The mechanisms of inhibitory effect of 8-methoxypsoralen and long-wave ultraviolet light on experimental contact sensitization.

A suppressive effect of treatment with 8-methoxypsoralen (8-MOP) and long-wave ultraviolet light (PUVA) on the induction and elicitation of contact sensitivity to dinitrochlorbenzene was observed in guinea pigs. The mechanisms by which PUVA exerts its inhibitory effect seemed to be variable depending on the site and extent of the treatment. When PUVA was given at the induction site, only one treatment using topical 1% 8-MOP and 7,6 J/cm2 UVA is a small area was enough to inhibit sensitization, and the effect was long-lasting. This seemed to be caused by impairment of Langerhans cell function. On the other hand, when induction was attempted through nontreated skin, repeated exposures in larger areas were needed to suppress the challenge reactions. Nonspecific primary irritant dermatitis revealed a similar effect. This inhibitory effect was transient and possibly caused by lack of sufficient number of immune cells for the elicited reactions. An apparent enhancement of challenge reactions at the PUVA-treated site was too complicated to permit interpretation of its mechanism.