APOPTOSIS INDUCTION BY ULTRAVIOLET-LIGHT-A AND PHOTOCHEMOTHERAPY IN CUTANEOUS T-CELL LYMPHOMA - RELEVANCE TO MECHANISM OF THERAPEUTIC ACTION

The anti-tumor action of many chemotherapeutic agents has recently bee n attributed to the induction of apoptosis in the malignant cell popul ation. In this study, we investigated the ability of extracorporeal ph otopheresis (ExP) and in vitro PUVA (8-methoxypsoralen + ultraviolet A ) therapy to induce apoptosis in peripheral blood mononuclear cells fr om Sezary syndrome patients and normal controls. Flow cytometric analy sis of ExP- or PUVA-treated peripheral blood lymphocytes demonstrated two distinct cell populations within 24 h of treatment. One population was similar to untreated controls with the other exhibiting character istics of apoptoxic cell death, i.e., a loss of cell volume and an acc ompanying increase in cell density. This latter population was compris ed of cells with DNA strand breaks as determined by the Tdt-mediated d eoxyuridine triphosphate-biotin nick end labeling assay. Apoptosis was also confirmed morphologically by fluorescent and electron microscopy as well as by demonstration of characteristic DNA strand breaks (ladd ering) using gel electrophoresis. Apoptosis was not observed with 8-me thoxypsoralen (less than or equal to 300 ng per ml) alone; however, ul traviolet A alone at doses greater than or equal to 2 J per cm(2) indu ced apoptosis in lymphocytes. Peripheral blood T-cell subpopulations o f Sezary syndrome patients, including the malignant clone, were equall y susceptible to apoptosis subsequent to either photopheresis or PUVA treatment. In contrast, monocytes (CD14+/CD45+) appear to be resistant to apoptosis induction by ExP or PUVA treatment. Moreover, ExP-treate d and untreated monocytes phagocytized apoptotic, but not untreated, p eripheral blood mononuclear cells, ExP and PUVA have been shown to be efficacious and well-tolerated therapies in the treatment of dermatolo gic diseases and transplant rejection, These data suggest that inducti on of apoptosis may be an important event for therapeutic efficacy.