Pathogenic human monoclonal antibody against desmoglein 3

IgG against the Membrane-Proximal Portion of the Desmoglein 3 Ectodomain Induces Loss of Keratinocyte Adhesion, a Hallmark in Pemphigus Vulgaris

Pemphigus vulgaris is a severe autoimmune blistering disease characterized by IgG autoantibodies (auto-abs) against the desmosomal adhesion molecules desmoglein (DSG) 3 and DSG1. Underlying mechanisms leading to blister formation upon binding of DSG-specific IgG auto-abs are not fully understood. Numerous studies showed the pathogenicity of IgG auto-ab binding to the aminoterminal region 1 (EC1) of the DSG3 ectodomain. However, auto-abs in pemphigus vulgaris are polyclonal, including IgG against both aminoterminal- and membrane-proximal epitopes of the DSG3 ectodomain. In this study, the pathogenicity of a previously uncharacterized murine monoclonal IgG antibody, 2G4, directed against the membrane-proximal region (EC5) of the DSG3 ectodomain was characterized and tested in various specificity and functionality assays. The results clearly show that 2G4 is capable of inhibiting intercellular keratinocyte adhesion and of inducing cellular DSG3 redistribution by activation of the p38MAPK signal transduction pathway. In this study, we provide evidence that an IgG auto-abs directed against the membrane-proximal region EC5 of DSG3 induces acantholysis, the hallmark in pemphigus vulgaris. These findings challenge the current concept that IgG auto-abs targeting the NH₂-terminal portion of the DSG3 ectodomain are pathogenic only. Our study provides further aspects for a deeper understanding of desmosomal keratinocyte adhesion and improves our insight into the complex auto-ab‒induced blister formation in pemphigus vulgaris.

Pemphigus and Pemphigoid: From Disease Mechanisms to Druggable Pathways

Pemphigus and pemphigoid are paradigms for understanding the mechanisms of antibody-mediated autoimmune disease in humans. In pemphigus, IgG4-predominant autoantibodies cause intraepidermal blistering by direct interference with desmoglein interactions and subsequent disruption of desmosomes and signaling pathways. In pemphigoid, IgG1, IgG4, and IgE autoantibodies against basement membrane zone antigens directly interfere with hemidesmosomal adhesion, activating complement and Fc receptor‒mediated effector pathways. Unraveling disease mechanisms in pemphigus and pemphigoid has identified numerous opportunities for clinical trials, which hold promise to identify safer and more effective therapies for these potentially life-threatening diseases.

Targeting interleukin 4 receptor α: A new approach to the treatment of cutaneous autoimmune bullous diseases?

Bullous pemphigoid, mucous membrane pemphigoid, and pemphigus vulgaris are different cutaneous autoimmune blistering diseases, with complex pathogenic mechanisms. In all of them, a type-2 response is thought to have a central role. Interleukin 4 and Interleukin 13 are crucial cytokines in type-2 response. Treatment of these conditions is often challenging. Dupilumab, a recombinant fully human IgG4 monoclonal antibody with binding specificity to human interleukin-4 receptor IL-4Rα, has the potential to inhibit both IL-4 and IL-13. We propose IL-4Rα as a theoretical drug target for cutaneous autoimmune bullous diseases.

Walnut antigens can trigger autoantibody development in patients with pemphigus vulgaris through a "hit-and-run" mechanism

BACKGROUND

Environmental factors, as well as genetic predisposition, are known to be critical for the development of autoimmunity. However, the environmental agents that trigger autoimmune responses have remained elusive. One possible explanation is the "hit-and-run" mechanism in which the inciting antigens that initiate autoimmune responses are not present at the time of overt autoimmune disease.

OBJECTIVE

After our previous findings that some allergens can incite autoimmune responses, we investigated the potential role of environmental allergens in triggering autoantibody development in patients with an autoimmune skin disease, pemphigus vulgaris (PV).

METHODS

Revertant/germline mAbs (with mutations on variable regions of heavy and light chains reverted to germline forms) of 8 anti-desmoglein (Dsg) 3 pathogenic mAbs from patients with PV were tested for reactivity against a panel of possible allergens, including insects, pollens, epithelia, fungi, and food antigens.

RESULTS

All the PV germline mAbs were reactive to antigens from walnut, including the well-known allergen Jug r 2 and an uncharacterized 85-kDa protein component. Sera from patients with PV contained significantly greater levels of anti-Dsg3 autoantibodies than walnut-specific antibodies, suggesting that the autoreactive B-cell response in patients with PV might be initially triggered by walnut antigens but is subsequently driven by Dsg3.

CONCLUSION

Our findings suggest that walnut antigens/allergens can initiate autoantibody development in patients with PV through a "hit-and-run" mechanism. The revertant/germline mAb approach might provide a paradigm for the etiological study of other allergic and autoimmune diseases.

