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IL-17 and IL-23 are absolutely central to psoriasis pathogenesis as drugs targeting either cytokine are highly effective treatments for this disease. The efficacy of these drugs has been attributed to blocking the function of IL-17-producing T cells and their IL-23-induced expansion. However, we demonstrate that mast cells and neutrophils, not T cells, are the predominant cell types that contain IL-17 in human skin. IL-17+ mast cells and neutrophils are found at higher densities than IL-17+ T cells in psoriasis lesions and frequently release IL-17 in the process of forming specialized structures called extracellular traps (MCETs and NETs, respectively). Furthermore, we find that IL-23 and IL-1β can induce MCET formation and degranulation of human mast cells. Release of IL-17 from innate immune cells may be central to the pathogenesis of psoriasis, representing a fundamental mechanism by which the IL-23-IL-17 axis mediates host defense and autoimmunity.
Much attention in recent years has focused on the production of IL-17A and IL-17F, often referred to together as IL-17, by a subset of T cells called Th17 cells (1). The differentiation of Th17 cells is supported by IL-6 and TGF-β, while IL-23, IL-1β, and IL-21 promote their expansion (2-4). Interestingly, polymorphisms at loci encoding components of IL-23 and its receptor—IL23A, IL12B, and IL23R—have been associated with an increased risk of developing psoriasis (5-9). Psoriasis lesions contain increased amounts of IL-17 mRNA and increased numbers of Th17 cells (2, 10, 11). Similarly, increased tissue expression of IL-17 and numbers of Th17 cells are seen in rheumatoid arthritis, Crohn’s disease, autoimmune uveitis, lupus erythematosus, ankylosing spondylitis, asthma, and multiple sclerosis (12). The pathophysiologic relevance of the IL-23-IL-17 axis in autoinflammatory disease is highlighted by the clinical effectiveness of antibodies targeting IL-23/IL-12 p40 and IL-17 in treating psoriasis, as well as other aforementioned diseases (13-16).
While IL-17 production by T cells is widely studied, it is increasingly appreciated that diverse types of innate immune cells can also produce IL-17 (17). Recent studies demonstrate that IL-17+ mast cells and neutrophils can be found in complicated atherosclerotic plaques (18). Similarly, IL-17+ mast cells are also present in inflamed synovium of rheumatoid arthritis (19). Mast cells, particularly a subset containing tryptase and chymase (MCTC), are enriched in the papillary dermis of psoriasis lesions (20, 21). Mast cells frequently degranulate in early eruptive and recurring psoriasis lesions and have been described as ‘ghost cells’ (22-24). Interestingly, mast cell numbers are decreased in psoriasis lesions after successful treatment with anthralin, PUVA, or cyclosporine (25-27). Neutrophils are also enriched in psoriasis lesions, especially in the epidermis where they aggregate in Munro’s microabscesses (MM) in the stratum corneum and spongiform pustules of Kogoj (SPK) in the stratum spinosum (28). While the precise function of neutrophils is unknown in psoriasis, a critical pathogenic role is supported by case reports of psoriasis remission during drug-induced agranulocytosis, and its reappearance after normalization of neutrophil numbers (29). Furthermore, razoxane, a drug effective against all forms of psoriasis and psoriatic arthritis, causes a dose-dependent depression of neutrophil counts (30). Very recently, a study indicated that mast cells and neutrophils contain IL-17 in psoriasis plaques of 3 patients (31). However, the significance of these numbers relative to normal appearing skin as well as the mechanism of IL-17 release by innate immune cells in psoriasis remains unknown.
