Isolation and Characterization of Human Peripheral Blood Dendritic Cells

Dendritic cells (DC) are potent APC that can be purified from cultured peripheral blood non-T cells. Because no specific cell surface marker has been found, the lineage of DC remains obscure. The purpose of these studies was to determine the circulating blood cells that could give rise to functional human DC. DC were enriched when purified by standard techniques from non-T cells that were treated with L-leucyl-L-leucine methyl ester, known to be toxic to monocytes and cytolytic cells. To determine whether monocytes or B cells could give rise to DC, fresh non-T cells were sorted into CD14⁺ monocytes, CD19⁺ B cells, and CD14^- and CD19^- cells. Although there was some enrichment for APC function by cultured nonadherent CD14^- or CD19^- cells, a marked enrichment for cells with dendritic morphology and potent APC function was found in the population that was sorted by the absence of expression of CD14, CD19, CD3, and CD16. More than 90% of the CD14^-CD19^-CD16^-CD3^- sorted cells, and of control DC, expressed the myeloid markers CD13 and CD33. Therefore, fresh non-T cells were sorted based on the expression of these myeloid markers. In comparison with CD33^-CD14^- B cells, some of the CD33⁺ cells expressed CD14 dimly. However, they were easily distinguished from monocytes, which intensely expressed CD14. CD33⁺CD14^dim cells developed dendritic processes and were more potent APC than control DC, CD33⁺CD14⁺, or CD33^-CD14^- cells. Although freshly isolated CD33⁺CD14^dim DC expressed a number of cell surface molecules also expressed by CD14⁺ monocytes, they demonstrated lower levels of lysosomal enzymes and a lack of FcR-mediated phagocytosis in comparison with monocytes. Differentiation of morphology and phenotype of CD33⁺CD14^dim cells occurred within 6 to 36 h in culture. However, the CD33⁺CD14^dim cells could effectively function as APC without prolonged preincubation to develop dendritic morphology. These data indicate that human blood DC arise from precursors that express the myeloid lineage markers CD13 and CD33, but are functionally distinct from classic CD14⁺ monocytes.