In vivo blockade of pemphigus vulgaris acantholysis by inhibition of intracellular signal transduction cascades

Unbiased screening identifies regulators of cell-cell adhesion and treatment options in pemphigus

Cell-cell junctions, and specifically desmosomes, are crucial for robust intercellular adhesion. Desmosomal function is compromised in the autoimmune blistering skin disease pemphigus vulgaris. We combine whole-genome knockout screening and a promotor screen of the desmosomal gene desmoglein 3 in human keratinocytes to identify novel regulators of intercellular adhesion. Kruppel-like-factor 5 (KLF5) directly binds to the desmoglein 3 regulatory region and promotes adhesion. Reduced levels of KLF5 in patient tissue indicate a role in pemphigus vulgaris. Autoantibody fractions from patients impair intercellular adhesion and reduce KLF5 levels in in vitro and in vivo disease models. These effects were dependent on increased activity of histone deacetylase 3, leading to transcriptional repression of KLF5. Inhibiting histone deacetylase 3 increases KLF5 levels and protects against the deleterious effects of autoantibodies in murine and human pemphigus vulgaris models. Together, KLF5 and histone deacetylase 3 are regulators of desmoglein 3 gene expression and intercellular adhesion and represent potential therapeutic targets in pemphigus vulgaris.

Desmosomes at a glance

Desmosomes are relatives of ancient cadherin-based junctions, which emerged late in evolution to ensure the structural integrity of vertebrate tissues by coupling the intermediate filament cytoskeleton to cell-cell junctions. Their ability to dynamically counter the contractile forces generated by actin-associated adherens junctions is particularly important in tissues under high mechanical stress, such as the skin and heart. Much more than the simple cellular 'spot welds' depicted in textbooks, desmosomes are in fact dynamic structures that can sense and respond to changes in their mechanical environment and external stressors like ultraviolet light and pathogens. These environmental signals are transmitted intracellularly via desmosome-dependent mechanochemical pathways that drive the physiological processes of morphogenesis and differentiation. This Cell Science at a Glance article and the accompanying poster review desmosome structure and assembly, highlight recent insights into how desmosomes integrate chemical and mechanical signaling in the epidermis, and discuss desmosomes as targets in human disease.

Pemphigus and pemphigoids: Clinical presentation, diagnosis and therapy

Pemphigus and pemphigoid are two potentially life-threatening groups of autoimmune diseases, characterized by autoantibodies targeting structural components of desmosomes or hemidesmosomes, respectively. Affected patients typically show itchy/painful plaques or blistering skin lesions and/or impairing mucosal blistering and erosions, which may strongly impact their quality of life. Since the milestone work of Walter Lever in 1953, who differentiated these two groups of diseases by histopathological analysis of the level of antibody-mediated skin cleavage, enormous progresses occurred. Achievements made in laboratory diagnostics now allow to identify antigen specific structural proteins of the skin that are targeted by pathogenic autoantibodies. These progresses were accompanied by an increased understanding of the pathogenesis of these diseases thanks to the establishment of animal models reproducing disease and on studies on skin and blood of affected individuals, which have been leading to novel and disease-specific treatments. Yet, given their phenotypical overlap with more common dermatological diseases, correct diagnosis and appropriate treatment are often delayed, in some cases leading to irreversible sequelae, including organ dysfunction (i.e., loss of vision in mucous membrane pemphigoid). Here, we provide a concise overview of the clinical appearance, diagnosis and therapeutic management of pemphigus and pemphigoid diseases.

Pemphigus- und Pemphigoid-Erkrankungen: Klinik, Diagnostik und Therapie: Pemphigus and pemphigoids: Clinical presentation, diagnosis and therapy

Pemphigus und Pemphigoid sind seltene Autoimmunkrankheiten der Haut mit potenziell lebensbedrohlichem Verlauf. Autoantikörper gegen epidermale und junktionale Strukturproteine (Desmosomen sowie Hemidesmosomen) führen bei Betroffenen typischerweise zu juckenden, schmerzhaften Plaques oder Blasen an der Haut und/oder Blasenbildung und Erosionen der Schleimhäute mit möglicher Einschränkung der Lebensqualität. Seit der bahnbrechenden Arbeit von Walter Lever im Jahr 1953, dem es gelang, mittels histopathologischer Untersuchung diese beiden Krankheitsgruppen anhand des Musters der Antikörper‐vermittelten Blasenbildung zu differenzieren, wurden enorme Fortschritte im Verständnis der Erkrankungen erzielt. Die Errungenschaften in der Labordiagnostik ermöglichten die Identifikation von Zielstrukturen zur präzisen Unterscheidung verschiedener Varianten der bullösen Autoimmunerkrankungen. Diese Fortschritte gingen dank der Entwicklung von Tiermodellen mit einem besseren Verständnis der Pathogenese einher. Außerdem haben Studien an Haut und Blut betroffener Patienten zu neuen und krankheitsspezifischen Behandlungen geführt. Aufgrund ihrer Seltenheit und der klinischen Ähnlichkeit mit anderen dermatologischen Erkrankungen verzögern sich die korrekte Diagnosestellung und die Einleitung einer entsprechenden Therapie häufig, was in einigen Fällen zu irreversiblen Folgeerscheinungen, einschließlich Funktionsstörungen von Organen (zum Beispiel Verlust des Sehvermögens beim Schleimhautpemphigoid) führt. Wir geben hier einen Überblick über das klinische Erscheinungsbild, den Diagnosealgorithmus und das therapeutische Management von Pemphigus‐ und Pemphigoid‐Erkrankungen.

Mouse models of pemphigus: valuable tools to investigate pathomechanisms and novel therapeutic interventions

Autoimmune blistering diseases (AIBD) are paradigms of autoantibody-mediated organ-specific autoimmune disorders that involve skin and/or mucous membranes. Compared to other autoimmune diseases, the pathogenicity of autoantibodies in AIBD is relatively well described. Pemphigus is a potentially lethal autoantibody driven autoimmune disorder with a strong HLA class II association. It is mainly characterized by IgG against the desmosomal adhesion molecules desmoglein 3 (Dsg3) and Dsg1. Several murine pemphigus models were developed subsequently, each allowing the analysis of a characteristic feature, such as pathogenic IgG or Dsg3-specific T or B cells. Thus, the models can be employed to preclinically evaluate potentially novel therapies. We here thoroughly summarize past and recent efforts in developing and utilizing pemphigus mouse models for pathomechanistic investigation and therapeutic interventions.

