Toward effective HIV vaccination. Induction of binary epitope reactive antibodies with broad HIV neutralizing activity

Department of Pathology and Laboratory Medicine, Chemical Immunology Research Center, University of Texas-Houston Medical School, Houston, Texas 77030, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 10/2009; 284(44):30627-42. DOI: 10.1074/jbc.M109.032185
Source: PubMed


We describe murine monoclonal antibodies (mAbs) raised by immunization with an electrophilic gp120 analog (E-gp120) expressing the rare ability to neutralize genetically heterologous human immunodeficiency virus (HIV) strains. Unlike gp120, E-gp120 formed covalent oligomers. The reactivity of gp120 and E-gp120 with mAbs to reference neutralizing epitopes was markedly different, indicating their divergent structures. Epitope mapping with synthetic peptides and electrophilic peptide analogs indicated binary recognition of two distinct gp120 regions by anti-E-gp120 mAbs, the 421-433 and 288-306 peptide regions. Univalent Fab and single chain Fv fragments expressed the ability to recognize both peptides. X-ray crystallography of an anti-E-gp120 Fab fragment revealed two neighboring cavities, the typical antigen-binding cavity formed by the complementarity determining regions (CDRs) and another cavity dominated by antibody heavy chain variable (V(H)) domain framework (FR) residues. Substitution of the FR cavity V(H) Lys-19 residue by an Ala residue resulted in attenuated binding of the 421-433 region peptide probe. The CDRs and V(H) FR replacement/silent mutation ratios exceeded the ratio for a random mutation process, suggesting adaptive development of both putative binding sites. All mAbs studied were derived from V(H)1 family genes, suggesting biased recruitment of the V gene germ line repertoire by E-gp120. The conserved 421-433 region of gp120 is essential for HIV binding to host CD4 receptors. This region is recognized weakly by the FR of antibodies produced without exposure to HIV, but it usually fails to induce adaptive synthesis of neutralizing antibodies. We present models accounting for improved CD4-binding site recognition and broad HIV neutralizing activity of the mAbs, long sought goals in HIV vaccine development.

