In contrast to Langerhans cells, which make interleukin (IL)-12, differentiated macrophages that infiltrate the epidermis 72 h after ultraviolet B (UV) irradiation potently produce IL-10 mRNA and secrete IL-10 protein. We asked whether differentiated UV macrophages in the epidermis acquired their activated, IL-10hi status as a result of entering the epidermis or as a result of encountering UV-induced changes in the dermal microenvironment. In this study, sequential section immunostaining directly showed dynamic and reciprocal changes of infiltrating CD11b+ macrophages and CD1a+ Langerhans cell loss in human epidermis and dermis after in vivo UV exposure in relation to the microanatomic localization of newly appearing dermal cells that stain for IL-10 mRNA by in situ hybridization. Using quantitative reverse transcriptase polymerase chain reaction on purified dermal cell subsets, the first significant rise in IL-10 mRNA occurred 6 h after UV in the dermal CD11b+ (CD1-, 3-, 24-, 56-) monocytic/macrophagic population. Significant induction of IL-10 mRNA 24 h post-UV was limited to the CD11b+ CD1- subset (p = 0.006). The fold increase of IL-10 mRNA relative to 0 h by the CD11b+ dermal monocytic/macrophagic population peaked at 24-48 h and tapered thereafter. Intense IL-10 production by macrophages in the epidermis appeared to follow dermal changes, with maximum production at 72 h, indicating migration/activation of this population from the dermis, and the remainder of dermal cells, depleted of monocyte/macrophages and Langerhans cell-like antigen-presenting cells, showed no increase in IL-10 at any time point post-UV. IL-10 protein-producing CD11b+ macrophages in the dermis were also documented by flow cytometry. IL-12 mRNA was differentially regulated from IL-10 after UV, in that IL-12 was consistently downregulated in the CD11b+ monocytic/macrophagic population (p < 0.0002). Taken together, monocytic/macrophagic cells with high IL-10 and low IL-12 expression initially appear in the dermis as early as 6 h, and then appear in the epidermis, implicating the dermis as the primary site of activation/signaling for IL-10 upregulation in cutaneous antigen-presenting cells.