Structure and specificity of an antibody targeting a proteolytically cleaved IgG hinge

Impairment of IgG Fc functions promotes tumor progression and suppresses NK cell antitumor actions

Natural killer (NK) cells mediate antibody dependent cytotoxic killing of cancer cells via cross-linking FcγR on NK cells with IgG-Fc. Studies have shown that the single-hinge cleaved IgGs (scIgGs) have dysfunctional Fc and failed engagement with FcγRs on immune cells. However, little is known about how scIgGs impact on antitumor immunity in the tumor microenvironment. In this study, we revealed a significant association of tumor scIgGs with tumor progression and poor outcomes of breast cancer patients (n = 547). Using multiple mouse tumor models, we demonstrated that tumor scIgGs reduced NK cell cytotoxic activities and resulted in aggressive tumor progression. We further showed that an anti-hinge specific monoclonal antibody (AHA) rescued the dysfunctional Fc in scIgGs by providing a functional Fc and restored NK cell cytotoxic activity. These findings point to a novel immunotherapeutic strategy to enhance Fc engagement with FcγRs for activation of anticancer immunity.

Proteinase-nicked IgGs: an unanticipated target for tumor immunotherapy

The host immune system adopts multiple mechanisms involving antibodies to confront cancer cells. Accordingly, anti-tumor mAbs have become mainstays in cancer treatment. However, neither host immunity nor mAb therapies appear capable of controlling tumor growth in all cases. Structural instability of IgG was overlooked as a factor contributing to immunosuppression in the tumor microenvironment. Recently, physiological proteinases were identified that disable IgG immune effector functions. Evidence shows that these proteinases cause localized IgG impairment by selective cleavage of a single IgG peptide bond in the hinge-region. The recognition of IgG cleavage in the tumor microenvironment provides alternatives for tumor immunotherapy.

Molecular characterization of human anti-hinge antibodies derived from single-cell cloning of normal human B cells

Anti-hinge antibodies (AHAs) are an autoantibody subclass that, following proteolytic cleavage, recognize cryptic epitopes exposed in the hinge regions of immunoglobulins (Igs) and do not bind to the intact Ig counterpart. AHAs have been postulated to exacerbate chronic inflammatory disorders such as inflammatory bowel disease and rheumatoid arthritis. On the other hand, AHAs may protect against invasive microbial pathogens and cancer. However, despite more than 50 years of study, the origin and specific B cell compartments that express AHAs remain elusive. Recent research on serum AHAs suggests that they arise during an active immune response, in contrast to previous proposals that they derive from the preexisting immune repertoire in the absence of antigenic stimuli. We report here the isolation and characterization of AHAs from memory B cells, although anti-hinge-reactive B cells were also detected in the naive B cell compartment. IgG AHAs cloned from a single human donor exhibited restricted specificity for protease-cleaved F(ab')₂ fragments and did not bind the intact IgG counterpart. The cloned IgG-specific AHA-variable regions were mutated from germ line-derived sequences and displayed a high sequence variability, confirming that these AHAs underwent class-switch recombination and somatic hypermutation. Consistent with previous studies of serum AHAs, several of these clones recognized a linear, peptide-like epitope, but one clone was unique in recognizing a conformational epitope. All cloned AHAs could restore immune effector functions to proteolytically generated F(ab')₂ fragments. Our results confirm that a diverse set of epitope-specific AHAs can be isolated from a single human donor.

Efficient generation of single domain antibodies with high affinities and enhanced thermal stabilities

Single domain antibodies (sdAbs), made of natural single variable regions of camelid or cartilaginous fish antibodies, or unpaired variable regions of mouse or human IgGs, are some of the more promising biologic modalities. However, such conventional sdAbs have difficulties of either using unwieldy animals for immunization or having high affinity deficiencies. Herein, we offer a versatile method to generate rabbit variable domain of heavy chain (rVH) derived sdAbs with high affinities (K_D values of single digit nM or less) and enhanced thermal stabilities (equal to or even higher than those of camelid derived sdAbs). It was found that a variety of rVH binders, including those with high affinities, were efficiently acquired using an rVH-displaying phage library produced at a low temperature of 16 °C. By a simple method to introduce an additional disulfide bond, their unfolding temperatures were increased by more than 20 °C without severe loss of binding affinity. Differential scanning calorimetry analysis suggested that this highly efficient thermal stabilization was mainly attributed to the entropic contribution and unique thermodynamic character of the rVHs.

Generation of therapeutic immunoconjugates via Residue-Specific Conjugation Technology (RESPECT) utilizing a native cysteine in the light chain framework of Oryctolagus cuniculus

The conjugation of toxins, dyes, peptides, or proteins to monoclonal antibodies is often performed via free thiol groups generated by either partial reduction methods or engineering free cysteine residues into the antibody sequence. Antibodies from the rabbit Oryctolagus cuniculus have an additional intrachain disulfide bond, whereby the light chain variable kappa domain is bridged to the constant kappa region between cysteine residues at positions 80 and 171, respectively. Chimerization of rabbit antibodies with human constant domains allows for the generation of a free thiol group at the light chain position 80 (C80) that can be used for site-specific conjugation. An efficient process for the purification and simultaneous removal of cysteinylation at the C80 site was developed. The unpaired C80 was shown to be efficiently conjugated using several different maleimido-based ligands. REsidue SPEcific Conjugation Technology (RESPECT) antibody-drug conjugates prepared using rabbit-human chimeric anti-human mesothelin rabbit antibodies and maleimido-PEG₂-auristatin conjugated to C80 were shown to be highly potent and specific in vitro and effective in vivo in reduction of tumor growth in a highly aggressive mesothelin-expressing xenograft tumor model.