Single-Cell Analysis Suggests that Ongoing Affinity Maturation Drives the Emergence of Pemphigus Vulgaris Autoimmune Disease

Pemphigus vulgaris (PV) is an autoimmune disease characterized by blistering sores on skin and mucosal membranes, caused by autoantibodies primarily targeting the cellular adhesion protein, desmoglein-3 (Dsg3). To better understand how Dsg3-specific autoantibodies develop and cause disease in humans, we performed a cross-sectional study of PV patients before and after treatment to track relevant cellular responses underlying disease pathogenesis, and we provide an in-depth analysis of two patients by generating a panel of mAbs from single Dsg3-specific memory B cells (MBCs). Additionally, we analyzed a paired sample from one patient collected 15-months prior to disease diagnosis. We find that Dsg3-specific MBCs have an activated phenotype and show signs of ongoing affinity maturation and clonal selection. Monoclonal antibodies (mAbs) with pathogenic activity primarily target epitopes in the extracellular domains EC1 and EC2 of Dsg3, though they can also bind to the EC4 domain. Combining antibodies targeting different epitopes synergistically enhances in vitro pathogenicity.

Affinity Maturation Drives Epitope Spreading and Generation of Proinflammatory Anti-Citrullinated Protein Antibodies in Rheumatoid Arthritis

OBJECTIVE

Rheumatoid arthritis (RA) is characterized by the presence of anti-citrullinated protein antibodies (ACPAs); nevertheless, the origin, specificity, and functional properties of ACPAs remain poorly understood. The aim of this study was to characterize the evolution of ACPAs by sequencing the plasmablast antibody repertoire at serial time points in patients with established RA.

METHODS

Blood samples were obtained at up to 4 serial time points from 8 individuals with established RA who were positive for ACPAs by the anti-cyclic citrullinated peptide test. CD19+CD3-IgD-CD14-CD20-CD27+CD38++ plasmablasts were isolated by single-cell sorting and costained with citrullinated peptide tetramers to identify ACPA-expressing plasmablasts. Cell-specific oligonucleotide barcodes were utilized, followed by large-scale sequencing and bioinformatics analysis, to obtain error-corrected, paired heavy- and light-chain antibody gene sequences for each B cell.

RESULTS

Bioinformatics analysis revealed 170 persistent plasmablast lineages in the RA blood, of which 19% included multiple isotypes. Among IgG- and IgA-expressing plasmablasts, significantly more IgA-expressing than IgG-expressing persistent lineages were observed (P < 0.01). Shared complementarity-determining region 3 sequence motifs were identified across subjects. A subset of the plasmablast lineages included members derived from later time points with divergent somatic hypermutations that encoded antibodies that bind an expanded set of citrullinated antigens. Furthermore, these recombinant, differentially mutated plasmablast antibodies formed immune complexes that stimulated higher macrophage production of tumor necrosis factor (TNF) compared to antibodies representing earlier time point-derived lineage members that were less mutated.

CONCLUSION

These findings demonstrate that established RA is characterized by a persistent IgA ACPA response that exhibits ongoing affinity maturation. This observation suggests the presence of a persistent mucosal antigen that continually promotes the production of IgA plasmablasts and their affinity maturation and epitope spreading, thus leading to the generation of ACPAs that bind additional citrullinated antigens and more potently stimulate macrophage production of TNF.

IgG4-mediated autoimmune diseases: a niche of antibody-mediated disorders

Immunoglobulin 4 (IgG4) is one of four human IgG subclasses and has several unique functional characteristics. It exhibits low affinity for complement and for most Fc receptors. It furthermore has generally high affinity for its antigen, with binding occurring in a monovalent fashion, as IgG4 can exchange Fab-arms with other IgG4 molecules. Because of these characteristics, IgG4 is believed to block its targets and prevent inflammation, which, depending on the setting, can have a protective or pathogenic effect. One example of IgG4 pathogenicity is muscle-specific kinase (MuSK) myasthenia gravis (MG), in which patients develop IgG4 MuSK autoantibodies, resulting in muscle weakness. As a consequence of the distinct IgG4 characteristics, the pathomechanism of MuSK MG is very different from IgG1-and IgG3-mediated autoimmune diseases, such as acetylcholine receptor MG. In recent years, new autoantibodies in a spectrum of autoimmune diseases have been discovered. Interestingly, some were found to be predominantly IgG4. These IgG4-mediated autoimmune diseases share many pathomechanistic aspects with MuSK MG, suggesting that IgG4-mediated autoimmunity forms a separate niche among the antibody-mediated disorders. In this review, we summarize the group of IgG4-mediated autoimmune diseases, discuss the role of IgG4 in MuSK MG, and highlight interesting future research questions for IgG4-mediated autoimmunity.

