Impact of structural modifications of IgG antibodies on effector functions

Immunoglobulin G (IgG) antibodies are a critical component of the adaptive immune system, binding to and neutralizing pathogens and other foreign substances. Recent advances in molecular antibody biology and structural protein engineering enabled the modification of IgG antibodies to enhance their therapeutic potential. This review summarizes recent progress in both natural and engineered structural modifications of IgG antibodies, including allotypic variation, glycosylation, Fc engineering, and Fc gamma receptor binding optimization. We discuss the functional consequences of these modifications to highlight their potential for therapeutical applications.

The Influence of Human IgG Subclass and Allotype on Complement Activation

Complement activation via the classical pathway is initiated when oligomeric Igs on target surfaces are recognized by C1 of the complement cascade. The strength of this interaction and activation of the complement system are influenced by structural variation of the Ab, including Ab isotype, subclass, and glycosylation profile. Polymorphic variants of IgG have also been described to influence Fc-dependent effector functions. Therefore, we assessed complement binding, deposition, and complement-dependent cytotoxicity (CDC) of 27 known IgG allotypes with anti-trinitrophenyl specificity. Differences between allotypes within subclasses were minor for IgG1, IgG3, and IgG4 allotypes, and more substantial for IgG2. Allelic variant IGHG2*06, containing a unique serine at position 378 in the CH3 domain, showed less efficient complement activation and CDC compared with other IgG2 polymorphisms. We also observed variable cell lysis between IgG1 and IgG3, with IgG3 being superior in lysis of human RBCs and Ramos cells, and IgG1 being superior in lysis of Raji and Wien133 cells, demonstrating that a long-standing conundrum in the literature depends on cellular context. Furthermore, we compared IgG1 and IgG3 under different circumstances, showing that Ag density and Ab hinge length, but not complement regulators, define the context dependency of Ab-mediated CDC activity. Our results point toward a variation in the capacity of IgG subclasses to activate complement due to single amino acid changes and hinge length differences of allotypes to activate complement, which might give new insights on susceptibility to infectious, alloimmune, or autoimmune diseases and aid the design of Ab-based therapeutics.

Mutation of the TGN1412 anti-CD28 monoclonal antibody lower hinge confers specific FcγRIIb binding and retention of super-agonist activity

The agonistic action of several immunomodulatory monoclonal antibodies (mAbs) requires both target antigen binding and clustering of this mAb:target complex by the Fcs interacting with Fcγ receptors (FcγRs), in particular FcγRIIb, on neighboring bystander cells. Fc mutations were made in the immunoglobulin G4 (IgG4)-based TGN1412 anti-CD28 mAb to define the role of FcγR interactions in its "super-agonist" activity. The dual mutation, IgG4-ED269,270 AA, ablated interaction with all human FcγRs and agonistic action was consequentially lost, confirming the FcγR dependence on the action of TGN1412. The IgG4 lower hinge region (F234 L235 G236 G237 ) was modified by L235 E mutation (F234 E235 G236 G237 ), a mutation commonly used to ablate FcγR binding, including in approved therapeutic mAbs. However, rather than ablating all FcγR binding, IgG4-L235 E conferred specific binding to FcγRIIb, the inhibitory Fc receptor. Furthermore, in combination with the core hinge-stabilizing mutation (IgG4-S228 P, L235 E), this mutation increased affinity for FcγRIIb compared with wild-type IgG4. In addition to having FcγRIIb specificity, these engineered TGN1412 antibodies retained their super-agonistic ability, demonstrating that CD28- and FcγRIIb-specific binding are together sufficient for agonistic function. The FcγRIIb-specific nature of IgG4-L235 E has utility for mAb-mediated immune agonism therapies that are dependent on FcγRIIb interaction and of anti-inflammatory mAbs in allergy and autoimmunity that harness FcγRIIb inhibitory signaling.

Targeting two distinct epitopes on human CD73 with a bispecific antibody improves anticancer activity

Background

Immunosuppressive extracellular adenosine is generated by the enzymatic activity of CD73. In preclinical models, antibodies (Abs) targeting different epitopes on CD73 exert anticancer activity through distinct mechanisms such as inhibition of enzymatic activity, engagement of Fc receptors, and spatial redistribution of CD73.

