Bullous pemphigoid IgG induces BP180 internalization via a macropinocytic pathway

Targeting antibody-mediated complement-independent mechanism in bullous pemphigoid with diacerein

BACKGROUND

Bullous pemphigoid (BP) is an antibody-mediated blistering disease predominantly affecting the elderly. The pathogenesis involves both complement-dependent and complement-independent mechanisms. The therapeutic potential of targeting complement-independent mechanism has not yet been determined. The mainstay of treatment, corticosteroid, has many side effects, indicating the needs of better treatments.

OBJECTIVE

We tempted to establish an in vitro model of BP which resembles complement-independent mechanism and to examine the therapeutic potential of a novel anti-inflammatory agent, diacerein.

METHODS

Cultured HaCaT cells were treated with purified antibodies from BP patients, with or without diacerein to measure the cell interface presence of BP180, protein kinase C, and the production of proinflammatory cytokines. An open-label, randomized, phase 2 trial was conducted to compare topical diacerein and clobetasol ointments in patients with mild-to-moderate BP (NCT03286582).

RESULTS

The reduced presentation of BP180 at cell interface after treating with BP autoantibodies was noticed in immunofluorescence and western blotting studies. The phenomenon was restored by diacerein. Diacerein also reduced the autoantibody-induced increase of pro-inflammatory cytokines. Reciprocal changes of BP180 and protein kinase C at the cell interface were found after treating with BP autoantibodies. This phenomenon was also reversed by diacerein in a dose-dependent manner. The phase 2 trial showed that topical diacerein reduced the clinical symptoms which were comparable to those of topical clobetasol.

CONCLUSION

Diacerein inhibited BP autoantibody-induced reduction of BP180 and production of proinflammatory cytokines in vitro and showed therapeutic potential in patients with BP. It is a novel drug worthy of further investigations.

From Molecular Insights to Clinical Perspectives in Drug-Associated Bullous Pemphigoid

Bullous pemphigoid (BP), the most common autoimmune blistering disease, is characterized by the presence of autoantibodies targeting BP180 and BP230 in the basement membrane zone. This leads to the activation of complement-dependent and independent pathways, resulting in proteolytic cleavage at the dermoepidermal junction and an eosinophilic inflammatory response. While numerous drugs have been associated with BP in the literature, causality and pathogenic mechanisms remain elusive in most cases. Dipeptidyl peptidase 4 inhibitors (DPP4i), in particular, are the most frequently reported drugs related to BP and, therefore, have been extensively investigated. They can potentially trigger BP through the impaired proteolytic degradation of BP180, combined with immune dysregulation. DPP4i-associated BP can be categorized into true drug-induced BP and drug-triggered BP, with the latter resembling classic BP. Antineoplastic immunotherapy is increasingly associated with BP, with both B and T cells involved. Other drugs, including biologics, diuretics and cardiovascular and neuropsychiatric agents, present weaker evidence and poorly understood pathogenic mechanisms. Further research is needed due to the growing incidence of BP and the increasing identification of new potential triggers.

Extracellular matrix in skin diseases: The road to new therapies

BACKGROUND

The extracellular matrix (ECM) is a vital structure with a dynamic and complex organization that plays an essential role in tissue homeostasis. In the skin, the ECM is arranged into two types of compartments: interstitial dermal matrix and basement membrane (BM). All evidence in the literature supports the notion that direct dysregulation of the composition, abundance or structure of one of these types of ECM, or indirect modifications in proteins that interact with them is linked to a wide range of human skin pathologies, including hereditary, autoimmune, and neoplastic diseases. Even though the ECM's key role in these pathologies has been widely documented, its potential as a therapeutic target has been overlooked.

AIM OF REVIEW

This review discusses the molecular mechanisms involved in three groups of skin ECM-related diseases - genetic, autoimmune, and neoplastic - and the recent therapeutic progress and opportunities targeting ECM.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This article describes the implications of alterations in ECM components and in BM-associated molecules that are determinant for guaranteeing its function in different skin disorders. Also, ongoing clinical trials on ECM-targeted therapies are discussed together with future opportunities that may open new avenues for treating ECM-associated skin diseases.

Targeting type 2 inflammation in bullous pemphigoid: current and emerging therapeutic approaches

Bullous pemphigoid (BP) is one of the most common autoimmune bullous diseases and mainly affects an elderly population with multi-morbidity. Due to the frailty of many BP patients, existing treatment options are limited. The blisters associated with BP result from IgG and IgE autoantibodies binding to the central components of hemidesmosome, BP180, and BP230, stimulating a destructive inflammatory process. The known characteristic features of BP, such as intense pruritus, urticarial prodrome, peripheral eosinophilia, elevated IgE, as well as recent expanding evidence from in vitro and in vivo studies implicate type 2 inflammation as an important driver of BP pathogenesis. Type 2 inflammation is an inflammatory pathway involving a subset of CD4+ T cells that secrete IL-4, IL-5, and IL-13, IgE-secreting B cells, and granulocytes, such as eosinophils, mast cells, and basophils. It is believed that effectors in type 2 inflammation may serve as novel and effective treatment targets for BP. This review focuses on recent understandings of BP pathogenesis with a particular emphasis on the role of type 2 inflammation. We summarize current clinical evidence of using rituximab (B-cell depletion), omalizumab (anti-IgE antibody), and dupilumab (anti-IL-4/13 antibody) in the treatment of BP. The latest advances in emerging targeted therapeutic approaches for BP treatment are also discussed.

