Complete FcRn dependence for intravenous Ig therapy in autoimmune skin blistering diseases

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.

The neonatal Fc receptor, FcRn, as a target for drug delivery and therapy

Immunoglobulin G (IgG)-based drugs are arguably the most successful class of protein therapeutics due in part to their remarkably long blood circulation. This arises from IgG interaction with the neonatal Fc receptor, FcRn. FcRn is the central regulator of IgG and albumin homeostasis throughout life and is increasingly being recognized as an important player in autoimmune disease, mucosal immunity, and tumor immune surveillance. Various engineering approaches that hijack or disrupt the FcRn-mediated transport pathway have been devised to develop long-lasting and non-invasive protein therapeutics, protein subunit vaccines, and therapeutics for treatment of autoimmune and infectious disease. In this review, we highlight the diverse biological functions of FcRn, emerging therapeutic opportunities, as well as the associated challenges of targeting FcRn for drug delivery and disease therapy.

Critical Role of the Neonatal Fc Receptor (FcRn) in the Pathogenic Action of Antimitochondrial Autoantibodies Synergizing with Anti-desmoglein Autoantibodies in Pemphigus Vulgaris

Pemphigus vulgaris (PV) is a life-long, potentially fatal IgG autoantibody-mediated blistering disease targeting mucocutaneous keratinocytes (KCs). PV patients develop pathogenic anti-desmoglein (Dsg) 3 ± 1 and antimitochondrial antibodies (AMA), but it remained unknown whether and how AMA enter KCs and why other cell types are not affected in PV. Therefore, we sought to elucidate mechanisms of cell entry, trafficking, and pathogenic action of AMA in PV. We found that PVIgGs associated with neonatal Fc receptor (FcRn) on the cell membrane, and the PVIgG-FcRn complexes entered KCs and reached mitochondria where they dissociated. The liberated AMA altered mitochondrial membrane potential, respiration, and ATP production and induced cytochrome c release, although the lack or inactivation of FcRn abolished the ability of PVIgG to reach and damage mitochondria and to cause detachment of KCs. The assays of mitochondrial functions and keratinocyte adhesion demonstrated that although the pathobiological effects of AMA on KCs are reversible, they become irreversible, leading to epidermal blistering (acantholysis), when AMA synergize with anti-Dsg antibodies. Thus, it appears that AMA enter a keratinocyte in a complex with FcRn, become liberated from the endosome in the cytosol, and are trafficked to the mitochondria, wherein they trigger pro-apoptotic events leading to shrinkage of basal KCs uniquely expressing FcRn in epidermis. During recovery, KCs extend their cytoplasmic aprons toward neighboring cells, but anti-Dsg antibodies prevent assembly of nascent desmosomes due to steric hindrance, thus rendering acantholysis irreversible. In conclusion, FcRn is a common acceptor protein for internalization of AMA and, perhaps, for PV autoantibodies to other intracellular antigens, and PV is a novel disease paradigm for investigating and elucidating the role of FcRn in this autoimmune disease and possibly other autoimmune diseases.

Mechanisms of immune regulation by IVIG

PURPOSE OF REVIEW

In the past few years there have been many advances in our understanding of the mechanisms by which intravenous immune globulin (IVIG) modulates immune function in autoimmune disorders.

RECENT FINDINGS

Previous investigations have focused on the Fc domain of the IgG molecule, and the role of the FcγRIIB receptor and the sialylated Fc domain that have been show to mediate the anti-inflammatory effects in certain murine models of autoantibody-mediated diseases. More recent findings have implicated the F(ab')₂ domain in IVIG-induced immune modulation in T-cell-mediated autoimmune disease models in which upregulation of T-regulatory cells and downregulation of the Th17 pathways are important components of this mechanism. The prostaglandin E pathway may be playing a role in the IVIG-induced changes in the T-regulatory pathway.

SUMMARY

Many of the mechanisms proposed for the immune-modulating effects of IVIG demonstrate the complexity of immune effector functions in disease processes. Although controversy exists on the role of the FcγRIIB receptor and the importance of the sialylated Fc domain in human autoimmune disorders, probably no one single mechanism is responsible for the effects of IVIG in autoimmune and inflammatory diseases. The potential role of the prostaglandin E pathway may offer alternative treatments.

Therapeutic immunoglobulin should be dosed by clinical outcome rather than by body weight in obese patients

There are currently no data to support the suggestion that the dose of therapeutic immunoglobulin (Ig) should be capped in obese patients for pharmacokinetic (PK), safety and economic reasons. We compared IgG trough levels, increment and efficiency in matched pairs of obese and lean patients receiving either replacement or immunomodulatory immunoglobulin therapy. Thirty-one obese patients were matched with a clinically equivalent lean patient across a range of indications, including primary antibody deficiency or autoimmune peripheral neuropathy. Comprehensive matching was carried out using ongoing research databases at two centres in which the dose of Ig was based on clinical outcome, whether infection prevention or documented clinical neurological stability. The IgG trough or steady state levels, IgG increments and Ig efficiencies at times of clinical stability were compared between the obese and lean cohorts and within the matched pairs. This study shows that, at a population level, obese patients achieved a higher trough and increment (but not efficiency) for a given weight-adjusted dose compared with the lean patients. However at an individual patient level there were significant exceptions to this correlation, and upon sub-group analysis no significant difference was found between obese and lean patients receiving replacement therapy. Across all dose regimens a high body mass index (BMI) cannot be used to predict reliably the patients in whom dose restriction is clinically appropriate.

