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.

Extracellular cleavage of collagen XVII is essential for correct cutaneous basement membrane formation

In skin, basal keratinocytes in the epidermis are tightly attached to the underlying dermis by the basement membrane (BM). The correct expression of hemidesmosomal and extracellular matrix (ECM) proteins is essential for BM formation, and the null-expression of one molecule may induce blistering diseases associated with immature BM formation in humans. However, little is known about the significance of post-translational processing of hemidesmosomal or ECM proteins in BM formation. Here we show that the C-terminal cleavage of hemidesmosomal transmembrane collagen XVII (COL17) is essential for correct BM formation. The homozygous p.R1303Q mutation in COL17 induces BM duplication and blistering in humans. Although laminin 332, a major ECM protein, interacts with COL17 around p.R1303, the mutation leaves the binding of both molecules unchanged. Instead, the mutation hampers the physiological C-terminal cleavage of COL17 in the ECM. Consequently, non-cleaved COL17 ectodomain remnants induce the aberrant deposition of laminin 332 in the ECM, which is thought to be the major pathogenesis of the BM duplication that results from this mutation. As an example of impaired cleavage of COL17, this study shows that regulated processing of hemidesmosomal proteins is essential for correct BM organization in skin.

Context-Dependent Regulation of Collagen XVII Ectodomain Shedding in Skin

Pemphigoid is a common autoimmune blistering disorder in which autoantibodies target transmembrane collagen XVII (COL17), a component of hemidesmosomes in basal keratinocytes. The ectodomain of COL17 can be cleaved from the cell surface within the juxtamembranous extracellular NC16A domain, and, interestingly, certain autoantibodies of pemphigoid patients preferentially react with the shed ectodomain. These findings suggest that COL17 ectodomain shedding generates neoepitopes on the shed form; however, the regulatory mechanism of the shedding in in vivo skin and the pathogenicity of the neoepitope-targeting antibodies still are uncertain. To address these issues, we produced rabbit antibodies specifically reacting with N-terminal cleavage sites of the shed COL17 ectodomain. The antibodies showed that certain amounts of the human COL17 ectodomain are cleaved physiologically at Gln(525) in in vivo skin. In contrast, migrating human keratinocytes cleave COL17 at Leu(524) but not at Gln(525). The passive transfer of antibodies reacting with an N-terminal cleavage site of the mouse COL17 ectodomain into neonatal wild-type mice failed to induce blister formation, even though the antibodies bound to the dermal-epidermal junctions, indicating that cleavage site-specific antibodies have reduced or absent pathogenicity for blister formation. This study shows the ectodomain shedding of COL17 to be a physiological event in in vivo human skin that probably generates nonpathologic epitopes on the cleavage sites.

[Clinical and molecular findings of pachyonychia congenita type 2 (PC-2)]

Pachyonychia congenita is a group of autosomal dominant inheritance pattern disorders characterized by hypertrophic nail dystrophy There are two main clinical subtypes: type 1 and 2. Pachyonychia congenita type 2 is readily differentiated from type 1 by multiple steatocysts and/or presence of natal teeth and can be confirmed by mutations of KRT6B and KRT17. We report the case of a 33-year-o/d female patient with the missense mutation in KRT17 gene (c.280C> T, p.Arg94Cys) and discuss the several clinical features found with this mutation in the literature.

In vivo analysis of IgE autoantibodies in bullous pemphigoid: a study of 100 cases

BACKGROUND

Bullous pemphigoid (BP) is an acquired autoimmune blistering disease characterized by subepidermal blister formation, in vivo linear deposition of immunoglobulin G (IgG) and complements at the dermal-epidermal junction (DEJ). The circulating IgG autoantibodies are directed against two epidermal hemidesmosomal glycoproteins: BP180, also known as type XVII collagen (COL17), and BP230. In addition, recent studies have shown that IgE autoantibodies may be involved in the pathogenesis of BP, although in vivo IgE deposition in lesional skin has not been fully characterized in large numbers of BP patients.

OBJECTIVE

This study investigated the incidence of in vivo deposition of IgE autoantibodies at the DEJ in lesional skin from a large number of BP patients.

METHODS

Peri-lesional skin samples from 100 patients who met the clinical and histopathological criteria for BP were investigated by direct immunofluorescence for the deposition of autoantibodies and complement. Patients' sera were also investigated by enzyme-linked immunosorbent assay and indirect immunofluorescence.

