Sunscreen lotions prevent ultraviolet radiation-induced suppression of antitumor immune responses.

Abstract

Exposure to subcarcinogenic doses of ultraviolet (UV) radiation suppresses tumor immunity, thus permitting the emergence and growth of highly immunogenic skin cancers in mice. Sunscreens prevent UV carcinogenesis; however, there are conflicting reports regarding their ability to block UV-induced tumor immune suppression. In this study we critically evaluated the effects of UV spectrum and dose on the tumor immune protective capacity of 4 marketed sunscreen lotions with labeled sun protection factors (SPF) 8-45. Effective tumor immune suppression doses (TISD), i.e., the lowest dose tested to induce outgrowth of transplanted nonmelanoma skin tumors in 100% of UV-exposed C3H mice, were established for 3 different UV sources. TISD were significantly lower for unfiltered (FS) and Kodacel-filtered (KFS) UVB-type FS20 sunlamps compared with a filtered xenon arc lamp solar simulator. Sunscreen tumor immune protection levels matched those predicted by their labeled SPF when sunscreen-protected mice were exposed to a fixed TISD of solar simulator UV radiation. SPF 30 and 45 sunscreens also blocked activation of tumor antigen-specific suppressor T-lymphocytes in mice exposed to solar simulator UV radiation. In comparison, sunscreens with SPF > or = 15 provided partial to complete protection, as measured by tumor incidence, for mice exposed to UV radiation from KFS. All sunscreens tested reduced tumor growth rates in KFS UV-exposed mice. None of the sunscreens tested provided measurable tumor immune protection for mice exposed to FS UV radiation. Thus, sunscreen lotions provide an extent of tumor immune protection consistent with their labeled SPF when appropriate testing conditions are employed.