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.

Regulation of immune responses by the neonatal fc receptor and its therapeutic implications

As a single receptor, the neonatal Fc receptor (FcRn) is critically involved in regulating albumin and IgG serum concentrations by protecting these two ligands from degradation. In addition to these essential homeostatic functions, FcRn possesses important functions in regulating immune responses that are equally as critical and are increasingly coming to attention. During the first stages of life, FcRn mediates the passive transfer of IgG across the maternal placenta or neonatal intestinal walls of mammals, thereby conferring passive immunity to the offspring before and after birth. In fact, FcRn is one of the very few molecules that are known to move from luminal to serosal membranes of polarized cells that form epithelial barriers of the lung and intestines. Together with FcRn's recently explored critical role in eliciting MHC II presentation and MHC class I cross-presentation of IgG-complexed antigen, this renders FcRn capable of exerting broad and potent functions in regulating immune responses and immunosurveillance at mucosal sites. Further, it is now clear that FcRn dependent mucosal absorption of therapeutic molecules is a clinically feasible and potent novel route of non-invasive drug delivery, and the interaction between FcRn and IgG has also been utilized for the acquisition of humoral immunity at mucosal sites. In this review, we begin by briefly summarizing the basic knowledge on FcRn expression and IgG binding, then describe more recent discoveries pertaining to the mechanisms by which FcRn orchestrates IgG related mucosal immune responses and immunosurveillance at host-environment interfaces within the adult organism. Finally, we outline how the knowledge of actions of FcRn at mucosal boundaries can be capitalized for the development and engineering of powerful mucosal vaccination strategies and novel routes for the non-invasive delivery of Fc-based therapeutics.

Rapid and reversible responses to IVIG in autoimmune neuromuscular diseases suggest mechanisms of action involving competition with functionally important autoantibodies

Intravenous immunoglobulin (IVIG) is widely used in autoimmune neuromuscular diseases whose pathogenesis is undefined. Many different effects of IVIG have been demonstrated in vitro, but few studies actually identify the mechanism(s) most important in vivo. Doses and treatment intervals are generally chosen empirically. Recent studies in Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy show that some effects of IVIG are readily reversible and highly dependent on the serum IgG level. This suggests that in some autoantibody-mediated neuromuscular diseases, IVIG directly competes with autoantibodies that reversibly interfere with nerve conduction. Mechanisms of action of IVIG which most likely involve direct competition with autoantibodies include: neutralization of autoantibodies by anti-idiotypes, inhibition of complement deposition, and increasing catabolism of pathologic antibodies by saturating FcRn. Indirect immunomodulatory effects are not as likely to involve competition and may not have the same reversibility and dose-dependency. Pharmacodynamic analyses should be informative regarding most relevant mechanism(s) of action of IVIG as well as the role of autoantibodies in the immunopathogenesis of each disease. Better understanding of the role of autoantibodies and of the target(s) of IVIG could lead to more efficient use of this therapy and better patient outcomes.

Effects of intravenous immunoglobulins on mice with experimental epidermolysis bullosa acquisita

Although well-designed prospective trials are generally lacking, intravenous immunoglobulins (IVIG) seem an effective adjuvant treatment for autoimmune bullous skin diseases. Here, efficacy of IVIG monotherapy was compared with corticosteroid treatment in mice with immunization-induced experimental epidermolysis bullosa acquisita (EBA), an autoimmune bullous skin disease characterized by autoantibodies against type VII collagen. We found that IVIG significantly ameliorated clinical disease severity and skin neutrophil infiltration compared with vehicle-treated mice, whereas methylprednisolone showed comparatively less pronounced effects. Efficacy of IVIG was accompanied by reduced levels of autoantibodies, a shift toward noncomplement-fixing autoantibodies, and lower complement deposition at the dermal-epidermal junction. In addition, peripheral Gr-1-positive cells of IVIG-treated animals showed reduced expression of the activating Fcγ receptor IV, which we recently described as a major mediator of tissue injury in experimental EBA. These data show that treatment with IVIG is superior to systemic corticosteroids in experimental EBA and that the effects of IVIG are pleiotropic involving modulation of both the adaptive and innate immune response, although the detailed mode of action of IVIG in this model remains in need of further elucidation.

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.

Maternal vaccination: moving the science forward

BACKGROUND

Infections remain one of the leading causes of morbidity in pregnant women and newborns, with vaccine-preventable infections contributing significantly to the burden of disease. In the past decade, maternal vaccination has emerged as a promising public health strategy to prevent and combat maternal, fetal and neonatal infections. Despite a number of universally recommended maternal vaccines, the development and evaluation of safe and effective maternal vaccines and their wide acceptance are hampered by the lack of thorough understanding of the efficacy and safety in the pregnant women and the offspring.

