NOS2-derived low levels of NO drive psoriasis pathogenesis.

Psoriasis is an IL-23/Th17-mediated skin disorder with a strong genetic predisposition. The impact of its susceptibility gene nitric oxide synthase 2 (NOS2) remains unknown. Here, we demonstrate strong NOS2 mRNA expression in psoriatic epidermis, an effect that is IL-17 dependent. However, its complete translation to protein is prevented by the IL-17-induced miR-31 implying marginally upregulated NO levels in psoriatic skin. We demonstrate that lower levels of NO, as opposed to higher levels, increase keratinocyte proliferation and mediate IL-17 downstream effects. We hypothesized that the psoriatic phenotype may be alleviated by either eliminating or increasing cellular NO levels. In fact, using the imiquimod psoriasis mouse model, we found a profound impact on the psoriatic inflammation in both IMQ-treated NOS2 KO mice and wild-type mice treated with IMQ and the NO-releasing berdazimer gel. In conclusion, we demonstrate that IL-17 induces NOS2 and fine-tunes its translation towards a window of proinflammatory and hyperproliferative effects and identify NO donor therapy as a new treatment modality for psoriasis.

MTH1 Inhibitors for the Treatment of Psoriasis.

Inflammatory diseases, including psoriasis, are characterized by changes in redox regulation. The MTH1 prevents the incorporation of oxidized nucleotides during DNA replication. Using MTH1 small-molecule inhibitors, we found induced apoptosis through 8-oxodeoxyguanosine triphosphate accumulation and DNA double-strand breaks after oxidative stress in normal and malignant keratinocytes. In psoriasis, we detected increased MTH1 expression in lesional skin and PBMCs compared with that in the controls. Using the imiquimod psoriasis mouse model, we found that MTH1 inhibition diminished psoriatic histological characteristics and normalized the levels of neutrophils and T cells in the skin and skin-draining lymph nodes. The inhibition abolished the expression of T helper type 17‒associated cytokines in the skin, which was in line with decreased levels of IL-17-producing γδ T cells in lymph nodes. In human keratinocytes, MTH1 inhibition prevented the upregulation of IL-17‒downstream genes, which was independent of ROS-induced apoptosis. In conclusion, our data support MTH1 inhibition using small molecules suitable for topical application as a promising therapeutic approach to psoriasis.

Enhanced Inflammasome Activity in Patients with Psoriasis Promotes Systemic Inflammation.

Psoriasis is linked to systemic inflammation and cardiovascular comorbidities, but studies of the underlying cellular mechanisms are lacking. The NLRP3 inflammasome is genetically associated with psoriasis, and its activation is increasingly linked with cardiovascular disease. In this study, we show that patients with psoriasis exhibited higher plasma levels of inflammasome-generated IL-1ß and IL-18, without any correlation to skin lesion severity. Increased constitutive expression of the inflammasome sensors NLRP3, NLRP1, and AIM2 was found in peripheral blood cells of the patients and also of those with mild disease, and this was accompanied by an increased caspase-1 reactivity in the myeloid blood subsets. TNF-α was found to activate selectively the NLRP3 inflammasome without the requirement for a priming signal. TNF-α was found to signal through the TNFR‒caspase-8‒caspase-1 alternative inflammasome pathway, which proceeds independently of pyroptosis. Patients who received anti-TNF therapy had normalized plasma IL-1ß and IL-18 levels as well as normalized caspase-1 reactivity. This was in contrast to the patients treated with methotrexate who exhibited persistent, increased caspase-1 reactivity. Thus, we show that the TNF-α-mediated activation of NLRP3 inflammasomes in patients with psoriasis may contribute to systemic inflammation. Anti-TNF therapy normalized inflammasome function, suggesting a mechanism for the cardiovascular risk‒reducing effect.

IL-17 and IL-22 Promote Keratinocyte Stemness in the Germinative Compartment in Psoriasis.

Psoriasis is an inflammatory skin disorder characterized by the hyperproliferation of basal epidermal cells. It is regarded as T-cell mediated, but the role of keratinocytes (KCs) in the disease pathogenesis has reemerged, with genetic studies identifying KC-associated genes. We applied flow cytometry on KCs from lesional and nonlesional epidermis to characterize the phenotype in the germinative compartment in psoriasis, and we observed an overall increase in the stemness markers CD29 (2.4-fold), CD44 (2.9-fold), CD49f (2.8-fold), and p63 (1.4-fold). We found a reduced percentage of cells positive for the early differentiation marker cytokeratin 10 and a greater fraction of CD29+ and involucrin+ cells in the psoriasis KCs than in nonlesional KCs. The up-regulation of stemness markers was more pronounced in the K10+ cells. Furthermore, the psoriasis cells were smaller, indicating increased proliferation. Treatment with IL-17 and IL-22 induced a similar expression pattern of an up-regulation of p63, CD44, and CD29 in normal KCs and increased the colony-forming efficiency and long-term proliferative capacity, reflecting increased stem cell-like characteristics in the KC population. These data suggest that IL-17 and IL-22 link the inflammatory response to the immature differentiation and epithelial regeneration by acting directly on KCs to promote cell stemness.

Genome-Wide DNA Methylation Profiling Identifies Differential Methylation in Uninvolved Psoriatic Epidermis.

Psoriasis is a chronic inflammatory skin disease with both local and systemic components. Genome-wide approaches have identified more than 60 psoriasis-susceptibility loci, but genes are estimated to explain only one-third of the heritability in psoriasis, suggesting additional, yet unidentified, sources of heritability. Epigenetic modifications have been linked to psoriasis and altered DNA methylation patterns in psoriatic versus healthy skin have been reported in whole-skin biopsies. In this study, focusing on epigenetic modifications in the psoriatic uninvolved skin, we compared the lesional and non-lesional epidermis from psoriasis patients with epidermis from healthy controls. We performed an exhaustive genome-wide DNA methylation profiling using reduced representation bisulfite sequencing, which interrogates the methylation status of approximately 3-4 million CpG sites. More than 2,000 strongly differentially methylated sites were identified and a striking overrepresentation of the Wnt and cadherin pathways among the differentially methylated sites was found. In particular, we observe a strong differential methylation in several psoriasis candidate genes. A substantial number of differentially methylated sites present in the uninvolved versus healthy epidermis suggests the presence of a pre-psoriatic state in the clinically healthy skin type. Our exploratory study represents a starting point for identifying biomarkers for psoriasis-prone skin before disease onset.