RESULTS

18% of BP patients were found to show IgE deposition at the DEJ. Disease severity, clinical course and outcome did not differ between IgE-positive and IgE-negative patients. In 3 IgE-positive cases, IgG was undetectable in vivo, and these cases showed atypical manifestations.

CONCLUSION

The results of in vivo IgE deposition may not be useful in predicting the disease course of BP, although predominant IgE deposition could alter the pattern of clinical manifestations.

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.

Anti-BP180-type mucous membrane pemphigoid: report of two cases

We describe two patients with anti-BP180-type mucous membrane pemphigoid (MMP), who were correctly diagnosed and treated in early stages through the cooperation of dentists and dermatologists. Patient 1 was a 74-year-old woman who visited our dental department due to blisters over the oral mucosa and eruptions on the skin. She had also experienced bleeding of the gingiva and palate mucosa. Biopsy specimens from the oral mucosa revealed detachment of epithelial basement membrane and subepithelial lamina propria with slight chronic inflammation. Direct immunofluorescence (DIF) revealed linear IgG and IgA deposits along the basement membrane zone (BMZ). Indirect immunofluorescence (IIF) using 1 M-NaCl split normal human skin showed binding of IgG and IgA on the epidermal side. On immunoblot analysis, IgG and IgA autoantibodies reacted with the C-terminal protein of BP180. These findings indicated a diagnosis of anti-BP180-type MMP. Patient 2 was a 59-year-old woman who was referred to our dental department with a history of blisters and large erosions on the gingiva. Biopsy specimens from the oral mucosa revealed partial junctional separation at the level of the basement membrane. DIF showed linear depositions of IgG and C3 along the BMZ. IIF, using 1 M-NaCl split normal human skin, revealed circulating anti-BMZ-IgG antibodies bound to the epidermal side. These findings indicated a diagnosis of anti-BP180-type MMP. Both patients were treated successfully with systemic or topical steroids and oral health care. In conclusion, appropriate clinical examination and cooperation among medical specialists are important for the early diagnosis and treatment of patients with recurrent and chronic stomatitis and for their good prognosis.

Highly prevalent SERPINB7 founder mutation causes pseudodominant inheritance pattern in Nagashima-type palmoplantar keratosis

BACKGROUND

Nagashima-type palmoplantar keratosis (NPPK) is a distinct autosomal recessive genodermatosis characterized by diffuse transgressive palmoplantar keratoderma (PPK). Very recently, putative loss-of-function mutations in SERPINB7, which encodes a member of the serine protease inhibitor superfamily and is abundantly expressed in the epidermis, have been identified as a cause of NPPK.

OBJECTIVES

To confirm further the role of SERPINB7 mutations in the pathogenesis of NPPK.

METHODS

We analysed 10 Japanese families with NPPK using Sanger and/or whole-exome sequencing.

RESULTS

We identified one novel and three recurrent null mutations in SERPINB7. In all the families, the NPPK trait was inherited in an autosomal recessive manner; in one of the families, there was pseudodominant inheritance, which had not been described in NPPK.

CONCLUSIONS

These data clearly provide further evidence that NPPK is caused by loss-of-function mutations in SERPINB7.

Paper-based ELISA for the detection of autoimmune antibodies in body fluid-the case of bullous pemphigoid

Bullous pemphigoid (BP), a common autoimmune blistering disease, is increasing in incidence and conveys a high mortality. Detection of autoantibodies targeting the noncollagenous 16A (NC16A) domain of type XVII collagen using enzyme-linked immunosorbent assay (ELISA) has demonstrated high sensitivity and specificity for diagnosing BP. We have developed a rapid, low-cost, and widely applicable ELISA-based system to detect the NC16A autoimmune antibody and then diagnose and monitor BP disease activity using a piece of filter paper, a wax-printer, and NC16A antigens. Both sera and/or blister fluids from 14 untreated BP patients were analyzed. The control group included healthy volunteers and patients with other blistering disorders such as pemphigus vulgaris. In our established paper-based ELISA (P-ELISA) system, only 2 μL of serum or blister fluid and 70 min were required to detect anti-NC16A autoimmune antibodies. The relative color intensity was significantly higher in the BP group than in the control groups when using either serum (P < 0.05) or blister fluid (P < 0.001) specimens from BP patients. The results of P- ELISA were moderately correlated with the titer of the commercial ELISA kit (MBL, Japan) (rho = 0.5680, P = 0.0011). This newly developed system allows for rapid and convenient diagnosis and/or monitoring of BP disease activity.

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.