There Is (Scientific) Strength in Numbers: A Comprehensive Quantitation of Fc Gamma Receptor Numbers on Human and Murine Peripheral Blood Leukocytes

Osteoporosis: interferon-gamma-mediated bone remodeling in osteoimmunology

As the world population ages, osteoporosis, the most common disease of bone metabolism, affects more than 200 million people worldwide. The etiology is an imbalance in bone remodeling process resulting in more significant bone resorption than bone remodeling. With the advent of the osteoimmunology field, the immune system's role in skeletal pathologies is gradually being discovered. The cytokine interferon-gamma (IFN-γ), a member of the interferon family, is an important factor in the etiology and treatment of osteoporosis because it mediates bone remodeling. This review starts with bone remodeling process and includes the cellular and key signaling pathways of bone remodeling. The effects of IFN-γ on osteoblasts, osteoclasts, and bone mass are discussed separately, while the overall effects of IFN-γ on primary and secondary osteoporosis are summarized. The net effect of IFN-γ on bone appears to be highly dependent on the environment, dose, concentration, and stage of cellular differentiation. This review focuses on the mechanisms of bone remodeling and bone immunology, with a comprehensive discussion of the relationship between IFN-γ and osteoporosis. Finding the paradoxical balance of IFN-γ in bone immunology and exploring the potential of its clinical application provide new ideas for the clinical treatment of osteoporosis and drug development.

A Dual-domain Engineered Antibody for Efficient HBV Suppression and Immune Responses Restoration

Chronic hepatitis B (CHB) remains a major public health concern because of the inefficiency of currently approved therapies in clearing the hepatitis B surface antigen (HBsAg). Antibody-based regimens have demonstrated potency regarding virus neutralization and HBsAg clearance. However, high dosages or frequent dosing are required for virologic control. In this study, a dual-domain-engineered anti-hepatitis B virus (HBV) therapeutic antibody 73-DY is developed that exhibits significantly improved efficacy regarding both serum and intrahepatic viral clearance. In HBV-tolerant mice, administration of a single dose of 73-DY at 2 mg kg-1 is sufficient to reduce serum HBsAg by over 3 log10 IU mL-1 and suppress HBsAg to < 100 IU mL-1 for two weeks, demonstrating a dose-lowering advantage of at least tenfold. Furthermore, 10 mg kg-1 of 73-DY sustainably suppressed serum viral levels to undetectable levels for ≈ 2 weeks. Molecular analyses indicate that the improved efficacy exhibited by 73-DY is attributable to the synergy between fragment antigen binding (Fab) and fragment crystallizable (Fc) engineering, which conferred sustained viral suppression and robust viral eradication, respectively. Long-term immunotherapy with reverse chimeric 73-DY facilitated the restoration of anti-HBV immune responses. This study provides a foundation for the development of next-generation antibody-based CHB therapies.

Tissue niche occupancy determines the contribution of fetal- versus bone-marrow-derived macrophages to IgG effector functions

Understanding the mechanisms underlying cytotoxic immunoglobulin G (IgG) activity is critical for improving therapeutic antibody activity and inhibiting autoantibody-mediated tissue pathology. While prior research highlights the important role of the mononuclear phagocytic system for removing opsonized target cells, it remains unclear which monocyte or macrophage subsets stemming from fetal or post-natal bone-marrow (BM)-associated definitive hematopoiesis are involved in target cell depletion. By using a titrated irradiation approach as well as Kupffer-cell-specific deletion of activated Fcγ receptor signaling, we establish conditions under which the contribution of BM-derived monocytes versus yolk-sac-derived liver-resident macrophages to cytotoxic IgG activity can be studied. Our results demonstrate that liver-resident macrophages originating from either fetal or adult hematopoiesis play a central role in IgG-mediated depletion of opsonized target cells from the peripheral blood under steady-state conditions, highlighting the impact of the tissue niche and not macrophage origin for cytotoxic antibody activity.

Fcγ receptors and immunomodulatory antibodies in cancer

The discovery of both cytotoxic T lymphocyte-associated antigen 4 (CTLA4) and programmed cell death protein 1 (PD1) as negative regulators of antitumour immunity led to the development of numerous immunomodulatory antibodies as cancer treatments. Preclinical studies have demonstrated that the efficacy of immunoglobulin G (IgG)-based therapies depends not only on their ability to block or engage their targets but also on the antibody's constant region (Fc) and its interactions with Fcγ receptors (FcγRs). Fc-FcγR interactions are essential for the activity of tumour-targeting antibodies, such as rituximab, trastuzumab and cetuximab, where the killing of tumour cells occurs at least in part due to these mechanisms. However, our understanding of these interactions in the context of immunomodulatory antibodies designed to boost antitumour immunity remains less explored. In this Review, we discuss our current understanding of the contribution of FcγRs to the in vivo activity of immunomodulatory antibodies and the challenges of translating results from preclinical models into the clinic. In addition, we review the impact of genetic variability of human FcγRs on the activity of therapeutic antibodies and how antibody engineering is being utilized to develop the next generation of cancer immunotherapies.

Protocol for isolation and spectral flow cytometry analysis of immune cells from the murine exocrine and endocrine pancreas

Diabetes mellitus is a disease of the hormone-secreting endocrine pancreas. However, increasing evidence suggests that the exocrine pancreas is also involved in the pathogenesis of diabetes. In this protocol, we describe how to harvest both isolated islets and exocrine tissue from one mouse pancreas, followed by a detailed explanation of how to isolate and analyze immune cells using full-spectrum flow cytometry.

Human neutrophil Fc gamma receptors: different buttons for different responses

Neutrophils are fundamental cells in host defense. These leukocytes are quickly recruited from the blood to sites of infection or tissue damage. At these sites, neutrophils initiate several innate immune responses, including phagocytosis, production of reactive oxygen species, degranulation to release proteases and other antimicrobial compounds, production of inflammatory mediators, and formation of neutrophil extracellular traps. In addition to their role in innate immunity, neutrophils are now recognized as cells that also regulate adaptive immunity, via interaction with dendritic cells and lymphocytes. Neutrophils also respond to adaptive immunity by interacting with antibody molecules. Indeed, antibody molecules allow neutrophils to have antigen-specific responses. Neutrophils express different receptors for antibodies. The receptors for immunoglobulin G molecules are known as Fcγ receptors. Upon Fcγ receptor aggregation on the cell membrane, these receptors trigger distinct signal transduction cascades that activate particular cellular responses. In this review, we describe the major Fcγ receptors expressed on human neutrophils and discuss how each Fcγ receptor activates a choice of signaling pathways to stimulate particular neutrophil responses.

In vitro analysis of anti-HPA-1a dependent platelet phagocytosis and its inhibition using a new whole blood phagocytosis assay (WHOPPA)