Innate immune activation as cofactor in pemphigus disease manifestation

Molecular mechanisms underlying auto-antibody-induced acantholysis in pemphigus vulgaris are subject of current research to date. To decipher the discrepancy between ubiquitous antibody binding to the epidermal desmosomes, but discontinuous disease manifestation, we were able to identify Ultraviolet A (UVA) as a cofactor for acantholysis. UVA induces interleukin (IL)-1 secretion in keratinocytes, mirroring innate immune system activation. In an in vitro keratinocyte dissociation assay increased fragmentation was observed when UVA was added to anti-Desmoglein 3 Immunoglobulins (anti-Dsg3 IgG). These results were confirmed in skin explants where UVA enhanced anti-Dsg3-mediated loss of epidermal adhesion. The UVA-mediated effect was blocked in vitro by the pan-caspase-inhibitor zVAD-fmk. Thus, we introduce UVA as a caspase-dependent exogenous cofactor for acantholysis which suggests that local innate immune responses largely contribute to overt clinical blister formation upon autoantibody binding to epidermal cells in pemphigus vulgaris.

What protein kinases are crucial for acantholysis and blister formation in pemphigus vulgaris? A systematic review

Pemphigus vulgaris (PV) is a potentially fatal autoimmune blistering disease characterized by cell-cell detachment (or acantholysis) and blister formation. While the signaling mechanisms that associate with skin/mucosal blistering are being elucidated, specific treatment strategies targeting PV-specific pathomechanisms, particularly kinase signaling, have yet to be established. Hence, the aim of this review was to systematically evaluate molecules in the class of kinases that are essential for acantholysis and blister formation and are therefore candidates for targeted therapy. English articles from PubMed and Scopus databases were searched, and included in vitro, in vivo, and human studies that investigated the role of kinases in PV. We selected studies, extracted data and assessed risk of bias in duplicates and the results were reported according to the methodology outlined by the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA). The risk of bias assessment was performed on in vivo studies utilizing SYRCLE's risk of bias tool. Thirty-five studies were included that satisfied the pathogenicity criterion of kinases in PV, the vast majority being experimental models that used PV sera (n = 13) and PV-IgG (n = 22). Inhibition of kinase activity (p38MAPK, PKC, TK, c-Src, EGFR, ERK, mTOR, BTK, and CDK2) was achieved mostly by pharmacological means. Overall, we found substantial evidence that kinase inhibition reduced PV-associated phosphorylation events and keratinocyte disassociation, prevented acantholysis, and blocked blister formation. However, the scarce adherence to standardized reporting systems and the experimental protocols/models used did limit the internal and external validity of these studies. In summary, this systematic review highlighted the pathogenic intracellular events mediated by kinases in PV acantholysis and presented kinase signaling as a promising avenue for translational research. In particular, the molecules identified and discussed in this study represent potential candidates for the development of mechanism-based interventions in PV.

Mechanisms Causing Acantholysis in Pemphigus-Lessons from Human Skin

Pemphigus vulgaris (PV) is an autoimmune bullous skin disease caused primarily by autoantibodies (PV-IgG) against the desmosomal adhesion proteins desmoglein (Dsg)1 and Dsg3. PV patient lesions are characterized by flaccid blisters and ultrastructurally by defined hallmarks including a reduction in desmosome number and size, formation of split desmosomes, as well as uncoupling of keratin filaments from desmosomes. The pathophysiology underlying the disease is known to involve several intracellular signaling pathways downstream of PV-IgG binding. Here, we summarize our studies in which we used transmission electron microscopy to characterize the roles of signaling pathways in the pathogenic effects of PV-IgG on desmosome ultrastructure in a human ex vivo skin model. Blister scores revealed inhibition of p38MAPK, ERK and PLC/Ca2+ to be protective in human epidermis. In contrast, inhibition of Src and PKC, which were shown to be protective in cell cultures and murine models, was not effective for human skin explants. The ultrastructural analysis revealed that for preventing skin blistering at least desmosome number (as modulated by ERK) or keratin filament insertion (as modulated by PLC/Ca2+) need to be ameliorated. Other pathways such as p38MAPK regulate desmosome number, size, and keratin insertion indicating that they control desmosome assembly and disassembly on different levels. Taken together, studies in human skin delineate target mechanisms for the treatment of pemphigus patients. In addition, ultrastructural analysis supports defining the specific role of a given signaling molecule in desmosome turnover at ultrastructural level.

Mechanism-based therapeutic targets of pemphigus vulgaris: A scoping review of pathogenic molecular pathways

Pemphigus vulgaris (PV) is a potentially fatal autoimmune blistering disease characterised by cell-cell detachment or acantholysis. The mechanisms which follow antibody (Ab) binding and culminate in acantholytic changes and skin/mucosal blistering have not been fully clarified. Current treatment strategies are not specific to PV pathophysiology and although life-saving, harbour considerable side effects. We aimed to systematically assess the molecules amenable to targeted treatments that follow Ab binding and are associated with PV acantholysis. The resulting scoping review was conducted under PRISMA-ScR guidelines with clear inclusion and exclusion criteria and focused specifically on kinases, caspases, proteases, hydrolytic enzymes and other molecules of interest postulated to take part in the pathophysiology of PV. The review process resulted in the identification of 882 articles, of which 56 were eligible for qualitative synthesis. From the included articles, the majority (n = 42) used PV-IgG as the pathogenic agent, mainly via in vitro (n = 16) and in vivo (n = 10) models. Twenty-five molecules were found to play a pathogenic role in PV, including uPA, ADAM10, EGFR, Src, PKC, cdk2, ERK, PLC, calmodulin, NOS, p38MAPK and caspase-3. Selective inhibition of these molecules resulted in varying degrees of reduction in acantholysis and blistering. The pathogenic molecules identified in this review represent potential candidates for clinical translation.

Autoantibody-Specific Signalling in Pemphigus

Pemphigus is a severe autoimmune disease impairing barrier functions of epidermis and mucosa. Autoantibodies primarily target the desmosomal adhesion molecules desmoglein (Dsg) 1 and Dsg 3 and induce loss of desmosomal adhesion. Strikingly, autoantibody profiles in pemphigus correlate with clinical phenotypes. Mucosal-dominant pemphigus vulgaris (PV) is characterised by autoantibodies (PV-IgG) against Dsg3 whereas epidermal blistering in PV and pemphigus foliaceus (PF) is associated with autoantibodies against Dsg1. Therapy in pemphigus is evolving towards specific suppression of autoantibody formation and autoantibody depletion. Nevertheless, during the acute phase and relapses of the disease additional treatment options to stabilise desmosomes and thereby rescue keratinocyte adhesion would be beneficial. Therefore, the mechanisms by which autoantibodies interfere with adhesion of desmosomes need to be characterised in detail. Besides direct inhibition of Dsg adhesion, autoantibodies engage signalling pathways interfering with different steps of desmosome turn-over. With this respect, recent data indicate that autoantibodies induce separate signalling responses in keratinocytes via specific signalling complexes organised by Dsg1 and Dsg3 which transfer the signal of autoantibody binding into the cell. This hypothesis may also explain the different clinical pemphigus phenotypes.

