The neonatal Fc receptor (FcRn) is not required for IVIg or anti-CD44 monoclonal antibody–mediated amelioration of murine immune thrombocytopenia

Anti-Inflammatory and Immunomodulatory Effect of High-Dose Immunoglobulins in Children: From Approved Indications to Off-Label Use

BACKGROUND

The large-scale utilization of immunoglobulins in patients with inborn errors of immunity (IEIs) since 1952 prompted the discovery of their key role at high doses as immunomodulatory and anti-inflammatory therapy, in the treatment of IEI-related immune dysregulation disorders, according to labelled and off-label indications. Recent years have been dominated by a progressive imbalance between the gradual but constant increase in the use of immunoglobulins and their availability, exacerbated by the SARS-CoV-2 pandemic.

OBJECTIVES

To provide pragmatic indications for a need-based application of high-dose immunoglobulins in the pediatric context.

SOURCES

A literature search was performed using PubMed, from inception until 1st August 2023, including the following keywords: anti-inflammatory; children; high dose gammaglobulin; high dose immunoglobulin; immune dysregulation; immunomodulation; immunomodulatory; inflammation; intravenous gammaglobulin; intravenous immunoglobulin; off-label; pediatric; subcutaneous gammaglobulin; subcutaneous immunoglobulin. All article types were considered.

IMPLICATIONS

In the light of the current imbalance between gammaglobulins' demand and availability, this review advocates the urgency of a more conscious utilization of this medical product, giving indications about benefits, risks, cost-effectiveness, and administration routes of high-dose immunoglobulins in children with hematologic, neurologic, and inflammatory immune dysregulation disorders, prompting further research towards a responsible employment of gammaglobulins and improving the therapeutical decisional process.

Anti-HLA Class I alloantibodies in platelet transfusion refractoriness: From mechanisms and determinants to therapeutic prospects

Patients with hematological disorders and severe thrombocytopenia require extensive and iterative platelet transfusion support. In these patients, platelet transfusion refractoriness represents a serious adverse transfusion event with major outcomes for patient care. Recipient alloantibodies against the donor HLA Class I antigens expressed at the cell surface of platelets result in a rapid removal of transfused platelets from the circulation and thus, therapeutic and prophylactic transfusion failure leading to a major bleeding risk. In this case, the only way to support the patient relies on the selection of HLA Class I compatible platelets, an approach restricted by the limited number of HLA-typed donors available and the difficulty of meeting the demand in an emergency. However, not all patients with anti-HLA Class I antibodies develop refractoriness to platelet transfusions, raising the question of the intrinsic characteristics of the antibodies and the immune-mediated mechanisms of platelet clearance associated with a refractory state. In this review, we examine the current challenges in platelet transfusion refractoriness and detail the key features of the antibodies involved that should be considered. Finally, we also provide an overview of future therapeutic strategies.

New insights into IVIg mechanisms and alternatives in autoimmune and inflammatory diseases

PURPOSE OF REVIEW

Intravenous immunoglobulin (IVIg) is an effective treatment for an increasing number of autoimmune and inflammatory conditions. However, IVIg continues to be limited by problems of potential shortages and cost. A number of mechanisms have been described for IVIg, which have been captured in newly emergent IVIg mimetic and IVIg alternative therapies. This review discusses the recent developments in IVIg mimetics and alternatives.

RECENT FINDINGS

Newly emergent IVIg mimetics and alternatives capture major proposed mechanisms of IVIg, including FcγR blockade, FcRn inhibition, complement inhibition, immune complex mimetics and sialylated IgG. Many of these emergent therapies have promising preclinical and clinical trial results.

SUMMARY

Significant research has been undertaken into the mechanism of IVIg in the treatment of autoimmune and inflammatory disease. Understanding the major IVIg mechanisms has allowed for rational development of IVIg mimetics and alternatives for several IVIg-treatable diseases.