From rabbit antibody repertoires to rabbit monoclonal antibodies

In this review, we explain why and how rabbit monoclonal antibodies have become outstanding reagents for laboratory research and increasingly for diagnostic and therapeutic applications. Starting with the unique ontogeny of rabbit B cells that affords highly distinctive antibody repertoires rich in in vivo pruned binders of high diversity, affinity and specificity, we describe the generation of rabbit monoclonal antibodies by hybridoma technology, phage display and alternative methods, along with an account of successful humanization strategies.

Humanization of rabbit monoclonal antibodies via grafting combined Kabat/IMGT/Paratome complementarity-determining regions: Rationale and examples

Rabbit monoclonal antibodies (RabMAbs) can recognize diverse epitopes, including those poorly immunogenic in mice and humans. However, there have been only a few reports on RabMAb humanization, an important antibody engineering step usually done before clinical applications are investigated. To pursue a general method for humanization of RabMAbs, we analyzed the complex structures of 5 RabMAbs with their antigens currently available in the Protein Data Bank, and identified antigen-contacting residues on the rabbit Fv within the 6 Angstrom distance to its antigen. We also analyzed the supporting residues for antigen-contacting residues on the same heavy or light chain. We identified "HV4" and "LV4" in rabbit Fvs, non-complementarity-determining region (CDR) loops that are structurally close to the antigen and located in framework 3 of the heavy chain and light chain, respectively. Based on our structural and sequence analysis, we designed a humanization strategy by grafting the combined Kabat/IMGT/Paratome CDRs, which cover most antigen-contacting residues, into a human germline framework sequence. Using this strategy, we humanized 4 RabMAbs that recognize poorly immunogenic epitopes in the cancer target mesothelin. Three of the 4 humanized rabbit Fvs have similar or improved functional binding affinity for mesothelin-expressing cells. Interestingly, 4 immunotoxins composed of the humanized scFvs fused to a clinically used fragment of Pseudomonas exotoxin (PE38) showed stronger cytotoxicity against tumor cells than the immunotoxins derived from their original rabbit scFvs. Our data suggest that grafting the combined Kabat/IMGT/Paratome CDRs to a stable human germline framework can be a general approach to humanize RabMAbs.

Evading pre-existing anti-hinge antibody binding by hinge engineering

Antigen-binding fragments (Fab) and F(ab')₂ antibodies serve as alternative formats to full-length anti-bodies in therapeutic and immune assays. They provide the advantage of small size, short serum half-life, and lack of effector function. Several proteases associated with invasive diseases are known to cleave antibodies in the hinge-region, and this results in anti-hinge antibodies (AHA) toward the neoepitopes. The AHA can act as surrogate Fc and reintroduce the properties of the Fc that are otherwise lacking in antibody fragments. While this response is desired during the natural process of fighting disease, it is commonly unwanted for therapeutic antibody fragments. In our study, we identify a truncation in the lower hinge region of the antibody that maintains efficient proteolytic cleavage by IdeS protease. The resulting neoepitope at the F(ab')₂ C-terminus does not have detectable binding of pre-existing AHA, providing a practical route to produce F(ab')₂ in vitro by proteolytic digestion when the binding of pre-existing AHA is undesired. We extend our studies to the upper hinge region of the antibody and provide a detailed analysis of the contribution of C-terminal residues of the upper hinge of human IgG1, IgG2 and IgG4 to pre-existing AHA reactivity in human serum. While no pre-existing antibodies are observed toward the Fab of IgG2 and IgG4 isotype, a significant response is observed toward most residues of the upper hinge of human IgG1. We identify a T₂₂₅L variant and the natural C-terminal D₂₂₁ as solutions with minimal serum reactivity. Our work now enables the production of Fab and F(ab')₂ for therapeutic and diagnostic immune assays that have minimal reactivity toward pre-existing AHA.

Tumor evasion of humoral immunity mediated by proteolytic impairment of antibody triggered immune effector function

Immune suppression is recognized as a hallmark of cancer and this notion is largely based on studies on cellular immunity. Our recent studies have demonstrated a potential new mechanism of cancer suppression of immunity by impairment of antibody effector function mediated by proteolytic enzymes in the tumor microenvironment.

