Humoral Immune Effector Mechanisms

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Humoral Immune Effector Mechanisms
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Ig of Different Isotypes
How do the functional differences impact the
immune response?
The focus will be on Ig and its interactions
with immune effector systems and receptors
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Complement - an important effector system has
important functions related to immune defense

• Lysis of cells. This is the original function
  identified
• and causes hypotonic cell death by making hole.
  It is
• not effective against organisms with cell walls
  such as fungi
• and Gram positive bacteria

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Complement - an important effector system has
important functions related to immune defense

• Opsonization. Macrophage and PMNs have FcRs and
• at least two different kinds of complement
  receptors that
• aid in phagocytosis. C3b, a cleavage product
  formed during
• activation is the major player. Antigen coated
  with C3b binds
• to cells bearing complement receptors and if the
  cell is
• a phagocyte the antigen will be phagocytosed.

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Complement - an important effector system has
important functions related to immune defense

• Inflammation. Peptides generated during
  activation
• play a role in inflammation. The anaphylatoxins
  of which
• C5a is the most potent bind receptors on mast
  cells and
• basophils and cause degranulation with the
  release of
• pharmacologically active mediators which induce
  smooth-
• muscle contraction and increases in vascular
  permeability.
• C3a, C5a and C5b67 act as chemoattractants and
  induce
• monocytes and neutrophils to adhere to vascular
  endothelial
• cells, extravasate through the endothelial lining
  of the
• capillaries and migrate to the site of complement
  activation
• in the tissue.

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Complement - an important effector system has
important functions related to immune defense
4. Immune clearance. Removes immune complexes
from the circulation and deposits them in the
liver where they are degraded. C3b facilitates
immune complex binding to CR1 on RBCs. In the
liver and spleen the complexes are stripped
from the RBC and phagocytosed. Complement also
helps to solubilize immune complexes.
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Complement - an important effector system has
important functions related to immune defense

• Enhanced immune response. CD21, part of the co-
• receptor on the B cell, binds cleaved C3.
  Recently it has
• also been shown that C3 is required for optimal
  expansion
• of T cells during a systemic viral infection.

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Complement - an important effector system has
important functions related to immune defense

• Virus neutralization. Complement mediates viral
• neutralization by facilitating viral aggregation
  and by coating
• the viral surface.

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Classical pathway Requires Ab Mannose binding
lectin pathway Alternative pathway
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IgM
The first isotype made during the immune response
Exists in pentameric/hexameric forms
One consequence of polymerization is higher
avidity
The higher avidity helps to compensate in part
for the low affinity of IgM made early in the
immune response before somatic mutation has taken
place. Also each molecule has ten/twelve C1q
binding sites
IgG
In contrast to IgM, complement activation by IgG
requires aggregation.
Monomeric IgG has an exposed C1q binding site but
it is of low affinity adjacent sites are
required in order to achieve an interaction of
sufficient avidity to trigger the cascade.
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IgM is a very rigid molecule
Binding of Ag by IgM leads to a conformational
change exposing the C1q binding sites.
There are multiple sites in each IgM molecule so
one IgM can bind C1q and activate the
complement cascade
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C1
C1 q binding site
C1q binding site in CH2
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MBL - resembles C1q in structure except
specificity is for carbohydrate
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C3a and C5a are anaphylatoxins C3a - 77 amino
acid peptide that causes smooth muscle
contraction, increases vascular permeability and
mast cell and basophil degranulation C5a- a77
amino acid peptide is also a chemoattractant and
activator of WBC
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C3a and C5a are anaphylatoxins Also amplify the
inflammatory response by inducing the synthesis
of pro-inflammatory cytokines. Their receptors
are present on many cell types including
leukocytes, mast cells, macrophages, endothelial
cells, astrocytes and microglial cells
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C3a and C5a are anaphylatoxins Anaphylatoxin
triggered cascade of events that contribute to
the pathogenesis of a number of disease and
conditions including hypersensitivity reactions,
endotoxin shock, multiples organ failure and
respiratory distress syndrome. Neutralization
of the effects of anaphylatoxin is therefore of
substantial clinical significance
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C3a and C5a are anaphylatoxins Neutralization of
the effects of anaphylatoxin is therefore of
substantial clinical significance It has now
been shown that C3a and C5a bind to
immunoglobulin molecules through the constant
region of the Fab. (Nature Medicine, published
online March 3, 2003).
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Intravenous immunoglobulin (IVIG) and F(ab)2
prepared from it inhibit C3a and C5a induced
histamine release
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Binding is to F(ab)2 and not Fc and is not
inhibited by binding to Ag
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Cleavage products of C3b are opsonins
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Complement receptors (CD21 CR2) potentiate the
immune response
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Fcg receptors are important humoral effector
molecules
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FcRs function for the phagocytosis of Ab coated
particles by macrophages and killing by NK
cells.
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Phagocytic Index (PI) (percentage of cells
containing at least on particle)X(mean number of
particles per positive cell)
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For the Arthus reaction Ab is injected into the
skin followed by
intravenous Ag
It had been thought to be complement mediated but
it can be virtually absent in g chain knock-out
mice
However, there are strain differences and some
strains show more complement dependence than
others
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In this experiment the mice were treated with
suboptimal amounts of antibody
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Immune complexes can determine whether
macrophages bias to TH1 or TH2
Activated macrophages treated with IFN-g and LPS
are biased to IFN- g production when stimulated
by Ag alone, but produce IL-4 when stimulated by
immune complexes. This bias is stable and is
seen when CD4 cells were stimulated 7 days
later with OVA alone. Intracellular cytokine
expression reflects what is seen in the
secretions (J. Immunol 1683701)
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Immune complexes facilitate DC priming of CTLs
and induce DC maturation in the absence of T
cell help
J. Immunol. 1682240-2246, 2002
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There is increased binding and uptake of Ag
present in ICs
J. Immunol. 1682240-2246, 2002
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FcRn plays a major role in determining serum
persistence
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CD89 - FcaRI receptor for IgA
Found on cells of the myeloid lineage including
neutrophils, eosinophils, most monocytes,
interstitial dendritic cells, and Kuppfer
cells. Can be expressed associated with FcRg.
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Uptake of FITC-labeled E. coli opsonized with IgA
into Kupffer cells of
transgenic mice expressing CD89
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When the CD89 is occupied, it inhibits the
phagocytosis of E. coli opsonized with IgG
Buffer Unopsonized E. coli
Buffer opsonized E. coli
Opsonized E. coli irrelevant Fab
Anti-CD89 Fab opsonized E. coli
In this case an anti-CD89 was used in place of
IgA to mimic its effects
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A rat mast cell line was engineered to express
FcaRI
Addition of IgA but not of IgG inhibited IgE
mediated granule release. However, extensive
crosslinking with F(ab)2 and rabbit
anti-mouse resulted in granule release. Therefore
a model emerges of FcaRI acting as an inhibitory
receptor at low occupany, and as a inflammatory
receptor when extensively crosslinked. Both
activities required FcRg with a functional ITAM.
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