Methods

Using controlled Fab arm exchange, we generated biparatopic bispecific antibodies (bsAbs) from parental anti-CD73 Abs with distinct anticancer activities. The resulting anticancer activity was evaluated using in vitro and in vivo models.

Results

We demonstrate that different anticancer activities can be combined in a biparatopic bsAb. Remarkably, the bsAb significantly improved the enzyme inhibitory activity compared with the parental Abs, which led to neutralization of adenosine-mediated T-cell suppression as demonstrated by proliferation and interferon gamma (IFN-γ) production and prolonged survival of tumor-bearing mice. Additionally, the bsAb caused more efficient internalization of cell surface CD73 and stimulated potent Fc-mediated engagement of human immune effector cells in vitro and in vivo.

Conclusions

Our data collectively demonstrate that complementary anticancer mechanisms of action of distinct anti-CD73 Abs can be combined and enhanced in a biparatopic bsAb. The multiple mechanisms of action and superior activity compared with the monospecific parental Abs make the bsAb a promising candidate for therapeutic targeting of CD73 in cancer. This concept may greatly improve future Ab design.

Overcoming Challenges for CD3-Bispecific Antibody Therapy in Solid Tumors

Immunotherapy of cancer with CD3-bispecific antibodies is an approved therapeutic option for some hematological malignancies and is under clinical investigation for solid cancers. However, the treatment of solid tumors faces more pronounced hurdles, such as increased on-target off-tumor toxicities, sparse T-cell infiltration and impaired T-cell quality due to the presence of an immunosuppressive tumor microenvironment, which affect the safety and limit efficacy of CD3-bispecific antibody therapy. In this review, we provide a brief status update of the CD3-bispecific antibody therapy field and identify intrinsic hurdles in solid cancers. Furthermore, we describe potential combinatorial approaches to overcome these challenges in order to generate selective and more effective responses.

FcγR Binding and ADCC Activity of Human IgG Allotypes

Antibody dependent cellular cytotoxicity (ADCC) is an Fc-dependent effector function of IgG important for anti-viral immunity and anti-tumor therapies. NK-cell mediated ADCC is mainly triggered by IgG-subclasses IgG1 and IgG3 through the IgG-Fc-receptor (FcγR) IIIa. Polymorphisms in the immunoglobulin gamma heavy chain gene likely form a layer of variation in the strength of the ADCC-response, but this has never been studied in detail. We produced all 27 known IgG allotypes and assessed FcγRIIIa binding and ADCC activity. While all IgG1, IgG2, and IgG4 allotypes behaved similarly within subclass, large allotype-specific variation was found for IgG3. ADCC capacity was affected by residues 291, 292, and 296 in the CH2 domain through altered affinity or avidity for FcγRIIIa. Furthermore, allotypic variation in hinge length affected ADCC, likely through altered proximity at the immunological synapse. Thus, these functional differences between IgG allotypes have important implications for therapeutic applications and susceptibility to infectious-, allo- or auto-immune diseases.

DuoBody-CD3xCD20 induces potent T-cell-mediated killing of malignant B cells in preclinical models and provides opportunities for subcutaneous dosing

Background

DuoBody®-CD3xCD20 (GEN3013) is a full-length human IgG1 bispecific antibody (bsAb) recognizing CD3 and CD20, generated by controlled Fab-arm exchange. Its Fc domain was silenced by introduction of mutations L234F L235E D265A.

Methods

T-cell activation and T-cell-mediated cytotoxicity were measured by flow cytometry following co-culture with tumour cells. Anti-tumour activity of DuoBody-CD3xCD20 was assessed in humanized mouse models in vivo. Non-clinical safety studies were performed in cynomolgus monkeys.

