Variations in the phagosomal environment of human neutrophils and mononuclear phagocyte subsets

Abstract

AbstractThe phagosomal microenvironment has a major influence on the enzyme activity and biology within these organelles. Here we directly compared the phagosomal pH of human neutrophils, monocytes, dendritic cell (DC) and monocyte-derived cells. An unexpected observation was the striking difference in phagosomal environment between the three monocytes subsets. Classical monocytes and neutrophils had alkaline phagosomes, yet non-classical monocytes had more acid phagosomes. Intermediate monocytes had an intermediate phenotype. We next investigated the differences between primary DCs versus in vitro monocyte-derived DCs and established that both these cells had acidic phagosomal environments. We also confirmed reports of an alkaline phagosome in “M1” macrophages, and an acidic one in “M2” macrophages. Across all phagocytes, alkalinisation was dependent upon the activity of the NADPH oxidase, as when it was absent in cells from a patient with chronic granulomatous disease (CGD) or was abolished by an inhibitor of the oxidase, diphenyleneiodonium (DPI). An increased alkalinity in the phagosome was associated with more oxidase activity. These data highlight the heterogenous nature of phagocytic vacuoles within the family of mononuclear phagocytes that will dictate the function of these compartments.Key pointsPhagosomal function depends upon the action of enzymes released into them from cytoplasmic granules.The substantial differences in the phagosomal pH in the different phagocytes will affect their compliment of enzymes and their functions.