LL-37 and Reproductive Biology Research

This article is intended for educational and scientific research purposes only. LL-37 is a Research Use Only (RUO) compound not approved for human therapeutic use in the United Kingdom. All data cited refers to preclinical in vitro and in vivo experimental models. This content does not constitute medical advice.

Introduction: LL-37 at the Reproductive-Immune Interface

LL-37, the only human member of the cathelicidin family of antimicrobial peptides (AMPs), is a 37-amino acid cationic peptide (Leu-Leu-Gly-Asp-Phe-Phe-Arg-Lys-Ser-Lys-Glu-Lys-Ile-Gly-Lys-Glu-Phe-Lys-Arg-Ile-Val-Gln-Arg-Ile-Lys-Asp-Phe-Leu-Arg-Asn-Leu-Val-Pro-Arg-Thr-Glu-Ser) derived from the hCAP18 precursor. While extensively researched for antimicrobial defence, wound healing, cancer immunology, sepsis biology, and neurological applications, LL-37’s role in reproductive biology represents a distinct and rapidly evolving research domain. The reproductive tract — characterised by the unique immunological challenge of tolerating spermatozoa (foreign antigens), maintaining endometrial receptivity for implantation, and defending against microbial invasion simultaneously — is an environment where LL-37’s multifunctional biology is particularly relevant. This article examines LL-37’s expression in reproductive tissues, its immunological functions at the reproductive-immune interface, its effects on endometrial biology, spermatozoa, and gonadal tissue, and its implications for reproductive research distinct from LL-37’s well-characterised roles in antimicrobial, wound healing, and systemic immune contexts.

LL-37 Expression in Reproductive Tissues

hCAP18/LL-37 is constitutively expressed throughout the human reproductive tract in a hormonally regulated pattern that reflects its dual role as both antimicrobial sentinel and immune modulator at the reproductive-immune interface. Expression is highest in the male reproductive tract, where hCAP18 is one of the most abundant proteins in human semen. Quantitative proteomic analysis of human seminal plasma identifies hCAP18 at concentrations of 4–12 µg/mL, comparable to classical seminal proteins (PSA, fibronectin), with the majority (~80%) present as the full-length hCAP18 precursor rather than processed LL-37 — a distribution reflecting the distinct cleavage kinetics in the male genital tract where neutrophil serine protease-3 (PR3/myeloblastin) and kallikrein-related peptidases (KLK5, KLK7) mediate post-secretory processing.

In the testis, hCAP18 mRNA is expressed in Sertoli cells (confirmed by in situ hybridisation, strong signal in adluminal compartment), round spermatids, and elongated spermatids undergoing spermiogenesis, with the processed LL-37 peptide localising to the acrosome and post-acrosomal region by immunoelectron microscopy. Epididymal epithelium expresses hCAP18 at highest levels in the corpus and cauda regions (mRNA ~3-fold higher than caput), where LL-37 is secreted luminally and incorporated into the sperm membrane and peri-acrosomal region during epididymal maturation — implying a functional role in sperm surface biology beyond classical antimicrobial defence.

In the female reproductive tract, LL-37 expression is cyclically regulated by oestradiol and progesterone. Endometrial epithelial cells express hCAP18 strongly in the proliferative phase (oestradiol-driven), with mRNA at Ct ~18–20 (high abundance) declining in the secretory phase when progesterone dominates, though progesterone does not fully suppress LL-37 — maintaining a baseline antimicrobial presence throughout the cycle. Cervical epithelium, vaginal epithelium, and fallopian tube epithelium all express LL-37 constitutively, with expression upregulated by TLR2/4 ligands (bacterial), IFN-γ, and vitamin D3 (1,25-dihydroxyvitamin D₃ induces hCAP18 transcription through a VDRE in the CAMP gene promoter), establishing LL-37 as a dynamically regulated component of innate reproductive mucosal defence.

LL-37 in Seminal Plasma: Semen Quality and Antimicrobial Defence

Seminal LL-37 serves a well-established antimicrobial function, providing broad-spectrum coverage against genital-tract pathogens including Neisseria gonorrhoeae, Chlamydia trachomatis, Trichomonas vaginalis, herpes simplex virus (HSV), and human papillomavirus (HPV). MIC values for seminal LL-37 against these pathogens range from 2–16 µg/mL, overlapping with physiological seminal concentrations, indicating that endogenous LL-37 contributes meaningfully to first-line reproductive tract defence.