IL-17 orchestrates innate immune responses against extracellular pathogens by inducing expression of antimicrobial peptides (AMPs) and neutrophil-tropic chemokines CXCL1, CXCL2, and IL-8 (32-35). The same AMPs and chemokines are found at extremely high levels in psoriatic epidermis (36-38). Not surprisingly, mice and humans with deficits in IL-17 production or signaling are highly susceptible to infection with extracellular bacteria and fungi (39-42). Similarly, effective antimicrobial activity by neutrophils and mast cells depends on formation of structures called extracellular traps, termed NETs and MCETs, respectively (43-45). Extracellular traps (ETs) are formed through a specialized process of cell death termed ETosis (46), where chromatin extends into fine, web-like threads to which proteins are bound. In particular, NETs can contain myeloperoxidase (MPO), proteinase 3, and AMPs such as cathelicidin (LL-37) (47). NETosis can be triggered by extracellular bacteria and fungi or their components (48, 49). MCETs contain tryptase, LL-37, and chromatin (45), forming in response to bacteria, H2O2, or PMA (45, 48). In humans, NETs have been visualized in physiologic host responses to infections (44, 50) and have been implicated in the pathology of ANCA-induced vasculitis (51). Additionally, a recent study showed that lupus nephritis is associated with an inability to degrade NETs in blood (52). Furthermore, while MCETs have been studied in a human mast cell line and mouse bone-marrow derived mast cells (45), it is unclear whether MCETosis occurs in human tissue under normal or pathologic conditions.
To understand the complex pathophysiology of psoriasis, it is imperative to define the precise cellular sources of IL-17 and the mechanisms mediating IL-17 release. Therefore, we investigate the production of IL-17 by innate immune cells in psoriasis and explore a potential role for extracellular traps formed by these cells in human skin. Surprisingly, we observe that most IL-17+ cells in normal and psoriatic skin are mast cells, not T cells. Additionally, neutrophils in well-developed psoriasis lesions also express IL-17. We observe that mast cells and neutrophils release IL-17 into skin though ETosis, as well as conventional degranulation. Furthermore, a subtype of human neutrophils, low density granulocytes (LDG), isolated from psoriatic blood are increased in psoriatic compared to control blood. Interestingly, mast cells readily form ETs with resultant IL-17 release in control human skin explants treated with IL-23 and IL-1β, suggesting a novel mechanism driving ETosis and IL-17 release from mast cells. These findings suggest that mast cells and neutrophils may play more prominent roles than previously appreciated in psoriasis and similar diseases by releasing IL-17 through extracellular trap formation.
All subjects provided written, informed consent. Skin punch biopsies (6 μm) were obtained from patients with normal healthy skin (NN), normal-looking symptomless psoriatic skin (PN), and psoriatic plaques (PP). Before biopsy, cases were required to be off all systemic therapies for at least 2 weeks and off topical anti-psoriatic medications for at least 1 week. Control donors were identified from respondents to advertisements in the area (Ann Arbor, MI), had no personal or family history of psoriasis, and were free of inflammatory skin disease at the time of biopsy. This study was approved by the Institutional Review Board of the University of Michigan Medical School.
Five-micrometer sections of skin were deparaffinized on a hot plate at 65°C for 1 hour. They were rehydrated by incubation and heat retrieval in Cell Conditioning Solution (CC1, Ventana Med, Tuscon, AZ). The sections were blocked with horse serum (0.2%; Invitrogen, Carlsbad, CA) for 30 min at room temperature. Then simultaneous staining was first performed for 30 min at room temperature with goat anti–IL-17 (100 μg/ml; R&D Systems, Minneapolis, MN) and another primary Ab, either rabbit anti-CD3 (0.4 μg/ml; Ventana Med, Tuscon, AZ), rabbit anti-CD4 (1:25; Cell Marque, Rocklin, CA), mouse anti-CD8 (1:25; Leica Biosystems, Newcastle, U.K.), mouse anti-mast cell tryptase (1:150; Dako USA, Carpinteria, CA), mouse anti-mast cell chymase (1:1000; AbD Serotec, Oxford, U.K.), or rabbit anti-MPO (1:1500; Dako USA, Carpinteria, CA). Triple staining was conducted with IL-17, MPO, and mouse anti-human cathelicidin [LL-37] antibodies (1:200; Abcam, Cambridge, MA). This was followed by 30 min incubation in room temperature with matched secondary Abs: chicken anti-goat IgG Alexa-Fluor 488 (1:300; Invitrogen, Carlsbad, CA), chicken anti-mouse IgG Alexa-Fluor 594 (1:300; Invitrogen, Carlsbad, CA), or chicken anti-rabbit IgG Alexa-Fluor 594 (1:300; Invitrogen, Carlsbad, CA). For triple staining, donkey anti-rabbit Alexa-Fluor 350 (1:300; Invitrogen, Carlsbad, CA) was also used. ProLong Gold antifade reagent with DAPI was added to the slides prior to mounting with cover slips (Invitrogen, Carlsbad, CA). Images were captured with a fluorescence microscope (BX50; Olympus, Essex, U.K.) using DP Controller and DP Manager (Olympus) software, and a confocal microscope (FV-500; Olympus, Essex, U.K.).
