The human peptide LL‑37 can kick‑start a cell‑cleaning process called autophagy in macrophages, but only when it’s in its natural form. Small chemical changes at the start of the peptide (like adding acetyl or formyl groups) or cutting off the first two leucine building blocks stop this effect. The peptide made by neutrophils is often modified and therefore doesn’t trigger autophagy, while the version released by macrophages stays mostly unchanged and works.

The human cathelicidin peptide LL-37 induces autophagy in human macrophages. Different post-translational modifications (PTMs) such as citrullination, acetylation, and formylation impact LL-37, yet their effect on autophagy remains unknown. Thus, we set out to study how the cellular source could impact PTM of LL-37 and subsequent effects on autophagy initiation. Neutrophil-released LL-37 failed to induce autophagy, unlike macrophage-released LL-37. Mass spectrometry analysis revealed modifications on neutrophil-derived LL-37, especially at the N terminus, while macrophage-derived LL-37 remained mostly native. Native LL-37 initiated autophagy, while formylated and acetylated versions did not. Truncated peptides lacking the N-terminal di-leucine motif or substituted with di-alanine did not initiate autophagy. Native LL-37 failed to initiate autophagy in macrophages with genetic inactivation of dipeptidyl peptidase-1. An intact N-terminal di-leucine motif in LL-37 was crucial for autophagy initiation, and modifications abrogated the effects. This pathway presents a novel way to regulate the effects of LL-37 in infection or inflammation.