Vitamin D and LL-37: Sunlight's Antimicrobial Link

LL-37

VDRE in CAMP promoter

The CAMP gene contains a vitamin D response element that directly links vitamin D levels to LL-37 antimicrobial peptide production in humans.

Liu et al., Science, 2006

Liu et al., Science, 2006

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The link between sunlight and infection resistance has been observed for over a century. Before antibiotics, tuberculosis patients were treated with heliotherapy, exposure to sunlight, with measurable improvements in outcomes. The molecular explanation for this observation arrived in 2006, when Liu et al. demonstrated that Toll-like receptor activation in human macrophages triggers a vitamin D-dependent pathway that induces cathelicidin (LL-37) production, which then kills intracellular Mycobacterium tuberculosis.^[1] This discovery connected three previously separate fields: vitamin D biology, innate immunity, and antimicrobial peptide research.

As the pillar article on LL-37: The Immune Peptide That Does Everything describes, LL-37 is the only human cathelicidin with antimicrobial, immunomodulatory, and wound-healing functions. This article examines the specific molecular pathway through which vitamin D controls LL-37 production, the clinical evidence connecting vitamin D status to antimicrobial defense, and the limitations of translating this pathway into therapeutic interventions.

The Molecular Pathway: How Vitamin D Turns On LL-37

The CAMP Gene and Its Vitamin D Response Element

The gene encoding LL-37's precursor protein (hCAP-18) is called CAMP, located on chromosome 3p21.31. In 2005, Gombart et al. identified a consensus vitamin D response element (VDRE) in the CAMP promoter region. This VDRE is located within an AluSx short interspersed nuclear element (SINE), a type of transposable element that is specific to primates. When 1,25-dihydroxyvitamin D3 (the active form of vitamin D) binds the vitamin D receptor (VDR), the VDR-RXR heterodimer binds to this VDRE and directly upregulates CAMP transcription.

This evolutionary detail matters for research interpretation. Because the VDRE is primate-specific, mice, rats, and most other animal models cannot upregulate cathelicidin in response to vitamin D. Studies in mouse models of vitamin D and infection must be interpreted with this caveat. Lowry et al. (2020) addressed this by creating a humanized mouse model carrying the human CAMP gene with its VDRE, which confirmed that vitamin D-induced cathelicidin expression functions in vivo as predicted from cell culture studies.^[2]

The TLR-Vitamin D-Cathelicidin Axis

Liu et al. (2006) described a pathway that explains how infection triggers vitamin D-mediated antimicrobial defense. When macrophages detect a pathogen through Toll-like receptors (TLR2/1 in the case of mycobacteria), two things happen simultaneously: the cell upregulates CYP27B1, the enzyme that converts circulating 25-hydroxyvitamin D (25(OH)D) to the active 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), and it upregulates VDR expression. The locally produced 1,25(OH)2D3 binds VDR, which then activates CAMP transcription and LL-37 production.^[1]

This pathway is elegant in its logic: the cell detects a pathogen, locally activates vitamin D (independent of systemic levels), and produces an antimicrobial peptide to kill the pathogen. But there is a critical bottleneck: the cell needs sufficient 25(OH)D as substrate for CYP27B1. If circulating 25(OH)D levels are low (vitamin D deficiency), the local conversion cannot produce enough 1,25(OH)2D3 to drive CAMP transcription, and LL-37 production fails.

The RXR-alpha Requirement

Svensson et al. (2016) clarified an additional molecular detail. They showed that vitamin D-induced CAMP upregulation in human keratinocytes requires retinoid X receptor alpha (RXR-alpha), the obligate heterodimer partner for VDR. This means the pathway is not simply VDR + vitamin D = LL-37. It requires the coordinated activity of at least two nuclear receptors (VDR and RXR-alpha) and is potentially influenced by retinoid (vitamin A) status as well, since RXR can also heterodimerize with retinoid acid receptors.^[3]

Clinical Evidence: Where Vitamin D-LL-37 Matters

Tuberculosis

The tuberculosis connection provided the original evidence for the pathway's clinical relevance. Liu et al. (2006) showed that sera from African Americans (who have higher rates of vitamin D deficiency) were less efficient at supporting TLR-induced cathelicidin production than sera from Caucasian Americans, correlating with lower 25(OH)D levels.^[1]

Al-Jaberi et al. (2022) extended this to a clinical population. They demonstrated that macrophages from patients with vitamin D deficiency showed reduced vitamin D-induced cathelicidin production and impaired killing of M. tuberculosis. The study confirmed that the cellular defect was specifically at the cathelicidin induction step, not a generalized macrophage dysfunction.^[4]

