Biology Reference
In-Depth Information
to gp120,
171
while increasing the binding of a potent broadly neu-
tralizing antibody b12. In further studies, they focused on the reduc-
tion of binding of a wide range of non-neutralizing antibodies by
incorporating seven more glycosylation sites in addition to the four
alanine mutations.
172
It was interesting to note that these hyperglyco-
sylated Env proteins were not recognized by non-neutralizing anti-
bodies directed toward CD4 BS (such as b3, b6, CD4-IgG2, 15e,
F91, and F105); however, binding of the neutralizing antibody b12
remained, albeit at lower affinity, largely unaffected. Furthermore,
hyperglycosylation affected the exposure of conformational epitopes
recognized by mAbs 17b, 48d, and X5. It remains to be determined
whether these modified molecules can alter the immunogenicity of
Env and, in particular, if the increased b12 reactivity can be correlated
with enhanced neutralizing activity.
Directing the Immune Response Toward
the CD4 Binding Site by Introducing
Deletions in Bridging Sheet
It has been difficult to induce antibody responses directed against the
CD4 BS by vaccination with gp120. From gp120 structure analysis,
it is apparent that Env is folded into inner and outer domains (Fig. 7).
The inner domain (with respect to the N and C terminus) comprises
two helices, a small five-stranded
sheet sandwiched at its proximal
end, and a projection at the distal end from which the V1/V2 loop
emanates. The outer domain is a stalked double barrel that lies along-
side the inner domain, such that the outer barrel and inner bundle
axes are approximately parallel. The gp120 inner and outer domains
are attached by a bridging sheet that further limits the accessibility of
the CD4 binding pocket. This bridging sheet is composed of four
anti-parallel
β
-21) (Fig. 8). The
CD4 binding site is buried deep between the inner and outer domains
of this molecule and, hence, possibly not accessible to antibodies.
Structural data suggest that the V2 loop may fold over the bridging
sheet and, due to its three-dimensional position, the bridging sheet is
β
strands (namely
β
-2,
β
-3,
β
-20, and
β
