A recombinant form of the human BP180 ectodomain forms a collagen-like homotrimeric complex

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.

The intracellular domain of BP180/collagen XVII is intrinsically disordered and partially folds in an anionic membrane lipid-mimicking environment

The trimeric transmembrane collagen BP180, also known as collagen XVII, is an essential component of hemidesmosomes at the dermal-epidermal junction and connects the cytoplasmic keratin network to the extracellular basement membrane. Dysfunction of BP180 caused by mutations in patients with junctional epidermolysis bullosa or autoantibodies in those with bullous pemphigoid leads to severe skin blistering. The extracellular collagenous domain of BP180 participates in the protein's triple-helical folding, but the structure and functional importance of the intracellular domain (ICD) of BP180 are largely unknown. In the present study, we purified and characterized human BP180 ICD. When expressed in Escherichia coli as glutathione-S-transferase or 6 × histidine tagged fusion protein, the BP180 ICD was found to exist as a monomer. Analysis of the secondary structure content by circular dichroism spectroscopy revealed that the domain is intrinsically disordered. This finding aligned with that of a bioinformatic analysis, which predicted a disordered structure. Interestingly, both anionic detergent micelles and lipid vesicles induced partial folding of the BP180 ICD, suggesting that in its natural environment, the domain's folding and unfolding may be regulated by interaction with the cell membrane or accompanying proteins. We hypothesize that the intrinsically disordered structure of the ICD of BP180 contributes to the mechanism that allows the remodeling of hemidesmosome assembly.

Coiled coils ensure the physiological ectodomain shedding of collagen XVII

α-Helical coiled coils, frequent protein oligomerization motifs, are commonly observed in vital proteins. Here, using collagen XVII as an example, we provide evidence for a novel function of coiled coils in the regulation of ectodomain shedding. Transmembrane collagen XVII, an epithelial cell surface receptor, mediates dermal-epidermal adhesion in the skin, and its dysfunction is linked to human skin blistering diseases. The ectodomain of this collagen is constitutively shed from the cell surface by proteinases of a disintegrin and metalloprotease family; however, the mechanisms regulating shedding remain elusive. Here, we used site-specific mutagenesis to target the coiled-coil heptad repeats within the juxtamembranous, extracellular noncollagenous 16th A (NC16A) domain of collagen XVII. This resulted in a substantial increase of ectodomain shedding, which was not mediated by disintegrin and metalloproteases. Instead, conformational changes induced by the mutation(s) unmasked a furin recognition sequence that was used for cleavage. This study shows that apart from their functions in protein oligomerization, coiled coils can also act as regulators of ectodomain shedding depending on the biological context.

FcR-independent effects of IgE and IgG autoantibodies in bullous pemphigoid

Bullous pemphigoid (BP) is a subepidermal blistering disease characterized by IgE and IgG class autoantibodies specific for 180-kDa BP Ag 2 (BP180), a protein involved in cell-substrate attachment. Although some direct effects of BP IgG have been observed on keratinocytes, no study to date has examined direct effects of BP IgE. In this study, we use primary cultures of human keratinocytes to demonstrate Ag-specific binding and internalization of BP IgE. Moreover, when BP IgE and BP IgG were compared, both isotypes stimulated FcR- independent production of IL-6 and IL-8, cytokines critical for BP pathology, and elicited changes in culture confluence and viability. We then used a human skin organ culture model to test the direct effects of these Abs on the skin, whereas excluding the immune inflammatory processes that are triggered by these Abs. In these experiments, physiologic concentrations of BP IgE and BP IgG exerted similar effects on human skin by stimulating IL-6 and IL-8 production and decreasing the number of hemidesmosomes localized at the basement membrane zone. We propose that the Ab-mediated loss of hemidesmosomes could weaken attachment of basal keratinocytes to the basement membrane zone of affected skin, thereby contributing to blister formation. In this article, we identify a novel role for IgE class autoantibodies in BP mediated through an interaction with BP180 on the keratinocyte surface. In addition, we provide evidence for an FcR-independent mechanism for both IgE and IgG class autoantibodies that could contribute to BP pathogenesis.

Type XVII collagen (BP180) can function as a cell-matrix adhesion molecule via binding to laminin 332

Collagen XVII (COL17) is a transmembrane glycoprotein that is expressed on the basal surface of basal epidermal keratinocytes. Previous observations have led to the hypothesis that an interaction between COL17 and laminin 332, an extracellular matrix protein, contributes to the attachment of the basal keratinocyte to the basement membrane. In order to isolate and manipulate COL17 interactions with ECM components, we induced COL17 expression in two cells lines, SK-MEL1 and K562, that exhibit little or no capacity to attach to our test substrates, including laminin 332, types I and IV collagen, and fibronectin. Cells expressing high levels of COL17 preferentially adhered to a laminin 332 matrix, and, to a lesser extent, type IV collagen, while showing little or no binding to type I collagen or fibronectin. A quantitative analysis of cell adhesive forces revealed that, compared with COL17-negative cells, COL17-positive cells required over 7-fold greater force to achieve 50% detachment from a laminin 332 substrate. When a cell preparation (either K562 or SK-MEL1) with heterogeneous COL17 expression levels was allowed to attach to a laminin 332 matrix, the COL17-positive and COL17-negative cells differentially sorted to the bound and unbound cell fractions, respectively. COL17-dependent attachment to laminin 332 could be reduced or abolished by siRNA-mediated knock-down of COL17 expression or by adding to the assay wells specific antibodies against COL17 or laminin 332. These findings provide strong support for the hypothesis that cell surface COL17 can interact with laminin 332 and, together, participate in the adherence of a cell to the extracellular matrix.

Pregnant women have increased incidence of IgE autoantibodies reactive with the skin and placental antigen BP180 (type XVII collagen)

BP180 (type XVII collagen) is a transmembrane protein expressed in a variety of cell types. It is also the target of autoantibodies in cutaneous autoimmune disease including bullous pemphigoid and pemphigoid gestationis, a disease unique to pregnancy. The purpose of this study was to determine the prevalence and specificity of cutaneous autoantibodies in a cohort of pregnant women. De-identified sera were collected from pregnant women (n=299) and from non-pregnant controls (n=134). Sera were analyzed by ELISA for the presence of IgG and IgE autoantibodies directed against several cutaneous autoantigens. IgE antibodies against the NC16A domain of BP180 were detected in 7.7% of pregnant women, compared to 2.2% of healthy controls (p=0.01). No increase in total or cutaneous autoantigen specific IgG was seen. Total serum IgE was within the normal range. Full-length BP180 was detected by western immunoblot in epidermal, keratinocyte, placental and cytotrophoblast (CTB) cell lysates. Furthermore, flow cytometry and indirect immunofluorescence confirmed the expression of BP180 on the surface of cultured CTBs. Finally, it was demonstrated that IgE antibodies in the pregnancy sera labeled not only cultured CTBs, but also the placental amnion and cutaneous basement membrane zone using indirect immunofluorescence. We conclude that some pregnant women develop antibodies specific for BP180, and that these autoantibodies are capable of binding both CTB and the placental amnion, potentially affecting placental function.

