Cathelicidin: Functions in Immunity and Health

Cathelicidin is a peptide produced by the human body that serves as a front-line weapon against infection, but its roles extend far beyond simply killing bacteria. In humans, the active form is called LL-37, and it functions as part germ-killer, part immune coordinator, and part tissue repair signal. Research over the past two decades has linked cathelicidin to conditions as varied as inflammatory bowel disease, psoriasis, lupus, and atherosclerosis, making it one of the more versatile molecules in the innate immune system.

How Cathelicidin Becomes Active

Your body does not produce cathelicidin in its ready-to-use form. Immune cells, skin cells, and cells lining the gut and lungs all manufacture a larger precursor protein called hCAP-18. The business end of the molecule is at one end, and it has to be physically cut loose before it can do anything useful. In neutrophils, the enzyme responsible for this cleavage is proteinase 3, a protein stored in specialized granules. Once the neutrophil releases its contents in response to an infection, proteinase 3 snips hCAP-18 to liberate the 37-amino-acid peptide known as LL-37.¹ This activation step matters because the intact precursor is not antimicrobial on its own. The body essentially keeps a loaded weapon on safety until it reaches the right location.

Killing Microbes by Punching Holes

Once released, LL-37 targets bacteria in a direct, physical way. Rather than interfering with a single biochemical pathway the way conventional antibiotics do, it attacks the structural integrity of bacterial membranes. Structural studies show that LL-37 molecules assemble into a four-unit channel when they encounter a bacterial membrane. These narrow channels allow water to pass through, and time-lapse imaging of treated bacteria shows discontinuities forming in the outer membrane, followed by broader cell-wall damage and death.² Alongside channel formation, LL-37 disrupts the way lipids are packed in the membrane and depolarizes it, which collapses the electrical gradient bacteria need to survive.³

This physical mode of attack works against both major classes of bacteria. The mouse equivalent, called CRAMP, shows potent activity against a range of gram-negative strains at low concentrations.⁴ Because membrane disruption is such a fundamental assault, it is harder for bacteria to evolve resistance compared with traditional antibiotics that target a single protein. That said, bacteria are not defenseless.

How Bacteria Fight Back

Some bacteria have evolved countermeasures that blunt cathelicidin’s effectiveness. These strategies include altering the electrical charge on their own surfaces to repel the positively charged peptide, modifying the fluidity of their membranes so the peptide cannot insert properly, producing enzymes that chop up the peptide before it can act, and running efflux pumps that physically eject it.⁵ These resistance mechanisms are one reason why cathelicidin alone is not sufficient to eliminate all pathogens and why the immune system deploys a layered defense.

Coordinating the Immune Response

LL-37 does far more than kill microbes directly. It acts as a chemical signal that calls other immune cells to the site of infection. Neutrophils carry a receptor called FPR2 that recognizes LL-37, and when the peptide binds to it, the neutrophils become activated and produce structures called neutrophil extracellular traps, sticky webs of DNA and proteins that snare bacteria.⁶

LL-37 also regulates how intensely the immune system reacts. Macrophages, the immune cells that engulf pathogens, can undergo a form of inflammatory self-destruction called pyroptosis when they encounter bacterial toxins. LL-37 suppresses this process through two mechanisms: it neutralizes the bacterial signal that starts the cascade and blocks the receptor that amplifies it. This dual braking effect could have implications for managing sepsis, where runaway inflammation causes more harm than the infection itself.⁷

The Vitamin D Connection

One of the most clinically relevant regulators of cathelicidin is vitamin D. When immune cells encounter a pathogen, vitamin D signaling ramps up production of cathelicidin, linking your vitamin D status directly to the strength of this particular defense. Both innate and adaptive immune signals work through the vitamin D receptor to boost cathelicidin output.⁸ This is part of why vitamin D deficiency has been associated with increased susceptibility to respiratory infections. The connection is not abstract: without adequate vitamin D, cells simply produce less of this antimicrobial peptide.