The number of cells expressing one or both markers of interest in each 200x field was manually counted by two independent blinded observers on Adobe Photoshop®. From these numbers, we calculated the proportions of cells of different populations; e.g. the proportion of IL-17+ of all tryptase+ cells is calculated as (# IL-17+tryptase+cells) / (# IL-17+tryptase+ cells + IL-17−tryptase+ cells). Each data point in Fig. Fig.22 and and55 represents the average of 1-6 independent fields for each patient sample.
Punch biopsies were obtained from healthy donors without psoriasis as above and were quartered longitudinally. Each divided section was incubated in R10 media (RPMI-1640; Gibco) with 0.4 mM Ca2+ and 10% FCS) with soy bean trypsin inhibitor [SBTI] (100 μg/ml; Sigma) or R10 media with SBTI containing either Substance P (10 μM; Sigma), Compound 48/80 (10 μg/ml; Sigma), IFN-γ (200 ng/ml; Miltenyi Biotec), IL-1β (2.5 ng/ml; R&D Systems), IL-23 (10 ng/ml; R&D Systems), or both IL-1β and IL-23 together for 2 hours or 3 days. The tissues were fixed in 10% neutral buffered formalin and subjected to immunofluorescence staining as above. The proportions of intact mast cells and degranulated mast cells were counted by direct visualization through the microscope by two independent observers.
Normal-density neutrophils and LDG were as described (53). The purity of the neutrophil fraction was at least 95% and was determined by CD15, CD14, CD10, and negative CD3 expression by FACS. In some experiments, neutrophils were sorted after the dextran gradient isolation to >99.5% purity using FACS ARIA (Becton Dickinson, San Jose) gating on CD15+CD3− cells with neutrophil size and granularity characteristics as in Figure 4A. For LDG, PBMC were isolated using Ficoll/Hypaque, RBCs were lysed using hypotonic/hypertonic saline, then T and B lymphocytes, NK cells, monocytes, and residual RBCs were removed by negative selection. LDG were identified as CD15+/CD14lo or CD10+/CD14lo (CD10+/CD15hi/CD14lo).
Total cell lysates were obtained by Laemmli extraction. A total of 10 μg of each sample and 40 ng of the recombinant cytokines were loaded. Samples were separated on 15% SDS-polyacrylamide gels and transferred to Immobilon-P membranes (Millipore, Billerica, MA). Western blots were blocked with PBS/0.02% Tween 20 (PBS-T) for IL-17A or TBS/0.02% Tween 20 (TBS-T) for IL-17F containing 5% non-fat dry milk for 1 □hour at room temperature and incubated with mouse anti-human IL-17A antibody (1:250; R&D Systems, Minneapolis, MN) or mouse anti-human IL-17F antibody (1:1000; eBioscience, San Diego, CA) overnight at 4°C. The blots were washed three times with PBS-T and then incubated with horseradish-peroxidase-conjugated mouse secondary antibody (1:2500; GE Healthcare, Piscataway, NJ) for 1.5 □hours at room temperature. Blots were washed again and protein expression was detected by chemiluminescence using ECL (Thermo Scientific, Rockford, IL) and Kodak X-Omat film (Kodak, Rochester, NY). Anti-β-actin (1:2500; Sigma) was used as a loading control.