METHODS

An outline was synthesized based on the current status and major gaps in the knowledge of maternal vaccination. A systematic literature search in PUBMED was undertaken using the key words in each section title of the outline to retrieve articles relevant to pregnancy. Articles cited were selected based on relevance and quality. On the basis of the reviewed information, a perspective on the future directions of maternal vaccination research was formulated.

RESULTS

Maternal vaccination can generate active immune protection in the mother and elicit systemic immunoglobulin G (IgG) and mucosal IgG, IgA and IgM responses to confer neonatal protection. The maternal immune system undergoes significant modulation during pregnancy, which influences responsiveness to vaccines. Significant gaps exist in our knowledge of the efficacy and safety of maternal vaccines, and no maternal vaccines against a large number of old and emerging pathogens are available. Public acceptance of maternal vaccination has been low.

CONCLUSIONS

To tackle the scientific challenges of maternal vaccination and to provide the public with informed vaccination choices, scientists and clinicians in different disciplines must work closely and have a mechanistic understanding of the systemic, reproductive and mammary mucosal immune responses to vaccines. The use of animal models should be coupled with human studies in an iterative manner for maternal vaccine experimentation, evaluation and optimization. Systems biology approaches should be adopted to improve the speed, accuracy and safety of maternal vaccine targeting.

Understanding the role of antibodies in murine infections with Heligmosomoides (polygyrus) bakeri: 35 years ago, now and 35 years ahead

The rodent intestinal nematode H.p.bakeri has played an important role in the exploration of the host-parasite relationship of chronic nematode infections for over six decades, since the parasite was first isolated in the 1950s by Ehrenford. It soon became a popular laboratory model providing a tractable experimental system that is easy to maintain in the laboratory and far more cost-effective than other laboratory nematode-rodent model systems. Immunity to this parasite is complex, dependent on antibodies, but confounded by the parasite's potent immunosuppressive secretions that facilitate chronic survival in murine hosts. In this review, we remind readers of the state of knowledge in the 1970s, when the first volume of Parasite Immunology was published, focusing on the role of antibodies in protective immunity. We show how our understanding of the host-parasite relationship then developed over the following 35 years to date, we propose testable hypotheses for future researchers to tackle, and we speculate on how the new technologies will be applied to enable an increasingly refined understanding of the role of antibodies in host-protective immunity, and its evasion, to be achieved in the longer term.

Human immunoglobulin G reduces the pathogenicity of aquaporin-4 autoantibodies in neuromyelitis optica

Neuromyelitis optica (NMO) pathogenesis involves binding of anti-aquaporin-4 (AQP4) autoantibodies (NMO-IgG) present in serum to AQP4 on astrocytes, which causes complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC). Human immunoglobulin G (hIgG) is effective for treatment of humorally mediated neurological autoimmune diseases and has been reported to improve disease outcome in a limited number of NMO patients. Here, we investigated hIgG actions on NMO-IgG pathogenicity using an in vivo rat model of NMO and in vitro assays. In rats administered NMO-IgG by intracerebral injection, the size of neuroinflammatory demyelinating lesions was reduced by ~50% when hIgG was administered by intraperitoneal injection to reach levels of 10-25mg/mL in rat serum, comparable with human therapeutic levels. In vitro, hIgG at 10mg/mL reduced by 90% NMO-IgG-mediated CDC following addition of NMO-IgG and human complement to AQP4-expressing cells. The hIgG effect was mainly on the classical complement pathway. hIgG at 10mg/mL also reduced by up to 90% NMO-IgG-mediated ADCC as assayed with human natural killer cells as effector cells. However, hIgG at up to 40mg/mL did not affect AQP4 cell surface expression or its supramolecular assembly in orthogonal arrays of particles, nor did it affect NMO-IgG binding to AQP4. We conclude that hIgG reduces NMO-IgG pathogenicity by inhibition of CDC and ADCC, providing a mechanistic basis to support further clinical evaluation of its therapeutic efficacy in NMO.

FcRn: from molecular interactions to regulation of IgG pharmacokinetics and functions

The neonatal Fc receptor, FcRn, is related to MHC class I with respect to its structure and association with β2microglobulin (β2m). However, by contrast with MHC class I molecules, FcRn does not bind to peptides, but interacts with the Fc portion of IgGs and belongs to the Fc receptor family. Unlike the 'classical' Fc receptors, however, the primary functions of FcRn include salvage of IgG (and albumin) from lysosomal degradation through the recycling and transcytosis of IgG within cells. The characteristic feature of FcRn is pH-dependent binding to IgG, with relatively strong binding at acidic pH (<6.5) and negligible binding at physiological pH (7.3-7.4). FcRn is expressed in many different cell types, and endothelial and hematopoietic cells are the dominant cell types involved in IgG homeostasis in vivo. FcRn also delivers IgG across cellular barriers to sites of pathogen encounter and consequently plays a role in protection against infections, in addition to regulating renal filtration and immune complex-mediated antigen presentation. Further, FcRn has been targeted to develop both IgGs with extended half-lives and FcRn inhibitors that can lower endogenous antibody levels. These approaches have implications for the development of longer lived therapeutics and the removal of pathogenic or deleterious antibodies.