Ca2+ signalling is critical for autoantibody-induced blistering of human epidermis in pemphigus

BACKGROUND

Pemphigus is a severe bullous autoimmune skin disease. Pemphigus foliaceus (PF) is characterized by antidesmoglein (Dsg) 1 IgG causing epidermal blistering; mucosal pemphigus vulgaris (mPV) by anti-Dsg3 IgG inducing erosions in the mucosa; and mucocutaneous pemphigus vulgaris (PV) by affecting both, with autoantibodies targeting Dsg1 and Dsg3.

OBJECTIVES

To characterize the Ca²⁺ flux pathway and delineate its importance in pemphigus pathogenesis and clinical phenotypes caused by different antibody profiles.

METHODS

Immunoprecipitation, Ca²⁺ flux analysis, Western blotting, immunofluorescence staining, dissociation assays and a human skin ex vivo model were used.

RESULTS

PV IgG and PF IgG, but neither Dsg3-specific monoclonal antibody (AK23) nor mPV IgG, caused Ca²⁺ influx in primary human keratinocytes. Phosphatidylinositol 4-kinase α interacts with Dsg1 but not with Dsg3. Its downstream target - phospholipase-C-γ1 (PLC) - was activated by PV IgG and PF IgG but not AK23 or mPV IgG. PLC releases inositol 1,4,5-trisphosphate (IP3) causing IP3 receptor (IP3R) activation and Ca²⁺ flux from the endoplasmic reticulum into the cytosol, which stimulates Ca²⁺ release-activated channels (CRAC)-mediated Ca²⁺ influx. Inhibitors against PLC, IP3R and CRAC effectively blocked PV IgG and PF IgG-induced Ca²⁺ influx; ameliorated alterations of Dsg1 and Dsg3 localization, and reorganization of keratin and actin filaments; and inhibited loss of cell adhesion in vitro. Finally, inhibiting PLC or IP3R was protective against PV IgG-induced blister formation and redistribution of Dsg1 and Dsg3 in human skin ex vivo.

CONCLUSIONS

Ca²⁺ -mediated signalling is important for epidermal blistering and dependent on the autoantibody profile, which indicates different roles for signalling complexes organized by Dsg1 and Dsg3. Interfering with PLC and Ca²⁺ signalling may be a promising approach to treat epidermal manifestations of pemphigus.

Role of PKC and ERK Signaling in Epidermal Blistering and Desmosome Regulation in Pemphigus

Desmosomes reinforce cohesion of epithelial cells at the interface between adjacent cells. They include the cadherin-type adhesion molecules desmoglein 1 (Dsg1) and Dsg3. Pemphigus vulgaris (PV) is an autoimmune disease in which circulating autoantibodies (PV-IgG) targeting Dsg1 and 3 cause characteristic epidermal blister formation. It has been shown that PV-IgG binding induced activation of kinases such as ERK and PKC, and inhibition of these signaling pathways prevented loss of cell cohesion in cell cultures. However, the role of Erk and PKC in blister formation and regulation of desmosome ultrastructure in human skin are unknown. Accordingly, we assessed the role of PKC and ERK signaling pathways in blister formation and regulation of desmosome ultrastructure in human epidermis. Here we performed electron microscopy analyses using human skin explants injected with PV-IgG together with inhibitors for PKC or ERK signaling. Inhibition of PKC was not effective to prevent suprabasal blister formation or ultrastructural alterations of desmosomes. In contrast, inhibition of ERK signaling significantly ameliorated blister formation and decrease in the number of desmosomes whereas shortening and splitting of desmosomes and keratin filament insertion were not different from samples treated with PV-IgG alone. However, apical desmosomes between basal and suprabasal cells remained unaltered when ERK signaling was inhibited. Therefore, our results show that inhibition of ERK but not PKC signaling appears to be effective to ameliorate blistering and alterations of desmosome ultrastructure triggered by PV-IgG in human skin.

Recurrence of Pemphigus Vulgaris Under Nivolumab Therapy

For many types of cancer, immune checkpoint inhibitors have proven to be a highly effective treatment. The monoclonal anti-PD-1 antibody nivolumab stimulates the immune system and is one of the newest treatment options for non-small cell lung cancer. In doing so, immune checkpoint inhibitors can trigger many skin lesions that have not yet been completely investigated in their entirety. In this case report, pemphigus vulgaris is identified as a potential adverse event that occurs under the treatment with nivolumab. In addition to the standard methods, we examined our patient's samples with ex vivo confocal laser scanning microscopy. This is a new and innovative diagnostic method that uses vertical scanning to provide fast, high-resolution imaging of freshly excised tissue, even using fluorescently labeled antibodies.

The involvement of ADAM10 in acantholysis in mucocutaneous pemphigus vulgaris depends on the autoantibody profile of each patient

BACKGROUND

Acantholysis in pemphigus vulgaris (PV) may be triggered by desmoglein (Dsg) and non-Dsg autoantibodies. The autoantibody profile of each patient results in distinct intracellular signalling patterns.

OBJECTIVES

Based on our previous findings, we aimed to elucidate whether PV acantholysis in a mouse model may be mediated by activation of a disintegrin and metalloproteinase 10 (ADAM10).

METHODS

We used three PV-IgG fractions from different patients containing high or low levels of anti-Dsg1 and anti-Dsg3 antibodies, and the presence or not of anti-desmocollin (Dsc) antibodies, using a passive transfer mouse model of PV.

RESULTS

Although all of the PV-IgG fractions produced suprabasal acantholysis, only those containing anti-Dsg1/3, but not anti-Dsc2/3 antibodies, induced ADAM10 activation in a Src-dependent way, and an increase in the epidermal growth factor (EGF) receptor ligands EGF and betacellulin (BTC). In contrast, the presence of anti-Dsc2/3 antibodies, in addition to anti-Dsg1/3, triggered earlier and ADAM10-independent epidermal detachment, with no increase in EGF and BTC, which was associated with an earlier and more intense acantholysis.

