Skin Cells Use Defensin Peptides Not Just to Kill Bacteria — but to Call in Neutrophils Through a Newly Discovered Receptor

The researchers generated two mutant mouse lines: one lacking the entire defensin gene cluster specifically in keratinocytes, and another lacking the Mrgpra2a/b receptors. They assessed skin microbiome composition, challenged mice with S. aureus skin infections, analyzed neutrophil abscess formation, and measured cytokine/chemokine release (IL-1β, CXCL2) upon defensin-Mrgpra2 activation.

This study redefines defensins from simple antimicrobial agents to immune signaling molecules. By revealing how skin epithelial cells communicate with neutrophils through defensins, it opens new therapeutic avenues — boosting this pathway could help treat skin infections (especially antibiotic-resistant Staph), while understanding its disruption could explain conditions like eczema where the skin microbiome is disturbed.

Antimicrobial peptides have long been considered part of innate immunity's 'chemical barrier,' but this study elevates them to signaling molecules that coordinate immune cell responses. Published in the top journal Immunity, it changes how the field thinks about host defense — peptides aren't just killers, they're messengers that orchestrate the immune system's response to infection.

The study was conducted entirely in mice, and the human orthologs of Mrgpra2a/b (MRGPRX2) have somewhat different expression patterns and functions. The specific defensins responsible for Mrgpra2 activation in vivo were not individually identified. Whether this axis operates similarly in other tissues beyond skin was not explored.