Defensin peptides made by skin cells activate previously orphan receptors (Mrgpra2a/b) on neutrophils, and this signaling axis is essential for maintaining a healthy skin microbiome and fighting off Staph infections.
Orphan receptors de-orphanedMrgpra2a/b on neutrophils were previously 'orphan' receptors with no known ligand — this study identified defensin peptides as their natural activators, revealing a new immune signaling pathway
What the researchers found
The study identified Mrgpra2a/b — previously orphan G-protein-coupled receptors on neutrophils — as the receptors for keratinocyte-derived defensins. This represents a novel epithelial-to-immune cell signaling axis.
Mice lacking either the entire defensin gene cluster in keratinocytes or the Mrgpra2a/b receptors developed skin dysbiosis characterized by reduced microbial diversity and expansion of Staphylococcus species. Both mutant lines showed impaired neutrophil abscess formation — a hallmark of antibacterial immunity — and increased susceptibility to S. aureus skin infections. Mechanistically, defensin activation of Mrgpra2 triggered neutrophil release of IL-1β and CXCL2, which are critical for amplifying and propagating the antibacterial immune response.
Why it matters
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.
How the study worked
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.
What this study cannot tell us
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.
How to read the evidence
This is a rigorous preclinical study published in Immunity (a top-tier immunology journal) using multiple knockout mouse lines with complementary approaches. The evidence for the defensin-Mrgpra2 axis is strong, though human translation remains to be demonstrated.
When this study was published
Published in 2022, this is a recent and highly cited study that has already influenced the field's understanding of antimicrobial peptide biology.
The bigger picture
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.
Questions still open
- Does the human ortholog MRGPRX2 serve the same defensin-sensing function on human neutrophils, and could it be targeted therapeutically?
- Could defects in this defensin-Mrgpra2 signaling axis contribute to skin conditions like atopic dermatitis, where both microbiome dysbiosis and impaired neutrophil function are observed?
- Can synthetic defensin analogs be designed to specifically activate this pathway without antimicrobial activity, to boost immune responses independently?
Common questions
What are defensins and what new role was discovered for them?
What happened to mice that couldn't make defensins in their skin?
Read the original research
Keratinocyte-derived defensins activate neutrophil-specific receptors Mrgpra2a/b to prevent skin dysbiosis and bacterial infection.
Immunity, 55(9), 1645-1662.e7
Citation
Dong, Xintong; Limjunyawong, Nathachit; Sypek, Elizabeth I; Wang, Gaofeng; Ortines, Roger V; Youn, Christine; Alphonse, Martin P; Dikeman, Dustin; Wang, Yu; Lay, Mark; Kothari, Ruchita; Vasavda, Chirag; Pundir, Priyanka; Goff, Loyal; Miller, Lloyd S; Lu, Wuyuan; Garza, Luis A; Kim, Brian S; Archer, Nathan K; Dong, Xinzhong. (2022). Keratinocyte-derived defensins activate neutrophil-specific receptors Mrgpra2a/b to prevent skin dysbiosis and bacterial infection.. Immunity, 55(9), 1645-1662.e7. https://doi.org/10.1016/j.immuni.2022.06.021