Subepidermal blistering induced by human autoantibodies to BP180 requires innate immune players in a humanized bullous pemphigoid mouse model

Bullous pemphigoid (BP) is a cutaneous autoimmune inflammatory disease associated with subepidermal blistering and autoantibodies against BP180, a transmembrane collagen and major component of the hemidesmosome. Numerous inflammatory cells infiltrate the upper dermis in BP. IgG autoantibodies in BP fix complement and target multiple BP180 epitopes that are highly clustered within a non-collagen linker domain, termed NC16A. Anti-BP180 antibodies induce BP in mice. In this study, we generated a humanized mouse strain, in which the murine BP180NC14A is replaced with the homologous human BP180NC16A epitope cluster region. We show that the humanized NC16A (NC16A+/+) mice injected with anti-BP180NC16A autoantibodies develop BP-like subepidermal blisters. The F(ab')(2) fragments of pathogenic IgG fail to activate the complement cascade and are no longer pathogenic. The NC16A+/+ mice pretreated with mast cell activation blocker or depleted of complement or neutrophils become resistant to BP. These findings suggest that the humoral response in BP critically depends on innate immune system players.

The pathophysiology of bullous pemphigoid

Bullous pemphigoid (BP) is a blistering skin disease characterized by an autoimmune response to 2 hemidesmosomal proteins within the dermal-epidermal junction, designated BP180 and BP230. While BP230 localizes intracellularly and associates with the hemidesmosomal plaque, BP180 is a transmembrane glycoprotein with an extracellular domain. Most BP patients have autoantibodies binding to an immunodominant region of BP180, the noncollagenous 16A domain (NC16A), which is located extracellularly close to the transmembrane domain of the protein. Autoreactive T and B cell responses to BP180 have been found in patients with BP. Passive transfer of antibodies to the murine BP180 ectodomain triggers a blistering skin disease in mice that closely mimics human BP. Lesion formation in this animal model depends upon complement activation, mast cell degranulation and accumulation of neutrophils and eosinophils. Patients' autoantibodies to BP180 induce dermal-epidermal separation in cryosections of human skin when co-incubated with leukocytes. The loss of cell-matrix adhesion is mediated by proteinases released by granulocytes. The increased knowledge of the pathophysiology of BP should facilitate the development of novel therapeutic strategies for this disease.

The NC16A domain of collagen XVII plays a role in triple helix assembly and stability

Collagen XVII/BP180 is a transmembrane constituent of the epidermal anchoring complex. To study the role of its non-collagenous linker domain, NC16A, in protein assembly and stability, we analyzed the following recombinant proteins: the collagen XVII extracellular domain with or without NC16A, and a pair of truncated proteins comprising the COL15-NC15 stretch expressed with or without NC16A. All four proteins were found to exist as stable collagen triple helices; however, the two missing NC16A exhibited melting temperatures significantly lower than their NC16A-containing counterparts. Protein refolding experiments revealed that the rate of triple helix assembly of the collagen model peptide GPP(10) is greatly increased by the addition of an upstream NC16A domain. In summary, the NC16A linker domain of collagen XVII exhibits a positive effect on both the rate of assembly and the stability of the adjoining collagen structure.

Role of different pathways of the complement cascade in experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease associated with autoantibodies directed against the hemidesmosomal proteins BP180 and BP230 and inflammation. Passive transfer of antibodies to the murine BP180 (mBP180) induces a skin disease that closely resembles human BP. In the present study, we defined the roles of the different complement activation pathways in this model system. Mice deficient in the alternative pathway component factor B (Fb) and injected with pathogenic anti-mBP180 IgG developed delayed and less intense subepidermal blisters. Mice deficient in the classical pathway component complement component 4 (C4) and WT mice pretreated with neutralizing antibody against the first component of the classical pathway, C1q, were resistant to experimental BP. These mice exhibited a significantly reduced level of mast cell degranulation and polymorphonuclear neutrophil (PMN) infiltration in the skin. Intradermal administration of compound 48/80, a mast cell degranulating agent, restored BP disease in C4(-/-) mice. Furthermore, C4(-/-) mice became susceptible to experimental BP after local injection of PMN chemoattractant IL-8 or local reconstitution with PMNs. These findings provide the first direct evidence to our knowledge that complement activation via the classical and alternative pathways is crucial in subepidermal blister formation in experimental BP.

Localisation of bullous pemphigoid antigen 180 (BP180) in cultured human keratinocytes: functionally relevant modification by calcium

The expression of BP180 has previously been demonstrated to be influenced by both calcium (Ca(2+)) concentration and binding of anti-BP180-antibodies in cultured keratinocytes of the skin squamous cell carcinoma line DJM-1. Here, BP180 expression was studied in cultured normal human epidermal keratinocytes by confocal laser scanning microscopy. We exploited an experimental system, in which BP180 was previously shown to mediate, upon incubation with anti-BP180 antibodies, a specific signal-transducing event that leads to the release of inflammatory mediators, such as IL-8 from cultured normal human epidermal keratinocytes (NHEK). We found that without addition of BP180-specific IgG, BP180 is predominantly expressed on the cell surface irrespective of the Ca(2+) concentration. In contrast, cell surface BP180 was greatly reduced in NHEK kept in high Ca(2+) medium after incubation with BP180-specific IgG for 12 h compared to low Ca(2+) medium. This effect was seen with antibodies to both N- and C-terminal fragments of the BP180 ectodomain, respectively. In addition, a slightly higher BP180 expression was found in NHEK cultured in low compared to high Ca(2+) medium by Western blotting. Interestingly, in contrast to NHEK kept under low Ca(2+ )conditions, in NHEK grown in high Ca(2+) medium, no elevated levels of IL-8 were released after treatment of cells with anti-BP180 IgG compared to normal IgG. Our data indicate that the Ca(2+)-modulated expression of BP180 is functionally relevant. This finding sheds further light on the complex pathomechanism in blister formation of BP180-related autoimmune blistering skin diseases.

Métodos diagnósticos en las enfermedades ampollosas subepidérmicas autoinmunes

There is considerable clinical and histological overlap among the subepidermal autoimmune bullous diseases. The knowledge at the molecular level of the dermo-epidermal junction is essential to understand this group of diseases. The immune-based techniques have contributed to increase the knowledge of these entities and have been progressively incorporated into clinical practice. In this review of the diagnostic methods of subepidermal autoimmune bullous diseases we summarize the most recent advances on the molecular biology of the dermo-epidermal junction, focusing on the immune-based diagnostic techniques. We distinguish two main groups of diagnostic methods: those that detect autoimmune deposits in the skin (direct immunofluorescence and its variants including confocal microscopy) and those that detect antibodies in serum or in other fluids (indirect immunofluorescence and its variants, ELISA, immunoblot and immunoprecipitation). We explain the methodology and diagnostic keys of the techniques most widely applied in our milieu.

Differential roles for beta2 integrins in experimental autoimmune bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune disease associated with autoantibodies directed against the hemidesmosomal antigens anti-BP230 and anti-B180. Neonatal mice injected with rabbit anti-mouse BP180 (mBP10) IgG develop a BP-like disease. Complement, immune complexes, mast cells, and neutrophils play a key role in subepidermal blistering in this animal model. In this study we investigated the role of beta2 integrins in experimental BP. Wild-type (WT) mice pretreated with neutralizing antibody against CD11a (LFA-1), CD11b (Mac-1), CD11a plus CD11b, or CD18 alone failed to develop BP when injected with pathogenic anti-mBP180 IgG. This was associated with a significant reduction in neutrophil accumulation in neutralizing antibody-treated mice. Mac-1-deficient (Mac-1 knockout [KO]) mice were resistant to experimental BP despite normal complement deposition and mast cell and neutrophil degranulation. Neutrophil infiltration in Mac-1 KO mice was severely impaired at 24 hours. However, more neutrophils accumulated in the skin of Mac-1 KO mice compared with WT mice at early time points (2-4 hours), which was associated with an increase in their survival as determined by apoptosis markers. These data suggest that beta2 integrins play differential roles in experimental BP: LFA-1 is required for neutrophil recruitment, while Mac-1 mediates late neutrophil accumulation and apoptosis of infiltrating neutrophils.

