IgE, IgE Receptors and Anti-IgE Biologics: Protein Structures and Mechanisms of Action.

The evolution of IgE in mammals added an extra layer of immune protection at body surfaces to provide a rapid and local response against antigens from the environment. The IgE immune response employs potent expulsive and inflammatory forces against local antigen provocation, at the risk of damaging host tissues and causing allergic disease. Two well-known IgE receptors, the high-affinity FcεRI and low-affinity CD23, mediate the activities of IgE. Unlike other known antibody receptors, CD23 also regulates IgE expression, maintaining IgE homeostasis. This mechanism evolved by adapting the function of the complement receptor CD21. Recent insights into the dynamic character of IgE structure, its resultant capacity for allosteric modulation, and the potential for ligand-induced dissociation have revealed previously unappreciated mechanisms for regulation of IgE and IgE complexes. We describe recent research, highlighting structural studies of the IgE network of proteins to analyze the uniquely versatile activities of IgE and anti-IgE biologics.

Signaling by Antibodies: Recent Progress.

IgG antibodies mediate a diversity of immune functions by coupling of antigen specificity through the Fab domain to signal transduction via Fc-Fc receptor interactions. Indeed, balanced IgG signaling through type I and type II Fc receptors is required for the control of proinflammatory, anti-inflammatory, and immunomodulatory processes. In this review, we discuss the mechanisms that govern IgG-Fc receptor interactions, highlighting the diversity of Fc receptor-mediated effector functions that regulate immunity and inflammation as well as determine susceptibility to infection and autoimmunity and responsiveness to antibody-based therapeutics and vaccines.

Compromised Humoral Functional Evolution Tracks with SARS-CoV-2 Mortality.

The urgent need for an effective SARS-CoV-2 vaccine has forced development to progress in the absence of well-defined correlates of immunity. While neutralization has been linked to protection against other pathogens, whether neutralization alone will be sufficient to drive protection against SARS-CoV-2 in the broader population remains unclear. Therefore, to fully define protective humoral immunity, we dissected the early evolution of the humoral response in 193 hospitalized individuals ranging from moderate to severe. Although robust IgM and IgA responses evolved in both survivors and non-survivors with severe disease, non-survivors showed attenuated IgG responses, accompanied by compromised Fcɣ receptor binding and Fc effector activity, pointing to deficient humoral development rather than disease-enhancing humoral immunity. In contrast, individuals with moderate disease exhibited delayed responses that ultimately matured. These data highlight distinct humoral trajectories associated with resolution of SARS-CoV-2 infection and the need for early functional humoral immunity.

A Functional Role for Antibodies in Tuberculosis.

While a third of the world carries the burden of tuberculosis, disease control has been hindered by a lack of tools, including a rapid, point-of-care diagnostic and a protective vaccine. In many infectious diseases, antibodies (Abs) are powerful biomarkers and important immune mediators. However, in Mycobacterium tuberculosis (Mtb) infection, a discriminatory or protective role for humoral immunity remains unclear. Using an unbiased antibody profiling approach, we show that individuals with latent tuberculosis infection (Ltb) and active tuberculosis disease (Atb) have distinct Mtb-specific humoral responses, such that Ltb infection is associated with unique Ab Fc functional profiles, selective binding to FcγRIII, and distinct Ab glycosylation patterns. Moreover, compared to Abs from Atb, Abs from Ltb drove enhanced phagolysosomal maturation, inflammasome activation, and, most importantly, macrophage killing of intracellular Mtb. Combined, these data point to a potential role for Fc-mediated Ab effector functions, tuned via differential glycosylation, in Mtb control.

Antibodies elicited in humans upon chimeric hemagglutinin-based influenza virus vaccination confer FcγR-dependent protection in vivo.

