LL-37

LL-37 (human cathelicidin antimicrobial peptide, CAMP) is the sole human cathelicidin: a 37-residue cationic amphipathic peptide released by proteolytic cleavage of the precursor hCAP-18 (human cationic antimicrobial protein, 18 kDa). Expression is broad — neutrophils, macrophages, dendritic cells, natural killer cells, and epithelial cells, with high representation in skin, lung, and gastrointestinal tract, plus the respiratory tract, testes, and ocular surface.

It is a core effector of innate immunity with pleiotropic activity: direct membrane-disruptive killing of bacteria, fungi, enveloped viruses, and parasites; neutralisation of lipopolysaccharide; chemotaxis of leukocytes; inhibition of neutrophil apoptosis; stimulation of angiogenesis and keratinocyte migration; and cytokine modulation. Because the bactericidal mechanism targets membrane charge rather than a specific protein, resistance development is considered less likely than with conventional antibiotics, which is why it is discussed in the context of the WHO's classification of antimicrobial resistance as a top-ten global health threat.

Immunomodulation is bidirectional rather than suppressive: pro-inflammatory cytokine release and immune cell recruitment where needed, paired with LPS binding and anti-inflammatory signalling that limit runaway inflammation. It is chemotactic for T-cells, monocytes, neutrophils, and mast cells.

Evidence base: two randomised placebo-controlled trials in hard-to-heal venous leg ulcers (Gronberg et al., 2014; a multicentric trial published 2021) and one randomised double-blind trial in diabetic foot ulcers (2023), all topical. There are no published human clinical trials, pharmacokinetic data, or dose-finding studies for injectable LL-37. Injectable protocols are extrapolated from preclinical work and practice patterns. Known limitations include partial inactivation by serum proteins, susceptibility to proteolysis (trypsin), a defined cytotoxicity ceiling for host cells, and synthesis cost owing to peptide length.

The contraindication list is long and mechanistically coherent: LL-37 is overexpressed in psoriasis, rosacea, and lupus, and the pro-inflammatory cytokine release that drives its antimicrobial utility is the same mechanism behind reported autoimmune flare and psoriasis exacerbation.

Bactericidal mechanism: LL-37 is a cationic amphipathic peptide. Electrostatic attraction to the anionic bacterial membrane surface (mammalian membranes are net neutral) is followed by insertion of the hydrophobic face into the lipid bilayer, oligomerisation, and pore or channel formation. Loss of membrane integrity and leakage of cytoplasmic contents produce lysis. Because the target is membrane charge architecture rather than a mutable protein, resistance development is constrained — bacteria cannot readily alter surface charge without losing viability.

Immunomodulatory activity:

LPS neutralisation. Direct binding of lipopolysaccharide prevents endotoxin-driven activation of immune cells and the systemic inflammatory cascade underlying septic shock. In animal sepsis models, protection was comparable to antibiotics with substantially lower endotoxin and inflammation levels.

Chemotaxis. LL-37 recruits neutrophils, monocytes, T-cells, and mast cells to infection sites.

Inhibition of neutrophil apoptosis, extending effector cell survival during active infection.

Angiogenesis and keratinocyte migration, supporting re-epithelialisation and wound-bed perfusion — the basis of the topical trial results.

Cytokine modulation: increased anti-inflammatory signalling with management of pro-inflammatory output, via binding of specific cell receptors and signalling pathways. The net effect is immune balance rather than suppression.

Anti-biofilm activity. Biofilm-embedded bacteria are estimated at 100 to 1000 times more antibiotic-resistant than planktonic cells. LL-37 both inhibits biofilm formation and penetrates and disrupts established matrix, exposing bacteria to immune clearance.

Antibiotic synergy. In vitro, membrane disruption by LL-37 improves access of beta-lactams and vancomycin to intracellular targets.

Vitamin D regulation. The cathelicidin gene promoter carries a vitamin D response element, so LL-37 expression is directly vitamin D-dependent (Liu et al., 2006). Deficiency lowers endogenous LL-37 and helps explain increased infection susceptibility. A 2022 Frontiers in Immunology paper suggested vitamin D-mediated upregulation of LL-37 may limit severe inflammatory responses in viral infection.

Caveats for systemic use: partial inactivation by serum proteins, proteolytic degradation (trypsin; wound fluid appears protective), and host cell cytotoxicity above approximately 4.5 to 45 mcg/mL (1 to 10 micromolar) define a therapeutic window that injectable protocols aim to stay well beneath.