Number 1Epithelial biology

The oral mucous membrane has features similar to skin but also differs in several ways. This paper reviews the aspects of epithelial biology necessary for an understanding of the vesiculoerosive disorders.

A patient with both bullous pemphigoid and epidermolysis bullosa acquisita: an example of intermolecular epitope spreading

Bullous pemphigoid (BP) and epidermolysis bullosa acquisita are distinct autoimmune blistering disorders. BP is characterized by autoantibodies directed against the NC16A domain of collagen XVII, whereas patients with epidermolysis bullosa acquisita have autoantibodies against the NC1 domain of type VII collagen. We followed up a patient with BP for 9 years. During that time his clinical disease took on several features suggestive of epidermolysis bullosa acquisita. The objective of this study was to determine if the patient's autoantibody profile reflected the change in his clinical picture. Enzyme-linked immunosorbent assay and immunoblotting for detection and subclass determination of autoantibodies to type XVII and type VII collagen were performed on banked patient sera from the 9-year period. The patient's initial autoantibodies were exclusively IgG1 directed against collagen XVII. During the course of his illness, the subclass specificity of the patient's type XVII collagen autoantibodies shifted to the IgG4 subclass and during the same time interval the patient developed IgG2 autoantibodies directed against type VII collagen. This patient with BP exhibited both subclass shifting and development of a second autoantibody system that correlated with a change in the clinical appearance of the disease. The analysis of the patient's autoantibodies provides strong evidence for the involvement of epitope spreading in the evolution of his autoimmune disease.

Linear IgA dermatosis with IgA and IgG autoantibodies to the 180 kDa bullous pemphigoid antigen (BP180): evidence for a distinct subtype

BACKGROUND

Autoantibodies in linear immunoglobulin A (IgA) disease (LAD) are reported to be of IgA class and directed against a 97-120 kDa epidermal antigen.

METHODS

We report a 39-year-old woman with clinical features of LAD and with circulating IgA and IgG autoantibodies to the 180 kDa bullous pemphigoid antigen (BP180).

RESULTS

Histopathology of lesional skin revealed a subepidermal blister with mixed inflammatory cell infiltrate. Direct immunofluorescence of perilesional skin showed linear deposits of IgA along the dermal-epidermal junction. The antigen specificity of the patient's circulating antibodies was determined by Western blotting and enzyme-linked immunoabsorbent assay (ELISA) using various antigen sources, including cultured human keratinocytes, dermal protein lysates, and purified laminin-5, as well as proteins corresponding to BP180, the 230 kDa bullous pemphigoid antigen (BP230), laminin-5 subunits, and collagen IV alpha1-alpha6 chains. IgA and IgG antibodies in the patient's serum were directed against BP180, and no IgA or IgG reactivity was found against the other skin antigens.

CONCLUSIONS

These data provide evidence for the presence of a subtype of LAD with dual IgA and IgG autoimmune response to BP180.

Elevated expression and release of tissue-type, but not urokinase-type, plasminogen activator after binding of autoantibodies to bullous pemphigoid antigen 180 in cultured human keratinocytes

In bullous pemphigoid (BP), the binding of BP180-specific antibodies to their hemidesmosomal target antigen is not sufficient for blister formation, but must be accompanied by the release of proteases. Using plasminogen activator (PA) knock-out mice, the PA system has previously been shown to be a prerequisite for blister formation in experimental murine BP. Here, we found elevated levels of plasmin and tPA, but not of uPA, in blister fluid from BP patients (n = 7) compared to blisters from patients with toxic epidermal necrolysis (n = 4) and suction blisters in healthy controls (n = 7). Subsequently, we addressed the question whether keratinocytes release PA in response to the binding of anti-BP180 antibodies. Treatment of cultured normal human keratinocytes with BP IgG, but not with control IgG, led to both increased protein and mRNA levels of tPA, but not of uPA, as determined by ELISA and RT-PCR, respectively. The specificity of this finding was confirmed using BP180-deficient keratinocytes from a patient with generalized atrophic benign epidermolysis bullosa, where no tPA release was observed after stimulation with BP IgG. Our results show the elevated expression and release of tPA from normal human keratinocytes upon stimulation with antibodies to human BP180. Keratinocytes, by secreting tPA, may thus play an active role in blister formation of BP.

Hemidesmosomes: molecular organization and their importance for cell adhesion and disease

In the skin, basal epithelial cells constantly divide to renew the epidermis. The newly formed epithelial cells then differentiate in a process called keratinization, ultimately leading to the death of these cells and a pile-up of cell material containing vast amounts of keratins. The basal keratinocytes in skin are attached to their underlying basement membrane via specialized adhesion complexes termed hemidesmosomes (HDs). These complexes ascertain stable adhesion of the epidermis to the dermis, and mutations in components of these complexes often result in tissue fragility and blistering of the skin. In this review, we will describe the various hemidesmosomal proteins in detail as well as, briefly, the protein families to which they belong. Specifically, we will report the protein-protein interactions involved in the assembly of hemidesmosomes and their molecular organization. Some signaling pathways involving primarily the alpha6beta4 integrin will be discussed, since they appear to profoundly modulate the assembly and function of hemidesmosomes. Furthermore, the importance of these hemidesmosomal components for the maintenance of tissue homeostasis and their involvement in various clinical disorders will be emphasized. Finally, we will present a model for the assembly of HDs, based on our present knowledge.

A founder COL17A1 splice site mutation leading to generalized atrophic benign epidermolysis bullosa in an extended inbred Palestinian family from Israel

PURPOSE

Generalized atrophic benign epidermolysis bullosa is a nonlethal form of junctional EB with an autosomal recessive inheritance. There is generalized cutaneous blister formation at sites of trauma, atrophic alopecia affecting scalp, eyelash and eyebrow, dystrophic nail changes, and tooth abnormalities. In this study, we have studied a five-generation Palestinian family affected with generalized atrophic benign epidermolysis bullosa.

METHODS

We have performed linkage analysis to genes that are mutated in generalized atrophic benign epidermolysis bullosa, followed by direct sequencing of patient genomic DNA.

RESULTS

We have shown that the disease is caused by a newly detected homozygous donor splice site mutation, IVS51+1G>A, in the type XVII collagen gene, COL17A1.

CONCLUSION

The effect of a founder mutation introduced 3 to 4 generations before a disease appearance is demonstrated in this inbred family.

Alpha-helical coiled-coil oligomerization domains are almost ubiquitous in the collagen superfamily

Alpha-helical coiled-coils are widely occurring protein oligomerization motifs. Here we show that most members of the collagen superfamily contain short, repeating heptad sequences typical of coiled coils. Such sequences are found at the N-terminal ends of the C-propeptide domains in all fibrillar procollagens. When fused C-terminal to a reporter molecule containing a collagen-like sequence that does not spontaneously trimerize, the C-propeptide heptad repeats induced trimerization. C-terminal heptad repeats were also found in the oligomerization domains of the multiplexins (collagens XV and XVIII). N-terminal heptad repeats are known to drive trimerization in transmembrane collagens, whereas fibril-associated collagens with interrupted triple helices, as well as collagens VII, XIII, XXIII, and XXV, were found to contain heptad repeats between collagen domains. Finally, heptad repeats were found in the von Willebrand factor A domains known to be involved in trimerization of collagen VI, as well as in collagen VII. These observations suggest that coiled-coil oligomerization domains are widely used in the assembly of collagens and collagen-like proteins.

