The type I interferon system is locally activated in psoriatic lesions

The expression of mRNAs encoding interferons (IFNs) and IFN-inducible proteins has been studied in psoriatic lesions and in noninvolved skin. The specific mRNAs have been detected by in situ hybridization using antisense RNAs. Signals for the expression of IFN-gamma mRNA have been found exclusively in cells of psoriatic lesions, and most likely represent a subpopulation of infiltrating leukocytes. Weak signals of IFN-alpha mRNA have been detected throughout the hyperkeratotic epidermis, although specific signals for IFN-beta mRNA expression were not detectable. The expression of two IFN-alpha-inducible gene products, namely the MxA protein and the 2'-5' oligoadenylate (2-5A) synthetase, have been studied as markers for the local activation of the IFN-alpha system. Expression of MxA mRNA and protein was observed in psoriatic keratinocytes, but not in normal appearing keratinocytes adjacent to the lesions. Similarly, 2-5A synthetase expression was markedly elevated in psoriatic keratinocytes. The results of the present study indicate that the IFN-alpha system is selectively activated in psoriatic lesions, although it remains silent in noninvolved skin. The implications of this finding are discussed within the boundaries of current understanding of the cytokine network.