Development and characterisation of monoclonal antibodies specific for the murine inhibitory FcγRIIB (CD32B)

Inhibitory Fcγ receptor deletion enhances CD8 T cell stemness increasing anti-PD-1 therapy responsiveness against glioblastoma

BACKGROUND

Certain cancers present challenges for treatment because they are resistant to immune checkpoint blockade (ICB), attributed to low tumor mutational burden and the absence of T cell-inflamed features. Among these, glioblastoma (GBM) is notoriously resistant to ICB. To overcome this resistance, the identification of T cells with heightened stemness marked by T-cell factor 1 (TCF1) expression has gained attention. Several studies have explored ways to preserve stem-like T cells and prevent terminal exhaustion. In this study, we investigate a target that triggers stem-like properties in CD8 T cells to enhance the response to ICB in a murine GBM model.

METHODS

Using Fcgr2b-/- mice and a murine GL261 GBM model, we confirmed the efficacy of anti-programmed cell death protein-1 (PD-1) immunotherapy, observing improved survival. Analysis of immune cells using fluorescence-activated cell sorting and single-cell RNA sequencing delineated distinct subsets of tumor-infiltrating CD8 T cells in Fcgr2b-/- mice. The crucial role of the stem-like feature in the response to anti-PD-1 treatment for reinvigorating CD8 T cells was analyzed. Adoptive transfer of OT-I cells into OVA-expressing GL261 models and CD8 T cell depletion in Fcgr2b-/- mice confirmed the significance of Fcgr2b-/- CD8 T cells in enhancing the antitumor response. Last, S1P1 inhibitor treatment confirmed that the main source of tumor antigen-specific Fcgr2b-/- CD8 T cells is the tumor-draining lymph nodes (TdLNs).

RESULTS

In a murine GBM model, anti-PD-1 monotherapy and single-Fc fragment of IgG receptor IIb (FcγRIIB) deletion exhibit limited efficacy. However, their combination substantially improves survival by enhancing cytotoxicity and proliferative capacity in tumor-infiltrating Fcgr2b-/- CD8 T cells. The improved response to anti-PD-1 treatment is associated with the tumor-specific memory T cells (Ttsms) exhibiting high stemness characteristics within the tumor microenvironment (TME). Ttsms in the TdLN thrives in a protective environment, maintaining stem-like characteristics and serving as a secure source for tumor infiltration. This underscores the significance of FcγRIIB ablation in triggering Ttsms and enhancing ICB therapy against GBM.

CONCLUSIONS

Deletion of FcγRIIB on CD8 T cells leads to the generation of a Ttsms, which is localized in TdLN and protected from the immunosuppressive TME. Incorporating these highly stemness-equipped Ttsms enhances the response to anti-PD-1 therapy in immune-suppressed brain tumors.

Integrated analyses revealed the potential role and immune link of mitochondrial dysfunction between periodontitis and type 2 diabetes mellitus

There is a reciprocal comorbid relationship between periodontitis and type 2 diabetes mellitus (T2DM). Recent studies have suggested that mitochondrial dysfunction (MD) could be the key driver underlying this comorbidity. The aim of this study is to provide novel understandings into the potential molecular mechanisms between MD and the comorbidity, and identify potential therapeutic targets for personalized clinical management. MD-related differentially expressed genes (MDDEGs) were identified. Enrichment analyses and PPI network analysis were then conducted. Six algorithms were used to explore the hub MDDEGs, and these were validated by ROC analysis and qRT-PCR. Co-expression and potential drug targeting analyses were then performed. Potential biomarkers were identified using LASSO regression. The immunocyte infiltration levels in periodontitis and T2DM were evaluated via CIBERSORTx and validated in mouse models. Subsequently, MD-related immune-related genes (MDIRGs) were screened by WGCNA. The in vitro experiment verified that MD was closely associated with this comorbidity. GO and KEGG analyses demonstrated that the connection between periodontitis and T2DM was mainly enriched in immuno-inflammatory pathways. In total, 116 MDDEGs, eight hub MDDEGs, and two biomarkers were identified. qRT-PCR revealed a distinct hub MDDEG expression pattern in the comorbidity group. Altered immunocytes in disease samples were identified, and their correlations were explored. The in vivo examination revealed higher infiltration levels of inflammatory immunocytes. The findings of this study provide insight into the mechanism underlying the gene-mitochondria-immunocyte network and provide a novel reference for future research into the function of mitochondria in periodontitis and T2DM.

