Interleukin-18 Exacerbates Skin Inflammation and Affects Microabscesses and Scale Formation in a Mouse Model of Imiquimod-Induced Psoriasis
Psoriasis is a chronic inflammatory skin disease affecting 2% to 3% of the global population. Despite extensive research, the exact etiology of psoriasis remains unclear, though it is widely believed to involve a complex interplay of cytokines and chemokines produced by various immune and tissue cells. Among these cytokines, interleukin-18 (IL-18), a potent pro-inflammatory cytokine of the IL-1 family, has been implicated in the pathogenesis of psoriasis. Previous studies have shown that IL-18 levels are elevated in early active and progressive plaque-type psoriatic lesions, and serum or plasma levels of IL-18 correlate with the Psoriasis Area and Severity Index (PASI). However, the specific mechanisms by which IL-18 exacerbates psoriatic inflammation remain poorly understood. This study aimed to investigate the role of IL-18 in a mouse model of imiquimod (IMQ)-induced psoriasis, using IL-18 knockout (KO) mice and wild-type (WT) mice to elucidate the cytokine’s effects on skin inflammation, pathological features, and cytokine profiles.
Introduction
Psoriasis is characterized by abnormal keratinocyte proliferation, immune cell infiltration, and the production of pro-inflammatory cytokines. The disease is thought to be driven by T helper 1 (Th1) and T helper 17 (Th17) cells, which secrete cytokines such as interferon-gamma (IFNγ) and interleukin-17 (IL-17), respectively. IL-18, a cytokine structurally and functionally similar to IL-1β, plays a critical role in activating Th1 cells and promoting Th17 cell development in collaboration with IL-23. IL-18 is known to synergize with IL-23 to enhance psoriasis-like epidermal hyperplasia and inflammation. Despite these findings, the precise mechanisms by which IL-18 contributes to psoriasis pathogenesis remain unclear. This study sought to address this gap by examining the effects of IL-18 in an IMQ-induced psoriasis mouse model, focusing on skin inflammation, cytokine profiles, and pathological features such as microabscesses and scale formation.
Methods
Mouse Model and Treatment
C57BL/6 mice and IL-18 KO mice (C57BL/6 background) were used in this study. The mice were divided into four groups: WT control group (n=12), IL-18 KO control group (n=12), WT + IMQ group (n=13), and IL-18 KO + IMQ group (n=11). The dorsal skin of the mice was shaved to expose a 2 cm x 3 cm area. IMQ-treated mice received a daily application of 62.5 mg of 5% IMQ cream, while control mice received an equivalent volume of vaseline cream. Skin samples were collected from all mice on day 8 after treatment.
Evaluation of Skin Inflammation
The severity of IMQ-induced skin inflammation was assessed using a scoring system based on the clinical PASI. Erythema, scaling, and thickening were scored independently from 0 to 4, with cumulative scores ranging from 0 to 12. The scoring was performed by three independent investigators.
Histopathological Analysis
Skin samples were subjected to hematoxylin and eosin (H&E) staining to evaluate pathological features such as acanthosis, dermal cell infiltration, Munro microabscesses, and scale formation. Measurements of acanthosis thickness, scale area, and microabscess area were performed using Image-Pro Plus software. Cell counts in the dermal layer were also conducted.
Cytokine Profiling
Total RNA was extracted from skin biopsies and reverse-transcribed into cDNA. Real-time polymerase chain reaction (PCR) was used to quantify the expression levels of psoriasis-related cytokines, including IL-1β, IL-4, IL-12, IL-15, IL-17, IL-23, IL-27, and IFNγ. Immunohistochemistry (IHC) was performed to assess protein levels of IL-1β, IL-27, CXCL1, and Ly6g.
Results
IL-18 Exacerbates IMQ-Induced Skin Inflammation
IMQ-treated WT mice developed erythema, epidermal thickening, and silvery scales, while IL-18 KO mice showed milder skin lesions. PASI scores revealed that IMQ-induced WT mice exhibited significant erythema, scaling, and thickening from day 2 to day 7, with a regression on day 8. In contrast, IL-18 KO mice showed a continuous increase in symptoms over the 8-day period. Cumulative PASI scores were significantly lower in IL-18 KO mice compared to WT mice from day 3 to day 7, indicating that IL-18 exacerbates IMQ-induced skin inflammation.