Antibody responses against highly conserved epitopes on the stalk domain of influenza virus hemagglutinin (HA) confer broad protection; however, such responses are limited. To effectively induce stalk-specific immunity against conserved HA epitopes, sequential immunization strategies have been developed based on chimeric HA (cHA) constructs featuring different head domains but the same stalk regions. Immunogenicity studies in small animal models, as well as in humans, revealed that cHA immunogens elicit stalk-specific IgG responses with broad specificity against heterologous influenza virus strains. However, the mechanisms by which these antibodies confer in vivo protection and the contribution of their Fc effector function remain unclear. To characterize the role of Fc-FcγR (Fcγ receptor) interactions to the in vivo protective activity of IgG antibodies elicited in participants in a phase I trial of a cHA vaccine candidate, we performed passive transfer studies of vaccine-elicited IgG antibodies in mice humanized for all classes of FcγRs, as well as in mice deficient for FcγRs. IgG antibodies elicited upon cHA vaccination completely protected FcγR humanized mice against lethal influenza virus challenge, while no protection was evident in FcγR-deficient mice, suggesting a major role for FcγR pathways in the protective function of vaccine-elicited IgG antibodies. These findings have important implications for influenza vaccine development, guiding the design of vaccination approaches with the capacity to elicit IgG responses with optimal Fc effector function.

Afucosylated IgG responses in humans - structural clues to the regulation of humoral immunity.

Healthy immune responses require efficient protection without excessive inflammation. Recent discoveries on the degree of fucosylation of a human N-linked glycan at a conserved site in the immunoglobulin IgG-Fc domain might add an additional regulatory layer to adaptive humoral immunity. Specifically, afucosylation of IgG-Fc enhances the interaction of IgG with FcγRIII and thereby its activity. Although plasma IgG is generally fucosylated, afucosylated IgG is raised in responses to enveloped viruses and Plasmodium falciparum proteins expressed on infected erythrocytes, as well as during alloimmune responses. Moreover, while afucosylation can exacerbate some infectious diseases (e.g., COVID-19), it also correlates with traits of protective immunity against malaria and HIV-1. Herein we discuss the implications of IgG afucosylation for health and disease, as well as for vaccination.

Single-dose HPV vaccine immunity: is there a role for non-neutralizing antibodies?

A single dose of human papillomavirus (HPV) vaccine against HPV infection (prerequisite for cervical cancer) appears to be as efficacious as two or three doses, despite inducing lower antibody titers. Neutralizing antibodies are thought to be the primary mediator of protection, but the threshold for protection is unknown. Antibody functions beyond neutralization have not been explored for HPV vaccines. Here, we discuss the immune mechanisms of HPV vaccines, with a focus on non-neutralizing antibody effector functions. In the context of single-dose HPV vaccination where antibody is limiting, we propose that non-neutralizing antibody functions may contribute to preventing HPV infection. Understanding the immunological basis of protection for single-dose HPV vaccination will provide a rationale for implementing single-dose HPV vaccine regimens.

Distinctive antibody responses to Mycobacterium tuberculosis in pulmonary and brain infection.

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), remains a global health burden. While Mtb is primarily a respiratory pathogen, it can spread to other organs, including the brain and meninges, causing TB meningitis (TBM). However, little is known about the immunological mechanisms that leads to differential disease across organs. Attention has focused on differences in T cell responses in the control of Mtb in the lungs, but emerging data point to a role for antibodies, as both biomarkers of disease control and as antimicrobial molecules. Given an increasing appreciation for compartmentalized antibody responses across the blood brain barrier, here we characterized the antibody profiles across the blood and brain compartments during TBM, and determined whether Mtb-specific humoral immune responses differed between Mtb infection of the lung (pulmonary TB) and TBM. Using a high throughput systems serology approach, we deeply profiled the antibody responses against 10 different Mtb antigens, including lipoarabinomannan (LAM) and purified protein derivative (PPD), in HIV-negative adults with pulmonary TB (n=10) vs TBM (n=60). Antibody studies included analysis of immunoglobulin isotypes (IgG, IgM, IgA) and subclass levels (IgG1-4), the capacity of Mtb-specific antibodies to bind to Fc receptors or C1q, and to activate innate immune effectors functions (complement and NK cells activation, monocyte or neutrophil phagocytosis). Machine learning methods were applied to characterize serum and CSF responses in TBM, identify prognostic factors associated with disease severity, and define the key antibody features that distinguish TBM from pulmonary TB. In individuals with TBM, we identified CSF-specific antibody profiles that marked a unique and compartmentalized humoral response against Mtb, characterized by an enrichment of Mtb-specific antibodies able to robustly activate complement and drive phagocytosis by monocytes and neutrophils, all of which were associated with milder TBM severity at presentation. Moreover, individuals with TBM exhibited Mtb-specific antibodies in the serum with an increased capacity to activate phagocytosis by monocytes, compared to individuals with pulmonary TB, despite having lower IgG titers and Fcγ receptors (FcγR)-binding capacity. Collectively, these data point to functionally divergent humoral responses depending on the site of infection (i.e. lungs vs brain), and demonstrate a highly compartmentalized Mtb-specific antibody response within the CSF during TBM. Moreover, our results suggest that phagocytosis- and complement-mediating antibodies may promote attenuated neuropathology and milder TBM disease.