Mechanisms Causing Loss of Keratinocyte Cohesion in Pemphigus

The autoimmune blistering skin disease pemphigus is caused by IgG autoantibodies against desmosomal cadherins, but the precise mechanisms are in part a matter of controversial discussions. This review focuses on the currently existing models of the disease and highlights the relevance of desmoglein-specific versus nondesmoglein autoantibodies, the contribution of nonautoantibody factors, and the mechanisms leading to cell dissociation and blister formation in response to autoantibody binding. As the review brings together the majority of laboratories currently working on pemphigus pathogenesis, it aims to serve as a solid basis for further investigations for the entire field.

Setting the target for pemphigus vulgaris therapy

Despite the rising incidence of autoimmunity, therapeutic options for patients with autoimmune disease still rely on decades-old immunosuppressive strategies that risk severe and potentially fatal complications. Thus, novel therapeutic approaches for autoimmune diseases are greatly needed in order to minimize treatment-related toxicity. Such strategies would ideally target only the autoreactive immune components to preserve beneficial immunity. Here, we review how several decades of basic, translational, and clinical research on the immunology of pemphigus vulgaris (PV), an autoantibody-mediated skin disease, have enabled the development of targeted immunotherapeutic strategies. We discuss research to elucidate the pathophysiology of PV and how the knowledge afforded by these studies has led to the preclinical and clinical testing of targeted approaches to neutralize autoantibodies, to induce antigen-specific tolerance, and to specifically eliminate autoreactive B cells in PV.

Rituximab therapy in pemphigus and other autoantibody-mediated diseases

Rituximab, a monoclonal antibody targeting the B cell marker CD20, was initially approved in 1997 by the United States Food and Drug Administration (FDA) for the treatment of non-Hodgkin lymphoma. Since that time, rituximab has been FDA-approved for rheumatoid arthritis and vasculitides, such as granulomatosis with polyangiitis and microscopic polyangiitis. Additionally, rituximab has been used off-label in the treatment of numerous other autoimmune diseases, with notable success in pemphigus, an autoantibody-mediated skin blistering disease. The efficacy of rituximab therapy in pemphigus has spurred interest in its potential to treat other autoantibody-mediated diseases. This review summarizes the efficacy of rituximab in pemphigus and examines its off-label use in other select autoantibody-mediated diseases.

Non-infectious environmental antigens as a trigger for the initiation of an autoimmune skin disease

Pemphigus represents a group of organ specific autoimmune blistering disorders of the skin mediated by pathogenic autoantibodies with well-defined antigenic targets. While most of these diseases are sporadic, endemic forms of disease do exist. The endemic form of pemphigus foliaceus (also known as fogo selvagem, FS) exhibits epidemiological features that suggest exposure to hematophagous insect bites are a possible precipitating factor of this autoimmune disease, and provides a unique opportunity to study how environmental factors contribute to autoimmune disease development. FS patients and healthy individuals from endemic regions show an autoreactive IgM response that starts in early childhood and becomes restricted to IgG4 autoantibodies in FS patients. In searching for triggering environmental antigens, we have found that IgG4 and IgE autoantibodies from FS patients cross-react with a salivary antigen from sand flies. The presence of these cross-reactive antibodies and antibody genetic analysis confirming that these antibodies evolve from the same naïve B cells provides compelling evidence that this non-infectious environmental antigen could be the initial target of the autoantibody response in FS. Consequently, FS serves as an ideal model to study the impact of environmental antigens in the development of autoimmune disease.

Pathogenicity and Epitope Characteristics Do Not Differ in IgG Subclass-Switched Anti-Desmoglein 3 IgG1 and IgG4 Autoantibodies in Pemphigus Vulgaris

Pemphigus vulgaris (PV) is characterized by IgG1 and IgG4 autoantibodies to desmoglein (Dsg) 3, causing suprabasal blistering of skin and mucous membranes. IgG4 is the dominant autoantibody subclass in PV and correlates with disease activity, whereas IgG1 can be associated with remittent disease. It is unknown if switching the same variable region between IgG4 and IgG1 directly impacts pathogenicity. Here, we tested whether three pathogenic PV monoclonal antibodies (mAbs) from three different patients demonstrate differences in antigen affinity, epitope specificity, or pathogenicity when expressed as IgG1 or IgG4. F706 anti-Dsg3 IgG4 and F779 anti-Dsg3 IgG1, previously isolated as heterohybridomas, and Px43, a monovalent anti-Dsg3/Dsg1 IgG antibody isolated by phage display, were subcloned to obtain paired sets of IgG1 and IgG4 mAbs. Using ELISA and cell surface staining assays, F706 and F779 demonstrated similar antigen binding affinities of IgG1 and IgG4, whereas Px43 showed 3- to 8-fold higher affinity of IgG4 versus IgG1 by ELISA, but identical binding affinities to human skin, perhaps due to targeting of a quaternary epitope best displayed in tissues. All 3 mAb pairs targeted the same extracellular cadherin (EC) domain on Dsg3, caused Dsg3 internalization in primary human keratinocytes, and caused suprabasal blisters in human skin at comparable doses. We conclude that switching IgG1 and IgG4 subclasses of pathogenic PV mAbs does not directly affect their antigen binding or pathogenic properties.

