Cytokine network and dysregulated apoptosis in atopic dermatitis

Activation and skin-selective homing of peripheral blood memory/effector T cells and effector functions in the skin represent sequential immunological events in the pathogenesis of atopic dermatitis (AD). T cells infiltrating the skin utilize the cutaneous lymphocyte-associated antigen (CLA) and oth er receptors to recognize and cross the vascular endothelium. In the periph eral blood of AD patients, both CD4(+) and CD8(+) subsets of CLA+CD45RO(+) T cells are in an activated state with high CD25, HLA-DR and CD40-ligand ex pression. They express upregulated Fas and Fas-ligand and undergo activatio n-induced apoptosis. After homing to skin these T cells Term dermal infiltr ates which play a key role in the pathogenesis of the disease. Skin-infiltr ating T cells in AD are protected from activation-induced cell death, altho ugh they express both Fns and Fas-Ligand. They are protected from apoptosis by cytokines such as IL-2, IIA, and IL-15 and extracellular matrix compone nts such as fibronectin and transferrin. CLA, skin-homing T cells may play a role in peripheral blood eosinophilia and hyper ISE production by high IL -5 and IL-13 expression, respectively; These T cells secrete IFN-gamma in t he skin, which upregulates Fas on keratinocytes and renders them susceptibl e to apoptosis. Keratinocyte apoptosis is induced by Fas-ligand, either sol uble or expressed on the surface of T cells, leading to eczema formation. H ere we discuss the mechanisms of skin-selective T cell homing and activatio n, and emphasize the concept of dysregulated apoptosis of T cells, eosinoph ils, and keratinocytes as essential pathogenetic episodes in AD and other e czematous disorders.