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a serious bleeding condition mostly caused by the reaction between maternal anti-HPA-1a antibodies and fetal platelets. This reaction leads to Fc-dependent platelet phagocytosis. Although several serological methods have been developed to identify maternal antibodies, a reliable laboratory parameter as a prognostic tool for FNAIT severity is still lacking. In this study, we developed whole blood platelet phagocytosis assay (WHOPPA), a flow cytometry-based phagocytosis assay that uses a pH-sensitive fluorescent dye (pHrodo-SE) to analyze anti-HPA-1a-dependent platelet phagocytosis in whole blood. WHOPPA revealed a high phagocytosis rate for the anti-HPA-1a opsonized platelets by monocytes but not by neutrophils. Analysis of different monocyte populations showed that all monocyte subsets, including classical (CD14++CD16-), intermediate (CD14++CD16+), and nonclassical (CD14+CD16++) monocytes, were able to engulf opsonized platelets. A unique monocyte subset, termed shifted monocytes (CD14+CD16-), showed the highest phagocytosis rate and was detected after platelet engulfment. FcγR inhibition tests revealed that except for FcγRIIa, FcγRI and FcγRIII on monocytes were responsible for the phagocytosis of anti-HPA-1a opsonized platelets. Analysis of anti-HPA-1a antibodies from FNAIT cases (n = 7) showed the phagocytosis of HPA-1aa but not of HPA-1bb platelets by monocytes. The phagocytosis rate was highly correlated with bound antibodies measured by flow cytometry (p < 0001; r = 0.9214) and MAIPA assay (p < 0.001; r = 0.7692). The phagocytosis rates were equal for type I and II anti-HPA-1a antibodies recognizing the plexin-semaphoring-integrin (PSI) domain and PSI/epidermal growth factor 1 domain of β3 integrin, respectively. By contrast, type III anti-HPA-1a antibodies reacting with αvβ3 integrin did not induce platelet phagocytosis. Furthermore, effector-silenced mAbs against HPA-1a inhibited the phagocytosis of anti-HPA-1a opsonized platelets. In conclusion, WHOPPA is a reliable in vitro platelet phagocytosis assay that mimics the phagocytosis of anti-HPA-1a opsonized platelets in whole blood. This assay allows to prove platelet phagocytosis ex vivo and evaluate the inhibitory capacity of different inhibitors as therapeutically strategies for the prevention of fetal thrombocytopenia in FNAIT in the future.

Function-structure approach reveals novel insights on the interplay of Immunoglobulin G 1 proteoforms and Fc gamma receptor IIa allotypes

Human Fc gamma receptor IIa (FcγRIIa) or CD32a has two major allotypes with a single amino acid difference at position 131 (histidine or arginine). Differences in FcγRIIa allotypes are known to impact immunological responses such as the clinical outcome of therapeutic monoclonal antibodies (mAbs). FcγRIIa is involved in antibody-dependent cellular phagocytosis (ADCP), which is an important contributor to the mechanism-of-action of mAbs by driving phagocytic clearance of cancer cells. Hence, understanding the impact of individual mAb proteoforms on the binding to FcγRIIa, and its different allotypes, is crucial for defining meaningful critical quality attributes (CQAs). Here, we report a function-structure based approach guided by novel FcγRIIa affinity chromatography-mass spectrometry (AC-MS) assays to assess individual IgG1 proteoforms. This allowed to unravel allotype-specific differences of IgG1 proteoforms on FcγRIIa binding. FcγRIIa AC-MS confirmed and refined structure-function relationships of IgG1 glycoform interactions. For example, the positive impact of afucosylation was higher than galactosylation for FcγRIIa Arg compared to FcγRIIa His. Moreover, we observed FcγRIIa allotype-opposing and IgG1 proteoform integrity-dependent differences in the binding response of stress-induced IgG1 proteoforms comprising asparagine 325 deamidation. The FcγRIIa-allotype dependent binding differences resolved by AC-MS were in line with functional ADCP-surrogate bioassay models. The molecular basis of the observed allotype specificity and proteoform selectivity upon asparagine 325 deamidation was elucidated using molecular dynamics. The observed differences were attributed to the contributions of an inter-molecular salt bridge between IgG1 and FcγRIIa Arg and the contribution of an intra-molecular hydrophobic pocket in IgG1. Our work highlights the unprecedented structural and functional resolution of AC-MS approaches along with predictive biological significance of observed affinity differences within relevant cell-based methods. This makes FcγRIIa AC-MS an invaluable tool to streamline the CQA assessment of therapeutic mAbs.

Opsonization-independent antigen-specific recognition by myeloid phagocytes expressing monoclonal antibodies

This report demonstrates a novel class of innate immune cells designated "variable immunoreceptor-expressing myeloids" (VIREMs). Using single-cell transcriptomics and genome-wide epigenetic profiling, we establish that VIREMs are myeloid cells unrelated to lymphocytes. We visualize the phenotype of B-VIREMs that are capable of genetically recombining and expressing antibody genes, the exclusive hallmark function of B lymphocytes. These cells, designated B-VIREMs, display monoclonal antibody cell surface signatures and regularly circulate in the blood of healthy individuals. Single-cell data reveal clonal expansion of circulating B-VIREMs as a dynamic response to disease stimuli. Live-cell imaging models suggest that B-VIREMs load their own Fc receptors with endogenous antibodies during vesicle transport to the cell surface. A first cloned B-VIREM-derived antibody (Vab1) specifically binds stomatin, a ubiquitous scaffold protein that is strictly expressed intracellularly, allowing Vab1-bearing macrophages to phagocytose cell debris without requiring prior opsonization. Our results suggest important antigen-specific tissue maintenance functionalities in these innate immune cells.

Effect of posttranslational modifications and subclass on IgG activity: from immunity to immunotherapy

Humoral immune responses are characterized by complex mixtures of polyclonal antibody species varying in their isotype, target epitope specificity and affinity. Posttranslational modifications occurring during antibody production in both the antibody variable and constant domain create further complexity and can modulate antigen specificity and antibody Fc-dependent effector functions, respectively. Finally, modifications of the antibody backbone after secretion may further impact antibody activity. An in-depth understanding of how these posttranslational modifications impact antibody function, especially in the context of individual antibody isotypes and subclasses, is only starting to emerge. Indeed, only a minute proportion of this natural variability in the humoral immune response is currently reflected in therapeutic antibody preparations. In this Review, we summarize recent insights into how IgG subclass and posttranslational modifications impact IgG activity and discuss how these insights may be used to optimize therapeutic antibody development.

The Role of Cytokines in Neutrophil Development, Tissue Homing, Function and Plasticity in Health and Disease

Neutrophils are crucial innate immune cells and comprise 50-70% of the white blood cell population under homeostatic conditions. Upon infection and in cancer, blood neutrophil numbers significantly increase because of the secretion of various chemo- and cytokines by, e.g., leukocytes, pericytes, fibroblasts and endothelial cells present in the inflamed tissue or in the tumor microenvironment (TME). The function of neutrophils in cancer has recently gained considerable attention, as they can exert both pro- and anti-tumorigenic functions, dependent on the cytokine milieu present in the TME. Here, we review the effect of cytokines on neutrophil development, tissue homing, function and plasticity in cancer and autoimmune diseases as well as under physiological conditions in the bone marrow, bloodstream and various organs like the spleen, kidney, liver, lung and lymph nodes. In addition, we address several promising therapeutic options, such as cytokine therapy, immunocytokines and immunotherapy, which aim to exploit the anti-tumorigenic potential of neutrophils in cancer treatment or block excessive neutrophil-mediated inflammation in autoimmune diseases.

Mixed IgG Fc immune complexes exhibit blended binding profiles and refine FcR affinity estimates

Immunoglobulin G (IgG) antibodies coordinate immune effector responses by interacting with effector cells via fragment crystallizable γ (Fcγ) receptors. The IgG Fc domain directs effector responses through subclass and glycosylation variation. Although each Fc variant has been extensively characterized in isolation, during immune responses, IgG is almost always produced in Fc mixtures. How this influences effector responses has not been examined. Here, we measure Fcγ receptor binding to mixed Fc immune complexes. Binding of these mixtures falls along a continuum between pure cases and quantitatively matches a mechanistic model, except for several low-affinity interactions mostly involving IgG2. We find that the binding model provides refined estimates of their affinities. Finally, we demonstrate that the model predicts effector cell-elicited platelet depletion in humanized mice. Contrary to previous views, IgG2 exhibits appreciable binding through avidity, though it is insufficient to induce effector responses. Overall, this work demonstrates a quantitative framework for modeling mixed IgG Fc-effector cell regulation.

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.

