Phagocytosis by neutrophils - studies on phagosome dynamics and membrane traffic modulation by Streptococcus pyogenes

Abstract: Neutrophils are our most numerous and deadly white blood cells and without them we would succumb quickly to infections by pathogens. The main mechanism that the neutrophils employ for our protection is phagocytosis, where they eat and enclose their target inside a membrane-bound organelle, the phagosome. Neutrophil phagosomes are highly dynamic entities, and a large amount of antimicrobial substances are released to their interior within seconds of formation. In most cases this will kill the engulfed microbe, but there are exceptions. Streptococcus pyogenes is one of our most common pathogens and is responsible for a wide range of diseases. Recently it has been demonstrated that these bacteria are able to survive phagocytosis by neutrophils. This doctoral thesis is about explaining those mechanisms. What is described in this thesis is the development of new methods for studying phagosome biogenesis and maturation, including creating magnetic bacteria to isolate phagosomes and advanced microscopy for live measurements of phagosomal pH. The employment of cell lines and their differentiation into neutrophils is shown to be a useful research tool. Using these methods, novel findings regarding mechanisms for fusion between neutrophil granules and phagosomes are demonstrated. It is shown that S. pyogenes interfere with the intracellular membrane traffic of neutrophils, leading to a decreased delivery of antimicrobial substances and impaired acidification of S. pyogenes-containing phagosomes, helping them to survive phagocytosis.