Target-specific mechanics of phagocytosis: protrusive neutrophil response to zymosan differs from the uptake of antibody-tagged pathogens

The physical mechanisms that control target-specific responses of human neutrophils to distinct immune threats are poorly understood. Using dual-micropipette manipulation, we have quantified and compared the time courses of neutrophil phagocytosis of two different targets: zymosan (a prominent model of fungal infection), and antibody-coated (Fc) particles. Our single-live-cell/single-target approach exposes surprising differences between these two forms of phagocytosis. Unlike the efficient uptake of 3-μm Fc targets (within ~66 seconds), the engulfment of similarly sized zymosan is slow (~167 seconds), mainly due to the formation of a characteristic pedestal that initially pushes the particle outwards by ~1 μm. Despite a roughly twofold difference in maximum cortical tensions, the top 'pull-in' speeds of zymosan and Fc targets are indistinguishable at ~33 nm/second. Drug inhibition shows that both actin as well as myosin II partake in the regulation of neutrophil cortical tension and cytoplasmic viscosity; other than that, myosin II appears to play a minor role in both forms of phagocytosis. Remarkably, an intact actin cytoskeleton is required to suppress, in antibody-mediated phagocytosis, the initially protrusive deformation that distinguishes the neutrophil response to zymosan.