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
