phagocytosis

Phagocytosis

Phagocytosis is defined as the process of engulfment of solid particulate material by the cells (cell-eating). The cells performing this function are called phagocytes. There are 2 main types of phagocytic cells:

i) Polymorphonuclear neutrophils (PMNs) which appear early in acute inflammatory response, sometimes called as microphages.

ii) Circulating monocytes and fixed tissue mononuclear phagocytes, commonly called as macrophages.

The microbe undergoes the process of phagocytosis by polymorphs and macrophages and involves the following 3 steps:

1. Recognition and attachment

2. Engulfment

3. Killing and degradation

1. RECOGNITION AND ATTACHMENT

Phagocytosis is initiated by the expression of surface receptors on macrophages which recognise microorganisms: mannose receptor and scavenger receptor.

The process of phagocytosis is further enhanced when the microorganisms are coated with specific proteins, opsonins, from the serum or they get opsonised.

Opsonins establish a bond between bacteria and the cell membrane of phagocytic cell.

The main opsonins present in the serum and their corresponding receptors on the surface of phagocytic cells (PMNs or macrophages) are as under:

i) IgG opsonin is the Fc fragment of immunoglobulin G; it is the naturally occurring antibody in the serum that coats the bacteria while the PMNs possess receptors for the same.

ii) C3b opsonin is the fragment generated by activation of complement pathway. It is strongly chemotactic for attracting PMNs to bacteria.

iii) Lectins are carbohydrate-binding proteins in the plasma which bind to bacterial cell wall.

2. ENGULFMENT

The opsonised particle bound to the surface of phagocyte is ready to be engulfed. This is accomplished by formation of cytoplasmic pseudopods around the particle due to activation of actin filaments beneath cell wall, enveloping it in a phagocytic vacuole. Eventually, the plasma membrane enclosing the particle breaks from the cell surface so that membrane lined phagocytic vacuole or phagosome lies internalised and free in the cell cytoplasm. The phagosome fuses with one or more lysosomes of the cell and form bigger vacuole called phagolysosome.

3. KILLING AND DEGRADATION

Next comes the stage of killing and degradation of microorganism to dispose it off justifying the function of phagocytes as scavanger cells. The microorganisms after

being killed by antibacterial substances are degraded by hydrolytic enzymes. However, this mechanism fails to kill and degrade some bacteria like tubercle bacilli.

Disposal of microorganisms can proceed by following mechanisms:

A. Intracellular mechanisms:

i) Oxidative bactericidal mechanism by oxygen free radicals

a) MPO-dependent

b) MPO-independent

ii) Oxidative bactericidal mechanism by lysosomal granules

iii) Non-oxidative bactericidal mechanism

B. Extracellular mechanisms:

A. INTRACELLULAR MECHANISMS. There are intracellular metabolic pathways which more commonly kill microbes by oxidative mechanism and less often nonoxidative pathways.

i) Oxidative bactericidal mechanism by oxygen free radicals. An important mechanism of microbicidal killing is by oxidative damage by the production of reactive oxygen metabolites (O’2  H2O, OH’, HOCl, HOI, HOBr).

A phase of increased oxygen consumption (‘respiratory2burst’) by activated phagocytic leucocytes requires the essential presence of NADPH oxidase.

NADPH-oxidase present in the cell membrane of phagosome reduces oxygen to superoxide ion (O’2).

                                     

Superoxide is subsequently converted into H2O2 which has bactericidal properties:

                       

This type of bactericidal activity is carried out either via enzyme myeloperoxidase (MPO) present in the azurophilic granules of neutrophils and monocytes, or independent of enzyme MPO, as under:

a)       MPO-dependent killing. In this mechanism, the enzyme MPO acts on H2O2 in the presence of halides (chloride, iodide or bromide) to form hypohalous acid (HOCl, HOI, HOBr). This is called H2O2-MPO halide system and is more potent antibacterial system in polymorphs than H2O2 alone.

                         

b)       MPO-independent killing. Mature macrophages lack the enzyme MPO and they carry out bactericidal activity by producing OH- ions and superoxide singlet oxygen (O’) from H2O2  in the presence of O’2 (Haber-Weiss reaction) or in the presence of Fe++ (Fenton reaction):

                   

Reactive oxygen metabolites are particularly useful in eliminating microbial organisms that grow within phagocytes e.g. M. tuberculosis, Histoplasma capsulatum.

ii) Oxidative bactericidal mechanism by lysosomal granules. In this mechanism, the preformed granule-stored products of neutrophils and macrophages are discharged or

secreted into the phagosome and the extracellular environment. While the role of MPO is already highlighted above, others liberated by degranulation of macrophages and neutrophils are protease, trypsinase, phospholipase, and alkaline phosphatase. Progressive degranulation of neutrophils and macrophages along with oxygen free radicals degrades proteins i.e. induces proteolysis.

iii) Non-oxidative bactericidal mechanism. Some agents released from the granules of phagocytic cells do not require oxygen for bactericidal activity. These include the following:

a) Granules. Some of liberated lysosomal granules do not kill by oxidative damage but cause lysis of within phagosome.

These are lysosomal hydrolases, permeability increasing factors, cationic proteins (defensins), lipases, ptoteases,DNAases.

b) Nitric oxide. Nitric oxide reactive free radicals similar to oxygen free radicals are formed by nitric oxide synthase and is a potent mechanism of microbial killing. Nitric oxide is produced by endothelial cells as well as by activated macrophages.

B. EXTRACELLULAR MECHANISMS. Following mechanisms explain the bactericidal activity at extracellular level:

i) Granules. Degranulation of macrophages and neutrophils explained above continues to exert its effects of proteolysis outside the cells as well.

ii) Immune mechanisms. Immune-mediated lysis of microbes takes place outside the cells by mechanisms of cytolysis, antibody-mediated lysis and by cell-mediated cytotoxicity.

Reference- Harsh Mohan Text Book Of Pathology