Localization of class II molecules in storage vesicles, endosomes and lysosomes in human dendritic cells.

Class II molecules are a prerequisite for antigen presentation. We studied whether class II molecules can be found in the endocytic and/or lysosomal route of dendritic cells (DC), which are very potent antigen-presenting cells. Therefore first immunolabelling for HLA-DR alpha chain was applied on ultrathin cryosections of cells of which plasma membrane HLA-DR/DQ molecules were labelled in suspension, followed by incubation with the endocytic marker BSA-gold. Second, immunolabelling for HLA-DR alpha chains was applied on ultrathin cryosections of cells on which enzyme cytochemistry for acid phosphatase (APh) was performed to see whether the class II positive vesicles belong to the lysosomal compartment. Third, this immunolabelling was applied on cryosections of cells pretreated with the protein synthesis inhibitor cycloheximide (CHX) to see whether the class II positive vesicles are derived from biosynthesis. We found limited uptake of BSA-gold into endosomes and lysosomes, some of which also contained endocytozed HLA-DR/DQ. APh and HLA-DR were observed in the same vesicles but also vesicles containing either HLA-DR or APh were found. However, many class II positive vesicles were found, which were apparently not accessible to exogenous molecules. Moreover, the amount of class II positive vesicles decreased after treatment of the cells with CHX, suggesting that these vesicles form part of the biosynthetic route. These results imply that there is a cluster of class II positive vesicles, probably a storage compartment, that has connections with the lysosomal system. The concentration of lysosomes and class II positive vesicles in the juxtanuclear area of DC is probably of crucial importance in the processing of antigens.