Effect of monochromatic UVA light and 8-methoxypsoralen on human lymphocyte response to mitogen

The ultraviolet-A (UVA) action spectrum for inhibition of DNA synthesis by lymphocytes stimulated with phytohemagglutinin and sensitized with 8-methoxypsoralen (8-MOP) (100 ng/ml) was determined by exposing cells to 0.33 and 1.0 J/cm2 of sub-bands of UVA light at 322, 334, 346, 358 and 370 nm. For lymphocytes irradiated with 0.33 J/cm2, the wavelength for optimal inhibition of DNA synthesis was 334 nm (P less than 0.001). Relatively high doses of monochromatic UVA light (6J/cm2) significantly inhibited lymphocyte proliferation at all wavelengths tested: 64% at 370 nm, 90% at 358 nm, 78% at 346 nm and greater than 99% at both 334 and 322 nm (P less than 0.001 at each wavelength). The optimal wavelength for selective 8-MOP enhanced inhibition of DNA synthesis by lymphocytes in the 334-346 nm range is shown to support the usual assumption that crosslinks represent a lethal cellular event.