-37 expression might be part of the mechanism underlying the epidemiological correlations between vitamin D status and COVID-19 severity. This research is preliminary and should be held with appropriate caution — in vitro antiviral activity does not reliably translate to clinical benefit — but the mechanistic hypothesis is coherent.

The immunomodulatory functions of LL-37 are extensive and somewhat counterintuitive, because a molecule that kills bacteria directly is also, in certain contexts, a pro-inflammatory signal. LL-37 activates formyl peptide receptor 2 (FPR2/ALX) on immune cells, recruits mast cells and neutrophils, promotes monocyte differentiation, and in the right context amplifies the immune response to infection. It also modulates angiogenesis and promotes wound healing — effects that are relevant in skin and mucosal repair contexts. The fact that LL-37 can both kill microbes and direct the immune response to the site of infection makes it a multifunctional first-responder rather than a simple antimicrobial molecule.

The wound healing research is one of the most developed therapeutic directions for LL-37, because its combination of antimicrobial activity, angiogenesis promotion, and immune-cell recruitment is exactly the profile a chronic, non-healing wound needs — and protease-resistant analogs have been studied in chronic ulcers as a way to keep the peptide active long enough in the wound environment to help.

Two other research directions follow the same logic. Atopic dermatitis is one: the skin of people with atopic dermatitis tends to produce abnormally low levels of LL-37 and other antimicrobial peptides, which is thought to be part of why these patients are so prone to bacterial superinfection, particularly with Staphylococcus aureus. That observation has made restoring cathelicidin activity at the skin barrier a research interest, though it sits against an important complication — LL-37 can also drive inflammation if its levels swing too high, and it is found in excess in the skin of rosacea, where it appears to worsen rather than help the condition. The therapeutic question is therefore one of restoring balance rather than simply adding more, which is part of why no LL-37 dermatologic therapy has been established.

Periodontal disease is the other. The tissues around the teeth rely on cathelicidin as part of their defense against the bacterial biofilms that drive gum disease, and the clearest evidence comes from rare genetic conditions: people who cannot produce functional LL-37, such as those with severe congenital neutropenia, develop aggressive, early periodontal destruction. That natural experiment has prompted interest in LL-37 and its analogs as a way to support antimicrobial defense in the gum tissue, again typically in protease-stabilized forms designed to survive the enzyme-rich oral environment. These remain research directions rather than available treatments.

What makes LL-37 compelling is also what makes it hard to turn into a drug: it is a multifunctional, ancient molecule whose membrane-disrupting mechanism is difficult for microbes to outmaneuver, yet whose synthesis cost, fragility, and capacity to inflame human tissue have kept it short of an approved therapeutic. As resistance continues to erode the conventional antibiotic pipeline, the body's own template for killing microbes is increasingly being studied not as a curiosity but as a direction the next generation of antimicrobials may need to take.