Role of lipid nanodomains for inhibitory FcγRIIb function

The inhibitory Fcγ receptor FcγRIIb is involved in immune regulation and is known to localize to specific regions of the plasma membrane called lipid rafts. Previous studies suggested a link between the altered lateral receptor localization within the plasma membrane and the functional impairment of the FcγRIIb-I232T variant that is associated with systemic lupus erythematosus. Here, we conducted microsecond all-atom molecular dynamics simulations and IgG binding assays to investigate the lipid nano-environment of FcγRIIb monomers and of the FcγRIIb-I232T mutant within a plasma membrane model, the orientation of the FcγRIIb ectodomain, and its accessibility to IgG ligands. In contrast to previously proposed models, our simulations indicated that FcγRIIb does not favor a cholesterol- or a sphingolipid-enriched lipid environment. Interestingly, cholesterol was depleted for all studied FcγRIIb variants within a 2-3 nm environment of the receptor, counteracting the usage of raft terminology for models on receptor functionality. Instead, the receptor interacts with lipids that have poly-unsaturated fatty acyl chains and with (poly-) anionic lipids within the cytosolic membrane leaflet. We also found that FcγRIIb monomers adopt a conformation that is not suitable for binding to its IgG ligand, consistent with a lack of detectable binding of monomeric IgG in experiments on primary immune cells. However, our results propose that multivalent IgG complexes might stabilize FcγRIIb in a binding-competent conformation. We suggest differences in receptor complex formation within the membrane as a plausible cause of the altered membrane localization or clustering and the altered suppressive function of the FcγRIIb-I232T variant.

Immunoglobulin G subclasses confer protection against Staphylococcus aureus bloodstream dissemination through distinct mechanisms in mouse models

Antibodies bind target molecules with exquisite specificity. The removal of these targets is mediated by the effector functions of antibodies. We reported earlier that the monoclonal antibody (mAb) 3F6 promotes opsonophagocytic killing of Staphylococcus aureus in blood and reduces bacterial replication in animals. Here, we generated mouse immunoglobulin G (mIgG) subclass variants and observed a hierarchy in protective efficacy 3F6-mIgG2a > 3F6-mIgG1 ≥ 3F6-mIgG2b >> 3F6-mIgG3 following bloodstream challenge of C57BL/6J mice. This hierarchy was not observed in BALB/cJ mice: All IgG subclasses conferred similar protection. IgG subclasses differ in their ability to activate complement and interact with Fcγ receptors (FcγR) on immune cells. 3F6-mIgG2a-dependent protection was lost in FcγR-deficient, but not in complement-deficient C57BL/6J animals. Measurements of the relative ratio of FcγRIV over complement receptor 3 (CR3) on neutrophils suggest the preferential expression of FcγRIV in C57BL/6 mice and of CR3 in BALB/cJ mice. To determine the physiological significance of these differing ratios, blocking antibodies against FcγRIV or CR3 were administered to animals before challenge. Correlating with the relative abundance of each receptor, 3F6-mIgG2a-dependent protection in C57BL/6J mice showed a greater reliance for FcγRIV while protection in BALB/cJ mice was only impaired upon neutralization of CR3. Thus, 3F6-based clearance of S. aureus in mice relies on a strain-specific contribution of variable FcγR- and complement-dependent pathways. We surmise that these variabilities are the result of genetic polymorphism(s) that may be encountered in other mammals including humans and may have clinical implications in predicting the efficacy of mAb-based therapies.

On the complexity of IgE: The role of structural flexibility and glycosylation for binding its receptors

It is well established that immunoglobulin E (IgE) plays a crucial role in atopy by binding to two types of Fcε receptors (FcεRI and FcεRII, also known as CD23). The cross-linking of FcεRI-bound IgE on effector cells, such as basophils and mast cells, initiates the allergic response. Conversely, the binding of IgE to CD23 modulates IgE serum levels and antigen presentation. In addition to binding to FcεRs, IgE can also interact with other receptors, such as certain galectins and, in mice, some FcγRs. The binding strength of IgE to its receptors is affected by its valency and glycosylation. While FcεRI shows reduced binding to IgE immune complexes (IgE-ICs), the binding to CD23 is enhanced. There is no evidence that galectins bind IgE-ICs. On the other hand, IgE glycosylation plays a crucial role in the binding to FcεRI and galectins, whereas the binding to CD23 seems to be independent of glycosylation. In this review, we will focus on receptors that bind to IgE and examine how the glycosylation and complexation of IgE impact their binding.

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.

Recent advances in antigen targeting to antigen-presenting cells in veterinary medicine

Advances in antigen targeting in veterinary medicine have gained traction over the years as an alternative approach for diseases that remain a challenge for traditional vaccines. In addition to the nature of the immunogen, antigen-targeting success relies heavily on the chosen receptor for its direct influence on the elicited response that will ensue after antigen uptake. Different approaches using antibodies, natural or synthetic ligands, fused proteins, and DNA vaccines have been explored in various veterinary species, with pigs, cattle, sheep, and poultry as the most frequent models. Antigen-presenting cells can be targeted using a generic approach, such as broadly expressed receptors such as MHC-II, CD80/86, CD40, CD83, etc., or focused on specific cell populations such as dendritic cells or macrophages (Langerin, DC-SIGN, XCR1, DC peptides, sialoadhesin, mannose receptors, etc.) with contrasting results. Interestingly, DC peptides show high specificity to DCs, boosting activation, stimulating cellular and humoral responses, and a higher rate of clinical protection. Likewise, MHC-II targeting shows consistent results in enhancing both immune responses; an example of this strategy of targeting is the approved vaccine against the bovine viral diarrhea virus in South America. This significant milestone opens the door to continuing efforts toward antigen-targeting vaccines to benefit animal health. This review discusses the recent advances in antigen targeting to antigen-presenting cells in veterinary medicine, with a special interest in pigs, sheep, cattle, poultry, and dogs.

The spatial resolution limit of phagocytosis

Antibody-opsonized bacteria interact with Fc receptors in macrophages and trigger signaling cascades, which induce phagocytosis. The signaling pathways ultimately lead to actin polymerization that induces the protrusion of the membrane around the bacterium until it is completely engulfed. Although many proteins involved in the phagocytic cup formation have already been identified, it is still unclear how far the initial stimulus created by the bacterium-cell contact propagates in the cell. We hypothesize that this spreading distance is closely related to the spatial resolution limit of phagocytosis, the smallest distance in which two stimuli can be differentiated. Here, we probe this resolution limit by using holographic optical tweezers to attach pairs of immunoglobulin G-coated polystyrene microparticles (as models for opsonized bacteria) to murine macrophages in distances ranging from zero to several micrometers. By using 2-μm-sized particles, we found that the particles can be internalized jointly into one phagosome if they are attached to the cell very close together, but that they are taken up separately if they are attached far from each other. To explain this, we developed a model of the signaling process, which predicts the probabilities for separate uptake for different particle sizes and distances using cellular parameters including the average receptor distance. We tested the model by measuring the separate uptake probabilities for particles with a diameter of 1 to 3 μm and for cells with reduced numbers of Fcγ receptors and found very good agreement. Our model shows that the phagocytic uptake behavior can be explained by assuming an effective phagocytic signaling range of about 500 nm. Interestingly, this value corresponds to the lower size limit of phagocytosis. Our work provides quantitative access to spatial parameters of cellular signaling during phagocytosis and thereby contributes to a more quantitative understanding of cellular information processing.

Mixed IgG Fc immune complexes exhibit blended binding profiles and refine FcR affinity estimates

Immunoglobulin (Ig)G antibodies coordinate immune effector responses by selectively binding to target antigens and then interacting with various effector cells via the Fcγ receptors. The Fc domain of IgG can promote or inhibit distinct effector responses across several different immune cell types through variation based on subclass and Fc domain glycosylation. Extensive characterization of these interactions has revealed how the inclusion of certain Fc subclasses or glycans results in distinct immune responses. During an immune response, however, IgG is produced with mixtures of Fc domain properties, so antigen-IgG immune complexes are likely to almost always be comprised of a combination of Fc forms. Whether and how this mixed composition influences immune effector responses has not been examined. Here, we measured Fcγ receptor binding to immune complexes of mixed Fc domain composition. We found that the binding properties of the mixed-composition immune complexes fell along a continuum between those of the corresponding pure cases. Binding quantitatively matched a mechanistic binding model, except for several low-affinity interactions mostly involving IgG2. We found that the affinities of these interactions are different than previously reported, and that the binding model could be used to provide refined estimates of these affinities. Finally, we demonstrated that the binding model can predict effector-cell elicited platelet depletion in humanized mice, with the model inferring the relevant effector cell populations. Contrary to the previous view in which IgG2 poorly engages with effector populations, we observe appreciable binding through avidity, but insufficient amounts to observe immune effector responses. Overall, this work demonstrates a quantitative framework for reasoning about effector response regulation arising from IgG of mixed Fc composition.

