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Neutrophil extracellular traps (NETs) are a key antimicrobial feature of cellular innate immunity mediated by polymorphonuclear neutrophils (PMNs), the primary human leukocyte population. NETs trap and kill microbes but have also been linked to inflammation, e.g. atherosclerosis, arthritis or psoriasis by unknown mechanisms. We here characterize naRNA (NET-associated RNA), as a new canonical, abundant, and unexplored inflammatory NET component. naRNA, upon release by NET formation, drove further NET formation in naïve PMN, and induced macrophage and keratinocyte activation via TLR8 in humans and Tlr13 in mice, in vitro and in vivo. Importantly, in vivo naRNA strongly drove skin inflammation, whereas genetic ablation of RNA sensing drastically ameliorated psoriatic skin inflammation. Rather than accidentally assembling with LL37 on the NET, naRNA was intracellularly pre-associated in resting neutrophils as a ‘composite DAMP’, thus highlighting NET formation as a DAMP release process. This re-defines sterile NETs as an intentionally inflammatory agent, signaling and amplifying neutrophil activation. Moreover, in the many conditions previously linked to NETs and extracellular RNA, TLR-mediated naRNA sensing emerges as both potential cause and new intervention target.Graphical abstractCreated with biorender.com

Healthy skin maintains a diverse microbiome and a potent immune system to fight off infections. Here, we discovered that the epithelial-cell-derived antimicrobial peptides defensins activated orphan G-protein-coupled receptors (GPCRs) Mrgpra2a/b on neutrophils. This signaling axis was required for effective neutrophil-mediated skin immunity and microbiome homeostasis. We generated mutant mouse lines lacking the entire Defensin (Def) gene cluster in keratinocytes or Mrgpra2a/b. Def and Mrgpra2 mutant animals both exhibited skin dysbiosis, with reduced microbial diversity and expansion of Staphylococcus species. Defensins and Mrgpra2 were critical for combating S. aureus infections and the formation of neutrophil abscesses, a hallmark of antibacterial immunity. Activation of Mrgpra2 by defensin triggered neutrophil release of IL-1β and CXCL2 which are vital for proper amplification and propagation of the antibacterial immune response. This study demonstrated the importance of epithelial-neutrophil signaling via the defensin-Mrgpra2 axis in maintaining healthy skin ecology and promoting antibacterial host defense.

Healthy skin maintains a diverse microbiome and a potent immune system to fight off infections. Here, we discovered that epithelial cell-derived antimicrobial peptides defensins activate orphan GPCRs Mrgpra2a/b on neutrophils. This signaling axis is required for effective neutrophil-mediated skin immunity and microbiome homeostasis. We generated mutant mouse lines lacking the entire Defensin (Def) gene cluster or Mrgpra2a/b. Def and Mrgpra2 mutant animals both exhibited skin dysbiosis, with reduced microbial diversity and expansion of Staphylococcus species. Furthermore, we found that defensins and Mrgpra2 are critical for combatting S. aureus infections and the formation of neutrophil abscesses, a hallmark of antibacterial immunity. Activation of Mrgpra2 by defensin triggers neutrophils to release the pro-inflammatory cytokine IL-1β, which is vital for proper amplification and propagation of the antibacterial immune response. This study demonstrates the importance of epithelial-neutrophil signaling via the defensin-Mrgpra2 axis in maintaining healthy skin ecology and promoting antibacterial host defense.

Signal transducer and activator of transcription 3 (STAT3) is important for psoriasis pathogenesis because STAT3 signaling downstream of IL-6, IL-21, IL-22, and IL-23 contributes to T helper type 17 cell development and because transgenic mice with keratinocyte (KC) STAT3 expression (K14-Stat3C mice) develop psoriasis-like dermatitis. In this study, the relative contribution of STAT3 signaling in KCs versus in T cells was evaluated in the imiquimod model of psoriasis-like dermatitis. Mice with STAT3-inducible deletion in KCs (K5-Stat3-/- mice) had decreased psoriasis-like dermatitis and epidermal STAT3 phosphorylation compared with wild-type mice, whereas mice with constitutive deletion of STAT3 in all T cells were similar to wild-type mice. Interestingly, mice with KC-inducible deletion of IL-6Rα had similar findings to those of K5-Stat3-/- mice, identifying IL-6/IL-6R as a predominant upstream signal for KC STAT3-induced psoriasis-like dermatitis. Moreover, psoriasis-like dermatitis inversely associated with type 1 immune gene products, especially CXCL10, whereas CXCL10 limited psoriasis-like dermatitis, suggesting that KC STAT3 signaling promoted psoriasis-like dermatitis by restricting downstream CXCL10 expression. Finally, treatment of mice with the pan-Jak inhibitor, tofacitinib, reduced psoriasis-like dermatitis and epidermal STAT3 phosphorylation. Taken together, STAT3 signaling in KCs rather than in T cells was a more important determinant for psoriasis-like dermatitis in a mechanism that involved upstream KC IL-6R signaling and downstream inhibition of type 1 immunity‒associated CXCL10 responses.

Bacterial skin infections are a major societal health burden and are increasingly difficult to treat owing to the emergence of antibiotic-resistant strains such as community-acquired methicillin-resistant Staphylococcus aureus. Understanding the immunologic mechanisms that provide durable protection against skin infections has the potential to guide the development of immunotherapies and vaccines to engage the host immune response to combat these antibiotic-resistant strains. To this end, mouse skin infection models allow researchers to examine host immunity by investigating the timing, inoculum, route of infection and the causative bacterial species in different wild-type mouse backgrounds as well as in knockout, transgenic, and other types of genetically engineered mouse strains. To recapitulate the various types of human skin infections, many different mouse models have been developed. For example, four models frequently used in dermatological research are based on the route of infection, including (i) subcutaneous infection models, (ii) intradermal infection models, (iii) wound infection models, and (iv) epicutaneous infection models. In this article, we will describe these skin infection models in detail along with their advantages and limitations. In addition, we will discuss how humanized mouse models such as the human skin xenograft on immunocompromised mice might be used in bacterial skin infection research.