Characterization of patocytosis: endocytosis into macrophage surface-connected compartments

Previously, we described a unique macrophage endocytosis pathway in which aggregated low density lipoproteins and microcrystalline cholesterol induce and enter a labyrinth of membrane-bound compartments that remain connected to the cell surface. We now show that certain types of non-lipid particles such as polystyrene microspheres and colloidal gold also induce and enter macrophage surface-connected compartments (SCC), a process we call patocytosis. A common property among particles that stimulate patocytosis is their hydrophobic nature. Both aggregated LDL and microcrystalline cholesterol that we showed previously to stimulate patocytosis are hydrophobic. We now show that hydrophobic polystyrene microspheres and gold particles but not their hydrophilic counterparts triggered patocytosis. Uptake by patocytosis was limited to hydrophobic polystyrene microsphere particles less than 0.5 micron in diameter. Hydrophobic polystyrene microspheres greater than this size entered macrophages by phagocytosis. Actin-independent capping of hydrophobic polystyrene microspheres on the plasma membrane preceded actin-dependent uptake of the microspheres into SCC. Sequential rounds of microsphere uptake into SCC over two successive days could occur. There was some mixing of initial and subsequently accumulated microspheres in SCC. SCC formed from plasma membrane invaginations that connected with spaces created by unfolding of stacks of internal microvilli. Microsphere transport from plasma membrane invaginations into these spaces was inhibited by primaquine. Patocytosis is a unique endocytic process in macrophages triggered by small hydrophobic particles that provides a mechanism to sequester large amounts of these materials within a labyrinth of SCC.