Findings

DuoBody-CD3xCD20 induced highly potent T-cell activation and T-cell-mediated cytotoxicity towards malignant B cells in vitro. Comparison of DuoBody-CD3xCD20 to CD3 bsAb targeting alternative B-cell antigens, or to CD3xCD20 bsAb generated using alternative CD20 Ab, emphasized its exceptional potency. In vitro comparison with other CD3xCD20 bsAb in clinical development showed that DuoBody-CD3xCD20 was significantly more potent than three other bsAb with single CD3 and CD20 binding regions and equally potent as a bsAb with a single CD3 and two CD20 binding regions. DuoBody-CD3xCD20 showed promising anti-tumour activity in vivo, also in the presence of excess levels of a CD20 Ab that competes for binding. In cynomolgus monkeys, DuoBody-CD3xCD20 demonstrated profound and long-lasting B-cell depletion from peripheral blood and lymphoid organs, which was comparable after subcutaneous and intravenous administration. Peak plasma levels of DuoBody-CD3xCD20 were lower and delayed after subcutaneous administration, which was associated with a reduction in plasma cytokine levels compared to intravenous administration, while bioavailability was comparable.

Interpretation

Based on these preclinical studies, a clinical trial was initiated to assess the clinical safety of subcutaneous DuoBody-CD3xCD20 in patients with B-cell malignancies.

Funding

Genmab

CD3-Bispecific Antibody Therapy Turns Solid Tumors into Inflammatory Sites but Does Not Install Protective Memory

Immunotherapy of cancer with CD3-targeting bispecific antibodies (CD3 bsAb) is a fast developing field, and multiple tumor-associated antigens (TAA) are evaluated for hematologic and solid malignancies. The efficacy of these CD3 bsAb is usually examined in xenograft mouse tumor models with human T cells or in genetically engineered mouse models, where human TAA are introduced. These models often fail to fully recapitulate the natural tumor environment, especially for solid cancers, because of interspecies differences. Here, we investigated the systemic and intratumoral effects of a mouse CD3 bsAb in a fully immune-competent mouse melanoma model. Systemic administration of 0.5 mg/kg antibody induced a brief overall T-cell activation that was selectively sustained in the tumor microenvironment for several days. A fast subsequent influx of inflammatory macrophages into the tumor microenvironment was observed, followed by an increase in the number of CD4⁺ and CD8⁺ T cells. Although the capacity to directly kill melanoma cells in vitro was very modest, optimal tumor elimination was observed in vivo, even in the absence of CD8⁺ T cells, implying a redundancy in T-cell subsets for therapeutic efficacy. Finally, we took advantage of the full immune competence of our mouse model and tested immune memory induction. Despite a strong initial immunity against melanoma, treatment with the CD3 bsAb did not install protective memory responses. The observed mechanisms of action revealed in this immune-competent mouse model might form a rational basis for combinatorial approaches.

Kinetic mechanism of controlled Fab-arm exchange for the formation of bispecific immunoglobulin G1 antibodies

Bispecific antibodies (bsAbs) combine the antigen specificities of two distinct Abs and demonstrate therapeutic promise based on novel mechanisms of action. Among the many platforms for creating bsAbs, controlled Fab-arm exchange (cFAE) has proven useful based on minimal changes to native Ab structure and the simplicity with which bsAbs can be formed from two parental Abs. Despite a published protocol for cFAE and its widespread use in the pharmaceutical industry, the reaction mechanism has not been determined. Knowledge of the mechanism could lead to improved yields of bsAb at faster rates as well as foster adoption of process control. In this work, a combination of Förster resonance energy transfer (FRET), nonreducing SDS-PAGE, and strategic mutation of the Ab hinge region was employed to identify and characterize the individual steps of cFAE. Fluorescence correlation spectroscopy (FCS) was used to determine the affinity of parental (homodimer) and bispecific (heterodimer) interactions within the C_H3 domain, further clarifying the thermodynamic basis for bsAb formation. The result is a clear sequence of events with rate constants that vary with experimental conditions, where dissociation of the K409R parental Ab into half-Ab controls the rate of the reaction.

