The ultrastructural features of the surface of macrophages and the mechanics of phagocytosis

Macrophages are highly dynamic cells, both structurally and functionally, that perform many important roles in the body. They provide immunity by surveying tissues for the presence of infectious particles and physically engulfing them in a process called phagocytosis. They also restore damaged tissues by secreting molecular factors which stimulate growth and repair. In spite of their various roles, they possess conserved features which are manifested in generating different types of membrane protrusion. Amongst the well-described types are lamellipodia, filopodia and dorsal ruffles. In this thesis, I will present my investigation on a novel type of membrane protrusion amongst macrophages and phagocytes I referred to as membrane ridge. Due to its nanoscopic size, electron and superresolution microscopy techniques are required to interrogate its actin-rich architecture. These revealed its highly consistent thickness property which is strikingly conserved across different types of macrophages and phagocytic cells I examined. Moreover, by tracking its development, I found its thickness to be highly maintained from its inception up to its recovery back to the membrane surface, indicating a tightly regulated process in generating these structures. By subjecting macrophages to different chemical, mechanical and genetic manipulations, I found an inherent sensitivity of membrane ridges to these factors, suggesting their possible roles in diverse activities of macrophages such as inflammatory response, osmoregulation and homeostasis. Finally, I explored the phagocytosis of spherical particles by macrophages and found that target size, target surface property and possession of membrane ridges largely influence phagocytic uptake. Increase in size of a spherical target and loss of membrane ridges impair phagocytosis, whilst target opsonisation with antibodies enhances phagocytic uptake. Taken together, I described the nanoscopic architecture of a highly conserved membrane protrusion amongst phagocytic cells I referred to as membrane ridge and examined the effect of its loss along with size and surface properties of a spherical target in phagocytic process.