Mutations within a human IgG2 antibody form distinct and homogeneous disulfide isomers but do not affect Fc gamma receptor or C1q binding

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.

Vascular Considerations for Amyloid Immunotherapy

PURPOSE OF REVIEW

Amyloid beta (Aβ) plaque accumulation is a hallmark pathology contributing to Alzheimer's disease (AD) and is widely hypothesized to lead to cognitive decline. Decades of research into anti-Aβ immunotherapies provide evidence for increased Aβ clearance from the brain; however, this is frequently accompanied by complicated vascular deficits. This article reviews the history of anti-Aβ immunotherapies and clinical findings and provides recommendations moving forward.

RECENT FINDINGS

In 20 years of both animal and human studies, anti-Aβ immunotherapies have been a prevalent avenue of reducing hallmark Aβ plaques. In both models and with different anti-Aβ antibody designs, amyloid-related imaging abnormalities (ARIA) indicating severe cerebrovascular compromise have been common and concerning occurrence. ARIA caused by anti-Aβ immunotherapy has been noted since the early 2000s, and the mechanisms driving it are still unknown. Recent approval of aducanumab comes with renewed urgency to consider vascular deficits caused by anti-Aβ immunotherapy.

Coming together at the hinges: Therapeutic prospects of IgG3

The human IgG3 subclass is conspicuously absent among the formats for approved monoclonal antibody therapies and Fc fusion protein biologics. Concern about the potential for rapid degradation, reduced plasma half-life, and increased immunogenicity due to marked variation in allotypes has apparently outweighed the potential advantages of IgG3, which include high affinity for activating Fcγ receptors, effective complement fixation, and a long hinge that appears better suited for low abundance targets. This review aims to highlight distinguishing features of IgG3 and to explore its functional role in the immune response. We present studies of natural immunity and recombinant antibody therapies that elucidate key contributions of IgG3 and discuss historical roadblocks that no longer remain clearly relevant. Collectively, this body of evidence motivates thoughtful reconsideration of the clinical advancement of this distinctive antibody subclass for treatment of human diseases. Abbreviations: ADCC - Antibody-Dependent Cell-mediated CytotoxicityADE - Antibody-dependent enhancementAID - Activation-Induced Cytidine DeaminaseCH - Constant HeavyCHF - Complement factor HCSR - Class Switch RecombinationEM - Electron MicroscopyFab - Fragment, antigen bindingFc - Fragment, crystallizableFcRn - Neonatal Fc ReceptorFcγR - Fc gamma ReceptorHIV - Human Immunodeficiency VirusIg - ImmunoglobulinIgH - Immunoglobulin Heavy chain geneNHP - Non-Human Primate.

The solution structure of the human IgG2 subclass is distinct from those for human IgG1 and IgG4 providing an explanation for their discrete functions

Human IgG2 antibody displays distinct therapeutically-useful properties compared with the IgG1, IgG3, and IgG4 antibody subclasses. IgG2 is the second most abundant IgG subclass, being able to bind human FcγRII/FcγRIII but not to FcγRI or complement C1q. Structural information on IgG2 is limited by the absence of a full-length crystal structure for this. To this end, we determined the solution structure of human myeloma IgG2 by atomistic X-ray and neutron-scattering modeling. Analytical ultracentrifugation disclosed that IgG2 is monomeric with a sedimentation coefficient (s20, w0) of 7.2 S. IgG2 dimer formation was ≤5% and independent of the buffer conditions. Small-angle X-ray scattering in a range of NaCl concentrations and in light and heavy water revealed that the X-ray radius of gyration (Rg ) is 5.2-5.4 nm, after allowing for radiation damage at higher concentrations, and that the neutron Rg value of 5.0 nm remained unchanged in all conditions. The X-ray and neutron distance distribution curves (P(r)) revealed two peaks, M1 and M2, that were unchanged in different buffers. The creation of >123,000 physically-realistic atomistic models by Monte Carlo simulations for joint X-ray and neutron-scattering curve fits, constrained by the requirement of correct disulfide bridges in the hinge, resulted in the determination of symmetric Y-shaped IgG2 structures. These molecular structures were distinct from those for asymmetric IgG1 and asymmetric and symmetric IgG4 and were attributable to the four hinge disulfides. Our IgG2 structures rationalize the existence of the human IgG1, IgG2, and IgG4 subclasses and explain the receptor-binding functions of IgG2.

