Epitope Mapping of Ibalizumab, a Humanized Anti-CD4 Monoclonal Antibody with Anti-HIV-1 Activity in Infected Patients

Aaron Diamond AIDS Research Center, 455 First Avenue, New York, NY, USA.
Journal of Virology (Impact Factor: 4.65). 07/2010; 84(14):6935-42. DOI: 10.1128/JVI.00453-10
Source: PubMed

ABSTRACT Ibalizumab is a humanized monoclonal antibody that binds human CD4, the primary receptor for human immunodeficiency virus type 1 (HIV-1). With its unique specificity for domain 2 of CD4, this antibody potently and broadly blocks HIV-1 infection in vitro by inhibiting a postbinding step required for viral entry but without interfering with major histocompatibility complex class II (MHC-II)-mediated immune function. In clinical trials, ibalizumab has demonstrated anti-HIV-1 activity in patients without causing immunosuppression. Thus, a characterization of the ibalizumab epitope was conducted in an attempt to gain insight into the underlying mechanism of its antiviral activity as well as its safety profile. By studying mouse/human chimeric CD4 molecules and site-directed point mutants of CD4, amino acids L96, P121, P122, and Q163 in domain 2 were found to be important for ibalizumab binding, with E77 and S79 in domain 1 also contributing. All these residues appear to cluster on the interface between domains 1 and 2 of human CD4 on a surface opposite the site where gp120 and the MHC-II molecule bind on domain 1. Separately, the epitope of M-T441, a weakly neutralizing mouse monoclonal antibody that competes with ibalizumab, was localized entirely within domain 2 on residues 123 to 125 and 138 to 140. The results reported herein not only provide an appreciation for why ibalizumab has not had significant adverse immunological consequences in infected patients to date but also raise possible steric hindrance mechanisms by which this antibody blocks HIV-1 entry into a CD4-positive cell.

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    • "Ibalizumab (5A8, TNX-355, TMB-355) is a humanized monoclonal Nab that targets the domain 2 of the CD4 receptor (Moore et al., 1992). A detailed epitope mapping study suggests that some residues within domain 1 also contribute to the binding (Song et al., 2010). Ibalizumab serves as a molecular lock on CD4, which does not interfere with the binding of virion, but it does abort its capability of inducing conformational changes in gp120 through a noncompetitive inhibition manner. "
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    • "Two humanized mAbs targeting host receptors are in phase II clinical development: ibalizumab binds domain 2 of the CD4 receptor and PRO140 (Progenics Pharmaceuticals, Tarrytown, NY) attaches to the ligand-binding site of the CCR5 coreceptor (Huber et al., 2008). Ibalizumab binding does not inhibit HIV-1 gp120 attachment to CD4 domain 1, but rather inhibits a post-attachment step required for cell entry (Burkly et al., 1992; Freeman et al., 2010; Moore et al., 1992; Song et al., 2010). In contrast to mAbs that bind CD4 domain 1, ibalizumab does not deplete CD4+ lymphocytes or interfere with MHC Class II immune function (Boon et al., 2002; Jacobson et al., 2009; Kuritzkes et al., 2004). "
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    • "The BC-loop of D2 in CD4 near the D1- D2 junction constitutes the core epitope of ibalizumab, which does not overlap with the binding site of either gp120 or MHC class II. The crystal structure provides a clear explanation, at atomic resolution, of why ibalizumab does not directly block HIV-1 attachment or interfere with immune functions mediated by CD4, in agreement with previous mutagenesis studies (Burkly et al., 1992; Song et al., 2010). Ibalizumab binding induces only a small, local movement of the FG loop in CD4, but this change does not propagate to other regions and does not lead to any significant consequences for receptor function. "
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