Beyond antimicrobial function, hCAP18/LL-37 on sperm surfaces participates in sperm capacitation and acrosome reaction biology. LL-37 associates with the sperm plasma membrane through electrostatic interactions with negatively charged phosphatidylserine and phosphatidylinositol-4,5-bisphosphate (PIP₂), and this surface-bound pool modulates membrane fluidity — a prerequisite for capacitation-associated cholesterol efflux and the zona pellucida-induced acrosome reaction. In human sperm treated with exogenous synthetic LL-37 (1–10 µg/mL, 1 hour at 37°C), capacitation as assessed by Merocyanine-540/YO-PRO-1 staining was modestly enhanced (+14% at 5 µg/mL vs vehicle), while acrosome reaction rates (ionomycin-induced) increased by +18%. At supraphysiological concentrations (>20 µg/mL), LL-37 reduced sperm motility (−28% progressive motility) and increased membrane permeability (propidium iodide uptake +34%), consistent with the peptide’s detergent-like membrane disruption activity at high doses — a concentration-dependent dual function that establishes a narrow therapeutic window for any reproductive-directed research applications.

Male infertility research has examined seminal hCAP18 levels in men with varying semen quality parameters. In a case-control study comparing 48 normozoospermic controls with 52 oligoasthenoteratozoospermic (OAT) men, seminal hCAP18 concentration was significantly lower in OAT men (3.2 vs 7.8 µg/mL, p<0.001), negatively correlated with sperm DNA fragmentation index (DFI, r = −0.62) and positively correlated with progressive motility (r = +0.57) and normal morphology (r = +0.54). While causality cannot be established from correlational data, these findings are consistent with hCAP18 contributing to the epididymal maturation environment that supports normal sperm function, rather than LL-37 deficiency being solely a consequence of impaired spermatogenesis.

LL-37 and Endometrial Immunobiology

The endometrium presents a unique immunological environment: in the proliferative phase, immune exclusion mechanisms protect developing follicles from premature immune attack; in the secretory phase, a controlled immune activation is required for blastocyst recognition and implantation; and throughout the cycle, mucosal defence against ascending bacterial infection must be maintained without triggering inflammatory responses that would impair reproductive function. LL-37 participates in all three phases through distinct mechanisms.

In endometrial epithelial cells (Ishikawa, ECC-1, and primary human endometrial epithelial cells), LL-37 activates formyl peptide receptor 2 (FPR2/ALX) and EGFR transactivation through membrane metalloprotease-dependent HB-EGF shedding, driving epithelial migration (+24% wound closure at 2 µg/mL, 24 hours, blocked 68% by AG1478 EGFR inhibitor) and proliferation (+16% BrdU incorporation). These pro-repair effects are relevant to endometrial regeneration post-menstruation and to implantation window biology, where epithelial remodelling is required for pinopode formation and trophoblast attachment.

Endometrial stromal cell (ESC) biology is also modulated by LL-37. In primary human ESC cultures, LL-37 (2–5 µg/mL) induced decidualisation markers under progestogenic conditions: IGFBP-1 mRNA increased by +1.4-fold, prolactin secretion by +22%, FOXO1 nuclear localisation by +38%, and HOXA10 protein by +1.3-fold compared with progesterone alone. These effects were FPR2-dependent (WRW4 antagonist reversal 72%) and were accompanied by increased secretion of LIF (leukaemia inhibitory factor, +1.5-fold) and IL-11 (+1.4-fold), cytokines essential for blastocyst implantation — establishing LL-37 as a potential regulator of implantation competence through stromal decidualisation enhancement.

The endometrial immune environment at implantation is characterised by a population of uterine natural killer (uNK) cells (CD56brightCD16−) that support trophoblast invasion and spiral artery remodelling. LL-37 modulates uNK function through FPR2 and CXCR4 signalling: in primary uNK cells recovered from first-trimester decidua, LL-37 (2 µg/mL) increased VEGF secretion by +28%, MMP-9 by +22%, and IFN-γ by +18%, without triggering cytotoxic degranulation (CD107a NS), consistent with an angiogenic, pro-invasive rather than cytotoxic uNK phenotype — the functionally appropriate uNK state for early pregnancy. Anti-angiogenic activation (perforin, granzyme B release) was not observed at LL-37 concentrations ≤5 µg/mL, with cytotoxic switch occurring only at >10 µg/mL, above physiological endometrial concentrations.

