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.

Spatial transcriptomic landscape unveils immunoglobin-associated senescence as a hallmark of aging.

To systematically characterize the loss of tissue integrity and organ dysfunction resulting from aging, we produced an in-depth spatial transcriptomic profile of nine tissues in male mice during aging. We showed that senescence-sensitive spots (SSSs) colocalized with elevated entropy in organizational structure and that the aggregation of immunoglobulin-expressing cells is a characteristic feature of the microenvironment surrounding SSSs. Immunoglobulin G (IgG) accumulated across the aged tissues in both male and female mice, and a similar phenomenon was observed in human tissues, suggesting the potential of the abnormal elevation of immunoglobulins as an evolutionarily conserved feature in aging. Furthermore, we observed that IgG could induce a pro-senescent state in macrophages and microglia, thereby exacerbating tissue aging, and that targeted reduction of IgG mitigated aging across various tissues in male mice. This study provides a high-resolution spatial depiction of aging and indicates the pivotal role of immunoglobulin-associated senescence during the aging process.

Potent efficacy of an IgG-specific endoglycosidase against IgG-mediated pathologies.

Endo-ß-N-acetylglucosaminidases (ENGases) that specifically hydrolyze the Asn297-linked glycan on immunoglobulin G (IgG) antibodies, the major molecular determinant of fragment crystallizable (Fc) γ receptor (FcγR) binding, are exceedingly rare. All previously characterized IgG-specific ENGases are multi-domain proteins secreted as an immune evasion strategy by Streptococcus pyogenes strains. Here, using in silico analysis and mass spectrometry techniques, we identified a family of single-domain ENGases secreted by pathogenic corynebacterial species that exhibit strict specificity for IgG antibodies. By X-ray crystallographic and surface plasmon resonance analyses, we found that the most catalytically efficient IgG-specific ENGase family member recognizes both protein and glycan components of IgG. Employing in vivo models, we demonstrated the remarkable efficacy of this IgG-specific ENGase in mitigating numerous pathologies that rely on FcγR-mediated effector functions, including T and B lymphocyte depletion, autoimmune hemolytic anemia, and antibody-dependent enhancement of dengue disease, revealing its potential for treating and/or preventing a wide range of IgG-mediated diseases in humans.

Human gut mycobiota tune immunity via CARD9-dependent induction of anti-fungal IgG antibodies.

Antibodies mediate natural and vaccine-induced immunity against viral and bacterial pathogens, whereas fungi represent a widespread kingdom of pathogenic species for which neither vaccine nor neutralizing antibody therapies are clinically available. Here, using a multi-kingdom antibody profiling (multiKAP) approach, we explore the human antibody repertoires against gut commensal fungi (mycobiota). We identify species preferentially targeted by systemic antibodies in humans, with Candida albicans being the major inducer of antifungal immunoglobulin G (IgG). Fungal colonization of the gut induces germinal center (GC)-dependent B cell expansion in extraintestinal lymphoid tissues and generates systemic antibodies that confer protection against disseminated C. albicans or C. auris infection. Antifungal IgG production depends on the innate immunity regulator CARD9 and CARD9+CX3CR1+ macrophages. In individuals with invasive candidiasis, loss-of-function mutations in CARD9 are associated with impaired antifungal IgG responses. These results reveal an important role of gut commensal fungi in shaping the human antibody repertoire through CARD9-dependent induction of host-protective antifungal IgG.

Structures of Human Antibodies Bound to SARS-CoV-2 Spike Reveal Common Epitopes and Recurrent Features of Antibodies.

Neutralizing antibody responses to coronaviruses mainly target the receptor-binding domain (RBD) of the trimeric spike. Here, we characterized polyclonal immunoglobulin Gs (IgGs) and Fabs from COVID-19 convalescent individuals for recognition of coronavirus spikes. Plasma IgGs differed in their focus on RBD epitopes, recognition of alpha- and beta-coronaviruses, and contributions of avidity to increased binding/neutralization of IgGs over Fabs. Using electron microscopy, we examined specificities of polyclonal plasma Fabs, revealing recognition of both S1A and RBD epitopes on SARS-CoV-2 spike. Moreover, a 3.4 Å cryo-electron microscopy (cryo-EM) structure of a neutralizing monoclonal Fab-spike complex revealed an epitope that blocks ACE2 receptor binding. Modeling based on these structures suggested different potentials for inter-spike crosslinking by IgGs on viruses, and characterized IgGs would not be affected by identified SARS-CoV-2 spike mutations. Overall, our studies structurally define a recurrent anti-SARS-CoV-2 antibody class derived from VH3-53/VH3-66 and similarity to a SARS-CoV VH3-30 antibody, providing criteria for evaluating vaccine-elicited antibodies.