Clonal analysis of B-cell response in pemphigus course: toward more effective therapies

In this issue, Hammers et al. report the first longitudinal study of B-cell responses in pemphigus vulgaris. They show persistence of the same sets of anti-desmoglein 3 (Dsg3) B-cell clones over time in two patients with active disease, suggesting that escape from tolerance is not a frequent event. In addition, the disappearance of anti-Dsg3 B-cell clones in two patients during long-term rituximab-induced remission supports the therapeutic potential of rituximab.

Shared VH1-46 gene usage by pemphigus vulgaris autoantibodies indicates common humoral immune responses among patients

Pemphigus vulgaris (PV) is a potentially fatal blistering disease caused by autoantibodies (autoAbs) against desmoglein 3 (Dsg3). Here, we clone anti-Dsg3 antibodies (Abs) from four PV patients and identify pathogenic VH1-46 autoAbs from all four patients. Unexpectedly, VH1-46 autoAbs had relatively few replacement mutations. We reverted antibody somatic mutations to their germline sequences to determine the requirement of mutations for autoreactivity. Three of five VH1-46 germline-reverted Abs maintain Dsg3 binding, compared with zero of five non-VH1-46 germline-reverted Abs. Site-directed mutagenesis of VH1-46 Abs demonstrates that acidic amino-acid residues introduced by somatic mutation or heavy chain VDJ recombination are necessary and sufficient for Dsg3 binding. Our data suggest that VH1-46 autoantibody gene usage is commonly found in PV because VH1-46 Abs require few to no mutations to acquire Dsg3 autoreactivity, which may favour their early selection. Common VH gene usage indicates common humoral immune responses, even among unrelated patients.

MAPKAP kinase 2 (MK2)-dependent and -independent models of blister formation in pemphigus vulgaris

Pemphigus vulgaris (PV) is an autoimmune blistering disease characterized by autoantibodies to the keratinocyte adhesion protein desmoglein (Dsg) 3. Previous studies suggest that PV pathogenesis involves p38 mitogen activated protein kinase-dependent and -independent pathways. However, p38 is a difficult protein to study and therapeutically target because it has four isoforms and multiple downstream effectors. In the current study, we identify MAPKAP kinase 2 (MK2) as a downstream effector of p38 signaling in PV and describe MK2-dependent and -independent mechanisms of blister formation using passive transfer of human anti-Dsg IgG4 mAbs to neonatal mice. In human keratinocytes, PV mAbs activate MK2 in a dose-dependent manner. MK2 is also activated in human pemphigus skin blisters, causing translocation of MK2 from the nucleus to the cytosol. Small molecule inhibition of MK2 and silencing of MK2 expression block PV mAb-induced Dsg3 endocytosis in human keratinocytes. Additionally, small molecule inhibition and genetic deletion of p38α and MK2 inhibit spontaneous, but not induced, suprabasal blisters by PV mAbs in mouse passive transfer models. Collectively, these data suggest that MK2 is a key downstream effector of p38 that can modulate PV autoantibody pathogenicity. MK2 inhibition may be a valuable adjunctive therapy for control of pemphigus blistering.

Immunohistochemistry for Immunoglobulin G4 on Paraffin Sections for the Diagnosis of Pemphigus

Context.—Pemphigus is a group of autoimmune vesiculobullous diseases characterized by immunoglobulin G (IgG) antibodies directed against desmosomal adhesion proteins, with IgG4 being the predominant subclass in active diseases. Direct immunofluorescence for IgG performed on fresh-frozen tissue plays a crucial role in diagnosing pemphigus. However, the diagnosis might be hindered when frozen tissue is not available.

Objective.—To evaluate the usefulness of immunohistochemistry for IgG4 performed on paraffin sections as a diagnostic test for pemphigus.

Design.—Eighteen immunofluorescence-proven pemphigus cases (12 pemphigus vulgaris, 6 pemphigus foliaceus) were studied. Four normal skin specimens and 32 nonpemphigus vesiculobullous disease specimens served as controls. Paraffin sections of all cases were examined immunohistochemically for IgG4 expression. Positivity was defined as distinct, condensed, continuous immunoreactivity localized to the intercellular junctions of keratinocytes.