Recent mass spectrometry-based techniques and considerations for disulfide bond characterization in proteins

Disulfide bonds are important structural moieties of proteins: they ensure proper folding, provide stability, and ensure proper function. With the increasing use of proteins for biotherapeutics, particularly monoclonal antibodies, which are highly disulfide bonded, it is now important to confirm the correct disulfide bond connectivity and to verify the presence, or absence, of disulfide bond variants in the protein therapeutics. These studies help to ensure safety and efficacy. Hence, disulfide bonds are among the critical quality attributes of proteins that have to be monitored closely during the development of biotherapeutics. However, disulfide bond analysis is challenging because of the complexity of the biomolecules. Mass spectrometry (MS) has been the go-to analytical tool for the characterization of such complex biomolecules, and several methods have been reported to meet the challenging task of mapping disulfide bonds in proteins. In this review, we describe the relevant, recent MS-based techniques and provide important considerations needed for efficient disulfide bond analysis in proteins. The review focuses on methods for proper sample preparation, fragmentation techniques for disulfide bond analysis, recent disulfide bond mapping methods based on the fragmentation techniques, and automated algorithms designed for rapid analysis of disulfide bonds from liquid chromatography-MS/MS data. Researchers involved in method development for protein characterization can use the information herein to facilitate development of new MS-based methods for protein disulfide bond analysis. In addition, individuals characterizing biotherapeutics, especially by disulfide bond mapping in antibodies, can use this review to choose the best strategies for disulfide bond assignment of their biologic products. Graphical Abstract This review, describing characterization methods for disulfide bonds in proteins, focuses on three critical components: sample preparation, mass spectrometry data, and software tools.

Factors Affecting the FcRn-Mediated Transplacental Transfer of Antibodies and Implications for Vaccination in Pregnancy

At birth, neonates are particularly vulnerable to infection and transplacental transfer of immunoglobulin G (IgG) from mother to fetus provides crucial protection in the first weeks of life. Transcytosis of IgG occurs via binding with the neonatal Fc receptor (FcRn) in the placental synctiotrophoblast. As maternal vaccination becomes an increasingly important strategy for the protection of young infants, improving our understanding of transplacental transfer and the factors that may affect this will become increasingly important, especially in low-income countries where the burden of morbidity and mortality is highest. This review highlights factors of relevance to maternal vaccination that may modulate placental transfer—IgG subclass, glycosylation of antibody, total maternal IgG concentration, maternal disease, infant gestational age, and birthweight—and outlines the conflicting evidence and questions that remain regarding the complexities of these relationships. Furthermore, the intricacies of the Ab–FcRn interaction remain poorly understood and models that may help address future research questions are described.

Reformatting Rituximab into Human IgG2 and IgG4 Isotypes Dramatically Improves Apoptosis Induction In Vitro

The direct induction of cell death, or apoptosis, in target cells is one of the effector mechanisms for the anti CD20 antibody Rituximab. Here we provide evidence that Rituximab’s apoptotic ability is linked to the antibody IgG isotype. Reformatting Rituximab from the standard human IgG1 heavy chain into IgG2 or IgG4 boosted in vitro apoptosis induction in the Burkitt’s lymphoma B cell line Ramos five and four-fold respectively. The determinants for this behavior are located in the hinge region and CH1 domain of the heavy chain. By transplanting individual IgG2 or IgG4 specific amino acid residues onto otherwise IgG1 like backbones, thereby creating hybrid antibodies, the same enhancement of apoptosis induction could be achieved. The cysteines at position 131 of the CH1 domain and 219 in the hinge region, involved in IgG2 and IgG4 disulfide formation, were found to be of particular structural importance. Our data indicates that the hybrid antibodies possess a different CD20 binding mode than standard Rituximab, which appears to be key in enhancing apoptotic ability. The presented work opens up an interesting engineering route for enhancing the direct cytotoxic ability of therapeutic antibodies.

Conformation of the human immunoglobulin G2 hinge imparts superagonistic properties to immunostimulatory anticancer antibodies

Monoclonal antibody (mAb) drugs that stimulate antitumor immunity are transforming cancer treatment but require optimization for maximum clinical impact. Here, we show that, unlike other immunoglobulin isotypes, human IgG2 (h2) imparts FcγR-independent agonistic activity to immune-stimulatory mAbs such as anti-CD40, -4-1BB, and -CD28. Activity is provided by a subfraction of h2, h2B, that is structurally constrained due its unique arrangement of hinge region disulfide bonds. Agonistic activity can be transferred from h2 to h1 by swapping their hinge and CH1 domains, and substitution of key hinge and CH1 cysteines generates homogenous h2 variants with distinct agonistic properties. This provides the exciting opportunity to engineer clinical reagents with defined therapeutic activity regardless of FcγR expression levels in the local microenvironment.

IgG subclasses and allotypes: from structure to effector functions

Of the five immunoglobulin isotypes, immunoglobulin G (IgG) is most abundant in human serum. The four subclasses, IgG1, IgG2, IgG3, and IgG4, which are highly conserved, differ in their constant region, particularly in their hinges and upper CH2 domains. These regions are involved in binding to both IgG-Fc receptors (FcγR) and C1q. As a result, the different subclasses have different effector functions, both in terms of triggering FcγR-expressing cells, resulting in phagocytosis or antibody-dependent cell-mediated cytotoxicity, and activating complement. The Fc-regions also contain a binding epitope for the neonatal Fc receptor (FcRn), responsible for the extended half-life, placental transport, and bidirectional transport of IgG to mucosal surfaces. However, FcRn is also expressed in myeloid cells, where it participates in both phagocytosis and antigen presentation together with classical FcγR and complement. How these properties, IgG-polymorphisms and post-translational modification of the antibodies in the form of glycosylation, affect IgG-function will be the focus of the current review.

