Circulating natural killer cells in psoriasis

Background Psoriasis is an immunologically mediated, probably autoimmune, disease in which T-helper type 1 cytokines play an important role. Established autoimmune diseases, with similar mechanistic characteristics to psoriasis, include multiple sclerosis, rheumatoid arthritis, type 1 diabetes mellitus and systemic lupus erythematosus. Natural killer (NK) and natural killer-T (NK-T) cells are considered key to the pathogenesis of these conditions, which are characterized by reduced numbers of NK cells in peripheral blood. NK and NK-T cells have been implicated in the pathogenesis of psoriasis and are present in plaques of psoriasis. Objectives To investigate whether levels of NK and NK-T cells are reduced in the peripheral blood of patients with psoriasis. Methods Fourteen patients with untreated psoriasis, mean +/- SD age 46 +/- 13 years, and 13 healthy volunteers, mean +/- SD age 34 +/- 9 years, were venesected and peripheral blood mononuclear cells isolated, labelled with a panel of antibodies to T-cells and NK cells including CD3, CD56, CD57, CD16, CD94, CD158a, CD69 and cutaneous lymphocyte-associated antigen (CLA) and analysed using triple-colour flow cytometry. Results There were significantly fewer cells expressing the NK-cell markers CD16 (P < 0.001), CD56 (P < 0.003), CD94 (P < 0.001) and CD158a (P < 0.02) in patients with psoriasis compared with normal controls. However, circulating numbers of NK-T cells (CD3+ CD56+ CD57+), T-cells (CD3+), activated lymphocytes (CD69+) or CLA+ cells were not significantly different between patients with psoriasis and controls. Conclusions Circulating NK cells are reduced in psoriasis. This finding is similar to those in established autoimmune diseases such as rheumatoid arthritis. This observation provides some evidence that psoriasis may be an autoimmune disease in which NK cells play a role.