Phagocytosis: Our Current Understanding of a Universal Biological Process
Eileen Uribe-Querol and Carlos Rosales
Front Immunol. 2020; 11: 1066.

Phagocytosis is a cellular process for ingesting and eliminating particles larger than 0.5 μm in diameter, including microorganisms, foreign substances, and apoptotic cells. Phagocytosis is found in many types of cells and it is, in consequence an essential process for tissue homeostasis. However, only specialized cells termed professional phagocytes accomplish phagocytosis with high efficiency. Macrophages, neutrophils, monocytes, dendritic cells, and osteoclasts are among these dedicated cells. These professional phagocytes express several phagocytic receptors that activate signaling pathways resulting in phagocytosis. The process of phagocytosis involves several phases: i) detection of the particle to be ingested, ii) activation of the internalization process, iii) formation of a specialized vacuole called phagosome, and iv) maturation of the phagosome to transform it into a phagolysosome. In this review, we present a general view of our current understanding on cells, phagocytic receptors and phases involved in phagocytosis.

Phagosome maturation. The nascent phagosome gets transformed into a microbicidal vacuole, the phagolysosome, by sequential interactions with vesicles from the endocytic pathway. The process can be described in three stages of maturation: early (A), late (B), and phagolysosome (C). In this process, composition of the membrane changes to include molecules that control membrane fusion, such as the GTPases Rab5 and Rab7. The phagolysosome becomes increasingly acidic by the action of a proton-pumping V-ATPase and acquires various degradative enzymes, such as cathepsins, proteases, lysozymes, and lipases (scissors). EEA1, early endosome antigen 1; LAMP, lysosomal-associated membrane protein; NADPH, nicotinamide adenine dinucleotide phosphate oxidase.

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