Chung et al. (2020) reviewed the vitamin D-cathelicidin axis as a crossroads between protective immunity and pathological inflammation during infection. They noted that while cathelicidin induction by vitamin D is protective in tuberculosis, the same pathway could theoretically exacerbate granulomatous inflammation in susceptible individuals.^[5]

Respiratory Infections

Ramos-Martinez et al. (2018) conducted a clinical study supplementing asthma patients with vitamin D and measuring respiratory infection rates. They found that vitamin D supplementation reduced respiratory infections and that the reduction correlated with increased serum levels of both LL-37 and the anti-inflammatory cytokine IL-10. This study provided evidence that the vitamin D-cathelicidin pathway operates in human respiratory defense and that its clinical effects extend beyond direct antimicrobial killing to include immune regulation.^[6]

White et al. (2022) reviewed the broader evidence for vitamin D-induced antimicrobial peptides in antiviral innate immunity. They cataloged evidence for LL-37's activity against influenza, respiratory syncytial virus, and SARS-CoV-2, noting that vitamin D deficiency may impair antiviral defense at mucosal surfaces through reduced cathelicidin production.^[7]

COVID-19

Crane-Godreau et al. (2020) proposed that vitamin D deficiency and air pollution converge to suppress LL-37 expression, potentially increasing COVID-19 severity. Their epidemiological analysis linked geographic regions with low vitamin D status and high particulate matter exposure to worse COVID-19 outcomes.^[8] This hypothesis generated substantial interest but remains unconfirmed by randomized controlled trials specifically designed to test vitamin D supplementation for COVID-19 prevention through LL-37 induction.

The COVID-19 experience illustrates a broader challenge: epidemiological associations between vitamin D deficiency and infection risk are consistent and numerous, but randomized trials of vitamin D supplementation for infection prevention have produced mixed results. One explanation is that the cathelicidin pathway is substrate-limited only in deficient individuals. Supplementation above sufficiency thresholds may not further increase LL-37 production, which would explain why trials enrolling populations with mixed vitamin D status show attenuated effects. A second explanation involves timing: supplementation before infection may be protective by maintaining baseline cathelicidin levels, while supplementation during active infection may be too late to meaningfully alter the immune response. Most clinical trials have not been designed to distinguish between these scenarios.

Gut Health

Gubatan et al. (2020) demonstrated that cathelicidin mediates a protective role for vitamin D in ulcerative colitis. In human colonic epithelial cells, vitamin D-induced cathelicidin expression reduced inflammatory signaling and enhanced barrier function. In UC patients, higher serum cathelicidin levels correlated with clinical remission.^[9]

Skin Conditions

The vitamin D-cathelicidin axis has complex implications for skin disease. Connell et al. (2025) reviewed cathelicidin expression in atopic dermatitis and the therapeutic potential of vitamin D. In atopic dermatitis, cathelicidin expression is reduced compared to healthy skin and psoriatic skin, potentially contributing to the increased susceptibility to skin infections seen in AD patients. Vitamin D supplementation may help restore cathelicidin expression in this context.^[10]

Cabalin et al. (2023) confirmed this in a pediatric population, showing that oral vitamin D supplementation modulated epidermal expression of both the vitamin D receptor and cathelicidin in children with atopic dermatitis. The treatment enhanced antimicrobial peptide expression at the skin surface.^[11]

However, the picture reverses in psoriasis. In psoriatic skin, cathelicidin is already overexpressed, and LL-37-nucleic acid complexes drive the autoimmune inflammatory cascade. Whether vitamin D supplementation would worsen psoriasis by further increasing cathelicidin, or improve it through vitamin D's separate anti-inflammatory effects on T cells and dendritic cells, is not settled. Topical vitamin D analogs (calcipotriol) are a standard psoriasis treatment, suggesting the net effect is beneficial in skin despite the cathelicidin concern, but the mechanisms are complex. For more on how LL-37 drives psoriatic inflammation, see LL-37's Dual Role: Anti-Inflammatory and Pro-Inflammatory Effects.

Limitations and Open Questions

Individual variation in the pathway is poorly characterized. Polymorphisms in VDR, CYP27B1, CYP24A1 (which degrades active vitamin D), and the CAMP gene itself could all influence how effectively a given individual converts vitamin D status into LL-37 production. Population-level studies rarely account for this genetic heterogeneity.

Optimal serum 25(OH)D levels for cathelicidin induction have not been precisely defined. The threshold below which the pathway becomes substrate-limited is not known. Standard clinical thresholds for vitamin D sufficiency (30 ng/mL) were established for bone health, not immune function, and may not apply to cathelicidin biology.

