Dark effect of 8-methoxypsoralen (8-MOP) on lymphocytes.

In order to find, whether 8-MOP has an inhibiting effect on the proliferation of lymphocytes in the dark, we studied in vitro the lymphocyte proliferation by a mitogen (HgCl2) and by an antigen (tuberculin) from 16 psoriatic patients during the first oral PUVA treatment immediately before 8-MOP intake (03.--0.5 mg/kg body weight), 2 h later (time of maximal serum concentration) and after 24 h. The percentages of B- and T-lymphocytes were measured from 15 other psoriatic patients at equal times. There was a significant decrease (P less than 0.05) of the lymphocyte proliferation after stimulation with HgCl2 in culture, 2 h after 8-MOP, returning completely to the starting range after 24 h. On the contrary, neither the proliferation capacity of lymphocytes after stimulation with tuberculin nor the percentages of B- and T-lymphocytes were significantly changed. Obviously, 8-MOP, even without UVA irradiation, has a direct inhibiting effect on lymphocyte proliferation in concentrations therapeutically used. However, this effect is seen only with stimulation by a mitogen, not by an antigen. It cannot be explained by a change in the ratio between B- and T-lymphocytes.