Costimulatory molecule expression on freshly isolated Flt3L-mobilized circulating rhesus monkey pre-DC subsets. Rhesus monkey pre-DC subsets were flow-sorted before secondary staining for costimulatory molecule expression. Flow-sorted CD11c⁺ pre-DCs showed moderate levels of CD40, minimal levels of CD80, and moderate levels of CD86 expression. Flow-sorted CD123⁺⁺ pre-DCs also showed moderate expression of CD40 and minimal expression of CD86, indicating phenotypic immaturity. On overnight culture with CD40L, CD11c⁺ pre-DCs and CD123⁺⁺ pre-DCs up-regulated cell surface expression of CD40, CD80, and CD86. Open histograms indicate isotypes; shaded histograms, stained cells. Data are representative of 3 separate experiments for each subset.