Adaptive and innate immune pathogenesis of bullous pemphigoid: A review

Bullous pemphigoid (BP) is an autoimmune blistering disease that primarily affects elderly individuals. The presentation of BP is heterogeneous, typically manifesting as microscopic subepidermal separation with a mixed inflammatory infiltrate. The mechanism of pemphigoid development is unclear. B cells play a major role in pathogenic autoantibody production, and T cells, type II inflammatory cytokines, eosinophils, mast cells, neutrophils, and keratinocytes are also implicated in the pathogenesis of BP. Here, we review the roles of and crosstalk between innate and adaptive immune cells in BP.

Significance of anti-transcobalamin receptor antibodies in cutaneous arteritis revealed by proteome-wide autoantibody screening

Cutaneous arteritis (CA) is a single-organ vasculitis that exclusively affects the small to medium-sized arteries of the skin. Diagnosis depends on a histological investigation with skin biopsy, which could be burdensome for both patients and clinicians. Moreover, the pathogenesis of CA remains unstudied, and treatment has not yet been established. Herein, we applied our proteome-wide autoantibody screening method to explore autoantibodies in the serum of CA patients. As a result, anti-transcobalamin receptor (TCblR) antibodies (Abs) were specifically detected in 24% of CA patients. Patients with positive anti-TCblR Abs were spared from peripheral neuropathy compared to those with negative anti-TCblR Abs, showing characteristics as CA confined to the skin. In addition, we revealed that anti-TCblR Abs trigger the autocrine loop of interleukin-6 mediated by tripartite motif-containing protein 21 in human endothelial cells and induce periarterial inflammation in murine skin. Furthermore, we demonstrated that methylcobalamin, a ligand of TCblR, ameliorates inflammation caused by anti-TCblR Abs both in vitro and in vivo. Collectively, our investigation unveils the pathologic significance of anti-TCblR Abs in CA and their potential as a diagnostic marker and a pathophysiology-oriented therapeutic target.

Pruritogens in pemphigoid diseases: Possible therapeutic targets for a burdensome symptom

Pruritus is a hallmark feature in pemphigoid diseases, where it can be severe and greatly impact the quality of life of affected patients. Despite being a key symptom, the exact pathophysiological mechanisms involved in pruritus in pemphigoid are yet to be fully elucidated and effective therapies addressing them are limited. This review summarizes the present understanding of pruritus specific to pemphigoid diseases, especially the pruritogens that induce it, and the therapeutic options that have been explored so far. The majority of the available evidence is on bullous pemphigoid and epidermolysis bullosa acquisita. Histamine derived from basophils correlates with pruritus severity, with omalizumab demonstrating promising efficacy in pruritus for bullous pemphigoid. IL-4/-13 contribute to itch in bullous pemphigoid with dupilumab being evaluated in clinical trials. Other pruritogens of interest include substance P, tryptase, and thymic stromal lymphopoetin, with therapies targeting them requiring further investigation. Scratching behaviors contribute directly to blister formation through various mechanisms, such as pathological autoantibody recruitment, T helper cell type 1 polarization, and exposure of intracellular autoantigens. Treatments addressing these pathways may contribute to decreasing disease severity. Additional studies are needed to fully characterize how pruritus is regulated in pemphigoid diseases, to help pave the way to develop novel and effective therapeutics that will not only address pruritic symptoms but also decrease disease severity.

Successful pharmacological intervention at different levels of the complement system in an in vitro complement fixation model for bullous pemphigoid

Bullous pemphigoid (BP) is characterized by deposition of immunoglobulins and complement along the epidermal basement membrane (BM). In humans, there is a lack of functional studies targeting the complement system (CS). This study investigates activation of all complement pathways in BP skin biopsies. Moreover, pharmacological inhibition at different levels of the CS was investigated using anti-complement compounds in a complement fixation BP assay. In this retrospective study, 21 frozen biopsies from BP patients were stained by direct immunofluorescence for C1q, MBL, ficolin-2, C4d, properdin, C3c and C5b-9. Sera from 10 patients were analysed in a complement fixation assay in the presence of C1 inhibitor, anti-factor B monoclonal antibody (mAb), anti-C3 mAb and anti-C5 mAb and compared with dexamethasone. The two readouts were the quantity of complement deposited along the BM and the release of sC5b-9 in the supernatant. Our results show classical and alternative complement pathway activation in BP skin biopsies, but could not demonstrate significant lectin pathway activation. In contrast to dexamethasone, complement deposition along the BM could be selectively inhibited by anti-C1 and anti- factor B. More downstream, selective intervention at the level of C3 and C5 could effectively reduce complement deposition along the BM and the release of sC5b-9 in the supernatant. This study shows that selective intervention in either the classical, alternative or terminal pathway prevented deposition of complement along the BM in an in vitro BP model. The results of our study greatly encourage the clinical development of complement inhibitors for the treatment of BP.

Interleukin-26–DNA complexes promote inflammation and dermal-epidermal separation in a modified human cryosection model of bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune disease characterized by autoantibody-mediated activation of immune cells and subepidermal blister formation. Excess amounts of extracellular DNA are produced in BP, however, it remains unclear how extracellular DNA contributes to BP pathogenesis. Here we show a possible mechanism by which interleukin (IL)-26 binds to extracellular DNA released from neutrophils and eosinophils to support DNA sensing. Patients with BP exhibited high circulating levels of IL-26, forming IL-26–DNA complexes in the upper dermis and inside the blisters. IL-26–DNA complexes played a dual role in regulating local immunity and blister formation. First, they enhanced the production of inflammatory cytokines in monocytes and neutrophils. Second, and importantly, the complexes augmented the production and activity of proteases from co-cultured monocytes and neutrophils, which induced BP180 cleavage in keratinocytes and dermal-epidermal separation in a modified human cryosection model. Collectively, we propose a model in which IL-26 and extracellular DNA synergistically act on immune cells to enhance autoantibody-driven local immune responses and protease-mediated fragility of dermal-epidermal junction in BP.