Anti-inflammatory activity of CD44 antibodies in murine immune thrombocytopenia is mediated by Fcγ receptor inhibition

Monoclonal immunoglobulin G (IgG) antibodies to CD44 (anti-CD44) are anti-inflammatory in numerous murine autoimmune models, but the mechanisms are poorly understood. Anti-CD44 anti-inflammatory activity shows complete therapeutic concordance with IV immunoglobulin (IVIg) in treating autoimmune disease models, making anti-CD44 a potential IVIg alternative. In murine immune thrombocytopenia (ITP), there is no mechanistic explanation for anti-CD44 activity, although anti-CD44 ameliorates disease similarly to IVIg. Here, we demonstrate a novel anti-inflammatory mechanism of anti-CD44 that explains disease amelioration by anti-CD44 in murine ITP. Macrophages treated with anti-CD44 in vitro had dramatically suppressed phagocytosis through FcγRs in 2 separate systems of IgG-opsonized platelets and erythrocytes. Phagocytosis inhibition by anti-CD44 was mediated by blockade of the FcγR IgG binding site without changing surface FcγR expression. Anti-CD44 of different subclasses revealed that FcγR blockade was specific to receptors that could be engaged by the respective anti-CD44 subclass, and Fc-deactivated anti-CD44 variants lost all FcγR-inhibiting activity. In vivo, anti-CD44 functioned analogously in the murine passive ITP model and protected mice from ITP when thrombocytopenia was induced through an FcγR that could be engaged by the CD44 antibody's subclass. Consistent with FcγR blockade, Fc-deactivated variants of anti-CD44 were completely unable to ameliorate ITP. Together, anti-CD44 inhibits macrophage FcγR function and ameliorates ITP consistent with an FcγR blockade mechanism. Anti-CD44 is a potential IVIg alternative and may be of particular benefit in ITP because of the significant role that FcγRs play in human ITP pathophysiology.

Dissecting the mechanism of action of intravenous immunoglobulin in human autoimmune disease: Lessons from therapeutic modalities targeting Fcγ receptors

Since the first description of the administration of high doses of pooled serum IgG, also referred to as intravenous IgG (IVIg) therapy, as being able to ameliorate various autoimmune diseases, researchers have been investigating which molecular and cellular pathways underlie IVIg activity. Apart from trying to understand the obvious conundrum that IgG can trigger both autoimmune pathology and resolution of inflammation, the rapidly expanding use of IVIg has led to a lack of availability of this primary blood product, providing a strong rationale for developing recombinant alternatives. During the last decade, a tremendous number of novel insights into IVIg activity brought the goal of replacing IVIg within reach, at least in select indications, and has led to the initiation of several clinical trials. At the forefront of this effort is the modulation of autoantibody half-life and blocking access of autoantibodies to fragment cystallizable γ receptors (Fcγ receptors). In this rostrum article, we will briefly discuss current models of IVIg activity, followed by a more specific focus on novel therapeutic avenues that are entering the clinic and may replace IVIg in the future.

The use of IVIg in fetal and neonatal alloimmune thrombocytopenia- Principles and mechanisms

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare neonatal disorder that is caused by alloimmunization against platelet antigens during pregnancy. Although rare, affecting only 1 in 1000 live births, it can cause intracranial hemorrhage and other bleeding complications that can lead to miscarriage, stillbirth and life-long neurological complications. One of the gold-standard therapies for at risk pregnancies is the administration of IVIg. Although IVIg has been used in a variety of different disorders for over 40 years, its exact mechanism of action is still unknown. In FNAIT, the majority of its therapeutic effect is thought the be mediated through the neonatal Fc receptor, however other mechanisms cannot be excluded. Due to safety, supply and other concerns that are associated with IVIg use, alternative therapies that could replace IVIg are additionally being investigated. This includes the possibility of a prophylaxis regimen for FNAIT, similarly to what has been successfully used in hemolytic disease of the fetus and newborn for over 50 years.

