# Vitamin D, VDR and antimicrobial peptides: mechanisms and clinical limits

- Canonical: https://nutrifit.health/knowledge/vitamin-d-vdr-cathelicidin-antimicrobial-defense
- Language: en
- Published: 2026-10-09T17:05:12.265Z
- Updated: 2026-10-09T17:05:12.265Z
- Author: NutriFit Editorial

> Local vitamin D metabolism, VDR, CAMP and DEFB4: enzymes, compartments, LL-37 maturation and the limits of clinical infection prevention.

Vitamin D participates in immune regulation through local metabolism and altered gene transcription. In human myeloid cells, the “25(OH)D — CYP27B1 — calcitriol — VDR — CAMP” axis is important. It helps explain why vitamin D metabolite availability influences some innate responses. Its existence does not, however, establish that an additional capsule prevents infection in everyone or that raising blood concentrations into a chosen range makes immunity stronger.

Different research levels must be distinguished. Gene expression, precursor protein abundance, mature peptide production and clinical disease are different outcomes. Cellular findings can establish a mechanism, while randomised trials test a particular preparation, dose and regimen in a defined population. These results do not always coincide for vitamin D, making negative trials essential to interpretation.

## Three vitamin D forms and two spatial circuits

Cholecalciferol, calcidiol and calcitriol are not interchangeable. In skin, photochemical conversion of 7-dehydrocholesterol produces previtamin D₃, which subsequently isomerises to cholecalciferol. Hepatic 25-hydroxylation produces 25(OH)D₃, or calcidiol. Cheng and colleagues’ investigation of a genetic CYP2R1 defect established this enzyme’s physiological role as a human 25-hydroxylase. The 25(OH)D concentration is generally used to assess vitamin D status, but does not directly measure CAMP transcription within a macrophage.

CYP2R1 is a microsomal cytochrome P450 in endoplasmic reticulum membranes. CYP27B1, responsible for subsequent 1α-hydroxylation, belongs to the mitochondrial system. The general P450 monooxygenation principle incorporates one oxygen atom into the substrate while reducing the other to water: substrate + O₂ + NADPH + H⁺ → hydroxylated substrate + H₂O + NADP⁺. Haem binds oxygen, and electron-transfer proteins supply reducing equivalents. Microsomal and mitochondrial systems have different partners; ferredoxin and ferredoxin reductase participate in the mitochondrial circuit. Calcium does not directly convert the vitamin into the hormone in this reaction.

Renal CYP27B1 generates calcitriol for endocrine regulation of mineral metabolism. Immune cells can produce it locally from available 25(OH)D. Action within the same cell is termed intracrine, and action on neighbouring cells paracrine. Liu and colleagues’ 2006 work connected innate receptor recognition with increased expression of VDR and the vitamin D activation enzyme in human macrophages. Low substrate availability limited the response studied. This is a mechanistic link rather than a universal infection-frequency relationship with one blood test.

## Substrate availability and feedback

Most circulating metabolites bind vitamin D-binding protein and albumin. Total serum concentration, free fraction and availability to a specific cell are therefore different characteristics. Medium composition influences cellular experimental findings, while comparison with humans requires attention to transport, binding and local metabolism. A single total concentration cannot reconstruct each macrophage’s enzymatic circuit activity.

Renal CYP27B1 regulation relates to parathyroid hormone, FGF23 and mineral status. In immune cells, inflammatory and cytokine signals matter; this does not imply complete independence from every systemic constraint. CYP24A1 initiates vitamin D metabolite inactivation pathways, including 24-hydroxylation, helping limit signal duration. Enzyme expression, substrate availability and actual metabolite flux are also distinct.

Local synthesis can sometimes become systemically significant. In granulomatous diseases, extrarenal calcitriol production may contribute to hypercalcaemia. “Immune activation is safe because it is local” is consequently incorrect. Calcitriol medication and the precursor cholecalciferol have different pharmacological risks and should not substitute for one another.