FcγRIIB expressed on CD8+ T cells limits responsiveness to PD-1 checkpoint inhibition in cancer

Checkpoint inhibition using Fc-containing monoclonal antibodies has emerged as a powerful therapeutic approach to augment antitumor immunity. We recently showed that FcγRIIB, the only inhibitory IgG-Fc receptor, is expressed on a population of highly differentiated effector CD8+ T cells in the tumors of mice and humans, raising the possibility that CD8+ T cell responses may be directly modulated by checkpoint inhibitor binding to T cell-expressed FcγRIIB. Here, we show that despite exhibiting strong proliferative and cytokine responses at baseline, human FcγRIIBpos CD8+ T cells exhibited reduced responsiveness to both PD-1 and CTLA-4 checkpoint inhibition as compared with FcγRIIBneg CD8+ T cells in vitro. Moreover, frequencies of FcγRIIBpos CD8+ T cells were reduced after treatment of patients with melanoma with nivolumab in vivo. This reduced responsiveness was FcγRIIB dependent, because conditional genetic deletion of FcγRIIB on tumor-specific CD8+ T cells improved response to checkpoint blockade in B16 and LLC mouse models of cancer. The limited responsiveness of FcγRIIBpos CD8+ T cells was also dependent on an intact Fc region of the checkpoint inhibitor, in that treatment with Fc-devoid anti-PD-1 F(ab) fragments resulted in increased proliferation of FcγRIIBpos CD8+ T cells, without altering the response of FcγRIIBneg CD8+ T cells. Last, the addition of FcγRIIB blockade improved efficacy of PD-1 checkpoint inhibition in mouse models of melanoma, lung, and colon cancer. These results illuminate an FcγRIIB-mediated, cell-autonomous mechanism of CD8+ T cell suppression, which limits the efficacy of checkpoint inhibitors during antitumor immune responses in vivo.

FcγRIIB potentiates differentiation of myeloid-derived suppressor cells to mediate tumor immunoescape

Background: FcγRIIB, the sole inhibitory receptor of the Fc gamma receptor family, plays pivotal roles in innate and adaptive immune responses. However, the expression and function of FcγRIIB in myeloid-derived suppressor cells (MDSCs) remains unknown. This study aimed to investigate whether and how FcγRIIB regulates the immunosuppressive activity of MDSCs during cancer development. Methods: The MC38 and B16-F10 tumor-bearing mouse models were established to investigate the role of FcγRIIB during tumor progression. FcγRIIB-deficient mice, adoptive cell transfer, mRNA-sequencing and flow cytometry analysis were used to assess the role of FcγRIIB on immunosuppressive activity and differentiation of MDSCs. Results: Here we show that FcγRIIB was upregulated in tumor-infiltrated MDSCs. FcγRIIB-deficient mice showed decreased accumulation of MDSCs in the tumor microenvironment (TME) compared with wild-type mice. FcγRIIB was required for the differentiation and immunosuppressive activity of MDSCs. Mechanistically, tumor cell-derived granulocyte-macrophage colony stimulating factor (GM-CSF) increased the expression of FcγRIIB on hematopoietic progenitor cells (HPCs) by activating specificity protein 1 (Sp1), subsequently FcγRIIB promoted the generation of MDSCs from HPCs via Stat3 signaling. Furthermore, blockade of Sp1 dampened MDSC differentiation and infiltration in the TME and enhanced the anti-tumor therapeutic efficacy of gemcitabine. Conclusion: These results uncover an unrecognized regulatory role of the FcγRIIB in abnormal differentiation of MDSCs during cancer development and suggest a potential therapeutic target for anti-tumor therapy.

Synergistic effect of non-neutralizing antibodies and interferon-γ for cross-protection against influenza

Current influenza vaccines do not typically confer cross-protection against antigenically mismatched strains. To develop vaccines conferring broader cross-protection, recent evidence indicates the crucial role of both cross-reactive antibodies and viral-specific CD4+ T cells; however, the precise mechanism of cross-protection is unclear. Furthermore, adjuvants that can efficiently induce cross-protective CD4+ T cells have not been identified. Here we show that CpG oligodeoxynucleotides combined with aluminum salts work as adjuvants for influenza vaccine and confer strong cross-protection in mice. Both cross-reactive antibodies and viral-specific CD4+ T cells contributed to cross-protection synergistically, with each individually ineffective. Furthermore, we found that downregulated expression of Fcγ receptor IIb on alveolar macrophages due to IFN-γ secreted by viral-specific CD4+ T cells improves the activity of cross-reactive antibodies. Our findings inform the development of optimal adjuvants for vaccines and how influenza vaccines confer broader cross-protection.

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.

Genetic dissection of a major haplotype associated with arthritis reveal FcγR2b and FcγR3 to act additively

A haplotype with tightly linked Fc gamma receptor (FcγR) genes is known as a major locus controlling immune responses and autoimmune diseases, including arthritis. Here, we split a congenic fragment derived from the NOD mouse (Cia9) to study its effect on immune response and arthritis in mice. We found that arthritis susceptibility was indeed controlled by the FcγR gene cluster and a recombination between the FcγR2b and FcγR3 loci gave us the opportunity to separately study their impact. We identified the NOD-derived FcγR2b and FcγR3 alleles as disease-promoting for arthritis development without impact on antibody secretion. We further found that macrophage-mediated phagocytosis was directly correlated to FcγR3 expression in the congenic mice. In conclusion, we positioned FcγR2b and FcγR3 alleles as disease regulatory and showed that their genetic polymorphisms independently and additively control innate immune cell activation and arthritis.