A total of 1-2 × 106 neutrophils or LDG per mL were seeded in poly-L-lysine coated coverslips and incubated at 37°C, 5% CO2 for 15 min. Cells were washed with ice-cold PBS, fixed with cold 4% paraformaldehyde for 15min and blocked overnight with 10% FBS, 1% BSA, 0.05% Tween 20, and 2 mM EDTA in 1X PBS at 4°C overnight. Stimulated cells were incubated for 2 or 3 hours in 1X RPMI with GLUT and 2% BSA with 20 nM PMA (Sigma). For detection of NETs, fixed cells were washed with ice-cold 10% FBS in 1X PBS for 5min and incubated with primary antibodies for neutrophil elastase (Abcam, Cambridge, MA) or rabbit isotype control (Abcam, Cambridge, MA) for 45min at 4°C, washed twice and incubated with secondary antibodies for 45min at 4°C. Nuclear material was detected by incubating cells with Hoechst for 10min at RT. Coverslips were placed in slides with Prolong Antifade Reagent for further analysis using an inverted microscope (Olympus IX70, Center Valley, PA). Image overlay and segmentation was performed using Metamorph v7.7 software. Further analysis of the imaging data was performed using Photoshop®, where colocalization of neutrophil elastase and nuclear staining (Hoechst) was manually counted. The percentage of NETs was calculated as the average of 6 (4-6) high power fields of view normalized to the total number of cells per view.
Data are expressed as mean ± SD. NN, PN, and PP were compared using the one-way ANOVA with Tukey’s post hoc test using GraphPad Prism 5. Proportions of LDG in PBMC were compared using an unpaired two-tailed Student’s t-test. Mast cell explant conditions were compared to media using one-way ANOVA with Dunnett’s post hoc test. P < 0.05 was considered to be statistically significant.
Previous studies of IL-17 expression from human skin focused primarily on T cells stimulated ex vivo with PMA and ionomycin to increase signal intensity for subsequent flow cytometric analysis (2, 10, 11). While this approach is very useful to determine the differentiation state of T cells isolated from tissue, it does not define the identity and anatomic localization of cells actually containing intracellular IL-17 in vivo. Thus, we analyzed tissues isolated from psoriasis plaques (PP, n = 13), normal appearing skin from psoriasis patients (PN, n = 10), and normal control skin (NN, n = 10). To visualize IL-17+ T cells in their anatomic setting, we performed dual-color immunofluorescence and counted the numbers of cells per 200x field (cpf) expressing intracellular IL-17 and CD3 as a marker for T cells (Fig. 1A, Supplemental Fig. 1). Not surprisingly, the density of T cells is increased in PP skin compared to PN and NN skin (54 ± 35 vs. 12 ± 11 and 10 ± 7 cpf; PPP:PN < 0.001, PPP:NN < 0.001; Fig. 2A). However, the number of CD3+ T cells containing detectable IL-17 in PP lesions compared to PN and NN skin was not significantly different (1.1 vs. 0.8 vs. 0.5 cpf, Fig. 2B). To our surprise, we found that T cells represent only a minority of all IL-17+ cells in PP, PN, and NN skin (7.3 ± 7.2%, 7.3 ± 7.1%, 5.0 ± 5.0%, Fig. 2C). The proportion of IL-17+ T cells of all CD3+ T cells was calculated to be 3.4 ± 5.8% (PP), 10.1 ± 11.9% (PN), and 4.4 ± 4.9% (NN) (Fig. 2D). This finding is concordant with studies by our group and others defining the proportion of T cells capable of producing IL-17 after 3 days to 3 weeks of in vitro culture (2, 10, 11). However, the majority of IL-17+ cells, and especially those cells most intensely staining for IL-17, did not express CD3, suggesting that most cells containing IL-17 in psoriasis lesions and normal skin are not T cells.
