Polarization of DC endolysosomes toward the interacting CD8⁺ T cells.

Ultrastructural analysis of the interaction of DCs with CD4⁺ (A) or CD8⁺ (B-D) allospecific T cells. Cells were cocultured for 5 hours and processed for conventional thin section electron microscopy. Interaction between DCs and CD4⁺ T cells occurs primarily in correspondence with the DC areas occupied by the nucleus and is not characterized by polarization of cellular organelles (A). In DCs interacting with CD8⁺ T lymphocytes, both mitochondria and endolysosomal structures are polarized toward the areas of contact at the opposite site of the cell portion containing the nucleus (B). Panels C and D show details of a tight contact between the plasma membranes of the interacting cells (arrowhead in panel C) and of fusion sites of endolysosomes with the DC plasma membrane (arrows in panels C and D). (E) Immunoelectron microscopic analysis of the localization of IL-1β and cathepsin D in DCs interacting with CD8⁺ T cells. Double immunogold labeling with anticathepsin D (large gold particles) and anti–IL-1β (small gold particles) antibodies shows the colocalization of the 2 proteins in endolysosomal structures in the DC cytoplasm oriented toward the cell contact with a CD8⁺ T lymphocyte (arrow). Insets with enlargement of the areas in dotted boxes show groups of endolysosomes double-labeled for IL-1β (arrows) and cathepsin D (arrowheads). PM indicates plasma membrane; Nu, nucleus; M, mitochondria; and G, Golgi complex. Bars in panels A and B are 2 μm; in panels C and D, 0.5 μm; in insets, 0.1 μm.