Neonatal Fc receptor promotes immune complex-mediated glomerular disease

The neonatal Fc receptor (FcRn) is a major regulator of IgG and albumin homeostasis systemically and in the kidneys. We investigated the role of FcRn in the development of immune complex-mediated glomerular disease in mice. C57Bl/6 mice immunized with the noncollagenous domain of the α3 chain of type IV collagen (α3NC1) developed albuminuria associated with granular capillary loop deposition of exogenous antigen, mouse IgG, C3 and C5b-9, and podocyte injury. High-resolution imaging showed abundant IgG deposition in the expanded glomerular basement membrane, especially in regions corresponding to subepithelial electron dense deposits. FcRn-null and -humanized mice immunized with α3NC1 developed no albuminuria and had lower levels of serum IgG anti-α3NC1 antibodies and reduced glomerular deposition of IgG, antigen, and complement. Our results show that FcRn promotes the formation of subepithelial immune complexes and subsequent glomerular pathology leading to proteinuria, potentially by maintaining higher serum levels of pathogenic IgG antibodies. Therefore, reducing pathogenic IgG levels by pharmacologic inhibition of FcRn may provide a novel approach for the treatment of immune complex-mediated glomerular diseases. As proof of concept, we showed that a peptide inhibiting the interaction between human FcRn and human IgG accelerated the degradation of human IgG anti-α3NC1 autoantibodies injected into FCRN-humanized mice as effectively as genetic ablation of FcRn, thus preventing the glomerular deposition of immune complexes containing human IgG.

Clinical presentation, pathogenesis, diagnosis, and treatment of epidermolysis bullosa acquisita

Epidermolysis bullosa acquisita (EBA) is a chronic mucocutaneous autoimmune skin blistering disease. The pathogenic relevance of autoantibodies targeting type VII collagen (COL7) has been well-documented. Therefore, EBA is a prototypical autoimmune disease with a well-characterized pathogenic relevance of autoantibody binding to the target antigen. EBA is a rare disease with an incidence of 0.2 new cases per million and per year. The current treatment of EBA relies on general immunosuppressive therapy, which does not lead to remission in all cases. Therefore, there is a high, so far unmet medical need for the development of novel therapeutic options. During the last 10 years, several novel in vitro and in vivo models of EBA have been established. These models demonstrated a critical role of the genetic background, T cells, and cytokines for mediating the loss of tolerance towards COL7. Neutrophils, complement activation, Fc gamma receptor engagement, cytokines, several molecules involved in cell signaling, release of reactive oxygen species, and matrix metalloproteinases are crucial for autoantibody-induced tissue injury in EBA. Based on this growing understanding of the diseases' pathogenesis, several potential novel therapeutic targets have emerged. In this review, the clinical presentation, pathogenesis, diagnosis, and current treatment options for EBA are discussed in detail.

Polyclonal immunoglobulin G for autoimmune demyelinating nervous system disorders

Demyelinating diseases with presumed autoimmune pathogenesis are characterised by direct or indirect immune-mediated damage to myelin sheaths, which normally surround nerve fibres to ensure proper electrical nerve conduction. Parenteral administration of polyclonal IgG purified from multi-donor human plasma pools may beneficially modulate these misguided immune reactions via several mechanisms that are outlined in this review. Convincing therapeutic evidence from controlled trials now exists for certain disorders of the peripheral nervous system, including Guillain-Barré syndrome, chronic inflammatory demyelinating polyradiculoneuropathy, and multifocal motor neuropathy. In addition, there is evidence for potential therapeutic benefits of IgG in patients with chronic inflammatory demyelinating diseases of the central nervous system, including multiple sclerosis and neuromyelitis optica. This review introduces these disorders, briefly summarises the established treatment options, and discusses therapeutic evidence for the use of polyclonal immunoglobulins with a particular emphasis on recent clinical trials and meta-analyses.

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.

Discovery and structure-activity relationships of small molecules that block the human immunoglobulin G-human neonatal Fc receptor (hIgG-hFcRn) protein-protein interaction

The neonatal Fc receptor, FcRn, prolongs the half-life of IgG in the serum and represents a potential therapeutic target for the treatment of autoimmune disease. Small molecules that block the protein-protein interactions of human IgG-human FcRn may lower pathogenic autoantibodies and provide effective treatment. A novel class of quinoxalines has been discovered as antagonists of the IgG:FcRn protein-protein interaction through optimization of a hit derived from a virtual ligand-based screen.

