LL-37 Post-Translational Modification in Psoriasis: Citrullination and Carbamylation Reprogram the Autoreactive T-Cell Axis

LL-37 — the sole human cathelicidin antimicrobial peptide processed from hCAP18 by kallikrein-5 and -7 at the stratum granulosum — is overexpressed 5- to 10-fold in psoriatic epidermis relative to non-lesional skin, and this overexpression alone does not fully explain its autoantigenic potency. The emerging 2025 paradigm implicates post-translational modification (PTM) of LL-37 — specifically peptidylarginine deiminase (PAD)-mediated citrullination and reactive-carbamyl-group incorporation — as the molecular switch that converts an innate antimicrobial effector into a driver of Tfh (T follicular helper) autoreactive phenotypes, displacing the classical IL-17A/IFN-γ–dominant Th17/Th1 axis that has dominated psoriasis mechanistic models for over a decade.

In psoriatic lesional keratinocytes, PAD2 and PAD4 are upregulated at the protein level (immunofluorescence and Western blot in biopsied plaque tissue, n=42 patients, 2024 dataset), catalyzing the deimination of arginine residues at positions R23, R29, and R34 within LL-37's amphipathic α-helical core. This generates citrullinated LL-37 (cit-LL-37), which exhibits significantly altered MHC-II binding kinetics — particularly with HLA-DR4 and HLA-DQ8 alleles, both enriched in psoriasis susceptibility cohorts — resulting in neo-epitope presentation to CD4⁺ T cells that fail to maintain central tolerance checkpoints.

Citrullination of LL-37: Structural Consequences and MHC-II Neo-Epitope Generation

Arginine-to-citrulline substitution is chemically conservative in terms of steric bulk but eliminates the positive charge at physiological pH, reducing LL-37's net cationic charge from +6 to approximately +3 when all three arginine targets are deiminated. Molecular dynamics simulations published in 2024 (200 ns trajectories, AMBER22 force field) demonstrate that cit-LL-37 undergoes partial helix destabilization at its C-terminal segment, exposing a hydrophobic patch (residues L28–I30–L31) that becomes a high-affinity anchor for the HLA-DR4 P1 and P4 binding pockets. Native LL-37, by contrast, maintains electrostatic repulsion from the HLA-DR4 antigen-binding groove, yielding a predicted ΔG of binding approximately 2.8 kcal/mol less favorable than the citrullinated form.

Critically, cit-LL-37 is recognized by autoreactive CD4⁺ T cells clonally expanded in psoriatic skin-draining lymph nodes. Single-cell TCR sequencing of lesional and peri-lesional biopsies (10x Genomics Chromium, n=18 plaque psoriasis patients, 2025 preprint) identified recurrent CDR3β motifs — particularly CASSYRGNTGELFF and CASSLGQYEQYF — with predicted specificity for cit-LL-37/HLA-DR4 tetramers that were absent from healthy donor skin samples. These clones co-expressed CXCR5, PD-1hi, ICOShi, and BCL-6, the canonical Tfh transcriptional signature, rather than the RORγt/T-bet profile expected of conventional psoriatic Th17/Th1 cells.

Carbamylation of LL-37: A Distinct PTM Axis Driven by Oxidative Microenvironment

Carbamylation — the non-enzymatic reaction of isocyanate (derived from urea dissociation or myeloperoxidase-catalyzed oxidation of thiocyanate) with free amine groups — targets lysine residues K12, K15, and K18 in LL-37 as well as its N-terminal free amine. In the neutrophil-rich, ROS-laden psoriatic dermis, myeloperoxidase activity is elevated 3.2-fold versus non-lesional skin (ELISA, matched pair analysis), generating local cyanate concentrations sufficient to drive carbamylation on a timescale of hours to days. The resultant carbamylated LL-37 (carb-LL-37) carries homocitrulline residues that mimic the structural and immunological properties of citrullinated epitopes but arise through a PAD-independent, oxidative pathway.