Fc Characteristics Mediate Selective Placental Transfer of IgG in HIV-Infected Women.

The placental transfer of maternal IgG is critical for infant protection against infectious pathogens. However, factors that modulate the placental transfer of IgG remain largely undefined. HIV-infected women have impaired placental IgG transfer, presenting a unique "disruption model" to define factors that modulate placental IgG transfer. We measured the placental transfer efficiency of maternal HIV and pathogen-specific IgG in US and Malawian HIV-infected mothers and their HIV-exposed uninfected and infected infants. We examined the role of maternal HIV disease progression, infant factors, placental Fc receptor expression, IgG subclass, and glycan signatures and their association with placental IgG transfer efficiency. Maternal IgG characteristics, such as binding to placentally expressed Fc receptors FcγRIIa and FcγRIIIa, and Fc region glycan profiles were associated with placental IgG transfer efficiency. Our findings suggest that Fc region characteristics modulate the selective placental transfer of IgG, with implications for maternal vaccine design and infant health.

Longitudinal Analysis of the Human B Cell Response to Ebola Virus Infection.

Ebola virus (EBOV) remains a public health threat. We performed a longitudinal study of B cell responses to EBOV in four survivors of the 2014 West African outbreak. Infection induced lasting EBOV-specific immunoglobulin G (IgG) antibodies, but their subclass composition changed over time, with IgG1 persisting, IgG3 rapidly declining, and IgG4 appearing late. Striking changes occurred in the immunoglobulin repertoire, with massive recruitment of naive B cells that subsequently underwent hypermutation. We characterized a large panel of EBOV glycoprotein-specific monoclonal antibodies (mAbs). Only a small subset of mAbs that bound glycoprotein by ELISA recognized cell-surface glycoprotein. However, this subset contained all neutralizing mAbs. Several mAbs protected against EBOV disease in animals, including one mAb that targeted an epitope under evolutionary selection during the 2014 outbreak. Convergent antibody evolution was seen across multiple donors, particularly among VH3-13 neutralizing antibodies specific for the GP1 core. Our study provides a benchmark for assessing EBOV vaccine-induced immunity.

Progressive polyadenylation and m6A modification of Ighg1 mRNA maintain IgG1 antibody homeostasis in antibody-secreting cells.

Antigen-specific antibodies are generated by antibody-secreting cells (ASCs). How RNA post-transcriptional modification affects antibody homeostasis remains unclear. Here, we found that mRNA polyadenylations and N6-methyladenosine (m6A) modifications maintain IgG1 antibody production in ASCs. IgG heavy-chain transcripts (Ighg) possessed a long 3' UTR with m6A sites, targeted by the m6A reader YTHDF1. B cell-specific deficiency of YTHDF1 impaired IgG production upon antigen immunization through reducing Ighg1 mRNA abundance in IgG1+ ASCs. Disrupting either the m6A modification of a nuclear-localized splicing intermediate Ighg1 or the nuclear localization of YTHDF1 reduced Ighg1 transcript stability. Single-cell RNA sequencing identified an ASC subset with excessive YTHDF1 expression in systemic lupus erythematosus patients, which was decreased upon therapy with immunosuppressive drugs. In a lupus mouse model, inhibiting YTHDF1-m6A interactions alleviated symptoms. Thus, we highlight a mechanism in ASCs to sustain the homeostasis of IgG antibody transcripts by integrating Ighg1 mRNA polyadenylation and m6A modification.

Antigen-level resolution of commensal-specific B cell responses can be enabled by phage display screening coupled with B cell tetramers.

Induction of commensal-specific immunity contributes to tissue homeostasis, yet the mechanisms underlying induction of commensal-specific B cells remain poorly understood in part due to a lack of tools to identify these cells. Using phage display, we identified segmented filamentous bacteria (SFB) antigens targeted by serum and intestinal antibodies and generated B cell tetramers to track SFB-specific B cells in gut-associated lymphoid tissues. We revealed a compartmentalized response in SFB-specific B cell activation, with a gradient of immunoglobulin A (IgA), IgG1, and IgG2b isotype production along Peyer's patches contrasted by selective production of IgG2b within mesenteric lymph nodes. V(D)J sequencing and monoclonal antibody generation identified somatic hypermutation driven affinity maturation to SFB antigens under homeostatic conditions. Combining phage display and B cell tetramers will enable investigation of the ontogeny and function of commensal-specific B cell responses in tissue immunity, inflammation, and repair.

