FIG 9.
MAb 17b binds gp120 in the absence of bridging sheet hairpin 1 in a CD4-dependent manner. A binding ELISA was performed to test whether the CD4i MAbs and sCD4 bind wt gp120BaL or the following bridging sheet hairpin 1 gp120BaL mutants (truncation sequences shown in Fig. 3): ΔH1 (gp120 with truncated hairpin 1), ΔH1-V3 base (gp120 with truncated hairpin 1 and a partially truncated V3 loop), and ΔH1-ΔV3 (gp120 with truncated hairpin 1 and a fully truncated V3 loop). Only MAb 17b showed binding to the ΔH1 mutant and only in the presence of sCD4, provided that at least the base of the V3 loop was retained. Note that as sCD4 levels were detected by using a biotinylated anti-CD4 MAb and HRP-conjugated streptavidin (see Materials and Methods), a slightly different OD at 650 nm scale (y axis) is used. Although ELISAs are semiquantitative, the strict requirement for CD4 binding is considerable and statistically significant (P < 0.05), as indicated by an asterisk.