Another upstream regulator involves gut bacteria. Bile acid metabolites produced by commensal microbes can boost cathelicidin expression in lung epithelial cells through a specific signaling pathway. This means that the composition of your gut microbiome may influence antimicrobial defenses in your airways, an example of the gut-lung axis that researchers are increasingly recognizing.⁹

Cathelicidin in Skin Disease

The skin is one of the richest sources of cathelicidin, and the peptide’s behavior there reveals both its protective role and how its dysregulation can cause problems. In atopic dermatitis (eczema), cathelicidin production is suppressed. Skin biopsies from people with eczema show significantly lower levels of LL-37 compared with psoriasis lesions, where the peptide is abundant.¹⁰ This deficiency helps explain why eczema-prone skin is so susceptible to bacterial and viral infections. The antimicrobial barrier is weakened at exactly the sites where the skin is already inflamed and compromised.¹¹

Psoriasis presents the opposite problem. There, cathelicidin is overexpressed, and rather than simply fighting infection, the excess LL-37 activates a receptor on skin cells called TLR9, which drives further inflammation. Psoriatic skin shows elevated TLR9 levels compared with both non-lesional skin and eczema skin, and the presence of abundant LL-37 enables keratinocytes to respond to DNA fragments in a way that healthy skin would ignore.¹²

Rosacea adds another wrinkle. In this condition, the problem is not just that cathelicidin levels are high but that the enzyme kallikrein 5, which processes cathelicidin in the skin, is overactive. The result is an excess of processed LL-37 fragments that drive the characteristic redness, flushing, and visible blood vessels of rosacea by promoting inflammation and blood vessel growth.¹³

Protection in the Lungs and Gut

Cathelicidin plays a demonstrable role in clearing lung infections. Mice lacking the cathelicidin gene show impaired bacterial clearance and a sluggish early neutrophil response when infected with gram-negative pneumonia-causing bacteria.¹⁴ When synthetic LL-37 was delivered to the lungs of mice with Pseudomonas infections, bacterial clearance improved, and this appeared to work primarily by boosting the neutrophil response rather than through direct bacterial killing.¹⁵ In humans, circulating LL-37 levels are elevated in patients with bacterial respiratory infections, particularly those caused by bacteria that live inside host cells, suggesting the peptide is part of the active defense effort.¹⁶

In the gut, cathelicidin protects against colitis. Mice engineered to lack the cathelicidin gene develop more severe intestinal inflammation when challenged with a colitis-inducing agent.¹⁷ In human inflammatory bowel disease, the peptide may serve as a prognostic marker. Among patients with moderate or severe disease at diagnosis, those with higher initial LL-37 levels had significantly better recovery over the following six to eighteen months than those with lower levels.¹⁸

Wound Healing and New Blood Vessel Growth

Beyond fighting infection, LL-37 promotes tissue repair. One of its less obvious functions is stimulating the growth of new blood vessels, a process called angiogenesis that is critical during wound healing. Mice lacking the cathelicidin gene show reduced blood vessel formation in healing wounds, confirming that the peptide contributes directly to the revascularization step of tissue repair.¹⁹ This dual role, clearing infection while also rebuilding tissue, makes cathelicidin unusual among immune molecules. Many inflammatory signals damage surrounding tissue as a side effect; cathelicidin appears to do some of the cleanup work simultaneously.

When Cathelicidin Turns Against the Host

The same properties that make LL-37 useful in defense can become damaging in autoimmune disease. In systemic lupus erythematosus, LL-37 forms complexes with DNA released from damaged cells. These complexes are resistant to the enzymes that normally degrade stray DNA, and they activate a specific class of immune cells called plasmacytoid dendritic cells. Those cells then produce large amounts of type I interferon, a signaling molecule that drives the chronic inflammation characteristic of lupus. The persistent activation of this pathway is considered a crucial early event in lupus development, and LL-37 has been proposed as both a contributor to the disease process and a potential biomarker.²⁰

The pattern of LL-37 complexing with DNA fragments also shows up in atherosclerosis. Researchers have found elevated levels of LL-37 bound to mitochondrial DNA in atherosclerotic plaques and in the blood of people with cardiovascular disease. These complexes activate inflammatory receptors in blood vessel walls, promoting the recruitment of immune cells and the buildup of plaques. In mouse models, antibodies targeting the LL-37-DNA complex reduced the severity of atherosclerotic lesions. Meanwhile, atherosclerotic mice show increased cathelicidin levels in their blood, while mice with experimentally induced heart attacks show decreased levels, suggesting the peptide behaves differently depending on the type of cardiovascular insult.²¹²²

