A human monoclonal antibody to a complex epitope in the V3 region of gp120 of human immunodeficiency virus type 1 has broad reactivity within and outside clade B

Abstract

We have used virus neutralization and antibody-binding techniques to define the epitope for a human monoclonal antibody, designated 19b, within the V3 region of the gp120 surface glycoprotein of human immunodeficiency virus type 1. Unusually, the 19b epitope encompasses residues on both flanks of the V3 loop. However, 19b binding to gp120 is independent of sequences at the crown of the V3 loop, provided that they are compatible with the formation of a type II ␤ turn that is presumably necessary to juxtapose the antigenic residues on the V3 flanks. By comparing the V3 sequences of virus gp120s able and unable to bind 19b, we were able to define the canonical 19b epitope as-I-G-FY-T, where residues at the positions indicated by the gaps do not contribute directly to the 19b-binding site. A few conservative substitutions at the more critical residues are also compatible with 19b binding. Inspection of V3 sequences in the human immunodeficiency virus database indicated that the canonical 19b epitope is well conserved among isolates from the North American-European clade B and also among clade E isolates from Thailand and clade F isolates from Brazil. A minority of gp120s from clades A and C also possess the 19b epitope. Consistent with the theoretical predictions of its cross-clade reactivity, 19b was found to bind to gp120s from clades A, B, C, E, and F in immunoassays. However, 19b was not able to reduce the infectivity of primary viruses from clades A, E, and F that were predicted to possess the 19b epitope and only modestly reduced the infectivity of a clade C virus at low input virus concentrations. Cross-clade neutralization via V3-directed antibodies may, therefore, be difficult, even if the antibodies show broad reactivities in binding assays and the viruses theoretically possess the relevant binding site. Among neutralization sites on the envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1), the V3 region of the surface glycoprotein gp120 has attracted particular attention (20, 21, 24, 28-30, 42, 46, 50, 52). This is attributable not only to its being one of the few regions of gp120 identified to be a continuous neutralization epitope (20, 24, 50, 52) but also to the potency with which anti-V3 antibodies neutralize T-cell-line-adapted viruses such as HIV-1 LAI or HIV-1 MN. Thus anti-V3 murine monoclonal antibodies (MAbs) raised against recombinant gp120 or V3-based peptides can neutralize certain T-cell-line-adapted viruses at concentrations considerably below 1 g/ml (26, 28, 56, 57), as can some human MAbs (HuMAbs) (17-19, 54). Furthermore, under appropriate conditions, a passively administered V3 MAb could protect a chimpanzee from subsequent challenge by HIV-1 LAI (14). It has been widely considered that any vaccine immunogen designed to elicit humoral immunity to HIV-1 should be able to stimulate the production of virus-neutralizing V3 antibodies (4, 33, 42, 46). Furthermore, the V3 loop is a possible target for passive immunotherapy of HIV-1-infected individuals by using HuM Abs (60, 61

Identifying epitopes of HIV-1 that induce protective antibodies

“…So, with the exception of clades B and E, which show distinct antigenic differences 121 , there seems to be no particular preference for sera from patients infected with one clade to neutralize other viruses from the same clade; a diagramatic representation of this is shown in FIG. 5. Similarly, studies of HIV-1-specific human monoclonal antibodies, most of which come from clade-B-infected individuals (such as those listed in TABLE 1) show their comparable potency in neutralizing many viruses from various clades 32,86,122 . These data raise important questions about the relevance of HIV-1 genotypic classification in vaccine design.…”

Analysis of the neutralization breadth of the anti-V3 antibody F425-B4e8 and re-assessment of its epitope fine specificity by scanning mutagenesis

“…Defining the epitopes recognized by anti-V3 antibodies has traditionally been accomplished with V3 peptides in ELISA binding assays (e.g. Pepscan) and competition studies, or by determining the ability of the antibodies to bind a series of recombinant gp120s with varying V3 regions (for example (Eda et al, 2006b;Gorny et al, 1992;Gorny et al, 1998;Laisney and Strosberg, 1999;Moore et al, 1994;Moore et al, 1995;Schreiber et al, 1997;Seligman et al, 1996;White-Scharf et al, 1993)). In some instances, the results derived from these studies have been confirmed by analyzing peptide sequences selected with antibody from peptide phage display libraries (Keller et al, 1993) or by comparing the ability of antibodies to neutralize viruses with various V3 sequences (e.g.…”

The V3-directed immune response in natural human immunodeficiency virus type 1 infection is predominantly directed against a variable, discontinuous epitope presented by the gp120 V3 domain

“…The V3 loop is especially well exposed on monomeric or oligomeric gp120 and is accessible to various neutralizing MAbs but can simultaneously mask some discontinuous, conserved neutralization epitopes (29,51). From the work of several groups, it became evident that in natural infection, antibodies are preferentially produced against discontinuous epitopes on the gp120 glycoprotein (28,30,44,45). The V3 loops of clade B strains have a conserved GPGRAF sequence, called tip of the loop, as well as variable sequences on either side (22).…”