Phagosome - WikiHQ

300px|thumb | Phagocytosis of a bacterium, showing the formation of phagosome and phagolysosome

In cell biology, a phagosome is a vesicle formed around a particle engulfed by a phagocyte via phagocytosis. Professional phagocytes include macrophages, neutrophils, and dendritic cells (DCs).

A phagosome is formed by the fusion of the cell membrane around a microorganism, a senescent cell or an apoptotic cell. Phagosomes have membrane-bound proteins to recruit and fuse with lysosomes to form mature phagolysosomes. The lysosomes contain hydrolytic enzymes and reactive oxygen species (ROS) which kill and digest the pathogens. Phagosomes can also form in non-professional phagocytes, but they can only engulf a smaller range of particles, and do not contain ROS. The useful materials (e.g. amino acids) from the digested particles are moved into the cytosol, and waste is removed by exocytosis. Phagosome formation is crucial for tissue homeostasis and both innate and adaptive host defense against pathogens.

However, some bacteria can exploit phagocytosis as an invasion strategy. They either reproduce inside of the phagolysosome (e.g. Coxiella spp.) or escape into the cytoplasm before the phagosome fuses with the lysosome (e.g. Rickettsia spp.). Many Mycobacteria, including Mycobacterium tuberculosis and Mycobacterium avium paratuberculosis, can manipulate the host macrophage to prevent lysosomes from fusing with phagosomes and creating mature phagolysosomes. Such incomplete maturation of the phagosome maintains an environment favorable to the pathogens inside it.

Formation

Phagosomes are large enough to degrade whole bacteria, or apoptotic and senescent cells, which are usually >0.5μm in diameter. This means a phagosome is several orders of magnitude bigger than an endosome, which is measured in nanometres.

Phagosomes are formed when pathogens or opsonins bind to a transmembrane receptor, which are randomly distributed on the phagocyte cell surface. Upon binding, "outside-in" signalling triggers actin polymerisation and pseudopodia formation, which surrounds and fuses behind the microorganism. Protein kinase C, phosphoinositide 3-kinase, and phospholipase C (PLC) are all needed for signalling and controlling particle internalisation. Fc receptor (FcR), complement receptors (CR), mannose receptor and dectin-1 are phagocytic receptors, which means that they can induce phagocytosis if they are expressed in non-phagocytic cells such as fibroblasts. Other proteins such as Toll-like receptors are involved in pathogen pattern recognition and are often recruited to phagosomes but do not specifically trigger phagocytosis in non-phagocytic cells, so they are not considered phagocytic receptors.

Opsonisation

Opsonins are molecular tags such as antibodies and complements that attach to pathogens and up-regulate phagocytosis. Immunoglobulin G (IgG) is the major type of antibody present in the serum. It is part of the adaptive immune system, but it links to the innate response by recruiting macrophages to phagocytose pathogens. The antibody binds to microbes with the variable Fab domain, and the Fc domain binds to Fc receptors (FcR) to induce phagocytosis.

Complement-mediated internalisation has much less significant membrane protrusions, but the downstream signalling of both pathways converge to activate Rho GTPases. They control actin polymerisation which is required for the phagosome to fuse with endosomes and lysosomes.

Non-phagocytic cells

Other non-professional phagocytes have some degree of phagocytic activity, such as thyroid and bladder epithelial cells that can engulf erythrocytes and retinal epithelial cells that internalise retinal rods.

Structure

As the membrane of the phagosome is formed by the fusion of the plasma membrane, the basic composition of the phospholipid bilayer is the same. Endosomes and lysosomes then fuse with the phagosome to contribute to the membrane, especially when the engulfed particle is very big, such as a parasite. They also deliver various membrane proteins to the phagosome and modify the organelle structure.

Phagosomes can engulf artificial low-density latex beads and then purified along a sucrose concentration gradient, allowing the structure and composition to be studied. Vacuolar proton pumps (v-ATPase) are delivered to the phagosome to acidify the organelle compartment, creating a more hostile environment for pathogens and facilitating protein degradation. The bacterial proteins are denatured in low pH and become more accessible to the proteases, which are unaffected by the acidic environment. The enzymes are later recycled from the phagolysosome before egestion so they are not wasted. The composition of the phospholipid membrane also changes as the phagosome matures.