Efficient Generation of Bispecific Murine Antibodies for Pre-Clinical Investigations in Syngeneic Rodent Models

Therapeutic concepts exploiting tumor-specific antibodies are often established in pre-clinical xenograft models using immuno-deficient mice. More complex therapeutic paradigms, however, warrant the use of immuno-competent mice, that more accurately capture the relevant biology that is being exploited. These models require the use of (surrogate) mouse or rat antibodies to enable optimal interactions with murine effector molecules. Immunogenicity is furthermore decreased, allowing longer-term treatment. We recently described controlled Fab-arm exchange (cFAE) as an easy-to-use method for the generation of therapeutic human IgG1 bispecific antibodies (bsAb). To facilitate the investigation of dual-targeting concepts in immuno-competent mice, we now applied and optimized our method for the generation of murine bsAbs. We show that the optimized combinations of matched point-mutations enabled efficient generation of murine bsAbs for all subclasses studied (mouse IgG1, IgG2a and IgG2b; rat IgG1, IgG2a, IgG2b, and IgG2c). The mutations did not adversely affect the inherent effector functions or pharmacokinetic properties of the corresponding subclasses. Thus, cFAE can be used to efficiently generate (surrogate) mouse or rat bsAbs for pre-clinical evaluation in immuno-competent rodents.

Molecular Basis of Assembly and Activation of Complement Component C1 in Complex with Immunoglobulin G1 and Antigen

The classical complement pathway contributes to the natural immune defense against pathogens and tumors. IgG antibodies can assemble at the cell surface into hexamers via Fc:Fc interactions, which recruit complement component C1q and induce complement activation. Biophysical characterization of the C1:IgG complex has remained elusive primarily due to the low affinity of IgG-C1q binding. Using IgG variants that dynamically form hexamers efficient in C1q binding and complement activation, we could assess C1q binding in solution by native mass spectrometry and size-exclusion chromatography. Fc-domain deglycosylation, described to abrogate complement activation, affected IgG hexamerization and C1q binding. Strikingly, antigen binding by IgG hexamers or deletion of the Fab arms substantially potentiated complement initiation, suggesting that Fab-mediated effects impact downstream Fc-mediated events. Finally, we characterized a reconstituted 2,045.3 ± 0.4-kDa complex of intact C1 bound to antigen-saturated IgG hexamer by native mass spectrometry, providing a clear visualization of a complete complement initiation complex.

Opening the door to innovation

Open innovation is the new buzz, with initiatives popping up left and right. Here, we give a personal perspective on a very successful, knowledge-driven innovation initiated in an academia- industry alliance, which culminated in technology platforms that enable the generation of therapeutic antibodies with novel properties. To start, we provide a general background on open innovation in the drug development field.

Production of stable bispecific IgG1 by controlled Fab-arm exchange: scalability from bench to large-scale manufacturing by application of standard approaches

The manufacturing of bispecific antibodies can be challenging for a variety of reasons. For example, protein expression problems, stability issues, or the use of non-standard approaches for manufacturing can result in poor yield or poor facility fit. In this paper, we demonstrate the use of standard antibody platforms for large-scale manufacturing of bispecific IgG1 by controlled Fab-arm exchange. Two parental antibodies that each contain a single matched point mutation in the CH3 region were separately expressed in Chinese hamster ovary cells and manufactured at 1000 L scale using a platform fed-batch and purification process that was designed for standard antibody production. The bispecific antibody was generated by mixing the two parental molecules under controlled reducing conditions, resulting in efficient Fab-arm exchange of >95% at kg scale. The reductant was removed via diafiltration, resulting in spontaneous reoxidation of interchain disulfide bonds. Aside from the bispecific nature of the molecule, extensive characterization demonstrated that the IgG1 structural integrity was maintained, including function and stability. These results demonstrate the suitability of this bispecific IgG1 format for commercial-scale manufacturing using standard antibody manufacturing techniques.