The relevance of complement in pemphigoid diseases: A critical appraisal

Pemphigoid diseases are autoimmune chronic inflammatory skin diseases, which are characterized by blistering of the skin and/or mucous membranes, and circulating and tissue-bound autoantibodies. The well-established pathomechanisms comprise autoantibodies targeting various structural proteins located at the dermal-epidermal junction, leading to complement factor binding and activation. Several effector cells are thus attracted and activated, which in turn inflict characteristic tissue damage and subepidermal blistering. Moreover, the detection of linear complement deposits in the skin is a diagnostic hallmark of all pemphigoid diseases. However, recent studies showed that blistering might also occur independently of complement. This review reassesses the importance of complement in pemphigoid diseases based on current research by contrasting and contextualizing data from in vitro, murine and human studies.

Type XVII collagen: Relevance of distinct epitopes, complement-independent effects, and association with neurological disorders in pemphigoid disorders

Pemphigoid diseases (PD) are autoimmune skin blistering diseases characterized by autoantibodies directed against proteins of the cutaneous basement membrane zone (BMZ). One of the major antigens is type XVII collagen (BP180), a transmembrane glycoprotein, which is targeted in four PDs: bullous pemphigoid, mucous membrane pemphigoid, linear IgA dermatosis, and pemphigoid gestationis. To date, different epitopes on BP180 have been described to be recognized by PD disease patients’ autoantibodies. Different BP180 epitopes were associated with distinct clinical phenotypes while the underlying mechanisms are not yet fully understood. So far, the main effects of anti-BP180 reactivity are mediated by Fcγ-receptors on immune cells. More precisely, the autoantibody–antigen interaction leads to activation of complement at the BMZ and infiltration of immune cells into the upper dermis and, by the release of specific enzymes and reactive oxygen species, to the degradation of BP180 and other BMZ components, finally manifesting as blisters and erosions. On the other hand, inflammatory responses independent of Fcγ-receptors have also been reported, including the release of proinflammatory cytokines and internalization and depletion of BP180. Autoantibodies against BP180 can also be found in patients with neurological diseases. The assumption that the clinical expression of PD depends on epitope specificity in addition to target antigens, autoantibody isotypes, and antibody glycosylation is supported by the observation that epitopes of PD patients differ from those of PD patients. The aim of the present review is to describe the fine specificities of anti-BP180 autoantibodies in different PDs and highlight the associated clinical differences. Furthermore, the direct effects after binding of the autoantibodies to their target are summarized.

Insights Into the Pathogenesis of Bullous Pemphigoid: The Role of Complement-Independent Mechanisms

Bullous pemphigoid is an autoimmune blistering disease caused by autoantibodies targeting BP180 and BP230. While deposits of IgG and/or complement along the epidermal basement membrane are typically seen suggesting complement -mediated pathogenesis, several recent lines of evidence point towards complement-independent pathways contributing to tissue damage and subepidermal blister formation. Notable pathways include macropinocytosis of IgG-BP180 complexes resulting in depletion of cellular BP180, direct induction of pro-inflammatory cytokines from keratinocytes, as well as IgE autoantibody- and eosinophil-mediated effects. We review these mechanisms which open new perspectives on novel targeted treatment modalities.

Subunit-Specific Reactivity of Autoantibodies Against Laminin-332 Reveals Direct Inflammatory Mechanisms on Keratinocytes

Laminin-332 pemphigoid is a rare and severe autoimmune blistering disease, caused by IgG autoantibodies targeting laminin-332 in the dermal-epidermal basement zone. Laminin-332 pemphigoid is characterized by variable inflammatory infiltrate and the predominance of non-complement-fixing antibodies. Given these findings, we hypothesized that IgG autoantibodies to laminin-332 directly resulted in keratinocyte expression of inflammatory factors. We performed RNA-seq on primary human keratinocytes treated with IgG from patients with laminin-332 pemphigoid. Genes for numerous cytokines and chemokines were upregulated, including CSF2, CSF3, CXCL1, CXCL5, CXCL3, CXCL8, CXCL10, CXCL1, IL6, IL7, IL15, IL23, IL32, IL37, TGFB2 as well as metalloproteases. Considering the pro-inflammatory and proteolytic effect of autoantibodies from patients with laminin-332 pemphigoid identified in our initial experiment, we next questioned whether the reactivity against specific laminin subunits dictates the inflammatory and proteolytic keratinocyte response. Then, we treated keratinocytes with IgG from a separate cohort of patients with reactivity against individual subunits of laminin-332. We identified upregulation of IL-1α, IL-6, IL-8, CXCL1, MMP9, TSLP, and GM-CSF at the protein level, most notably in keratinocytes treated with IgG from laminin β3-reactive patients. We for the first time demonstrated a pro-inflammatory response, similar to that described in keratinocytes treated with IgG autoantibodies from patients with bullous pemphigoid, providing novel insight into the pathogenesis of laminin-332 pemphigoid and laminin-332 biology.