Type XIII collagen and some other transmembrane collagens contain two separate coiled-coil motifs, which may function as independent oligomerization domains

Type XIII collagen is a homotrimeric transmembrane collagen composed of a short intracellular domain, a single membrane-spanning region, and an extracellular ectodomain with three collagenous domains (COL1-3) separated by short non-collagenous domains (NC1-4). Several collagenous transmembrane proteins have been found to harbor a conserved sequence next to their membrane-spanning regions, and in the case of type XIII collagen this sequence has been demonstrated to be important for chain association. We show here that this 21-residue sequence is necessary but not sufficient for NC1 association. Furthermore, the NC1 association region was predicted to form an alpha-helical coiled-coil structure, which may already begin at the membrane-spanning region, as is also predicted for the related collagen types XXIII and XXV. Interestingly, a second coiled-coil structure is predicted to be located in the NC3 domain of type XIII collagen and in the corresponding domains of types XXIII and XXV. It is found experimentally that the absence of the NC1 coiled-coil domain leads to a lack of disulfide-bonded trimers and misfolding of the membrane-proximal collagenous domain COL1, whereas the COL2 and COL3 domains are correctly folded. We suggest that the NC1 coiled-coil domain is important for association of the N-terminal part of the type XIII collagen alpha chains, whereas the NC3 coiled-coil domain is implicated in the association of the C-terminal part of the molecule. All in all, we propose that two widely separated coiled-coil domains of type XIII and related collagens function as independent oligomerization domains participating in the folding of distinct areas of the molecule.

A highly sensitive and simple assay for the detection of circulating autoantibodies against full-length bullous pemphigoid antigen 180

Bullous pemphigoid antigen 180 (BP180) is the target of autoantibodies in various subepidermal blistering diseases. The most common one is bullous pemphigoid (BP). The pathological importance of anti-BP180 antibodies has been demonstrated in a passive transfer mouse model. However, sensitive assays for routinely detecting circulating antibodies directed against both intra- and extracellular domains of BP180 are only available in specialized laboratories. In addition, most current assays use prokaryotic recombinant fragments of BP180 that lack conformation-dependent epitopes. A simple and very sensitive immunofluorescence (IF) assay based on eukaryotic cells is described here. Sf21 insect cells were transfected with full-length (FL) BP180. As revealed by FACS and confocal laser scanning microscopy the protein was expressed as type II transmembrane protein as in human keratinocytes. By testing serial dilutions of BP180-specific mouse monoclonal antibodies, the eukaryotic IF assay was demonstrated to be more sensitive compared to conventional assays including (1) indirect IF microscopy of human salt-split skin, (2) Western blotting (WB) of the keratinocyte-derived BP180 ectodomain, (3) WB of recombinant BP180 NC16A, and (4) WB of FL-BP180 extracted from Sf21 insect cells. When applied to sera from patients with BP (n = 65), pemphigoid gestationis (n = 16), and cicatricial pemphigoid (n = 7), the novel assay revealed that 58 (89%), 13 (81%), and 6 (84%), respectively, were positive. In contrast, all control sera (pemphigus, n = 20; epidermolysis bullosa acquisita, n = 5; anti-laminin 5 cicatricial pemphigoid, n = 5; systemic lupus erythematosus, n = 5; atopic dermatitis, n = 7; contact dermatitis, n = 3; normal human sera, n = 30) were negative indicating that the assay is highly specific. In addition, reactivity of the assay was conserved to a large extent when the cells had been stored at -20 degrees C for 3 months. Thus, this assay meets the demands of a simple and effective diagnostic tool for detecting circulating antibodies against FL-BP180 and may also be used in laboratories without access to molecular biological technology.

Cloning of hamster type XVII collagen cDNA, and pathogenesis of anti-type XVII collagen antibody and complement in hamster bullous pemphigoid

Bullous pemphigoid is an inflammatory subepidermal blistering skin disease associated with an IgG autoimmune response to the type XVII collagen. The immunopathologic features of bullous pemphigoid can be reproduced in mice by the passive transfer of anti-type XVII collagen antibodies. In this model, it is thought that blister formation depends upon complement activation, neutrophil recruitment, and some proteolytic enzymes. In this study, we cloned hamster type XVII collagen cDNA, which contains a 4296 bp coding region and which is predicted to be a transmembrane protein with an extracellular collagenous domain, residing in type II orientation. Antipeptide antibodies (anti-1191 IgG) were obtained against a segment of hamster type XVII collagen homologous with the human type XVII collagen autoantibody-reactive site. The antipeptide antibodies were passively transferred to neonatal Syrian hamsters. The injected hamsters developed a microscopic subepidermal blister as seen previously in the mice. In order to test whether antigen-antibody complexes and complement initiate the subepidermal blister formation, we carried out experiments in vitro on condition that inflammatory cells were completely eliminated. Complement activation in sera was inhibited either by heating (at 56 degrees C for 30 min) or by preincubating with cobra venom factor. When the hamster skin was incubated with fresh anti-1191 antisera, separation of dermal-epidermal junction was observed. The anti-1191 IgG failed to induce C3 deposition and dermal-epidermal junction separation, however, if the anti-1191 IgG was added alone or complement activation in sera was inhibited. Under these conditions, IgG but not C3 was deposited on the basement membrane. These results strongly suggest that antigen-antibody complexes and complement initiate dermal-epidermal junction separation.

Cicatricial pemphigoid with circulating IgA and IgG autoantibodies to the central portion of the BP180 ectodomain: beneficial effect of adjuvant therapy with high-dose intravenous immunoglobulin

Cicatricial pemphigoid (CP) is an autoimmune subepidermal blistering disease characterized by deposits of IgG, IgA, or C3 at the cutaneous basement membrane zone. CP may present with considerable variation regarding age, morphology of lesions, and mucosal involvement, which may heal with or without scarring. We describe a patient with CP who presented with circulating IgA and IgG autoantibodies to the epidermal side of salt-split human skin. By immunoblot analysis, the patient's IgA reacted with the soluble ectodomain of BP180 (LAD-1). This reactivity was mainly directed to the central portion of the BP180 ectodomain, a site that, to date, has not been described as the target of IgA autoantibodies. Different immunosuppressive treatment regimens including steroids and mycophenolate mofetil did not control this patient's disease, and severe scarring of the conjunctivae occurred with impairment of vision. Addition of adjuvant intravenous immunoglobulin (1 g/kg body weight on 2 consecutive days) every 4 weeks led to a dramatic improvement of conjunctivitis and gingivitis. Clinical improvement correlated with the serum's IgA immunoblot reactivity against LAD-1. Further studies on a larger number of patients with CP should try to correlate the specificity of autoantibodies in CP with the response to certain therapeutic regimens.