CONCLUSIONS

All PV-IgG fractions produced suprabasal acantholysis, but our results reveal that depending on the levels of anti-Dsg antibodies or the presence of non-Dsg antibodies, such as anti-Dsc, more severe cell-cell epidermal detachment will occur at different times, and in an ADAM10-dependent manner or not. Acantholysis in these different groups of patients with PV may be a consequence of the activation of specific intracellular mechanisms downstream of Autoantibodies binding to Dsg or non-Dsg proteins, and therefore more specific therapeutic approaches in PV should be used. What's already known about this topic? Suprabasal acantholysis in pemphigus vulgaris (PV) may be triggered by both desmoglein (Dsg) and non-Dsg autoantibodies. The autoantibody profile of each patient is associated with a distinct intracellular signalling pattern. What does this study add? In patients with PV with anti-Dsg3 and anti-Dsg1, but not anti-desmocollin (Dsc)3 antibodies, ADAM10 activation is induced in an Src-dependent way, together with an increase in the epidermal growth factor receptor (EGFR) ligands EGF and betacellulin. The presence of anti-Dsc3 antibodies triggers an earlier and ADAM10-independent acantholysis, without increasing EGFR ligands, and is associated with more severe epidermal detachment. Lower levels of anti-Dsc3 antibodies are associated with less severe acantholysis. What is the translational message? In some patients with PV, the severity and the timing for cell-cell detachment seem to depend on the level of anti-Dsg1/3 antibodies, although other as yet uncharacterized antibodies may also participate. These patients with PV would exhibit inhibition of acantholysis by Src, ADAM10, EGF and EGFR inhibitors. In other patients, the presence of non-Dsg antibodies, such as anti-Dsc2/3, would produce an earlier and more severe ADAM10-independent suprabasal acantholysis.

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.

Role of Dsg1- and Dsg3-Mediated Signaling in Pemphigus Autoantibody-Induced Loss of Keratinocyte Cohesion

Pemphigus is an autoimmune dermatosis in which mucocutaneous blisters are induced primarily by autoantibodies against Desmoglein (Dsg) 1 and 3. Pemphigus vulgaris (PV) usually is associated with autoantibodies against Dsg3 whereas pemphigus foliaceus (PF) patients present autoantibodies against Dsg1. Several signaling pathways were proposed to cause loss of keratinocyte adhesion. However, relevance of different signaling pathways and role of Dsg1 and 3 to trigger signaling are not fully understood. Here, we show that Ca²⁺ chelation reduced PV-IgG- and PF-IgG-mediated loss of HaCaT keratinocyte cohesion whereas EGFR inhibition did not inhibit effects of PF-IgG. PV-IgG activated EGFR in a Src-dependent manner whereas both PV-IgG and PF-IgG caused Ca²⁺ influx independent of EGFR. ERK activation was Src-dependent in response to PV-IgG but not PF-IgG. To delineate the roles of Dsg isoforms to trigger signaling pathways, Dsg3- and Dsg2-deficient HaCaT keratinocyte cell lines were generated using CRISPR/Cas9. Dsg3- but not Dsg2-deficient cells were protected against PV-IgG-induced loss of cell adhesion. Ca²⁺ influx and ERK activation in response to PF-IgG were preserved in both cell lines.

Role of Src and Cortactin in Pemphigus Skin Blistering

Autoantibodies against desmoglein (Dsg) 1 and Dsg3 primarily cause blister formation in the autoimmune disease pemphigus vulgaris (PV). Src was proposed to contribute to loss of keratinocyte cohesion. However, the role and underlying mechanisms are unclear and were studied here. In keratinocytes, cell cohesion in response to autoantibodies was reduced in Src-dependent manner by two patient-derived PV-IgG fractions as well as by AK23 but not by a third PV-IgG fraction, although Src was activated by all autoantibodies. Loss of cell cohesion was progredient in a timeframe of 24 h and AK23, similar to PV-IgG, interfered with reconstitution of cell cohesion after Ca²⁺-switch, indicating that the autoantibodies also interfered with desmosome assembly. Dsg3 co-localized along cell contacts and interacted with the Src substrate cortactin. In keratinocytes isolated from cortactin-deficient mice, cell adhesion was impaired and Src-mediated inhibition of AK23-induced loss of cell cohesion for 24 h was significantly reduced compared to wild-type (wt) cells. Similarly, AK23 impaired reconstitution of cell adhesion was Src-dependent only in the presence of cortactin. Likewise, Src inhibition significantly reduced AK23-induced skin blistering in wt but not cortactin-deficient mice. These data suggest that the Src-mediated long-term effects of AK23 on loss of cell cohesion and skin blistering are dependent on cortactin-mediated desmosome assembly. However, in human epidermis PV-IgG-induced skin blistering and ultrastructural alterations of desmosomes were not affected by Src inhibition, indicating that Src may not be critical for skin blistering in intact human skin, at least when high levels of autoantibodies targeting Dsg1 are present.

Autoantibody Signaling in Pemphigus Vulgaris: Development of an Integrated Model

Pemphigus vulgaris (PV) is an autoimmune skin blistering disease effecting both cutaneous and mucosal epithelia. Blister formation in PV is known to result from the binding of autoantibodies (autoAbs) to keratinocyte antigens. The primary antigenic targets of pathogenic autoAbs are known to be desmoglein 3, and to a lesser extent, desmoglein 1, cadherin family proteins that partially comprise the desmosome, a protein structure responsible for maintaining cell adhesion, although additional autoAbs, whose role in blister formation is still unclear, are also known to be present in PV patients. Nevertheless, there remain large gaps in knowledge concerning the precise mechanisms through which autoAb binding induces blister formation. Consequently, the primary therapeutic interventions for PV focus on systemic immunosuppression, whose side effects represent a significant health risk to patients. In an effort to identify novel, disease-specific therapeutic targets, a multitude of studies attempting to elucidate the pathogenic mechanisms downstream of autoAb binding, have led to significant advancements in the understanding of autoAb-mediated blister formation. Despite this enhanced characterization of disease processes, a satisfactory explanation of autoAb-induced acantholysis still does not exist. Here, we carefully review the literature investigating the pathogenic disease mechanisms in PV and, taking into account the full scope of results from these studies, provide a novel, comprehensive theory of blister formation in PV.

