Effects of solar-simulated radiation dose fractionation on CD1a+ Langerhans cells and CD11b+ macrophages in human skin

Background There are few human studies investigating the immunosuppressive effects of exposure to solar‐simulated radiation (SSR) and its relationship with sunburn/erythema, and few comparative data on the importance of SSR exposure regimens. Objectives To evaluate whether SSR‐induced erythema is a reliable end‐point for assessing damage to antigen‐presenting cells (APCs) in human skin. Methods We compared the relationship between SSR‐induced erythema and alterations in epidermal CD1a+ Langerhans cells (LCs) and CD11b+ macrophages in human volunteers after single exposures to 0, 0·5, 1, 2 or 3 minimal erythema doses (MED). We also investigated whether SSR exposure leads to an accumulation or accommodation of the same end‐points by comparing the effects of a relatively low cumulative SSR dose (3 MED) given in varying daily dose fractions (4 × 0·75 MED, 2 × 1·5 MED and 1 × 3 MED). Results Single SSR exposures induced a dose‐dependent increase in erythema. CD1a+ LCs remaining in the irradiated epidermis showed a dose‐dependent increase in cell size and altered morphology. Significant depletion of CD1a+ LCs and presence of CD11b+ macrophages only occurred in sites irradiated with 2 MED and 3 MED. Dose fractionation had no effect on the final erythemal response but the 4 × 0·75 MED and 1 × 3 MED protocols were better tolerated than 2 × 1·5 MED for alterations in CD1a+ LC and CD11b+ cell numbers. In contrast, dose fractionation protected against alterations in CD1a+ LC morphology or cell size. Conclusions We found that erythema is a poor indicator of alterations in epidermal APCs and that dose fractionation is an important parameter in the immunological effects of ultraviolet radiation.