Histopathological Findings
H&E staining showed that IMQ-induced IL-18 KO mice exhibited milder acanthosis (98.46 ± 14.12 μm vs. 222.68 ± 71.10 μm) and dermal cell infiltration (572.25 ± 47.45 cells/field vs. 762.47 ± 59.59 cells/field) compared to WT mice. Control mice showed no significant inflammation or epidermal thickening.
IL-18 Influences Munro Microabscesses and Scale Formation
IMQ-induced IL-18 KO mice had significantly larger areas of Munro microabscesses (11,467.83 ± 5,112.09 μm² vs. 4,093.19 ± 2,591.88 μm²) and scales (100,935.24 ± 41,167.77 μm² vs. 41,604.41 ± 14,184.10 μm²) compared to WT mice. These findings suggest that IL-18 may play a role in the formation of these pathological features.
Cytokine Profiles
Real-time PCR analysis revealed that IMQ-induced IL-18 KO mice had significantly higher mRNA levels of IL-1β (343.04 vs. 54.99), IL-27 (1,360.68 vs. 373.15), and IL-4 (6,276.45 vs. 1,312.03) compared to WT mice. In contrast, IL-17 mRNA levels were decreased in IL-18 KO mice (89.29 vs. 188.34). IHC confirmed increased protein levels of IL-1β and IL-27 in IL-18 KO mice, consistent with the mRNA findings.
Neutrophil Infiltration and CXCL1 Expression
Immunohistochemical analysis showed that IMQ-induced IL-18 KO mice had significantly lower expression levels of CXCL1 (0.19 ± 0.01 vs. 0.75 ± 0.14), a chemokine involved in neutrophil recruitment. However, Ly6g, a marker for neutrophils, was significantly increased in the epidermis of IL-18 KO mice (0.23 vs. 0.01), suggesting a potential role for IL-18 in neutrophil infiltration.
Discussion
This study provides compelling evidence that IL-18 exacerbates IMQ-induced psoriasis-like skin inflammation in mice. The findings demonstrate that IL-18 deficiency results in milder skin lesions, reduced acanthosis, and decreased dermal cell infiltration. However, IL-18 KO mice exhibited larger areas of Munro microabscesses and scales, indicating that IL-18 may influence these pathological features. The cytokine profiling data suggest that IL-18 upregulates pro-inflammatory cytokines such as IL-17 while downregulating protective cytokines like IL-4 and IL-27. Additionally, IL-18 appears to promote neutrophil infiltration, as evidenced by increased Ly6g expression in the epidermis of IL-18 KO mice.
The elevated levels of IL-1β in IL-18 KO mice may reflect a compensatory mechanism due to the structural and functional similarities between IL-18 and IL-1β. Both cytokines are members of the IL-1 family and share signaling pathways, including the activation of nuclear factor-kappa B (NF-κB). The increased expression of IL-4 and IL-27 in IL-18 KO mice suggests that IL-18 may inhibit these protective cytokines, thereby exacerbating inflammation. IL-4 is known to have anti-inflammatory properties, while IL-27 can inhibit IL-18 activity by enhancing IL-18 binding protein (IL-18BP).
The role of neutrophils in psoriasis remains controversial. While neutrophils are a key component of Munro microabscesses, their exact contribution to disease severity is unclear. The increased Ly6g expression in IL-18 KO mice suggests that neutrophils may be more abundant in the epidermis, though this did not correlate with more severe skin lesions. This discrepancy warrants further investigation.
Conclusion
In conclusion, this study highlights the critical role of IL-18 in exacerbating IMQ-induced psoriasis-like skin inflammation in mice. IL-18 deficiency results in milder skin lesions, reduced acanthosis, and decreased dermal cell infiltration, but larger areas of Munro microabscesses and scales. The cytokine profiling data suggest that IL-18 upregulates pro-inflammatory cytokines and downregulates protective cytokines, thereby aggravating psoriatic inflammation. Additionally, IL-18 may influence neutrophil infiltration and the formation of pathological features such as microabscesses and scales. These findings provide valuable insights into the mechanisms by which IL-18 contributes to psoriasis pathogenesis and may inform the development of novel therapeutic strategies targeting IL-18 in psoriasis treatment.
doi.org/10.1097/CM9.0000000000000140
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