Immune checkpoint therapy-elicited sialylation of IgG antibodies impairs antitumorigenic type I interferon responses in hepatocellular carcinoma.

The reinvigoration of anti-tumor T cells in response to immune checkpoint blockade (ICB) therapy is well established. Whether and how ICB therapy manipulates antibody-mediated immune response in cancer environments, however, remains elusive. Using tandem mass spectrometric analysis of modification of immunoglobulin G (IgG) from hepatoma tissues, we identified a role of ICB therapy in catalyzing IgG sialylation in the Fc region. Effector T cells triggered sialylation of IgG via an interferon (IFN)-γ-ST6Gal-I-dependent pathway. DC-SIGN+ macrophages represented the main target cells of sialylated IgG. Upon interacting with sialylated IgG, DC-SIGN stimulated Raf-1-elicited elevation of ATF3, which inactivated cGAS-STING pathway and eliminated subsequent type-I-IFN-triggered antitumorigenic immunity. Although enhanced IgG sialylation in tumors predicted improved therapeutic outcomes for patients receiving ICB therapy, impeding IgG sialylation augmented antitumorigenic T cell immunity after ICB therapy. Thus, targeting antibody-based negative feedback action of ICB therapy has potential for improving efficacy of cancer immunotherapies.

Immunoglobulin G-dependent inhibition of inflammatory bone remodeling requires pattern recognition receptor Dectin-1.

Immunoglobulin G (IgG) antibodies are major drivers of inflammation during infectious and autoimmune diseases. In pooled serum IgG (IVIg), however, antibodies have a potent immunomodulatory and anti-inflammatory activity, but how this is mediated is unclear. We studied IgG-dependent initiation of resolution of inflammation in cytokine- and autoantibody-driven models of rheumatoid arthritis and found IVIg sialylation inhibited joint inflammation, whereas inhibition of osteoclastogenesis was sialic acid independent. Instead, IVIg-dependent inhibition of osteoclastogenesis was abrogated in mice lacking receptors Dectin-1 or FcγRIIb. Atomistic molecular dynamics simulations and super-resolution microscopy revealed that Dectin-1 promoted FcγRIIb membrane conformations that allowed productive IgG binding and enhanced interactions with mouse and human IgG subclasses. IVIg reprogrammed monocytes via FcγRIIb-dependent signaling that required Dectin-1. Our data identify a pathogen-independent function of Dectin-1 as a co-inhibitory checkpoint for IgG-dependent inhibition of mouse and human osteoclastogenesis. These findings may have implications for therapeutic targeting of autoantibody and cytokine-driven inflammation.

Tertiary lymphoid structures generate and propagate anti-tumor antibody-producing plasma cells in renal cell cancer.

The presence of intratumoral tertiary lymphoid structures (TLS) is associated with positive clinical outcomes and responses to immunotherapy in cancer. Here, we used spatial transcriptomics to examine the nature of B cell responses within TLS in renal cell carcinoma (RCC). B cells were enriched in TLS, and therein, we could identify all B cell maturation stages toward plasma cell (PC) formation. B cell repertoire analysis revealed clonal diversification, selection, expansion in TLS, and the presence of fully mature clonotypes at distance. In TLS+ tumors, IgG- and IgA-producing PCs disseminated into the tumor beds along fibroblastic tracks. TLS+ tumors exhibited high frequencies of IgG-producing PCs and IgG-stained and apoptotic malignant cells, suggestive of anti-tumor effector activity. Therapeutic responses and progression-free survival correlated with IgG-stained tumor cells in RCC patients treated with immune checkpoint inhibitors. Thus, intratumoral TLS sustains B cell maturation and antibody production that is associated with response to immunotherapy, potentially via direct anti-tumor effects.

Positive selection of IgG+ over IgM+ B cells in the germinal center reaction.

