Juntendo University researchers show that melanoma cells send out exosomes carrying a specific microRNA that silences LL-37, a protective antimicrobial peptide made by neighbouring skin cells — suppressing a local immune defence that would otherwise restrain the tumour.
Melanoma does not grow in isolation. Cancer cells constantly remodel the tissue around them to make it more hospitable to their own survival and spread, and a new study from the Atopy (Allergy) Research Center at Juntendo University Graduate School of Medicine in Japan, published in Cell Reports and reported on 1–2 September 2026, adds a specific and previously underexplored mechanism to that picture: melanoma cells appear to actively silence a natural antitumour defence carried by the ordinary skin cells — keratinocytes — that surround them.
Led by Professor François Niyonsaba, with co-authors Dr Quan Sun and Dr Ge Peng, the research team focused on LL-37, a peptide produced by keratinocytes that is best known as an antimicrobial defence molecule but which the team’s earlier work had also implicated in restraining tumour growth. Examining human melanoma tissue specimens, the researchers found that LL-37 levels were selectively and sharply reduced in keratinocytes located immediately around melanoma cells, while keratinocytes farther from the tumour retained normal LL-37 levels — a striking spatial pattern that pointed to something in the tumour’s immediate vicinity actively suppressing the peptide, rather than a generalised, tissue-wide effect.
The exosome mechanism
To find the cause, the team turned to exosomes — small membrane-bound vesicles that cells use to shuttle signalling molecules, including microRNAs, to their neighbours. When melanoma cells were cultured together with keratinocytes, LL-37 production by the keratinocytes dropped, and blocking exosome release with the drug manumycin A prevented that drop — direct evidence that melanoma-derived exosomes, not some other signal, were responsible. Detailed microRNA analysis identified the specific cargo: melanoma cells release exosomes carrying a microRNA called hsa-miR-221-5p, which keratinocytes take up and which then suppresses their LL-37 expression via a signalling pathway called EGFR, a local epithelial defence circuit that would otherwise help restrain tumour progression.
The researchers then tested the reverse experiment — restoring LL-37 rather than removing it — in mouse and cell models of melanoma, and found that LL-37 was able to actively restrict tumour growth when it was present at normal levels, reinforcing that its suppression by the tumour is not a side effect but appears to serve the tumour’s own progression directly. “Macrophages are the critical source” language seen in unrelated cancer studies does not apply here; in this study it is keratinocytes, not immune cells, that are being manipulated — underscoring that melanoma’s manipulation of its surrounding tissue extends beyond the immune system narrowly defined and into the ordinary epithelial cells that make up the skin itself.
Why it matters
Most research into the melanoma tumour microenvironment has historically focused on immune cells, blood vessels and connective tissue; the contribution of neighbouring keratinocytes — the most abundant cell type in the skin and the cells melanoma cells are, by definition, always surrounded by — has received comparatively little attention. This study helps close that gap, identifying a specific, mechanistically clear pathway (exosomal miR-221-5p suppressing keratinocyte LL-37 via EGFR signalling) rather than a vague correlational observation, which makes it a more immediately actionable target for future drug development: blocking exosome release, blocking the specific microRNA, or directly restoring LL-37 levels around a tumour are all plausible therapeutic angles the study opens up, though each would require substantial further preclinical and clinical validation before reaching patients. Melanoma remains one of the most aggressive skin cancers, capable of rapid metastasis if not caught early, and any credible new mechanistic lead into how it manipulates its immediate surroundings — rather than simply into how it evades the circulating immune system — adds a genuinely underexplored angle to a heavily studied disease.
-Dr Chaman Singh
Key facts
- Mechanism: melanoma cells release exosomes carrying microRNA hsa-miR-221-5p, which is taken up by neighbouring keratinocytes and suppresses their production of the antimicrobial/antitumour peptide LL-37 via EGFR signalling
- LL-37 was found to be selectively reduced only in keratinocytes immediately surrounding melanoma cells, not in keratinocytes farther away
- Blocking exosome release (with manumycin A) restored normal LL-37 levels; restoring LL-37 directly slowed tumour growth in mouse and cell models
- Published in Cell Reports (DOI: 10.1016/j.celrep.2026.117837); research led by Prof. François Niyonsaba, Juntendo University Graduate School of Medicine; reported 1–2 September 2026
