The use of targeted microbeads for quantitative analysis of the phagocytic properties of human monocyte-derived dendritic cells

The relationship between phagocytic capacity and morphology of dendritic cells (DCs) has not been investigated previously. Therefore, in order to approach this question, we have developed a novel assay, which is described here. The model of dendritic cells (DCs) used was based upon cytokine-induced differentiation of peripheral blood mononuclear cells, followed by culture on a fibronectin substratum. Under these conditions, standard current methods of quantifying phagocytosis are not applicable, as they rely upon flow cytometric analysis of fluid phase cells; and for adherent cells, quantitative efficiency of uptake is very difficult to measure. Furthermore, for both fluid phase and adherent cells, it is difficult to discriminate between internal and externally bound probe, and degradation of internalised probes can lead to artefacts. Therefore, in this study, these technical issues have been overcome by a simple and flexible assay. Phycoerythrin (PE)-conjugated antibodies are used to target microbeads to the DCs. Following an appropriate incubation period, secondary staining with fluorescein isothiocyanate (FITC)-conjugated antibody allows discrimination between internal and externally bound beads. Microscopic visualisation allows individual beads to be studied easily and thus phagocytosis quantified, whilst permitting parallel examination of morphological parameters. In particular, the relationship between bead uptake and the nature and distribution of the dendritic processes can be evaluated.