Using rat alveolar macrophages and polystyrene microspheres (1 to 9 um) possessing three different surface characteristics (unmodified polystyrene, IgG opsonized, and poloxamine coated) we investigated the dependence of phagocytosis on particle size. Dependence of two distinct steps of phagocytosis, particle attachment and internalization, on size and surface chemistry was separately assessed to elucidate the origins of size-recognition. Particle attachment to macrophages, quantified by flow cytometry, was found to depend on size and surface chemistry in a complex manner. Macrophages exhibited enhanced attachment to particles possessing diameters of 2-3 um. Further investigation, including manipulation of the physical structure of macrophage membranes and electron microscopy visualization, revealed that the ruffled topology of the membrane is responsible for size recognition and preferential attachment of 2-3 um particles to macrophages. Surprisingly, despite the intricate relationships between attachment and particle size and surface chemistry, the speed of particle internalization, measured with video microscopy, did not depend on either property.
Our studies show that particle attachment, orchestrated by the membrane topology, is solely responsible for recognition of particle size by macrophages. Several examples exist in biology of recognition at a molecular scale. However, the results presented here show a remarkable example of recognition on micron length scale. Such understanding of the interactions of polymeric particles with macrophages is essential in the design of particles to either reduce or enhance phagocytosis depending on drug delivery application.