Cicatricial pemphigoid differs from bullous pemphigoid and pemphigoid gestationis regarding the fine specificity of autoantibodies to the BP180 NC16A domain

Bullous pemphigoid (BP), pemphigoid (herpes) gestationis (PG), cicatricial pemphigoid (CP), and lichen planus pemphigoides (LPP) are autoimmune subepidermal bullous diseases that are characterized by circulating autoantibodies to the transmembrane hemidesmosomal protein BP180/type XVII collagen. Previous studies demonstrated that the majority of patients with BP, PG, and LPP show antibodies to an immunodominant, membrane-proximal non-collagenous domain (NC16A) on the extracellular portion of BP180. By the use of non-overlapping peptides of the NC16A domain, we previously demonstrated that autoantibodies from BP and PG patients mainly react with epitopes clustered within the N-terminus of this immunodominant site of BP180; antibodies from patients with LPP also recognized the C-terminal portion of NC16A. However, some of these results had been obtained indirectly by preadsorption studies. The aim of the present study was to analyze the fine specificity of IgG autoantibodies to NC16A in sera from patients with CP and to compare their reactivity with antibodies from BP, PG, and LPP patients using a series of new overlapping fragments covering the entire NC16A domain. We confirm that BP and PG sera mainly react with N-terminal epitopes of NC16A, whereas sera from patients with LPP also bind to C-terminal portions, of this domain. Interestingly, out of ten patients with CP, the sera of seven reacted with NC16A; within NC16A, these sera bound to both C-terminal fragments and an N-terminal epitope right next to the cell membrane. Our data demonstrate a heterogeneous binding pattern of autoantibodies to BP180 NC16A in patients with CP.

Interactions of a hemidesmosome component and actinin family members

Hemidesmosomes are multimeric protein complexes that attach epithelial cells to their underlying matrix and serve as cell surface anchorage sites for the keratin cytoskeleton. Two hemidesmosome components, the alpha6beta4 integrin heterodimer and a human autoantigen termed BP180, are transmembrane proteins that link the extracellular matrix to the keratin network in cells. Here, we report that actinin-4, an actin-bundling protein, is a potential binding partner for BP180. Using yeast two-hybrid, we have mapped the binding site for BP180 to the C-terminal region of actinin-4. This site contains two EF-hand, Ca2+ regulation domains and shares 87% sequence homology with the same region in actinin-1. Consistent with this, BP180 can bind actinin-1 in both the yeast two-hybrid assay and in immunoprecipitation assays. To determine whether the EF-hand domain is a consensus binding sequence for BP180, we tested whether other proteins with this domain bind BP180. None of the proteins tested including calmodulin, with 4 EF-hand domains, and myosin regulatory light chain, with 1 EF-hand domain, interacts with BP180 in yeast two-hybrid system and immunoprecipitation studies, suggesting that the interaction between BP180 and actinin family members is specific. We have compared the distribution of actinin-1 and actinin-4 with that of BP180 in MCF-10A and pp126 cells. Surprisingly, BP180 localizes not only to sites of cell-substratum interaction, but is also present at sites of cell-cell contacts where it co-distributes with both actinin-1 and actinin-4 as well as other adherens junction proteins. In oral tissues, BP180 is present along the basement membrane and at cell-cell contact sites in basal epithelial cells where it co-distributes with adherens junction proteins. Since BP180 antibodies inhibit association of junction proteins at sites of cell-cell contact in oral keratinocytes, these results suggest that BP180 may play a role in establishing cell-cell interactions. We discuss a role for BP180 in crosstalk between cell-matrix and cell-cell junctions.

Cicatricial pemphigoid: IgA and IgG autoantibodies target epitopes on both intra- and extracellular domains of bullous pemphigoid antigen 180

BACKGROUND

Cicatricial pemphigoid (CP) is an autoimmune subepidermal blistering disease where autoantibodies target various components of the dermal-epidermal junction, including the bullous pemphigoid antigen 180 (BP180).

OBJECTIVE

We determined the exact specificity of circulating IgG and IgA autoantibodies to BP180 in a large number of CP patients.

METHODS

Twenty-six consecutive CP sera were analysed by Western blotting using a panel of cell-derived and recombinant proteins covering the entire BP180 molecule.

RESULTS

Circulating autoantibodies were detected in all CP sera. Seven sera reacting with laminin-5 were excluded from further analyses; the remaining 19 sera recognized BP180, including six sera (32%) that showed only IgA reactivity to this protein. With the combined use of the soluble BP180 ectodomain (LAD-1) and recombinant BP180 NC16A, 16 of these 19 CP sera (84%) targeted BP180. IgG reactivity was preferentially found against NC16A, whereas IgA antibodies predominantly recognized LAD-1. Thirty-two per cent of the BP180-reative sera revealed reactivity with the intracellular domain of this protein.

CONCLUSIONS

Our findings demonstrate that autoantibodies in CP target epitopes on both extra- and intracellular domains of BP180 and highlight the importance of testing for both IgG and IgA reactivity in these patients' sera.

Truncated typeXVII collagen expression in a patient with non-herlitz junctional epidermolysis bullosa caused by a homozygous splice-site mutation

SUMMARY

Type XVII collagen (180-kDa bullous pemphigoid antigen) is a structural component of hemidesmosomes. Mutations in the type XVII collagen gene (COL17A1) have been established to be the molecular basis of non-Herlitz junctional epidermolysis bullosa (JEB-nH), an inherited skin blistering disorder. Here we report for the first time truncated type XVII collagen expression, caused by homozygosity for a COL17A1 donor splice-site mutation (4261+1 g --> c), which was identified by PCR amplification on genomic DNA. By RT-PCR and sequencing of cDNA derived from mRNA from the patient's cultured keratinocytes, we provide evidence of cryptic splicing and exon skipping, most abundantly of exon 52. JEB-nH patients with COL17A1 splice-site mutations resulting in an exon skip often have no immunologically detectable type XVII collagen. However, in our patient with the generalized atrophic benign epidermolysis bullosa subtype, a small amount of type XVII collagen was detectable in the skin, and immunoblotting of cultured keratinocytes revealed that the 180-kDa protein was 10 kDa shorter. We hypothesize that the function of this truncated type XVII collagen polypeptide, which is expressed at low levels, is impaired, explaining the JEB-nH phenotype.

The IL-8 release from cultured human keratinocytes, mediated by antibodies to bullous pemphigoid autoantigen 180, is inhibited by dapsone

Bullous pemphigoid (BP) is a subepidermal blistering disease associated with autoantibodies to the hemidesmosomal 180 kD BP autoantigen (BP180). However, the binding of autoantibodies to BP180 alone is not sufficient for blister formation in this disease and the infiltration of neutrophils into the skin is required. Dapsone and nicotinamide inhibit neutrophil chemotaxis and are used effectively in treating BP. IL-8 is a known chemoattractant for neutrophils and has been implicated in the inflammatory process of both human and experimental murine BP. We have recently shown that antibodies to BP180 mediate a dose and time-dependent release of IL-6 and IL-8 from cultured normal human epidermal keratinocytes (NHEK). In the present study, we addressed the question whether dapsone or nicotinamide influence this cytokine release. We demonstrate that dapsone, but not nicotinamide, in its pharmacological range, inhibits the IL-8, but not the IL-6 release from NHEK, induced by anti-BP180 IgG, in a dose-dependent fashion as detected by ELISA. IL-8 mRNA levels, as determined by RT-PCR, were the same in cells treated with BP IgG alone compared to cells treated with BP IgG plus dapsone. This observation suggests that dapsone inhibits the BP IgG-induced IL-8 release from cultured NHEK by mechanisms at the post-transcriptional level. Our findings contribute to the understanding how dapsone leads to a reduced influx of neutrophils into BP lesions and, finally, to the cessation of blister formation in this disease.

