Combinatorial Phage Antibody Libraries
89
Fig. 5
Tube representation of the structure of the whole IgG1 b12, a broadly
neutralizing human antibody against HIV-1. b12 was isolated from an Fab
phage display library prepared from the bone marrow RNA of an
asymptomatic HIV-1-seropositive individual. The structure was the ±rst to be
determined of an intact whole human IgG molecule. The light chains are
shown in pink and the heavy chains in blue. Carbohydrate residues in the Fc
portion of the molecule are also shown (see color plate p. xxii).
activity, and alanine-scanning mutagene-
sis of the CDR H3 loop of b12 supports
this prediction.
Several other novel antibodies against
HIV-1 have been selected by the use of
phage display of which we will men-
tion three. One antibody, Fab Z13, was
selected using a synthetic peptide cor-
responding to the membrane-proximal
region on the ectodomain of the trans-
membrane protein, gp41. This antibody
and two others, 2F5 and 4E10, all bind to
a similar region on gp41, and all of them
are capable of cross-neutralizing primary
isolates of HIV-1. Another antibody, Fab
X5, was selected against a ternary com-
plex of CCR5 (HIV-1 coreceptor), CD4,
and gp120. Fab X5 has been shown to
neutralize a wide range of primary iso-
lates of HIV-1. Interestingly, the whole
IgG X5 is actually poorer at neutralizing
many viral isolates than its Fab counter-
part, a property that has been attributed
to steric constraints in accessing its epi-
tope. A third antibody, scFv 4KG5, has
been selected, which binds to a novel epi-
tope on gp120 comprising the V1, V2, and
V3 variable loops. The binding of 4KG5
to gp120 is inhibited by all anti-CD4 re-
ceptor binding site (CD4BS) antibodies
tested (
n
=
14), and yet, its binding is
moderately enhanced in the presence of
the broadly neutralizing antibody b12 (de-
scribed above). Thus, 4KG5 has helped
distinguish, at the molecular level, b12
from other nonneutralizing anti-CD4BS
antibodies.
Following the generation of the ±rst
recombinant Fabs against HIV-1, two
more
libraries were
constructed
from
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h
eb
o
n
em
a
r
r
owo
fa
d
d
i
t
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a
lH
IV
-
1
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positive asymptomatic donors. Analysis
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