Since most IL-17+ cells were not T cells, we performed dual-color immunofluorescence for IL-17 with markers of innate immune cells. To identify subsets of skin mast cells, we stained for the mast cell specific enzymes, tryptase and chymase. Most mast cells in human skin express both enzymes and are designated MCTC cells. Co-staining revealed that most tryptase+ cells and chymase+ cells co-stained brightly for IL-17 (Fig. 1B, 1C, Supplemental Fig. 2, 3). We observed that the number of chymase+ mast cells significantly increased in PP skin compared to PN and NN skin (25 ± 7 vs. 16 ± 4 vs. 11 ± 3 cpf; nPP = 11, nPN = 10, nNN = 10; PPP:PN < 0.01, PPP:NN < 0.001, PPN:NN < 0.05; Fig. 2I). The density of chymase+IL-17+ cells was significantly increased in PP and PN skin compared to NN skin (23 ± 7 vs. 16 ± 4 vs. 7 ± 4 cpf; PPP:PN < 0.01, PPP:NN < 0.001, PPN:NN < 0.01; Fig. 2J). Importantly, the majority of IL-17+ cells were mast cells, regardless of disease status. Chymase+ mast cells accounted for nearly all of the IL-17+ cells, in contrast to CD3+ T cells in PP (89.0 ± 10.6% vs. 7.3 ± 7.2%; P = 5.9 × 10−14), PN (96.3 ± 3.6% vs. 7.3 ± 7.1%; P = 1.5 × 10−14), and NN skin (92.2 ± 12.7% vs. 5.0 ± 5.0%; P = 1.4 × 10−11; Fig. 2C, 2K). Additionally, the proportion of chymase+IL-17+ cells of all chymase+ cells was also significantly greater in PP and PN skin compared to NN skin (94.7 ± 5.5% vs. 95.5 ± 5.0% vs. 66.6 ± 24.5%; PPP:NN < 0.001, PPN:NN < 0.001; Fig. 2L). Experiments with tryptase staining revealed similar findings (Fig. 2E-H). These data demonstrate that mast cells, primarily MCTC cells, represent the majority of IL-17-containing cells in human skin. Furthermore, in psoriasis lesions, the density of MCTC mast cells is increased and a greater proportion of these cells contain IL-17.
We noticed that many CD3−IL-17+ cells in the epidermis of psoriasis plaques had trilobed nuclear morphology and were enriched in areas resembling SPKs and MMs. Dual-color immunofluorescence studies of IL-17 and MPO on PP skin revealed that most of these IL-17+ cells in the stratum spinosum and stratum corneum of well developed psoriasis lesions co-stained with MPO, verifying their identity as neutrophils (Fig. 1D, Supplemental Fig. 4). Quantification of PP, PN and NN sections stained with MPO and IL-17 confirmed the increased number of total neutrophils in PP skin compared to scant numbers in PN or NN skin (56 ± 47 vs. 4 ± 4 vs. 2 ± 3 cpf; nPP = 12, nPN = 10, nNN = 12; PPP:PN < 0.001, PPP:NN < 0.001; Fig. 2M) (28). Since neutrophils were enriched in the superficial MM and SPK, analysis of the cells was by necessity more epidermally-focused, but also included superficial dermal regions. A significant number of IL-17+ neutrophils were seen in PP lesions, but were rarely present in PN skin and were essentially absent in NN skin (18 ± 18 vs. 1 ± 1 vs. 0 ± 0 cpf; PPP:PN < 0.01, PPP:NN < 0.001; Fig 2N). Because neutrophils w present in PP lesions in epidermal clusters, the proportion of neutrophils of all IL-17+ cells in PP skin shows substantial variability but is significantly higher than in PN or NN skin (32.4 ± 20.6% vs. 8.1 ± 5.7% vs. 1.3 ± 4.3%; PPP:PN < 0.001, PPP:NN < 0.001; Fig. 2O). Neutrophils in PP or PN tended to contain more IL-17 compared to those in NN skin (33.8 ± 19.4% vs. 50.7 ± 45.9% vs. 0.8 ± 2.6%; PPN:NN < 0.01; Fig. 2P). Together, these data reveal that IL-17+ neutrophils are enriched in psoriasis lesions, primarily in focal epidermal collections.