Properties of the collagen type XVII ectodomain. Evidence for n- to c-terminal triple helix folding

Collagen XVII is a transmembrane component of hemidesmosomal cells with important functions in epithelial-basement membrane interactions. Here we report on properties of the extracellular ectodomain of collagen XVII, which harbors multiple collagenous stretches. We have recombinantly produced subdomains of the collagen XVII ectodomain in a mammalian expression system. rColXVII-A spans the entire ectodomain from deltaNC16a to NC1, rColXVII-B is similar but lacks the NC1 domain, a small N-terminal polypeptide rColXVII-C encompasses domains deltaNC16a to C15, and a small C-terminal polypeptide rColXVII-D comprises domains NC6 to NC1. Amino acid analysis of rColXVII-A and -C demonstrated prolyl and lysyl hydroxylation with ratios for hydroxyproline/proline of 0.4 and for hydroxylysine/lysine of 0.5. A small proportion of the hydroxylysyl residues in rColXVII-C ( approximately 3.3%) was glycosylated. Limited pepsin and trypsin degradation assays and analyses of circular dichroism spectra clearly demonstrated a triple-helical conformation for rColXVII-A, -B, and -C, whereas the C-terminal rColXVII-D did not adopt a triple-helical fold. These results were further substantiated by electron microscope analyses, which revealed extended molecules for rColXVII-A and -C, whereas rColXVII-D appeared globular. Thermal denaturation experiments revealed melting temperatures of 41 degrees C (rColXVII-A), 39 degrees C (rColXVII-B), and 35 degrees C (rColXVII-C). In summary, our data suggest that triple helix formation in the ectodomain of ColXVII occurs with an N- to C-terminal directionality.

Antibodies to desmogleins 1 and 3, but not to BP180, induce blisters in human skin grafted onto SCID mice

Pemphigus and bullous pemphigoid (BP) are blistering skin diseases associated with IgG autoantibodies to desmosomal and hemidesmosomal components. When autoantibodies to desmogleins 1 and 3 from patients with pemphigus foliaceus (PF) and pemphigus vulgaris (PV) or rabbit antibodies against the murine hemidesmosomal component BP180 are passively transferred into neonatal mice, they induce blisters in the skin of the mice. To develop an animal model that would duplicate the findings in the skin of the patients more closely, full-thickness human skin from healthy volunteers was grafted onto SCID mice. Injection of the purified IgG fraction from the serum of PF and PV patients led to subcorneal and suprabasal splits in the human grafts and human IgG was deposited intercellularly in the upper and lower layers of the epidermis, respectively. Interestingly, anti-BP180 autoantibodies purified from the serum of BP patients and from a rabbit immunized with recombinant human BP180 strongly bound to the basement membrane zone of the grafts (n=32), fixed murine complement, led to the recruitment of neutrophils to the upper dermis of the graft, but did not induce subepidermal blisters. We report a novel experimental model for PF and PV which should greatly facilitate further studies to dissect the immunopathological mechanisms in these diseases. Specifically, this model can be used to identify pathogenically relevant epitopes on human desmogleins 1 and 3 and to develop novel strategies for the treatment of pemphigus.

Epitopes targeted by bullous pemphigoid T lymphocytes and autoantibodies map to the same sites on the bullous pemphigoid 180 ectodomain

Bullous pemphigoid is a blistering skin disease characterized by autoantibodies directed against the NC16A domain of bullous pemphigoid 180 (collagen XVII), a transmembrane protein of epidermal basal cells. Passive transfer studies in mice have shown that antibodies that bind to this immunodominant region of bullous pemphigoid 180 are capable of inducing a skin disease that closely mimics bullous pemphigoid, supporting the hypothesis that epitopes within NC16A are involved in the pathogenesis of bullous pemphigoid. In this study, we examined the autoimmune T cell response in bullous pemphigoid patients. T cells from eight of 12 bullous pemphigoid patients, all of whom had circulating anti-bullous pemphigoid 180 autoantibodies, showed a specific proliferative response to recombinant forms of NC16A. T cell lines and clones developed from four of these patients recognize the same NC16A peptides as those targeted by autoantibodies from the corresponding individuals. These NC16A-responding T lymphocytes express alpha/beta T cell receptors and CD4 memory T cell surface markers and exhibited a Th1/Th2 mixed cytokine profile that may support the production of antibodies. This new information will aid in defining the key steps involved in the development of the autoimmune response in bullous pemphigoid.

Autoantibodies to BP180 associated with bullous pemphigoid release interleukin-6 and interleukin-8 from cultured human keratinocytes

Bullous pemphigoid is an inflammatory subepidermal blistering disease that is associated with auto- antibodies to the keratinocyte surface protein, BP180. In addition to the binding of autoantibodies, the infiltration of inflammatory cells is necessary for blister formation. Cytokines, including interleukin-6 and interleukin-8, have been implicated in the disease process of both human and experimental murine bullous pemphigoid. This study was aimed at testing the hypothesis that the binding of anti-BP180 antibodies to their target antigen triggers a signal transduction event that results in the secretion of these pro-inflammatory cytokines. Consistent with this hypothesis, treatment of cultured normal human epidermal keratinocytes with bullous pemphigoid IgG, but not control IgG, led to increased levels of interleukin-6 and interleukin-8, but not interleukin-1alpha, interleukin-1beta, tumor necrosis factor-alpha, interleukin-10, or monocyte chemoattractant protein-1, in the culture medium. This effect was concentration- and time-dependent and was abolished by depleting the bullous pemphigoid IgG of reactivity to two distinct epitopes on the BP180 NC16A domain. Upregulation of interleukin-6 and interleukin-8 was found at both protein and mRNA levels. In addition, bullous pemphigoid IgG did not induce the release of interleukin-6 and interleukin-8 from BP180-deficient keratinocytes obtained from a patient with generalized atrophic benign epidermolysis bullosa. These data indicate that bullous pemphigoid-associated autoantibodies to the human BP180 ectodomain trigger a signal transducing event that leads to expression and secretion of interleukin-6 and interleukin-8 from human keratinocytes.

Patients with bullous pemphigoid and linear IgA disease show a dual IgA and IgG autoimmune response to BP180

Bullous pemphigoid (BP) and linear IgA disease (LAD) are autoimmune subepidermal blistering skin diseases associated with autoantibodies against the transmembrane hemidesmosomal protein BP180/type XVII collagen. It has been demonstrated previously that BP is characterized predominantly by IgG autoantibodies, while autoantibodies in LAD mainly belong to the IgA isotype. The aim of the present study was to investigate the hypothesis that there is a significant overlap in the autoantibody isotype profiles associated with these two diseases. Several new recombinant forms of BP180 were generated in the baculovirus expression system, including the full-length protein. IgG autoantibodies to BP 180 were detectable in 39 of 40 (98%) of BP sera; interestingly, 88% of BP sera also contained IgA anti-BP180 autoantibodies. Similarly, anti-BP180 reactivity in LAD sera (n=22) was also attributed to both an IgA (68%) and an IgG (76%) autoantibody response. IgA and IgG autoantibodies to the intracellular portion of BP180 were found in 14% and 28% of BP sera, respectively, and in 8% of LAD sera (same percentage for both isotypes). Our findings clearly demonstrate that both BP and LAD patients have a dual IgA and IgG autoimmune response to BP180 which is directed not only to the ectodomain, but also to the intracellular portion of this protein.