Global shape and ligand binding efficiency of the HIV-1-neutralizing antibodies differ from those of antibodies that cannot neutralize HIV-1

Asymmetric disposition of Fab arms in the structures solved for the broadly neutralizing monoclonal antibody (nmAb) IgG1 b12 raised the question of whether the unusual shape observed for b12 is common for all IgG1 mAbs or if there is a difference in the overall shape of nmAbs versus non-nmAbs. We compared small angle x-ray scattering (SAXS) data-based models and limited proteolysis profiles of some IgG1 mAbs known to be having and lacking HIV-1 neutralizing potency. In non-nmAbs, the Fab arms were found to be symmetrically disposed in space relative to central Fc, but in most nmAbs, the Fab arms were asymmetrically disposed, as seen for IgG1 b12. The only exceptions were 2G12 and 4E10, where both Fab arms were closed above Fc, suggesting some Fab-Fc and/or Fab-Fab interaction in the nmAbs that constrained extension of the Fab-Fc linker. Interestingly, these observations were correlated with differential proteolysis profiles of the mAbs by papain. Under conditions when papain could cut both Fab arms of non-nmAbs, only one Fab arm could be removed from neutralizing ones (except for 2G12 and 4E10). Chromatography and small angle x-ray scattering results of papain-digested products revealed that 1) the Fab-Fc or Fab-Fab interactions in unliganded mAbs are retained in digested products, and 2) whereas anti-gp120 non-nmAbs could bind two gp120 molecules, nmAbs could bind only one gp120. Additional experiments showed that except for 2G12 and 4E10, unopen shapes of nmAbs remain uninfluenced by ionic strength but can be reversibly opened by low pH of buffer accompanied by loss of ligand binding ability.

Three-dimensional structure of the human myeloma IgG2

Human immunoglobulin G, subclass 2 (hIgG2), plays an important role in immunity to bacterial pathogens and in numerous pathological conditions. However, there is a lack of information regarding the three-dimensional (3D) structure of the hIgG2 molecule. We used electron microscopy (EM), differential scanning microcalorimetry (DSC) and fluorescence for structural analysis of the hIgG2. DSC and fluorescence indicated two types of interaction between C_H1 domain of Fab (antigen-binding fragment/subunit) and C_H2 domain of Fc (complement fixation fragment/subunit) simultaneously present in the sample: close interaction, which increases the thermostability of both, C_H1 and C_H2 domains, and weak (or no) interaction, which is typical for most IgGs but not hIgG2. Thermodynamics could not determine if both types of interactions are present within a single molecule. To address this question, EM was used. We employed a single-particle reconstruction and negative staining approach to reveal the three-dimensional structure of the hIgG2. A three-dimensional model of hIgG2 was created at 1.78 nm resolution. The hIgG2 is asymmetrical: one Fab subunit is in close proximity to the upper portion of the Fc subunit (C_H2 domain) and the other Fab is distant from Fc. The plane of Fab subunits is nearly perpendicular to Fc. EM structure of the hIgG2 is in good agreement with thermodynamic data: a Fab distant from Fc should exhibit a lower melting temperature while a Fab interacting with Fc should exhibit a higher melting temperature. Both types of Fab subunits exist within one molecule resembling an A/B hIgG2 isoform introduced earlier on physicochemical level by Dillon et al. (2008). In such an arrangement, the access to the upper portion of Fc subunit is partially blocked by a Fab subunit. That might explain for instance why hIgG2 mildly activates complement and binds poorly to Fc receptors. Understanding of the three-dimensional structure of the hIgG2 should lead to better design of antibody-based therapeutics.

Complementary MS methods assist conformational characterization of antibodies with altered S-S bonding networks

As therapeutic monoclonal antibodies (mAbs) become a major focus in biotechnology and a source of the next-generation drugs, new analytical methods or combination methods are needed for monitoring changes in higher order structure and effects of post-translational modifications. The complexity of these molecules and their vulnerability to structural change provide a serious challenge. We describe here the use of complementary mass spectrometry methods that not only characterize mutant mAbs but also may provide a general framework for characterizing higher order structure of other protein therapeutics and biosimilars. To frame the challenge, we selected members of the IgG2 subclass that have distinct disulfide isomeric structures as a model to evaluate an overall approach that uses ion mobility, top-down MS sequencing, and protein footprinting in the form of fast photochemical oxidation of proteins (FPOP). These three methods are rapid, sensitive, respond to subtle changes in conformation of Cys → Ser mutants of an IgG2, each representing a single disulfide isoform, and may be used in series to probe higher order structure. The outcome suggests that this approach of using various methods in combination can assist the development and quality control of protein therapeutics.