Soluble Fas Ligand Is Essential for Blister Formation in Pemphigus

Pemphigus is a blistering disease characterized by pemphigus autoantibodies (PVIgG) directed mostly against desmogleins (Dsgs), resulting in the loss of keratinocyte adhesion (acantholysis). Yet, the mechanisms underlying blister formation remain to be clarified. We have shown previously that anti-Fas ligand (FasL) antibody (Ab) prevents PVIgG-induced caspase-8 activation and Dsg cleavage in human keratinocytes, and that sera from pemphigus patients contain abnormally increased levels of FasL. Here, we demonstrate that recombinant FasL induces the activation of caspases prior to Dsg degradation, and anti-FasL Ab prevents acantholysis in cultured keratinocytes. Moreover, the silencing of FasL reduces PVIgG-induced caspase-8 activation and Dsg3 cleavage. Following injection of PVIgG into mice, FasL is upregulated at 1–3 h and is followed by caspase-8-mediated keratinocyte apoptosis, before blister formation. The administration of anti-FasL Ab after PVIgG injection blocks blister formation in mice. Furthermore, we injected PVIgG into two different gene-targeted mutant mice that selectively lack either secreted soluble FasL (sFasL), FasL^Δs/Δs mice, or the membrane-bound form of FasL (mFasL), FasL^Δm/Δm mice. After PVIgG treatment, blisters are only visible in FasL^Δm/Δm animals, lacking mFasL, but still producing sFasL, similar to wild-type (C57BL/6) animals. By contrast, a significant decrease in the relative acantholytic area is observed in the FasL^Δs/Δs animals. These results demonstrate that soluble FasL plays a crucial role in the mechanisms of blister formation, and blockade of FasL could be an effective therapeutic approach for pemphigus.

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.

Crosstalk between Signaling Pathways in Pemphigus: A Role for Endoplasmic Reticulum Stress in p38 Mitogen-Activated Protein Kinase Activation?

Pemphigus consists of a group of chronic blistering skin diseases mediated by autoantibodies (autoAbs). The dogma that pemphigus is caused by keratinocyte dissociation (acantholysis) as a distinctive and direct consequence of the presence of autoAb targeting two main proteins of the desmosome—desmoglein (DSG) 1 and/or DSG3—has been put to the test. Several outside-in signaling events elicited by pemphigus autoAb in keratinocytes have been described, among which stands out p38 mitogen-activated protein kinase (p38 MAPK) engagement and its apoptotic effect on keratinocytes. The role of apoptosis in the disease is, however, debatable, to an extent that it may not be a determinant event for the occurrence of acantholysis. Also, it has been verified that compromised DSG trans-interaction does not lead to keratinocyte dissociation when p38 MAPK is inhibited. These examples of conflicting results have been followed by recent work revealing an important role for endoplasmic reticulum (ER) stress in pemphigus’ pathogenesis. ER stress is known to activate the p38 MAPK pathway, and vice versa. However, this relationship has not yet been studied in the context of activated signaling pathways in pemphigus. Therefore, by reviewing and hypothetically connecting the role(s) of ER stress and p38 MAPK pathway in pemphigus, we highlight the importance of elucidating the crosstalk between all activated signaling pathways, which may in turn contribute for a better understanding of the role of apoptosis in the disease and a better management of this life-threatening condition.

Different signaling patterns contribute to loss of keratinocyte cohesion dependent on autoantibody profile in pemphigus

Pemphigus is an autoimmune blistering skin disease caused primarily by autoantibodies against desmoglein (Dsg)1 and 3. Here, we characterized the mechanisms engaged by pemphigus IgG from patients with different clinical phenotypes and autoantibody profiles. All pemphigus vulgaris (PV) and pemphigus foliaceus (PF) IgG and AK23, a monoclonal mouse antibody against Dsg3, caused loss of cell cohesion, cytokeratin retraction and p38MAPK activation. Strong alterations in Dsg3 distribution were caused by mucosal (aDsg3 antibodies), mucocutaneous (aDsg1 + aDsg3) as well as atypical (aDsg3) PV-IgG. All PV-IgG fractions and AK23 compromised Dsg3 but not Dsg1 binding and enhanced Src activity. In contrast, rapid Ca²⁺ influx and Erk activation were induced by mucocutaneous PV-IgG and pemphigus foliaceus (PF) IgG (aDsg1) whereas cAMP was increased by mucosal and mucocutaneous PV-IgG only. Selective inhibition of p38MAPK, Src or PKC blocked loss of keratinocyte cohesion in response to all autoantibody fractions whereas Erk inhibition was protective against mucocutaneous PV-IgG and PF-IgG only. These results demonstrate that signaling patterns parallel the clinical phenotype as some mechanisms involved in loss of cell cohesion are caused by antibodies targeting Dsg3 whereas others correlate with autoantibodies against Dsg1. The concept of key desmosome regulators may explain observations from several experimental models of pemphigus.

Mechanisms of Acantholysis in Pemphigus: Mechanical or Inflammatory?

Pemphigus is a rare autoimmune disease, involving the skin and mucous epithelia, characterized by flaccid blisters and erosions. Histologically, the basic abnormality in all forms of pemphigus is the separation of keratinocytes from one another, a process known as acantholysis. There is direct evidence that autoantibodies against desmoglein, a transmembrane desmosomal component, are critical in its pathogenesis, but the exact mechanism that induces acantholysis is yet unknown. Actually, different studies suggest three possible mechanisms: sterical impedance, intracellular signalling and apoptosis. Understanding these processes should show new therapeutic perspective.

Desmosomes in acquired disease

Desmosomes are cell-cell junctions that mediate adhesion and couple the intermediate filament cytoskeleton to sites of cell-cell contact. This architectural arrangement integrates adhesion and cytoskeletal elements of adjacent cells. The importance of this robust adhesion system is evident in numerous human diseases, both inherited and acquired, which occur when desmosome function is compromised. This review focuses on autoimmune and infectious diseases that impair desmosome function. In addition, we discuss emerging evidence that desmosomal genes are often misregulated in cancer. The emphasis of our discussion is placed on the way in which human diseases can inform our understanding of basic desmosome biology and in turn, the means by which fundamental advances in the cell biology of desmosomes might lead to new treatments for acquired diseases of the desmosome.

Cell adhesion in epidermal development and barrier formation

Cell-cell adhesions are necessary for structural integrity and barrier formation of the epidermis. Here, we discuss insights from genetic and cell biological studies into the roles of individual cell-cell junctions and their composite proteins in regulating epidermal development and function. In addition to individual adhesive functions, we will discuss emerging ideas on mechanosensation/transduction of junctions in the epidermis, noncanonical roles for adhesion proteins, and crosstalk/interdependencies between the junctional systems. These studies have revealed that cell adhesion proteins are connected to many aspects of tissue physiology including growth control, differentiation, and inflammation.

