Isotype modulates epitope specificity, affinity, and antiviral activities of anti–HIV-1 human broadly neutralizing 2F5 antibody

Abstract

The constant heavy chain (CH1) domain affects antibody affinity and fine specificity, challenging the paradigm that only variable regions contribute to antigen binding. To investigate the role of the CH1 domain, we constructed IgA2 from the broadly neutralizing anti–HIV-1 2F5 IgG1, and compared 2F5 IgA2 and IgG binding affinity and functional activities. We found that 2F5 IgA2 bound to the gp41 membrane proximal external region with higher affinity than IgG1. Functionally, compared with IgG1, 2F5 IgA2 more efficiently blocked HIV-1 transcytosis across epithelial cells and CD4 + cell infection by R5 HIV-1. The 2F5 IgG1 and IgA2 acted synergistically to fully block HIV-1 transfer from Langerhans to autologous CD4 + T cells and to inhibit CD4 + T-cell infection. Epitope mapping performed by screening a random peptide library and in silico docking modeling suggested that along with the 2F5 IgG canonical ELDKWA epitope on gp41, the IgG1 recognized an additional 3D-conformational epitope on the gp41 C-helix. In contrast, the IgA2 epitope included a unique conformational motif on the gp41 N -helix. Overall, the CH1 region of 2F5 contributes to shape its epitope specificity, antibody affinity, and functional activities. In the context of sexually transmitted infections such as HIV-1/AIDS, raising a mucosal IgA-based vaccine response should complement an IgG-based vaccine response in blocking HIV-1 transmission.

Immunoglobulin isotype influences affinity and specificity

“…Consistent with this view, X-ray crystallographic studies of Fab fragments showed that V region sequences were separated from the first domain of the C region (CH1) by long polypeptide chains that lacked ordered structure and seemed to insulate the V regions from the C regions, while tethering the two regions into one molecule (4). However, this neat view of one molecule with two independent functional regions is at odds with several observations in the literature, and recent studies, including the study by Tudor et al in PNAS (5), suggest that the basic model of Ig structure-function needs to be reconsidered and revised.…”

“…In addition, two other groups, including the report by Tudor et al (5), described additional examples in which Abs expressing identical V region sequences manifested altered specificity and/or affinity (13). Given that five independent groups have now reported that C region can affect V region affinity and/or specificity (5,7,8,10,13)…”

Thermodynamic Analysis of the Binding of 2F5 (Fab and Immunoglobulin G Forms) to Its gp41 Epitope Reveals a Strong Influence of the Immunoglobulin Fc Region on Affinity

“…In contrast, numerous authors have reported that both variable (V) and constant (C) Ig regions contribute to the binding affinity and specificity of antibodies (11,(17)(18)(19)(20)(21)(22)(23). Most of these studies have focused on affinity differences between distinct Ig isotypes.…”

“…For instance, the IgG isotype has been shown to affect the Ag-binding and HIV-neutralizing activity of monoclonal (24) and polyclonal (25) antibodies. Furthermore, 2F5 IgA2 and IgG1 display significantly different epitope specificity, antibody affinity, and functional activities (11). It remains unclear whether allosteric influence between C and V regions is limited to the Fab domain or extends further to the hinge and Fc regions of Ig.…”

“…The atomic interactions of the complex between the core epitope and antibody 2F5 have been described in detail (8 -10). An additional complexity of the 2F5 epitope has been suggested, including other gp41 regions (11) and an influence of the proximal lipid bilayer (12,13), which strongly affects its immunogenicity (14). Understanding the interaction of this antibody with its epitope requires not only a detailed knowledge of the structure but also knowing the physicochemical characteristics of the antigen (Ag)-antibody complex described by the kinetic rate constants, equilibrium constants, and thermodynamics of binding.…”