Summary points

The binding behavior of mixed Fc immune complexes is a blend of the binding properties for each constituent IgG species.An equilibrium, multivalent binding model can be generalized to incorporate immune complexes of mixed Fc composition.Particularly for low-affinity IgG-Fcγ receptor interactions, immune complexes provide better estimates of affinities.The FcγR binding model predicts effector-elicited cell clearance in humanized mice.

Enhancing HIV-1 Neutralization by Increasing the Local Concentration of Membrane-Proximal External Region-Directed Broadly Neutralizing Antibodies

Broadly neutralizing antibodies (bNAbs) against the membrane-proximal external region (MPER) of the gp41 component of the human immunodeficiency virus type 1 (HIV-1) envelope (Env) are characterized by long, hydrophobic, heavy chain complementarity-determining region 3s (HCDR3s) that interact with the MPER and some viral membrane lipids to achieve increased local concentrations. Here, we show that increasing the local concentration of MPER-directed bNAbs at the cell surface via binding to the high-affinity Fc receptor FcγRI potentiates their ability to prevent viral entry in a manner analogous to the previously reported observation wherein the lipid-binding activity of MPER bNAbs increases their concentration at the viral surface membrane. However, binding of MPER-directed bNAb 10E8 to FcγRI abolishes the neutralization synergy that is seen with the N-heptad repeat (NHR)-targeting antibody D5_AR and NHR-targeting small molecule enfuvirtide (T20), possibly due to decreased accessibility of the NHR in the FcγRI-10E8-MPER complex. Taken together, our results suggest that lipid-binding activity and FcγRI-mediated potentiation function in concert to improve the potency of MPER-directed bNAbs by increasing their local concentration near the site of viral fusion. Therefore, lipid binding may not be a strict requirement for potent neutralization by MPER-targeting bNAbs, as alternative methods can achieve similar increases in local concentrations while avoiding potential liabilities associated with immunologic host tolerance. IMPORTANCE The trimeric glycoprotein Env, the only viral protein expressed on the surface of HIV-1, is the target of broadly neutralizing antibodies and the focus of most vaccine development efforts. Broadly neutralizing antibodies targeting the membrane proximal external region (MPER) of Env show lipid-binding characteristics, and modulating this interaction affects neutralization. In this study, we tested the neutralization potencies of variants of the MPER-targeting antibody 10E8 with different viral-membrane-binding and host FcγRI-binding capabilities. Our results suggest that binding to both lipid and FcγRI improves the neutralization potency of MPER-directed antibodies by concentrating the antibodies at sites of viral fusion. As such, lipid binding may not be uniquely required for MPER-targeting broadly neutralizing antibodies, as alternative methods to increase local concentration can achieve similar improvements in potency.

The Fc-effector function of COVID-19 convalescent plasma contributes to SARS-CoV-2 treatment efficacy in mice

COVID-19 convalescent plasmas (CCPs) are chosen for plasma therapy based on neutralizing titers and anti-Spike immunoglobulin levels. However, CCP characteristics that promote SARS-CoV-2 control are complex and incompletely defined. Using an in vivo imaging approach, we demonstrate that CCPs with low neutralizing (ID50 ≤ 1:250), but moderate to high Fc-effector activity, in contrast to those with poor Fc function, delay mortality and/or improve survival of SARS-CoV-2-challenged K18-hACE2 mice. The impact of innate immune cells on CCP efficacy depended on their residual neutralizing activity. Fractionation of a selected CCP revealed that IgG and Ig(M + A) were required during therapy, but the IgG fraction alone sufficed during prophylaxis. Finally, despite reduced neutralization, ancestral SARS-CoV-2-elicited CCPs significantly delayed Delta and Beta-induced mortality suggesting that Fc-effector functions contribute to immunity against VOCs. Thus, Fc activity of CCPs provide a second line of defense when neutralization is compromised and can serve as an important criterion for CCP selection.

The emerging landscape of novel 4-1BB (CD137) agonistic drugs for cancer immunotherapy

The clinical development of 4-1BB agonists for cancer immunotherapy has raised substantial interest during the past decade. The first generation of 4-1BB agonistic antibodies entering the clinic, urelumab (BMS-663513) and utomilumab (PF-05082566), failed due to (liver) toxicity or lack of efficacy, respectively. The two antibodies display differences in the affinity and the 4-1BB receptor epitope recognition, as well as the isotype, which determines the Fc-gamma-receptor (FcγR) crosslinking activity. Based on this experience a very diverse landscape of second-generation 4-1BB agonists addressing the liabilities of first-generation agonists has recently been developed, with many entering clinical Phase 1 and 2 studies. This review provides an overview focusing on differences and their scientific rationale, as well as challenges foreseen during the clinical development of these molecules.

CD16+ fibroblasts foster a trastuzumab-refractory microenvironment that is reversed by VAV2 inhibition

The leukocyte Fcγ receptor (FcγR)-mediated response is important for the efficacy of therapeutic antibodies; however, little is known about the role of FcγRs in other cell types. Here we identify a subset of fibroblasts in human breast cancer that express CD16 (FcγRIII). An abundance of these cells in HER2⁺ breast cancer patients is associated with poor prognosis and response to trastuzumab. Functionally, upon trastuzumab stimulation, CD16⁺ fibroblasts reduce drug delivery by enhancing extracellular matrix stiffness. Interaction between trastuzumab and CD16 activates the intracellular SYK-VAV2-RhoA-ROCK-MLC2-MRTF-A pathway, leading to elevated contractile force and matrix production. Targeting of a Rho family guanine nucleotide exchange factor, VAV2, which is indispensable for the function of CD16 in fibroblasts rather than leukocytes, reverses desmoplasia provoked by CD16⁺ fibroblasts. Collectively, our study reveals a role for the fibroblast FcγR in drug resistance, and suggests that VAV2 is an attractive target to augment the effects of antibody treatments.

Toward Understanding How Staphylococcal Protein A Inhibits IgG-Mediated Phagocytosis

IgG molecules are crucial for the human immune response against bacterial infections. IgGs can trigger phagocytosis by innate immune cells, like neutrophils. To do so, IgGs should bind to the bacterial surface via their variable Fab regions and interact with Fcγ receptors and complement C1 via the constant Fc domain. C1 binding to IgG-labeled bacteria activates the complement cascade, which results in bacterial decoration with C3-derived molecules that are recognized by complement receptors on neutrophils. Next to FcγRs and complement receptors on the membrane, neutrophils also express the intracellular neonatal Fc receptor (FcRn). We previously reported that staphylococcal protein A (SpA), a key immune-evasion protein of Staphylococcus aureus, potently blocks IgG-mediated complement activation and killing of S. aureus by interfering with IgG hexamer formation. SpA is also known to block IgG-mediated phagocytosis in absence of complement, but the mechanism behind it remains unclear. In this study, we demonstrate that SpA blocks IgG-mediated phagocytosis and killing of S. aureus and that it inhibits the interaction of IgGs with FcγRs (FcγRIIa and FcγRIIIb, but not FcγRI) and FcRn. Furthermore, our data show that multiple SpA domains are needed to effectively block IgG1-mediated phagocytosis. This provides a rationale for the fact that SpA from S. aureus contains four to five repeats. Taken together, our study elucidates the molecular mechanism by which SpA blocks IgG-mediated phagocytosis and supports the idea that in addition to FcγRs, the intracellular FcRn is also prevented from binding IgG by SpA.

