The endocytic activity of dendritic cells.

Abstract

I n this issue, Sallusto et al. (1) report that dendritic cells . can vigorously internalize solutes by macropinocytosis, delivering these to MHC class II-rich vesicles (MIIVs). In the most familiar form of endocytosis, clathrin-coated pits invaginate to form small (<0.1/~m in diameter) coated resides (Fig. 1). Ligands adsorb to receptors in the coated pit and are subsequently delivered to lysosomes, where the ligands are degraded down to the level of amino acids. During macropinocytosis, which occurs at sites of membrane ruffling, larger droplets of fluid enter the cell, and an actin-based cytoskeleton rather than clathrin seems to be required (Fig. 1). Sallusto et al. (1) describe solutes that rapidly enter dendritic cells within macropinosomes, both in the fluid phase or attached to the membrane. In both cases, the solutes are delivered to MIIVs, where they presumably can be converted to peptides and recycled to the cell surface as MHC II-peptide complexes. This set of events would explain a previous report from these same authors on the efficiency with which dendritic cells present protein antigens (2). In that study, the presentation by dendritic cells of antigens, apparently from the fluid phase, was as effective as presentation by B cell lines that adsorb the same antigen via surface Ig receptors. These new observations represent a striking turn of events. In most previous studies of dendritic cells, little or no uptake