This past year we continued studies of a novel pathway by which human monocyte-derived macrophages take up certain types of particles. The particles induce a complex of membrane-delimited cytoplasmic compartments where they become sequestered. These compartments remain connected to the surface of the macrophage. We showed previously that these surface- connected compartments(SCC) form when monocyte-macrophages process some types of cholesterol-containing particles. We show now that SCC can be induced by non-cholesterol-containing particles such as latex beads, but this depends on the size of the beads. Latex beads greater than or equal to 0.5u/m entered macrophages by phagocytosis, whereas latex beads 0.01- 0.5u/m induced and became sequestered within SCC. Sequestration and phagocytosis showed differential susceptibility to a protein kinase A inhibitor, H-89. Sequestration of small beads but not phagocytosis of large beads was inhibited. Small beads, but not large beads, aggregated on the surface of the macrophage within 30 minutes of incubation. Sequestration of the aggregated beads within SCC then rapidly followed. At 20 degrees C, aggregation of beads on the surface of the macrophage occurred, but subsequent uptake of the aggregated beads into SCC was blocked. Sequestration was inhibited by cytochalasin D but not by nocodazole. This suggests that actin microfilaments but not microtubules function in the process of sequestration. The acid protease, cathepsin D, was present in some deep cytoplasmic regions of SCC. MMP-9 and other neutral proteases were not detected within SCC. Uptake of particles into SCC represents an unusual mode of endocytosis that can be distinguished from the process of phagocytosis. Sequestration provides a means for monocyte-derived macrophages to store temporarily large amounts of material. This material can be later processed by the macrophages.
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