Cell interactions in psoriasis.

Products of the HLA-D gene, the la or DR antigens, have been shown to control interactions between certain cells with immune functions. Three HLA-D alleles have been reported to be associated with a markedly increased relative risk in psoriatic individuals. We report the existence of apparently unique anatomic interactions between lymphocytes and basal keratinocytes, between Langerhans' cells and basal keratinocytes, and between Langerhans' cells and lymphocytes, deduced on the basis of (1) characteristic cytoplasmic processes extending from one cell into the cytoplasm of adjacent cells, with frequent absence of the intervening plasma membrane at the apexes of these processes, and (2) intimate apposition of the plasma membranes between the interaction cells over a large surface area. These interactions were noted in five of ten untreated psoriatic patients (three of four patients with Koebner's phenomenon) and in one of six treated psoriatic patients, but in none of 17 controls. Intercellular space abnormalities, probably secondary to excessive proteolytic enzyme release, and basal keratinocyte herniations in psoriasis may result from these anatomic interactions.