A novel mouse strain optimized for chronic human antibody administration.

Therapeutic human IgG antibodies are routinely tested in mouse models of oncologic, infectious, and autoimmune diseases. However, assessing the efficacy and safety of long-term administration of these agents has been limited by endogenous anti-human IgG immune responses that act to clear human IgG from serum and relevant tissues, thereby reducing their efficacy and contributing to immune complex­mediated pathologies, confounding evaluation of potential toxicity. For this reason, human antibody treatment in mice is generally limited in duration and dosing, thus failing to recapitulate the potential clinical applications of these therapeutics. Here, we report the development of a mouse model that is tolerant of chronic human antibody administration. This model combines both a human IgG1 heavy chain knock-in and a full recapitulation of human Fc receptor (FcγR) expression, providing a unique platform for in vivo testing of human monoclonal antibodies with relevant receptors beyond the short term. Compared to controls, hIgG1 knock-in mice mount minimal anti-human IgG responses, allowing for the persistence of therapeutically active circulating human IgG even in the late stages of treatment in chronic models of immune thrombocytopenic purpura and metastatic melanoma.

FcγRIIA-specific DARPins as novel tools in blood cell analysis and platelet aggregation.

Fc receptors are involved in a variety of physiologically and disease-relevant responses. Among them, FcγRIIA (CD32a) is known for its activating functions in pathogen recognition and platelet biology, and, as potential marker of T lymphocytes latently infected with HIV-1. The latter has not been without controversy due to technical challenges complicated by T-B cell conjugates and trogocytosis as well as a lack of antibodies distinguishing between the closely related isoforms of FcγRII. To generate high-affinity binders specific for FcγRIIA, libraries of designed ankyrin repeat proteins (DARPins) were screened for binding to its extracellular domains by ribosomal display. Counterselection against FcγRIIB eliminated binders cross-reacting with both isoforms. The identified DARPins bound FcγRIIA with no detectable binding for FcγRIIB. Their affinities for FcγRIIA were in the low nanomolar range and could be enhanced by cleavage of the His-tag and dimerization. Interestingly, complex formation between DARPin and FcγRIIA followed a two-state reaction model, and discrimination from FcγRIIB was based on a single amino acid residue. In flow cytometry, DARPin F11 detected FcγRIIA+ cells even when they made up less than 1% of the cell population. Image stream analysis of primary human blood cells confirmed that F11 caused dim but reliable cell surface staining of a small subpopulation of T lymphocytes. When incubated with platelets, F11 inhibited their aggregation equally efficient as antibodies unable to discriminate between both FcγRII isoforms. The selected DARPins are unique novel tools for platelet aggregation studies as well as the role of FcγRIIA for the latent HIV-1 reservoir.

A human immune system (HIS) mouse model that dissociates roles for mouse and human FcR+ cells during antibody-mediated immune responses.

Human immune system (HIS) mice provide a model to study human immune responses in vivo. Currently available HIS mouse models may harbor mouse Fc Receptor (FcR)-expressing cells that exert potent effector functions following administration of human Ig. Previous studies showed that the ablation of the murine FcR gamma chain (FcR-γ) results in loss of antibody-dependent cellular cytotoxicity and antibody-dependent cellular phagocytosis in vivo. We created a new FcR-γ-deficient HIS mouse model to compare host (mouse) versus graft (human) effects underlying antibody-mediated immune responses in vivo. FcR-γ-deficient HIS recipients lack expression and function of mouse activating FcRs and can be stably and robustly reconstituted with human immune cells. By screening blood B-cell depletion by rituximab Ig variants, we found that human FcγRs-mediated IgG1 effects, whereas mouse activating FcγRs were dominant in IgG4 effects. Complement played a role as an IgG1 variant (IgG1 K322A) lacking complement binding activity was largely ineffective. Finally, we provide evidence that FcγRIIIA on human NK cells could mediate complement-independent B-cell depletion by IgG1 K322A. We anticipate that our FcR-γ-deficient HIS model will help clarify mechanisms of action of exogenous administered human antibodies in vivo.

