No abstract

The identification and epitope mapping of broadly neutralizing anti-human immunodeficiency virus type 1 (HIV-1) antibodies (Abs) is important for vaccine design, but, despite much effort, very few such Abs have been forthcoming. Only one broadly neutralizing anti-gp41 monoclonal Ab (MAb), 2F5, has been described. Here we report on two MAbs that recognize a region immediately C-terminal of the 2F5 epitope. Both MAbs were generated from HIV-1-seropositive donors, one (Z13) from an antibody phage display library, and one (4E10) as a hybridoma. Both MAbs recognize a predominantly linear and relatively conserved epitope, compete with each other for binding to synthetic peptide derived from gp41, and bind to HIV-1 MN virions. By flow cytometry, these MAbs appear to bind relatively weakly to infected cells and this binding is not perturbed by pretreatment of the infected cells with soluble CD4. Despite the apparent linear nature of the epitopes of Z13 and 4E10, denaturation of recombinant envelope protein reduces the binding of these MAbs, suggesting some conformational requirements for full epitope expression. Most significantly, Z13 and 4E10 are able to neutralize selected primary isolates from diverse subtypes of HIV-1 (e.g., subtypes B, C, and E). The results suggest that a rather extensive region of gp41 close to the transmembrane domain is accessible to neutralizing Abs and could form a useful target for vaccine design.Eliciting broadly neutralizing antibodies (Abs) to human immunodeficiency virus (HIV-1) is a major goal of vaccine research but one that has proved extraordinarily elusive (8,11,77). This probably reflects the low antigenicity and immunogenicity of the HIV-1 envelope spike and most especially of relatively conserved regions of the spike. It is clear that much of the protein surface of the gp120 and gp41 protein molecules in the heterotrimeric envelope spike (gp120 3 -gp41 3 ) is directly or indirectly occluded from Ab binding by protein-protein interaction. Thus, for example, extensive surfaces on gp41 appear to be involved in interaction with other gp41 molecules and with gp120 (62, 63). Reciprocally, a portion of the surface of gp120 is occluded by the interaction with gp41 and by trimer formation (28,32,75,76). The relatively low immunogenicity of HIV-1 envelope trimers is also inferred from the low titers of neutralizing Abs, particularly cross-isolate neutralizing Abs, elicited during natural infection (31,39,40). This follows since a good correlation has been established between Ab neutralization and binding to envelope spikes, at least for T-cell-line-adapted viruses (50, 57, 61), suggesting that the deficit in neutralization originates from a deficit in spike binding. Low immunogenicity presumably arises, at least in part, from the weakly stimulating properties of the exposed regions of the envelope trimer. These include extensive regions of carbohydrate. A caveat here is that one cannot generally be sure of the eliciting antigen: Abs reactive with the trimer may have been elicited by other...

Anti-human immunodeficiency virus type 1 (HIV-1) antibodies whose binding to gp120 is enhanced by CD4 binding (CD4i antibodies) are generally considered nonneutralizing for primary HIV-1 isolates. However, a novel CD4i-specific Fab fragment, X5, has recently been found to neutralize a wide range of primary isolates.To investigate the precise nature of the extraordinary neutralizing ability of Fab X5, we evaluated the abilities of different forms (immunoglobulin G [IgG], Fab, and single-chain Fv) of X5 and other CD4i monoclonal antibodies to neutralize a range of primary HIV-1 isolates. Our results show that, for a number of isolates, the size of the neutralizing agent is inversely correlated with its ability to neutralize. Thus, the poor ability of CD4i-specific antibodies to neutralize primary isolates is due, at least in part, to steric factors that limit antibody access to the gp120 epitopes. Studies of temperature-regulated neutralization or fusion-arrested intermediates suggest that the steric effects are important in limiting the binding of IgG to the viral envelope glycoproteins after HIV-1 has engaged CD4 on the target cell membrane. The results identify hurdles in using CD4i epitopes as targets for antibody-mediated neutralization in vaccine design but also indicate that the CD4i regions could be efficiently targeted by small molecule entry inhibitors.Human immunodeficiency virus type 1 (HIV-1) entry into host cells is initiated by the binding of the gp120 subunit of the viral envelope glycoprotein (Env) complex to the host cell receptor (CD4) (8,20). This interaction induces conformational changes in gp120 resulting in the exposure of a conserved high-affinity binding site for the coreceptor (the chemokine receptors CCR5 or CXCR4) (46,47,54,56,59). A second obligatory binding step between the gp120-CD4 complex and the coreceptor is then thought to induce additional conformational changes that ultimately result in the fusion of viral and host cell membranes (9, 18).Neutralizing antibodies are believed to act, at least in part, by binding to the exposed Env surface and obstructing the initial interaction between a trimeric array of gp120 molecules on the virion surface and receptor molecules on the target cell (36,37,57). In response, HIV-1 has evolved a number of strategies to evade recognition by neutralizing antibodies, particularly those directed to the conserved CD4 and coreceptor binding sites of Env. The extent of protection of these sites from antibody recognition is limited by the necessity to preserve the accessibility for receptor interaction. In the case of the CD4bs this has led to the following structural features: (i) it is partially obscured from antibody recognition by the V1/V2 loop and associated carbohydrate structures; (ii) the flanking residues are variable and modified by glycosylation; (iii) it is recessed to an extent that limits direct access by an antibody variable region; (iv) clusters of residues within the CD4bs that do not directly interact with CD4 are subject to variation...

The promise of bispecific antibodies (bsAbs) to yield more effective therapeutics is well recognized; however, the generation of bsAbs in a practical and cost-effective manner has been a formidable challenge. Here we present a technology for the efficient generation of bsAbs with normal IgG structures that is amenable to both antibody drug discovery and development. The process involves separate expression of two parental antibodies, each containing single matched point mutations in the CH3 domains. The parental antibodies are mixed and subjected to controlled reducing conditions in vitro that separate the antibodies into HL half-molecules and allow reassembly and reoxidation to form highly pure bsAbs. The technology is compatible with standard large-scale antibody manufacturing and ensures bsAbs with Fcmediated effector functions and in vivo stability typical of IgG1 antibodies. Proof-of-concept studies with HER2×CD3 (T-cell recruitment) and HER2×HER2 (dual epitope targeting) bsAbs demonstrate superior in vivo activity compared with parental antibody pairs.immunotherapy | pharmacokinetics | anti-tumor

Two humanized IgG4 antibodies, natalizumab and gemtuzumab, are approved for human use, and several others, like TGN1412, are or have been in clinical development. Although IgG4 antibodies can dynamically exchange half-molecules, Fab-arm exchange with therapeutic antibodies has not been demonstrated in humans. Here, we show that natalizumab exchanges Fab arms with endogenous human IgG4 in natalizumab-treated individuals. Gemtuzumab, in contrast, contains an IgG4 core-hinge mutation that blocks Fab-arm exchange to undetectable levels both in vitro and in a mouse model. The ability of IgG4 therapeutics to recombine with endogenous IgG4 may affect their pharmacokinetics and pharmacodynamics. Although pharmacokinetic modeling lessens concerns about undesired cross-linking under normal conditions, unpredictability remains and mutations that completely prevent Fab-arm exchange in vivo should be considered when designing therapeutic IgG4 antibodies.