Psoriasis: dysregulation of innate immunity.

The current understanding of the function of natural killer (NK) T cells in innate immunity and their potential to control acquired specific immunity, as well as the remarkable efficacy of antitumour necrosis factor-alpha biological treatments in psoriasis, forces us to refine the current T-cell hypothesis of psoriasis pathogenesis, and to give credit to the role of innate immunity. Psoriasis might be envisioned to be a genetically determined triggered state of otherwise dormant innate immunity. This aggravated state of innate immunity is represented by the activity of NK T cells, dendritic cells, neutrophils and keratinocytes, leading to the recruitment and activation of preferentially type 1 T cells, possibly in an antigen-independent way. Keratinocytes in psoriasis then are sensitive to the effects of T-cell activation and cytokine production, interferon (IFN)-gamma, by responding with psoriasiform hyperplasia. The chronic inflammation of psoriatic lesions suggests that this might be due to a deficiency in downregulation processes (e.g. a defect in the regulatory T-cell repertoire) and/or the persistence of an unknown trigger resulting in an exaggerated innate immune response.