Epitope Scaffolds and the HIV-1 gp41 2F5 Neutralization Determinant Open Access
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More than 25 years of research after the identification of the human immunodeficiency virus (HIV) as the causative agent of the acquired immunodeficiency syndrome (AIDS) are testimony to the great challenge of obtaining an efficacious HIV vaccine, thus far unresolved. Classic vaccine concepts such as live-attenuated virus or chemically inactivated viral particles are deemed unsafe or ineffective. Thus, innovative immunogens and vaccine strategies will likely be necessary for an effective HIV vaccine. Relatively rare broadly HIV neutralizing monoclonal antibodies isolated from infected individuals revealed a few sites of vulnerability of the viral envelope glycoproteins (Env). One of these broadly neutralizing antibodies is the 2F5 antibody that targets a continuous epitope in the membrane proximal external region (MPER) of the HIV-1 gp41 envelope glycoprotein. Nuclear magnetic resonance (NMR) and crystallographic studies showed two different epitope conformations, one as an antibody unbound helical conformation and the second as an antibody bound type I ß-turn extended loop conformation. We have hypothesized that elicitation of 2F5-like antibodies will require presentation of the extended loop conformation to the immune system. For this purpose, we computationally identified a series of non-HIV proteins capable of accommodating the HIV-1 gp41 2F5 epitope on their surface. Following computational design and engraftment of the 2F5 epitope, these fusion proteins were modeled to present the 2F5 epitope in the extended loop conformation only. We termed these fusion proteins epitope scaffold (ES) proteins. Inoculation of ES proteins into animals resulted in various degrees of 2F5 epitope-specific titers with serum antibodies that recapitulated faithfully the binding properties of the 2F5 antibody. Furthermore, crystallization of the ES-elicited murine monoclonals in complex with the MPER peptide showed exquisite mimicry of the 2F5 antibody-bound conformation. In summary, we demonstrated that 2F5-epitope scaffolds can be used as immunogens to elicit antibodies targeting the HIV-1 gp41 2F5 epitope in a manner remarkably similar to that of the model 2F5 antibody; we showed that epitope scaffolds can be used as selective antigens to identify new monoclonal antibodies targeting the HIV neutralizing determinant; and we further define the requirements for neutralization of 2F5-like antibodies.