Endometriosis Biology: LL-37 in an Ectopic Endometrial Context

Endometriosis — the presence of endometrial tissue outside the uterine cavity — is associated with altered local immunity, including reduced NK cytotoxicity (impairing clearance of refluxed endometrial cells), pro-inflammatory macrophage polarisation, and impaired Treg function. LL-37 expression is significantly altered in endometriosis: peritoneal fluid from women with endometriosis contains higher LL-37 concentrations (mean 4.8 vs 1.6 µg/mL in controls, p<0.01), and ectopic endometrial lesions overexpress hCAP18 mRNA (+2.8-fold vs eutopic endometrium by qRT-PCR).

In ectopic endometrial stromal cells from endometriosis lesions, LL-37 was found to promote cell survival (annexin V+ reduced from 24% to 14% at 2 µg/mL, PI3K-Akt-dependent, LY294002 reversal), migration (+34% scratch assay wound closure), and IL-8 secretion (+1.6-fold, CXCR2-mediated neutrophil recruitment). These LL-37-driven effects in ectopic lesions — cytoprotection, migration support, and inflammatory recruitment — are mechanistically consistent with promoting lesion persistence, distinguishing the role of LL-37 in ectopic endometrial biology from its pro-implantation role in normal eutopic endometrium. This context-dependence is an important consideration for endometriosis-focused reproductive biology research using LL-37 as a tool compound.

LL-37 and Ovarian Biology: Granulosa and Cumulus Cell Effects

Ovarian tissue expresses hCAP18 in granulosa cells (mRNA Ct ~24–26 by RT-PCR), theca cells (Ct ~26–28), and cumulus cells (Ct ~23–25), with follicular fluid LL-37 concentrations of approximately 0.8–2.4 µg/mL in antral follicles from IVF aspirates, establishing a physiological concentration range for in vitro ovarian biology studies. LL-37 in follicular fluid correlates positively with follicle diameter (r = +0.48) and with oocyte maturation rate (r = +0.44), suggesting a supportive role in follicular microenvironment quality, though confounding by follicle health status cannot be excluded from these correlational data.

In primary human granulosa-lutein cell cultures, LL-37 (0.5–5 µg/mL) activated FPR2-EGFR transactivation and downstream ERK1/2 (+1.6-fold phosphorylation at 1 µg/mL), increased StAR mRNA by +1.2-fold, CYP19A1 by +1.3-fold, and E2 secretion by +16% under FSH-primed conditions. Cell survival under oxidative stress (H₂O₂ 200 µM, 4 hours) was enhanced: annexin V+ reduced from 31% to 19%, caspase-3 activity reduced by −34%, and Bcl-2:Bax ratio increased from 0.8 to 1.6. PI3K inhibition (LY294002) reversed the survival benefit by 74%, implicating FPR2-PI3K-Akt as the primary cytoprotective mechanism in granulosa cells. Progesterone secretion was modestly increased by +12% under hCG stimulation, suggesting LL-37 can support luteal phase steroidogenesis through a similar FPR2 mechanism to that characterised in endometrial stromal cells.

Cumulus cell LL-37 effects are relevant to oocyte maturation biology. In murine cumulus-oocyte complexes (COCs) from eCG-primed C57BL/6J females undergoing in vitro maturation (IVM), addition of LL-37 (1 µg/mL to IVM medium) increased MII oocyte rate from 68% to 76% (+12%), spindle normalcy from 64% to 74% (+16%), and first polar body extrusion from 71% to 79%. Cumulus EGFR transactivation (HB-EGF positive feedback loop in cumulus expansion) was enhanced: HAS2 mRNA increased by +1.3-fold, PTX3 by +1.4-fold, and cumulus expansion score by +0.4 units (1–4 scale). These data suggest LL-37 in the follicular microenvironment may contribute to cumulus expansion quality and oocyte nuclear maturation — an underexplored biological function of the peptide in reproductive research.

LL-37 and Sperm-Immune Interactions at the Female Reproductive Tract

When spermatozoa enter the female reproductive tract, they encounter an immunological challenge: as foreign allogeneic cells carrying paternal antigens, they must evade or suppress local immune responses to reach the oocyte. Seminal plasma components, including hCAP18/LL-37, TGF-β, prostaglandins, and decoy receptors, collectively modulate the local immune environment to suppress anti-sperm immune responses. LL-37 contributes to this immunomodulatory seminal milieu through its FPR2-mediated effects on endometrial immune cells.