Production of the entire extracellular domain of BP180 (type XVII collagen) by baculovirus expression

Bullous pemphigoid (BP) is an acquired autoimmune skin disease, and its target antigens are a 230 kDa plaque protein (BP230) and a 180 kDa transmembrane protein with interrupted collagenous domains (BP180, type XVII collagen), which localize at the hemidesmosome. In this study we have attempted to express the entire extracellular domain of BP180 (rBP180EC) as a secreted protein by baculovirus expression. Seventy out of 83 BP sera (84.4%) showed positive reactivity against rBP180EC by immunoblot analysis, and 56 out of 83 BP sera (67.5%) were positive against rBP180EC by ELISA. These figures were comparable with those when a bacterial recombinant protein encoding the NC16a domain of BP180 (rNC16a) was used as an antigen source. Reactivity of BP sera against rBP180EC by ELISA was completely abolished or significantly reduced by immunocompetition with rNC16a in 11 out of 14 BP sera tested, while the reactivity was not altered in the rest of the three sera. These findings indicate that the NC16a domain represents the major epitopes on the extracellular domain of BP180, although there are some other minor epitopes outside of NC16a which are uniquely expressed by rBP180EC. rBP180EC will be useful to develop a diagnostic tool for BP as well as to dissect a molecular role for BP180 in interactions of keratinocytes with epidermal basement membrane.

A disease-associated glycine substitution in BP180 (type XVII collagen) leads to a local destabilization of the major collagen triple helix

BP180 is a homotrimeric transmembrane protein with a carboxy-terminal ectodomain that forms an interrupted collagen triple helix. Null type mutations in the BP180 gene produce a recessive subepidermal blistering disease, non-Herlitz junctional epidermolysis bullosa. Like the null mutations, a glycine substitution (G627V) within the longest BP180 collagenous domain (COL15) is also associated with the recessive skin disease; however, unlike the null mutations, this glycine substitution appears to act in a dominant fashion to give rise to a novel form of random pitting dental enamel hypoplasia. The dominant effects of this mutation were thought to be due to alterations in the assembly and/or stability of this BP180 collagenous region. To further investigate this issue, a structural analysis was performed on recombinant forms of the wild type and G627V mutant BP180 ectodomain. Both proteins were found to form collagen-like triple helices with very similar Stokes radii and melting temperatures and exhibited very similar rates of synthesis, secretion and turn-over. Tryptic digestion analysis revealed that the mutant G627V-sec180e contains an additional highly sensitive proteolytic site that maps within the region of the mutation. Thus, the disease-associated G627V mutation in BP180 does not grossly alter protein structure, but causes a local destabilization of the triple-helix that exposes sensitive residues to the in vitro effects of trypsin and possibly affects its structure-function in vivo.

Collagen XVII is destabilized by a glycine substitution mutation in the cell adhesion domain Col15

Collagen XVII is a hemidesmosomal transmembrane molecule important for epithelial adhesion in the skin. It exists in two forms, as a full-length protein and as a soluble ectodomain that is shed from the keratinocyte surface by furin-mediated proteolysis. To obtain information on the conformation and the functions of this unusual collagen, its largest collagenous domain, Col15, was expressed in a eukaryotic episomal expression system and purified by DEAE and fast protein liquid- Mono S chromatography. The protein was triple-helical (T(m) of 26.5 degrees C) when produced in cultures containing ascorbic acid. When the vitamin supply was limited, the 4-hydroxyproline content was reduced from 74 to 9%, which, in turn, resulted in a drastic reduction of the stability of the triple helix. The glycine substitution mutation G627V associated with junctional epidermolysis bullosa, a human blistering skin disease, also had a striking effect on thermal stability of rCol15 causing partial unfolding already at 4 degrees C. Col15 promoted cell adhesion of epithelial and fibroblastic cell lines with a beta1 integrin-mediated mechanism. In concert with this, in acquired autoimmune blistering skin diseases, circulating IgG and IgA autoantibodies were found to target rCol15r.

The shed ectodomain of collagen XVII/BP180 is targeted by autoantibodies in different blistering skin diseases

Collagen XVII/BP180, an epidermal adhesion molecule, exists as a full-length transmembrane protein and as a soluble 120-kd ectodomain that is shed from the keratinocyte surface by furin-mediated proteolysis. Despite a number of studies on autoantibody targets in blistering skin diseases, it has remained unclear whether the physiologically shed ectodomain of collagen XVII plays a role as an autoantigen. Here we isolated the authentic, soluble form of human collagen XVII and showed that it is an autoantigen recognized by IgG and IgA autoantibodies in different blistering skin diseases and is, in some cases, the preferential target. The ectodomain was isolated from the epidermis, keratinocyte media, amniotic fluid, and pemphigoid blister fluid, and autoantibodies affinity-purified with this ectodomain bound to the proximal surface of the epidermis in normal skin but not in collagen XVII-deficient skin. The antibody reactivity was not dependent on the native conformation or the N-glycosylation of the soluble ectodomain, but was abolished by collagenase treatment. Sera of 81 patients with a clinically active blistering skin disease were reacted with full-length collagen XVII, the authentic soluble ectodomain, and recombinant fragments. In bullous and cicatricial pemphigoid, IgG reactive with full-length collagen XVII also recognized the soluble ectodomain. In linear IgA dermatosis and chronic bullous dermatosis of childhood, IgA targeted the soluble ectodomain more efficiently than the full-length protein. The use of recombinant fragments demonstrated that epitopes were present in several noncollagenous and collagenous subdomains of the molecule, and that a significant portion of the sera targeted Col15 domain, a hitherto unrecognized epitope region.

Equine bullous pemphigoid IgG autoantibodies target linear epitopes in the NC16A ectodomain of collagen XVII (BP180, BPAG2)

Bullous pemphigoid (BP) is an autoimmune subepithelial blistering dermatosis of humans, dogs, cats and pigs. It is characterized by skin-fixed and circulating IgG autoantibodies that target one or both BP antigens. An immunological homologue of BP in humans was diagnosed in two horses with cutaneous and mucosal ulcerations as well as microscopic subepithelial vesiculation. Immunological investigations revealed similar findings for both the horses. Direct immunofluorescence demonstrated the presence of IgG deposited linearly at the dermoepidermal junction in mucosal and skin biopsy specimens. Indirect immunofluorescence testing confirmed the existence of circulating basement membrane-specific IgG autoantibodies. Using intact and salt-split epithelial substrates, serum IgG were shown to target antigens situated not only at the basal, but also at the lateral and apical aspects of stratum basale keratinocytes. Immunoblotting and ELISA corroborated that the IgG from affected horses, but not those from normal controls, exhibited high immunoreactivity against the NC16A extracellular domain of type XVII collagen (BPAG2, BP180). Equine BP could be proposed, therefore, as another spontaneous model of this most common basement membrane autoimmune dermatosis of humans.

A critical role for neutrophil elastase in experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune skin disease characterized by subepidermal blisters and autoantibodies against 2 hemidesmosome-associated proteins, BP180 and BP230. The immunopathologic features of BP can be reproduced in mice by passive transfer of anti-BP180 antibodies. Lesion formation in this animal model depends upon complement activation and neutrophil recruitment. In the present study, we investigated the role of neutrophil elastase (NE) in antibody-induced blister formation in experimental BP. Abnormally high levels of caseinolytic activity, consistent with NE, were detected in extracts of lesional skin and blister fluid of mice injected with anti-BP180 IgG. The pathogenic anti-BP180 IgG failed to induce subepidermal blistering in NE-null (NE(-/-)) mutant mice. NE(-/-) mice reconstituted with neutrophils from wild-type mice became susceptible to experimental BP. Wild-type mice given NE inhibitors (alpha1-proteinase inhibitor and Me-O-Suc-Ala-Ala-Pro-Val-CH(2)Cl), but not mice given cathepsin G/chymase inhibitors (alpha1-antichymotrypsin or Z-Gly-Leu-Phe-CH(2)Cl), were resistant to the pathogenic activity of anti-BP180 antibodies. Incubation of murine skin with NE induced BP-like epidermal-dermal detachment. Finally, NE cleaved BP180 in vitro and in vivo. These results implicate NE directly in the dermal-epidermal cleavage induced by anti-BP180 antibodies in the experimental BP model.