Pharmacokinetic prediction of an antibody in mice based on an in vitro cell-based approach using target receptor-expressing cells

In vivo pharmacokinetics (PK) studies using mice and monkeys are the main approaches for evaluating and predicting the PK of antibodies, and there is a strong demand for methods that do not require animal experiments. In this work, we focused on quantitatively predicting the nonlinear PK of an antibody based on cell-based assays. An anti-mouse Fc gamma receptor IIB antibody was used as a model antibody. To determine the PK parameters related to nonspecific elimination in vivo, the plasma concentration profile at 100 mg/kg, at which target-specific clearance is saturated, was analyzed by a 2-compartment model. To estimate the parameters related to target-specific elimination, the Michaelis–Menten constant (K_m) and the maximum elimination rate (V_max) were determined by an uptake assay using Chinese hamster ovary (CHO) cells expressing the target receptor. Finally, the integration of all of these parameters permitted the PK to be predicted at doses ranging from 1 to 100 mg/kg regardless of whether target-specific clearance was saturated or nonsaturated. The findings presented herein show that in vitro assays using target-expressing cells are useful tools for obtaining PK parameters and predicting PK profiles and, in some cases, eliminate the need for in vivo PK studies using experimental animals.

FcγRII (CD32) modulates antibody clearance in NOD SCID mice leading to impaired antibody-mediated tumor cell deletion

BACKGROUND

Immune compromised mice are increasingly used for the preclinical development of monoclonal antibodies (mAb). Most common are non-obese diabetic (NOD) severe combined immunodeficient (SCID) and their derivatives such as NOD SCID interleukin-2 γ-/- (NSG), which are attractive hosts for patient-derived xenografts. Despite their widespread use, the relative biological performance of mAb in these strains has not been extensively studied.

METHODS

Clinically relevant mAb of various isotypes were administered to tumor and non-tumor-bearing SCID and NOD SCID mice and the mAb clearance monitored by ELISA. Expression analysis of surface proteins in both strains was carried out by flow cytometry and immunofluorescence microscopy. Further analysis was performed in vitro by surface plasmon resonance to assess mAb affinity for Fcγ receptors (FcγR) at pH 6 and pH 7.4. NOD SCID mice genetically deficient in different FcγR were used to delineate their involvement.

RESULTS

Here, we show that strains on the NOD SCID background have significantly faster antibody clearance than other strains leading to reduced antitumor efficacy of clinically relevant mAb. This rapid clearance is dependent on antibody isotype, the presence of Fc glycosylation (at N297) and expression of FcγRII. Comparable effects were not seen in the parental NOD or SCID strains, demonstrating the presence of a compound defect requiring both genotypes. The absence of endogenous IgG was the key parameter transferred from the SCID as reconstituting NOD SCID or NSG mice with exogenous IgG overcame the rapid clearance and recovered antitumor efficacy. In contrast, the NOD strain was associated with reduced expression of the neonatal Fc Receptor (FcRn). We propose a novel mechanism for the rapid clearance of certain mAb isotypes in NOD SCID mouse strains, based on their interaction with FcγRII in the context of reduced FcRn.

CONCLUSIONS

This study highlights the importance of understanding the limitation of the mouse strain being used for preclinical evaluation, and demonstrates that NOD SCID strains of mice should be reconstituted with IgG prior to studies of mAb efficacy.

Recombinant Factor VIII Fc Inhibits B Cell Activation via Engagement of the FcγRIIB Receptor

The development of neutralizing antibodies (inhibitors) against factor VIII (FVIII) is a major complication of hemophilia A treatment. The sole clinical therapy to restore FVIII tolerance in patients with inhibitors remains immune tolerance induction (ITI) which is expensive, difficult to administer and not always successful. Although not fully understood, the mechanism of ITI is thought to rely on inhibition of FVIII-specific B cells (1). Its efficacy might therefore be improved through more aggressive B cell suppression. FcγRIIB is an inhibitory Fc receptor that down-regulates B cell signaling when cross-linked with the B cell receptor (BCR). We sought to investigate if recombinant FVIII Fc (rFVIIIFc), an Fc fusion molecule composed of FVIII and the Fc region of immunoglobulin G1 (IgG1) (2), is able to inhibit B cell activation more readily than FVIII. rFVIIIFc was able to bind FVIII-exposed and naïve B cells from hemophilia A mice as well as a FVIII-specific murine B cell hybridoma line (413 cells). An anti-FcγRIIB antibody and FVIII inhibited binding, suggesting that rFVIIIFc is able to interact with both FcγRIIB and the BCR. Furthermore, incubation of B cells from FVIII-exposed mice and 413 cells with rFVIIIFc resulted in increased phosphorylation of SH-2 containing inositol 5-phosphatase (SHIP) when compared to FVIII. B cells from FVIII-exposed hemophilia A mice also exhibited decreased extracellular signal-regulated kinase (ERK) phosphorylation when exposed to rFVIIIFc. These differences were absent in B cells from naïve, non-FVIII exposed hemophilic mice suggesting an antigen-dependent effect. Finally, rFVIIIFc was able to inhibit B cell calcium flux induced by anti-Ig F(ab)₂. Our results therefore indicate that rFVIIIFc is able to crosslink FcγRIIB and the BCR of FVIII-specific B cells, causing inhibitory signaling in these cells.

