A combined method for both endogenous myeloperoxidase and acid phosphatase cytochemistry as well as immunoperoxidase surface labelling discriminating human peripheral blood-derived dendritic cells and monocytes.

On light microscopical (LM) level dendritic cells (DC) isolated from lymphoid organs can be discriminated from macrophages (M phi) by the presence of acid phosphatase (APh) activity in a spot near the nucleus and constitutional expression of class II antigens. The aim of our study was to investigate whether DC and monocytes (Mo) enriched from human peripheral blood could be discriminated on the electron microscopical (EM) level. Therefore we developed a triple method by which we compared the presence of myeloperoxidase (MPO) containing vesicles, the localization of APh containing vesicles and expression of MHC class II and RFD1 (a DC-associated class II-like antigen) plasma-membrane antigens. DC, functionally characterized as potent stimulators in a MLR, are MPO-negative, whereas Mo show MPO in cytoplasmic granules. Although both DC and Mo show little APh activity at LM level, both types of cells show APh activity at the EM level but at different locations. In DC APh containing vesicles are present in a distinct juxtanuclear area, in contrast to Mo, which show APh activity in lysosomes scattered throughout the whole cytoplasm. Moreover, on both LM and EM level, DC are strongly class II positive, whereas Mo show variable labelling intensity for class II, while RFD1 was only found on DC.