Antibody function in neutralization and protection against HIV-1

Abstract

The ability to induce neutralizing antibodies is generally thought to be of great importance for vaccine efficacy. In HIV-1 research this quality has been elusive as the HIV-1 virus has evolved multiple mechanisms to evade neutralizing antibodies. This thesis traces studies with four broadly neutralizing human monoclonal antibodies against HIV-1 (i.e. b12, 2G12, 2F5 and 4E10) covering all prominent HIV-1 neutralizing epitopes, and examines the mechanisms by which antibodies protect against HIV-1 infection in vivo. The ability to neutralize HIV-1 infection in vitro is demonstrated to be an important correlate of protection in vivo for b12, which recognizes an important epitope overlapping the CD4 binding site of HIV-1. Studies using neutralizing antibody variants of b12 harboring mutations in their Fc region however demonstrated that also antibody Fc-mediated effector functions contribute to protection. Interestingly, the ability of antibody to interact with specific receptors (Fc?R) on cells of the innate immune system was found to be critical. The studies strongly suggest that neutralization of HIV-1 alone is not sufficient and that antibodies also need to engage cellular mechanisms which target and destroy HIV-1 infected cells to achieve complete protection. Our studies provided promise for vaccine development by identification of the requirements for protection, but also disappointment, as the high antibody titers needed seemed unachievable for induction with a vaccine. Significantly, we demonstrated that a strong drawback of the traditional design of antibody protection studies in animals is that high viral doses are used that contain much more virus than is contained in human transmission events. By using an innovative experimental design, it was found that low titers of the neutralizing antibody b12 provided significant protection against repeated challenges with, physiologically relevant, low doses of virus. Importantly, the serum neutralizing titers of the protected animals were determined to be low enough so that it can be reasonably expected that such titers can be achieved by vaccination. Protection studies with 2G12, which recognizes a cluster of mannose residues on gp120 via an unusual binding mechanism, brought an additional surprise as 2G12 was found to protect at very low serum neutralizing titers even when assessed in traditional high dose challenge experiments. The unusual protective ability of 2G12 underscores the realization that the gp120 glycan shield represents an important target for vaccine design. Finally, we examined protection by 2F5 and 4E10, which are directed against a site in the highly conserved membrane proximal external region (MPER) of gp41. 2F5 and 4E10 were able to provide complete protection against a SHIV mucosal challenge. The MPER therefore represents a third possible site of attack against HIV-1 and target for vaccine design. In summary, these studies have increased our insight into the mechanisms by which antibodies contribute to protection against HIV-1 infection. Our studies thus identify three sites of vulnerability in HIV-1 and show that protection against HIV-1 infection can be achieved by antibody levels that likely can be obtained by vaccination. Thereby, this thesis provides a new promise and a long-sought optimism for HIV-1 vaccine development.