Signaling through the Inhibitory Fc Receptor FcγRIIB Induces CD8+ T Cell Apoptosis to Limit T Cell Immunity

Effector CD8+ T cells are important mediators of adaptive immunity, and receptor-ligand interactions that regulate their survival may have therapeutic potential. Here, we identified a subset of effector CD8+ T cells that expressed the inhibitory fragment crystallizable (Fc) receptor FcγRIIB following activation and multiple rounds of division. CD8+ T cell-intrinsic genetic deletion of Fcgr2b increased CD8+ effector T cell accumulation, resulting in accelerated graft rejection and decreased tumor volume in mouse models. Immunoglobulin G (IgG) antibody was not required for FcγRIIB-mediated control of CD8+ T cell immunity, and instead, the immunosuppressive cytokine fibrinogen-like 2 (Fgl2) was a functional ligand for FcγRIIB on CD8+ T cells. Fgl2 induced caspase-3/7-mediated apoptosis in Fcgr2b+, but not Fcgr2b-/-, CD8+ T cells. Increased expression of FcγRIIB correlated with freedom from rejection following withdrawal from immunosuppression in a clinical trial of kidney transplant recipients. Together, these findings demonstrate a cell-intrinsic coinhibitory function of FcγRIIB in regulating CD8+ T cell immunity.

The Use of Anti-CD40 mAb in Cancer

Immunomodulatory monoclonal antibody (mAb) therapy is at the forefront of developing cancer therapeutics with numerous targeted agents proving highly effective in selective patients at stimulating protective host immunity, capable of eradicating established tumours and leading to long-term disease-free states. The cell surface marker CD40 is expressed on a range of immune cells and transformed cells in malignant states whose signalling plays a critical role in modulating adaptive immune responses. Anti-CD40 mAb therapy acts via multiple mechanisms to stimulate anti-tumour immunity across a broad range of lymphoid and solid malignancies. A wealth of preclinical research in this field has led to the successful development of multiple anti-CD40 mAb agents that have shown promise in early-phase clinical trials. Significant progress has been made to enhance the engagement of antibodies with immune effectors through their interactions with Fcγ receptors (FcγRs) by the process of Fc engineering. As more is understood about how to best optimise these agents, principally through the fine-tuning of mAb structure and choice of synergistic partnerships, our ability to generate robust, clinically beneficial anti-tumour activity will form the foundation for the next generation of cancer therapeutics.

Immune Checkpoints as Therapeutic Targets in Autoimmunity

Antibodies that block the immune checkpoint receptors PD1 and CTLA4 have revolutionized the treatment of melanoma and several other cancers, but in the process, a new class of drug side effect has emerged—immune related adverse events. The observation that therapeutic blockade of these inhibitory receptors is sufficient to break self-tolerance, highlights their crucial role in the physiological modulation of immune responses. Here, we discuss the rationale for targeting immune checkpoint receptors with agonistic agents in autoimmunity, to restore tolerance when it is lost. We review progress that has been made to date, using Fc-fusion proteins, monoclonal antibodies or other novel constructs to induce immunosuppressive signaling through these pathways. Finally, we explore potential mechanisms by which these receptors trigger and modulate immune cell function, and how understanding these processes might shape the design of more effective therapeutic agents in future.

New revelations from an old receptor: Immunoregulatory functions of the inhibitory Fc gamma receptor, FcγRIIB (CD32B)

The Fc gamma receptor IIB (FcγRIIB/CD32B) was generated million years ago during evolution. It is the sole inhibitory receptor for IgG, and has long been associated with the regulation of humoral immunity and innate immune homeostasis. However, new and surprising functions of FcγRIIB are emerging. In particular, FcγRIIB has been shown to perform unexpected activatory roles in both immune-signaling and monoclonal antibody (mAb) immunotherapy. Furthermore, although ITIM signaling is an integral part of FcγRIIB regulatory activity, it is now clear that inhibition/activation of immune responses can occur independently of the ITIM. In light of these new findings, we present an overview of the established and noncanonical functions of FcγRIIB and discuss how this knowledge might be exploited therapeutically.