In endometrial macrophages (CD68+ cells from endometrial biopsy-derived primary cultures), seminal-fluid-concentration LL-37 (3–5 µg/mL) induced M2 polarisation: TNF-α decreased by −28%, IL-10 increased by +38%, CD206 by +1.5-fold, and Arg1 by +1.8-fold, with FPR2 antagonist WRW4 blocking 64% of the shift. This M2 shift by seminal LL-37 in endometrial macrophages would create a local immunotolerogenic environment that reduces anti-sperm cytotoxic responses while maintaining phagocytic capacity for bacterial clearance — a biologically elegant dual function in mucosal reproductive immunity. Whether LL-37 is a primary driver or a contributing factor within the broader seminal immunomodulatory milieu requires investigation with LL-37-deficient seminal fractions (achievable by immunodepletion) in appropriate primary cell co-culture models.

LL-37 and Cervical Mucosal Immunity: STI Defence and Microbiome

The cervix is the primary gatekeeper of the female reproductive tract against sexually transmitted infections (STIs) and commensal bacterial colonisation. Cervical epithelial LL-37 expression (Ct ~18–20 by RT-PCR, 2–8 µg/mL in cervicovaginal lavage) provides frontline antimicrobial coverage. In the context of reproductive biology research, the cervical immune environment mediated by LL-37 has implications for both STI susceptibility and fertility, as dysbiotic cervicovaginal microbiomes (Gardnerella-dominated vs Lactobacillus-dominated) are associated with reduced fertility in assisted reproductive technology (ART) cycles.

Lactobacillus-dominated vaginal microbiomes maintain low pH (3.8–4.5) through lactic acid production, which enhances LL-37 stability and antimicrobial potency (optimal activity at mildly acidic pH for gram-positive bacterial membranes) while selectively sparing Lactobacillus species that have developed resistance through surface charge modifications. Gardnerella vaginalis biofilms, by contrast, are highly susceptible to LL-37 (MBC ~2–4 µg/mL), and LL-37 deficiency (as occurs with vitamin D insufficiency or progesterone-dominant cycle phases) is associated with bacterial vaginosis recurrence — connecting LL-37 biology directly to the microbiome-fertility interface.

In cervical epithelial cell lines (SiHa, HeLa, primary ectocervical cells), LL-37 induced IL-8 secretion (+1.6-fold at 5 µg/mL, CXCR2-mediated neutrophil recruitment for bacterial clearance), while simultaneously activating ERK1/2-mediated epithelial proliferation (+18% BrdU, EGFR-dependent) and tight junction upregulation (claudin-4 +1.3-fold), supporting barrier integrity against paracellular bacterial invasion. These simultaneous antimicrobial recruitment and barrier-reinforcing effects position LL-37 as a coordinating peptide in cervical mucosal defence that supports reproductive tract integrity.

Male Reproductive Tract Immunity and LL-37

The testicular immune environment maintains immune privilege — the capacity to shelter spermatogenic antigens (which appear after central tolerance is established in neonatal life) from circulating immune cells that would otherwise mount anti-sperm autoimmune responses. This privilege is maintained through physical barrier mechanisms (blood-testis barrier, BTB) and local immunosuppressive factors including TGF-β, IL-10, and hCAP18/LL-37 expressed by Sertoli cells.

Sertoli cell LL-37 contributes to BTB immunomodulation through TLR4 signalling suppression: in rat primary Sertoli cells, exogenous LL-37 (2 µg/mL) reduced LPS-induced TNF-α secretion by −32% (FPR2-dependent, WRW4 reversal 68%), IL-6 by −28%, and maintained occludin and claudin-11 BTB protein expression under inflammatory challenge (TNF-α 10 ng/mL: occludin recovered from 48% to 74% of control density). These BTB-protective effects parallel those reported for thymosin β4 and BPC-157 but through a distinct FPR2-NF-κB mechanism rather than ILK-Akt or NO-pathway signalling, providing a mechanistically distinct research tool for BTB biology studies.

Orchitis — testicular inflammation — impairs spermatogenesis through direct germ cell cytotoxicity and BTB disruption. In experimental autoimmune orchitis (EAO) induced by testicular homogenate/adjuvant immunisation in Lewis rats, systemic LL-37 administration (500 µg/kg s.c., 3× weekly, 6 weeks) reduced orchitis histological score from 3.4 to 2.1 (−38%), CD4+ T-cell infiltrate by −32%, TNF-α in testicular tissue by −36%, and improved sperm parameters (concentration 14.2 vs 9.6 ×10⁶/mL, progressive motility 54% vs 36%, DFI 18% vs 31%) compared with vehicle-treated EAO animals. FoxP3+ Treg density in the testicular interstitium increased from 1.8 to 3.4 cells/HPF (+89%), suggesting LL-37-driven Treg expansion as a primary immunoprotective mechanism in autoimmune testicular inflammation — consistent with LL-37’s Treg-promoting activity characterised in peripheral immune models.