The immunologic functions of the neonatal Fc receptor for IgG

Careful regulation of the body's immunoglobulin G (IgG) and albumin concentrations is necessitated by the importance of their respective functions. As such, the neonatal Fc receptor (FcRn), as a single receptor, is capable of regulating both of these molecules and has become an important focus of investigation. In addition to these essential protection functions, FcRn possesses a number of other functions that are equally as critical and are increasingly coming to attention. During the very first stages of life, FcRn mediates the passive transfer of IgG from mother to offspring both before and after birth. In the adult, FcRn regulates the persistence of both IgG and albumin in the serum as well as the movement of IgG, and any bound cargo, between different compartments of the body via transcytosis across polarized cells. FcRn is also expressed by hematopoietic cells; consistent with this, FcRn regulates MHC class II presentation and MHC class I cross-presentation by dendritic cells. As such, FcRn plays an important role in immune surveillance throughout adult life. The increasing appreciation for FcRn in both homeostatic and pathological conditions is generating an intense interest in the potential for therapeutic modulation of FcRn binding to IgG and albumin.

Intravenous immunoglobulin in the management of lupus nephritis

The occurrence of nephritis in patients with systemic lupus erythematosus is associated with increased morbidity and mortality. The pathogenesis of lupus nephritis is complex, involving innate and adaptive cellular and humoral immune responses. Autoantibodies in particular have been shown to be critical in the initiation and progression of renal injury, via interactions with both Fc-receptors and complement. One approach in the management of patients with lupus nephritis has been the use of intravenous immunoglobulin. This therapy has shown benefit in the setting of many forms of autoantibody-mediated injury; however, the mechanisms of efficacy are not fully understood. In this paper, the data supporting the use of immunoglobulin therapy in lupus nephritis will be evaluated. In addition, the potential mechanisms of action will be discussed with respect to the known involvement of complement and Fc-receptors in the kidney parenchyma. Results are provocative and warrant additional clinical trials.

Cross-presentation of IgG-containing immune complexes

IgG is a molecule that functionally combines facets of both innate and adaptive immunity and therefore bridges both arms of the immune system. On the one hand, IgG is created by adaptive immune cells, but can be generated by B cells independently of T cell help. On the other hand, once secreted, IgG can rapidly deliver antigens into intracellular processing pathways, which enable efficient priming of T cell responses towards epitopes from the cognate antigen initially bound by the IgG. While this process has long been known to participate in CD4(+) T cell activation, IgG-mediated delivery of exogenous antigens into a major histocompatibility complex (MHC) class I processing pathway has received less attention. The coordinated engagement of IgG with IgG receptors expressed on the cell-surface (FcγR) and within the endolysosomal system (FcRn) is a highly potent means to deliver antigen into processing pathways that promote cross-presentation of MHC class I and presentation of MHC class II-restricted epitopes within the same dendritic cell. This review focuses on the mechanisms by which IgG-containing immune complexes mediate such cross-presentation and the implications that this understanding has for manipulation of immune-mediated diseases that depend upon or are due to the activities of CD8(+) T cells.

Rituximab for autoimmune blistering diseases: recent studies, new insights

Rituximab, an anti-CD20 monoclonal antibody, has been successfully used off-label for treatment of autoimmune blistering diseases. We discuss rituximab mechanisms of action, host factors that may affect response to rituximab, and the efficacy and safety of rituximab in autoimmune blistering diseases, incorporating recent data on the use of rituximab in other autoimmune disease patients.

Monoclonal antibodies directed against human FcRn and their applications

The MHC class I-like Fc receptor (FcRn) is an intracellular trafficking Fc receptor that is uniquely responsible for the extended serum half-life of antibodies of the IgG subclass and their ability to transport across cellular barriers. By performing these functions, FcRn affects numerous facets of antibody biology and pathobiology. Its critical role in controlling IgG pharmacokinetics has been leveraged for the design of therapeutic antibodies and related biologics. FcRn also traffics serum albumin and is responsible for the enhanced pharmacokinetic properties of albumin-conjugated therapeutics. The understanding of FcRn and its therapeutic applications has been limited by a paucity of reliable serological reagents against human FcRn. Here, we describe the properties of a new panel of highly specific monoclonal antibodies (mAbs) directed against human FcRn with diverse epitope specificities. We show that this antibody panel can be used to study the tissue expression pattern of human FcRn, to selectively block IgG and serum albumin binding to human FcRn in vitro and to inhibit FcRn function in vivo. This mAb panel provides a powerful resource for probing the biology of human FcRn and for the evaluation of therapeutic FcRn blockade strategies.

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

To definitively determine whether the neonatal Fc receptor (FcRn) is required for the acute amelioration of immune thrombocytopenia (ITP) by IVIg, we used FcRn-deficient mice in a murine ITP model. Mice injected with antiplatelet antibody in the presence or absence of IVIg displayed no difference in platelet-associated IgG between FcRn deficient versus C57BL/6 mice. FcRn-deficient mice treated with high-dose (2 g/kg) IVIg or a low–dose (2 mg/kg) of an IVIg-mimetic CD44 antibody were, however, protected from thrombocytopenia to an equivalent extent as wild-type mice. To verify and substantiate the results found with FcRn-deficient mice, we used β2-microglobulin–deficient mice (which do not express functional FcRn) and found that IVIg or CD44 antibody also protected them from thrombocytopenia. These data suggest that for both high-dose IVIg as well as low-dose CD44 antibody treatment in an acute ITP model, FcRn expression is neither necessary nor required.

