Clathrin‐mediated phagosome resolution regenerates lysosomes and maintains degradative capacity of macrophages after phagocytosis

Abstract

Phagocytosis is the selective internalization of large particles and is critical for immunity, development and tissue homeostasis. The conversion of phagosomes into microbicidal compartments is a highly regulated process, which has been studied in great detail. Conversely, the fate of phagolysosomes, after their cargo is digested, is poorly understood. By utilizing fluorescent bacteria and labelling phagosomal markers, we followed mature phagolysosomes to their resolution. Despite previous assumptions, phagolysosomes are not exocytosed, but instead undergo a continuous dismembering process resulting in the shedding of myriads of vesicles containing degraded cargo. These phagosome‐derived vesicles resemble lysosomes, based on the acquisition of lysosomal markers, acidification and fusogenicity with newly formed phagosomes. Combining pharmacological and genetic strategies, we found that phagosomal fragmentation is dependent on cargo degradation, clathrin and the integrity of the actin and microtubule cytoskeleton. In addition, we provide evidence that phagosome‐derived vesicles are required to recycle lysosomal components allowing for the formation of new phagosomes and consequently maintaining the capacity of macrophages to ingest targets. Thus, we propose that phagolysosome resolution recycles cellular resources for the continuous generation of new phagosomes and the prevention of this phase will critically impair the immune and housekeeping functions of the phagocytes. Support or Funding Information Canadian Institutes of Health Research Canada Research Chairs Program Ryerson University Natural Sciences and Engineering Council of Canada Clathrin mediates phagosome resolution. A. Macrophages engulfed mRFP1‐expressing E. coli. At 1 h post‐phagocytosis, phagosomes are rod‐shaped, like E. coli and mRFP1 is mostly associated with phagosomes. After 4 h of phagosome maturation, phagosomes shrunk and mRFP1 is found in small vesicular compartments. Clathrin inhibitors, pitstop and ikarugamycin, impair phagosome fragmentation. B. Quantification of phagosome‐derived vesicles, where b is significantly different from other conditions.

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“…Less is known about the final stage of phagosome resolution and how the phagosome is resorbed to allow the phagocyte to return to homeostasis. However, studies have shown that bacteria‐containing phagolysosomes in macrophages undergo fragmentation through vesicle budding and constriction (Levin et al , 2016 ; Botelho et al , 2020 ; preprint: Lancaster et al , 2020 ).…”