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Background  New experimental approaches to the study of the neutrophil phagosome and bacterial killing prompted a reassessment of the usefulness of all- trans  retinoic acid (ATRA)-differentiated HL-60 cells as a neutrophil model. HL-60 cells are special in that they possess azurophilic granules while lacking the specific granules with their associated oxidase components. The resulting inability to mount an effective intracellular respiratory burst makes these cells more dependent on other mechanisms when killing internalized bacteria.    Methodology/Principal Findings  In this work phagocytosis and phagosome-related responses of ATRA-differentiated HL-60 cells were compared to those earlier described in human neutrophils. We show that intracellular survival of wild-type  S. pyogenes  bacteria in HL-60 cells is accompanied by inhibition of azurophilic granule–phagosome fusion. A mutant  S. pyogenes  bacterium, deficient in M-protein expression, is, on the other hand, rapidly killed in phagosomes that avidly fuse with azurophilic granules.    Conclusions/Significance  The current data extend our previous findings by showing that a system lacking in oxidase involvement also indicates a link between inhibition of azurophilic granule fusion and the intraphagosomal fate of  S. pyogenes  bacteria. We propose that differentiated HL-60 cells can be a useful tool to study certain aspects of neutrophil phagosome maturation, such as azurophilic granule fusion.

Phagocytosis of Streptococcus pyogenes by All-Trans Retinoic Acid-Differentiated HL-60 Cells: Roles of Azurophilic Granules and NADPH Oxidase