An action spectrum for ultraviolet radiation-induced immunosuppression in humans.

Background: The immune-suppressive effects of sunlight play a central role in skin carcinogenesis. Ultraviolet (UV) B radiation is highly immunosuppressive even at suberythemal doses, and longwave UVA is now also recognized to cause immunosuppression in humans. The relative contributions of UVA and UVB to immunosuppression by incidental daily sun exposure are, however, unclear.

Objectives: We previously determined wavelength dependencies for immunosuppression by UVB and UVA wavebands in humans. We now aimed to calculate relative and solar immune-suppressive effectiveness across the UVB and UVA spectra.

Methods: We used the nickel model of recall contact hypersensitivity to determine UV immunosuppression dose responses and minimum immune suppression doses (MISDs) at 11 narrowbands from 289 to 392 nm. The relative immune-suppressive effectiveness of each narrowband was then determined as 1/MISD vs. wavelength. This curve was multiplied by the solar spectrum to show the relative immune-suppressive effectiveness of each waveband in sunlight.

Results: We found peaks of immune-suppressive effectiveness in the UVB waveband at 300 nm and in the UVA at 370 nm. Because of the far greater amount of longwave UVA in sunlight, the relative solar immune-suppressive effectiveness of UVA was threefold higher than that of UVB at doses equivalent to sun exposure from normal daily activities.

Conclusions: Longwave UVA, which abuts the visible light spectrum and is less effectively filtered by sunscreens than UVB, is likely to be the largest contributor to immunosuppression resulting from incidental daily sun exposure.