FcγRIV is required for IgG2c mediated enhancement of RBC alloimmunization

Passive immunization with anti-D can prevent maternal alloimmunization to RhD thereby preventing hemolytic disease of the fetus and newborn. Unexpectedly, anti-D fails in some cases and some monoclonal anti-D preparations paradoxically enhances alloimmunization. The underlying mechanisms modulating humoral alloimmunization by anti-D are unknown. We previously reported that IgG antibody subclasses differentially regulate alloimmunity in response to red blood cell (RBC) transfusions in a mouse model; in particular, IgG2c significantly enhanced RBC alloantibody responses. Initial mechanistic studies revealed that IgG2c:RBC immune complexes were preferentially consumed by the splenic dendritic cell (DC) subsets that play a role in RBC alloimmunization. The deletion of activating Fc-gamma receptors (FcγRs) (i.e., FcγRI, FcγRIII, and FcγRIV) on DCs abrogated IgG2c-mediated enhanced alloimmunization. Because DCs express high levels of FcγRIV, which has high affinity for the IgG2c subclass, we hypothesized that FcγRIV was required for enhanced alloimmunization. To test this hypothesis, knockout mice and blocking antibodies were used to manipulate FcγR expression. The data presented herein demonstrate that FcγRIV, but not FcγRI or FcγRIII, is required for IgG2c-mediated enhancement of RBC alloantibody production. Additionally, FcγRI is alone sufficient for IgG2c-mediated RBC clearance but not for increased alloimmunization, demonstrating that RBC clearance can occur without inducing alloimmunization. Together, these data, combined with prior observations, support the hypothesis that passive immunization with an RBC-specific IgG2c antibody increases RBC alloantibody production through FcγRIV ligation on splenic conventional DCs (cDCs). This raises the question of whether standardizing antibody subclasses in immunoprophylaxis preparations is desirable and suggests which subclasses may be optimal for generating monoclonal anti-D therapeutics.

Impaired B cell Expression of the Inhibitory Fcγ Receptor IIB in Myasthenia Gravis

Myasthenia gravis (MG) is an autoimmune disease in which pathogenic immunoglobulin G (IgG) antibodies (Abs) bind to acetylcholine receptors (AChR) or to functionally related molecules at the neuromuscular junction. B cell expression of the inhibitory IgG receptor, FcγRIIB, maintains peripheral immune tolerance and its absence renders B cells hyperresponsive to autoantigen. Here, we report that FcγRIIB expression levels are substantially reduced in B lineage cells derived from immunotherapy-naïve patients with AChR-Ab⁺ early-onset MG (EOMG). In contrast, genetic variants associated with impaired FcγRIIB expression are not enriched in MG, indicating post-transcriptional dysregulation. FcγR-targeted therapies could have therapeutic benefits in MG. This article is protected by copyright. All rights reserved.

Induction, decay, and determinants of functional antibodies following vaccination with the RTS,S malaria vaccine in young children

BACKGROUND

RTS,S is the first malaria vaccine recommended for implementation among young children at risk. However, vaccine efficacy is modest and short-lived. Antibodies play the major role in vaccine-induced immunity, but knowledge on the induction, decay, and determinants of antibody function is limited, especially among children. Antibodies that promote opsonic phagocytosis and other cellular functions appear to be important contributors to RTS,S immunity.

METHODS

We studied a phase IIb trial of RTS,S/AS02 conducted in young children in malaria-endemic regions of Mozambique. We evaluated the induction of antibodies targeting the circumsporozoite protein (CSP, vaccine antigen) that interact with Fcγ-receptors (FcRγs) and promote phagocytosis (neutrophils, monocytes, THP-1 cells), antibody-dependent respiratory burst (ADRB) by neutrophils, and natural killer (NK) cell activity, as well as the temporal kinetics of responses over 5 years of follow-up (ClinicalTrials.gov registry number NCT00197041).

RESULTS

RTS,S vaccination induced CSP-specific IgG with FcγRIIa and FcγRIII binding activity and promoted phagocytosis by neutrophils, THP-1 monocytes, and primary human monocytes, neutrophil ADRB activity, and NK cell activation. Responses were highly heterogenous among children, and the magnitude of neutrophil phagocytosis by antibodies was relatively modest, which may reflect modest vaccine efficacy. Induction of functional antibodies was lower among children with higher malaria exposure. Functional antibody magnitude and the functional activity of antibodies largely declined within a year post-vaccination, and decay were highest in the first 6 months, consistent with the decline in vaccine efficacy over that time. Decay rates varied for different antibody parameters and decay was slower for neutrophil phagocytosis. Biostatistical modelling suggested IgG1 and IgG3 contribute in promoting FcγR binding and phagocytosis, and IgG targeting the NANP-repeat and C-terminal regions CSP were similarly important for functional activities.

CONCLUSIONS

Results provide new insights to understand the modest and time-limited efficacy of RTS,S in children and the induction of antibody functional activities. Improving the induction and maintenance of antibodies that promote phagocytosis and cellular functions, and combating the negative effect of malaria exposure on vaccine responses are potential strategies for improving RTS,S efficacy and longevity.

Harnessing IgG Fc glycosylation for clinical benefit

The effector activity of IgG antibodies is regulated at several levels, including IgG subclass, modifications of the Fc glycan, and the distribution of Type I and II Fcγ receptors (FcγR) on effector cells. Here, we explore how Fc glycosylation, particularly sialylation and fucosylation, tunes cellular responses to immune complexes. We review the current understanding of the pathways and mechanisms underlying this biology, address FcγR in antigen presentation, and discuss aspects of the clinical understanding of Fc glycans in therapies and disease.

Heterogeneity in IgG-CD16 signaling in infectious disease outcomes

In this review, we discuss how IgG antibodies can modulate inflammatory signaling during viral infections with a focus on CD16a-mediated functions. We describe the structural heterogeneity of IgG antibody ligands, including subclass and glycosylation that impact binding by and downstream activity of CD16a, as well as the heterogeneity of CD16a itself, including allele and expression density. While inflammation is a mechanism required for immune homeostasis and resolution of acute infections, we focus here on two infectious diseases that are driven by pathogenic inflammatory responses during infection. Specifically, we review and discuss the evolving body of literature showing that afucosylated IgG immune complex signaling through CD16a contributes to the overwhelming inflammatory response that is central to the pathogenesis of severe forms of dengue disease and coronavirus disease 2019 (COVID-19).

Targeting Myeloid Checkpoint Molecules in Combination With Antibody Therapy: A Novel Anti-Cancer Strategy With IgA Antibodies?

Immunotherapy with therapeutic antibodies has shown a lack of durable responses in some patients due to resistance mechanisms. Checkpoint molecules expressed by tumor cells have a deleterious impact on clinical responses to therapeutic antibodies. Myeloid checkpoints, which negatively regulate macrophage and neutrophil anti-tumor responses, are a novel type of checkpoint molecule. Myeloid checkpoint inhibition is currently being studied in combination with IgG-based immunotherapy. In contrast, the combination with IgA-based treatment has received minimal attention. IgA antibodies have been demonstrated to more effectively attract and activate neutrophils than their IgG counterparts. Therefore, myeloid checkpoint inhibition could be an interesting addition to IgA treatment and has the potential to significantly enhance IgA therapy.

HIF activation enhances FcγRIIb expression on mononuclear phagocytes impeding tumor targeting antibody immunotherapy

BACKGROUND

Hypoxia is a hallmark of the tumor microenvironment (TME) and in addition to altering metabolism in cancer cells, it transforms tumor-associated stromal cells. Within the tumor stromal cell compartment, tumor-associated macrophages (TAMs) provide potent pro-tumoral support. However, TAMs can also be harnessed to destroy tumor cells by monoclonal antibody (mAb) immunotherapy, through antibody dependent cellular phagocytosis (ADCP). This is mediated via antibody-binding activating Fc gamma receptors (FcγR) and impaired by the single inhibitory FcγR, FcγRIIb.