BP180/Collagen XVII: A Molecular View

BP180 is a type II collagenous transmembrane protein and is best known as the major autoantigen in the blistering skin disease bullous pemphigoid (BP). The BP180 trimer is a central component in type I hemidesmosomes (HD), which cause the adhesion between epidermal keratinocytes and the basal lamina, but BP180 is also expressed in several non-HD locations, where its functions are poorly characterized. The immunological roles of intact and proteolytically processed BP180, relevant in BP, have been subject to intensive research, but novel functions in cell proliferation, differentiation, and aging have also recently been described. To better understand the multiple physiological functions of BP180, the focus should return to the protein itself. Here, we comprehensively review the properties of the BP180 molecule, present new data on the biochemical features of its intracellular domain, and discuss their significance with regard to BP180 folding and protein-protein interactions.

Granzyme B inhibition reduces disease severity in autoimmune blistering diseases

Pemphigoid diseases refer to a group of severe autoimmune skin blistering diseases characterized by subepidermal blistering and loss of dermal-epidermal adhesion induced by autoantibody and immune cell infiltrate at the dermal-epidermal junction and upper dermis. Here, we explore the role of the immune cell-secreted serine protease, granzyme B, in pemphigoid disease pathogenesis using three independent murine models. In all models, granzyme B knockout or topical pharmacological inhibition significantly reduces total blistering area compared to controls. In vivo and in vitro studies show that granzyme B contributes to blistering by degrading key anchoring proteins in the dermal-epidermal junction that are necessary for dermal-epidermal adhesion. Further, granzyme B mediates IL-8/macrophage inflammatory protein-2 secretion, lesional neutrophil infiltration, and lesional neutrophil elastase activity. Clinically, granzyme B is elevated and abundant in human pemphigoid disease blister fluids and lesional skin. Collectively, granzyme B is a potential therapeutic target in pemphigoid diseases.

Presence of Cutaneous Complement Deposition Distinguishes between Immunological and Histological Features of Bullous Pemphigoid-Insights from a Retrospective Cohort Study

The practical implications of complement deposition in direct immunofluorescence (DIF) microscopy and its influence on the disease phenotype are poorly understood. We aimed to investigate whether the presence of complement deposition in DIF microscopy gives rise to differences in the morphological, immunological, and histological characteristics of patients with BP (bullous pemphigoid). We performed a retrospective study encompassing patients with BP in a specialized tertiary referral center. Logistic regression model was utilized to identify variables independently associated with complement deposition. The study included 233 patients with BP, of whom 196 (84.1%) demonstrated linear C3 deposition along the dermal-epidermal junction (DEJ) in DIF analysis. BP patients with C3 deposition had higher mean (SD) levels (645.2 (1418.5) vs. 172.5 (243.9) U/mL; p < 0.001) and seropositivity rate (86.3% vs.64.9%; p = 0.002) of anti-BP180 NC16A and less prevalent neutrophilic infiltrate in lesional skin specimens (29.8% vs. 52.4%; p = 0.041). C3 deposition was found positively associated with the detection of anti-BP180 NC16A autoantibodies (OR, 4.25; 95% CI, 1.38–13.05) and inversely associated with the presence of neutrophils in lesional skin (OR, 3.03; 95% CI, 1.09–8.33). To conclude, complement deposition influences the immunological and histological features of BP. These findings are in line with experimental data describing the pathogenic role of complement in BP.

Update on IgG4-mediated autoimmune diseases: New insights and new family members

Antibodies of IgG4 subclass are exceptional players of the immune system, as they are considered to be immunologically inert and functionally monovalent, and as such may be part of classical tolerance mechanisms. IgG4 antibodies are found in a range of different diseases, including IgG4-related diseases, allergy, cancer, rheumatoid arthritis, helminth infection and IgG4 autoimmune diseases, where they may be pathogenic or protective. IgG4 autoimmune diseases are an emerging new group of diseases that are characterized by pathogenic, antigen-specific autoantibodies of IgG4 subclass, such as MuSK myasthenia gravis, pemphigus vulgaris and thrombotic thrombocytopenic purpura. The list of IgG4 autoantigens is rapidly growing and to date contains 29 candidate antigens. Interestingly, IgG4 autoimmune diseases are restricted to four distinct organs: 1) the central and peripheral nervous system, 2) the kidney, 3) the skin and mucous membranes and 4) the vascular system and soluble antigens in the blood circulation. The pathogenicity of IgG4 can be validated using our classification system, and is usually excerted by functional blocking of protein-protein interaction.

Dermal Fibroblasts Internalize Phosphatidylserine-Exposed Secretory Melanosome Clusters and Apoptotic Melanocytes

Pigmentation in the dermis is known to be caused by melanophages, defined as melanosome-laden macrophages. In this study, we show that dermal fibroblasts also have an ability to uptake melanosomes and apoptotic melanocytes. We have previously demonstrated that normal human melanocytes constantly secrete melanosome clusters from various sites of their dendrites. After adding secreted melanosome clusters collected from the culture medium of melanocytes, time-lapse imaging showed that fibroblasts actively attached to the secreted melanosome clusters and incorporated them. Annexin V staining revealed that phosphatidylserine (PtdSer), which is known as an 'eat-me' signal that triggers the internalization of apoptotic cells by macrophages, is exposed on the surface of secreted melanosome clusters. Dermal fibroblasts were able to uptake secreted melanosome clusters as did macrophages, and those fibroblasts express TIM4, a receptor for PtdSer-mediated endocytosis. Further, co-cultures of fibroblasts and melanocytes demonstrated that dermal fibroblasts internalize PtdSer-exposed apoptotic melanocytes. These results suggest that not only macrophages, but also dermal fibroblasts contribute to the collection of potentially toxic substances in the dermis, such as secreted melanosome clusters and apoptotic melanocytes, that have been occasionally observed to drop down into the dermis from the epidermis.