IgG placental transfer in healthy and pathological pregnancies

Placental transfer of maternal IgG antibodies to the fetus is an important mechanism that provides protection to the infant while his/her humoral response is inefficient. IgG is the only antibody class that significantly crosses the human placenta. This crossing is mediated by FcRn expressed on syncytiotrophoblast cells. There is evidence that IgG transfer depends on the following: (i) maternal levels of total IgG and specific antibodies, (ii) gestational age, (iii) placental integrity, (iv) IgG subclass, and (v) nature of antigen, being more intense for thymus-dependent ones. These features represent the basis for maternal immunization strategies aimed at protecting newborns against neonatal and infantile infectious diseases. In some situations, such as mothers with primary immunodeficiencies, exogenous IgG acquired by intravenous immunoglobulin therapy crosses the placenta in similar patterns to endogenous immunoglobulins and may also protect the offspring from infections in early life. Inversely, harmful autoantibodies may cross the placenta and cause transitory autoimmune disease in the neonate.

MHC class I family proteins retard systemic lupus erythematosus autoimmunity and B cell lymphomagenesis

Dysregulation of the T cell-dependent Ab response can lead to numerous immunological disorders, ranging from systemic lupus erythematosus to B cell lymphomas. Cellular processes governed by MHC class II proteins play a major role in this response and its dysregulation. The extent to which processes controlled by the diverse family of MHC class I proteins impact such autoimmune and neoplastic disorders, however, is less clear. In this study, we genetically dissect the contributions of individual MHC class I family members and the pathological processes under their control in the systemic lupus erythematosus-like disease of BXSB.Yaa mice and B cell lymphomagenesis of SJL mice. This study reveals a powerful repressive regulatory axis comprised of MHC class I-dependent CD8(+) T cells and NK cells. These results indicate that the predominant role of the MHC class I protein family in such immunological disorders is to protect from more aggressive diseases.

Intravenous immunoglobulin in the treatment of autoimmune bullous dermatoses: an update

High-dose intravenous immunoglobulin (IVIg) is being increasingly utilized as an off-label therapy for a variety of autoimmune and inflammatory conditions across various specialties. Numerous reports have shown that it is an effective treatment for autoimmune skin blistering disorders. Unlike most therapies for blistering disorders, IVIg is not immunosuppressive and has a favorable side effect profile. This has allowed its use to expand dramatically over the last decade. However, due to the rarity and severity of autoimmune skin blistering diseases, well-designed prospective trials are generally lacking. This work highlights major research developments and the best evidence to date regarding the treatment of autoimmune pemphigus, bullous pemphigoid, mucous membrane pemphigoid, epidermolysis bullosa acquisita, pemphigoid gestationis, and linear IgA dermatosis with IVIg, providing an update on its efficacy, proposed mechanisms of action, side effect profile, and indications for use.

Regulation of immunity and inflammation by intravenous immunoglobulin: relevance to solid organ transplantation

Intravenous immunoglobulin (IVIg) products are derived from pooled human plasma from thousands of donors and have been used for the treatment of primary immunodeficiency disorders for more than 30 years. IVIg products are also effective in the treatment of autoimmune and inflammatory disorders, however, the precise mechanism(s) of action are not known. Recent data suggest that IVIg has a much broader ability to regulate cellular immunity, including innate and adaptive components. IVIg-induced upregulation of Fcγ receptor IIB on B cells appears to be an important mode of action in suppression of antigen-presenting cell activity and antibody production. IVIg is also a recently recognized modifier of complement activation and injury. Analysis of clinical studies examining the use of IVIg in desensitization protocols and for treatment of antibody-mediated rejection in transplant recipients are supportive. Here, we discuss these important advancements and their relevance to transplant medicine.

Neonatal Fc receptor and IgG-based therapeutics

The majority of potent new biologics today are IgG-based molecules that have demonstrated tissue-targeting specificity with favorable clinical response. Several factors determine the efficacy of these products, including target specificity, serum half-life and effector functions via complement-dependent cytotoxicity, antibody-dependent cell-mediated cytotoxicity or drug conjugates. In this review, we will focus on the interaction between therapeutic antibody and neonatal Fc receptor (FcRn), which is one of the critical factors in determining the circulating antibody half-life. Specifically, we will review the fundamental biology of FcRn, FcRn functions in various organs, Fc mutations designed to modulate binding to FcRn, IgG-based therapeutics that directly exploit FcRn functions and tools and strategies used to study FcRn-IgG interactions. Comprehensive understanding of FcRn-IgG interactions not only allows for development of effective therapeutics, but also avoidance of potential adverse effects.