Neural nitric oxide synthase participates in pemphigus vulgaris acantholysis through upregulation of Rous sarcoma, mammalian target of rapamycin and focal adhesion kinase

Pemphigus vulgaris (PV) is an autoimmune blistering skin disease characterized by suprabasal acantholysis produced as a consequence of desmoglein (Dsg) and non-Dsg autoantibodies binding to several targeting molecules localized on the membrane of keratinocytes. Nitric oxide (NO) may exert a pathogenic function in several immunological processes. We have previously demonstrated that neural nitric oxide synthase (nNOS) plays part in PV acantholysis. Also, our group has described a relevant role for HER [human epidermal growth factor receptor (EGFR) related] isoforms and several kinases such as Src (Rous sarcoma), mammalian target of rapamycin (mTOR) and focal adhesion kinase (FAK), as well as caspases in PV development. Using a passive transfer mouse model of PV, we aimed to investigate the relationship between the increase in nNOS and EGFR, Src, mTOR and FAK kinase upregulation observed in PV lesions. Our results revealed a new function for nNOS, which contributes to EGFR-mediated PV acantholysis through the upregulation of Src, mTOR and FAK. In addition, we found that nNOS participates actively in PV at least in part by increasing caspase-9 and caspase-3 activities. These findings underline the important issue that in PV acantholysis, caspase activation is a nNOS-linked process downstream of Src, mTOR and FAK kinase upregulation.

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.

Signaling dependent and independent mechanisms in pemphigus vulgaris blister formation

Pemphigus vulgaris (PV) is an autoimmune epidermal blistering disease caused by autoantibodies directed against the desmosomal cadherin desmoglein-3 (Dsg3). Significant advances in our understanding of pemphigus pathomechanisms have been derived from the generation of pathogenic monoclonal Dsg3 antibodies. However, conflicting models for pemphigus pathogenicity have arisen from studies using either polyclonal PV patient IgG or monoclonal Dsg3 antibodies. In the present study, the pathogenic mechanisms of polyclonal PV IgG and monoclonal Dsg3 antibodies were directly compared. Polyclonal PV IgG cause extensive clustering and endocytosis of keratinocyte cell surface Dsg3, whereas pathogenic mouse monoclonal antibodies compromise cell-cell adhesion strength without causing these alterations in Dsg3 trafficking. Furthermore, tyrosine kinase or p38 MAPK inhibition prevents loss of keratinocyte adhesion in response to polyclonal PV IgG. In contrast, disruption of adhesion by pathogenic monoclonal antibodies is not prevented by these inhibitors either in vitro or in human skin explants. Our results reveal that the pathogenic activity of polyclonal PV IgG can be attributed to p38 MAPK-dependent clustering and endocytosis of Dsg3, whereas pathogenic monoclonal Dsg3 antibodies can function independently of this pathway. These findings have important implications for understanding pemphigus pathophysiology, and for the design of pemphigus model systems and therapeutic interventions.

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.

An adult passive transfer mouse model to study desmoglein 3 signaling in pemphigus vulgaris

Evidence has accumulated that changes in intracellular signaling downstream of desmoglein 3 (Dsg3) may play a significant role in epithelial blistering in the autoimmune disease pemphigus vulgaris (PV). Currently, most studies on PV involve passive transfer of pathogenic antibodies into neonatal mice which have not finalized epidermal morphogenesis, and do not permit analysis of mature hair follicles (HFs) and stem cell niches. To investigate Dsg3 antibody-induced signaling in the adult epidermis at defined stages of the HF cycle, we here developed a model with passive transfer of the monospecific pathogenic Dsg3 antibody AK23 into adult 8-week-old C57Bl/6J mice. Validated using histopathological and molecular methods, we found that this model faithfully recapitulates major features described in PV patients and PV models. Two hours after AK23 transfer we observed widening of intercellular spaces between desmosomes and EGFR activation, followed by increased Myc expression and epidermal hyperproliferation, desmosomal Dsg3 depletion and predominant blistering in HFs and oral mucosa. These data confirm that the adult passive transfer mouse model is ideally suited for detailed studies of Dsg3 antibody-mediated signaling in adult skin, providing the basis for investigations on novel keratinocyte-specific therapeutic strategies.

Pemphigus autoimmunity: hypotheses and realities

The goal of contemporary research in pemphigus vulgaris and pemphigus foliaceus is to achieve and maintain clinical remission without corticosteroids. Recent advances of knowledge on pemphigus autoimmunity scrutinize old dogmas, resolve controversies, and open novel perspectives for treatment. Elucidation of intimate mechanisms of keratinocyte detachment and death in pemphigus has challenged the monopathogenic explanation of disease immunopathology. Over 50 organ-specific and non-organ-specific antigens can be targeted by pemphigus autoimmunity, including desmosomal cadherins and other adhesion molecules, PERP cholinergic and other cell membrane (CM) receptors, and mitochondrial proteins. The initial insult is sustained by the autoantibodies to the cell membrane receptor antigens triggering the intracellular signaling by Src, epidermal growth factor receptor kinase, protein kinases A and C, phospholipase C, mTOR, p38 MAPK, JNK, other tyrosine kinases, and calmodulin that cause basal cell shrinkage and ripping desmosomes off the CM. Autoantibodies synergize with effectors of apoptotic and oncotic pathways, serine proteases, and inflammatory cytokines to overcome the natural resistance and activate the cell death program in keratinocytes. The process of keratinocyte shrinkage/detachment and death via apoptosis/oncosis has been termed apoptolysis to emphasize that it is triggered by the same signal effectors and mediated by the same cell death enzymes. The natural course of pemphigus has improved due to a substantial progress in developing of the steroid-sparing therapies combining the immunosuppressive and direct anti-acantholytic effects. Further elucidation of the molecular mechanisms mediating immune dysregulation and apoptolysis in pemphigus should improve our understanding of disease pathogenesis and facilitate development of steroid-free treatment of patients.

The extent of desmoglein 3 depletion in pemphigus vulgaris is dependent on Ca(2+)-induced differentiation: a role in suprabasal epidermal skin splitting?