## VDR is a transcriptional receptor rather than a kinase

VDR belongs to the nuclear receptor family. Its DNA-binding domain contains zinc-coordination structures, while its ligand-binding domain recognises calcitriol. Rochel and colleagues’ crystallographic work demonstrated the active conformation of the natural-ligand complex. Zinc supports DNA-recognition structure; it is not a phosphate donor, and its involvement does not establish benefits from additional zinc for this axis.

VDR interacts with retinoid X receptor RXR and chromatin regulatory regions, including vitamin D response elements. Ligand binding changes its coregulator interaction surface. Coactivators and chromatin-remodelling complexes help transcriptional machinery access genes. The acetyltransferases p300/CBP use acetyl-CoA as an acetyl-group donor to modify proteins; this is a separate enzymatic reaction not performed by VDR itself.

Response depends on chromatin accessibility, coregulator availability and cellular state. Receptor binding to DNA does not guarantee the same expression change across all genes. “Calcitriol switches on all antimicrobial immunity” loses this selectivity. VDR polymorphisms can be investigated as response modifiers, but a variant–disease association does not establish an individual dose or replace a clinical trial.

## CAMP and DEFB4 are regulated differently

In 2005, Gombart, Borregaard and Koeffler showed that human CAMP is a direct VDR target in the cells studied. Its regulatory element is associated with a repeat element acquired during primate evolution. Similar induction was not observed in the mouse cells investigated. The human pathway cannot therefore be automatically transferred to an ordinary mouse model simply because both species possess cathelicidins.

CAMP encodes the hCAP18 precursor; mature LL-37 emerges through proteolytic processing. In Sørensen and colleagues’ 2001 study, proteinase-3, a serine protease, performed this processing in neutrophil material after exocytosis; detecting the precursor inside phagolysosomes was not accompanied by detectable cleavage under the conditions examined. Increased CAMP mRNA consequently does not equal more functional LL-37 in every cell and compartment. Processing location, available proteases and subsequent peptide degradation matter.

For DEFB4, associated with human β-defensin-2, a VDR-only explanation is insufficient. Liu and colleagues showed in 2009 that IL-1β and VDR pathways needed to converge in the human monocyte response studied. NF-κB regulation helped explain the difference from CAMP. This is cooperation between inflammatory and hormonal signals rather than universal NF-κB suppression by vitamin D. Outcomes can differ between cells, signals and chromatin states.

[![Mitochondrial CYP27B1, calcitriol, nuclear VDR/RXR, CAMP transcription, cytoplasmic hCAP18 synthesis and separate processing into LL-37; DEFB4 also requires IL-1β/NF-κB.](https://api.nutrifit.health/api/v2/public/knowledge/files/images/nutrifit-scientific-20261009-vitamin-d-vdr-cathelicidin-antimicrobial-defense-vdr-axis-cb42410f3819.png)](https://api.nutrifit.health/api/v2/public/knowledge/files/images/nutrifit-scientific-20261009-vitamin-d-vdr-cathelicidin-antimicrobial-defense-vdr-axis-cb42410f3819.png)

## Peptides, membranes and intracellular disposal

LL-37 is a cationic peptide with amphipathic properties: different regions interact with water and lipid environments. Electrostatic interactions with membrane components help explain antimicrobial effects, but activity depends on medium composition, protein binding and membrane state. Higher concentration does not imply proportionally greater beneficial activity; the peptide can also interact with host cells. Its biology should not be reduced to a safe “natural antibiotic”.

Cellular research also connects the vitamin D pathway with autophagy and intracellular compartment maturation. Autophagy involves membrane structure formation, material capture and delivery for lysosomal degradation. An increase in one autophagosome marker cannot distinguish enhanced flux from impaired subsequent disposal; evidence of process completion is needed. These mechanisms can support cellular defence without establishing that a supplement replaces anti-infective treatment.

Cathelicidin also participates in immune interactions with nucleic acids, and excessive or incorrectly localised responses can sustain inflammation. The connection between chromatin, neutrophil proteins and autoantigens is discussed in [NETs, immune thrombosis and autoimmune risks](/knowledge/neutrophil-extracellular-traps-nets-thrombosis-autoimmunity). This context shows why enhancing one effector is not a general measure of “immune quality”.

