UVB-induced (6-4) photoproducts in hairless mouse epidermis studied by quantitative immunohistochemistry

DNA photoproducts in epidermal basal cells caused by in vivo exposure to various wavelengths of ultraviolet B (UVB) light were studied by immunohistochemical and microfluorometric methods. Hairless mice were irradiated with UVB doses ranging between 6.3 and 100 mJ/cm2 at 280, 290, 297, 302 and 313 nm. A dose of 25 mJ/cm2 at 297 nm corresponded to a minimal erythema dose in the Hr/Hr mice. Immediately after the exposure, frozen skin sections were stained in situ with affinity-purified antiserum having a major specificity against UV-induced (6-4) photoproducts. Microfluorometric measurements of nuclear immunofluorescence intensities were then performed to quantitate the DNA lesions in epidermal basal cell nuclei. All wavelengths except 313 nm produced DNA photoproducts, and the induction was both dose and wavelength dependent. The most effective wavelength was 290 nm, followed by 297, 280 and 302 nm. The quantitative determinations indicated variations in the DNA photoproduct content between nuclei exposed to the same dose and wavelength. This may be due to shielding by other nuclei or to differences in the sensitivity to UVB irradiation among basal cells, resulting in non-random distribution of DNA-damaged cells. Hence, measurable amounts of UVB-induced (6-4) photoproducts are produced in hairless mouse epidermis following in vivo exposure with biologically relevant doses of narrow-banded UVB light.