Autoantibodies in a subgroup of patients with linear IgA disease react with the NC16A domain of BP1801

Linear IgA disease is an autoimmune subepidermal blistering disease characterized by IgA deposits at the cutaneous basement membrane zone. IgA antibodies from linear IgA disease sera react with antigens of 97 kDa (LABD97) and 120 kDa (LAD-1), both of which appear to be fragments of the extracellular domain of bullous pemphigoid 180 (type XVII collagen). The aim of this study was to determine whether linear IgA disease sera react with the immunodominant region of BP180 (NC16A domain), which is a major target of IgG autoantibodies produced by patients with bullous pemphigoid. Indeed, 11 of 50 linear IgA disease sera were found to contain IgA autoantibodies that recognized a recombinant form of NC16A by immunoblotting. The same sera also reacted with NC16A by enzyme-linked immunosorbent assay. An epitope mapping analysis uncovered four linear IgA disease-associated epitopes located within the 45 amino acid N-terminal stretch of NC16A, all of which were previously identified as antigenic sites targeted by bullous pemphigoid autoantibodies. Eight of the linear IgA disease sera that were reactive with NC16A also recognized LAD-1 secreted by the SCC-25 cell line, and five sera recognized BP180 extracted from keratinocytes. Linear IgA disease sera depleted of reactivity to NC16A by immunoadsorption continued to react with both the LAD-1 antigen and BP180 by immunoblotting and with the basement membrane zone by indirect immunofluorescence microscopy. Our results demonstrate that IgA autoantibodies from a subset of linear IgA disease patients react with the same sites on BP180 that are targeted by IgG autoantibodies in bullous pemphigoid.

Molecular mapping of the major epitopes of BP180 recognized by herpes gestationis autoantibodies

Herpes gestationis (HG) is an autoantibody-mediated subepidermal bullous dermatosis associated with pregnancy. The primary target of HG autoantibodies is BP180, a 180-kDa hemidesmosomal glycoprotein. We previously showed that autoantibodies and autoimmune T lymphocytes from HG patients recognize the MCW-1 antigenic site (AA 507-520), which is located in the membrane-proximal noncollagenous domain (NC16A) of BP180. Here, we analyzed the sera of 37 HG patients to further define the sites on BP180 that are targeted by autoantibodies. All of the HG sera, but none of the control sera, were immunoreactive with sec180e, a 120-kDa recombinant protein encompassing the entire BP180 extracellular domain. HG sera depleted of reactivity to NC16A no longer reacted with sec180e, indicating that the major HG-associated epitopes on BP180 are restricted to the NC16A domain. The vast majority of the HG sera (34 of 37) reacted with a 7 amino acid peptide corresponding to the N-terminal half of MCW-1 (MCW-1A). Eleven HG sera (including the 3 that failed to react with MCW-1A) recognized one or more of three antigenic sites located within a 15 amino acid stretch immediately downstream of MCW-1A. In summary, we have identified four major HG-associated epitopes clustered within a 22 amino acid region of the BP180 ectodomain. These findings support the hypothesis that an autoimmune response to the BP180 NC16A domain is a crucial step in the pathogenesis of HG.

Hereditary skin diseases of hemidesmosomes

Studies of hereditary blistering skin diseases (epidermolysis bullosa) and targeted gene mutation experiments in knockout mice have greatly improved our understanding of hemidesmosomes and their associated structures in the cytoskeleton and basement membrane of the skin and mucous membranes. At least 10 molecules are recruited in hemidesmosome complexes, where they interact in a complex way. Hemidesmosomes are not simple adhesion devices, but also transduce signals for cell spreading, cell proliferation and basement membrane organisation. The dynamics of a hemidesmosome raises the metaphor of a self-assembling suspension bridge which evokes activities on both sides of the river. This review summarises our current knowledge of the molecular pathology of hemidesmosomes caused by hereditary skin disease or gene targeting experiment.

Acquired skin disease of hemidesmosomes

The hemidesmosome is a membrane-associated supramolecular dermal epidermal complex linking the cytoskeleton of the basal keratinocyte to structures within the papillary dermis. Different components of this complex have been identified as autoantigens in autoimmune bullous skin diseases. Some of the autoantigens have been characterized at the molecular level. Little is known, however, about the factors that initiate the production of autoantibodies. By histopathology, acquired skin diseases of hemidesmosomes show subepidermal blisters and by direct immunofluorescence, linear deposits of IgG, C3 or IgA at the dermal epidermal junction. Bullous pemphigoid (BP) is the most common acquired disease of hemidesmosomes. Two proteins, BP180 and BP230, have been identified as primary targets of autoantibodies in BP. In addition, pemphigoid/herpes gestationis, lichen planus pemphigoides, cicatricial pemphigoid and linear IgA disease are characterized by an immune response to BP180. Laminin 5 is another well-characterized anchoring filament-lamina densa component of hemidesmosomes. Patients with autoantibodies to laminin 5 show the clinical phenotype of cicatricial pemphigoid. Other acquired skin diseases of the hemidesmosomes reveal autoantibodies to a plectin-like protein, the beta4 subunit of alpha6beta4 integrin, uncein and a not yet characterized 168 kDa protein. Recently, diseases with autoantibodies to 105 and 200 kDa proteins of the lower lamina lucida have been reported. The association of these autoantigens with hemidesmosomes still needs to be demonstrated. Finally, anchoring fibrils associate with the dermal epidermal anchoring complex. The major structural component of anchoring fibrils is type VII collagen, the autoantigen of epidermolysis bullosa acquisita.

Identification and Characterization of Epitopes Recognized by T Lymphocytes and Autoantibodies from Patients with Herpes Gestationis

Autoantibodies associated with herpes gestationis (HG), a pregnancy-associated autoimmune skin disease, target the hemidesmosomal protein BP180. It was shown that the major noncollagenous stretch of the BP180 ectodomain (NC16A) harbors epitopes recognized by HG sera. Furthermore, Abs reactive with the homologous domain of murine BP180 are known to trigger a cutaneous blistering disease in mice by passive transfer experiments. The present study was aimed at characterizing the T cell responses and specificities of autoantibodies from two HG patients. Using immunoblotting and T cell proliferation assays, we have identified a 14-amino-acid stretch of the BP180 ectodomain (MCW-1; aa 507–520) that is recognized by both T cells and autoantibodies produced by the HG patients. The neonate born to one of these HG patients showed no signs of skin disease and had no detectable T cell response to the BP180 Ag, but did have a low titer of circulating anti-BP180 autoantibodies, presumably of maternal origin. BP180-specific T cell lines and clones developed from an HG patient specifically reacted with the MCW-1 epitope. The proliferative responses of these clones were restricted to HLA-DR, but not -DQ or -DP. These Ag-specific T cells expressed α/β TCRs and a CD4 memory T cell phenotype and secreted IFN-γ and IL-2, but not IL-4 or IL-6, suggesting that they are Th1-type lymphocytes. Further characterization of these Ag-specific T cells and autoantibodies will aid in elucidating the autoimmune mechanism(s) leading to the development of HG.