Inhibition of cell mediated immune responses by 8-methoxypsoralen and long-wave ultraviolet light: a possible explanation for the clinical effects of photoactivated psoralen

Human thymus-derived lymphocytes proliferate when cultured with lymphocytes or epidermal cells from unrelated individuals because such cells express HLA-D antigens, which are recognized as foreign by thymus-derived lymphocytes. The current study demonstrates that these responses are inhibited if either the stimulator cells or responder cells are pretreated with the combination of 8-methoxypsoralen and long-wave ultraviolet light. Additional studies revealed that photoactivated 8-methoxypsoralen has a lethal effect on lymphocytes and monocytes but not on the majority of epidermal cells. These observations suggest that the dramatic beneficial effect of PUVA on patients with psoriasis and other skin disorders may be due to a toxic effect on immunocompetent cells in the epidermis, which results in inhibition of cell mediated immune responses.