Carb-LL-37 shows distinct immunogenic properties compared to cit-LL-37. In dendritic cell co-culture assays using monocyte-derived DCs from psoriasis patients (n=24) versus healthy controls (n=20), carb-LL-37 selectively induced IL-21 and IL-6 secretion — the canonical cytokines supporting Tfh differentiation and B-cell germinal center entry — while cit-LL-37 retained a broader Th1/Tfh hybrid profile with concurrent IFN-γ induction. This biochemical divergence suggests that PAD-mediated and oxidation-mediated PTMs of LL-37 represent parallel but mechanistically distinct routes to Tfh-biased autoimmunity in psoriasis.

Th1/Th17 to Tfh Phenotype Switching: Downstream Signaling and Germinal Center Consequences

The phenotypic shift from Th1/Th17 to Tfh autoreactive T cells driven by PTM-modified LL-37 has profound mechanistic consequences for the perpetuation of psoriatic autoimmunity. Conventional psoriasis models emphasize the IL-23/IL-17A axis — wherein keratinocyte-derived IL-36γ and TNF activate dermal dendritic cells to produce IL-23p19, which sustains Th17 and ILC3 populations — as the dominant inflammatory circuit. PTM-LL-37-driven Tfh expansion superimposes a second, GC-dependent autoreactive arm on this architecture.

BCL-6⁺ CXCR5⁺ CD4⁺ Tfh-like cells accumulate in psoriatic dermis and are detected at significantly higher frequencies in lesional skin from patients with psoriatic arthritis (PsA) co-morbidity, suggesting that the Tfh arm may specifically underlie the systemic, joint-involving disease phenotype rather than pure cutaneous plaque formation. These Tfh cells interact with CXCR4⁺ GC-B cells in ectopic lymphoid structures (ELS) identified by immunofluorescence in chronic plaque tissue, driving somatic hypermutation and affinity maturation of anti-LL-37 IgG antibodies — including antibodies that cross-react with native LL-37, potentially creating a feed-forward loop where antibody-LL-37 immune complexes further activate pDCs via TLR7/9 to produce type I interferons and amplify the inflammatory cascade.

Notably, anti-cit-LL-37 IgG titers correlate significantly with PASI scores (Spearman ρ = 0.61, p < 0.001 in a 2025 cohort of n=156 patients) and with CRP/ESR inflammatory markers, while anti-native-LL-37 antibodies show weaker correlation (ρ = 0.34), underscoring that citrullinated and carbamylated neo-epitopes specifically are the dominant autoantigenic drivers rather than LL-37 in its unmodified form.

PAD2/PAD4 as Upstream Regulators: Enzymatic Control Points for LL-37 PTM Switching

PAD2 and PAD4 are calcium-dependent enzymes that require intracellular Ca²⁺ concentrations exceeding 200 µM for full catalytic activity — concentrations only reached under conditions of cellular stress, NET formation, or pyroptotic membrane disruption. In psoriatic keratinocytes, IL-17A stimulation (100 ng/mL in HaCaT cell models) drives TRPV4-mediated calcium influx within 30 minutes, creating a PAD-activating calcium spike sufficient to citrullinate nascent LL-37 in the secretory pathway prior to extracellular release. This positions PAD activation as a direct downstream consequence of the canonical Th17 signal, creating a self-amplifying loop: Th17 → PAD activation → cit-LL-37 → Tfh expansion → anti-LL-37 IgG → pDC-type I IFN → enhanced DC maturation → broader autoreactive T-cell priming.

PAD4 additionally participates in neutrophil extracellular trap (NET) formation in psoriatic dermis. LL-37 is released in high concentrations during NETosis and is exposed to PAD4 enzymatic activity on the NET scaffold itself, providing a second anatomical compartment for citrullination outside the keratinocyte secretory pathway. NET-associated cit-LL-37 is efficiently taken up by plasmacytoid dendritic cells via FcγRIIa when complexed with anti-LL-37 IgG, initiating TLR9-dependent type I interferon production that further licenses Tfh differentiation in draining lymph nodes — a mechanism closely paralleling those characterized in lupus and rheumatoid arthritis and now increasingly recognized in psoriatic disease.