Results.—The immunostains were independently evaluated in a masked manner by 3 pathologists, with a 100% interobserver agreement. Nine of 12 pemphigus vulgaris cases (sensitivity 75.0%), and 4 of 6 pemphigus foliaceus cases (sensitivity 66.7%), were positive for IgG4 immunostain. The overall sensitivity was 72.2%. One control specimen (bullous pemphigoid) showed IgG4 positivity (specificity 97.2%). In specimens demonstrating acantholysis, 8 of 10 pemphigus vulgaris cases (sensitivity 80.0%) and 4 of 4 pemphigus foliaceus cases (sensitivity 100.0%) were positive for IgG4. The overall sensitivity for specimens with acantholytic lesions was 85.7%.

Conclusion.—Immunohistochemistry for IgG4 provides a reasonably sensitive and highly specific test for diagnosing pemphigus, especially when frozen tissue is not available, and active acantholytic lesions are examined.

Methods for revealing therapeutic targets in case of true acantholytic pemphigus

The authors present a review of current approaches to studying the pathogenesis of true acantholytic pemphigus that may reveal new targets for therapeutic treatment. They discuss methods used for the first time to study this lethal dermatosis, and the development of previously used methods that are still important for studying true acantholytic pemphigus.

The enigmatic autoimmune response in endemic pemphigus foliaceus

Endemic pemphigus foliaceus, known as Fogo Selvagem (FS) in Brazil, is a disease characterized by subcorneal blisters and IgG4 anti-dermoglein 1 (Dsg1) autoantibodies. Epidemiological studies of FS strongly an environmental etiology. A 15-year surveillance of the Limao Verde Amerindian reservation in Brazil has uncovered information on the transition of the autoimmune response from the pre-clinical stage to disease state. This incubation time may evolve over several years. The serological markers of the pre-clinical state of FS are IgM anti-Dsg1, IgE and non-IgG4 autoantibodies against Dsg1. The disease stage of FS is characterized by the rise of pathogenic IgG4 anti-Dsg1 autoantibodies. In this review, the authors reviewed the literature on the relevance of the humoral autoimmune response of FS as well as the possible environmental triggers of anti-Dsg1 autoantibody formation. Based on epidemiological observations, the authors hypothesize that the pathogenic IgG4 response in FS may be triggered by hematophagous insect bites.

Structure and function of immunoglobulins

Immunoglobulins are heterodimeric proteins composed of 2 heavy and 2 light chains. They can be separated functionally into variable domains that bind antigens and constant domains that specify effector functions, such as activation of complement or binding to Fc receptors. The variable domains are created by means of a complex series of gene rearrangement events and can then be subjected to somatic hypermutation after exposure to antigen to allow affinity maturation. Each variable domain can be split into 3 regions of sequence variability termed the complementarity-determining regions (CDRs) and 4 regions of relatively constant sequence termed the framework regions. The 3 CDRs of the heavy chain are paired with the 3 CDRs of the light chain to form the antigen-binding site, as classically defined. The constant domains of the heavy chain can be switched to allow altered effector function while maintaining antigen specificity. There are 5 main classes of heavy chain constant domains. Each class defines the IgM, IgG, IgA, IgD, and IgE isotypes. IgG can be split into 4 subclasses, IgG1, IgG2, IgG3, and IgG4, each with its own biologic properties, and IgA can similarly be split into IgA1 and IgA2.

Clinical activity of pemphigus vulgaris relates to IgE autoantibodies against desmoglein 3

Pemphigus vulgaris (PV) is a severe autoimmune bullous skin disease and is primarily associated with IgG against desmoglein 3 (dsg3), a desmosomal adhesion protein. In light of the recent association of autoreactive T helper (Th) 2 cells with active PV, the present study sought to relate the occurrence of Th2-regulated dsg3-specific autoantibody subtypes, i.e. IgE and IgG4, in 93 well-characterized PV patients. Patients with acute onset PV (n=37) showed the highest concentrations of serum IgE and IgG4 autoantibodies, which were significantly lower in PV patients in remission (n=14). Furthermore, there was a strong correlation between dsg3-reactive IgE and IgG4 in acute onset, but not in chronic active (n=42) or remittent patients. Additionally, intercellular IgE deposits were detected in the epidermis of acute onset PV. Thus, dsg3-specific IgE and IgG4 autoantibodies are related to acute onset disease which provides additional support to the concept that PV is a Th2-driven autoimmune disorder.

Significance of autoimmunity to non-desmoglein targets in pemphigus

The pathogenesis of pemphigus vulgaris (PV) is a highly controversial, "hot" topic that has received considerable enrichment in recent years by both clinical and basic researchers. On the one hand, the classical view of desmogleins (Dsg) as main targets of this autoimmune disease is supported by the characterization of pathogenic anti-Dsg3 antibodies from both patients and animal models. On the other hand, fundamental doubt has been raised towards this monopathogenic view by several independent factors: (1) pemphigus lesions can be induced in Dsg3-knockout (KO) mice; (2) pemphigus sera contain multiple autoantibodies against different adhesion molecules and also cholinergic receptors; (3) experimental inhibition of PV IgG induced acantholysis can be obtained by interference with different signaling cascades regulating both calcium homeostasis and apoptosis; and (4) cholinergic agonists exhibit anti-acantholytic activity both in vitro and in vivo. The field is open for controlled clinical trials and further basic research to unfold the true story of the pemphigus enigma and provide the basis for a better treatment of pemphigus patients.