Intracellular accumulation of PD-1 molecules in circulating T lymphocytes in advanced malignant melanoma: an implication for immune evasion mechanism

BACKGROUND

The blockade of cell surface PD-1 (_(sur)PD-1) by monoclonal antibodies, represented by nivolumab, provides the strategy to treat advanced malignant melanoma (AMM). The intracellular presence of PD-1 molecules have been reported in some T cell subsets, however, their kinetic association with those expressed on the cell surface, let alone their significance in antitumor immunity has been ill-investigated.

METHODS

Intracellular PD-1 expression status in T cell subsets in AMM cases during nivolumab administration was chronologically characterized. The kinetics of PD-1 molecules within AMM-derived T cells was assessed in vitro in conjunction with their functional properties.

RESULTS

Increase in _(sur)PD-1 and intracellular PD-1 (_(int)PD-1⁺) expression was characteristic for AMM T cells. After short-term culture, virtually _(sur)PD-1^- nivolumab-treated AMM T cells restore _(sur)PD-1 expression, which could not be explained by the detachment of nivolumab from PD-1 epitopes alone. The blockade of trans-Golgi network resulted in the decrease in the extent of _(sur)PD-1 recovery, suggesting the translocation of accumulated _(int)PD-1 to the cell surface. Antigen-specific PD-1⁺ T cells significantly increased in _(int)PD-1⁺ cells after treatment. In addition, a surge in _(int)PD-1⁺CD4⁺ T cells was observed prior to the emergence of skin rash as an immune-related adverse event (irAE).

CONCLUSIONS

Accumulated _(int)PD-1 in T cells may contribute to enhanced immune evasion in AMM. Evaluation of intracellular PD-1 expression would be useful for better management of nivolumab-treated AMM patients in view of predicting treatment response and the incidence of irAE. Our findings further support the necessity of periodical administration of nivolumab for treating AMM.

Adenovirus-Mediated LAMA3 Transduction Enhances Hemidesmosome Formation and Periodontal Reattachment during Wound Healing

A robust dento-epithelial junction prevents external pathogenic factors from entering connective tissue and could be crucial for periodontal reattachment after periodontal surgery. The junctional epithelium (JE) is attached to the tooth surface through the hemidesmosome (HD) and internal basal lamina, where the primary component is laminin-332. Destruction of the JE leads to the loss of periodontal attachment. Traditional treatments are effective in eliminating local inflammation of the gingiva; however, few directly promote periodontal reattachment and HD formation. Here, we designed a gene-therapy strategy using the adenovirus-mediated human laminin-332 α3 chain (LAMA3) gene (Ad-LAMA3) transduced into a human-immortalized epidermal cell line (HaCaT) to study the formation of HD in vitro. Ad-LAMA3 promoted early adhesion and fast migration of HaCaT cells and increased expression of LAMA3 and type XVII collagen (BP180) significantly. Furthermore, HaCaT cells could facilitate formation of mature HDs after LAMA3 overexpression. In vivo experiments demonstrated that the JE transduced with Ad-LAMA3 could increase expression of LAMA3 and BP180 and “biological sealing” between the tooth and gingival epithelium. These results suggested that adenovirus-mediated LAMA3 transduction is a novel therapeutic strategy that promotes the stability and integration of the JE around the tooth during wound healing.

Resolution in bullous pemphigoid

Pemphigoid diseases are a group of autoimmune blistering skin diseases defined by an immune response against certain components of the dermal-epidermal adhesion complex. They are prototypical, autoantibody-driven, organ-specific diseases with the emergence of inflammatory skin lesions dependent on the recruitment of immune cells, particularly granulocytes, into the skin. During an acute flare of disease, inflammatory skin lesions typically progressing from erythema through urticarial plaques to subepidermal blisters erosions erupt and, finally, completely resolve, thus illustrating that resolution of inflammation is continuously executed in pemphigoid disease patients and can be directly monitored on the skin. Despite these superb conditions for examining resolution in pemphigoid diseases as paradigm diseases for antibody-induced tissue inflammation, the mechanisms of resolution in pemphigoid are underinvestigated and still largely elusive. In the last decade, mouse models for pemphigoid diseases were developed, which have been instrumental to identify several key pathways for the initiation of inflammation in these diseases. More recently, also protective pathways, specifically IL-10 and C5aR2 signalling on the molecular level and T_regs on the cellular level, counteracting skin inflammation have been highlighted and may contribute to the continuous execution of resolution in pemphigoid diseases. The upstream orchestrators of this process are currently under investigation. Pemphigoid disease patients, particularly bullous pemphigoid patients, who are predominantly above 75 years of age, often succumb to the side effects of the immunosuppressive therapeutics nowadays still required to suppress the disease. Pemphigoid disease patients may therefore represent a group of patients benefiting most substantially from the introduction of non-immunosuppressive, proresolving therapeutics into the treatment regimens for their disease.

The Intersection of IgE Autoantibodies and Eosinophilia in the Pathogenesis of Bullous Pemphigoid

Bullous pemphigoid (BP) is an autoimmune blistering disease characterized by autoantibodies targeting cellular adhesion molecules. While IgE autoantibodies are occasionally reported in other autoimmune blistering diseases, BP is unique in that most BP patients develop an IgE autoantibody response. It is not known why BP patients develop self-reactive IgE and the precise role of IgE in BP pathogenesis is not fully understood. However, clinical evidence suggests an association between elevated IgE antibodies and eosinophilia in BP patients. Since eosinophils are multipotent effector cells, capable cytotoxicity and immune modulation, the putative interaction between IgE and eosinophils is a primary focus in current studies aimed at understanding the key components of disease pathogenesis. In this review, we provide an overview of BP pathogenesis, highlighting clinical and experimental evidence supporting central roles for IgE and eosinophils as independent mediators of disease and via their interaction. Additionally, therapeutics targeting IgE, the Th2 axis, or eosinophils are also discussed.

