Phagocytic ProcessesOutline

1
Phagocytic Processes-Outline

• The Defenders
• Professional phagocytes
• The strategy
• The process
• Extravasation and chemotaxis
• Recognition and phagocytosis
• Killing
• Digestion
• Role in resistance to microbes
• Role in resistance to cancer
• Role in wound healing/bone remodeling

2
Professional Phagocytes - (phagocytosis
ingestion of particles)

• Monocytes-blood
• Macrophages - Reside in tissues, kill microbes,
  and process and present antigen
• Kupffer cells
• Alveolar macrophages
• Microglial cells
• Osteoclasts
• Neutrophils - Kill microbes, do not present
  antigen
• Dendritic cells - Primary function is antigen
  processing and presentation

3
Macrophages in action
Bacteria
Lymphocyte
4
The Strategy

• The anti-microbial strategy involving phagocytes
  could be described as a distributed (not a
  centralized) defense
• Phagocytes are resident in some tissues and
  quickly move into others as infection, trauma, or
  other stimuli attract them and inflammation
  begins
• This contrasts with the strategy for initiating
  acquired immune responses, which is centralized
• Microbial antigens (either from lymph or ingested
  by dendritic cells accumulate in secondary
  lymphoid tissues, e.g., lymph nodes).
  Lymphocytes travel through, and those specific
  for the antigen respond

5
The process - extravasation

• Endothelial cells in regions of infection,
  trauma, etc. become activated and express E- and
  P-selectin and ICAM-1, which bind to ligands on
  leukocytes causing them to stop and begin the
  process of diapedesis. They squeeze between
  endothelial cells, secrete proteases to break
  down the basement membrane, then enter the
  tissues.

Fig. 8.11
6
The process - chemotaxis

• Leukocytes in tissues move toward sites of
  infection by crawling toward higher
  concentrations of chemoattractants such as
  chemokines like IL-8 or bacterial products such
  as N-formyl peptides

A macrophage extends pseudopods to ingest a yeast
cell
7
Recognition

• A variety of surface receptors on phagocytes bind
  to microbial components

Fig. 8.8
8
Recognition

• Professional phagocytes have receptors for
  complement components that greatly enhance
  recognition and binding of microbes

Fig. 1.15
9
Recognition

• Phagocytic cells have receptors that bind to
  antibody-coated particles. In some cases (e.g.,
  bacteria with capsules), binding and phagocytosis
  are minimal without antibody, complement, or
  C-reactive protein

Fig. 7.22
10
Phagocytosis

• After recognition of microbial components by
  receptors on the surface of the phagocyte, the
  microbe is ingested by means of membrane
  invagination to produce a phagosome. This fuses
  with lysosomes which have a low pH and
  degradative enzymes to kill the microbe in a
  structure that is then termed a phagolysosome

Fig. 1.13
11
Phagocytosis

• Opsonization with antibody generally enhances the
  process of phagocytosis. However, some microbes
  (e.g., Salmonella, Listeria, Mycobacteria) can
  withstand the phagolysosome environment in
  unactivated macrophages

Figure 7.23
12
Phagocytosis

• Most bacteria are effectively retained in
  phagosomes and destroyed when these fuse with
  lysosomes. However, Listeria monocytogenes can
  escape into the cytoplasm in unactivated
  macrophages (left panel). However, activation by
  exposure to cytokines or LPS prevents this
  escape. From Higginbotham, J.N., Lin, T.L., and
  Pruett, S.B. Clin. Exp. Immunol. 88492-498

13
Phagocytosis

• The phagosome begins simply as a pinched off
  region of cytoplasmic membrane, but it quickly
  acquires many unique proteins that are involved
  in its functions. This is revealed in the 2-D
  gel shown the proteins were identified by mass
  spectroscopy (this process is used to evaluate
  the cellular proteome).

From Garin et al., J. Cell. Biol. 152165, 2001
14
Phagocytosis

• The Hollywood version

15
Killing

• Mechanisms include agents that are resident in
  lysosomes of neutrophils and macrophages as well
  as processes that are triggered by phagocytosis
  (respiratory burst leading to generation of
  reactive oxygen species).

16
Killing

• The respiratory burst generates large quantities
  of toxic oxygen metabolites that are potent
  anti-microbial agents. They are also often
  lethal to the phagocyte and may damage
  surrounding tissues as well

NADPH Oxidase
O2 NADPH NADP O2- H O2- H O2
H202 H202 Cl- OCl- H2O
Superoxide Dismutase
Myeloperoxidase
17
Killing

• Neutrophils generate reactive oxygen species at
  near maximum levels when triggered, but
  macrophage production of reactive oxygen species
  can be increased substantially if the cells are
  exposed to certain T cell-derived cytokines such
  as IFN-g. Thus, some microbes are controlled but
  not completely cleared by the innate immune
  system, and an acquired cell-mediated immune
  response is required for clearance.

18
Killing

• Reactive nitrogen intermediates, including nitric
  oxide (NO) are unequivocally involved in
  anti-microbial actions of macrophages in mice.
  There is considerable evidence that this is the
  case in humans as well, but the conditions
  required for macrophage activation for this
  function in humans are more rigorous and not yet
  well understood.

19
Digestion

• Microbes are broken down by enzymes in the
  phagolysosome to basic constituents (amino acids,
  etc.) for reuse by the macrophage or excretion.
  In this process peptides are generated that
  associate with MHC class II proteins which are
  then transported to the cell surface for
  presentation to T cells. Dendritic cells are the
  most effective of the phagocytes in this regard.
  Macrophages can also perform this function, but
  they may require activation by cytokines for
  optimal performance.

20
Role in resistance to microbes
Fig. 9.10

• Defects in phagocytes often are associated with
  profound immune deficits

21
Role in resistance to microbes

• The effector mechanisms induced by antibody
  (neutralization and opsonization) ultimately
  depend on phagocytes (except at mucosal surfaces
  where inhibition of microbial attachment is
  critical).

22
Role in resistance to cancer

• Tumor cells per se do not activate macrophages
  sufficiently to cause them to become tumoricidal.
  However, macrophages that are activated by other
  means can kill cancer cells by some of the same
  mechanisms used to kill bacteria as well as the
  production of TNF-a, which induces apoptosis in
  some cancer cells.
• Thus, macrophages may or may not be important in
  normal immune surveillance for cancer, but they
  may be useful in immunotherapy, if properly
  activated.

23
Role in wound healing/bone remodeling/cellular
homeostasis

• Macrophages are largely responsible for
  recognizing, clearing, and recycling damaged or
  old leukocytes and red blood cells.
• Macrophages are responsible for clearing
  apoptotic cells before the internal contents of
  the cells can be release and induce an
  inflammatory response.
• Macrophages (or related cells called osteoclasts)
  are involved in tissue repair by clearing dead
  tissue and connective tissue components and in
  bone remodeling, healing, and growth, by
  degrading regions of bone so further growth can
  occur.