Anti-FcγRIIB (CD32) Antibodies Differentially Modulate Murine FVIII-Specific Recall Response in vitro

Fc gamma receptors (FcγRs) for IgG regulate adaptive immune responses by modulating activating and inhibitory signalling pathways within immune cells. Data from a haemophilia A mouse model demonstrate that genetic deletion or blockade of the inhibitory FcγR (CD32) suppresses the formation of antibody-secreting cells (ASCs) in vitro. Mechanisms preventing the FVIII-specific recall response, however, remain unclear. Here, the potential role of CD32 inhibition was studied by differentially modulating receptor activity with selected anti-CD32 monoclonal antibodies (mAbs). Splenocytes from immunized FVIII^-/- mice were restimulated with FVIII in the absence or presence of different anti-CD32 mAbs over 6 days. At day 6, cytokine release was quantified from cell culture supernatant and the formation of FVIII-specific ASCs assessed. Binding of FVIII-containing immune complexes (F8-ICs) to bone marrow-derived dendritic cells (BMdDCs) was also investigated. The antagonistic CD32 mAb AT128 suppressed the formation of FVIII-specific ASCs and reduced secretion of IFN-γ and IL-10. In contrast, the agonistic mAbs AT130-2 and AT130-5, and their F(ab')₂ fragments, allowed the formation of FVIII-specific ASCs, even though the full IgG of AT130-2 reduced binding of F8-ICs to CD32. Data suggest that an inhibitory signal is transmitted when F8-ICs bind to CD32 and that this signal is required during memory B cell (MBC) activation to support formation of FVIII-specific ASCs. If the inhibitory signal is lacking due to CD32 deletion or blockade with antagonistic anti-CD32 mAbs, FVIII-specific T cell stimulation and ASC formation are suppressed, whereas agonistic stimulation of CD32 restores T cell stimulation and ASC formation.

CD32 inhibition and high dose of rhFVIII suppress murine FVIII-specific recall response by distinct mechanisms in vitro

Development of neutralising antibodies (inhibitors) against factor VIII (FVIII) is a frequent and severe complication of replacement therapy in haemophilia A. Previous data from haemophilia A mouse model demonstrates that both CD32 inhibition and high doses of rhFVIII prevent the differentiation of FVIII-specific memory B cells (MBCs) into antibody secreting cells (ASCs). Here, cellular targets responsible for the suppression of ASC formation by means of CD32 inhibition and high dose of rhFVIII were analysed. We investigated apoptosis on FVIII-specific MBCs using a pan caspases inhibitor, and screened for defects in rhFVIII presentation by analysing T cell release of Th1- and Th2-cytokines in vitro. Although high dose of rhFVIII suppressed ASC formation, cytokine response was not affected. Upon re-stimulation of splenocytes with high dose of rhFVIII, prevention of apoptosis fully restored the FVIII-specific recall response. In contrast, genetic deletion or inhibition of CD32 significantly altered Th1- and Th2-response. CD32 blockade and inhibition of apoptosis resulted in a partial rescue of FVIII-specific ASCs. Normal cytokine secretion could not be restored. In conclusion, suppression of FVIII-specific recall response by CD32 and high doses of rhFVIII is mediated by distinct mechanisms. High dose of rhFVIII induces apoptosis in FVIII-specific MBCs but does not influence FVIII-specific T cell response. CD32 blockade, however, may suppress the FVIII-specific recall response by two ways: i) increasing apoptosis of FVIII-specific MBCs and ii) disturbing FVIII-specific T cell response by modulating presentation of rhFVIII to CD4⁺ T cells in vitro.

Lack of Fc Gamma Receptor IIIA Promotes Rather than Suppresses Humoral and Cellular Immune Responses after Mucosal or Parenteral Immunization with Antigen and Adjuvants

The Fcγ receptor IIIA (FcγRIIIA) has traditionally been known as a positive regulator of immune responses. Consistent with this, mice deficient in FcγRIIIA are protected from various inflammation-associated pathologies including several autoimmune diseases. In contrast to this accepted dogma, we show here that mice lacking FcγRIIIA developed increased rather than reduced both humoral and cellular immune responses to mucosal (sublingual) immunization with ovalbumin (OVA) given together with the strong mucosal adjuvant cholera toxin as well as to parenteral (subcutaneous) immunization with OVA in complete Freund's adjuvant. After either route of immunization, in comparison with concomitantly immunized wild-type mice, FcγRIIIA-/- mice had increased serum anti-OVA IgG (IgG1 but not IgG2) antibody responses as well as augmented cellular responses that included memory B cells and effector T cells. The increments in immune responses in FcγRIIIA-/- mice were similar to those seen in FcγRIIB-/- mice. Furthermore, OVA-pulsed FcγRIIIA-/- DCs, similar to OVA-specific FcγRIIB-/- DCs, had enhanced capacity to activate OVA-specific OT-II T cells, which was even further pronounced when DCs were pulsed with IgG1-complexed OVA. Our data support an inhibitory-regulatory role of FcγRIIIA on vaccine/adjuvant-induced immune responses and demonstrate that lack of FcγRIIIA can promote rather than suppress both humoral and cellular immune responses.