GM-CSF and IL-4 are not involved in IVIG-mediated amelioration of ITP in mice: a role for IL-11 cannot be ruled out

Previously, we have reported that interleukin (IL)-4, granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-11, but not IL-33, are up-regulated in two strains of mice with immune thrombocytopenia (ITP) that are responsive to intravenous immunoglobulin (IVIg) treatment. Previously, IL-4 was ruled out in the mechanism of IVIg; however, other publications have suggested this cytokine as a major player in the mechanism of IVIg action. Thus, we sought to further investigate a role for IL-4 and, in addition, GM-CSF and IL-11 in the mechanism of action of IVIg using a murine model of ITP. A passive platelet antibody model was used to generate ITP in IL-4 receptor knock-out (IL-4R^-/- ), IL-11 receptor knock-out (IL-11Rα^-/- ) and GM-CSF knock-out (Csf2^-/- ) mice. We also used a neutralizing antibody to IL-11 and recombinant human IL-11 (rhIL-11) in addition to depleting basophils in vivo to study the effect of IVIg to ameliorate ITP. Our results showed that basophils, IL-4 and GM-CSF were unimportant in both ITP induction and its amelioration by IVIg. The role of IL-11 in these processes was less clear. Even though IL-11Rα^-/- mice with ITP responded to IVIg similarly to wild-type (WT) mice, treatment of ITP WT mice with rhIL-11 instead of IVIg showed an increase in platelet numbers and WT mice administered anti-IL-11 showed a significant reduction in the ability of IVIg to ameliorate the ITP. Our findings indicate that neither IL-4, basophils or GM-CSF have roles in IVIg amelioration of ITP; however, a role for IL-11 requires further study.

High-Dose Intravenous Immunoglobulin in Skin Autoimmune Disease

The immunomodulatory potential and low incidence of severe side effects of high-dose intravenous immunoglobulin (IVIg) treatment led to its successful application in a variety of dermatological autoimmune diseases over the last two decades. IVIg is usually administered at a dose of 2 g per kg body weight distributed over 2–5 days every 4 weeks. They are most commonly used as a second- or third-line treatment in dermatological autoimmune disease (pemphigus vulgaris, pemphigus foliaceus, bullous pemphigoid, mucous membrane pemphigoid, epidermolysis bullosa acquisita, dermatomyositis, systemic vasculitis, and systemic lupus erythematosus). However, first-line treatment may be warranted in special circumstances like concomitant malignancy, a foudroyant clinical course, and contraindications against alternative treatments. Furthermore, IVIg can be considered first line in scleromyxedema. Production of IVIg for medical use is strictly regulated to ensure a low risk of pathogen transmission and comparable quality of individual batches. More common side effects include nausea, headache, fatigue, and febrile infusion reactions. Serious side effects are rare and include thrombosis and embolism, pulmonary edema, renal failure, aseptic meningitis, and severe anaphylactic reactions. Regarding the mechanism of action, one can discriminate between functions of the Fcγ region and the F(ab)2 region and their effects on a cellular level. These functions are not mutually exclusive, and more than one pathway may contribute to the beneficial effects. Here, we present a historical background, details on manufacturing, hypotheses on the mechanisms of action, information on the clinical application in the abovementioned conditions, and a brief outlook on future directions of IVIg treatment in dermatology.

IVIG-mediated effector functions in autoimmune and inflammatory diseases

Intravenous immunoglobulin (IVIG) is a pooled preparation of normal IgG obtained from several thousand healthy donors. It is widely used in the immunotherapy of a large number of autoimmune and inflammatory diseases. The mechanisms of action of IVIG are complex and, as discussed in this review, experimental and clinical data provide an indicator that the therapeutic benefit of IVIG therapy is due to several mutually non-exclusive mechanisms affecting soluble mediators as well as cellular components of the immune system. These mechanisms depend on Fc and/or F(ab')2 fragments. A better understanding of the effector functions of IVIG should help in identification of biomarkers of responses to IVIG in autoimmune patients.

