Disappearance of epidermal Langerhans cells during PUVA therapy

The numbers and morphological appearance of epidermal Langerhans cells (LCs) were studied in twenty-five patients with psoriasis receiving treatment with 8-methoxypsoralen (8-MOP) and long wavelength UV irradiation (UV-A) (PUVA). After a single exposure, LCs showed loss of fine dendritic processes. Repeated treatments resulted in a reduction of the number of LCs from the mean pretreatment value of 713/mm2 to less than 60/mm2 after seven treatments. The number of LCs remained low while treatment continued for up to 4 weeks. This finding may explain the impaired contact hypersensitivity observed in patients with psoriasis receiving PUVA therapy.