Pathogenic epitopes of autoantibodies in pemphigus reside in the amino-terminal adhesive region of desmogleins which are unmasked by proteolytic processing of prosequence

Pemphigus targets desmogleins (Dsgs), which are thought to be synthesized as inactive precursor proteins with prosequences that are cleaved by substilisin-like proprotein convertases, such as furin, to yield mature adhesive molecules. We hypothesized that some pemphigus pathogenic antibodies (Abs), which presumably interfere with adhesion, only bind the mature form. A pathogenic and three non-pathogenic anti-Dsg1 monoclonal Abs (mAbs) isolated from a pemphigus foliaceus (PF) patient, were used for immunoprecipitation and ELISA of recombinant precursor and mature Dsg1. The pathogenic Ab binds mature Dsg1, whereas non-pathogenic Abs bind either only the precursor or both the precursor and mature Dsg1. Competition ELISA showed that the majority of PF sera target the same or nearby epitopes defined by the pathogenic anti-Dsg1 mAb that blocked >20% binding of 29 out of 40 PF sera. Furthermore, the immunoreactivity of 45 PF sera against the mature Dsg1 was 3.2 fold stronger than that against the precursor Dsg1 by ELISA. Similar results were observed in anti-Dsg3 Abs in 47 pemphigus vulgaris sera, suggesting that most pemphigus sera target epitopes that are unmasked by proteolytic processing. These findings support the idea that at least some pathogenic pemphigus autoantibodies induce the loss of cell adhesion by directly binding the trans-interaction site of Dsgs.

Immunoglobulin G4: an odd antibody

Despite its well-known association with IgE-mediated allergy, IgG4 antibodies still have several poorly understood characteristics. IgG4 is a very dynamic antibody: the antibody is involved in a continuous process of half-molecules (i.e. a heavy and attached light-chain) exchange. This process, also referred to as 'Fab-arm exchange', results usually in asymmetric antibodies with two different antigen-combining sites. While these antibodies are hetero- bivalent, they will behave as monovalent antibodies in most situations. Another aspect of IgG4, still poorly understood, is its tendency to mimic IgG rheumatoid factor (RF) activity by interacting with IgG on a solid support. In contrast to conventional RF, which binds via its variable domains, the activity of IgG4 is located in its constant domains. This is potentially a source of false positives in IgG4 antibody assay results. Because regulation of IgG4 production is dependent on help by T-helper type 2 (Th2) cells, the IgG4 response is largely restricted to non-microbial antigens. This Th2-dependency associates the IgG4 and IgE responses. Another typical feature in the immune regulation of IgG4 is its tendency to appear only after prolonged immunization. In the context of IgE-mediated allergy, the appearance of IgG4 antibodies is usually associated with a decrease in symptoms. This is likely to be due, at least in part, to an allergen-blocking effect at the mast cell level and/or at the level of the antigen-presenting cell (preventing IgE-facilitated activation of T cells). In addition, the favourable association reflects the enhanced production of IL-10 and other anti-inflammatory cytokines, which drive the production of IgG4. While in general, IgG4 is being associated with non-activating characteristics, in some situations IgG4 antibodies have an association with pathology. Two striking examples are pemphigoid diseases and sclerosing diseases such as autoimmune pancreatitis. The mechanistic basis for the association of IgG4 with these diseases is still enigmatic. However, the association with sclerosing diseases may reflect an excessive production of anti-inflammatory cytokines triggering an overwhelming expansion of IgG4-producing plasma cells. The bottom line for allergy diagnosis: IgG4 by itself is unlikely to be a cause of allergic symptoms. In general, the presence of allergen-specific IgG4 indicates that anti-inflammatory, tolerance-inducing mechanisms have been activated. The existence of the IgG4 subclass, its up-regulation by anti-inflammatory factors and its own anti-inflammatory characteristics may help the immune system to dampen inappropriate inflammatory reactions.