Generation of two high affinity anti-mouse FcRn antibodies: Inhibition of IgG recycling in wild type mice and effect in a mouse model of immune thrombocytopenia

Primary immune thrombocytopenia (ITP) is an autoimmune disease characterized by pathogenic immunoglobulin G (IgG) autoantibodies that bind to platelets, causing their phagocytic removal and leading to reductions in platelet number. The neonatal Fc receptor (FcRn) selectively salvages and recycles IgG, including pathogenic IgG, thereby extending the half-life of IgG in plasma. Two anti-mouse FcRn monoclonal antibodies (mAb) (4470 and 4464) were generated to evaluate the effect of inhibiting IgG recycling. Statistically significant reductions in plasma IgG concentration were observed upon administration of 4470 (10, 30 and 100 mg/kg) in wild-type mice. In a passive mouse model of ITP, 4464 alleviated the reduction in platelet number and/or preserved newly produced platelets when dosed prophylactically as well as in a therapeutic dosing regimen once platelet numbers had already been reduced. These results support the investigation of anti-FcRn therapy as a potential treatment for ITP.

Massive immune response against IVIg interferes with response against other antigens in mice: A new mode of action?

Administration of high dose intravenous immunoglobulin (IVIg) is widely used in the clinic to treat autoimmune and severe inflammatory diseases. However, its mechanisms of action remain poorly understood. We assessed the impact of IVIg on immune cell populations using an in vivo ovalbumin (Ova)-immunization mouse model. High dose IVIg significantly reduced the Ova-specific antibody response. Intriguingly, the results obtained indicate an immediate and massive immune reaction against IVIg, as shown by the activation and expansion of B cells and CD4+ T cells in the spleen and draining lymph nodes and the production of IVIg-specific antibodies. We propose that IVIg competes at the T-cell level with the response against Ova to explain the immunomodulatory properties of IVIg. Two monoclonal antibodies did not succeeded in reproducing the effects of IVIg. This suggests that in addition to the mouse response against human constant domains, the enormous sequence diversity of IVIg may significantly contribute to this massive immune response against IVIg. While correlation of these findings to IVIg-treated patients remains to be explored, our data demonstrate for the first time that IVIg re-directs the immune response towards IVIg and away from a specific antigen response.

Structural basis for pH-insensitive inhibition of immunoglobulin G recycling by an anti-neonatal Fc receptor antibody

The neonatal Fc receptor FcRn plays a critical role in the trafficking of IgGs across tissue barriers and in retaining high circulating concentrations of both IgG and albumin. Although generally beneficial from an immunological perspective in maintaining IgG populations, FcRn can contribute to the pathogenesis of autoimmune disorders when an abnormal immune response targets normal biological components. We previously described a monoclonal antibody (DX-2507) that binds to FcRn with high affinity at both neutral and acidic pH, prevents the simultaneous binding of IgG, and reduces circulating IgG levels in preclinical animal models. Here, we report a 2.5 Å resolution X-ray crystal structure of an FcRn-DX-2507 Fab complex, revealing a nearly complete overlap of the IgG-Fc binding site in FcRn by complementarity-determining regions in DX-2507. This overlap explains how DX-2507 blocks IgG binding to FcRn and thereby shortens IgG half-life by preventing IgGs from recycling back into circulation. Moreover, the complex structure explains how the DX-2507 interaction is pH-insensitive unlike normal Fc interactions and how serum albumin levels are unaffected by DX-2507 binding. These structural studies could inform antibody-based therapeutic approaches for limiting the effects of IgG-mediated autoimmune disease.