Antimicrobial: broad-spectrum activity against gram-positive (Staphylococcus, Streptococcus) and gram-negative (E. coli, Pseudomonas, Klebsiella) bacteria, fungi (Candida species), and enveloped viruses (herpes simplex, influenza); documented activity against over 38 bacterial species, 16 fungal species, and 16 viruses. In vitro efficacy against MRSA, vancomycin-resistant Enterococcus, and Pseudomonas aeruginosa biofilms. Bactericidal against *S. aureus*, described as the most common cause of upper respiratory tract infection. Inhibition and disruption of biofilms, relevant to chronic sinusitis, chronic wound infection, and device-associated infection.

Immunomodulatory: chemotaxis of T-cells, monocytes, neutrophils, and mast cells; inhibition of neutrophil apoptosis; LPS neutralisation with protection in animal sepsis models comparable to antibiotics at lower endotoxin and inflammation levels; cytokine balance via receptor and pathway binding. Described as beneficial in sepsis.

Wound healing: two randomised controlled trials of topical LL-37 demonstrated accelerated closure via keratinocyte-driven re-epithelialisation, angiogenesis, and concurrent antimicrobial protection; fibroblast stimulation with enhanced migration and proliferation.

Antibiotic synergy: in vitro synergy with beta-lactams and vancomycin through membrane permeabilisation, potentially overcoming resistance and permitting lower antibiotic doses.

Cardiovascular: lowers heart disease risk; protects against atherosclerosis.

Pulmonary: prevention of lung injury via inhibition of pro-inflammatory cytokines and promotion of anti-inflammatory pathways.

Musculoskeletal: osteoblast recruitment and stimulation of growth factors and cytokines for bone regeneration; protection against collagen damage from inflammatory arthritis.

Oncology — internally contradictory claims: inhibition of tumour cell proliferation, migration, and invasion, an anti-tumour immune environment reducing metastasis, and anti-cancer effects comparable to chemotherapeutics in various human cancer cell lines, set against the contraindication that LL-37 can drive proliferation of tumour cells in breast, ovarian, and lung cancers. The contraindication is operative.

Translational gap: all human data are topical. No evidence confirms that subcutaneous LL-37 reaches infection sites at effective concentrations, persists in circulation given serum protein inactivation and proteolysis, or reproduces the topical dose-response. In the diabetic foot ulcer trial, LL-37 cream improved granulation but did not significantly reduce IL-1 alpha, TNF-alpha, or aerobic bacterial colonisation.

No published pharmacokinetic data exists for injectable LL-37, so onset and duration are inferred from user reports and the topical trials rather than clinical pharmacology.

User-reported course for acute infection or immune support: perceptible immune activation within 2 to 3 days, sometimes with mild flu-like symptoms consistent with cytokine release; symptomatic improvement by the end of the first week. Chronic or biofilm-associated infections — recurrent sinusitis, Lyme co-infections, device-related or chronic wound infection — run 2 to 6 weeks before meaningful improvement. Users combining LL-37 with conventional antibiotics report the combination outperforming antibiotics alone, consistent with in vitro synergy data.

Local reactions (erythema, swelling, warmth, burning or stinging on injection, pruritus) are expected in the first days and typically resolve within hours; they reflect local chemotaxis and immune activation rather than allergy.

Topical benchmark: in Gronberg et al. (2014), measurable healing was evident within 3 to 4 weeks of twice-weekly application, with the 0.5 mg/mL group showing a healing rate constant approximately six times placebo (p = 0.003) versus approximately three times for 1.6 mg/mL (p = 0.088). The inverted dose-response, and necrosis or severe inflammatory reactions at 3.2 mg/mL, indicate that response does not scale with dose.

Transient exacerbation of psoriasis or rosacea is reported by affected users, consistent with LL-37 overexpression in those lesions.

Use is episodic: initiated for a defined infectious, immune, or wound-healing indication and discontinued on resolution. Long-term effects of chronic supplementation are unknown.

Evidence status. All published human dosing is topical: 0.5 mg/mL and 1.6 mg/mL twice weekly for three weeks in venous leg ulcers, and LL-37 cream twice weekly for four weeks in diabetic foot ulcers. No human dose-finding, pharmacokinetic, or safety data exist for the subcutaneous route. The protocol below reflects clinical practice patterns and what users report.

Reconstitution. 5 mg vial with 2 mL bacteriostatic water: 2.5 mg/mL, i.e. 250 mcg per 10 insulin units (25 mcg per unit). Draws: 100 mcg = 4 units (0.04 mL); 125 mcg = 5 units (0.05 mL); 200 mcg = 8 units (0.08 mL); 250 mcg = 10 units (0.10 mL). A 5 mg vial covers 25 days at 200 mcg daily.