Mutation of Y407 in the CH3 domain dramatically alters glycosylation and structure of human IgG

Antibody engineering is increasingly being used to influence the properties of monoclonal antibodies to improve their biotherapeutic potential. One important aspect of this is the modulation of glycosylation as a strategy to improve efficacy. Here, we describe mutations of Y407 in the CH3 domain of IgG1 and IgG4 that significantly increase sialylation, galactosylation, and branching of the N-linked glycans in the CH2 domain. These mutations also promote the formation of monomeric assemblies (one heavy-light chain pair). Hydrogen-deuterium exchange mass spectrometry was used to probe conformational changes in IgG1-Y407E, revealing, as expected, a more exposed CH3–CH3 dimerization interface. Additionally, allosteric structural effects in the CH2 domain and in the CH2–CH3 interface were identified, providing a possible explanation for the dramatic change in glycosylation. Thus, the mutation of Y407 in the CH3 domain remarkably affects both antibody conformation and glycosylation, which not only alters our understanding of antibody structure, but also reveals possibilities for obtaining recombinant IgG with glycosylation tailored for clinical applications.

Quantitative analysis of the interaction strength and dynamics of human IgG4 half molecules by native mass spectrometry

Native mass spectrometry (MS) is a powerful technique for studying noncovalent protein-protein interactions. Here, native MS was employed to examine the noncovalent interactions involved in homodimerization of antibody half molecules (HL) in hinge-deleted human IgG4 (IgG4Δhinge). By analyzing the concentration dependence of the relative distribution of monomer HL and dimer (HL)(2) species, the apparent dissociation constant (K(D)) for this interaction was determined. In combination with site-directed mutagenesis, the relative contributions of residues at the CH3-CH3 interface to this interaction could be characterized and corresponding K(D) values quantified over a range of 10(-10)-10(-4) M. The critical importance of this noncovalent interaction in maintaining the intact dimeric structure was also proven for the full-length IgG4 backbone. Using time-resolved MS, the kinetics of the interaction could be measured, reflecting the dynamics of IgG4 HL exchange. Hence, native MS has provided a quantitative view of local structural features that define biological properties of IgG4.

Therapeutic IgG4 antibodies engage in Fab-arm exchange with endogenous human IgG4 in vivo

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.

When binding is enough: nonactivating antibody formats

Most therapeutic antibodies currently used in the clinic are based on the human IgG1 format, which is a bivalent molecule that efficiently interacts with the immune system's effector functions. In clinical applications where binding to the target alone is sufficient for therapeutic efficacy; however, engagement of the immune system is not required and may even cause unwanted side-effects. Likewise, bivalent binding to the target may negatively influence the therapeutic efficacy of an antibody. Here we discuss the state of the art for antibody-based therapeutics, designed to be nonactivating (i.e. do not engage the innate immune system's effector functions), in both monovalent and bivalent formats.

In vivo blockade of OX40 ligand inhibits thymic stromal lymphopoietin driven atopic inflammation

Thymic stromal lymphopoietin (TSLP) potently induces deregulation of Th2 responses, a hallmark feature of allergic inflammatory diseases such as asthma, atopic dermatitis, and allergic rhinitis. However, direct downstream in vivo mediators in the TSLP-induced atopic immune cascade have not been identified. In our current study, we have shown that OX40 ligand (OX40L) is a critical in vivo mediator of TSLP-mediated Th2 responses. Treating mice with OX40L-blocking antibodies substantially inhibited immune responses induced by TSLP in the lung and skin, including Th2 inflammatory cell infiltration, cytokine secretion, and IgE production. OX40L-blocking antibodies also inhibited antigen-driven Th2 inflammation in mouse and nonhuman primate models of asthma. This treatment resulted in both blockade of the OX40-OX40L receptor-ligand interaction and depletion of OX40L-positive cells. The use of a blocking, OX40L-specific mAb thus presents a promising strategy for the treatment of allergic diseases associated with pathologic Th2 immune responses.

Antigen selection from an HIV-1 immune antibody library displayed on yeast yields many novel antibodies compared to selection from the same library displayed on phage

Phage display of antibody libraries has been widely used for over a decade to generate monoclonal antibodies. Yeast display has been developed more recently. Here the two approaches were directly compared using the same HIV-1 immune scFv cDNA library expressed in phage and yeast display vectors and using the same selecting antigen (HIV-1 gp120). Yeast display was shown to sample the immune antibody repertoire considerably more fully than phage display, selecting all the scFv identified by phage display and twice as many novel antibodies. Positive phage display selection appeared to largely reflect those antibodies that as phage-scFv gave the highest signal in phage ELISAs assessing antigen binding. This signal is thought to reflect the efficiency of expression of folded scFv at the phage surface. Increased access to immune repertoires may increase the rescue of novel antibodies of therapeutic or analytical value that often form a minor part of a typical antibody response.