Therapeutic Potential of Intravenous Immunoglobulin in Acute Brain Injury

Acute ischemic and traumatic injury of the central nervous system (CNS) is known to induce a cascade of inflammatory events that lead to secondary tissue damage. In particular, the sterile inflammatory response in stroke has been intensively investigated in the last decade, and numerous experimental studies demonstrated the neuroprotective potential of a targeted modulation of the immune system. Among the investigated immunomodulatory agents, intravenous immunoglobulin (IVIg) stand out due to their beneficial therapeutic potential in experimental stroke as well as several other experimental models of acute brain injuries, which are characterized by a rapidly evolving sterile inflammatory response, e.g., trauma, subarachnoid hemorrhage. IVIg are therapeutic preparations of polyclonal immunoglobulin G, extracted from the plasma of thousands of donors. In clinical practice, IVIg are the treatment of choice for diverse autoimmune diseases and various mechanisms of action have been proposed. Only recently, several experimental studies implicated a therapeutic potential of IVIg even in models of acute CNS injury, and suggested that the immune system as well as neuronal cells can directly be targeted by IVIg. This review gives further insight into the role of secondary inflammation in acute brain injury with an emphasis on stroke and investigates the therapeutic potential of IVIg.

Immune recruitment or suppression by glycan engineering of endogenous and therapeutic antibodies

Human serum IgG contains multiple glycoforms which exhibit a range of binding properties to effector molecules such as cellular Fc receptors. Emerging knowledge of how the Fc glycans contribute to the antibody structure and effector functions has opened new avenues for the exploitation of defined antibody glycoforms in the treatment of diseases. Here, we review the structure and activity of antibody glycoforms and highlight developments in antibody glycoengineering by both the manipulation of the cellular glycosylation machinery and by chemoenzymatic synthesis. We discuss wide ranging applications of antibody glycoengineering in the treatment of cancer, autoimmunity and inflammation. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Immunomodulation of immunothrombocytopenia

Current treatment strategies in patients with immunothrombocytopenia (ITP) include immunosuppression and the stimulation of platelet production. Research over the last decade has emphasized the important role of Fc-receptors as key molecules responsible for autoantibody-mediated platelet depletion, allowing for a more specific targeting of this pathway instead of a generalized suppression of the immune system. This short review will discuss approaches aiming at interfering with key steps in this pathway, such as preventing the interaction of the platelet autoantibody immune complex with phagocytic cells, enhancing the clearance of autoantibodies, and interfering with or modulating the signaling pathways responsible for innate immune effector cell activation.

Pathways Responsible for Human Autoantibody and Therapeutic Intravenous IgG Activity in Humanized Mice

Immunoglobulin G (IgG) antibodies are major drivers of autoimmune pathology, but they are also used in the form of intravenous IgG (IVIg) therapy to suppress autoantibody activity. To identify the pathways underlying human autoantibody and IVIg activity, we established a humanized mouse model of an autoantibody-dependent autoimmune disease responding to treatment with IVIg preparations. We show that the human IgG subclass strongly impacts autoantibody activity and that the Fc-receptor genotype of the human donor immune system further modulates autoantibody activity. Human mononuclear phagocytes were responsible for autoantibody activity, and IVIg therapy was able to suppress disease pathology in an Fc-fragment-dependent manner. While highly sialylated IgG glycovariants were essential for IVIg activity, it was independent of the Fc-receptor genotype and did not result in a general block of activating or the neonatal Fc-receptor. These findings may help in the development of strategies to block autoantibody and enhance therapeutic IVIg activity in humans.

