This study demonstrates for the first time that the phosphorylation level of STAT1(Tyr701) and STAT1(Ser727) is increased in lesional psoriatic skin. In addition, specific signalling pathways leading to this phosphorylation have been identified. Together, our data indicate an important role of STAT1 in the pathogenesis of psoriasis.

The inflammasome is a cytosolic multiprotein complex with two major functions: recognizing pathogen-associated molecular patterns and reacting to these through activation of the proinflammatory cytokines IL-1β and IL-18. In this study, we characterized the expression of inflammasome components in psoriatic skin and other common inflammatory skin diseases. Human skin biopsy specimens, cultured primary human keratinocytes, and peripheral blood mononuclear cells (PBMCs) were analyzed using quantitative reverse transcriptase-PCR (RT-PCR) and semiquantitative western blotting. mRNA expression of the inflammasome components NALP1, NALP3, ASC, caspase-1, caspase-4, and caspase-5 was detected in psoriatic skin. Interestingly, we found an extensive, 20-fold upregulation (P<0.01) of caspase-5 mRNA in lesional compared with nonlesional psoriatic skin, whereas caspase-1, caspase-4, and ASC (apoptosis-associated speck-like protein with CARD domain) mRNAs were upregulated by only 1.5- to 2.6-fold (P<0.01). Caspase-5 mRNA was not increased in biopsies from other inflammatory skin diseases, suggesting that this finding could be psoriasis specific. In vitro experiments revealed that caspase-5 mRNA was induced in primary keratinocytes as well as PBMCs stimulated with IFN-γ. Inhibition studies suggested that caspase-5 mRNA upregulation was mediated through the NF-κB pathway. Our findings suggest that caspase-5 and the inflammasome may have an important role in the inflammatory response in psoriasis.