Disruption of desmosome assembly by monovalent human pemphigus vulgaris monoclonal antibodies

The intercellular interactions of the desmosomal cadherins, desmoglein and desmocollin, are required for epidermal cell adhesion. Pemphigus vulgaris (PV) is a potentially fatal autoimmune blistering disease characterized by autoantibodies against desmoglein (Dsg) 3. During calcium-induced desmosome assembly, treatment of primary human keratinocytes with pathogenic monovalent anti-Dsg3 mAbs produced from a PV patient causes a decrease of Dsg3 and desmoplakin but not desmocollin (Dsc) 3 in the Triton-insoluble fraction of cell lysates within 2 hours. Immunofluorescence and antibody ELISA studies suggest that pathogenic mAbs cause internalization of cell-surface Dsg3 but not Dsc3 through early endosomes. Electron microscopy demonstrated a lack of well-formed desmosomes in keratinocytes treated with pathogenic compared to nonpathogenic mAbs. In contrast, pathogenic mAbs caused late depletion of Dsg3 from preformed desmosomes at 24 hours, with effects on multiple desmosomal proteins including Dsc3 and plakoglobin. Together, these studies indicate that pathogenic PV mAbs specifically cause internalization of newly synthesized Dsg3 during desmosome assembly, correlating with their pathogenic activity. Monovalent human PV anti-Dsg mAbs reproduce the effects of polyclonal PV IgG on Dsg3 and will facilitate future studies to further dissect the cellular mechanisms for the loss of cell adhesion in pemphigus.

Targeting Pemphigus Autoantibodies through their Heavy-Chain Variable Region Genes

Pemphigus vulgaris (PV) is a potentially fatal blistering disease characterized by autoantibodies against cell surface adhesion proteins desmoglein (Dsg) 3 and Dsg1. Previous studies using phage display to clone Dsg-reactive monoclonal antibodies from a PV patient demonstrated that a limited number of antibody variable region genes encode the autoantibody repertoire, with different genes for pathogenic and non-pathogenic mAbs. Here, we investigated the feasibility of specific autoantibody targeting in pemphigus. We produced rabbit anti-idiotypic antibodies against two pathogenic and two non-pathogenic PV mAbs. Antisera inhibited binding of the immunizing mAb to Dsgs by ELISA as well as pathogenicity against cultured human keratinocytes. Antisera also inhibited other mAbs using the same variable region heavy chain (V(H)) genes, despite different light chains or somatic mutations. Additionally, peptide phage display identified peptide sequences that bound PV mAbs in a V(H)-specific manner. To evaluate the therapeutic potential of V(H) gene-targeted reagents, preimmune sera and antisera were used to adsorb pathogenic antibodies from PV sera. Pooled antisera significantly reduced pathogenic activity from the original PV patient's serum and bound pathogenic antibodies from two other PV sera, suggesting shared autoantibody V(H) gene usage among PV patients. Together, these data suggest novel V(H) gene-targeted approaches toward PV treatment.

Beyond steric hindrance: the role of adhesion signaling pathways in the pathogenesis of pemphigus

Epidermal cell adhesion depends on the intercellular interactions of transmembrane cadherin glycoproteins, which form the basis of adherens junctions and desmosomes. Pemphigus is a blistering disease of the skin and mucous membranes characterized by autoantibodies against the cell surface desmosomal cadherins, desmoglein (Dsg) 3 and Dsg1. An unanswered question in pemphigus pathophysiology is the mechanism of acantholysis, or loss of keratinocyte cell adhesion. One longstanding theory for pemphigus pathogenesis is the concept of steric hindrance, in which pathogenic pemphigus autoantibodies cause loss of intercellular adhesion by directly interfering with desmosomal cadherin trans-interactions. However, several recent studies have demonstrated that modulation of p38MAPK, Rho family GTPase, c-myc, protein kinase C, and phospholipase C signaling pathways prevents keratinocyte dissociation induced by pemphigus autoantibodies. As it is unlikely that desmosomal signaling would occur only in response to pemphigus autoantibodies, these studies suggest that numerous different signaling molecules may play a role in desmosomal homeostasis. Many of these same signaling pathways regulate classical cadherins in adherens junctions. Given the recent discovery of bidirectional crosstalk between adherens junctions and desmosomes, it would be valuable to understand how signaling pathways implicated in pemphigus pathogenesis may be involved in more general mechanisms of desmosome and adherens junction regulation. In this review, we will summarize the evidence supporting a role for steric hindrance and signaling mechanisms in the pathogenesis of pemphigus acantholysis and discuss potential analogues in the classical cadherin literature.

The relevance of the IgG subclass of autoantibodies for blister induction in autoimmune bullous skin diseases

Autoimmune bullous skin diseases are characterized by autoantibodies and T cells specific to structural proteins maintaining cell-cell and cell-matrix adhesion in the skin. Existing clinical and experimental evidence generally supports a pathogenic role of autoantibodies for blister formation. These autoantibodies belong to several IgG subclasses, which associate with different functional properties and may thus determine the pathogenic potential of IgG antibodies. In pemphigus diseases, binding of IgG to keratinocytes is sufficient to cause intraepidermal blisters without engaging innate immune effectors and IgG4 autoantibodies seem to mainly mediate acantholysis. In contrast, in most subepidermal autoimmune blistering diseases, complement activation and recruitment and activation of leukocytes by autoantibodies are required for blister induction. In these conditions, tissue damage is thought to be mainly mediated by IgG1, but not IgG4 autoantibodies. This review summarizes the current knowledge on the pathogenic relevance of the IgG subclass of autoantibodies for blister formation. Characterization of the pathogenically relevant subclass(es) of autoantibodies not only provides mechanistic insights, but should greatly facilitate the development of improved therapeutic modalities of autoimmune blistering diseases.