Internalization of HIV-1 by Phagocytes Is Increased When Virions Are Opsonized with Multimeric Antibody in the Presence of Complement.

The low abundance of envelope spikes and the inability of IgG to aggregate virions render HIV-1 an inadequate target for antibody-mediated clearance by phagocytes. In an attempt to improve the ability of antibody to mediate the internalization of HIV-1 virions, we generated multimers of the broadly neutralizing HIV-1-specific monoclonal antibody (MAb) VRC01 using site-directed mutagenesis of the Fc segment. We then measured virion internalization using primary human monocytes and neutrophils. We found that, in the absence of complement, immune complexes consisting of HIV-1 virions and VRC01 multimers were slightly more efficiently internalized than were complexes formed with monomeric VRC01. The presence of complement, however, greatly augmented internalization of immune complexes formed with the multimeric MAb but had little impact on monomeric MAb-mediated internalization. Multimerization and the presence of complement overcome the limited ability of monomeric antibody to mediate internalization of HIV-1 virions and may thus provide a therapeutic approach to clearing virus. IMPORTANCE Antibody-mediated internalization of HIV-1 by phagocytes, a potential mechanism for clearing virus, is very inefficient. In an effort to improve viral clearance, we produced a multimeric form of the broadly neutralizing monoclonal antibody VRC01. We found that VRC01 antibody multimers (primarily hexamers) were only slightly more efficient in mediating HIV-1 internalization than was monomeric VRC01. However, the addition of complement resulted in substantially greater internalization of multimer-opsonized virus. In contrast, complement had little if any impact on internalization of monomer-opsonized virus. Therefore, antibody multimerization in combination with complement may overcome the limited ability of monomeric antibody to mediate internalization of HIV-1 virions. Our findings may provide a therapeutic approach to clearing virus.

Improving Antibody Therapeutics by Manipulating the Fc Domain: Immunological and Structural Considerations.

Interactions between the crystallizable fragment (Fc) domain of antibodies and a plethora of cellular Fc receptors (FcRs) or soluble proteins form a critical link between humoral and innate immunity. In particular, the immunoglobulin G Fc domain is critical for the clearance of target cells by processes that include (a) cytotoxicity, phagocytosis, or complement lysis; (b) modulation of inflammation; (c) antigen presentation; (d) antibody-mediated receptor clustering; and (e) cytokine release. More than 30 Fc-engineered antibodies aimed primarily at tailoring these effects for optimal therapeutic outcomes are in clinical evaluation or have already been approved. Nonetheless, our understanding of how FcR engagement impacts various immune cell phenotypes is still largely incomplete. Recent insights into FcR biology coupled with advances in Fc:FcR structural analysis, Fc engineering, and mouse models that recapitulate human biology are helping to fill in existing knowledge gaps. These advances will provide a blueprint on how to fine-tune the Fc domain to achieve optimal therapeutic efficacy.

HIV-Associated Alterations of the Biophysical Features of Maternal Antibodies Correlate With Their Reduced Transfer Across the Placenta.

BACKGROUND: Human immunodeficiency virus (HIV) infection during pregnancy is associated with reduced transplacental transfer of maternal antibodies and increased risk of severe infections in children who are exposed and uninfected with HIV. The basis of this reduced transfer of maternal immunity has not yet been defined but could involve modifications in the biophysical features of antibodies. The objective of this study was to assess the impact of maternal HIV infection on the biophysical features of serum IgG and transplacental antibody transfer. METHODS: Maternal serum IgG subclass levels, Fc glycosylation, Fc receptor (FcR) binding, and transplacental transfer of pathogen-specific maternal IgG were measured in pregnant women with HIV (WWH) and pregnant women testing negative for HIV (WNH) in Cape Town, South Africa. RESULTS: Maternal antibody profiles were strikingly different between pregnant WWH and WNH. Antibody binding to FcγR2a and FcγR2b, IgG1 and IgG3 antibodies, and agalactosylated antibodies were all elevated in WWH, whereas digalactosylated and sialylated antibodies were reduced compared to pregnant WNH. Antibody features that were elevated in WWH were also correlated with reduced transplacental transfer of vaccine antigen-specific antibodies. CONCLUSIONS: HIV infection is associated with marked alterations of biophysical features of maternal IgG and reduced placental transfer, potentially impairing antimicrobial immunity.