Therapeutic antibody targeting of indoleamine-2,3-dioxygenase (IDO2) inhibits autoimmune arthritis

Rheumatoid arthritis (RA) is a debilitating inflammatory autoimmune disease with no known cure. Recently, we identified the immunomodulatory enzyme indoleamine-2,3-dioxygenase 2 (IDO2) as an essential mediator of autoreactive B and T cell responses driving RA. However, therapeutically targeting IDO2 has been challenging given the lack of small molecules that specifically inhibit IDO2 without also affecting the closely related IDO1. In this study, we develop a novel monoclonal antibody (mAb)-based approach to therapeutically target IDO2. Treatment with IDO2-specific mAb alleviated arthritis in two independent preclinical arthritis models, reducing autoreactive T and B cell activation and recapitulating the strong anti-arthritic effect of genetic IDO2 deficiency. Mechanistic investigations identified FcγRIIb as necessary for mAb internalization, allowing targeting of an intracellular antigen traditionally considered inaccessible to mAb therapy. Taken together, our results offer preclinical proof of concept for antibody-mediated targeting of IDO2 as a new therapeutic strategy to treat RA and other autoantibody-mediated diseases.

Harnessing co-stimulatory TNF receptors for cancer immunotherapy: Current approaches and future opportunities

Co-stimulatory tumor necrosis factor receptors (TNFRs) can sculpt the responsiveness of T cells recognizing tumor-associated antigens. For this reason, agonist antibodies targeting CD137, CD357, CD134 and CD27 have received considerable attention for their therapeutic utility in enhancing anti-tumor immune responses, particularly in combination with other immuno-modulatory antibodies targeting co-inhibitory pathways in T cells. The design of therapeutic antibodies that optimally engage and activate co-stimulatory TNFRs presents an important challenge of how to promote effective anti-tumor immunity while avoiding serious immune-related adverse events. Here we review our current understanding of the expression, signaling and structural features of CD137, CD357, CD134 and CD27, and how this may inform the design of pharmacologically active immuno-modulatory antibodies targeting these receptors. This includes the integration of our emerging knowledge of the role of Fcγ receptors (FcγRs) in facilitating antibody-mediated receptor clustering and forward signaling, as well as promoting immune effector cell-mediated activities. Finally, we bring our current preclinical and clinical knowledge of co-stimulatory TNFR antibodies into the context of opportunities for next generation molecules with improved pharmacologic properties.

Mouse and human FcR effector functions

Mouse and human FcRs have been a major focus of attention not only of the scientific community, through the cloning and characterization of novel receptors, and of the medical community, through the identification of polymorphisms and linkage to disease but also of the pharmaceutical community, through the identification of FcRs as targets for therapy or engineering of Fc domains for the generation of enhanced therapeutic antibodies. The availability of knockout mouse lines for every single mouse FcR, of multiple or cell-specific--'à la carte'--FcR knockouts and the increasing generation of hFcR transgenics enable powerful in vivo approaches for the study of mouse and human FcR biology. This review will present the landscape of the current FcR family, their effector functions and the in vivo models at hand to study them. These in vivo models were recently instrumental in re-defining the properties and effector functions of FcRs that had been overlooked or discarded from previous analyses. A particular focus will be made on the (mis)concepts on the role of high-affinity IgG receptors in vivo and on results from antibody engineering to enhance or abrogate antibody effector functions mediated by FcRs.

IgG subclasses determine pathways of anaphylaxis in mice

BACKGROUND

Animal models have demonstrated that allergen-specific IgG confers sensitivity to systemic anaphylaxis that relies on IgG Fc receptors (FcγRs). Mouse IgG_2a and IgG_2b bind activating FcγRI, FcγRIII, and FcγRIV and inhibitory FcγRIIB; mouse IgG₁ binds only FcγRIII and FcγRIIB. Although these interactions are of strikingly different affinities, these 3 IgG subclasses have been shown to enable induction of systemic anaphylaxis.

OBJECTIVE

We sought to determine which pathways control the induction of IgG₁-, IgG_2a-, and IgG_2b-dependent passive systemic anaphylaxis.

METHODS

Mice were sensitized with IgG₁, IgG_2a, or IgG_2b anti-trinitrophenyl mAbs and challenged with trinitrophenyl-BSA intravenously to induce systemic anaphylaxis that was monitored by using rectal temperature. Anaphylaxis was evaluated in mice deficient for FcγRs injected with mediator antagonists or in which basophils, monocytes/macrophages, or neutrophils had been depleted. FcγR expression was evaluated on these cells before and after anaphylaxis.

RESULTS

Activating FcγRIII is the receptor primarily responsible for all 3 models of anaphylaxis, and subsequent downregulation of this receptor was observed. These models differentially relied on histamine release and the contribution of mast cells, basophils, macrophages, and neutrophils. Strikingly, basophil contribution and histamine predominance in mice with IgG₁- and IgG_2b-induced anaphylaxis correlated with the ability of inhibitory FcγRIIB to negatively regulate these models of anaphylaxis.

CONCLUSION

We propose that the differential expression of inhibitory FcγRIIB on myeloid cells and its differential binding of IgG subclasses controls the contributions of mast cells, basophils, neutrophils, and macrophages to IgG subclass-dependent anaphylaxis. Collectively, our results unravel novel complexities in the involvement and regulation of cell populations in IgG-dependent reactions in vivo.