Pemphigus vulgaris (PV) is an autoimmune disease of the skin and mucous membranes and is characterized by development of autoantibodies against the desmosomal cadherins desmoglein (Dsg) 3 and Dsg1 and formation of intraepidermal suprabasal blisters. Depletion of Dsg3 is a critical mechanism in PV pathogenesis. Because we did not detect reduced Dsg3 levels in keratinocytes cultured for longer periods under high-Ca(2+) conditions, we hypothesized that Dsg depletion depends on Ca(2+)-mediated keratinocyte differentiation. Our data indicate that depletion of Dsg3 occurs specifically in deep epidermal layers both in skin of patients with PV and in an organotypic raft model of human epidermis incubated using IgG fractions from patients with PV. In addition, Dsg3 depletion and loss of Dsg3 staining were prominent in cultured primary keratinocytes and in HaCaT cells incubated in high-Ca(2+) medium for 3 days, but were less pronounced in HaCaT cultures after 8 days. These effects were dependent on protein kinase C signaling because inhibition of protein kinase C blunted both Dsg3 depletion and loss of intercellular adhesion. Moreover, protein kinase C inhibition blocked suprabasal Dsg3 depletion in cultured human epidermis and blister formation in a neonatal mouse model. Considered together, our data indicate a contribution of Dsg depletion to PV pathogenesis dependent on Ca(2+)-induced differentiation. Furthermore, prominent depletion in basal epidermal layers may contribute to the suprabasal cleavage plane observed in PV.

A glass of red wine to keep vascular disease at bay, but what about pemphigus vulgaris?

Pemphigus vulgaris is a rare autoimmune blistering disease, involving the skin and mucous epithelia, which is characterized by flaccid blisters and erosions. It is caused by the presence of autoantibodies directed against desmoglein, a glycoprotein that plays a critical role in cell-cell attachment. Upon a predisposing genetic background, different agents have been shown to act as triggers for the pathogenesis of pemphigus. The most evident association is with drug intake, while the role of diet is often underestimated. The aim of this article is to review the possible role of tannins, a group of phenolic metabolites that are widely distributed in almost all plant foods and beverages, particularly red wine, as a trigger for pemphigus vulgaris.

Protective endogenous cyclic adenosine 5'-monophosphate signaling triggered by pemphigus autoantibodies

Pemphigus vulgaris (PV) is an autoimmune skin disease mediated by autoantibodies directed against the cadherin-type cell adhesion molecules desmoglein (Dsg) 3 and Dsg1 and is characterized by loss of keratinocyte cohesion and epidermal blistering. Several intracellular signaling pathways, such as p38MAPK activation and RhoA inhibition, have been demonstrated to be altered following autoantibody binding and to be causally involved in loss of keratinocyte cohesion. In this paper, we demonstrate that cAMP-mediated signaling completely prevented blister formation in a neonatal pemphigus mouse model. Furthermore, elevation of cellular cAMP levels by forskolin/rolipram or β receptor agonist isoproterenol blocked loss of intercellular adhesion, depletion of cellular Dsg3, and morphologic changes induced by Ab fractions of PV patients (PV-IgG) in cultured keratinocytes. Incubation with PV-IgG alone increased cAMP levels, indicating that cAMP elevation may be a cellular response pathway to strengthen intercellular adhesion. Our data furthermore demonstrate that this protective pathway may involve protein kinase A signaling because protein kinase A inhibition attenuated recovery from PV-IgG-induced cell dissociation. Finally, cAMP increase interfered with PV-IgG-induced signaling by preventing p38MAPK activation both in vitro and in vivo. Taken together, our data provide insights into the cellular response mechanisms following pemphigus autoantibody binding and point to a possible novel and more specific therapeutic approach in pemphigus.

Efficacy of intravenous immunoglobulin (IVIG) affinity-purified anti-desmoglein anti-idiotypic antibodies in the treatment of an experimental model of pemphigus vulgaris

Pemphigus vulgaris is a rare life-threatening autoimmune bullous disease caused by immunoglobulin G (IgG) autoantibodies directed against desmogleins 1 and 3. Previously, we showed that intravenous immunoglobulin (IVIG) ameliorates anti-desmoglein-induced experimental pemphigus vulgaris in newborn naive mice. The aim of this study was to examine the efficacy of anti-anti-desmoglein-specific IVIG in a similar model. Pemphigus-vulgaris-specific IVIG (PV-sIVIG) was affinity-purified from IVIG on a column of single-chain variable fragment (scFv) anti-desmogleins 1 and 3. The anti-idiotypic activity of PV-sIVIG was confirmed by enzyme-linked immunosorbent assay, inhibition assay. After induction of pemphigus by injection of anti-desmogleins 1 and 3 scFv to newborn mice, the animals were treated with PV-sIVIG, IVIG (low or high dose) or IgG from a healthy donor (n = 10 each). The skin was examined 24-48 h later, and samples of affected areas were analysed by histology and immunofluorescence. In vitro study showed that PV-sIVIG significantly inhibited anti-desmogleins 1 and 3 scFv binding to recombinant desmoglein-3 in a dose-dependent manner. Specificity was confirmed by inhibition assay. In vivo analysis revealed cutaneous lesions of pemphigus vulgaris in mice injected with normal IgG (nine of 10 mice) or low-dose IVIG (nine of 10 mice), but not in mice treated with PV-sIVIG (none of 10) or high-dose IVIG (none of 10). On immunopathological study, PV-sIVIG and regular IVIG prevented the formation of acantholysis and deposition of IgG in intercellular spaces. In conclusion, the PV-sIVIG preparation is more effective than native IVIG in inhibiting anti-desmoglein-induced pemphigus vulgaris in mice and might serve as a future therapy in patients with the clinical disease.

Desmosomes: adhesive strength and signalling in health and disease

Desmosomes are intercellular junctions whose primary function is strong intercellular adhesion, known as hyperadhesion. In the present review, we discuss how their structure appears to support this function as well as how they are assembled and down-regulated. Desmosomal components also have signalling functions that are important in tissue development and remodelling. Their adhesive and signalling functions are both compromised in genetic and autoimmune diseases that affect the heart, skin and mucous membranes. We conclude that much work is required on structure–function relationships within desmosomes in vivo and on how they participate in signalling processes to enhance our knowledge of tissue homoeostasis and human disease.

Tyrosine kinases in inflammatory dermatologic disease

Tyrosine kinases (TKs) are enzymes that catalyze the phosphorylation of tyrosine residues on protein substrates. They are key components of signaling pathways that drive an array of cellular responses including proliferation, differentiation, migration, and survival. Specific TKs have recently been identified as critical to the pathogenesis of several autoimmune and inflammatory diseases. Small-molecule inhibitors of TKs are emerging as a novel class of therapy that may provide benefit in certain patient subsets. In this review, we highlight TK signaling implicated in inflammatory dermatologic diseases, evaluate strategies aimed at inhibiting these aberrant signaling pathways, and discuss prospects for future drug development.

