Antimicrobial Peptide LL-37 May Trigger Autoimmune Muscle Disease Through Interferon Activation | Research

The takeaway

The host defense peptide LL-37 is elevated in muscle tissue of polymyositis and dermatomyositis patients and may trigger the type I interferon response that drives these autoimmune muscle diseases.

LL-37 elevated + all patients vitamin D deficient
Elevated LL-37 from neutrophils in diseased muscle tissue, combined with universal vitamin D deficiency in myositis patients, supports a link between LL-37 dysregulation and autoimmune muscle disease

What the researchers found

The antimicrobial peptide LL-37 was elevated in muscle tissue of patients with polymyositis (PM) and dermatomyositis (DM) compared to healthy controls, primarily released by infiltrating neutrophils. Plasmacytoid dendritic cells (BDCA-2+, the main type I interferon producers) and the interferon marker MxA were also elevated in affected muscle. All PM/DM patients with short disease duration had low vitamin D levels. The findings support the hypothesis that LL-37 may trigger the type I interferon system in these autoimmune muscle diseases, similar to its known role in lupus (SLE).

Why it matters

LL-37 is normally a protective antimicrobial peptide, but this study reveals its darker side: in autoimmune diseases, it may trigger harmful immune responses. The finding that LL-37 activates the same interferon pathway in myositis as it does in lupus suggests a common peptide-driven mechanism across multiple autoimmune diseases. The vitamin D connection is also notable, since vitamin D regulates LL-37 expression.

The numbers in context

Muscle biopsies: 3 PM + 5 DM (short duration), 3 PM + 1 DM (long duration) · Skin biopsies: 5 DM symptomatic, 3 PM + 3 DM non-symptomatic · 6 SLE controls · 5 muscle + 6 skin healthy controls · All PM/DM patients: low vitamin D

How the study worked

Case-control study comparing tissue biopsies from PM and DM patients with SLE patients and healthy controls. Muscle and skin biopsies were immunohistochemically stained for LL-37, neutrophils (CD66b), plasmacytoid dendritic cells (BDCA-2), MxA (interferon marker), and macrophages (CD68, CD163). Double staining confirmed LL-37 expression in neutrophils. Serum vitamin D (25(OH)D3) levels measured in 8 myositis patients and 40 healthy controls.

Who was studied

Patients with polymyositis and dermatomyositis (short and long disease duration), systemic lupus erythematosus patients, and healthy controls

What this study cannot tell us

Very small sample sizes (3-5 patients per group) limit statistical power and generalizability. Cross-sectional design cannot establish whether LL-37 elevation is a cause or consequence of the inflammatory process. The study cannot prove that LL-37 triggers myositis — only that it's present at higher levels and correlates with interferon markers.

How to read the evidence

This is a small observational study with 3-5 patients per group. While the tissue analysis is detailed and the hypothesis is plausible, the very small sample sizes and cross-sectional design can only suggest associations, not prove causation.

When this study was published

Published in 2017, this study contributes to the growing understanding of LL-37's role in autoimmunity. Research on cathelicidin-driven autoimmune pathology has continued, though no LL-37-targeted therapies have reached clinical trials for myositis.

The bigger picture

LL-37's role in autoimmunity is a double-edged sword story: the same peptide that protects against infections can, under certain conditions, trigger harmful immune responses. Its involvement has already been established in lupus and psoriasis. This study extends the pattern to autoimmune myositis, suggesting LL-37 may be a common pathogenic factor across multiple autoimmune diseases. The vitamin D connection raises the intriguing possibility that vitamin D deficiency could worsen autoimmunity by dysregulating LL-37.

Questions still open

Would vitamin D supplementation reduce LL-37 levels and improve outcomes in polymyositis and dermatomyositis patients?
Is there a therapeutic approach that could block LL-37's interferon-triggering activity while preserving its antimicrobial function?
Does the LL-37/interferon pathway explain why autoimmune diseases like lupus, myositis, and psoriasis sometimes co-occur?

Common questions

How can an antimicrobial peptide cause autoimmune disease?

LL-37 normally kills bacteria, but it can also form complexes with DNA released from dying cells. These LL-37-DNA complexes activate plasmacytoid dendritic cells, which produce large amounts of type I interferon — an immune signal that, in excess, can trigger the immune system to attack the body's own tissues.

Could taking vitamin D help prevent autoimmune muscle disease?

This study found all myositis patients had low vitamin D, and vitamin D regulates LL-37 production. While it's plausible that maintaining adequate vitamin D could help, this study can't prove vitamin D deficiency causes myositis. Vitamin D supplementation is generally recommended for autoimmune disease patients, but as a supportive measure, not a treatment.

Read the original research

The host defense peptide LL-37 a possible inducer of the type I interferon system in patients with polymyositis and dermatomyositis.

Journal of autoimmunity, 78, 46-56

View on PubMed ↗Publisher’s paper ↗

Citation

Lu, Xin; Tang, Quan; Lindh, Monica; Dastmalchi, Maryam; Alexanderson, Helene; Popovic Silwerfeldt, Karin; Agerberth, Birgitta; Lundberg, Ingrid E; Wick, Cecilia. (2017). The host defense peptide LL-37 a possible inducer of the type I interferon system in patients with polymyositis and dermatomyositis.. Journal of autoimmunity, 78, 46-56. https://doi.org/10.1016/j.jaut.2016.12.003