Tyrosine Kinases in Autoimmune and Inflammatory Skin Diseases

Tyrosine kinases relay signals from diverse leukocyte antigen receptors, innate immune receptors, and cytokine receptors, and therefore mediate the recruitment and activation of various leukocyte populations. Non-receptor tyrosine kinases of the Jak, Src, Syk, and Btk families play major roles in various immune-mediated disorders, and small-molecule tyrosine kinase inhibitors are emerging novel therapeutics in a number of those diseases. Autoimmune and inflammatory skin diseases represent a broad spectrum of immune-mediated diseases. Genetic and pharmacological studies in humans and mice support the role of tyrosine kinases in several inflammatory skin diseases. Atopic dermatitis and psoriasis are characterized by an inflammatory microenvironment which activates cytokine receptors coupled to the Jak-Stat signaling pathway. Jak kinases are also implicated in alopecia areata and vitiligo, skin disorders mediated by cytotoxic T lymphocytes. Genetic studies indicate a critical role for Src-family kinases and Syk in animal models of autoantibody-mediated blistering skin diseases. Here, we review the various tyrosine kinase signaling pathways and their role in various autoimmune and inflammatory skin diseases. Special emphasis will be placed on identification of potential therapeutic targets, as well as on ongoing preclinical and clinical studies for the treatment of inflammatory skin diseases by small-molecule tyrosine kinase inhibitors.

New Insights Into the Pathogenesis of Bullous Pemphigoid: 2019 Update

There are several lines of evidence indicating that the physiopathological bases of bullous pemphigoid (BP), the most common subepidermal autoimmune bullous disease, are hallmarked by the production of autoantibodies directed against the hemidesmosomal anchoring proteins BP180 and BP230. In contrast to the robustness of the latter assumption, the multifaceted complexity of upstream and downstream mechanisms implied in the pathogenesis of BP remains an area of intense speculation. So far, an imbalance between T regulatory cells and autoreactive T helper (Th) cells has been regarded as the main pathogenic factor triggering the autoimmune response in BP patients. However, the contributory role of signaling pathways fostering the B cell stimulation, such as Toll-like receptor activation, as well as that of ancillary inflammatory mechanisms responsible for blister formation, such as Th17 axis stimulation and the activation of the coagulation cascade, are still a matter of debate. In the same way, the pathomechanisms implied in the loss of dermal-epidermal adhesion secondary to autoantibodies binding are not fully understood. Herein, we review in detail the current concepts and controversies on the complex pathogenesis of BP, shedding light on the most recent theories emerging from the literature.

Complement Activation in Autoimmune Bullous Dermatoses: A Comprehensive Review

Autoimmune bullous dermatoses (AIBD) are characterized by circulating autoantibodies that are either directed against epidermal antigens or deposited as immune complexes in the basement membrane zone (BMZ). The complement system (CS) can be activated by autoantibodies, thereby triggering activation of specific complement pathways. Local complement activation induces a pathogenic inflammatory response that eventually results in the formation of a sub- or intraepidermal blister. Deposition of complement components is routinely used as a diagnostic marker for AIBD. Knowledge from different animal models mimicking AIBD and deposition of complement components in human skin biopsies provides more insight into the role of complement in the pathogenesis of the different AIBD. This review outlines the role of the CS in several AIBD including bullous pemphigoid, epidermolysis bullosa acquisita, mucous membrane pemphigoid (MMP), pemphigus, linear IgA-disease, and dermatitis herpetiformis. We also discuss potential therapeutic approaches targeting key complement components, pathways and pathogenic complement-mediated events.

Bullous Pemphigoid IgG Induces Cell Dysfunction and Enhances the Motility of Epidermal Keratinocytes via Rac1/Proteasome Activation

Bullous pemphigoid (BP) is an autoimmune disease characterized by the formation of blisters, in which autoantibodies mainly target type XVII collagen (ColXVII) expressed in basal keratinocytes. BP IgG is known to induce the internalization of ColXVII from the plasma membrane of keratinocytes through macropinocytosis. However, the cellular dynamics following ColXVII internalization have not been completely elucidated. BP IgG exerts a precise effect on cultured keratinocytes, and the morphological/functional changes in BP IgG-stimulated cells lead to the subepidermal blistering associated with BP pathogenesis. Based on the electron microscopy examination, BP IgG-stimulated cells exhibit alterations in the cell membrane structure and the accumulation of intracellular vesicles. These morphological changes in the BP IgG-stimulated cells are accompanied by dysfunctional mitochondria, increased production of reactive oxygen species, increased motility, and detachment. BP IgG triggers the cascade leading to metabolic impairments and stimulates cell migration in the treated keratinocytes. These cellular alterations are reversed by pharmacological inhibitors of Rac1 or the proteasome pathway, suggesting that Rac1 and proteasome activation are involved in the effects of BP IgG on cultured keratinocytes. Our study highlights the role of keratinocyte kinetics in the direct functions of IgG in patients with BP.