Functional Compartmentalization of Antibodies in the Central Nervous System During Chronic HIV Infection.

The central nervous system (CNS) has emerged as a critical HIV reservoir. Thus, interventions aimed at controlling and eliminating HIV must include CNS-targeted strategies. Given the inaccessibility of the brain, efforts have focused on cerebrospinal fluid (CSF), aimed at defining biomarkers of HIV-disease in the CNS, including HIV-specific antibodies. However, how antibodies traffic between the blood and CNS, and whether specific antibody profiles track with HIV-associated neurocognitive disorders (HAND) remains unclear. Here, we comprehensively profiled HIV-specific antibodies across plasma and CSF from 20 antiretroviral therapy (ART) naive or treated persons with HIV. CSF was populated by IgG1 and IgG3 antibodies, with reduced Fc-effector profiles. While ART improved plasma antibody functional coordination, CSF profiles were unaffected by ART and were unrelated to HAND severity. These data point to a functional sieving of antibodies across the blood-brain barrier, providing previously unappreciated insights for the development of next-generation therapeutics targeting the CNS reservoir.

Therapeutic antibody glycosylation impacts antigen recognition and immunogenicity.

In this study we show that glycosylation is relevant for immune recognition of therapeutic antibodies, and that defined glycan structures can modulate immunogenicity. Concerns regarding immunogenicity arise from the high heterogeneity in glycosylation that is difficult to control and can deviate from human glycosylation if produced in non-human cell lines. While non-human glycosylation is thought to cause hypersensitivity reactions and immunogenicity, less is known about effects of Fc-associated glycan structures on immune cell responses. We postulated that glycosylation influences antigen recognition and subsequently humoral responses to therapeutic antibodies by modulating 1) recognition and uptake by dendritic cells (DCs), and 2) antigen routing, processing and presentation. Here, we compared different glycosylation variants of the antibody rituximab (RTX) in in vitro assays using human DCs and T cells as well as in in vivo studies. We found that human DCs bind and internalize unmodified RTX stronger compared to its aglycosylated form suggesting that glycosylation mediates uptake after recognition by glycan-specific receptors. Furthermore, we show that DC-uptake of RTX increases or decreases if glycosylation is selectively modified to recognize activating (by mannosylation) or inhibitory lectin receptors (by sialylation). Moreover, glycosylation seems to influence antigen presentation by DCs because specific glycovariants tend to induce either stronger or weaker T cell activation. Finally, we demonstrate that antibody glycosylation impacts anti-drug antibody (ADA) responses to RTX in vivo. Hence, defined glycan structures can modulate immune recognition and alter ADA responses. Glyco-engineering may help to decrease clinical immunogenicity and ADA-associated adverse events such as hypersensitivity reactions.

Role of anti-HBs in functional cure of HBeAg+ chronic hepatitis B patients infected with HBV genotype A.

BACKGROUND & AIMS: HBsAg-specific antibody responses are difficult to detect during chronic hepatitis B infection (CHB) and are often overlooked. The aim of this study was to examine whether anti-HBs may be involved in functional cure (FC) by profiling anti-HBs responses in patients with CHB using a panel of specific assays. METHODS: Longitudinal serum samples were obtained from 25 patients with CHB who were infected with HBV genotype A and were undergoing nucleos(t)ide analogue (NA) treatment: 14 achieved FC while 11 remained infected (non-FC). Anti-HBs immune complexes (HBsAg-IC), FcγRIIIa dimer binding, epitope specificity and neutralisation efficacy were measured. RESULTS: HBsAg-IC peaks were detected prior to HBsAg loss in 10/14 FC patients. These HBsAg-IC peaks overlapped with either an alanine aminotransferase (ALT) flare (8/10 patients), or a rise in ALT (2/10 patients). HBsAg-IC peaks were detected in 7/11 non-FC patients, but were not associated with an ALT flare. FCγRIIIa binding was detected in 9/14 FC patients, independent from detection of overlapping HBsAg-IC/ALT peaks. FC patients had stable HBsAg epitope occupancy across the study, whereas non-FC patients had a reduction in HBsAg epitope occupancy within the first 12-24 weeks of NA treatment. Convalescent sera from FC patients recognised more HBsAg epitopes and neutralised HBV infection more potently than anti-HBs derived from vaccinees. Neutralisation potency appeared to increase post-HBsAg loss in 4/5 FC patients examined. CONCLUSIONS: Using these assays, we confirm that anti-HBs responses are present and fluctuate over time in this cohort of patients with HBeAg+ CHB, who were infected with HBV genotype A and treated with NAs. Key anti-HBs profiles associated with either FC or failure to achieve FC were also identified, suggesting a role for anti-HBs responses in FC. LAY SUMMARY: Using a panel of assays to characterise hepatitis B surface antibody (anti-HBs) responses in a group of patients with chronic hepatitis B, we identified anti-HBs profiles associated with either functional cure, or failure to achieve functional cure. Functional cure was associated with immune complex peaks which overlapped with alanine aminotransferase flares. Conversely, in those who did not achieve functional cure, immune complex peaks were present, but were not associated with alanine aminotransferase flares, and a decline in anti-HBs diversity was observed early during treatment.

