Lecture on Phagocytosis

1. Phagocytosis
Md. Murad Khan
Lecturer
Department of Microbiology
Jagannath University, Dhaka.

2. 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.

3. 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

4. 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.

5. The steps of phagocytosis

6. 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.

7. Opsonization and Attachment

8. 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.

9. 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.

10. 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.

11. 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.

12. Intracellular killing and digestion

13. 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.

14. 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.

15. 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.

16. 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.

17. Bacterial Virulence Factors Subvert Host
Defenses
Phagosome
maturation stalled
(M. tuberculosis; Legionella)
Ingestion phase
impaired
(Yersinia)
Resistance to
lysosomal degradation
(Salmonella)
Modification of
phagocytic receptors
(P. aeruginosa)
Escape from phagosome
into cytosol (Listeria, Shigella)

18. THANKS TO ALL