Phagosome maturation: going through the acid test

Abstract

Eukaryotic cells engulf a variety of particles during their lifetime, including potentially pathogenic microorganisms and apoptotic cells. A person clears approximately 200 billion cells each day, making the removal of apoptotic cells one of the most common types of phagocytosis. Understanding how bacteria and apoptotic cells are phagocytosed and processed is a fundamentally important biological problem, both for normal homeostasis and disease. Internalized particles are present in membrane-bound organelles that are termed phagosomes. The phagosome functions as more than just as an organelle for 'garbage disposal': proteins from the ingested target are degraded into peptides and presented on major histocompatibility complex (MHC) class II molecules (in the case of bacteria) for the generation of an immune response, whereas apoptotic cell-derived antigens are typically cross-presented on MHC class I molecules and are tolerogenic. Phagosome maturation is the process by which a particle-containing phagosome 'matures' through a series of increasingly acidic membrane-bound structures, becoming an acidic phagolysosome before fusion with lysosomes. Proteomics approaches have identified a number of candidates localized to the phagosome, including the GTPases RAB5 and RAB7. Recent studies in model systems, such as Drosophila melanogaster, Dictyostelium discoideum and Caenorhabditis elegans, have developed genetic models for the identification and characterization of proteins that are required for phagosome maturation. Studies in the nematode have led to the identification of a pathway for the maturation of apoptotic cell-containing phagosomes. Following phagocytosis, apoptotic cells (and other particles) exist in phagosomes. The proteins on the intracellular face of the phagosome membrane change as the phagosome matures. Soon after uptake, the phagosome is coated with the GTPase RAB5, which is subsequently exchanged for RAB7 and eventually for lysosomal markers, such as LAMP1. The regulation of phagosome maturation is complex, requiring a series of guanine nucleotide-exchange factors (GEFs), GTPase-activating proteins (GAPs) and effectors. How RAB5 is regulated on the phagosome in vivo is just beginning to be described. The HOPS complex, a RAB7 activator and effector, is required for the maturation of the phagosome from the RAB7-positive stage. A number of different bacterial pathogens have evolved mechanisms for co-opting phagosome maturation as a means of immune evasion or as a replicative niche. These bacteria target the machinery that regulates maturation, in some cases converting the phagosome into other types of organelles. Future studies are expected to focus on signalling pathways that determine whether the immune response to an internalized particle would be immunogenic (in response to bacteria) or tolerogenic (in response to apoptotic cells). The identification of novel players, and their placement within a pathway for phagosome maturation, might be important for future development of new therapeutics that target intracellular pathogens (such as Mycobacterium tuberculosis).