Crystal Structure of the Broadly Cross-Reactive HIV-1-Neutralizing Fab X5 and Fine Mapping of Its Epitope,

The human monoclonal antibody Fab X5 neutralizes a broad range of HIV-1 primary isolates. The crystal structure of X5 has been determined at 1.9 A resolution. There are two crystallographically independent Fab fragments in the asymmetric unit. The crystallographic R value for the final model is 0.22. The antibody-combining site features a long (22 amino acid residues) CDR H3 with a protruding hook-shaped motif. The X5 structure and site-directed mutagenesis data suggest that X5 amino acid residues W100 and Y100F in the CDR H3 motif may be critical for the binding of Fab X5 to gp120. X5 bound to a complex of a CD4 mimetic and gp120 with approximately the same kinetics and affinity as to a CD4-gp120 complex, suggesting that specific interactions between CD4 and X5 are unlikely to contribute to the binding of X5 to gp120-CD4 complexes. Binding of X5 to alanine scanning mutants of gp120JR-CSF complexed with CD4 suggested a critical role of the highly conserved amino acid residues at positions 423 and 432. The X5 structure and fine mapping of its epitope may assist in the elucidation of the mechanisms of viral entry and neutralization, and the development of HIV-1 inhibitors and vaccines.

Improved Breadth and Potency of an HIV-1-Neutralizing Human Single-chain Antibody by Random Mutagenesis and Sequential Antigen Panning

Several human monoclonal antibodies can neutralize a range of human immunodeficiency virus type 1 (HIV-1) primary isolates but their potency and related ability to suppress generation of HIV-1 escape mutants is significantly lower than the activity of antiretroviral drugs currently in clinical use. Recently, a human Fab, X5, was identified and found to neutralize primary isolates from different clades. Further improvement of the potency and breadth of HIV-1 neutralization by this antibody could be critical for its potential use in the treatment of HIV-1-infected patients. However, increasing potency of an antibody by selection from libraries may lead to a decrease in the breadth of neutralization. In an attempt to solve this problem, we subjected a random mutagenesis library of the scFv X5 to sequential rounds of selection on non-homologous HIV-1 envelope glycoproteins (Envs) dubbed sequential antigen panning (SAP). By using SAP, we identified two scFv antibodies, m6 and m9, that were tested with a panel of 33 diverse primary HIV-1 infectious isolates in an assay based on a reporter cell-line expressing high levels of CD4, CCR5 and CXCR4. The IC(50) was less than 50 microg/ml for 21 (m6) and 19 (m9) out of 29 isolates from group M (subtypes A-C, F, G and CRF-01AE) and one isolate from group N; three isolates from group O were not significantly inhibited at 50 microg/ml. The average IC(50) values for the two antibodies were significantly (p<0.001, n=29) lower compared to scFv X5. Their inhibitory activity does not appear to be related to the HIV-1 subtype, coreceptor usage or the disease stage. m9 inhibited infection of peripheral blood mononuclear cells by the primary isolates JRCSF, 89.6 and BR020 with IC(90) of 4, 6 and 25 microg/ml, respectively; for a single-round infection by pseudovirus, the IC(90) for JRSCF, 89.6, YU2 and HXBc2 was 15, 5, 15 and 5 microg/ml, respectively. In these two assays the IC(90) for m9 was, on average, two- to threefold lower than for scFv X5. These results demonstrate that both the potency and the breadth of HIV-1 neutralization of one of the few known potent broadly cross-reactive human monoclonal antibodies, scFv X5, could be improved significantly. However, only experiments in animal models and clinical trials in humans will show whether these new scFvs and the approach for their identification have potential in the development of prophylactics and therapeutics for HIV-1 infections.