Fully human monoclonal antibody inhibitors of the neonatal fc receptor reduce circulating IgG in non-human primates

The therapeutic management of antibody-mediated autoimmune disease typically involves immunosuppressant and immunomodulatory strategies. However, perturbing the fundamental role of the neonatal Fc receptor (FcRn) in salvaging IgG from lysosomal degradation provides a novel approach - depleting the body of pathogenic immunoglobulin by preventing IgG binding to FcRn and thereby increasing the rate of IgG catabolism. Herein, we describe the discovery and preclinical evaluation of fully human monoclonal IgG antibody inhibitors of FcRn. Using phage display, we identified several potent inhibitors of human-FcRn in which binding to FcRn is pH-independent, with over 1000-fold higher affinity for human-FcRn than human IgG-Fc at pH 7.4. FcRn antagonism in vivo using a human-FcRn knock-in transgenic mouse model caused enhanced catabolism of exogenously administered human IgG. In non-human primates, we observed reductions in endogenous circulating IgG of >60% with no changes in albumin, IgM, or IgA. FcRn antagonism did not disrupt the ability of non-human primates to mount IgM/IgG primary and secondary immune responses. Interestingly, the therapeutic anti-FcRn antibodies had a short serum half-life but caused a prolonged reduction in IgG levels. This may be explained by the high affinity of the antibodies to FcRn at both acidic and neutral pH. These results provide important preclinical proof of concept data in support of FcRn antagonism as a novel approach to the treatment of antibody-mediated autoimmune diseases.

Targeting the Fc receptor in autoimmune disease

INTRODUCTION

The Fc receptors (FcRs) and their interactions with immunoglobulin and innate immune opsonins, such as C-reactive protein, are key players in humoral and cellular immune responses. As the effector mechanism for some therapeutic monoclonal antibodies, and often a contributor to the pathogenesis and progression of autoimmunity, FcRs are promising targets for treating autoimmune diseases.

AREAS COVERED

This review discusses the nature of different FcRs and the various mechanisms of their involvement in initiating and modulating immunocyte functions and their biological consequences. It describes a range of current strategies in targeting FcRs and manipulating their interaction with specific ligands, while presenting the pros and cons of these approaches. This review also discusses potential new strategies including regulation of FcR expression and receptor crosstalk.

EXPERT OPINION

FcRs are appealing targets in the treatment of inflammatory autoimmune diseases. However, there are still knowledge limitations and technical challenges, the most important being a better understanding of the individual roles of each of the FcRs and enhancement of the specificity in targeting particular cell types and specific FcRs.

Therapeutic effect of IVIG on inflammatory arthritis in mice is dependent on the Fc portion and independent of sialylation or basophils

High-dose i.v. Ig (IVIG) is used to treat various autoimmune and inflammatory diseases; however, the mechanism of action remains unclear. Based on the K/BxN serum transfer arthritis model in mice, IVIG suppression of inflammation has been attributed to a mechanism involving basophils and the binding of highly sialylated IgG Fc to DC-SIGN-expressing myeloid cells. The requirement for sialylation was examined in the collagen Ab-induced arthritis (CAbIA) and K/BxN serum transfer arthritis models in mice. High-dose IVIG (1-2 g/kg body weight) suppressed inflammatory arthritis when given prophylactically. The same doses were also effective in the CAbIA model when given subsequent to disease induction. In this therapeutic CAbIA model, the anti-inflammatory effect of IVIG was dependent on IgG Fc but not F(ab')2 fragments. Removal of sialic acid residues by neuraminidase had no impact on the anti-inflammatory activity of IVIG or Fc fragments. Treatment of mice with basophil-depleting mAbs did not abrogate the suppression of either CAbIA or K/BxN arthritis by IVIG. Our data confirm the therapeutic benefit of IVIG and IgG Fc in Ab-induced arthritis but fail to support the significance of sialylation and basophil involvement in the mechanism of action of IVIG therapy.

