Polymeric microspheres have found numerous applications in drug delivery, vaccination, personal care and medical diagnostics (1
). Microspheres provide an effective means of protecting, releasing and potentially targeting drugs for various biomedical applications. Drug-carrying microspheres have been administered in animals and humans via intravascular (4
), inhalation (5
), nasal (6
), subcutaneous (7
) and other routes. Regardless of the route of administration, microspheres are eventually cleared from the body by phagocytosis, which is one of the body’s innate modes of defense against invading pathogens and other non-indigenous particulate matter (8
). Macrophages, the body’s phagocytic cells, bind particles through a receptor mediated process and internalize them by actin-driven engulfment (9
). Though this process is undesired for most drug delivery microspheres, there are some cases, such as vaccines, when efficient targeting to macrophages is required (10
). Regardless of whether phagocytosis of microspheres is to be avoided or sought, understanding of the relationships between particle properties and phagocytic uptake is crucial.
Phagocytosis by professional phagocytes has been a topic of extensive research over the last several decades (9
). Most prominently, studies have elucidated the role of Fc receptors FcγRI, FcγRIIA
in activating Arp2/3 dependent actin polymerization via several intermediate steps ultimately leading to phagosome formation (15
). Studies have also reported on phagocytosis via complement (CR1, CR3 and CR4) and scavenger (SR-AI/II, SR-BI and CD36, among others) receptors (9
). While the biochemical pathways of phagocytosis have been relatively well studied, the biophysical aspects of phagocytosis, such as the role of particle properties, have been far less studied. It has been established that particle parameters such as size, shape, surface chemistry and mechanical properties influence phagocytosis (18
). Among these, the role of surface chemistry is perhaps best studied. Modification of particle surfaces with polyethylene glycol and poloxamine is routinely used to reduce phagocytosis by postponing opsonization, or adsorption of proteins which increase phagocytosis (21
). However, an in depth understanding of the role of physical parameters and the biophysical origins of these roles is still emerging.
Several studies have been conducted to define the role of size in phagocytosis (22
). Tabata and Ikada studied phagocytosis of polystyrene microspheres (0.5–4.6 μm) by mouse peritoneal macrophages and reported that maximal phagocytosis was observed for an intermediate particle size (1.7 μm) (23
). Similar conclusions were drawn by Rudt et al.
for human blood granulocytes via indirect measurements of phagocytosis of particles ranging in diameter from 85 nm to 3.2 μm (26
). Simon studied the uptake of 0.5–8 μm polystyrene microspheres by human blood neutrophils and leukocytes and reported that phagocytosis decreased with increasing particle size (24
). Theoretical work has also been performed to understand the dependence of phagocytosis on particle size (27
). These studies predicted that phagocytosis should increase with particle size for hydrophobic particles and decrease for hydrophilic particles. Clearly, the precise dependence of phagocytosis on particle size and its mechanistic origin remain ambiguous. To elucidate previous unexplained or inconsistent results, we separately assess the two steps of phagocytosis, attachment and internalization. Herein, using rat alveolar macrophages as model macrophages and polystyrene particles (1–6 μm) as model microspheres, we report on the dependence of phagocytosis on size and postulate on its mechanistic origin.