## Clinical prevention: an early positive signal and its limits

Martineau and colleagues’ 2017 individual participant data meta-analysis included 10,933 participants from 25 randomised trials; the original dataset covered ages from birth to 95 years. For at least one acute respiratory infection, the adjusted odds ratio was 0.88, 95% CI 0.81–0.96. A stronger signal appeared with daily or weekly regimens without additional boluses and low baseline 25(OH)D. Varying doses, durations and infection definitions prevent conversion into one universal regimen. An odds ratio should also not be read as the same relative risk reduction.

Jolliffe and colleagues’ 2025 update changed the certainty. The primary vitamin D–placebo comparison comprised 40 studies and 61,589 participants: OR 0.94, 95% CI 0.88–1.00, p = 0.057. Statistically significant overall protection was not demonstrated; the analysis did not confirm effect modification by baseline status, age, dosing frequency or dose size. Funnel-plot asymmetry was discussed. This does not prove the absence of every possible subgroup effect, but weakens grounds for promising reliable overall prevention.

CORONAVIT randomised 6,200 UK residents aged at least 16 to an offer of testing followed by D₃ at 800 IU/day or 3,200 IU/day when 25(OH)D was below 75 nmol/L (1,550 participants per group), or no offer (3,100). Follow-up lasted six months and the trial was open-label. The primary outcome was at least one swab-test- or physician-confirmed respiratory infection. It occurred in 5.7% and 5.0% of the offer groups versus 4.6% of controls: OR 1.26 (0.96–1.66) and 1.09 (0.82–1.46). Benefit was not demonstrated. The trial tested a population strategy rather than perfect adherence to every assigned dose; control-group self-supplementation and pandemic conditions limit extrapolation.

## Tuberculosis and COVID-19: mechanism does not guarantee benefit

In Ganmaa’s double-blind, placebo-controlled trial, 8,851 Mongolian schoolchildren with negative baseline infection tests received D₃ at 14,000 IU weekly or placebo for three years. Baseline 25(OH)D was below 20 ng/ml in 95.6%. The primary outcome, a positive QuantiFERON test at follow-up, occurred in 147/4,074 (3.6%) versus 134/4,043 (3.3%): adjusted RR 1.10, 95% CI 0.87–1.38. Metabolite levels increased without improving infection prevention. This test reflects infection acquisition rather than directly measuring VDR or mature peptide function; clinical tuberculosis was a separate, rarer outcome.

The Vitamin D for COVID-19 Trial, published in 2026, included 1,747 adults with newly diagnosed infection and 277 cohabiting contacts in the USA and Mongolia. The double-blind randomised design assigned D₃ at 9,600 IU/day for two days followed by 3,200 IU/day for four weeks, or placebo. The primary outcome among infected participants was at least one healthcare visit, including hospitalisation, or death within four weeks. Incidence was 0.28 versus 0.29, OR 0.97, 95% CI 0.75–1.24. Benefit was not demonstrated. An uncertain long-symptom signal in a per-protocol analysis does not establish an effect, and most participants were relatively young; the findings do not establish treatment of all severe disease forms.

These doses describe studies rather than recommendations for self-administration. They show that increasing 25(OH)D, even from a low baseline, does not guarantee reduced infection risk. Laboratory biology and clinical prevention need separate evaluation.

## Testing, safety and the purpose of deficiency correction

25(OH)D and 1,25(OH)₂D answer different questions. The active metabolite is regulated and may not reflect stores; a normal result does not exclude low precursor availability. For 25(OH)D unit conversion, 1 ng/ml equals 2.5 nmol/L. Numerical conversion does not establish a universal immune threshold. The Endocrine Society’s 2024 disease-prevention guideline did not establish an optimal target concentration for this purpose and did not support routine general-population screening; it considers specific groups and does not replace management of established indications.

Correcting proven deficiency and preventing mineral-metabolism disorders retain their clinical purpose even when infection prevention is unconfirmed. Decisions depend on age, disease, medication and indications; high doses should not be justified by a wish to “switch on CAMP”. Excess vitamin D can cause hypercalcaemia and renal complications, while granulomatous conditions require particular attention to extrarenal activation.

