Biology Reference
In-Depth Information
(ii)
Triggered Env: targeting conformational epitopes
Conformational epitopes in Env induced upon interaction with CD4
are attractive targets for inducing neutralizing antibodies. It is well
documented that upon binding to CD4, HIV Env protein undergoes
significant conformational changes,
151,152
which are critical for mem-
brane fusion and viral entry in the target cells.
152
Furthermore, mAbs
such as 17b, 48d, 23e, 49e, 21c, E51, CG10, and X5 are better able
to recognize specific epitopes on gp120 after it interacts with soluble
CD4 (triggered Env).
46,48,49,153,154
Attempts have been made by vari-
ous groups to evaluate such triggered Env (gp120-CD4 complexes)
as potential vaccine candidates.
47,155
It was demonstrated that gp120-
CD4 complexes can induce broadly neutralizing antibody responses,
but they need to be stabilized using cross-linking reagents.
155
In a
proof of concept study, we have demonstrated that triggered Env
(gp120-CD4 complexes) induced strong immune responses against
both gp120 and CD4
145
and these antibodies neutralized both a pri-
mary isolate (SF162) as well as a T-cell adapted isolate (SF2).
156
To
elucidate the contribution of anti-Env antibodies to neutralization,
we affinity purified the anti-Env antibodies from anti-CD4 antibodies
and showed that these antibodies neutralized two subtype-B and one
subtype-C primary HIV-1 isolates that were tested.
156
Since gp120
SF2 alone was not able to induce primary isolate neutralizing anti-
bodies,
144
these preliminary data suggested that certain neutralizing
epitopes may be better exposed on triggered Env compared to un-
triggered gp120.
These studies illustrate the potential utility of the approach; how-
ever, the use of full-length CD4 raises the potential for induction of
autoimmune responses. One strategy to overcome this problem is to
identify the minimal binding domain of CD4. The recently reported
crystallographic structure of gp120, in complex with CD4 and the
Fab portion of mAb 17b,
157
has demonstrated that a large surface
(742 Å) of the domain D1 of CD4 binds to a large depression (800 Å)
on gp120. This CD4 interface is composed of 22 residues, con-
tributing to gp120 binding with mixed hydrophobic, electrostatic,
and H-bonding interactions. The large size and complexity of this