Access of antibody molecules to the conserved coreceptor binding site on glycoprotein gp120 is sterically restricted on primary human immunodeficiency virus type 1

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.

A novel human antibody against human immunodeficiency virus type 1 gp120 is V1, V2, and V3 loop dependent and helps delimit the epitope of the broadly neutralizing antibody immunoglobulin G1 b12

The V1/V2 and V3 loops are proximal to the CD4 binding site (CD4bs) of human immunodeficiency virus type 1 (HIV-1) gp120 and undergo conformational change upon CD4 receptor engagement by the HIV-1 envelope spike. Nearly all of the reported monoclonal antibodies (MAbs) against the CD4bs exhibit a very limited capacity to neutralize HIV-1. However, one such human MAb, immunoglobulin G1 (IgG1) b12, is uniquely able to neutralize primary isolates across subtypes with considerable potency. The molecular basis for the anti-HIV-1 activity of b12 is not fully understood but is relevant to vaccine design. Here we describe a novel human MAb, 4KG5, whose binding to monomeric gp120 is moderately enhanced by IgG1 b12. In sharp contrast, 4KG5 binding to gp120 is inhibited by soluble CD4 (sCD4) and by all other (n = 14) anti-CD4bs MAbs tested. 4KG5 is unable to recognize gp120 in which either V1, V2, or V3 has been deleted, and MAbs against the V2 or V3 loops inhibit the binding of 4KG5 to gp120. Moreover, 4KG5 is able to inhibit the binding of the CD4-induced MAbs 17b and X5 in the absence of sCD4, whereas 17b and X5 only weakly inhibit the binding of 4KG5 to gp120. Mutagenesis of gp120 provides further evidence of a discontinuous epitope of 4KG5 that is formed by the V1/V2 loop, the V3 loop, and a portion of the bridging sheet (C4). 4KG5 was isolated as a single-chain Fv from a phage display library constructed from the bone marrow of an HIV-1-seropositive subject (FDA2) whose serum neutralizes HIV-1 across subtypes. Despite its source, we observed no significant neutralization with 4KG5 against the autologous (R2) virus and several other strains of HIV-1. The results suggest a model in which antibody access to the CD4bs on the envelope spike of HIV-1 is restricted by the orientation and/or dynamics of the V1/V2 and V3 loops, and b12 avoids these restrictions.

Broadly cross-reactive HIV-1-neutralizing human monoclonal Fab selected for binding to gp120-CD4-CCR5 complexes

HIV-1 entry into cells involves formation of a complex between gp120 of the viral envelope glycoprotein (Env), a receptor (CD4), and a coreceptor, typically CCR5. Here we provide evidence that purified gp120(JR-FL)-CD4-CCR5 complexes exhibit an epitope recognized by a Fab (X5) obtained by selection of a phage display library from a seropositive donor with a relatively high broadly neutralizing serum antibody titer against an immobilized form of the trimolecular complex. X5 bound with high (nM) affinity to a variety of Envs, including primary isolates from different clades and Envs with deleted variable loops (V1, -2, -3). Its binding was significantly increased by CD4 and slightly enhanced by CCR5. X5 inhibited infection of peripheral blood mononuclear cells by a selection of representative HIV-1 primary isolates from clades A, B, C, D, E, F, and G with an efficiency comparable to that of the broadly neutralizing antibody IgG1 b12. Furthermore, X5 inhibited cell fusion mediated by Envs from R5, X4, and R5X4 viruses. Of the five broadly cross-reactive HIV-1-neutralizing human monoclonal antibodies known to date, X5 is the only one that exhibits increased binding to gp120 complexed with receptors. These findings suggest that X5 could possibly be used as entry inhibitor alone or in combination with other antiretroviral drugs for the treatment of HIV-1-infected individuals, provide evidence for the existence of conserved receptor-inducible gp120 epitopes that can serve as targets for potent broadly cross-reactive neutralizing antibodies in HIV-1-infected patients, and have important conceptual and practical implications for the development of vaccines and inhibitors.