At low and medium power we had observed that the nuclei of IL-17+ neutrophils in MM frequently displayed the morphology of neutrophil extracellular traps. Indeed, high power microscopic examination of dual-color immunofluorescence revealed that IL-17 and MPO were frequently associated with NET-like formations of DNA, visualized with DAPI staining. These NETs were frequently observed in psoriatic lesional epidermis (Fig. 3A), particularly in MMs and SPKs, as evidenced by the thin extensions of nuclear material costaining with MPO (Fig. 3B). To confirm the functional relevance of NETs, tri-color immunofluorescence staining showed that dermal NETs frequently contained both IL-17 and cathelicidin/LL-37 (Fig. 3C). Further visualization through confocal microscopy confirmed that the morphology of NETs varied from cell to cell even within aggregates (Fig. 3D) suggesting that newly arriving neutrophils may be undergoing NETosis. Hematoxylin and eosin stained slides confirmed that not all neutrophils in a MM had altered nuclear morphology (Fig. 3E). The NETs with the longest chromatin projections tended to costain with MPO, but often not with IL-17. In contrast, NETs with shorter extrusions often stained with IL-17, presumably representing less mature NETs. Because IL-17 is not seen in all NETs in PP, we hypothesize that IL-17 does not bind to extruded chromatin nearly as strongly or abundantly as MPO.
To explore the sites of IL-17 production, human neutrophils isolated using standard dextran protocol were further enriched by flow cytometric sorting to >99.5% purity (Fig. 4A). Lysates of these flow-sorted cells contained IL-17A, but not IL-17F protein, as detected using Western blotting (Fig. 4B).
Seminal studies almost three decades ago described a subpopulation of neutrophils in psoriasis blood which are polar in shape and have ruffled cell membranes (54). Similarly, a recent study revealed a population of low density granulocytes (LDG) in the blood of patients with systemic lupus erythematosus which share some properties with activated neutrophils (53). We found increased numbers of LDG in the blood of patients with psoriasis compared to control blood (34.6 ± 5.1% vs. 11.0 ± 2.0%, P < 0.01; Fig. 4C). We measured NET formation in blood neutrophils from psoriasis or controls and visualized NETosis using stains for neutrophil elastase and DAPI. In four sets from patients and controls, we observed that psoriasis LDG tended to form NETs without any stimulation, in contrast to control or psoriasis neutrophils. After 2 hours incubation in PMA, control neutrophils, psoriasis neutrophils, or LDG all formed NETs. However, compared to control neutrophils, psoriasis neutrophils tended to form more NETs after culture without PMA treatment although this was not statistically significant (Fig. 4D). These findings indicate that populations of circulating neutrophils in psoriasis readily undergo NETosis.
On immunofluorescence studies, we noticed that MCTC in the papillary dermis of psoriasis plaques were often not intact. On initial inspection, they appeared to be degranulated, consistent with scanning electon microscopy studies showing degranulated MC in developing or recurrent psoriasis plaques (22, 23). However, high power microscopic visualization of the superficial papillary dermis revealed that while many of these MCTC had undergone conventional degranulation, a significant number of cells revealed mast cell tryptase and chymase co-localized with linear DNA (Fig. 5A, 5B). We identified these structures as MCETs, based on their similarity to MCETs formed in vitro (45). While essentially all MC in PP contain IL-17, a minority of MCETs co-stained brightly for IL-17, and many were IL-17−. This suggests that IL-17 is released at low levels upon MCETosis, diffusing to a level below our threshold of detection faster than tryptase or chymase. The shapes of MCETs in skin are more compact than those formed in vitro, likely due to the physical constraints of surrounding tissue. MCETs or degranulated MC were not frequently seen in NN or PN skin samples (Supplemental Fig. 2, 3). This data demonstrates MCETs can form in human psoriasis lesions and represent a distinct mechanism of IL-17 release by mast cells in vivo.