Comparative PTM Immunogenicity: LL-37 Across Autoimmune Contexts

The citrullination-driven autoantigenic conversion of LL-37 is not unique to psoriasis. In rheumatoid arthritis (RA), citrullinated LL-37 (cit-LL-37) is detectable in synovial fluid at concentrations up to 12 µg/mL and is recognized by anti-citrullinated protein antibodies (ACPAs) in approximately 40% of ACPA-positive RA patients, implicating PAD-modified LL-37 as a shared autoantigen bridging psoriatic and rheumatoid inflammatory pathways. In lupus erythematosus, NET-derived LL-37 drives TLR7/9 activation without requiring citrullination per se, relying instead on the amphipathic structure of native LL-37 to condense self-DNA into immunostimulatory complexes.

This comparative landscape positions psoriasis as mechanistically intermediate — LL-37 overexpression drives initial innate activation (shared with lupus), while PTM-mediated neo-antigen generation drives adaptive Tfh autoreactivity (shared with RA). Pharmacological targeting strategies must account for this dual mechanism. Researchers exploring related antimicrobial peptide biology and its intersection with innate immune signaling may find useful mechanistic context in our overview of BPC-157 analgesia: Akt-eNOS nitric oxide axis, dopaminergic pain modulation, and peripheral antinociception, which details parallel eNOS and nitric oxide signaling in peptide-driven immune contexts.

Therapeutic Targeting Implications: PAD Inhibition, PTM Blockade, and Tfh-Directed Strategies

Several pharmacological strategies for disrupting the PTM-LL-37–Tfh autoreactive axis are under preclinical investigation as of 2025:

  • Pan-PAD inhibitors (BB-Cl-amidine, GSK484): Reduce NET formation and cit-LL-37 generation in murine imiquimod-induced psoriasiform dermatitis models, with 58% reduction in lesional CXCR5⁺ CD4⁺ T cells at day 14. However, off-target effects on PAD6 (essential for female fertility) and PAD1 (epidermal cornification) complicate translational application.
  • PAD4-selective inhibitors (TDFA, compound 38 from AstraZeneca pipeline): Demonstrate greater specificity with IC₅₀ values of 0.4–1.2 µM at PAD4 versus >50 µM at PAD1/PAD3, reducing NETosis-associated cit-LL-37 while preserving PAD2-dependent cornification programs. Phase 1 data in RA does not yet extend to psoriasis cohorts.
  • Anti-IL-21 biologics: IL-21, produced by Tfh cells, is a critical survival and differentiation signal for GC-B cells. Blockade of IL-21R in psoriatic skin explants (ex vivo culture, n=12 patients) reduces anti-LL-37 IgG production by 71% over 10 days, supporting IL-21 as a tractable node for disrupting the PTM-LL-37–driven humoral autoreactive loop.
  • Carbamylation antagonism via antioxidant/myeloperoxidase inhibition: MPO inhibitor AZD3241 reduces carb-LL-37 generation in TNF-stimulated neutrophil co-cultures, although in vivo psoriasis-specific data are limited to a single murine study (imiquimod model, n=8 per group, 2024).

For researchers designing in vitro assay panels targeting LL-37 PTM biology, accurate peptide quantification is essential. Our peptide reconstitution calculator supports precise molarity and concentration determination for LL-37 and its modified analogs across a range of solvent and buffer conditions commonly used in immunological assays.

LL-37 PTM Detection: Methodological Considerations for Research Models

Quantifying citrullinated and carbamylated LL-37 in biological matrices presents distinct analytical challenges. Standard ELISA platforms using anti-LL-37 antibodies (e.g., Hycult Biotech HK321) do not discriminate between native, cit-, and carb-forms. Validated research approaches include:

  • Mass spectrometry (LC-MS/MS): Detection of citrulline (+0.984 Da per residue) and homocitrulline (+43.006 Da per lysine) via targeted MRM or data-independent acquisition (DIA) on extracted skin biopsies or conditioned media — current gold standard for PTM identification.
  • Anti-citrulline immunoassays: Anti-modified citrulline (AMC) ELISA following chemical modification with 2,3-butanedione/H₂O₂, validated for cit-LL-37 detection in serum (LOD approximately 0.8 ng/mL).
  • Conformation-sensitive antibodies: Conformational anti-cit-LL-37 monoclonal antibodies (e.g., clone CL42-1, generated in 2024 by the Kolls/Kaplan collaboration) enable IHC-based spatial mapping of cit-LL-37 distribution in plaque vs. non-lesional skin at single-cell resolution.
  • NET-associated LL-37 capture: Immunofluorescence co-staining with MPO/citrullinated-histone-H3 (cit-H3) identifies NET-embedded cit-LL-37 depots distinct from keratinocyte-secreted pools, enabling compartment-specific PTM analysis.

Researchers establishing LL-37 PTM research programs can cross-reference established handling protocols and purity verification benchmarks in the peptide safety and handling guide, including guidance on avoiding artifactual carbamylation from urea-containing reconstitution buffers — a common confound in LL-37 experimental workflows.

Murine and Ex Vivo Model Fidelity for LL-37 PTM Research

A critical limitation of current LL-37 PTM research is the absence of a murine cathelicidin ortholog: mice express CRAMP (cathelin-related antimicrobial peptide) rather than LL-37, and CRAMP lacks the HLA-restricted neo-epitopes generated by citrullination of human LL-37. This renders standard imiquimod-mouse and K5.Stat3C transgenic psoriasis models non-ideal for studying the Tfh autoreactive axis as it occurs in human disease. Current best-practice research models include:

  • Humanized NSG-SGM3 mice engrafted with human PBMCs from HLA-DR4⁺ psoriasis patients, challenged intradermally with cit-LL-37 in Freund's incomplete adjuvant (FIA) — demonstrating robust CXCR5⁺ CD4⁺ expansion in skin-draining lymph nodes by day 21 (preliminary data, 2025).
  • Ex vivo psoriatic skin explant cultures: Maintain lesional immune architecture for 5–7 days, permitting pharmacological intervention studies (PAD inhibition, anti-IL-21) with intact cell-cell crosstalk between keratinocytes, dermal DCs, and resident T cells.
  • HLA-DR4 transgenic mice (DRB1*04:01-Tg): Permit cit-LL-37 peptide immunization studies with human-relevant MHC restriction, enabling TCR repertoire analysis of citrulline-specific Tfh clones in a model that is partially tractable for mechanistic PAD inhibitor studies.

The intersection of innate peptide biology with adaptive autoreactive phenotyping in psoriasis research parallels broader themes in immunomodulatory peptide science. Researchers working at this interface may also find mechanistic comparisons in our brief on Thymosin Alpha-1 and checkpoint inhibitor synergy: TLR-2/9 dendritic cell priming and tumor microenvironment remodeling, which addresses related DC maturation and T-cell phenotype polarization mechanisms. For liver-metabolic peptide contexts that share PAD-associated inflammatory signaling, see also Tesamorelin MASLD liver fat: oxidative phosphorylation upregulation, VEGFA/CSF1 suppression, and fibrosis prevention.

Full mechanistic profiles of these and related peptides are catalogued in the peptide research database, which includes receptor binding tables, PTM annotations, and curated literature for over 200 research-grade peptides.

2025 Research Landscape: Open Questions and Priority Gaps

Despite rapid progress in characterizing cit-LL-37 and carb-LL-37 immunogenicity, several mechanistic gaps remain unresolved as of mid-2025:

  • Whether the Tfh expansion driven by PTM-LL-37 is clonally distinct from or overlapping with the Th17 and Th1 populations targeted by currently approved biologics (secukinumab, ixekizumab, risankizumab) — critical for predicting whether PTM-directed strategies will be additive or synergistic with existing IL-17A/IL-23 blockade.
  • The identity of the carbamyl-LL-37-reactive B-cell clones and whether anti-carb-LL-37 antibodies exhibit cross-reactivity with structural proteins in the joint synovium or enthesis — which could mechanistically explain the psoriasis-to-PsA transition in susceptible patients.
  • The temporal dynamics of PTM accumulation in psoriatic lesions — whether citrullination precedes or follows carbamylation, and whether PAD inhibitor intervention during the early plaque formation window (weeks 0–4) can prevent Tfh imprinting before clonal expansion is established.
  • HLA-restriction mapping beyond DR4/DQ8 — psoriasis susceptibility extends to HLA-Cw6 and additional class II alleles, and whether cit-LL-37 neo-epitopes are presented across the full spectrum of psoriasis-associated HLA haplotypes remains incompletely characterized.

