Relation of antipsoriatic and Langerhans cell depleting effects of systemic psoralen photochemotherapy: a clinical, enzyme histochemical, and electron microscopic study.

The possible relation between the therapeutic action and Langerhans cell (LC) depleting effect of psoralen photochemotherapy (PUVA) was investigated in 9 psoriatic patients by parallel recording of the clinical status, using the psoriasis area and severity index ( PASI ), and the epidermal LC counts of uninvolved skin, using light (ATPase staining) and electron microscopy (EM). Both light and electron microscopically recorded LC numbers decreased significantly during the PUVA course of, on the average, 21 exposures and a mean cumulative UVA dose of 77 J/cm2. At the end of the treatment period, both the PASI score and the light microscopically recorded LC density had reduced to about one-tenth of original, i.e., from 7.5 +/- 5.7 to 0.8 +/- 0.8 points and from 787 +/- 70 to 60 +/- 48 cells/mm2, respectively. After cessation of the PUVA course, the PASI scores remained low during the 3-7 week follow-up period, while the LC counts returned to normal, and even exceeded the starting value by 12%. Although a possible functional impairment of LC in the post-PUVA period cannot be excluded, the data are interpreted as speaking against the hypothesis that the therapeutic action of PUVA would be mediated through its LC effects.