VDR and antimicrobial peptides explain an important part of human immunobiology. The justified clinical conclusion is narrower: substrate availability matters to some mechanisms, but universal protection, one target level and replacement of treatment do not follow from those mechanisms.

## FAQ

### Does VDR activation guarantee infection prevention?

No. Cellular mechanism and clinical prevention are different outcomes; large randomised trials provide no universal guarantee.

### Are CAMP and DEFB4 regulated identically?

No. In the human monocyte response studied, DEFB4 required convergence of IL-1β and VDR signals involving NF-κB.

### Are study doses recommendations?

No. They describe specific trials. Deficiency correction depends on indications; a wish to enhance CAMP does not justify high doses.

## Sources

- [Gombart AF, Borregaard N, Koeffler HP.. Human cathelicidin antimicrobial peptide (CAMP) gene is a direct target of the vitamin D receptor and is strongly up-regulated in myeloid cells by 1,25-dihydroxyvitamin D3. FASEB J. 2005.](https://pubmed.ncbi.nlm.nih.gov/15985530/)
- [Liu PT, Stenger S, Li H et al. Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science. 2006.](https://pubmed.ncbi.nlm.nih.gov/16497887/)
- [Liu PT, Schenk M, Walker VP et al. Convergence of IL-1beta and VDR activation pathways in human TLR2/1-induced antimicrobial responses. PLoS One. 2009.](https://pubmed.ncbi.nlm.nih.gov/19503839/)
- [Sørensen OE, Follin P, Johnsen AH et al. Human cathelicidin, hCAP-18, is processed to the antimicrobial peptide LL-37 by extracellular cleavage with proteinase 3. Blood. 2001.](https://pubmed.ncbi.nlm.nih.gov/11389039/)
- [Cheng JB, Levine MA, Bell NH et al. Genetic evidence that the human CYP2R1 enzyme is a key vitamin D 25-hydroxylase. Proc Natl Acad Sci U S A. 2004.](https://pubmed.ncbi.nlm.nih.gov/15128933/)
- [Rochel N, Wurtz JM, Mitschler A et al. The crystal structure of the nuclear receptor for vitamin D bound to its natural ligand. Mol Cell. 2000.](https://pubmed.ncbi.nlm.nih.gov/10678179/)
- [Martineau AR, Jolliffe DA, Hooper RL et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017.](https://pubmed.ncbi.nlm.nih.gov/28202713/)
- [Jolliffe DA, Camargo CA, Sluyter JD et al. Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. Lancet Diabetes Endocrinol. 2025.](https://pubmed.ncbi.nlm.nih.gov/39993397/)
- [Ganmaa D, Uyanga B, Zhou X et al. Vitamin D Supplements for Prevention of Tuberculosis Infection and Disease. N Engl J Med. 2020.](https://pubmed.ncbi.nlm.nih.gov/32706534/)
- [Jolliffe DA, Holt H, Greenig M et al. Effect of a test-and-treat approach to vitamin D supplementation on risk of all cause acute respiratory tract infection and covid-19: phase 3 randomised controlled trial (CORONAVIT). BMJ. 2022.](https://pubmed.ncbi.nlm.nih.gov/36215226/)
- [Ganmaa D, Cook KA, Khudyakov P et al. A Randomized Trial of Vitamin D Supplementation and COVID-19 Clinical Outcomes and Long COVID: The Vitamin D for COVID-19 Trial. J Nutr. 2026.](https://pubmed.ncbi.nlm.nih.gov/41826107/)
- [Demay MB, Pittas AG, Bikle DD et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024.](https://pubmed.ncbi.nlm.nih.gov/38828931/)
- [Yuk JM, Shin DM, Lee HM et al. Vitamin D3 induces autophagy in human monocytes/macrophages via cathelicidin. Cell Host Microbe. 2009.](https://pubmed.ncbi.nlm.nih.gov/19748465/)
- [Lande R, Gregorio J, Facchinetti V et al. Plasmacytoid dendritic cells sense self-DNA coupled with antimicrobial peptide. Nature. 2007.](https://pubmed.ncbi.nlm.nih.gov/17873860/)