Background Generalized pustular psoriasis (GPP) is a rare, debilitating, and often life-threatening inflammatory disease characterized by episodic infiltration of neutrophils into the skin, pustule development, and systemic inflammation, which can manifest in the presence or absence of chronic plaque psoriasis (PV). Current treatments are unsatisfactory warranting a better understanding of GPP pathogenesis. Objective To understand better the disease mechanism of GPP to allow improved targeted therapies. Methods We performed a gene expression study on formalin-fixed paraffin-embedded GPP (n=28) and PV (n=12) lesional biopsies and healthy control (n=20) skin. Differential gene expression was analyzed using gene ontology and enrichment analysis. Gene expression was validated with qRT-PCR and immunohistochemistry, and a potential disease mechanism investigated using primary human cell culture. Results Compared with healthy skin, GPP lesions yielded 479 and PV 854 differentially expressed genes respectively, with 184 upregulated in both diseases. We detected significant contributions of IL-17A, TNF, IL-1, IL-36 and interferons in both diseases; although GPP lesions furnished higher IL-1 and IL-36 and lower IL-17A and interferon-γ mRNA expression than PV. We detected prominent IL-36 expression by keratinocytes proximal to neutrophilic pustules and show that both neutrophils and neutrophil proteases activate IL-36. Suggesting another mechanism regulating IL-36 activity, the protease inhibitors serpin A1 and A3, which inhibit elastase and cathepsin G respectively, were upregulated in both diseases and inhibited activation of IL-36. Conclusions Our data indicate sustained activation of IL-1 and IL-36 in GPP, inducing neutrophil chemokine expression, infiltration and pustule formation, suggesting that the IL-1/IL-36 inflammatory axis is a potent driver of disease pathology in GPP.

Neutrophil extracellular traps (NETs) represent an important defense mechanism against microorganisms. Clearance of NETs is impaired in a subset of patients with systemic lupus erythematosus (SLE), while NETosis is increased in neutrophils and, particularly, in low-density granulocytes derived from lupus patients. NETs are toxic to the endothelium, expose immunostimulatory molecules, activate plasmacytoid dendritic cells and may participate in organ damage through incompletely characterized pathways. In order to better understand the role of NETs in fostering dysregulated inflammation, we examined inflammasome activation in response to NETs or to LL-37, an antibacterial protein externalized on the NETs. Both NETs and LL-37 activate caspase-1, the central enzyme of the inflammasome, in both human and murine macrophages, resulting in release of active IL-1β and IL-18. LL-37 activation of the NLRP3 inflammasome utilizes P2×7 receptor-mediated potassium efflux. NET and LL-37-mediated activation of the inflammasome is enhanced in macrophages derived from lupus patients. In turn, IL-18 is able to stimulate NETosis in human neutrophils. These results suggest that enhanced formation of NETs in lupus patients can lead to increased inflammasome activation in adjacent macrophages. This leads to release of inflammatory cytokines which further stimulate NETosis, resulting in a feed-forward inflammatory loop that could potentially lead to disease flares and/or organ damage.

The innate immune system utilizes many approaches for defense against invading microorganisms, including complement-mediated lysis, engulfment, formation of neutrophil extracellular traps (NETs) and release of antimicrobial peptides (AMPs). While classically thought to be driven by adaptive immunity, the development of autoimmune diseases such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) is increasingly associated with dysregulated innate immune pathways. An emerging theme within this literature is the contribution of AMPs to the development of autoimmune disorders. This is best exemplified in atopic dermatitis and psoriasis where the defensins and the single human cathelicidin, LL-37, may contribute to disease. Further, in the past few years, a role for LL-37 has emerged in the pathogenesis of SLE, RA, atherosclerosis and possibly other diseases. This review discusses the role of LL-37 and its murine ortholog, mCRAMP, in the modulation of immune and inflammatory pathways and their effects on autoimmune and inflammatory diseases.

The mechanisms underlying caspase-1 activation and IL-1␤ processing during inflammatory activation of monocytes and macrophages are not well defined. Here, we describe an in vitro proteolytic processing assay that allows for comparison of caspase-1 regulatory components in a cell-free system separately from the confounding issue of IL-1␤ secretion. Analysis of in vitro IL-1␤ and caspase-1 processing in lysates from unstimulated Bac1 murine macrophages indicated a slow rate of basal caspase-1 activation and proteolytic maturation of IL-1␤. In contrast, brief (5 min) treatment of intact macrophages with extracellular ATP (as an activator of the P2X7 receptor) or nigericin before cell lysis markedly accelerated the in vitro processing of caspase-1 and IL-1␤. This acceleration of in vitro processing was strictly dependent on loss of intracellular K ϩ from the intact cells. The induction of in vitro caspase-1 activation by lysis per se or by K ϩ loss before lysis was sensitive to pretreatment of intact macrophages with the tyrphostin AG-126 or bromoenol lactone, an inhibitor of Ca 2ϩ -independent phospholipase A2. Caspase-1 activation and IL-1␤ processing in lysates from unstimulated macrophages were also accelerated by addition of recombinant ASC, a previously identified adapter protein that directly associates with caspase-1. These data indicate that increased K ϩ efflux via P2X7 nucleotide receptor stimulation activates AG-126-and bromoenol lactone-sensitive signaling pathways in murine macrophages that result in stably maintained signals for caspase-1 regulation in cell-free assays.AG-126; ASC; bromoenol lactone; IL-1␤; inflammation (IL-1␤) is an important proinflammatory cytokine with circulating levels that are tightly regulated to prevent aberrant activation of pathways that can lead to chronic inflammation, septic shock, or death (9). IL-1␤ accumulates as a 33-kDa procytokine (proIL-1␤) in the cytoplasm of monocytes and macrophages, and its activation depends on cleavage to the active, mature 17-kDa form (mIL-1␤) by the enzyme caspase-1 (32, 44). Caspase-1 is synthesized as a low-activity 45-kDa zymogen (procaspase-1) that is proteolytically activated by cleavage of its COOH terminus into p10 and p20 subunits. These p10 and p20 subunits assemble to form a tetramer, a homodimer of heterodimers, that is highly active in its ability to cleave and activate proIL-1␤ (45). In vitro and overexpression studies have suggested that the activation of procaspase-1 depends on the oligomerization of two or more procaspase-1 molecules via caspase association recruitment domain (CARD) interactions between proteins able to bind to the CARD domain of procaspase-1 (12,31,35,36,40,46). Martinon et al. (24) recently reported that a four-protein complex, termed the inflammasome, can form in vitro and result in caspase-1 activation. Intermolecular procaspase-1 autocatalytic cleavage is then believed to generate the highly active caspase-1 tetramers (45). However, the intracellular signaling pathways that mediate the activation o...

Collectively, our data identify IFN-κ as a critical IFN in CLE pathology via promotion of enhanced IFN responses and photosensitivity. IFN-κ is a potential novel target for UVB prophylaxis and CLE-directed therapy.