Mixed Signals in Cancer

Cathelicidin’s relationship with cancer is genuinely contradictory. Depending on the tissue, LL-37 can either promote or suppress tumor growth. In ovarian, lung, and breast cancers, overexpression of LL-37 has been associated with tumor promotion and progression. In colon and gastric cancer, the opposite holds: LL-37 appears to suppress tumor development. The difference comes down to which receptors LL-37 binds on a given cancer cell, and those receptor profiles vary by cancer type.²³ This tissue-specificity makes cathelicidin a poor candidate for a one-size-fits-all cancer therapy, but it also means the peptide’s biology could eventually inform targeted approaches once the receptor interactions are better mapped.

LL-37 as a Clinical Biomarker

Because cathelicidin levels change in predictable ways during illness, researchers have explored whether measuring LL-37 in the blood could help predict outcomes. In critically ill patients admitted to an intensive care unit, those with the lowest cathelicidin levels on the first day had roughly four and a half times the odds of dying within 90 days compared with those in the highest group, even after adjusting for disease severity. Low cathelicidin also correlated with a greater likelihood of developing sepsis.²⁴

During the COVID-19 pandemic, a study found that the ratio of LL-37 to white blood cell count at hospital admission correlated inversely with disease severity. Patients with lower ratios were more likely to progress to critical illness.²⁵ These findings are preliminary and have not been validated in large multicenter trials, but they point to cathelicidin as a candidate biomarker for risk-stratifying patients early in serious infections.

An Ancient and Diverse Peptide Family

Cathelicidins are not unique to humans. They are found throughout the vertebrate lineage, from fish through amphibians, reptiles, birds, and mammals. What is shared across all these species is the precursor structure: a relatively conserved front end (the cathelin domain) attached to a highly variable active peptide at the other end.²⁶ Amphibian cathelicidins fill what was once a gap in the evolutionary record, connecting the fish and reptile branches of the family tree.²⁷

While humans have a single cathelicidin gene, other lineages have gone in very different directions. Cattle and related hoofed mammals have undergone repeated gene duplications, producing an entire array of distinct cathelicidin family members. Birds carry a cluster of cathelicidin genes organized into three subfamilies, with some species gaining extra copies and others losing them.²⁸ This diversity has practical importance for drug development, because different cathelicidin sequences have different properties. Peptides derived from snake cathelicidins, for example, have been engineered into shorter, more potent variants by rearranging which amino acids sit on the membrane-interacting face.²⁹

Therapeutic Efforts and Challenges

The appeal of turning cathelicidin into a drug is obvious: a molecule that kills bacteria, recruits immune cells, and promotes wound healing sounds like the ideal infection-fighter, especially as antibiotic resistance worsens. But natural LL-37 has limitations as a therapeutic. It is easily chewed up by enzymes in the body, it can be toxic to human cells at high concentrations, and the same inflammatory and angiogenic activities that help in wound healing could be harmful in the wrong context, as the atherosclerosis and lupus data suggest.

Researchers have attempted to address some of these problems through peptide engineering. One approach produces mirror-image versions of cathelicidin-derived peptides using D-amino acids, which human enzymes cannot easily degrade.³⁰ Other groups have trimmed the peptide down to shorter fragments that retain antimicrobial activity while reducing off-target effects. The diversity of cathelicidins across species provides a large natural library of sequences to mine, and computational tools that predict which amino acid arrangements will interact most effectively with bacterial membranes have accelerated the design process.

None of these engineered peptides has yet reached widespread clinical use, but several are in preclinical and early clinical testing for wound infections, cystic fibrosis lung infections, and biofilm-associated implant infections. The gap between promising lab results and an approved drug remains wide, partly because peptide drugs are expensive to manufacture and partly because delivering them to the right tissue at the right concentration is still a challenge. Still, the growing problem of antibiotic resistance keeps research investment flowing into this space, and cathelicidin-derived peptides remain among the most studied candidates.