Novel strategy for the selection of human recombinant Fab fragments to membrane proteins from a phage-display library

Traditionally, the selection of phage-display libraries is performed on purified antigens (Ags), immobilized to a solid substrate. However, this approach may not be applicable for some Ags, such as membrane proteins, which for structural integrity strongly rely on their native environment. Here we describe an approach for the selection of phage-libraries against membrane proteins. The envelope glycoproteins (Env) of the Human Immunodeficiency Virus type-1 (HIV-1) were used as a model for a type-1 integral membrane protein. HIV-1IHI Env, expressed on the surface of Rabbit Kidney cells (RK13) with a recombinant vaccinia virus (rVV), was solubilized using the non-ionic detergent n-Octyl beta-D-glucopyranoside (OG). Membrane associated Env was reconstituted into vesicles by the simultaneous removal of detergent and free monomeric Env subunits by gel-filtration. The resulting antigen preparation, termed OG-P1IHI, was captured on microtiter plates coated with Galanthus nivalis agglutinin (GNA) and used for rounds of selection (panning) of a well-characterized phage-display library derived from an HIV-1 seropositive donor. Simultaneously, an identical experiment was performed with OG-P1IHI vesicles disrupted by Nonidet P-40 (NP-P1IHI). Both membrane-associated and soluble Ags were selected for vaccinia-specific clones (OG-P1IHI: 59/75 and NP-P1IHI: 1/75) and HIV-1-specific clones (OG-P1IHI: 11/75 and NP-P1IHI: 65/75) using our approach. Hence, the novel panning strategy described here may be applicable for selection of phage-libraries against membrane proteins.

Broadly neutralizing antibodies targeted to the membrane-proximal external region of human immunodeficiency virus type 1 glycoprotein gp41

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.

Recognition properties of V3-specific antibodies to V3 loop peptides derived from HIV-1 gp120 presented in multiple conformations

To identify structural constraints and amino acid sequences important for antibody recognition of the third variable domain (V3) of HIV-1 gp120, we have studied the solution conformation of three 35-mer circular V3 loop peptides derived from HIV-1 strains which differ in syncytium- (SI) and non-syncytium-inducing (NSI) capacity. In addition to 2D NMR and CD analyses, fluid- and solid-phase immunoassays were performed using V3-specific antibodies to V3 peptides and gp120 derived from different strains of HIV-1. NMR and CD spectroscopy indicated that circular and linear V3 loops exist in water as a dynamic ensemble of multiple conformations. Amino acid substitutions and biochemical modifications of the V3 loop were found to affect antibody binding depending on the presentation of the antigens. From NMR observations and immunological experiments, we provide evidence for a V3 loop specific monoclonal antibody interaction which is directed predominantly against linear epitopes rather than against discontinuous epitopes. The absence of a single defined solution conformation of 35-mer circular V3 peptides should be taken into account when using V3-related peptides to investigate structural elements in the V3 domain of the gp120 envelope protein of HIV-1 involved in biological processes of the virus.

In vitro antigen challenge of human antibody libraries for vaccine evaluation: the human immunodeficiency virus type 1 envelope

Human antibody responses, or versions thereof, can be cloned as phage display libraries. In vaccine evaluation, the possibility therefore exists of challenging the human response in vitro, rather than in vivo, in order to assist in establishing the most promising vaccine leads. The characteristics of the antibodies retrieved directly indicate the strengths and weaknesses of the vaccine at the molecular level. We applied this approach to compare recombinant and native human immunodeficiency virus type 1 envelope preparations. We conclude that recombinant gp160, gp140, and, to a lesser extent, gp120 present epitopes around the CD4 binding site in a conformation different from that of the native multimer and contrary to expected vaccine requirements. Antibodies to the potently neutralizing b12 epitope were selected preferentially from an immune library by purified human immunodeficiency virus type 1 virions. This suggests that b12 is a major epitope on the virions, in contrast to recombinant envelope preparations, in which related, weakly neutralizing epitopes predominate. Although the majority of virions in the preparation used are expected to be noninfective, it appears that they predominantly express a native envelope configuration and would be able to elicit potent neutralizing antibodies.