A peptide immunization approach to counteract a Staphylococcus aureus protease defense against host immunity

Pathogens that induce acute and chronic infections, as well as certain cancers, employ numerous strategies to thwart host cellular and humoral immune defenses. One proposed evasion mechanism against humoral immunity is a localized expression of extracellular proteases that cleave the IgG hinge and disable host IgG functions. Host immunity appears to be prepared to counter such a proteolytic tactic by providing a group of autoantibodies, denoted anti-hinge antibodies that specifically bind to cleaved IgGs and provide compensating functional restoration in vitro. These respective counter-measures highlight the complex interrelationships among pathogens and host immunity and suggested to us a possible means for therapeutic intervention. In this study, we combined an investigation of pathogen-mediated proteolysis of host IgGs with an immunization strategy to boost host anti-hinge antibodies. In a Staphylococcus aureus infection model using an artificial tissue cage (wiffle ball) implanted into rabbits, cleaved rabbit IgGs were detected in abundance in the abscesses of untreated animals early after infection. However, in animals previously immunized with peptide analogs of the cleaved IgG hinge to generate substantial anti-hinge antibody titers, S. aureus colony formation was markedly reduced compared to control animals or those similarly immunized with a scrambled peptide sequence. The results of this study demonstrate that extensive local proteolysis of IgGs occurs in a test abscess setting and that immunization to increase host anti-hinge antibodies provided substantial acute protection against bacterial growth.

Structural Analysis and Optimization of Context-Independent Anti-Hypusine Antibodies

Context-independent anti-hypusine antibodies that bind to the post-translational modification (PTM), hypusine, with minimal dependence on flanking amino acid sequences, were identified. The antibodies bind to both hypusine and deoxyhypusine or selectively to hypusine but not to deoxyhypusine. Phage display was used to further enhance the affinity of the antibodies. Affinity maturation of these anti-hypusine antibodies improved their performance in affinity capture of the only currently known hypusinated protein, eukaryotic translation initiation factor 5A. These anti-hypusine antibodies may have utility in the identification of novel hypusinated proteins. Crystal structures of the corresponding Fab fragments were determined in complex with hypusine- or deoxyhypusine-containing peptides. The hypusine or deoxyhypusine moiety was found to reside in a deep pocket formed between VH and VL domains of the Fab fragments. Interaction between the antibodies and hypusine includes an extensive hydrogen bond network. These are, to our knowledge, the first reported structures of context-independent anti-PTM antibodies in complex with the corresponding PTM.

A monoclonal antibody against hinge-cleaved IgG restores effector function to proteolytically-inactivated IgGs in vitro and in vivo

We report a chimeric monoclonal antibody (mAb) directed to a neo-epitope that is exposed in the IgG lower hinge following proteolytic cleavage. The mAb, designated 2095-2, displays specificity for IdeS-generated F(ab')₂ fragments, but not for full-length IgG or for closely-related F(ab')₂ fragments generated with other proteases. A critical component of the specificity is provided by the C-terminal amino acid of the epitope corresponding to gly-236 in the IgG1 (also IgG4) hinge. By its ability to bind to IdeS-cleaved anti-CD20 mAb, mAb 2095-2 fully restored antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) against WIL2-S cells to the otherwise inactive anti-CD20 IgG1 F(ab')₂ fragment. Similarly, 2095-2 reinstated ADCC against MDA-MB-231 cells to an anti-CD142 IgG1 F(ab')₂ fragment. mAb 2095-2 was also capable of eliciting both CDC and ADCC to IgG4 F(ab')₂ fragments, an IgG subclass that has weaker ADCC and CDC when intact relative to intact IgG1. The in vitro cell-based efficacy of 2095-2 was extended to the in vivo setting using platelets as a cell clearance surrogate. In a canine model, the co-administration of 2095-2 together with IdeS-generated, platelet-targeting anti-CD41/61 F(ab')₂ fragment not only restored platelet clearance, but did so at a rate and extent of clearance that exceeded that of intact anti-CD41/61 IgG at comparable concentrations. To further explore this unexpected amplification effect, we conducted a rat study in which 2095-2 was administered at a series of doses in combination with a fixed dose of anti-CD41/61 F(ab')₂ fragments. Again, the combination, at ratios as low as 1:10 (w/w) 2095-2 to F(ab')₂, proved more effective than the anti-CD41/61 IgG1 alone. These findings suggest a novel mechanism for enhancing antibody-mediated cell-killing effector functions with potential applications in pathologic settings such as tumors and acute infections where protease activity is abundant.

Second antibody modeling assessment (AMA-II)

To assess the state of the art in antibody 3D modeling, 11 unpublished high-resolution x-ray Fab crystal structures from diverse species and covering a wide range of antigen-binding site conformations were used as a benchmark to compare Fv models generated by seven structure prediction methodologies. The participants included: Accerlys Inc, Chemical Computer Group (CCG), Schrodinger, Jeff Gray's lab at John Hopkins University, Macromoltek, Astellas Pharma/Osaka University and Prediction of ImmunoGlobulin Structure (PIGS). The sequences of benchmark structures were submitted to the modelers and PIGS, and a set of models were generated for each structure. We provide here an overview of the organization, participants and main results of this second antibody modeling assessment (AMA-II). Also, we compare the results with the first antibody assessment published in this journal (Almagro et al., 2011;79:3050).