Standard protocol. 100 to 200 mcg subcutaneously once daily, no timing or food constraints. Duration 2 to 4 weeks for acute immune support, 4 to 6 weeks for chronic indications.

Per-indication tiers. Acute infection support: 200 mcg daily for 5 to 10 days. Chronic or biofilm infection: 100 to 200 mcg daily for 4 to 6 weeks. Immune support: 100 mcg daily for 2 to 4 weeks. An alternative practitioner pattern is 125 mcg daily for up to 50 days followed by a 2 to 4 week break before repeating.

Rationale for conservative dosing. Host cell cytotoxicity begins above approximately 4.5 to 45 mcg/mL (1 to 10 micromolar); injectable protocols sit well below this. Excess dose produces pronounced local reactions, cytokine-mediated flu-like symptoms, and potential autoimmune aggravation. The topical trials show an inverted dose-response — 0.5 mg/mL outperformed 1.6 mg/mL, and 3.2 mg/mL produced ulcer necrosis or severe inflammatory reactions — so titration here is for tolerability, not for effect. Start at 100 mcg and assess before increasing.

Systemic caveats. Partial inactivation by serum proteins and proteolytic degradation may reduce systemic efficacy relative to local application; no evidence confirms effective concentrations at infection sites after subcutaneous dosing.

A previous iteration of this page carried an eight-step ladder from 50 mcg to a 450 mcg ceiling over 8 to 12 weeks at 2.5 mL reconstitution, with a 4 to 8 week washout. The current protocol supersedes it with a lower ceiling and shorter courses.

Storage. Lyophilised: minus 20 degrees Celsius long term, 2 to 8 degrees short term, dry, light-protected. Reconstituted: 2 to 8 degrees, use within 28 days, protect from light, do not freeze.

Human data are exclusively topical.

Gronberg et al., 2014 (Wound Repair and Regeneration): randomised, placebo-controlled trial in 34 patients with hard-to-heal venous leg ulcers, allocated to vehicle, 0.5 mg/mL, or 1.6 mg/mL LL-37 applied twice weekly for three weeks with four weeks follow-up. Healing rate constant approximately six-fold over placebo at 0.5 mg/mL (p = 0.003) and approximately three-fold at 1.6 mg/mL (p = 0.088). No serious adverse events attributed to the peptide. A 3.2 mg/mL arm showed reduced efficacy with ulcer necrosis or severe inflammatory reactions. Inverted dose-response. Limitations: small sample, short duration, topical only.

Sigurdardottir et al., 2021 (Wound Repair and Regeneration): multicentric prospective randomised placebo-controlled trial in hard-to-heal venous leg ulcers, confirming topical wound healing benefit.

Pukkapote et al., 2023 (Archives of Dermatological Research; NCT04098562): randomised double-blind controlled trial of LL-37 cream in 25 diabetic foot ulcer patients (13 active, 12 placebo), twice weekly for 4 weeks. Consistently greater granulation index at days 7, 14, 21, and 28; enhanced healing in ulcers with mild infection. No significant reduction in IL-1 alpha or TNF-alpha and no reduction in aerobic bacterial colonisation. Limitations: small sample, short duration, topical only.

In vitro: bactericidal activity against MRSA, vancomycin-resistant Enterococcus, Pseudomonas aeruginosa biofilms, Candida species, herpes simplex virus, and influenza; activity documented against over 38 bacterial species, 16 fungal species, and 16 viruses. Synergy with beta-lactams and vancomycin via membrane permeabilisation. LPS neutralisation; in animal sepsis models, protection comparable to antibiotics with significantly lower endotoxin and inflammation. A 2022 Frontiers in Immunology paper suggested vitamin D-driven upregulation of LL-37 may limit severe inflammatory responses in viral infection, consistent with the vitamin D response element in the cathelicidin promoter.

Gaps: no human trial, dose-finding, pharmacokinetic, or safety data for injectable LL-37. Systemic efficacy is uncertain given partial inactivation by serum proteins and proteolytic susceptibility (trypsin), although wound fluid appears protective. Host cytotoxicity above 4.5 to 45 mcg/mL (1 to 10 micromolar) defines the therapeutic window. Synthesis cost is significant for a 37-residue peptide. The cell culture and plate data do not establish in vivo efficacy or optimal human dosing.