Depletion of endomembrane reservoirs drives phagocytic appetite exhaustion in macrophages

During phagocytosis, a phagocytic cup grows via F-actin remodeling and localized secretion to entrap a particle within a phagosome, which then fuses with endosomes and lysosomes to digest the particle, followed by phagosome resolution. As spatially limited systems, phagocytes have a maximal phagocytic capacity, at which point further uptake must be reduced. However, the processes responsible for phagocytic appetite exhaustion as phagocytes reach their maximal phagocytic capacity are poorly defined. We found that macrophages at their capacity have lower surface levels of Fcγ receptors but overexpression of these receptors did not increase their capacity, suggesting that receptor levels are not limiting. We found that surface membrane in-folding, membrane tension and cortical F-actin were all reduced in exhausted macrophages. Although this might contribute to appetite suppression, we also found that ‘free’ endosomes and lysosomes were severely depleted in exhausted macrophages. Consequently, focal exocytosis at sites of externally bound particles was reduced. In comparison, macrophages recovered their appetite if phagosome resolution was permitted. We propose that depletion of the endomembrane pools is a major determinant of phagocytic fatigue as macrophages reach their phagocytic capacity.