BP180 Is Critical in the Autoimmunity of Bullous Pemphigoid

Bullous pemphigoid (BP) is by far the most common autoimmune blistering dermatosis that mainly occurs in the elderly. The BP180 is a transmembrane glycoprotein, which is highly immunodominant in BP. The structure and location of BP180 indicate that it is a significant autoantigen and plays a key role in blister formation. Autoantibodies from BP patients react with BP180, which leads to its degradation and this has been regarded as the central event in BP pathogenesis. The consequent blister formation involves the activation of complement-dependent or -independent signals, as well as inflammatory pathways induced by BP180/anti-BP180 autoantibody interaction. As a multi-epitope molecule, BP180 can cause dermal-epidermal separation via combining each epitope with specific immunoglobulin, which also facilitates blister formation. In addition, some inflammatory factors can directly deplete BP180, thereby leading to fragility of the dermal-epidermal junction and blister formation. This review summarizes recent investigations on the role of BP180 in BP pathogenesis to determine the potential targets for the treatment of patients with BP.

Complement-independent blistering mechanisms in bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease that clinically demonstrates tense blisters with widespread erythema, histologically demonstrates subepidermal blistering and immunologically demonstrates the presence of circulating autoantibodies against hemidesmosomal molecules. Complement activation has long been regarded as necessary for the generation of the BP. However, certain evidence has recently come to support non-complemental blistering mechanisms. The story of BP blistering mechanisms is a complicated one. This review mainly focuses on a specific blistering mechanism that highlights the role of complements in BP blistering.

IgG4, complement, and the mechanisms of blister formation in pemphigus and bullous pemphigoid

Autoimmune bullous diseases are at the forefront of the research field on autoimmune diseases. Pemphigus and pemphigoid were historical entities in the world of descriptive dermatology for a long time. Recently, however, dermatologists and skin biologists have elegantly explained the novel pathomechanism of pemphigus and pemphigoid diseases. IgG4 is the major subclass of autoantibodies in autoimmune bullous diseases and is known to have little activity to activate complement. It is quite acceptable for pemphigus, because acantholysis in pemphigus has been demonstrated to be complement-independent. On the other hand, subepidermal blister formation in bullous pemphigoid has been believed to be complement-dependent. Therefore, the role of IgG4 autoantibodies on blister formation in bullous pemphigoid remains controversial. Here, we examine the progress of research on the mechanisms of blister formation in autoimmune bullous diseases. We focus on the complement-dependent and independent blistering in bullous pemphigoid using comparisons between pemphigus diseases. In addition, we review the current understanding of the role of IgG4 antibodies in bullous pemphigoid.

Macropinocytosis of type XVII collagen induced by bullous pemphigoid IgG is regulated via protein kinase C

Macropinocytosis is an endocytic pathway that is involved in the nonselective fluid uptake of extracellular fluid. Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease associated with autoantibodies to type XVII collagen (COL17), which is a component of hemidesmosome. When keratinocytes are treated with BP-IgG, COL17 internalizes into cells by way of the macropinocytosis. We investigated the mechanism of COL17 macropinocytosis using DJM-1 cells, a cutaneous squamous cell carcinoma cell line. First, non-hemidesmosomal COL17 was preferentially depleted by stimulation with the BP-IgG in the DJM-1 cells. To investigate the signaling involved in COL17-macropinocytosis, the inhibition of small GTPase family members Rac1 and Cdc42 was found to strongly repress COL17 internalization; in addition, the Rho inhibitor also partially blocked that internalization, suggesting these small GTPases are involved in signaling to mediate COL17-macropinocytosis. Western blotting using Phostag-SDS-PAGE demonstrated high levels of COL17 phosphorylation in DJM-1 cells under steady-state condition. Treatment with BP-IgG increased the intracellular calcium level within a minute, and induced the overabundant phosphorylation of COL17. The overabundant phosphorylation of COL17 was suppressed by a protein kinase C (PKC) inhibitor. In addition, PKC inhibitor repressed COL17 endocytosis using cell culture and organ culture systems. Finally, the depletion of COL17 was not observed in the HEK293 cells transfected COL17 without intracellular domain. These results suggest that COL17 internalization induced by BP-IgG may be mediated by a PKC pathway. In summary, BP-IgG initially binds to COL17 distributed on the plasma membrane, and COL17 may be internalized by means of a macropinocytic pathway related to the phosphorylation of the intracellular domain by PKC.

Anti-BP180-type mucous membrane pemphigoid immunoglobulin G shows heterogeneity of internalization of BP180/collagen XVII into keratinocyte cytoplasm

Anti-BP180-type mucous membrane pemphigoid (BP180-MMP) is a rare autoimmune subepidermal blistering disease that targets the C terminus of BP180/collagen XVII. Currently, the pathomechanism of BP180-MMP is not well understood. We reported previously that immunoglobulin G (IgG) from patients with bullous pemphigoid (BP) can induce internalization of BP180 via a macropinocytic pathway, which depletes BP180 and weakens epidermal cell-matrix integrity. The purpose of the present study was to elucidate the pathomechanism of BP180-MMP. Immunohistochemistry of biopsy specimens from two patients with BP180-MMP revealed that one patient had BP180 internalization, but the other did not. In live-cell imaging using IgG from patients with BP180-MMP on several keratinocyte cell lines, IgG from only three out of the seven patients was associated with BP180 internalization into the cytoplasm. Our results suggest that IgG from patients with BP180-MMP shows heterogeneity of internalization of BP180. This variability in BP180 internalization in patients with BP or BP180-MMP may lead to differences in clinical presentation.

