Comparative Analysis of the Morphological, Cytochemical, Immunophenotypical, and Functional Characteristics of Normal Human Peripheral Blood Lineage/CD16/HLA-DR/CD14 Cells, CD14 Monocytes, and CD16 Dendritic Cells

Human peripheral blood (PB) CD14^lo/HLA-DR⁺ cells were initially described as a subset of mature monocytes. Recently, it has been suggested that these represent a part of a new subset of dendritic cells (DC), characterized by the coexpression of MDC-8/HLA-DR/CD16. The aim of the present paper was to analyze the morphological, cytochemical, phenotypical, and functional characteristics of PB CD16⁺/HLA-DR⁺ cells compared to both PB CD14⁺ monocytes and CD16⁻ DC. In contrast to CD14⁺ monocytes, purified CD16⁺/HLA-DR⁺ cells displayed cytoplasmic veils and lacked cytoplasmic myeloperoxidase and α-naphthyl acetate esterase. Normal human PB CD16⁺/HLA-DR⁺ cells also displayed phenotypic characteristics different from those of CD14⁺ monocytes: they lacked the CD64 Fcγ receptor, showed lower levels of CD32, and expressed higher amounts of CD16 compared to CD14⁺ monocytes. They also displayed a different pattern of expression of other antigens, including CD14, HLA-DR, CD45RA, CD45RO, complement receptors and complement regulatory surface proteins, adhesion and costimulatory molecules, and cytokine receptors, among others. When compared to CD16⁻ DC, CD16⁺/HLA-DR⁺ cells showed reactivity for CD16, dim positivity for CD14, higher expression of both Ig- and complement-receptors and lower reactivity for HLA-DR, adhesion, and costimulatory molecules (with the exception of CD86). The CD16⁺/HLA-DR⁺ cell subset displayed a higher Ig/complement-mediated phagocytic/oxidative activity than CD16⁻ DC, although this activity was significantly lower than that of mature monocytes. Regarding cytokine production at the single cell level, LPS plus IFN-γ-stimulated PB CD16⁺/HLA-DR⁺ cells produced significant amounts of IL1β, IL6, IL12, TNFα, and IL8; however, the percentage of cytokine-producing cells and the amount of cytokine/cell were lower in CD16⁺/HLA-DR⁺ cells than in CD14⁺ monocytes. In addition, upon comparing CD16⁺/HLA-DR⁺ cells with CD33⁺⁺⁺/CD16⁻ DC, we found that the percentage of cytokine-producing cells and the amount of cytokine/cell were significantly different in both cell subsets. In summary, our results show that CD16⁺/HLA-DR⁺ cells clearly display different morphologic, cytochemical, immunophenotypical, and functional characteristics compared to both mature monocytes and CD16⁻ DC. Interestingly, these cells are more frequent than other DC in normal human adult PB and cord blood samples, while they are less represented in normal bone marrow.