Regulation of Monoclonal Antibody Immunotherapy by FcγRIIB

Monoclonal antibodies (mAb) are revolutionising the treatment of many different diseases. Given their differing mode of action compared to most conventional chemotherapeutics and small molecule inhibitors, they possess the potential to be independent of common modes of treatment resistance and can typically be combined readily with existing treatments without dose-limiting toxicity. However, treatments with mAb rarely result in cure and so a full understanding of how these reagents work and can be optimised is key for their subsequent improvement. Here we review how an understanding of the biology of the inhibitory Fc receptor, FcγRIIB (CD32B), is leading to the development of improved mAb treatments.

Development and Characterization of Monoclonal Antibodies Specific for Mouse and Human Fcγ Receptors

FcγRs are key regulators of the immune response, capable of binding to the Fc portion of IgG Abs and manipulating the behavior of numerous cell types. Through a variety of receptors, isoforms, and cellular expression patterns, they are able to fine-tune and direct appropriate responses. Furthermore, they are key determinants of mAb immunotherapy, with mAb isotype and FcγR interaction governing therapeutic efficacy. Critical to understanding the biology of this complex family of receptors are reagents that are robust and highly specific for each receptor. In this study, we describe the development and characterization of mAb panels specific for both mouse and human FcγR for use in flow cytometry, immunofluorescence, and immunocytochemistry. We highlight key differences in expression between the two species and also patterns of expression that will likely impact on immunotherapeutic efficacy and translation of therapeutic agents from mouse to clinic.

FcγR requirements leading to successful immunotherapy

Monoclonal antibody (mAb) immunotherapy is currently experiencing an unprecedented amount of success, delivering blockbuster sales for the pharmaceutical industry. Having experienced several false dawns and overcoming technical issues which limited progress, we are now entering a golden period where mAbs are becoming a mainstay of treatment regimes for diseases ranging from cancer to autoimmunity. In this review, we discuss how these mAbs are most likely working and focus in particular on the key receptors that they interact with to precipitate their therapeutic effects. Although their targets may vary, their engagement with Fcγ receptors (FcγRs) on numerous immune effector cells is almost universal, and here we review their roles in delivering successful immunotherapy.

Effect of trastuzumab interchain disulfide bond cleavage on Fcγ receptor binding and antibody-dependent tumour cell phagocytosis

The Fc domain of human IgG1 binds to Fcγ receptors (FcγRs) to induce effector functions such as phagocytosis. There are four interchain disulfide bonds between the H and L chains. In this study, the disulfide bonds within the IgG1 trastuzumab (TRA), which is specific for HER2, were cleaved by mild S-sulfonation or by mild reduction followed by S-alkylation with three different reagents. The cleavage did not change the binding activities of TRA to HER2-bearing SK-BR-3 cells. The binding activities of TRA to FcγRIIA and FcγRIIB were greatly enhanced by modification with mild reduction and S-alkylation with ICH2CONH2 or N-(4-aminophenyl) maleimide, while the binding activities of TRA to FcγRI and FcγRIIIA were decreased by any of the four modifications. However, the interchain disulfide bond cleavage by the different modifications did not change the antibody-dependent cell-mediated phagocytosis (ADCP) of SK-BR-3 cells by activated THP-1 cells. The order of FcγR expression levels on the THP-1 cells was FcγRII > FcγRI > FcγRIII and ADCP was inhibited by blocking antibodies against FcγRI and FcγRII. These results imply that the effect of the interchain disulfide bond cleavage on FcγRs binding and ADCP is dependent on modifications of the cysteine residues and the FcγR isotypes.

Deletion or inhibition of Fc gamma receptor 2B (CD32) prevents FVIII-specific activation of memory B cells in vitro

Development of inhibitory antibodies against factor VIII (FVIII) is a severe complication of replacement therapy in haemophilia A. Patients with inhibitors are treated with high FVIII doses in the context of immune tolerance therapy (ITT). Data from haemophilia A mouse model suggest that high FVIII concentrations prevent the formation of antibody secreting cells (ASCs) from memory B cells (MBCs) by inducing apoptosis. Fc gamma receptor 2B (CD32) is an important regulator of B cell function, mediating inhibitory signals after cross-linking with the B cell receptor. Here, the role of CD32 in the regulation of FVIII-specific MBCs was investigated using F8-/- and F8-/-CD32-/- knockout mice and monoclonal antibodies (mAbs). The initial immune response was similar between F8-/- and F8-/-CD32-/- mice, including concentration of anti-FVIII antibodies and number of FVIII-specific ASCs in spleen and bone marrow. In contrast, formation of ASCs from MBCs upon rhFVIII re-stimulation in vitro was abolished in F8-/-CD32-/- mice, whereas FVIII/anti-FVIII immune complexes significantly enhanced ASC formation in F8-/- mice. Inhibition of CD32 by mAbs or F(ab)2 fragments prevented ASC formation in a dose-dependent manner. Transfer of B cell-depleted splenocytes using CD45R (B220) depletion from CD32-competent mice did not restore ASC formation in F8-/-CD32-/- cells confirming that CD32 is required on B cells. We conclude that CD32 is a crucial regulator of FVIII-specific B cells and is required for the differentiation of MBCs into ASCs. Inhibition of CD32 could potentially improve the efficacy of FVIII in the context of ITT.