Desmosomes in vivo

The structure, function, and regulation of desmosomal adhesion in vivo are discussed. Most desmosomes in tissues exhibit calcium-independent adhesion, which is strongly adhesive or “hyperadhesive”. This is fundamental to tissue strength. Almost all studies in culture are done on weakly adhesive, calcium-dependent desmosomes, although hyperadhesion can be readily obtained in confluent cell culture. Calcium dependence is a default condition in vivo, found in wounds and embryonic development. Hyperadhesion appears to be associated with an ordered arrangement of the extracellular domains of the desmosomal cadherins, which gives rise to the intercellular midline identified in ultrastructural studies. This in turn probably depends on molecular order in the desmosomal plaque. Protein kinase C downregulates hyperadhesion and there is preliminary evidence that it may also be regulated by tyrosine kinases. Downregulation of desmosomes in vivo may occur by internalisation of whole desmosomes rather than disassembly. Hyperadhesion has implications for diseases such as pemphigus.

Antimitochondrial autoantibodies in pemphigus vulgaris: a missing link in disease pathophysiology

A loss of epidermal cohesion in pemphigus vulgaris (PV) results from autoantibody action on keratinocytes (KCs) activating the signaling kinases and executioner caspases that damage KCs, causing their shrinkage, detachment from neighboring cells, and rounding up (apoptolysis). In this study, we found that PV antibody binding leads to activation of epidermal growth factor receptor kinase, Src, p38 MAPK, and JNK in KCs with time pattern variations from patient to patient. Both extrinsic and intrinsic apoptotic pathways were also activated. Although Fas ligand neutralizing antibody could inhibit the former pathway, the mechanism of activation of the latter remained unknown. PV antibodies increased cytochrome c release, suggesting damage to mitochondria. The immunoblotting experiments revealed penetration of PVIgG into the subcellular mitochondrial fraction. The antimitochondrial antibodies from different PV patients recognized distinct combinations of antigens with apparent molecular sizes of 25, 30, 35, 57, 60, and 100 kDa. Antimitochondrial antibodies were pathogenic because their absorption abolished the ability of PVIgG to cause keratinocyte detachment both in vitro and in vivo. The downstream signaling of antimitochondrial antibodies involved JNK and late p38 MAPK activation, whereas the signaling of anti-desmoglein 3 (Dsg3) antibody involved JNK and biphasic p38 MAPK activation. Using KCs grown from Dsg3(-/-) mice, we determined that Dsg3 did not serve as a surrogate antigen allowing antimitochondrial antibodies to enter KCs. The PVIgG-induced activation of epidermal growth factor receptor and Src was affected neither in Dsg3(-/-) KCs nor due to absorption of antimitochondrial antibodies. These results demonstrated that apoptolysis in PV is a complex process initiated by at least three classes of autoantibodies directed against desmosomal, mitochondrial, and other keratinocyte self-antigens. These autoantibodies synergize with the proapoptotic serum and tissue factors to trigger both extrinsic and intrinsic pathways of cell death and break the epidermal cohesion, leading to blisters. Further elucidation of the primary signaling events downstream of PV autoantigens will be crucial for the development of a more successful therapy for PV patients.

Pemphigus in the XXI Century: New life to an old story

In this new century of pemphigus research, the search for novel treatments is switching from a monospecific approach, focused on immunosuppression, to a polyspecific approach that includes drugs acting on novel pathophysiologic pathways. Current research argues that acantholysis in pemphigus occurs as an active process resulting from intracellular signaling triggered as a result of IgG binding to the keratinocyte membrane antigens in a receptor-ligand fashion. Recent progress regarding the pathophysiology of pemphigus acantholysis led to, or was accompanied by, breakthrough discoveries of safer treatments. Both the identification of cell-surface receptors to acetylcholine among the nondesmoglein (Dsg) targets for pemphigus antibodies, and the elucidation of the cholinergic control of keratinocyte cell adhesion provide an explanation for the therapeutic efficacy of cholinomimetics in patients with pemphigus. In patients' skin, Fas-L, TNFalpha, and, probably, IL-1alpha act as autocrine/paracrine co-factors for anti-keratinocyte IgG. Thus, it appears that an array of interconnected signaling cascades is responsible for acantholysis and cell death in pemphigus. Future studies should define the signaling pathways mediating acantholysis that occur in individual pemphigus patients and identify the membrane proteins (receptors) triggering signaling along a specific pathway upon their ligation by autoantibodies. It will be important to determine which pathway 1) leads directly to a loss of cell-cell adhesion (primary pathway), 2) which is being activated due to cell shrinkage/detachment (secondary pathway), 3) which contributes to utilization of altered proteins and organelles (scavenging pathway), and 4) which represents the cell defense (protective pathway). To dissect out the signaling pathways originating from binding of pemphigus IgG to non-Dsg targets on the keratinocyte plasma membrane experiments should be performed in cultures of murine keratinocytes grown from the Dsg3-/- mice or human keratinocytes with the knocked-down expression of the Dsg1 and/or Dsg3 gene by the RNA interference.

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.

An imbalance in Akt/mTOR is involved in the apoptotic and acantholytic processes in a mouse model of pemphigus vulgaris

Pemphigus vulgaris (PV) is an autoimmune blistering disease characterized by the presence of IgG autoantibodies against Dsg3. Our aim was to investigate the molecular events implicated in the development and localization of apoptosis and acantholysis in PV. We used a passive transfer mouse model together with immunohistochemical (IHC) techniques and the TUNEL assay, with quantification analysis in the basal layer of the epidermis. The activated signalling molecules analysed and apoptotic cells detected showed an identical localization. Herein, we found for the first time in vivo an increased expression of activated HER receptor isoforms in the basal layer in PV lesions. Besides, we observed the almost total lack of activated Akt compared with a higher level of activated mTOR within the basal cells of the epidermis. Our observations strongly support that the restriction of acantholysis to the basal layer may be due, at least in part, to the selective and increased presence of activated HER receptor isoforms in these cells. After phosphorylation of HER receptor isoforms, intracellular signalling pathways are activated in the basal layer. In addition, the imbalance in Akt/mTOR that takes place in the basal cells may provide intracellular signals necessary for the development of apoptosis and acantholysis.

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.