Positive selection of high-affinity B cells within germinal centers (GCs) drives affinity maturation of antibody responses. Here, we examined the mechanism underlying the parallel transition from immunoglobulin M (IgM) to IgG. Early GCs contained mostly unswitched IgM+ B cells; IgG+ B cells subsequently increased in frequency, dominating GC responses 14-21 days after antigen challenge. Somatic hypermutation and generation of high-affinity clones occurred with equal efficiency among IgM+ and IgG+ GC B cells, and inactivation of Ig class-switch recombination did not prevent depletion of IgM+ GC B cells. Instead, high-affinity IgG+ GC B cells outcompeted high-affinity IgM+ GC B cells via a selective advantage associated with IgG antigen receptor structure but independent of the extended cytoplasmic tail. Thus, two parallel forms of GC B-cell-positive selection, based on antigen receptor variable and constant regions, respectively, operate in tandem to ensure high-affinity IgG antibodies predominate in mature serum antibody responses.

A SARS-CoV-2 antibody broadly neutralizes SARS-related coronaviruses and variants by coordinated recognition of a virus-vulnerable site.

Potent neutralizing SARS-CoV-2 antibodies often target the spike protein receptor-binding site (RBS), but the variability of RBS epitopes hampers broad neutralization of multiple sarbecoviruses and drifted viruses. Here, using humanized mice, we identified an RBS antibody with a germline VH gene that potently neutralized SARS-related coronaviruses, including SARS-CoV and SARS-CoV-2 variants. X-ray crystallography revealed coordinated recognition by the heavy chain of non-RBS conserved sites and the light chain of RBS with a binding angle mimicking the angiotensin-converting enzyme 2 (ACE2) receptor. The minimum footprints in the hypervariable region of RBS contributed to the breadth of neutralization, which was enhanced by immunoglobulin G3 (IgG3) class switching. The coordinated binding resulted in broad neutralization of SARS-CoV and emerging SARS-CoV-2 variants of concern. Low-dose therapeutic antibody treatment in hamsters reduced the virus titers and morbidity during SARS-CoV-2 challenge. The structural basis for broad neutralizing activity may inform the design of a broad spectrum of therapeutics and vaccines.

Anti-commensal IgG Drives Intestinal Inflammation and Type 17 Immunity in Ulcerative Colitis.

Inflammatory bowel disease is a chronic, relapsing condition with two subtypes, Crohn's disease (CD) and ulcerative colitis (UC). Genome-wide association studies (GWASs) in UC implicate a FCGR2A variant that alters the binding affinity of the antibody receptor it encodes, FcγRIIA, for immunoglobulin G (IgG). Here, we aimed to understand the mechanisms whereby changes in FcγRIIA affinity would affect inflammation in an IgA-dominated organ. We found a profound induction of anti-commensal IgG and a concomitant increase in activating FcγR signaling in the colonic mucosa of UC patients. Commensal-IgG immune complexes engaged gut-resident FcγR-expressing macrophages, inducing NLRP3- and reactive-oxygen-species-dependent production of interleukin-1ß (IL-1ß) and neutrophil-recruiting chemokines. These responses were modulated by the FCGR2A genotype. In vivo manipulation of macrophage FcγR signal strength in a mouse model of UC determined the magnitude of intestinal inflammation and IL-1ß-dependent type 17 immunity. The identification of an important contribution of IgG-FcγR-dependent inflammation to UC has therapeutic implications.

Human Memory B Cells Harbor Diverse Cross-Neutralizing Antibodies against BK and JC Polyomaviruses.

Human polyomaviruses cause a common childhood infection worldwide and typically elicit a neutralizing antibody and cellular immune response, while establishing a dormant infection in the kidney with minimal clinical manifestations. However, viral reactivation can cause severe pathology in immunocompromised individuals. We developed a high-throughput, functional antibody screen to examine the humoral response to BK polyomavirus. This approach enabled the isolation of antibodies from all peripheral B cell subsets and revealed the anti-BK virus antibody repertoire as clonally complex with respect to immunoglobulin sequences and isotypes (both IgM and IgG), including a high frequency of monoclonal antibodies that broadly neutralize BK virus subtypes and the related JC polyomavirus. Cryo-electron microscopy of a broadly neutralizing IgG single-chain variable fragment complexed with BK virus-like particles revealed the quaternary nature of a conserved viral epitope at the junction between capsid pentamers. These features unravel a potent modality for inhibiting polyomavirus infection in kidney transplant recipients and other immunocompromised patients.

Whence and wherefore IgE?

