Background:  Immunogenic, broadly reactive epitopes of the HIV-1 envelope glycoprotein could serve as important targets of the adaptive humoral immune response. However, variability in exposed epitopes and a combination of highly effective envelope-cloaking strategies have made the identification of such epitopes problematic. 

Methods:  We used the JC53bl-13 HIV entry assay to evaluate the immunogenicity and antigenic cross-reactivity of HIV-1 and HIV-2 envelope co-receptor binding sites. HIV-1 and HIV-2 viruses, with and without soluble CD4 (sCD4) pretreatment, were used to detect CD4-induced (CD4i) neutralizing antibodies in plasma from 143 patients infected by HIV-1 of 8 different subtypes and to detect cross-reactivity between HIV-2 viral glycoproteins and 25 HIV-1 monoclonal antibodies. Neutralizing antibody results were corroborated by direct binding assays using monomeric HIV-1 and HIV-2 glycoproteins with and without sCD4. 

Results:  The chemokine co-receptor binding site of HIV-1 subtypes A, B, C, D, F, G, H, and CRF02 elicited high titers of antibody during natural human infection and these antibodies bound and neutralized viruses as divergent as HIV-2 when pretreated with sCD4. Plasma from HIV-1 subtype D patients had especially high titers of CD4i antibodies that neutralized HIV-2 at dilutions exceeding 1:100,000. Human CD4i monoclonal antibodies elicited by HIV-1 infection, including 17b, 21c, 19e, 31H, E51, ED49, ED47, and X5, also bound and potently neutralized sCD4 pretreated HIV-2 virions. In vivo, we found naturally occurring variants of HIV-1 in human plasma that contained envelope glycoproteins with spontaneously exposed co-receptor binding surfaces. These viruses could infect CD4 negative Cf2Th-synCCR5 cells in the absence of added sCD4 and were neutralized by CD4i antibodies. 

Conclusions:  The co-receptor binding surface of the HIV-1 envelope glycoprotein is inherently highly immunogenic and antigenically broadly cross-reactive. Thus, despite remarkable evolutionary diversity among primate lentiviruses, functional constraints on receptor binding create opportunities for broad humoral immune recognition, which in turn serves to constrain the viral quasi-species.

Keywords: neutralizing antibodies; CD4i antibodies; HIV co-receptors