The endothelial expresson and upregulation of ICAM-1 and epidermal keratinocyte expression of ICAM-1 are well documented in psoriasis. ICAM-1 mediates the adhesion and trafficking of circulating activated skin-seeking CD45RO+ memory CD4+ T lymphocytes from the vessel into the dermis and epidermis of psoriatic skin by binding to its ligand LFA-1(lymphocyte function-associated antigen-1) expressed on lymphocyte membranes.

The purpose of this study was to investigate the expression of ICAM-1 in vascular endothelium and keratinocyte of psoriatic skin and the difference of ICAM-1 expression between early and fully developed psoriatic lesions.

We have studied the expression of ICAM-1 in twelve psoriatic patients who had not been treated for psoriatic lesions for 1 month and three normal human skin samples by immunohistochemical staining using monoclonal antibody against ICAM-1.

Immunohistochemical staining revealed anti-ICAM-1 antibody positively stained only in the subpapillary endothelial cells of normal skin. But in all psoriatic lesions studied, anti- ICAM-1 antibody was stained positively in the endothelium of papillary and subpapillary plexus, and in fully developed psoriatic lesions, anti-ICAM-1 antibody was stained focally in epidermal keratinocytes.

The results suggest that ICAM-1 expression on papillary microvessels and keratinocytes may play an important role in the transendothelial and transepidermal migration of lymphocytes from the vessel into the dermis and epidermis of psoriatic skin.