Despite the near ubiquitous presence of Ig-based antibodies in vertebrates, IgE is unique to mammals. How and why it emerged remains mysterious. IgE expression is greatly constrained compared to other IgH isotypes. While other IgH isotypes are relatively abundant, soluble IgE has a truncated half-life, and IgE plasma cells are mostly short-lived. Despite its rarity, IgE is consequential and can trigger life-threatening anaphylaxis. IgE production reflects a dynamic steady state with IgG memory B cells feeding short-lived IgE production. Emerging evidence suggests that IgE may also potentially be produced in longer-lived plasma cells as well, perhaps as an aberrancy stemming from its evolutionary roots from an antibody isotype that likely functioned more like IgG. As a late derivative of an ancient systemic antibody system, the benefits of IgE in mammals likely stems from the antibody system's adaptive recognition and response capability. However, the tendency for massive, systemic, and long-lived production, common to IgH isotypes like IgG, were likely not a good fit for IgE. The evolutionary derivation of IgE from an antibody system that for millions of years was good at antigen de-sensitization to now functioning as a highly specialized antigen-sensitization function required heavy restrictions on antibody production-insufficiency of which may contribute to allergic disease.

Stellabody: A novel hexamer-promoting mutation for improved IgG potency.

Advances in antibody engineering are being directed at the development of next generation immunotherapeutics with improved potency. Hexamerisation of IgG is a normal physiological aspect of IgG biology and recently described mutations that facilitate this process have a substantial impact upon monoclonal antibody behavior resulting in the elicitation of dramatically enhanced complement-dependent cytotoxicity, Fc receptor function, and enhanced antigen binding effects, such as targeted receptor agonism or microbe neutralization. Whereas the discovery of IgG hexamerisation enhancing mutations has largely focused on residues with exposure at the surface of the Fc-Fc and CH2-CH3 interfaces, our unique approach is the engineering of the mostly buried residue H429 in the CH3 domain. Selective substitution at position 429 forms the basis of Stellabody technology, where the choice of amino acid results in distinct hexamerisation outcomes. H429F results in monomeric IgG that hexamerises after target binding, so called "on-target" hexamerisation, while the H429Y mutant forms pH-sensitive hexamers in-solution prior to antigen binding. Moreover, Stellabody technologies are broadly applicable across the family of antibody-based biologic therapeutics, including conventional mAbs, bispecific mAbs, and Ig-like biologics such as Fc-fusions, with applications in diverse diseases.

The importance of IgG glycosylation-What did we learn after analyzing over 100,000 individuals.

All four subclasses of immunoglobulin G (IgG) antibodies have glycan structures attached to the protein part of the IgG molecules. Glycans linked to the Fc portion of IgG are found in all IgG antibodies, while about one-fifth of IgG antibodies in plasma also have glycans attached to the Fab portion of IgG. The IgG3 subclass is characterized by more complex glycosylation compared to other IgG subclasses. In this review, we discuss the significant influence that glycans exert on the structural and functional properties of IgG. We provide a comprehensive overview of how the composition of these glycans can affect IgG's effector functions by modulating its interactions with Fcγ receptors and other molecules such as the C1q component of complement, which in turn influence various immune responses triggered by IgG, including antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). In addition, the importance of glycans for the efficacy of therapeutics like monoclonal antibodies and intravenous immunoglobulin (IVIg) therapy is discussed. Moreover, we offer insights into IgG glycosylation characteristics and roles derived from general population, disease-specific, and interventional studies. These studies indicate that IgG glycans are important biomarkers and functional effectors in health and disease.

Fc-FcγR interactions during infections: From neutralizing antibodies to antibody-dependent enhancement.

Advances in antibody technologies have resulted in the development of potent antibody-based therapeutics with proven clinical efficacy against infectious diseases. Several monoclonal antibodies (mAbs), mainly against viruses such as SARS-CoV-2, HIV-1, Ebola virus, influenza virus, and hepatitis B virus, are currently undergoing clinical testing or are already in use. Although these mAbs exhibit potent neutralizing activity that effectively blocks host cell infection, their antiviral activity results not only from Fab-mediated virus neutralization, but also from the protective effector functions mediated through the interaction of their Fc domains with Fcγ receptors (FcγRs) on effector leukocytes. Fc-FcγR interactions confer pleiotropic protective activities, including the clearance of opsonized virions and infected cells, as well as the induction of antiviral T-cell responses. However, excessive or inappropriate activation of specific FcγR pathways can lead to disease enhancement and exacerbated pathology, as seen in the context of dengue virus infections. A comprehensive understanding of the diversity of Fc effector functions during infection has guided the development of engineered antiviral antibodies optimized for maximal effector activity, as well as the design of targeted therapeutic approaches to prevent antibody-dependent enhancement of disease.