Because IL-1β and IL-23 promote increased production of IL-17 from T cells, we evaluated whether these cytokines might similarly promote production or release of IL-17 from mast cells in situ in skin. Tissue sections from normal skin biopsies (n = 3) incubated for 3 days in control media contained mostly intact mast cells at a density comparable to tissue sections from the same patient fixed immediately after biopsy. After 3 days of treatment with IL-1β and IL-23, there was no increased expression of IL-17. However, there were significant increases in degranulated and ETosed MC in treated tissue, as measured by staining for tryptase, chymase, and IL-17 (Fig. 5C). Treatment of normal skin with either IL-1β or IL-23 alone led to fewer degranulated/ETosed mast cells than treatment with both compounds together as measured with tryptase (Pmedia:IL-23 < 0.05, Pmedia:IL-1β/IL-23 < 0.05; Fig. 5D) or chymase (Pmedia:IL-23 < 0.01, Pmedia:IL-1β/IL-23 < 0.01; Fig. 5E). Treatment with Substance P or Compound 48/80, classic mast cell degranulating agents, elicited degranulation of mast cells but did not yield significant ETs. Consistent findings were observed in separate experiments from 4 independent healthy skin donors in the 3-day cultures. These findings indicate that IL-1β and IL-23 can rapidly induce MC degranulation and MCETosis in human skin, resulting in release of IL-17 and other mast cell products.
Our findings suggest prominent roles for IL-17 released by mast cells and neutrophils in psoriasis as outlined in our model (Fig. 6). In normal skin that encounters trauma or microbial infection, mast cells detect stimuli such as necrotic tissue or microbial invasion through TLRs and other PRRs. This detection can lead to direct release of preformed mediators including TNF-α, IL-17, CXCL2 (55), chymase and tryptase through degranulation or MCETosis. These mediators promote endothelial activation, vascular permeability, and rapid influx of neutrophils. Encounter with extracellular bacteria or products of necrotic cells will promote NETosis with release of mediators including IL-17, which can further amplify neutrophil accumulation by increasing CXCL1, CXCL2, and IL-8 expression. IL-17 produced by neutrophils has also been shown to be critical for neutrophil accumulation (56). Interestingly, these CXCR2 agonists participate in NETosis in vivo as treatment with CXCR2 antagonists block NETosis and improve lung function in cystic fibrosis (57). Activation of CXCR2 by through increased release of CXCL1, CXCL2, and IL-8 may similarly mediate NETosis in psoriasis. While the relevant targets of IL-17 in psoriatic skin are unclear, IL-17 can increase the expression of AMPs human β-defensin-2 (HBD-2), S100A7, S100A8, S100A9, and cathelicidin/LL-37 in keratinocytes (33-35). These AMPs can stimulate immune cell infiltration, and NET-derived LL-37-DNA complexes may promote IFN-α release from plasmacytoid dendritic cells (pDC) (58). Indeed, MCET- or NET-derived nucleic acids may be responsible for the pDC activation seen after tape-stripping of human skin, a model for sterile mechanical skin injury (59, 60), accounting in part for the proposed role of mast cells in the Koebner phenomenon (24). IFN-α and TNF-α can stimulate influx of inflammatory DC and macrophages, which in the presence of IFN-γ and other danger signals produce cytokines including IL-23 and IL-1β. These cytokines can also stimulate IL-17 release by T cells, neutrophils, and mast cells to further promote the cycle of inflammation.