Fusion may take minutes to hours depending on the contents of the phagosome; FcR or mannose receptor-mediated fusion last less than 30 minutes, but phagosomes containing latex beads may take several hours to fuse with lysosomes.

Smaller lumenal molecules are transferred by fusion faster than larger molecules, which suggests that a small aqueous channel forms between the phagosome and other vesicles during "kiss-and-run", through which only limited exchange is allowed.

Rab5 recruits PI-3 kinase and other tethering proteins such as Vps34 to the phagosome membrane, so endosomes can deliver proteins to the phagosome. Rab5 is partially involved in the transition to Rab7, via the CORVET complex and the HOPS complex in yeast. Rab11 is involved in membrane recycling.

Function

Pathogen degradation

Macrophages and neutrophils are professional phagocytes in charge of most of the pathogen degradation, but they have different bactericidal methods. Neutrophils have granules that fuse with the phagosome. The granules contain NADPH oxidase and myeloperoxidase, which produce toxic oxygen and chlorine derivatives to kill pathogens in an oxidative burst. Proteases and anti-microbial peptides are also released into the phagolysosome. Macrophages lack granules, and rely more on phagolysosome acidification, glycosidases, and proteases to digest microbes. Peptides from the bacteria are trafficked to the major histocompatibility complex (MHC). The peptide antigens are presented to lymphocytes, where they bind to T-cell receptors and activate T-cells, bridging the gap between innate and adaptive immunity. This is specific to mammals, birds, and jawed fish, as insects do not have adaptive immunity.

thumb|Phagocytosis -- amoeba

Nutrient

Ancient single-celled organisms such as amoeba use phagocytosis as a way to acquire nutrients, rather than an immune strategy. They engulf other smaller microbes and digest them within the phagosome of around one bacterium per minute, which is much faster than professional phagocytes. For the soil amoeba Dictyostelium discoideum, their main food source is the bacteria Legionella pneumophila, which causes Legionnaires' disease in humans. Phagosome maturation in amoeba is very similar to that in macrophages, so they are used as a model organism to study the process.

Tissue clearance

Phagosomes degrade senescent cells and apoptotic cells to maintain tissue homeostasis. Erythrocytes have one of the highest turnover rates in the body, and they are phagocytosed by macrophages in the liver and spleen. In the embryo, the process of removing dead cells is not well-characterised, but it is not performed by macrophages or other cells derived from hematopoietic stem cells. It is only in the adult that apoptotic cells are phagocytosed by professional phagocytes. Inflammation is only triggered by certain pathogen- or damage-associated molecular patterns (PAMPs or DAMPs), the removal of senescent cells is non-inflammatory. Autophagy is not limited to professional phagocytes, it is first discovered in rat hepatocytes by cell biologist Christian de Duve. Autophagosomes have a double membrane, the inner one from the engulfed organelle, and the outer membrane is speculated to be formed from the endoplasmic reticulum or the ER-Golgi intermediate compartment (ERGIC). The autophagosome also fuses with lysosomes to degrade its contents. When M. tuberculosis inhibit phagosome acidification, interferon gamma can induce autophagy and rescue the maturation process.

Bacterial evasion and manipulation

Many bacteria have evolved to evade the bactericidal properties of phagosomes or even exploit phagocytosis as an invasion strategy.

Mycobacterium tuberculosis target M2 macrophages at the lower parts of the respiratory pathway, which do not produce ROS. M. tuberculosis can also manipulate the signalling pathways by secreting phosphatases such as PtpA and SapM, which disrupt protein recruitment and block phagosome acidification.
Legionella pneumophila can re-model the phagosome membrane to imitate vesicles in other parts of the secretory pathway, so lysosomes do not recognise the phagosome and do not fuse with it. The bacterium secretes toxins that interfere with host trafficking, so the Legionella-containing vacuole recruits membrane proteins usually found on the endoplasmic reticulum or the ERGIC. This re-directs secretory vesicles to the modified phagosome and deliver nutrients to the bacterium.
Listeria monocytogenes secretes a pore-forming protein listeriolysin O so the bacterium can escape the phagosome into the cytosol. Listeriolysin is activated by the acidic environment of the phagosome. In addition, Listeria secretes two phospholipase C enzymes that facilitate phagosome escape.

References

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