Effects of ultraviolet B light on cutaneous immune responses of humans with deeply pigmented skin

The incidence of skin cancers of the basal and squamous cell types is extremely low among genetically black-skinned human beings, whereas these types of skin cancers are common among Caucasians, especially those who live in geographic areas of high sun exposure. Ultraviolet B light (UVB) is thought to be the primary oncogenic agent in sunlight. We have recently demonstrated that acute, low-dose exposure of Caucasian skin to UVB impairs the induction of contact hypersensitivity to dinitrochlorobenzene (DNCB) in approximately 40% of normal individuals. Importantly, this trait--termed UVB susceptibility--was found to be a characteristic of virtually 100% of patients with a history of biopsy-proved skin cancer, implying that UVB susceptibility may be a risk factor for this disease. Because melanin pigment is thought to be protective of some of the deleterious effects of UVB radiation, we have examined the capacity of a low-dose regimen of UVB to alter induction of contact hypersensitivity in individuals with genetically melanized or heavily tanned skin. Our results indicate that UVB radiation depletes heavily pigmented skin of Langerhans cells, just as it does in Caucasian skin. Moreover, UVB-susceptibility exists as a polymorphic trait in individuals with genetically determined black skin, as well as in individuals with heavily tanned skin, and the incidence of this trait is similar to that found among normal Caucasian subjects. Thus, melanin does not appear to protect against the deleterious effects of an acute, low-dose regimen of UVB on induction of cutaneous immunity, and the UVB susceptibility trait is equally well-represented in both black- and Caucasian-skinned individuals. We conclude that although UVB susceptibility may function as a risk factor for skin cancer in Caucasians, it does not function similarly in black-skinned human beings, probably because melanin effectively protects against the mutagenic properties of UVB radiation.