Macrophages in ovarian cancer and their interactions with monoclonal antibody therapies.

The unmet clinical need for effective treatments in ovarian cancer has yet to be addressed using monoclonal antibodies (mAbs), which have largely failed to overcome tumour-associated immunosuppression, restrict cancer growth, and significantly improve survival. In recent years, experimental mAb design has moved away from solely targeting ovarian tumours and instead sought to modulate the wider tumour microenvironment (TME). Tumour-associated macrophages (TAMs) may represent an attractive therapeutic target for mAbs in ovarian cancer due to their high abundance and close proximity to tumour cells and their active involvement in facilitating several pro-tumoural processes. Moreover, the expression of several antibody crystallisable fragment (Fc) receptors and broad phenotypic plasticity of TAMs provide opportunities to modulate TAM polarisation using mAbs to promote anti-tumoural phenotypes. In this review, we discuss the role of TAMs in ovarian cancer TME and the emerging strategies to target the contributions of these cells in tumour progression through the rationale design of mAbs.

B cells defined by immunoglobulin isotypes.

The ability of B cells to generate antibodies and provide long-lived protective immunity is the cornerstone of vaccination and has contributed to the success of modern medicine. The nine different antibody subclasses produced by humans have effector functions that differ according to antigen type and route of exposure. Expression of the appropriate isotype is critical for effective humoral immunity, and it is becoming clear that subclass specificity is to some extent reflected at the cellular level. Understanding the mechanisms that govern the induction, expansion, and maintenance of B cells expressing different antibody subclasses informs the strategic manipulation of responses to benefit human health. This article provides an overview of the mechanisms by which the different human antibody subclasses regulate immunity, presents an update on how antibody subclass expression is regulated at the cellular level and highlights key areas for future research.

Differential requirements for FcγR engagement by protective antibodies against Ebola virus.

Ebola virus (EBOV) continues to pose significant threats to global public health, requiring ongoing development of multiple strategies for disease control. To date, numerous monoclonal antibodies (mAbs) that target the EBOV glycoprotein (GP) have demonstrated potent protective activity in animal disease models and are thus promising candidates for the control of EBOV. However, recent work in a variety of virus diseases has highlighted the importance of coupling Fab neutralization with Fc effector activity for effective antibody-mediated protection. To determine the contribution of Fc effector activity to the protective function of mAbs to EBOV GP, we selected anti-GP mAbs targeting representative, protective epitopes and characterized their Fc receptor (FcγR) dependence in vivo in FcγR humanized mouse challenge models of EBOV disease. In contrast to previous studies, we find that anti-GP mAbs exhibited differential requirements for FcγR engagement in mediating their protective activity independent of their distance from the viral membrane. Anti-GP mAbs targeting membrane proximal epitopes or the GP mucin domain do not rely on Fc-FcγR interactions to confer activity, whereas antibodies against the GP chalice bowl and the fusion loop require FcγR engagement for optimal in vivo antiviral activity. This complexity of antibody-mediated protection from EBOV disease highlights the structural constraints of FcγR binding for specific viral epitopes and has important implications for the development of mAb-based immunotherapeutics with optimal potency and efficacy.