Decreased NK activity of nude mice receiving long-term ultraviolet irradiation: in relation to tumor induction.

We have serially evaluated the basal and poly I: C-enhanced NK activity of BALB/c X C57BL/6 F1 nude mice (CB6F1-nu/nu) receiving repeated ultraviolet (UV) exposures with the interval of 2-5 weeks until overt skin tumor(s) developed. Chronic UV exposure induced a dose dependent suppression of basal splenic NK activity. The latent period between the initial UV treatment and the appearance of the first overt skin tumor was 19 weeks. After that time, incidence of tumor development gradually increased until it became 69% at the last inspection (27th week). 89% of the tumors inspected were fibrosarcoma and the remaining revealed squamous cell carcinoma. The UV-induced suppression of NK activity was restored by poly I:C treatment before the tumor appearance; however, this restorative effect was significantly reduced thereafter.