METHODS

We applied a multi-OMIC approach coupled with in vitro functional assays and murine tumor models to assess the effects of hypoxia inducible factor (HIF) activation on mAb mediated depletion of human and murine cancer cells. For mechanistic assessments, siRNA-mediated gene silencing, Western blotting and chromatin immune precipitation were utilized to assess the impact of identified regulators on FCGR2B gene transcription.

RESULTS

We report that TAMs are FcγRIIbbright relative to healthy tissue counterparts and under hypoxic conditions, mononuclear phagocytes markedly upregulate FcγRIIb. This enhanced FcγRIIb expression is transcriptionally driven through HIFs and Activator protein 1 (AP-1). Importantly, this phenotype reduces the ability of macrophages to eliminate anti-CD20 monoclonal antibody (mAb) opsonized human chronic lymphocytic leukemia cells in vitro and EL4 lymphoma cells in vivo in human FcγRIIb+/+ transgenic mice. Furthermore, post-HIF activation, mAb mediated blockade of FcγRIIb can partially restore phagocytic function in human monocytes.

CONCLUSION

Our findings provide a detailed molecular and cellular basis for hypoxia driven resistance to antitumor mAb immunotherapy, unveiling a hitherto unexplored aspect of the TME. These findings provide a mechanistic rationale for the modulation of FcγRIIb expression or its blockade as a promising strategy to enhance approved and novel mAb immunotherapies.

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. Expected final online publication date for the Annual Review of Biomedical Engineering, Volume 24 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Next generation antibody-based therapies in neurology

Antibody (Ab)-based therapeutics are now standard in the treatment of neuroinflammatory diseases, and the spectrum of neurological diseases targeted by those approaches continues to grow. The efficacy of Ab-based drug-platforms is largely determined by the specificity-conferring antigen-binding fragment (Fab) and the crystallizable fragment (Fc) driving antibody function. The latter provides specific instructions to the immune system by interacting with cellular Fc receptors and complement components. Extensive engineering efforts enabled tuning of Fc functions to modulate effector functions and to prolong or reduce Ab serum half-lives. Technologies that improve bioavailability of Ab-based treatment platforms within the central nervous system parenchyma are being developed and could invigorate drug discovery for a number of brain diseases for which current therapeutic options are limited. These powerful approaches are currently being tested in clinical trials or have been successfully translated into the clinic. Here, we review recent developments in the design and implementation of Ab-based treatment modalities in neurological diseases.

Fc Binding by FcγRIIa Is Essential for Cellular Activation by the Anti-FcγRIIa mAbs 8.26 and 8.2

FcγR activity underpins the role of antibodies in both protective immunity and auto-immunity and importantly, the therapeutic activity of many monoclonal antibody therapies. Some monoclonal anti-FcγR antibodies activate their receptors, but the properties required for cell activation are not well defined. Here we examined activation of the most widely expressed human FcγR; FcγRIIa, by two non-blocking, mAbs, 8.26 and 8.2. Crosslinking of FcγRIIa by the mAb F(ab’)₂ regions alone was insufficient for activation, indicating activation also required receptor engagement by the Fc region. Similarly, when mutant receptors were inactivated in the Fc binding site, so that intact mAb was only able to engage receptors via its two Fab regions, again activation did not occur. Mutation of FcγRIIa in the epitope recognized by the agonist mAbs, completely abrogated the activity of mAb 8.26, but mAb 8.2 activity was only partially inhibited indicating differences in receptor recognition by these mAbs. FcγRIIa inactivated in the Fc binding site was next co-expressed with the FcγRIIa mutated in the epitope recognized by the Fab so that each mAb 8.26 molecule can contribute only three interactions, each with separate receptors, one via the Fc and two via the Fab regions. When the Fab and Fc binding were thus segregated onto different receptor molecules receptor activation by intact mAb did not occur. Thus, receptor activation requires mAb 8.26 Fab and Fc interaction simultaneously with the same receptor molecules. Establishing the molecular nature of FcγR engagement required for cell activation may inform the optimal design of therapeutic mAbs.

There Is Strength in Numbers: Quantitation of Fc Gamma Receptors on Murine Tissue-Resident Macrophages

Many of the effector functions of antibodies rely on the binding of antibodies/immune complexes to cellular Fcγ receptors (FcγRs). Since the majority of innate immune effector cells express both activating and inhibitory Fc receptors, the outcome of the binding of immune complexes to cells of a given population is influenced by the relative affinities of the respective IgG subclasses to these receptors, as well as by the numbers of activating and inhibitory FcγRs on the cell surface. A group of immune cells that has come into focus more recently is the various subsets of tissue-resident macrophages. The central functions of FcγRs on tissue macrophages include the clearance of opsonized pathogens, the removal of small immune complexes from the circulation and the depletion of antibody-opsonized cells in the therapy of autoimmunity and cancer. Despite these essential functions of FcγRs on tissue-resident macrophages, an in-depth quantification of FcγRs is lacking. Thus, the aim of our current study was to quantify the various Fcγ receptors on macrophages in murine liver, lung, kidney, brain, skin and spleen. Our study identified a pronounced heterogeneity between FcγR expression patterns of the different tissue macrophages, which may reflect their specialized functions within their unique niches in different organ environments.

Centrality of Myeloid-Lineage Phagocytes in Particle-Triggered Inflammation and Autoimmunity

Exposure to exogenous particles found as airborne contaminants or endogenous particles that form by crystallization of certain nutrients can activate inflammatory pathways and potentially accelerate autoimmunity onset and progression in genetically predisposed individuals. The first line of innate immunological defense against particles are myeloid-lineage phagocytes, namely macrophages and neutrophils, which recognize/internalize the particles, release inflammatory mediators, undergo programmed/unprogrammed death, and recruit/activate other leukocytes to clear the particles and resolve inflammation. However, immunogenic cell death and release of damage-associated molecules, collectively referred to as "danger signals," coupled with failure to efficiently clear dead/dying cells, can elicit unresolved inflammation, accumulation of self-antigens, and adaptive leukocyte recruitment/activation. Collectively, these events can promote loss of immunological self-tolerance and onset/progression of autoimmunity. This review discusses critical molecular mechanisms by which exogenous particles (i.e., silica, asbestos, carbon nanotubes, titanium dioxide, aluminum-containing salts) and endogenous particles (i.e., monosodium urate, cholesterol crystals, calcium-containing salts) may promote unresolved inflammation and autoimmunity by inducing toxic responses in myeloid-lineage phagocytes with emphases on inflammasome activation and necrotic and programmed cell death pathways. A prototypical example is occupational exposure to respirable crystalline silica, which is etiologically linked to systemic lupus erythematosus (SLE) and other human autoimmune diseases. Importantly, airway instillation of SLE-prone mice with crystalline silica elicits severe pulmonary pathology involving accumulation of particle-laden alveolar macrophages, dying and dead cells, nuclear and cytoplasmic debris, and neutrophilic inflammation that drive cytokine, chemokine, and interferon-regulated gene expression. Silica-induced immunogenic cell death and danger signal release triggers accumulation of T and B cells, along with IgG-secreting plasma cells, indicative of ectopic lymphoid tissue neogenesis, and broad-spectrum autoantibody production in the lung. These events drive early autoimmunity onset and accelerate end-stage autoimmune glomerulonephritis. Intriguingly, dietary supplementation with ω-3 fatty acids have been demonstrated to be an intervention against silica-triggered murine autoimmunity. Taken together, further insight into how particles drive immunogenic cell death and danger signaling in myeloid-lineage phagocytes and how these responses are influenced by the genome will be essential for identification of novel interventions for preventing and treating inflammatory and autoimmune diseases associated with these agents.

