In human dermis, ultraviolet radiation induces expansion of a CD36+ CD11b+ CD1- macrophage subset by infiltration and proliferation; CD1+ Langerhans-like dendritic antigen-presenting cells are concomitantly depleted.

Antigen-presenting (APC), suppressor T-cell-inducing macrophages infiltrate both human and murine epidermis after ultraviolet radiation (UVR) exposure. To determine their derivation, we prepared epidermal cell and dermal cell suspensions from human keratome biopsy specimens obtained from nonexposed skin and from UVB-irradiated sites (3 d after four times the minimal erythema dose). Simultaneous triple-marker flow cytometric analysis established the extended phenotype of macrophages infiltrating sunburned human epidermis (CD1a- CD1c- CD11b+ CD11c+ CD36+ Fc gamma RII+ DR+). This then enabled us to track dermal cells of this phenotype after UVR in relation to the heterogeneous DR+ populations in normal dermis. By both in situ immunohistology and cell suspension flow cytometry, UVR induced an expansion of bone marrow-derived DR+ cells in the perivasculature and sub-basement membrane zone of the papillary dermis. Despite an overall expansion of DR+ cells, the CD1a+ CD1c+ CD36- DR+ Langerhans-cell-like dendritic APC subset of dermal DR+ cells was depleted (p < 0.05), indicating that UVR-induced epidermal Langerhans cell loss (from 95% to 7% of DR+ epidermal cells) is not accounted for by Langerhans cell accumulation in the dermis. By contrast, UVR exposure induced a selective expansion of the dermal macrophage subset, which is phenotypically identical to the monocytic/macrophagic APCs that appear in the epidermis after UV injury (p < 0.01). Cell cycle analysis (to determine whether this expansion was accounted for entirely by infiltration) revealed no increase in the percentage of DR+ CD36+ UVR-exposed dermal cells in S/G2/M phase; however, the expanded DR+ CD36+ subset continued its already substantial level of proliferation unabated. Therefore, epidermal macrophages derive not only from transcapillary migration, but also from in situ proliferation of a dermal precursor. Taken together, these findings show that UVR creates an epidermal and dermal APC milieu which is dominated by monocytic/macrophagic cells, through depletion of cells of dentritic APC phenotype, and concomitant selective dermal expansion of a CD1a- CD1c- CD11b+ CD36+ Fc gamma RII+ DR+ (monocyte/macrophage) population.