Antagonistic human FcγRIIB (CD32B) antibodies have anti-tumor activity and overcome resistance to antibody therapy in vivo

Therapeutic antibodies have transformed cancer therapy, unlocking mechanisms of action by engaging the immune system. Unfortunately, cures rarely occur and patients display intrinsic or acquired resistance. Here, we demonstrate the therapeutic potential of targeting human (h) FcγRIIB (CD32B), a receptor implicated in immune cell desensitization and tumor cell resistance. FcγRIIB-blocking antibodies prevented internalization of the CD20-specific antibody rituximab, thereby maximizing cell surface accessibility and immune effector cell mediated antitumor activity. In hFcγRIIB-transgenic (Tg) mice, FcγRIIB-blocking antibodies effectively deleted target cells in combination with rituximab, and other therapeutic antibodies, from resistance-prone stromal compartments. Similar efficacy was seen in primary human tumor xenografts, including with cells from patients with relapsed/refractory disease. These data support the further development of hFcγRIIB antibodies for clinical assessment.

Fcγ receptor dependency of agonistic CD40 antibody in lymphoma therapy can be overcome through antibody multimerization

Immunomodulatory mAbs, led by the anti-CTLA4 mAb ipilimumab, are an exciting new class of drugs capable of promoting anticancer immunity and providing durable control of some tumors. Close analysis of a number of agents has revealed a critical yet variable role for Fcγ receptors in their efficacy. In this article, we reveal that agonistic anti-CD40 mAbs have an absolute requirement for cross-linking by inhibitory FcγRIIB when used systemically to treat established BCL1 syngeneic lymphoma, and therapy is lost when using a mouse IgG2a mAb not cross-linked by FcγRIIB. Furthermore, in FcγRIIB-deficient mice the lymphoma itself can provide FcγRIIB to cross-link anti-CD40 on neighboring cells, and only when this is blocked does therapy fail. The dependence on FcγRIIB for immunostimulatory activity was not absolute, however, because when anti-CD40 mAbs were administered systemically with the TLR3 agonist polyinosinic:polycytidylic acid or were given subcutaneously, activatory FcγR could also provide cross-linking. Using this mechanistic insight, we designed multimeric forms of anti-CD40 mAb with intrinsic FcγR-independent activity that were highly effective in the treatment of lymphoma-bearing mice. In conclusion, FcγR-independent anti-CD40 activation is a viable strategy in vivo. These findings have important translational implications, as humans, unlike mice, do not have IgG that binds strongly to FcγRIIB; therefore FcγR-independent derivatives represent an attractive therapeutic option.

FcγRIIB as a key determinant of agonistic antibody efficacy

Fc gamma Receptor (FcγR) IIB (CD32B) is an immunoreceptor tyrosine inhibitory motif (ITIM)-bearing Fc receptor that is involved in abrogating the signalling and function delivered from other receptors; archetypally those arising from other, activatory, FcγR and from the B cell receptor (BCR) for antigen. In the context of immunotherapy, it has convincingly been shown to limit a variety of clinically important therapeutic monoclonal antibodies (mAb) such as rituximab and trastuzumab in preclinical models. However, recent exploration of so-called immunomodulatory mAb, for example agonist mAb directed against various members of the TNFR super-family, has cast new light on the ability of FcγRIIB to regulate immune responses and immunotherapy. These data, accumulated by several independent groups, have shown the seemingly paradoxical ability of FcγRIIB to augment or even be absolutely required for the activity of this class of mAb. In this review we highlight the key role of FcγRIIB in regulating agonistic mAb, detail the likely mechanism of action and propose new ways in which this information may be exploited therapeutically.

Inhibitory FcγRIIb (CD32b) becomes activated by therapeutic mAb in both cis and trans and drives internalization according to antibody specificity

A major feature that distinguishes type I from type II anti-CD20 monoclonal antibodies (mAbs) and reduces their therapeutic efficacy is the tendency to internalize from the cell surface. We have shown previously that the extent of internalization correlates with the capacity of type I mAb to simultaneously engage both CD20 and the inhibitory Fcγ receptor, FcγRIIb, in a bipolar configuration. Here, we investigated whether mAbs directed at other B-cell surface receptors also engaged FcγRIIb and whether this interaction promoted internalization. Most mAbs engaged and activated FcγRIIb, with the strength of activation related to the level of mAb bound to the cell surface. However, engagement did not affect internalization of most mAb-ligated receptors, either in cell lines or primary chronic lymphocytic leukemia cells with the exception of CD19 and CD38. Furthermore, at high cell concentrations/density both cis and trans interactions between cell-surface bound mAb and FcγRIIb were evident, but trans interactions did not inhibit type I anti-CD20 mAb-mediated internalization. These data identify that FcγRIIb is engaged by many mAbs in both cis and trans configurations, triggering its activation, but that internalization via FcγRIIb occurs for only a select subset. These findings have implications when designing new antibody-based therapeutics.