Epitope-Dependent Pathogenicity of Antibodies Targeting a Major Bullous Pemphigoid Autoantigen Collagen XVII/BP180

In bullous pemphigoid, the common autoimmune blistering disorder, IgG autoantibodies target various epitopes on hemidesmosomal transmembrane collagen XVII (COL17)/BP180. Antibodies (Abs) targeting the extracellular noncollagenous 16th A domain of COL17 may be pathogenic; however, the pathogenic roles of Abs targeting non-noncollagenous 16th A regions are poorly understood. In this study using a pathogenic and a nonpathogenic monoclonal antibody (mAb) targeting the noncollagenous 16th A domain (mAb TS39-3) and the C-terminus domain (mAb C17-C1), respectively, we show that endocytosis of immune complexes after binding of Abs to cell surface COL17 is a key phenomenon that induces skin fragility. Passive transfer of IgG1 mouse mAb TS39-3 but not mAb C17-C1 induces dermal-epidermal separation in neonatal human COL17-expressing transgenic mice. Interestingly, mAb C17-C1 strongly binds with the dermal-epidermal junction of the recipient mice skin, suggesting that binding of Abs with COL17 is insufficient to induce skin fragility. In cultured normal human epidermal keratinocytes treated with these mAbs, mAb TS39-3 but not mAb C17-C1 internalizes immune complexes after binding with cell surface COL17 via macropinocytosis, resulting in reduced COL17 expression. This study shows that pathogenicity of Abs targeting COL17 is epitope dependent, which is associated with macropinocytosis-mediated endocytosis of immune complexes and finally results in the depletion of COL17 expression in basal keratinocytes.

Mechanisms of Disease: Pemphigus and Bullous Pemphigoid

Pemphigus and bullous pemphigoid are autoantibody-mediated blistering skin diseases. In pemphigus, keratinocytes in epidermis and mucous membranes lose cell-cell adhesion, and in pemphigoid, the basal keratinocytes lose adhesion to the basement membrane. Pemphigus lesions are mediated directly by the autoantibodies, whereas the autoantibodies in pemphigoid fix complement and mediate inflammation. In both diseases, the autoantigens have been cloned and characterized; pemphigus antigens are desmogleins (cell adhesion molecules in desmosomes), and pemphigoid antigens are found in hemidesmosomes (which mediate adhesion to the basement membrane). This knowledge has enabled diagnostic testing for these diseases by enzyme-linked immunosorbent assays and dissection of various pathophysiological mechanisms, including direct inhibition of cell adhesion, antibody-induced internalization of antigen, and cell signaling. Understanding these mechanisms of disease has led to rational targeted therapeutic strategies.

IgE-mediated mechanisms in bullous pemphigoid and other autoimmune bullous diseases

Autoimmune bullous diseases (AIBDs) are characterized by autoantibodies against structural proteins of the dermal-epidermal junction (in pemphigoid diseases) and the epidermal/ epithelial desmosomes (in pemphigus diseases). By far, the most common AIBD is bullous pemphigoid, which is immunopathologically characterized by autoantibodies against BP180 (type XVII collagen) and BP230. IgG and, to a lesser extent, IgA autoantibodies are the major autoantibody isotypes in these disorders. IgE autoantibodies are increasingly reported in particular in bullous pemphigoid. The development of specific and sensitive anti-BP180 IgE ELISA systems, the report of two experimental murine models employing IgE autoantibodies against BP180, and the successful treatment of bullous pemphigoid with the anti-IgE antibody omalizumab have raised interest in the role of IgE autoantibodies and the modulation of their production in AIBDs. Here, the relevance of IgE autoantibodies in the diagnosis, pathophysiology, and treatment decisions of AIBDs, with a focus on bullous pemphigoid, is reviewed.

Bullous pemphigoid autoantibodies directly induce blister formation without complement activation

Complement activation and subsequent recruitment of inflammatory cells at the dermal/epidermal junction are thought to be essential for blister formation in bullous pemphigoid (BP), an autoimmune blistering disease induced by autoantibodies against type XVII collagen (COL17); however, this theory does not fully explain the pathological features of BP. Recently, the involvement of complement-independent pathways has been proposed. To directly address the question of the necessity of the complement activation in blister formation, we generated C3-deficient COL17-humanized mice. First, we show that passive transfer of autoantibodies from BP patients induced blister formation in neonatal C3-deficient COL17-humanized mice without complement activation. By using newly generated human and murine mAbs against the pathogenic noncollagenous 16A domain of COL17 with high (human IgG1, murine IgG2), low (murine IgG1), or no (human IgG4) complement activation abilities, we demonstrate that the deposition of Abs, and not complements, is relevant to the induction of blister formation in neonatal and adult mice. Notably, passive transfer of BP autoantibodies reduced the amount of COL17 in lesional mice skin, as observed in cultured normal human keratinocytes treated with the same Abs. Moreover, the COL17 depletion was associated with a ubiquitin/proteasome pathway. In conclusion, the COL17 depletion induced by BP autoantibodies, and not complement activation, is essential for the blister formation under our experimental system.

Update on the pathogenesis of bullous pemphigoid: an autoantibody-mediated blistering disease targeting collagen XVII

Bullous pemphigoid (BP) is a common autoimmune blistering skin disorder that tends to affect the elderly. Autoantibodies (autoAbs) from BP patients react with two hemidesmosomal components: transmembrane collagen XVII (BP180 or BPAG2) and plakin family protein BP230 (BPAG1). Of these, collagen XVII (COL17) is thought to be a major autoantigen. The binding of autoAbs to COL17 following the activation of complements and inflammatory pathways eventually leads to the degradation of COL17, and this has been regarded as the main pathogenesis of BP. However, recent investigations have suggested other pathways, including a complement-independent pathway and a pathway involving IgE-autoAbs. BP-autoAbs can directly deplete COL17, leading to fragility of the dermal-epidermal junction. In addition, IgE-autoAbs to COL17 may be involved in the formation of itchy urticarial erythema associated with eosinophilic infiltration. This article summarizes the update on pathogenesis of BP, with a special focus on blister formation by autoAbs to COL17.