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.

Fc gamma receptor IIIb binding of individual antibody proteoforms resolved by affinity chromatography-mass spectrometry

The crystallizable fragment (Fc) of immunoglobulin G (IgG) activates key immunological responses by interacting with Fc gamma receptors (FcɣR). FcɣRIIIb contributes to neutrophil activation and is involved in antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). These processes present important mechanisms-of-actions of therapeutic antibodies. The very low affinity of IgG toward FcɣRIIIb (K_D ~ 10 µM) is a technical challenge for interaction studies. Additionally, the interaction is strongly dependent on IgG glycosylation, a major contributor to proteoform heterogeneity. We developed an affinity chromatography–mass spectrometry (AC-MS) assay for analyzing IgG-FcɣRIIIb interactions in a proteoform-resolved manner. This proved to be well suited to study low-affinity interactions. The applicability and selectivity of the method were demonstrated on a panel of nine different IgG monoclonal antibodies (mAbs), including no-affinity, low-affinity and high-affinity Fc-engineered or glycoengineered mAbs. Thereby, we could reproduce reported affinity rankings of different IgG glycosylation features and IgG subclasses. Additional post-translational modifications (IgG1 Met252 oxidation, IgG3 hinge-region O-glycosylation) showed no effect on FcɣRIIIb binding. Interestingly, we observed indications of an effect of the variable domain sequence on the Fc-binding that deserves further attention. Our new AC-MS method is a powerful tool for expanding knowledge on structure–function relationships of the IgG-FcɣRIIIb interaction. Hence, this assay may substantially improve the efficiency of assessing critical quality attributes of therapeutic mAbs with respect to an important aspect of neutrophil activation.

IgG Immune Complexes Inhibit Naïve T Cell Proliferation and Suppress Effector Function in Cytotoxic T Cells

Elevated levels of circulating immune complexes are associated with autoimmunity and with worse prognoses in cancer. Here, we examined the effects of well-defined, soluble immune complexes (ICs) on human peripheral T cells. We demonstrate that IgG-ICs inhibit the proliferation and differentiation of a subset of naïve T cells but stimulate the division of another naïve-like T cell subset. Phenotypic analysis by multi-parameter flow cytometry and RNA-Seq were used to characterize the inhibited and stimulated T cells revealing that the inhibited subset presented immature features resembling those of recent thymic emigrants and non-activated naïve T cells, whereas the stimulated subset exhibited transcriptional features indicative of a more differentiated, early memory progenitor with a naïve-like phenotype. Furthermore, we show that while IgG1-ICs do not profoundly inhibit the proliferation of memory T cells, IgG1-ICs suppress the production of granzyme-β and perforin in cytotoxic memory T cells. Our findings reveal how ICs can link humoral immunity and T cell function.

Ebola vaccine-induced protection in nonhuman primates correlates with antibody specificity and Fc-mediated effects

Although substantial progress has been made with Ebola virus (EBOV) vaccine measures, the immune correlates of vaccine-mediated protection remain uncertain. Here, five mucosal vaccine vectors based on human and avian paramyxoviruses provided nonhuman primates with varying degrees of protection, despite expressing the same EBOV glycoprotein (GP) immunogen. Each vaccine produced antibody responses that differed in Fc-mediated functions and isotype composition, as well as in magnitude and coverage toward GP and its conformational and linear epitopes. Differences in the degree of protection and comprehensive characterization of the response afforded the opportunity to identify which features and functions were elevated in survivors and could therefore serve as vaccine correlates of protection. Pairwise network correlation analysis of 139 immune- and vaccine-related parameters was performed to demonstrate relationships with survival. Total GP-specific antibodies, as measured by biolayer interferometry, but not neutralizing IgG or IgA titers, correlated with survival. Fc-mediated functions and the amount of receptor binding domain antibodies were associated with improved survival outcomes, alluding to the protective mechanisms of these vaccines. Therefore, functional qualities of the antibody response, particularly Fc-mediated effects and GP specificity, rather than simply magnitude of the response, appear central to vaccine-induced protection against EBOV. The heterogeneity of the response profile between the vaccines indicates that each vaccine likely exhibits its own protective signature and the requirements for an efficacious EBOV vaccine are complex.

Rationale and clinical development of CD40 agonistic antibodies for cancer immunotherapy

Introduction: CD40 signaling activates dendritic cells leading to improved T cell priming against tumor antigens. CD40 agonism expands the tumor-specific T cell repertoire and has the potential to increase the fraction of patients that respond to established immunotherapies.Areas covered: This article reviews current as well as emerging CD40 agonist therapies with a focus on antibody-based therapies, including next generation bispecific CD40 agonists. The scientific rationale for different design criteria, binding epitopes, and formats are discussed.Expert opinion: The ability of CD40 agonists to activate dendritic cells and enhance antigen cross-presentation to CD8⁺ T cells provides an opportunity to elevate response rates of cancer immunotherapies. While there are many challenges left to address, including optimal dose regimen, CD40 agonist profile, combination partners and indications, we are confident that CD40 agonists will play an important role in the challenging task of reprogramming the immune system to fight cancer.

Association of IgG1 Antibody Clearance with FcγRIIA Polymorphism and Platelet Count in Infliximab-Treated Patients

The FcγRIIA/CD32A is mainly expressed on platelets, myeloid and several endothelial cells. Its affinity is considered insufficient for allowing significant binding of monomeric IgG, while its H131R polymorphism (histidine > arginine at position 131) influences affinity for multimeric IgG2. Platelet FcγRIIA has been reported to contribute to IgG-containing immune-complexe clearance. Given our finding that platelet FcγRIIA actually binds monomeric IgG, we investigated the role of platelets and FcγRIIA in IgG antibody elimination. We used pharmacokinetics analysis of infliximab (IgG1) in individuals with controlled Crohn’s disease. The influence of platelet count and FcγRIIA polymorphism was quantified by multivariate linear modelling. The infliximab half-life increased with R allele number (13.2, 14.4 and 15.6 days for HH, HR and RR patients, respectively). It decreased with increasing platelet count in R carriers: from ≈20 days (RR) and ≈17 days (HR) at 150 × 10⁹/L, respectively, to ≈13 days (both HR and RR) at 350 × 10⁹/L. Moreover, a flow cytometry assay showed that infliximab and monomeric IgG1 bound efficiently to platelet FcγRIIA H and R allotypes, whereas panitumumab and IgG2 bound poorly to the latter. We propose that infliximab (and presumably any IgG1 antibody) elimination is partly due to an unappreciated mechanism dependent on binding to platelet FcγRIIA, which is probably tuned by its affinity for IgG2.

The high-affinity immunoglobulin receptor FcγRI potentiates HIV-1 neutralization via antibodies against the gp41 N-heptad repeat

Despite decades of research, an effective HIV-1 vaccine remains elusive. One potential vaccine target is the N-heptad repeat (NHR) region of gp41, which is the target of the FDA-approved drug enfuvirtide. However, monoclonal antibodies and antisera targeting this region have only been modestly neutralizing to date. Here, we show that the neutralization potency of the well-characterized anti-NHR antibody D5 is increased >5,000-fold by expression of FcγRI (CD64) on cells. Since FcγRI is expressed on macrophages and dendritic cells, which are implicated in the early establishment of HIV-1 infection following sexual transmission, these results may be important to HIV-1 vaccine development.