Frequently Asked Questions

What makes citrullinated LL-37 autoantigenic in psoriasis compared to native LL-37?

Native LL-37 carries a net charge of approximately +6 at physiological pH, which creates electrostatic repulsion from the HLA-DR4 antigen-binding groove and limits its presentation to CD4⁺ T cells. PAD2/PAD4-mediated citrullination at R23, R29, and R34 reduces net charge to approximately +3 and induces partial α-helical destabilization, increasing HLA-DR4 binding affinity by approximately 2.8 kcal/mol. The resulting neo-epitope is recognized by clonally expanded autoreactive CD4⁺ T cells bearing the CDR3β motifs CASSYRGNTGELFF and CASSLGQYEQYF, which co-express BCL-6, CXCR5, and ICOS — confirming a Tfh rather than Th17 phenotype.

How does carbamylation of LL-37 differ mechanistically from citrullination, and do both drive Tfh responses?

Citrullination is PAD2/PAD4-enzymatic, targeting arginine residues, and is driven by IL-17A-induced TRPV4-calcium signaling in keratinocytes and NET-associated PAD4 in neutrophils. Carbamylation is non-enzymatic, driven by myeloperoxidase-generated cyanate targeting lysine residues (K12, K15, K18) and the N-terminal amine, and predominates in the oxidative neutrophil-rich dermal compartment. Both modifications generate Tfh-biasing signals, but carb-LL-37 more selectively induces IL-21 and IL-6 from monocyte-derived DCs, while cit-LL-37 retains a hybrid Th1/Tfh profile with concurrent IFN-γ induction — suggesting distinct downstream humoral versus inflammatory consequences.

Why are standard murine psoriasis models inadequate for studying LL-37 post-translational modification and Tfh autoimmunity?

Mice do not express LL-37; their cathelicidin ortholog is CRAMP, which lacks the arginine residues at positions 23, 29, and 34 critical for PAD-mediated citrullination and subsequent HLA-DR4 neo-epitope formation. Additionally, standard mouse MHC class II molecules do not recapitulate the HLA-DR4/DQ8 restriction elements relevant to human psoriatic Tfh priming. Humanized NSG-SGM3 mice engrafted with HLA-DR4⁺ psoriatic PBMCs or HLA-DRB1*04:01 transgenic mice challenged with synthetic cit-LL-37 peptides currently represent the most mechanistically relevant preclinical models available.

Do approved psoriasis biologics (IL-17A, IL-23 blockers) suppress the PTM-LL-37 driven Tfh autoreactive axis?

This remains an open and clinically important question as of 2025. Secukinumab and ixekizumab (IL-17A neutralization) reduce PAD-activating IL-17A signaling in keratinocytes, potentially attenuating cit-LL-37 generation upstream. However, the Tfh-mediated GC-B-cell arm may be relatively insensitive to IL-17A/IL-23 blockade, as Tfh differentiation and GC reactions are primarily sustained by IL-21 and IL-4 rather than Th17-derived cytokines. Preliminary ex vivo data suggest that anti-LL-37 IgG titers do not significantly decline with secukinumab treatment over 16 weeks, implying the humoral autoreactive loop persists despite clinical PASI improvement — a finding with potential implications for residual disease in biologic non-responders and PsA progression.

This content is intended exclusively for licensed researchers, pharmacologists, and scientific institutions conducting research with regulated peptides. All mechanistic descriptions, study citations, and model data references are provided for scientific research purposes only and do not constitute clinical guidance, therapeutic recommendations, or medical advice. LL-37 and its modified analogs are research-use-only compounds and are not approved for human therapeutic administration outside of formally sanctioned clinical trial frameworks.

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