Amelioration of murine passive immune thrombocytopenia by IVIg and a therapeutic monoclonal CD44 antibody does not require the Myd88 signaling pathway

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder characterized by a low platelet count and the production of anti-platelet antibodies. The majority of ITP patients have antibodies to platelet integrin α_IIbβ₃ (GPIIbIIIa) which can direct platelet phagocytosis by macrophages. One effective treatment for patients with ITP is intravenous immunoglobulin (IVIg) which rapidly reverses thrombocytopenia. The exact mechanism of IVIg action in human patients is unclear, although in mouse models of passive ITP, IVIg can rapidly increase platelet counts in the absence of adaptive immunity. Another antibody therapeutic that can similarly increase platelet counts independent of adaptive immunity are CD44 antibodies. Toll-like receptors (TLRs) are pattern recognition receptors which play a central role in helping direct the innate immune system. Dendritic cells, which are notable for their expression of TLRs, have been directly implicated in IVIg function as an initiator cell, while CD44 can associate with TLR2 and TLR4. We therefore questioned whether IVIg, or the therapeutic CD44 antibody KM114, mediate their ameliorative effects in a manner dependent upon normal TLR function. Here, we demonstrate that the TLR4 agonist LPS does not inhibit IVIg or KM114 amelioration of antibody-induced thrombocytopenia, and that these therapeutics do not ameliorate LPS-induced thrombocytopenia. IVIg was able to significantly ameliorate murine ITP in C3H/HeJ mice which have defective TLR4. All known murine TLRs except TLR3 utilize the Myd88 adapter protein to drive TLR signaling. Employing Myd88 deficient mice, we found that both IVIg and KM114 ameliorate murine ITP in Myd88 deficient mice to the same extent as normal mice. Thus both IVIg and anti-CD44 antibody can mediate their ameliorative effects in murine passive ITP independent of the Myd88 signaling pathway. These data help shed light on the mechanism of action of IVIg and KM114 in the amelioration of murine ITP.

Antibody specific for the glycophorin A complex mediates intravenous immune globulin-resistant anemia in a murine model

BACKGROUND

Therapy for patients with autoimmune hemolytic anemia (AHA) remains a major challenge. Patients with glycophorin A (GPA)-specific immunoglobulin G antibodies can have severe hemolysis, which may occur by mechanisms independent from traditional macrophage-dependent Fcγ receptor (FcγR)-mediated extravascular hemolysis. As intravenous immune globulin (IVIG) is known to display its beneficial effects in FcγR-mediated cytopenias, and IVIG responses in AHA are inconsistent at best, we sought to gain insight into the mechanism of anemia by a GPA complex-specific monoclonal antibody (TER119) in a mouse model of immune hemolytic anemia and evaluate the therapeutic effect of IVIG.

STUDY DESIGN AND METHODS

The anemic effect of the TER119 antibody was studied in vitro by incubation of mouse RBC with the antibody and in vivo by infusing the antibody into normal mice versus mice genetically deficient for the Fc receptor γ chain (Fcγ), complement C3, mice naturally deficient in complement C5, and splenectomized mice. IVIG efficacy in anemia was determined by treating mice with an intensive IVIG dosing regimen.

RESULTS

The TER119-mediated anemia was independent of classical FcγR-, C3-, and C5-dependent mechanisms, but occurred by a mechanism consistent with RBC agglutination. In accordance with agglutination, the presence of the spleen accelerated the anemia observed but anemia could still occur in splenectomized mice. IVIG did not significantly affect the induction of anemia by TER119.

CONCLUSION

The mechanism of anemia induced by AHA-causing antibodies may be an important factor to consider in the response to therapy with IVIG.

Intravenous immunoglobulin therapy: how does IgG modulate the immune system?

Intravenous immunoglobulin (IVIG) preparations comprise pooled IgG antibodies from the serum of thousands of donors and were initially used as an IgG replacement therapy in immunocompromised patients. Since the discovery, more than 30 years ago, that IVIG therapy can ameliorate immune thrombocytopenia, the use of IVIG preparations has been extended to a wide range of autoimmune and inflammatory diseases. Despite the broad efficacy of IVIG therapy, its modes of action remain unclear. In this Review, we cover the recent insights into the molecular and cellular pathways that are involved in IVIG-mediated immunosuppression, with a particular focus on IVIG as a therapy for IgG-dependent autoimmune diseases.