Phagocytosis

Md. Murad Khan

Lecturer

Department of Microbiology

Jagannath University, Dhaka.

Definition

• Phagocytosis is a specific form of endocytosis by which cells internalize solid matter, including microbial pathogens. While most cells are capable of phagocytosis, it is the professional phagocytes of the immune system, including macrophages, neutrophils and immature dendritic cells, that truly excel in this process. In these cells, phagocytosis is a mechanism by which microorganisms can be contained, killed and processed for antigen presentation and represents a vital facet of the innate immune response to pathogens, and plays an essential role in initiating the adaptive immune response.

The Players

• Professional Phagocytes• A. Neutrophils (PMNs)

• live to eat and kill - make 1011/day

• B. Monocytes/Macrophages • have tissue counterparts, Kupffer cells, microglia, etc • can move in and out of tissues

• Nonprofessional Phagocytes• Epithelial and Endothelial cells

• may contribute to infection by allowing pathogens to get past vascular and mucosal cellular barriers and into tissues

• Receptors• A. Fc Receptors• B. Complement Receptors

• phagocytosis is a receptor-mediated event, these are essential for efficient phagocytosis

The steps of phagocytosis

• Detection and opsonization of the foreign particle and movement of the phagocyte to the area.• Attachment of the foreign particle to the phagocyte.• Engulfment or ingestion of the foreign particle into a vesicle called

a phagosome.• Fusion with lysosome and formation of the phagolysosome.• Intracellular killing and digestion.• In the case of macrophages, egestion and antigen presentation.

The steps of phagocytosis

Opsonization and Attachment

• The process of phagocytosis begins with the binding of opsonins (i.e. complement or antibody) and/or specific molecules on the pathogen surface (called pathogen-associated molecular pathogens [PAMPs]) to cell surface receptors on the phagocyte. • Numerous receptors are involved in phagocytosis. Complement

receptors and Fc receptors are particularly important for the recognition and phagocytosis of opsonised microbes and other solid matter.

Opsonization and Attachment

Engulfment or ingestion

• Attachment of the microbe to the phagocyte results in some sort of signal (the nature of which is still not clearly understood) that triggers ingestion of the microbe. Ingestion involves encircling the target particle with phagocytic membrane so that it is eventually taken inside the cytoplasm of the phagocyte engulfed in a membrane vesicle called a phagosome. This process requires ATP and is triggered by the attachment of the target to the phagocyte's cytoplasmic membrane.

Phagolysosome formation

• The phagosome containing the microorganism migrates into the cytoplasm and soon collides with a series of lysosomes forming a phagolysosome. When the membranes of the phagosome and lysosome meet, the contents of the lysosome explosively discharge, releasing a large number of toxic macromolecules and other compounds into the phagosome. The killing processes inside the phagolysosome are confined to the organelle of the phagolysosome, thus protecting the cytoplasm of the phagocyte from these toxic activities.

Intracellular killing and digestion

• Several minutes after phagolysosome formation, the first detectable effect on the microorganism is the loss of the ability to reproduce. Inhibition of macromolecular synthesis occurs sometime later and many pathogenic and non-pathogenic bacteria are dead 10 to 30 minutes after ingestion. The mechanisms phagocytes use to carry out this killing are diverse and complex, consisting of both metabolic products and lysosomal constituents. Each type of phagocyte (neutrophils, monocytes or macrophages) has a slightly different mix of killing methods. The killing mechanisms that phagocytes use can be organized into two broad groups: oxygen-dependent and oxygen-independent mechanisms.

Intracellular killing and digestion

• Several minutes after phagolysosome formation, the first detectable effect on the microorganism is the loss of the ability to reproduce. Inhibition of macromolecular synthesis occurs sometime later and many pathogenic and non-pathogenic bacteria are dead 10 to 30 minutes after ingestion. The mechanisms phagocytes use to carry out this killing are diverse and complex, consisting of both metabolic products and lysosomal constituents. Each type of phagocyte (neutrophils, monocytes or macrophages) has a slightly different mix of killing methods. The killing mechanisms that phagocytes use can be organized into two broad groups: oxygen-dependent and oxygen-independent mechanisms.

Intracellular killing and digestion

Intracellular killing and digestion

• Oxygen-dependent mechanisms• Binding of Fc receptors on neutrophils, monocytes and macrophages

causes an increase in oxygen uptake by the phagocyte called the respiratory burst. This influx of oxygen is used in a variety of mechanisms to cause damage to microbes inside the phagolysosome, but the common theme is the creation of highly reactive small molecules that damage the biomolecules of the pathogen. Binding of these receptors activates an NADPH oxidase that reduces O2 to O2

- (superoxide). Superoxide can further decay to hydroxide radical (OH .) or be converted into hydrogen peroxide (H2O2) by the enzyme superoxide dismutase.

Intracellular killing and digestion

• Oxygen-dependent mechanisms• In neutrophils, these oxygen species can act in concert with the enzyme

myeloperoxidase to form hypochlorous acid (HOCl) from H2O2 and chloride ion (Cl-). HOCl then reacts with a second molecule of H2O2 to form singlet oxygen (1O2), another reactive oxygen species. Macrophages in some mammalian species catalyze the production of nitric oxide (NO) by the enzyme nitric oxide synthase. NO is toxic to bacteria and directly inhibits viral replication. It may also combine with other oxygen species to form highly reactive peroxynitrate radicals. All of these toxic oxygen species are potent oxidizers and attack many targets in the pathogen. At high enough levels, reactive oxygen species overwhelm the protective mechanisms of the microbes, leading to their death.

Intracellular killing and digestion

• Oxygen-independent mechanisms• The pH of the phagolysosome can be as low as 4.0 and this alone can

inhibit the growth of many types of microorganisms. This low pH also enhances the activity of lysozyme, glycosylases, phospholipases and nucleases present in the phagolysosome that degrade various parts of the microbe. A variety of extremely basic proteins present in lysosomal granules strongly inhibit bacteria, yeast and even some viruses. In fact, a few molecules of any one of these cationic proteins can damage the membranes of a bacterial cell, causing death by an unknown mechanism. The phagolysosome of neutrophils also contains lactoferrin, an extremely powerful iron-chelating agent that sequesters most of the iron present, potentially inhibiting bacterial growth.

Egestion and antigen presentation

• Once microorganisms are destroyed, the unwanted organic material is expelled from the cell in a process called egestion. Egestion is the opposite of ingestion and the molecular mechanism is basically the reverse of phagocytosis with the microbial leftovers being dumped into the blood and lymph. Some of this microbial debris is not egested, but binds to special protein complexes (called Major Histocompatibility Complex molecules) on the membranes of macrophages for presentation to the immune system.

Bacterial Virulence Factors Subvert Host Defenses

Phagosomematuration stalled

(M. tuberculosis; Legionella)

Ingestion phaseimpaired(Yersinia)

Resistance tolysosomal degradation

(Salmonella)

Modification ofphagocytic receptors

(P. aeruginosa)

Escape from phagosome into cytosol (Listeria, Shigella)

THANKS TO ALL