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Available from: Sudhir Paul
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    • "2009; Planque et al., 2010). Despite the evident existence of a conformationally correct, solvent-exposed CD4BD core on the surface of gp120 (Nishiyama et al., 2009; Planque et al., 2010), immunization with this protein does not induce a robust adaptive anti-CD4BD core antibody response (Sun et al., 1989; Kelker et al., 2010), and these antibodies are also poorly represented in the antibody response to HIV infection (Planque et al., 2010). The down-regulatory interactions at the BCR FRs, therefore, may be the primary factor limiting the CD4BD core -directed neutralizing antibody response to the monomeric gp120 immunogen and oligomeric gp120 on the HIV surface. "
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    ABSTRACT: The immunodominant epitopes expressed by the HIV-1 envelope protein gp120 are hypermutable, defeating attempts to develop an effective HIV vaccine. Targeting the structurally conserved gp120 determinant that binds host CD4 receptors (CD4BD) and initiates infection is a more promising route to vaccination, but this has proved difficult because of the conformational flexibility of gp120 and immune evasion mechanisms used by the virus. Mimicking the outer CD4BD conformational epitopes is difficult because of their discontinuous nature. The CD4BD region composed of residues 421-433 (CD4BD(core)) is a linear epitope, but this region possesses B cell superantigenic character. While superantigen epitopes are vulnerable to a small subset of spontaneously produced neutralizing antibodies present in humans without infection (innate antibodies), their non-covalent binding to B cell receptors (BCRs) does not stimulate an effective adaptive response from B cells. Covalent binding at naturally occurring nucleophilic sites of the BCRs by an electrophilic gp120 (E-gp120) analog is a promising solution. E-gp120 induces the synthesis of neutralizing antibodies the CD4BD(core). The highly energetic covalent reaction is hypothesized to convert the abortive superantigens-BCR interaction into a stimulatory signal, and the binding of a spatially distinct epitope at the traditional combining site of the BCRs may furnish a second stimulatory signal. Flexible synthetic peptides can detect pre-existing CD4BD(core)-specific neutralizing antibodies. However, induced-fit conformational transitions of the peptides dictated by the antibody combining site structure may induce the synthesis of non-neutralizing antibodies. Successful vaccine targeting of the CD4BD will require a sufficiently rigid immunogen that mimics the native epitope conformation and bypasses B cell checkpoints restricting synthesis of the neutralizing antibodies.
    Full-text · Article · Dec 2012 · Frontiers in Immunology
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    • "In 2009 2.6 million people became newly infected and 1.8 million people died in the course of AIDS [1]. During the last decades several efforts to induce HIV-1 defending neutralizing antibodies (Abs) have failed [2]–[4] but also promising results were reported [5], [6]. One of the most potent neutralizing HIV-1 Abs isolated so far is the monoclonal Ab (mAb) 2F5 [7]–[11] which binds to the membrane proximal external region (MPER) of the virus envelope glycoprotein gp41 [12], [13]. "
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    ABSTRACT: The HIV-1 envelope protein harbors several conserved epitopes that are recognized by broadly neutralizing antibodies. One of these neutralizing sites, the MPER region of gp41, is targeted by one of the most potent and broadly neutralizing monoclonal antibody, 2F5. Different vaccination strategies and a lot of efforts have been undertaken to induce MPER neutralizing antibodies but little success has been achieved so far. We tried to consider the alternative anti-idiotypic vaccination approach for induction of 2F5-like antibodies. The previously developed and characterized anti-idiotypic antibody Ab2/3H6 was expressed as antibody fragment fusion protein with C-terminally attached immune-modulators and used for immunization of rabbits to induce antibodies specific for HIV-1. Only those rabbits immunized with immunogens fused with the immune-modulators developed HIV-1 specific antibodies. Anti-anti-idiotypic antibodies were affinity purified using a two-step affinity purification protocol which revealed that only little amount of the total rabbit IgG fraction contained HIV-1 specific antibodies. The characterization of the induced anti-anti-idiotypic antibodies showed specificity for the linear epitope of 2F5 GGGELDKWASL and the HIV-1 envelope protein gp140. Despite specificity for the linear epitope and the truncated HIV-1 envelope protein these antibodies were not able to exhibit virus neutralization activities. These results suggest that Ab2/3H6 alone might not be suitable as a vaccine.
    Full-text · Article · Jun 2012 · PLoS ONE
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    ABSTRACT: To identify an HIV epitope suitable for vaccine development. Diverse HIV-1 strains express few structurally constant regions on their surface vulnerable to neutralizing antibodies. The mostly conserved CD4-binding site (CD4BS) of gp120 is essential for host cell binding and infection by the virus. Antibodies that recognize the CD4BS are rare, and one component of the CD4BS, the 421-433 peptide region, expresses B-cell superantigenic character, a property predicted to impair the anti-CD4BS adaptive immune response. IgA samples purified from the plasma of patients with HIV infection were analyzed for the ability to bind synthetic mimetics containing the 416-433 gp120 region and full-length gp120. Infection of peripheral blood mononuclear cells by clinical HIV isolates was measured by p24 ELISA. IgA preparations from three patients with subtype B infection for 19-21 years neutralized heterologous, coreceptor CCR5-dependent subtype A, B, C, D, and AE strains with exceptional potency. The IgAs displayed specific binding of a synthetic 416-433 peptide mimetic dependent on recognition of the CD4-binding residues located in this region. Immunoadsorption, affinity chromatography, and mutation procedures indicated that HIV neutralization occurred by IgA recognition of the CD4BS. These observations identify the 421-433 peptide region as a vulnerable HIV site to which survivors of infection can produce powerful neutralizing antibodies. This indicates that the human immune system can bypass restrictions on the adaptive B cell response to the CD4BS, opening the route to targeting the 421-433 region for attaining control of HIV infection.
    Full-text · Article · Feb 2010 · AIDS (London, England)
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