Flexural versus plaque lesions in psoriasis: an immunohistochemical differentiation

Clinical presentation, therapeutic options, micro-environment and HLA-typing have been reported to be different in flexural psoriasis as compared to plaque psoriasis. We were interested in any difference concerning the pathogenesis of both conditions. By analysing T-cell subsets, NK-T cells and proliferation and differentiation markers, insight into the pathogenesis of both subtypes was obtained. Quantitative studies of T-cell subsets, cells expressing NK-receptors and markers of proliferation and differentiation in flexural and plaque psoriasis were investigated. Biopsies from 6 patients with both flexural and plaque lesions were obtained and processed for immunohistochemistry. Several T-cell subsets were stained: CD4+, CD8+, CD2+, CD25+, CD45RO+ and CD45RA+ T-cells. In addition cells expressing NK receptors were stained: CD94+ cells and CD161+ cells. T-cell subsets and cells expressing NK-receptors were analysed by immunohistochemical scoring. The proliferation marker Ki-67 and differentiation marker keratin-10 were revealed immunohistochemically by MIB-1 and RKSE60 respectively. Both markers were analysed using quantitative image analyses. The number of Ki-67 positive nuclei and the percentage of keratin-10 positive epidermal cells in flexural and plaque psoriasis were comparable. There is no difference between flexural and plaque psoriasis concerning other T-cell subsets. However a highly significant reduction is seen in flexural psoriasis with respect to CD161+ cells in the dermis. The similarity of chronic plaque psoriasis compared to flexural psoriasis with respect to T-cell subsets, epidermal proliferation and keratinization suggests that both conditions are pathogenetically identical. We propose the decreased quantity of lesional CD161+ cells in the dermis of flexural psoriatic lesions may result from chronic microbial challenge in flexural psoriasis.