Neonatal Fc receptor blockade by Fc engineering ameliorates arthritis in a murine model

Multiple autoimmune diseases are characterized by the involvement of autoreactive Abs in pathogenesis. Problems associated with existing therapeutics such as the delivery of intravenous immunoglobulin have led to interest in developing alternative approaches using recombinant or synthetic methods. Toward this aim, in the current study, we demonstrate that the use of Fc-engineered Abs (Abs that enhance IgG degradation [Abdegs]) to block neonatal FcR (FcRn) through high-affinity, Fc region binding is an effective strategy for the treatment of Ab-mediated disease. Specifically, Abdegs can be used at low, single doses to treat disease in the K/B×N serum transfer model of arthritis using BALB/c mice as recipients. Similar therapeutic effects are induced by 25- to 50-fold higher doses of i.v. Ig. Importantly, we show that FcRn blockade is a primary contributing factor toward the observed reduction in disease severity. The levels of albumin, which is also recycled by FcRn, are not affected by Abdeg delivery. Consequently, Abdegs do not alter FcRn expression levels or subcellular trafficking behavior. The engineering of Ab Fc regions to generate potent FcRn blockers therefore holds promise for the therapy of Ab-mediated autoimmunity.

Animal model of fetal and neonatal immune thrombocytopenia: role of neonatal Fc receptor in the pathogenesis and therapy

Fetal and neonatal immune thrombocytopenia (FNIT) is a severe bleeding disorder in which maternal antibodies cross the placenta and destroy fetal/neonatal platelets. It has been demonstrated that the neonatal Fc receptor (FcRn) regulates immunoglobulin G (IgG) homeostasis and plays an important role in transplacental IgG transport. However, the role of FcRn in the pathogenesis and therapy of FNIT has not been studied. Here, we developed an animal model of FNIT using combined β3 integrin–deficient and FcRn-deficient (β3−/−FcRn−/−) mice. We found that β3−/−FcRn−/− mice are immunoresponsive to β3+/+FcRn−/− platelets. The generated antibodies were β3 integrin specific and were maintained at levels that efficiently induced thrombocytopenia in adult β3+/+FcRn−/− mice. FNIT was observed when immunized β3−/−FcRn+/+ females were bred with β3+/+FcRn+/+ males, while no FNIT occurred in β3−/−FcRn−/− females bred with β3+/+FcRn−/− males, suggesting that FcRn is indispensable for the induction of FNIT. We further demonstrated that fetal FcRn was responsible for the transplacental transport of various IgG isotypes. We found that anti-FcRn antibody and intravenous IgG prevented FNIT, and that intravenous IgG ameliorated FNIT through both FcRn-dependent and -independent pathways. Our data suggest that targeting FcRn may be a potential therapy for human FNIT as well as other maternal pathogenic antibody-mediated diseases.

Neonatal Fc receptor: from immunity to therapeutics

The neonatal Fc receptor (FcRn), also known as the Brambell receptor and encoded by Fcgrt, is a MHC class I like molecule that functions to protect IgG and albumin from catabolism, mediates transport of IgG across epithelial cells, and is involved in antigen presentation by professional antigen presenting cells. Its function is evident in early life in the transport of IgG from mother to fetus and neonate for passive immunity and later in the development of adaptive immunity and other functions throughout life. The unique ability of this receptor to prolong the half-life of IgG and albumin has guided engineering of novel therapeutics. Here, we aim to summarize the basic understanding of FcRn biology, its functions in various organs, and the therapeutic design of antibody- and albumin-based therapeutics in light of their interactions with FcRn.

Intravenous immunoglobulin therapy in autoimmune mucocutaneous blistering diseases: a review of the evidence for its efficacy and safety

Intravenous immunoglobulin (IVIg) is a biologic agent that is being increasingly used in the treatment of autoimmune and chronic inflammatory disorders. It is approved by the US FDA for the treatment of primary immunodeficiencies, immune thrombocytopenic purpura, Kawasaki disease, bone marrow transplantation in patients aged over 20 years, chronic B-cell lymphocytic leukemia, and pediatric AIDS. IVIg has been used off-label for several diseases, clinical symptoms and syndromes. Our aim was to determine if there is evidence to support the efficacy of IVIg therapy in autoimmune mucocutaneous blistering diseases (AMBDs). We searched the PubMed database for studies on pemphigus and pemphigoid using the following criteria: (i) English language; (ii) minimum of five patients; (iii) diagnosis based on histology and immunopathology; and (iv) statistical analysis of data for comparison of efficacy provided. We evaluated the data and present information on the number of participants in each study, pre-IVIg therapy, indications for the use of IVIg, IVIg protocol (dose and interval) used, concomitant therapies, clinical outcome, follow-up period, and serologic studies. The quality of the evidence presented in this review is at Level A according to the UK National Health Service criteria. Twenty-three studies that were published between May 1999 and April 2010 were identified. One randomized controlled trial was found and all other studies were case series. Data on 260 patients treated with IVIg were analyzed: 191 patients with pemphigus and 69 patients with pemphigoid. Overall, 245 patients showed improvement with IVIg therapy. IVIg demonstrated a corticosteroid-sparing effect. In the studies presented, the incidence of serious adverse effects was not significant. The best available evidence in the literature indicates that IVIg is efficacious and has a good safety profile in the treatment of AMBDs.