Tissue-specific effects complicate systemic supplementation. Vitamin D-induced cathelicidin in the gut may be protective (as in UC), while the same increase in the skin could theoretically worsen conditions where cathelicidin is already pathologically elevated. See How LL-37 Activates Neutrophils and Dendritic Cells for how tissue context shapes LL-37's immune effects.

The primate specificity of the VDRE means that virtually all animal model data on vitamin D and cathelicidin must be interpreted with caution. The humanized mouse model from Lowry et al. (2020) addresses this partially, but differences in vitamin D metabolism, receptor expression, and immune cell composition between mice and humans remain confounding factors.^[2]

Interaction with other nutrients is understudied. The RXR-alpha requirement raises the question of whether vitamin A status influences the vitamin D-cathelicidin pathway. Zinc, which is required for cathelicidin processing, and omega-3 fatty acids, which modulate inflammatory pathways, may also interact with this axis. No controlled trials have examined these interactions systematically.

The Bottom Line

Vitamin D directly controls LL-37 production through a vitamin D response element in the CAMP gene promoter. This primate-specific pathway connects pathogen detection (via TLR signaling) to local vitamin D activation and cathelicidin-mediated microbial killing. Clinical evidence supports its relevance in tuberculosis, respiratory infections, inflammatory bowel disease, and atopic dermatitis. Vitamin D deficiency impairs this pathway at the substrate level. However, the mixed results of vitamin D supplementation trials likely reflect the pathway's substrate-limited nature: benefits are greatest in deficient individuals and may not extend to those already replete.

Frequently Asked Questions

How does vitamin D increase LL-37 production?

Vitamin D increases LL-37 through direct gene regulation. The CAMP gene (encoding LL-37's precursor protein hCAP-18) contains a vitamin D response element in its promoter. When active vitamin D (1,25-dihydroxyvitamin D3) binds the vitamin D receptor, the VDR-RXR-alpha complex binds this response element and upregulates CAMP transcription. Liu et al. (2006) showed that this pathway is activated when macrophages detect pathogens through Toll-like receptors, which trigger local conversion of circulating 25(OH)D to the active form.

Does vitamin D deficiency reduce antimicrobial defense?

Yes. The vitamin D-cathelicidin pathway requires circulating 25-hydroxyvitamin D as a substrate for local conversion to active vitamin D. When 25(OH)D levels are insufficient, macrophages cannot produce enough active vitamin D to drive LL-37 production. Al-Jaberi et al. (2022) showed that macrophages from vitamin D-deficient patients had reduced cathelicidin production and impaired ability to kill Mycobacterium tuberculosis compared to vitamin D-sufficient controls.

Can vitamin D supplements boost LL-37 levels?

Evidence suggests yes, particularly in deficient individuals. Ramos-Martinez et al. (2018) found that vitamin D supplementation in asthma patients increased serum LL-37 levels and reduced respiratory infections. Cabalin et al. (2023) showed oral vitamin D increased cathelicidin expression in children with atopic dermatitis. The effect appears most pronounced in those starting from a deficient baseline; supplementing already-sufficient individuals may not further increase LL-37.

Why can't mouse studies test vitamin D and cathelicidin?

The vitamin D response element in the human CAMP gene is located within a primate-specific Alu transposable element. Mice and most other mammals lack this element, so their cathelicidin genes do not respond to vitamin D. This means standard mouse models cannot replicate the human vitamin D-cathelicidin axis. Lowry et al. (2020) created a humanized mouse carrying the human CAMP gene to address this limitation.

Does sunlight exposure increase antimicrobial peptides?

Indirectly, yes. UVB radiation on skin converts 7-dehydrocholesterol to vitamin D3, which is then hydroxylated in the liver to 25(OH)D and locally in tissues to active 1,25(OH)2D3. This active vitamin D drives CAMP gene transcription and LL-37 production. The historical effectiveness of heliotherapy (sunlight treatment) for tuberculosis is now understood partly through this molecular pathway connecting UV exposure to antimicrobial peptide induction.

Is the vitamin D-LL-37 pathway relevant to COVID-19?

Crane-Godreau et al. (2020) proposed that vitamin D deficiency suppresses LL-37 and increases COVID-19 susceptibility, based on epidemiological correlations between low vitamin D status and worse outcomes. LL-37 has demonstrated antiviral activity against other respiratory viruses. However, this remains a hypothesis. No randomized controlled trial has specifically tested whether vitamin D supplementation prevents COVID-19 through LL-37 induction, and several proposed COVID-19 interventions based on immunological plausibility failed in trials.