The HIV-1 gp41 N-heptad repeat (NHR) region of the prehairpin intermediate, which is transiently exposed during HIV-1 viral membrane fusion, is a validated clinical target in humans and is inhibited by the Food and Drug Administration (FDA)-approved drug enfuvirtide. However, vaccine candidates targeting the NHR have yielded only modest neutralization activities in animals; this inhibition has been largely restricted to tier-1 viruses, which are most sensitive to neutralization by sera from HIV-1–infected individuals. Here, we show that the neutralization activity of the well-characterized NHR-targeting antibody D5 is potentiated >5,000-fold in TZM-bl cells expressing FcγRI compared with those without, resulting in neutralization of many tier-2 viruses (which are less susceptible to neutralization by sera from HIV-1–infected individuals and are the target of current antibody-based vaccine efforts). Further, antisera from guinea pigs immunized with the NHR-based vaccine candidate (ccIZN36)₃ neutralized tier-2 viruses from multiple clades in an FcγRI-dependent manner. As FcγRI is expressed on macrophages and dendritic cells, which are present at mucosal surfaces and are implicated in the early establishment of HIV-1 infection following sexual transmission, these results may be important in the development of a prophylactic HIV-1 vaccine.

Inhibitory affinity modulation of FcγRIIA ligand binding by glycosphingolipids by inside-out signaling

The interaction of the human FcγRIIA with immune complexes (ICs) promotes neutrophil activation and thus must be tightly controlled to avoid damage to healthy tissue. Here, we demonstrate that a fungal-derived soluble β-1,3/1,6-glucan binds to the glycosphingolipid long-chain lactosylceramide (LacCer) to reduce FcγRIIA-mediated recruitment to immobilized ICs under flow, a process requiring high-affinity FcγRIIA-immunoglobulin G (IgG) interactions. The inhibition requires Lyn phosphorylation of SHP-1 phosphatase and the FcγRIIA immunotyrosine-activating motif. β-glucan reduces the effective 2D affinity of FcγRIIA for IgG via Lyn and SHP-1 and, in vivo, inhibits FcγRIIA-mediated neutrophil recruitment to intravascular IgG deposited in the kidney glomeruli in a glycosphingolipid- and Lyn-dependent manner. In contrast, β-glucan did not affect FcγR functions that bypass FcγR affinity for IgG. In summary, we have identified a pathway for modulating the 2D affinity of FcγRIIA for ligand that relies on LacCer-Lyn-SHP-1-mediated inhibitory signaling triggered by β-glucan, a previously described activator of innate immunity.

Okubo et al. demonstrate that β-glucan binding to the glycosphingolipid lactosylceramide engages a Lyn kinase to SHP-1 phosphatase pathway that reduces FcγRIIA binding propensity for IgG, which suggests FcγRIIA affinity regulation by “inside-out” signaling. The β-glucan-lactosylceramide-Lyn axis prevents FcγRIIA-dependent neutrophil recruitment in vitro and to intravascular IgG deposits following glomerulonephritis.

Platelet FcγRIIA in immunity and thrombosis: Adaptive immunothrombosis

Sepsis and autoimmune diseases remain major causes of morbidity and mortality. The last decade has seen a new appreciation of platelets in host defense, in both immunity and thrombosis. Platelets are first responders in the blood to microbes or non-microbial antigens. The role of platelets in physiologic immunity is counterbalanced by their role in pathology, for example, microvascular thrombosis. Platelets encounter microbes and antigens via both innate and adaptive immune processes; platelets also help to shape the subsequent adaptive response. FcγRIIA is a receptor for immune complexes opsonized by IgG or pentraxins, and expressed in humans by platelets, granulocytes, monocytes and macrophages. With consideration of the roles of IgG and Fc receptors, the host response to microbes and autoantigens can be called adaptive immunothrombosis. Here we review newer developments involving platelet FcγRIIA in humans and humanized mice in immunity and thrombosis, with special attention to heparin-induced thrombocytopenia, systemic lupus erythematosus, and bacterial sepsis. Human genetic diversity in platelet receptors and the utility of humanized mouse models are highlighted.

Dendritic Cell Tumor Vaccination via Fc Gamma Receptor Targeting: Lessons Learned from Pre-Clinical and Translational Studies

Despite significant recent improvements in the field of immunotherapy, cancer remains a heavy burden on patients and healthcare systems. In recent years, immunotherapies have led to remarkable strides in treating certain cancers. However, despite the success of checkpoint inhibitors and the advent of cellular therapies, novel strategies need to be explored to (1) improve treatment in patients where these approaches fail and (2) make such treatments widely and financially accessible. Vaccines based on tumor antigens (Ag) have emerged as an innovative strategy with the potential to address these areas. Here, we review the fundamental aspects relevant for the development of cancer vaccines and the critical role of dendritic cells (DCs) in this process. We first offer a general overview of DC biology and routes of Ag presentation eliciting effective T cell-mediated immune responses. We then present new therapeutic avenues specifically targeting Fc gamma receptors (FcγR) as a means to deliver antigen selectively to DCs and its effects on T-cell activation. We present an overview of the mechanistic aspects of FcγR-mediated DC targeting, as well as potential tumor vaccination strategies based on preclinical and translational studies. In particular, we highlight recent developments in the field of recombinant immune complex-like large molecules and their potential for DC-mediated tumor vaccination in the clinic. These findings go beyond cancer research and may be of relevance for other disease areas that could benefit from FcγR-targeted antigen delivery, such as autoimmunity and infectious diseases.

A cell based assay for evaluating binding and uptake of an antibody using hepatic nonparenchymal cells

Evaluation of the binding and uptake of an antibody in liver non-parenchymal cells (NPC), including liver sinusoidal endothelial cells, is important for revealing its pharmacokinetic (PK) behavior, since NPC has important roles in eliminating an antibody from the blood via the Fc fragment of IgG receptor IIB (FcγRIIB). However, there is currently no in vitro quantitative assay using NPC. This study reports on the development of a cell-based assay for evaluating the binding and uptake of such an antibody using liver NPC of mice and monkeys. In mice, the FcγRIIB-expressing cells were identified in the CD146-positive and CD45-negative fraction by flow cytometry. A titration assay was performed to determine the PK parameters, and the obtained parameter was comparable to that determined by the fitting of the in vivo PK. This approach was also extended to NPC from monkeys. The concentration-dependent binding and uptake was measured to determine the PK parameters using monkey NPC, the FcγRIIB-expressing fraction of which was identified by CD31 and CD45. The findings presented herein demonstrate that the in vitro liver NPC assay using flow cytometry is a useful tool to determine the binding and uptake of biologics and to predict the PK.

Fcγ receptors on aging neutrophils

OBJECTIVE

Neutrophils are key effector cells of the innate immune system. They recognize antigens through membrane receptors, which are expressed during their maturation and activation. Neutrophils express FcγRII (CD32), FcγRIII (CD16), and FcγRI (CD64) after being activated by different factors such as cytokines and bacterial products. These receptors are involved with phagocytosis of IgG-opsonized microbes and enhance defense mechanisms. Based on that, our study seeks to compare the expression of FcγRII, FcγRIII, FcγRI, and CD11b on neutrophils from elderly and young subjects and their expression after in vitro activation with cytokines and LPS.

METHODOLOGY

Neutrophils were isolated from human peripheral blood and from mice bone marrow by density gradient. After isolation, FCγRs expression was immediately analyzed by flow cytometry or after in vitro stimulation.

RESULTS

In freshly isolated cells, the percentage of FcγRIIIb+ and CD11b+ neutrophils were higher in samples from young individuals; FcγRIIIa expression was more prominent on aged neutrophils; FcγRIA expression was similar in all samples analyzed. Exposure to CXCL8 and LPS resulted in a higher percentage of FcγRIa+ neutrophils on elderly individuals' samples but lower when compared with neutrophils from young donors. We observed that LPS caused an increase in FcγRIIa expression on aging human neutrophils. In contrast, FcγRIIIb expression in response to CXCL8 and LPS stimulation was not altered in the four groups. CD11b expression was lower in neutrophils from elderly individuals even in response to LPS and CXCL8. In mice, we observed differences only regarding CD11b expression, which was increased on aged neutrophils. LPS exposure caused an increase in all FcγRs.

CONCLUSIONS

Our results suggest that, in humans, the overall pattern of FcγR expression and integrin CD11b are altered during aging and immunosenescence might contribute to age-related infection.