Myeloperoxidase-derived hypohalous acids selectively halogenate α-defensins (HNP1-3) at conserved tyrosine residues in activated neutrophils, converting these antimicrobial peptides into potent immunomodulatory mediators that exhibit enhanced anti-toxin activity and dramatically amplified chemotactic and inflammatory signaling capacity. This post-translational modification occurs robustly in human neutrophil-dominated diseases including cystic fibrosis, bacterial pneumonia, and staphylococcal abscesses, revealing a previously unrecognized mechanism by which oxidative and non-oxidative neutrophil defense systems cooperate to regulate immune responses.

Neutrophils are immune cells that eliminate microbes using a powerful arsenal of reactive oxygen species, hypohalous acids, proteases, and antimicrobial peptides — yet how these cytotoxic effectors act in concert remains poorly understood. Here, we identify an unrecognized synergy between oxidative and non-oxidative neutrophil defenses. We show that myeloperoxidase-derived hypohalous acids selectively halogenate α-defensins (HNP1–3) at conserved tyrosine residues. This post-translational modification occurs in both human and rat neutrophils and is prominent in diseases marked by neutrophil-rich inflammation, including cystic fibrosis, bacterial pneumonia, and Staphylococcus aureus abscesses. Halogenation increases HNP1 hydrophobicity without altering its structure, reprogramming these peptides into potent immunomodulatory mediators. Using single-cell transcriptomics and a in vivo peritonitis model, we demonstrate that halogenated HNP1s amplify immune signaling.

Our analysis of the human haloproteome reveals a novel oxidative mechanism by which hypohalous acids rewire immune protein functions, uncovering an unexpected layer of cooperation between neutrophil effector systems. The discovery of halogenated HNP1s represents, to our knowledge, the first characterization of a protein halogenation that has biologically significance in human immunity.