In vivo effects of psoralens plus longwave ultraviolet radiation on immunity

Treatment with 8-methoxypsoralen plus UV radiation at 320-400 nm (PUVA) alters immune function in experimental animals and humans. In guinea pigs, the treatment can suppress the development of delayed hypersensitivity to an injected antigen and the development of contact hypersensitivity locally at the site of exposure to radiation and systemically at a nonexposed site. The mechanisms of these changes are unknown but may involve certain alterations that occur after treatment in Langerhans cells. In normal human subjects, PUVA treatment can alter the distribution of circulating T- and B-lymphocytes, and this effect appears to be dose related. The distribution and function of T-lymphocytes in patients with psoriasis are also altered by PUVA treatment. These effects are greater with high, cumulative exposure doses of the treatment. Suppression of contact hypersensitivity also has been observed in patients with mycosis fungoides and psoriasis treated with PUVA. Autoimmune phenomena do not appear to be a side effect of the treatment. Although immunosuppression may be a mechanism of action by which PUVA treatment exerts its therapeutic effect on disease, it can also result in adverse effects after long-term treatment.