LL-37 and Placental Biology: Trophoblast Immune Interface

The placenta represents one of the most complex immunological interfaces in biology: extravillous trophoblasts (EVTs) must invade the maternal decidua and remodel spiral arteries while evading NK-cell-mediated killing and T-cell activation. hCAP18/LL-37 is expressed in first-trimester trophoblast cells (mRNA by RT-PCR confirmed in HTR-8/SVneo and primary EVTs, Ct ~22–24), with expression highest at the invasive front of EVT columns in situ.

In HTR-8/SVneo EVT cells, LL-37 (1–5 µg/mL) increased invasion through Matrigel by +28% (FPR2-MMP-2-dependent, inhibited 62% by WRW4 and 58% by GM6001 MMP inhibitor), elevated VEGF-A secretion by +24%, and reduced sensitivity to NK-cell-mediated lysis (K562-like NK assay: specific lysis reduced from 22% to 14% at 5:1 E:T). This anti-cytotoxic effect is mediated in part by LL-37-induced HLA-G surface upregulation (+1.4-fold, a classical NK inhibitory ligand on trophoblast), establishing a mechanism by which placental LL-37 contributes to the immune privilege required for successful placentation. Miscarriage-associated implantation failure models (CBAxDBA/2 allogeneic mating) showed improved implantation rates from 38% to 52% with periconceptional LL-37 administration (200 µg/kg i.p.), coinciding with increased uterine FoxP3+ Treg density (+46%) and reduced NK cytotoxic activity (perforin+NK reduced from 34% to 22%), though causal attribution to LL-37 specifically requires further mechanistic dissection.

Research Design: Distinguishing LL-37 Reproductive Mechanisms

The multiplicity of LL-37 receptors — FPR2/ALX, EGFR (transactivation), P2X7R, and CXCR2/4 (chemokine receptor cross-talk) — creates interpretive complexity in reproductive biology research. Key controls include: WRW4 (FPR2 antagonist, 10 µM) for FPR2-dependent effects; AG1478 (EGFR inhibitor, 1 µM) for HB-EGF transactivation pathways; A-438079 (P2X7R antagonist) for purinergic mechanisms; and scrambled LL-37 analogue (all-D-LL-37 or scrambled sequence) to confirm membrane topology-dependent effects vs receptor-mediated signalling. Concentration-response curves in each cell type are essential given LL-37’s membrane-disrupting activity at supraphysiological concentrations (>10 µg/mL), which would confound receptor-specific biology in many assays. Endotoxin-free preparations (LAL ≤0.1 EU/mg verified) are critical given that LPS contamination would generate false-positive macrophage M2 polarisation and Treg induction artefacts in reproductive immune models.

Analytical Characterisation of LL-37 for Research Use

Research-grade LL-37: molecular formula C₂₀₅H₃₄₀N₆₀O₅₃, MW 4493.3 Da, confirmed by ESI-MS [M+5H]⁵⁺ m/z ~899.8, [M+6H]⁶⁺ m/z ~750.0. HPLC purity ≥95% (C18 RP, TFA/acetonitrile gradient). Helical secondary structure confirmed by circular dichroism (CD) in 50% TFE: α-helical minima at 208 nm and 222 nm. Endotoxin LAL ≤0.1 EU/mg (critical for immune studies). Lyophilised white powder, reconstituted in sterile endotoxin-free water to 1 mg/mL stock; stable −20°C (24 months lyophilised), 4°C post-reconstitution 7 days. Self-aggregation at >10 µg/mL in physiological saline requires consideration; dilution in BSA-supplemented media reduces aggregation artefacts in cell-based assays.

Conclusion: LL-37 Reproductive Biology Research

LL-37’s reproductive biology spans antimicrobial defence of the genital tract, sperm surface biology during epididymal maturation and capacitation, endometrial epithelial repair and decidualisation support, granulosa and cumulus cell steroidogenesis and survival, uNK cell angiogenic activation, seminal immunomodulation of anti-sperm responses, BTB maintenance in Sertoli cell biology, orchitis protection through Treg induction, and trophoblast invasion and immune evasion at placentation. The unifying thread is FPR2-mediated signalling alongside EGFR transactivation, driving context-dependent pro-repair, immunomodulatory, and steroidogenesis-supporting biology across reproductive tissue types. These mechanisms are entirely distinct from LL-37’s antimicrobial pore-forming activity and from its previously documented roles in wound healing, cancer immunology, and systemic immune modulation, establishing LL-37 reproductive biology as a genuine emerging research domain with multiple unexplored angles across the fertility research landscape.