408 Induction of Broadly Cross-reactive Primary Isolate Neutralizing Antibodies by Env-CD4 Complexes I. K. Srivastava*, E. Kan, V. Sharma, Y. Sun, J. B. Ulmer, S. W. Barnett Chiron Corp, Emeryville, CA
Background: The successful HIV vaccine should induce strong humoral and cellular responses at appropriate sites. HIV Env protein is the target for inducing broad primary isolate neutralizing antibody responses. However, gp120 monomer is rather limited in its ability to induce R5 isolate-specific neutralizing antibody responses. Therefore, we need to evaluate alternative strategies to expose functional epitopes on the envelope surface that may be the targets for inducing high avidity and primary isolate neutralizing responses. Viral entry into CD4+ T-cells is mediated by receptors (CD4) and co-receptors (CCR5). First, the virus binds to CD4, resulting in a conformational change that exposes the co-receptor binding site in HIV Env. Through this epitope, the virus securely attaches itself to the cell surface, which starts a cascade of events that leads to membrane fusion and virus entry into the cells. This process generates transient fusion intermediates that could be the target for inducing neutralizing antibodies. However, it is challenging to induce potent antibody responses against fusion intermediates. An alternative strategy may be to induce antibody responses against the co-receptor binding site and other cryptic epitopes that may be exposed after HIV Env binds to CD4.
Methods: To test this hypothesis we created stable gp120–CD4 complexes without cross linking, characterized the Env-CD4 complexes, and evaluated their immunogenicity in rabbits.
Results: Our results indicate that gp120–CD4 complexes induced strong and robust antibody responses in immunized animals. Furthermore, the sera collected from these animals were able to neutralize, at significant dilutions, all primary isolates tested. We have purified antibodies specific to gp120 over an affinity column and have tested these antibodies in neutralization assays. Preliminary data suggest that these affinity-purified antibodies also neutralized the heterologous primary isolates, indicating that neutralizing activity is associated with gp120. Un-complexed gp120SF2 was not able to induce any primary isolate neutralizing antibody responses.
Conclusions: Therefore, Env-CD4 complex is an effective strategy is to expose conformation-dependent functional epitopes on the HIV Env that may be the target for inducing broadly reactive neutralizing antibodies against multiple HIV-1 subtypes. Efforts are underway to identify a minimal CD4 domain capable of triggering the conformational change in the HIV Env.