Antibody Molecules, Genetic Engineering of
343
4.3
Polymers of Monomeric Antibodies
Although IgM is a naturally occurring
polymeric antibody (see Fig. 2), for sev-
eral applications it is desirable to have
other classes of antibodies in an IgM-like
(polymeric) structure. For example, IgM
does not bind the Fc receptors on phago-
cyte cells, whereas IgG effectively trig-
gers effector functions mediated through
gamma-speci±c Fc receptors. In addition,
since in IgG both heavy and light variable
regions are usually somatically mutated
(because of isotype switching to IgG), it
is expected that polymeric IgG may be
of higher intrinsic and functional af±nity
than the currently available IgM, resulting
in more sensitivity and/or speci±city. In
addition, many methods such as protein-
A binding are available that facilitate the
isolation of IgG.
Vectors have been developed for the
construction and expression of human
polymeric IgG. It was observed that the
18–amino acid carboxyl-terminal tailpiece
from human
µ
heavy chain is suf±cient
for polymer assembly. This ±nding was
exploited to produce IgM-like polymers of
IgG by fusing the 18–amino acid carboxyl-
terminal tailpiece from human
µ
to the
carboxyl-terminal of
γ
constant regions
(Fig. 7). Using this technique, IgM-like
polymers of IgG1, IgG2, IgG3, and IgG4
have been produced. IgGs obtained by
this approach possess up to 6 Fcs and 12
antigen-combining sites, greatly increas-
ing the avidity of their interactions with
other molecules. These polymeric anti-
bodies possess the Fc
γ
receptor–binding
properties of IgG. Not surprisingly, the
complement activity of normally active
IgG1 and IgG3 and somewhat less-active
IgG2 antibodies is dramatically enhanced
upon polymerization. An unexpected re-
sult is that IgG4, normally devoid of
complement activity, when polymerized
in the same fashion directs complement-
mediated lysis of target cells almost as
effectively as the other polymers. These
experiments demonstrate that polymer-
ization of monomeric antibodies such as
IgG is an effective approach to obtain
antibodies with broader and more power-
ful effector functions than their wild-type
counterparts.
An alternative strategy to make poly-
mers of IgG is to genetically fuse chicken
C
H
1C
H
2C
H
3
Bam
HI
Sca
I
Sal
I
IgG constant region
Sca
I
3' C4 with tp
Bam
HI
Bam
HI
Sca
I
Sal
I
C
H
1
Hinge C
H
2C
H
3
Hinge
Fig. 7
Strategy for the construction of polymeric IgG. Using appropriate
restriction sites, the
µ
tailpiece (
µ
tp) of human IgM is genetically fused
to the end of the heavy chain of human IgG.
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