In a typical host response to infection, TLR stimulation diminishes as pathogens and necrotic tissue are cleared, thereby interrupting the cycle of inflammation with decreased release of IL-17, TNF-α, IFN-α, IFN-γ, and IL-23. However, in people carrying polymorphisms in the psoriasis susceptibility genes IL12B, IL23A, or IL23R, IL-23 signaling may be dysregulated, resulting in inappropriately sustained release of IL-17 and other mediators from mast cells, neutrophils, and T cells. Likewise, polymorphisms in genes modifying signaling downstream of IL-17 (TRAF3IP2), TNF-α and TLRs (TNFAIP3, TNIP1) may fail to appropriately dampen or extinguish these pathways. Either scenario would result in prolonged inflammation in the absence of infection.
While our Western blot and immunofluorescence studies indicate the presence of IL-17A protein, mRNA from human neutrophils from several subjects showed either low or undetectable levels of IL-17 using real time RT-PCR and Affymetrix gene array experiments (M.J.K., data not shown). This may be explained by early production and storage of IL-17A by developing neutrophils prior to their release from bone marrow. Given their short lifespan, upon entering the circulation neutrophils may downregulate transcription of many mRNAs. An alternative explanation is that neutrophils and mast cells may bind and accumulate IL-17A protein produced by other cell types, instead of directly producing IL-17. However, while analysis of mRNA expression from skin mast cells is hindered by limited numbers, in vitro studies have shown that mast cells release IL-17 protein in response to multiple stimuli in a RORC-dependent manner (19).
IL-17 release from innate immune cells may provide a biologic basis for systemic features of psoriasis. Similar to its role in inflamed synovium of rheumatoid arthritis patients (19), mast cell-derived IL-17 may also be involved in psoriatic arthritis. The presence of IL-17+ mast cells in psoriatic plaques and complicated atherosclerotic plaques (18) suggests that mast cell release of IL-17 may contribute to the increased atherosclerosis and increased mortality associated with severe psoriasis (61). Indeed, inhibition of IL-17 attenuates both psoriasis and atherosclerotic plaque development (62).
Neutrophils, mast cells, and eosinophils form extracellular traps on exposure to bacteria, fungi, bacterial products, activated platelets, or IL-8 through activation of TLR, cytokine, and Fc receptors (47, 63). These receptors stimulate protein kinase C and ultimately result in the generation of ROS through activation of the NADPH oxidase complex (64). The ETosis-promoting activities of IL-23 and IL-1β were not previously suspected, and the precise signaling mechanisms regulating this process remain to be defined. Based on our results, we suggest modulation of mast cell and neutrophil ETosis and release of IL-17 is a novel therapeutic mechanism of action for drugs targeting IL-23.
In conclusion, this study demonstrates that increased numbers of mast cells and neutrophils in psoriasis lesions contribute to the release of the pathogenic cytokine IL-17 through the formation of extracellular traps, and that IL-23 and IL-1β provide a novel mechanistic stimulus for this phenomenon. These observations support a model in which mast cells and neutrophils play significant roles in the pathophysiology of psoriasis and potentially other autoinflammatory diseases driven by the IL-23-IL-17 axis. This model presents opportunities to better understand current treatments and develop novel therapies that inhibit ETosis and IL-17 release from mast cells and neutrophils.
We thank the subjects who participated in this study. We are grateful to Wendy Goodman (Case Western Reserve University, Cleveland, OH), and Doug Fullen (University of Michigan, Ann Arbor, MI) for advice on immunofluorescence. We thank numerous colleagues for critical feedback on the manuscript. We thank the University of Michigan Microscopy & Image Analysis Laboratory for assistance with microscopy.
1This work was supported by the Babcock Research Endowment (to A.T.B.), a Dermatology Foundation Career Development Award (to A.T.B.), by a National Psoriasis Foundation Research Grant (to A.T.B.), the Training Grant in Cell and Molecular Dermatology 5T32AR007197-33 (to C.J.R.), the University of Michigan/Centocor Posdoctoral Research Program in Immunology (to E.C.V. and M.J.K.), and PHS grant 5R01HL088419 (to M.J.K.).