Desmoglein autoimmunity in the pathogenesis of pemphigus

The most characteristic feature of pemphigus is a loss of cohesion between keratinocytes, resulting in formation of blisters and erosions on the mucosal membranes and the skin. Identification of circulating antibodies which bind to desmogleins (Dsg), transmembrane proteins involved in assembly of the desmosomes, led to the immediate realization that these antibodies may be pathogenic by interfering with desmosomal function. Despite extensive experimental evidence documenting the presence of the anti-Dsg response, its pathogenic relevance is still debated. At the current stage of the knowledge it seems likely that anti-Dsg imunoglobulins may play a role in pemphigus via interference with cellular Dsg trafficking and by activation of specific signalling pathways rather than by simple interference with desmosomal adhesion.

Searching for experimental models of Pemphigus vulgaris

The current knowledge on Pemphigus vulgaris (PV) pathophysiology suggests that blister formation relies on both PV IgG and non-IgG serum factors activity. PV autoimmunity seems to develop against both desmoglein 1/3 and acetylcholine receptors leading to transduction of signals to the cell mediated by phosphorilation events. Serum factors other than IgG also participate to PV acantholysis through apoptotic or cytokine-mediated mechanisms. Apart from the role played by each actor within the acantholysis, however, the current scenario arises important methodological issues. For example, the use of PV IgG or monoclonal anti-Dsg3 antibodies to experimentally reproduce the disease appears inadequate, as it does not take into account the role of non-IgG factors. On the basis of the above observations and those from our laboratories, here we propose that using whole sera from PV patients with active disease represents the most faithful manner to mimic the disease.

Changes in desmoglein 1 expression and subcellular localization in cultured keratinocytes subjected to anti-desmoglein 1 pemphigus autoimmunity

The complexity of pemphigus acantholysis together with the weak expression of desmoglein 1 (Dsg1) in cultured keratinocytes have made the study on the pathogenic action of anti-Dsg1 antibodies quite difficult. The pathophysiology of the acantholytic phenomenon could depend on the reduction of Dsg1 adhesion function occurring after its massive internalization or decrease of its synthesis. Here, we have investigated this hypothesis by using sera of patients having antibodies against Dsg1 or monoclonal anti-Dsg1 antibodies to simulate pemphigus autoimmunity in Dsg1-rich keratinocytes. Similar to pemphigus foliaceus (PF) and vulgaris (PV) sera, monoclonal anti-Dsg1 antibodies induced transient internalization of Dsg1 and reduced the adhesion strength among keratinocytes. However, binding of IgG to Dsg1 did not determine its early depletion from the adhesion complexes but reduced the amount of Dsg1 found in the Triton X-100 soluble pool of proteins. Taken together, our results represent the first demonstration that anti-Dsg1 antibodies induce similar alterations on the subcellular distribution of Dsg1 irrespective of the disease where they come from. Furthermore, the present study provides insight into the mechanisms underlying epithelial blistering observed in the skin type of pemphigus.

Treatment of pemphigus vulgaris with rituximab and intravenous immune globulin

BACKGROUND

Pemphigus vulgaris is a potentially fatal autoimmune mucocutaneous blistering disease. Conventional therapy consists of high-dose corticosteroids, immunosuppressive agents, and intravenous immune globulin.

METHODS

We studied patients with refractory pemphigus vulgaris involving 30% or more of their body-surface area, three or more mucosal sites, or both who had inadequate responses to conventional therapy and intravenous immune globulin. We treated the patients with two cycles of rituximab (375 mg per square meter of body-surface area) once weekly for 3 weeks and intravenous immune globulin (2 g per kilogram of body weight) in the fourth week. This induction therapy was followed by a monthly infusion of rituximab and intravenous immune globulin for 4 consecutive months. Titers of serum antibodies against keratinocytes and numbers of peripheral-blood B cells were monitored.

RESULTS

Of 11 patients, 9 had rapid resolution of lesions and a clinical remission lasting 22 to 37 months (mean, 31.1). All immunosuppressive therapy, including prednisone, could be discontinued before ending rituximab treatment in all patients. Two patients were treated with rituximab only during recurrences and had sustained remissions. Titers of IgG4 antikeratinocyte antibodies correlated with disease activity. Peripheral-blood B cells became undetectable shortly after initiating rituximab therapy but subsequently returned to normal values. Side effects that have been associated with rituximab were not observed, nor were infections.

CONCLUSIONS

The combination of rituximab and intravenous immune globulin is effective in patients with refractory pemphigus vulgaris.