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CHaPtEr 15 Immunoglobulin Function 227

Antibodies recognizing neo-epitopes can be used to monitor Another important feature of polysaccharide antigens is that
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the extent of activation of the complement pathway. A chimeric they are generally multivalent. Bacterial and perhaps viral polysac-
antibody, ensituximab, targeting neo-epitopes associated with charide epitope densities can approach values in the millions
colorectal and pancreatic carcinomas, represents an evolving area per square micrometer, which is probably one to several orders
of discovery of antibodies to target cancer neo-antigens. of magnitude greater than the epitope densities for protein
Studies in the 1970s on the sizes of epitopes associated with determinants on mammalian cells. Therefore multipoint attach-
synthetic peptide antigens yielded results suggesting that protein ment and functional affinity are likely to be critical factors in
epitopes would maximally involve six or seven amino acids. the mediation of immunity by antipolysaccharide antibodies or
However, the first structure of an antibody-variable module other carbohydrate-specific proteins.
in complex with a globular protein antigen, determined by
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X-ray crystallography, indicated that protein epitopes, defined IMMUNE COMPLEXES IN VIVO
on the basis of intermolecular contact, could be as large as
15–20 amino acids. A similar number of amino acids in the
antibody V domains constituted the paratope. And, even peptide KEY CoNCEPtS
antigen–antibody interaction can involve at least twelve peptide Immune Complexes
amino acids in contact with the antibody. Still, it is possible
that there are smaller epitopes on globular proteins, particularly • Immune complexes are aggregates of antibody and antigen.
for regions of proteins that protrude or have a high radius of • Immune complexes can form in tissues, or they can form in the circula-
tion and subsequently deposit in tissues.
curvature. • Immune complexes can activate complement or Fc receptor–bearing
Antibodies specific for both linear and conformational epitopes cells, leading to tissue damage.
have important practical applications. For example, a synthetic • The composition, size, charge, and antibody isotypes characterizing
peptide corresponding in amino acid sequence to a segment of a given population of immune complexes will influence the pathogenic
the polypeptide chain predicted from the nucleotide sequence potential of the complexes.
can be used to elicit antibodies. Antibodies with the potential
to recognize a linear epitope available in a denatured form of
the gene product can be used to identify the protein following Interactions between antibodies and antigens in vivo can result
expression, electrophoresis, and blotting under denaturing in the formation of molecular aggregates, referred to as immune
conditions. Some antibodies raised by challenge with synthetic complexes. Deposition of immune complexes in tissues, such as
peptides that bind to linear epitopes can recognize a protein in blood vessels, renal glomeruli, renal tubules, the thyroid gland,
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denatured form but will not bind to or alter the function of the and the choroid plexus, can result in pathological conditions.
native protein. Immune complexes can form in the circulation before deposition
Antibodies with the ability to neutralize protein function in a given tissue, or they can form directly in the affected tissue.
generally recognize conformations accessible to the native protein, A clinical situation associated with immune complex formation
usually at discontinuous epitopes. Thus antibodies specific for is therapy with immunoglobulin (Chapter 84). During administra-
peptides (that correspond in amino acid sequence to a portion tion, infused antibodies may bind endogenous antibodies and
of a native protein) or denatured protein that can cross-react the numbers of such complexes are greater at higher infusion
with the protein in a native (folded, functional) state can be rates. These “induced” immune complexes can promote adverse
extremely valuable. Such cross-reactivity is more likely to occur reactions, ranging from mild to severe, including headache, nausea,
when the region being recognized is relatively disordered in the myalgia, fever, chills, chest discomfort, skin and anaphylactic
native structure. 10 reactions, and aseptic meningitis.
Some variables, such as concentration, composition, size,
CARBOHYDRATE EPITOPES charge, and antibody isotype, will influence the magnitude and
sites of tissue deposition of immune complexes. The magnitude
The classic studies of Kabat on the binding of antibodies to of complement activation and the extent of interaction with Fc
dextran led to the concept that epitopes on carbohydrate antigens and complement receptors, in conjunction with the sites and
could be as large as six or seven monosaccharides. However, extent of tissue deposition, determine the biological properties
minimal carbohydrate epitopes can probably be as small as one of the complexes. Antigen–antibody lattice size is determined
or two monosaccharides. Even in the case of larger epitopes, it by antigen valence, epitope geometry, antibody valence, the
is typical for the terminal groups to play a dominant role in intrinsic affinity of paratope for epitope, antibody and antigen
determining antibody specificity for carbohydrate antigens. Recent flexibility, the ratio of antibody to antigen, and the absolute
studies have suggested that polysaccharide epitopes can sometimes concentrations of antibody and antigen. The potential diversity
also result from conformational properties of polysaccharides. of immune complex morphologies is illustrated in Fig. 15.4.
Interactions between antibodies and polysaccharides have These complexes, between a monoclonal antibody (mAb) specific
typically been characterized by relatively low intrinsic affinities for a bacterial polysaccharide and various antiidiotypic or
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in comparison to antibody–protein interactions. Relatively weak antiisotypic mAbs, are visualized with electron microscopy.
antibody–carbohydrate binding can result from biological Immune complexes have also been found to have immunoregu-
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constraints related to protection against self-recognition and latory effects, particularly with respect to antibody responses.
consequent tissue damage or from physical–chemical constraints Immune complexes can bind simultaneously to B-cell surfaces
related to the conformational freedom and high degree of solva- through antigen (to B-cell surface Ig), antibody (to Fc recep-
tion of unbound carbohydrates. However, antibodies produced tors), and associated complement components (to complement
in response to pathogens, such as HIV, may be much more effective receptors). The interaction with FcγRIIB, on the B-lymphocyte
at interacting with carbohydrate antigens. 11 membrane, has the effect of diminishing the B-cell response