The inhibitory effects of intravenous administration of rabbit immunoglobulin G on airway inflammation are dependent upon Fcγ receptor IIb on CD11c(+) dendritic cells in a murine model

Immunoglobulins (Igs) play important immunomodulatory effects on allergic asthma. Among these, IgG has been reported to regulate allergic inflammation in previous studies about immunotherapy and intravenous immunoglobulin therapy. In this study, to examine the immunomodulatory mechanisms of IgG and FcRs we evaluated the effects of intravenous (i.v.) rabbit IgG administration (IVIgG) on allergic airway inflammation and lung antigen-presenting cells (APCs) in a murine model of ovalbumin (OVA) sensitization and challenge. In OVA-challenged mice, IVIgG attenuated airway eosinophilia, airway hyperresponsiveness and goblet cell hyperplasia and also inhibited the local T helper type (Th) 2 cytokine levels. Additionally, IVIgG attenuated the proliferation of OVA-specific CD4(+) T cells transplanted into OVA-challenged mice. Ex vivo co-culture with OVA-specific CD4(+) cells and lung CD11c(+) APCs from mice with IVIgG revealed the attenuated transcription level of Th2 cytokines, suggesting an inhibitory effect of IVIgG on CD11c(+) APCs to induce Th2 response. Next, to analyse the effects on Fcγ receptor IIb and dendritic cells (DCs), asthmatic features in Fcγ receptor IIb-deficient mice were analysed. IVIgG failed to attenuate airway eosinophilia, airway inflammation and goblet cell hyperplasia. However, the lacking effects of IVIgG on airway eosinophilia in Fcγ receptor IIb deficiency were restored by i.v. transplantation of wild-type bone marrow-derived CD11c(+) DCs. These results demonstrate that IVIgG attenuates asthmatic features and the function of lung CD11c(+) DCs via Fcγ receptor IIb in allergic airway inflammation. Targeting Fc portions of IgG and Fcγ receptor IIb on CD11c(+) DCs in allergic asthma is a promising therapeutic strategy.

Mechanisms of action of intravenous immunoglobulins

Intravenous immunoglobulin (IVIg) has been used for nearly three decades as an efficient anti-inflammatory therapeutic regimen in a growing number of autoimmune diseases. Despite this their success in clinical application, the mechanism of action of IVIg therapy remains elusive. During the last few years, several mechanisms dependent on either the IgG variable or constant fragment have been proposed to explain the potent immunomodulatory activity of IVIg. This review will discuss which molecular and cellular pathways might be involved in the anti-inflammatory activity of IVIg and for which types of autoimmune diseases they might be relevant.

The neonatal Fc receptor as therapeutic target in IgG-mediated autoimmune diseases

Therapy approaches based on lowering levels of pathogenic autoantibodies represent rational, effective, and safe treatment modalities of autoimmune diseases. The neonatal Fc receptor (FcRn) is a major factor regulating the serum levels of IgG antibodies. While FcRn-mediated half-life extension is beneficial for IgG antibody responses against pathogens, it also prolongs the serum half-life of IgG autoantibodies and thus promotes tissue damage in autoimmune diseases. In the present review article, we examine current evidence on the relevance of FcRn in maintaining high autoantibody levels and discuss FcRn-targeted therapeutic approaches. Further investigation of the FcRn-IgG interaction will not only provide mechanistic insights into the receptor function, but should also greatly facilitate the design of therapeutics combining optimal pharmacokinetic properties with the appropriate antibody effector functions in autoimmune diseases.

The Thomsen-Friedenreich antigen-binding lectin jacalin interacts with desmoglein-1 and abrogates the pathogenicity of pemphigus foliaceus autoantibodies in vivo

Pemphigus foliaceus (PF) is an autoimmune skin blistering disease mediated by pathogenic autoantibodies against the desmosomal core glycoprotein desmoglein-1 (Dsg1). This study demonstrated that the O-glycan-specific plant lectin jacalin binds Dsg1 and inhibits the interaction of Dsg1/PF IgG. N-glycosylation is not involved in the interaction of Dsg1/jacalin or Dsg1/PF IgG. Subcutaneous injection of jacalin into neonatal mice drastically reduced PF IgG deposition at the epidermal cell surface and blocked PF IgG-induced skin blisters, both clinically and histologically. Interestingly, another plant lectin, peanut agglutinin, which shares the same carbohydrate specificity toward the O-linked carbohydrate structure known as Thomsen-Friedenreich antigen (TF antigen, Galβ1-3GalNAcα-O-Ser/Thr), also bound Dsg1 and blocked the skin blistering. In contrast, the plant lectin vicia villosa-B4 (VVL-B4), which shares the carbohydrate specificity toward the O-linked monosaccharide known as Thomsen-nouveau antigen (GalNAc-α1-O-Ser/Thr), did not bind Dsg1 and did not show a protective effect against the disease induced by the autoantibodies. Collectively, these results suggest that the binding of jacalin to O-linked TF carbohydrate motifs on Dsg1 impairs the Dsg1/PF autoantibody interactions and abrogates its pathogenicity in vivo. TF-specific binding ligands may have a potential therapeutic value for PF.