Eating pathogens : a study of macrophage phagocytosis and the effects of the extracellular matrix
Macrophages derive from multipotent precursor cells and play a critical role in the immune response. They can change states in response to environmental factors and much of these have been elucidated from a molecular biological approach. However, recent studies demonstrate that the biomechanical properties of the extra cellular matrix (ECM), such as stiffness, can affect the activation state of macrophages and possibly their ability to phagocytose. Phagocytosis, where a cell internalises a foreign object, by macrophages is a first line of defence against foreign bodies and is initiated by a receptor-ligand interaction which causes the polymerisation of actin to form the phagocytic cup which then goes on to become a phagosome. In this study, RAW264.7 and bone marrow derived macrophage (BMDM) cells were exposed to lipopolysaccharides (LPS) fluorescent bioparticles and zymosan fluorescent beads to study the phagocytosis process. The cells were investigated using time-lapse imaging and super-resolution microscopy. To understand the effects of ECM stiffness on phagocytosis, the cells were cultured on soft and stiff Polydimethylsiloxane (PDMS) elastomers to establish if substrate stiffness influences the phagocytic process. These results demonstrated that PDMS is not a good substrate for cell culture stiffness experiments. Using PDMS alone does not allow cells to adhere correctly and utilising ECM proteins as an adherence layer is also not feasible for stiffness studies as it negates the stiffness of the underlying substrate at the nanometre level. The live-imaging studies also revealed a possible new mechanism of phagosome formation.