In vivo binding site of pemphigus vulgaris antibodies and their fate during acantholysis

Super-Resolution Microscopy Reveals Altered Desmosomal Protein Organization in Tissue from Patients with Pemphigus Vulgaris

Pemphigus vulgaris (PV) is an autoimmune epidermal blistering disease in which autoantibodies (IgG) are directed against the desmosomal cadherin desmoglein 3 (Dsg3). In order to better understand how PV IgG alters desmosome morphology and function in vivo, PV patient biopsies were analyzed by structured illumination microscopy (SIM), a form of super-resolution fluorescence microscopy. In patient tissue, desmosomal proteins were aberrantly clustered and localized to PV IgG-containing endocytic linear arrays. Patient IgG also colocalized with markers for lipid rafts and endosomes. Additionally, steady-state levels of Dsg3 were decreased and desmosomes were reduced in size in patient tissue. Desmosomes at blister sites were occasionally split, with PV IgG decorating the extracellular faces of split desmosomes. Desmosome splitting was recapitulated in vitro by exposing cultured keratinocytes both to PV IgG and to mechanical stress, demonstrating that splitting at the blister interface in patient tissue is due to compromised desmosomal adhesive function. These findings indicate that Dsg3 clustering and endocytosis are associated with reduced desmosome size and adhesion defects in PV patient tissue. Further, this study reveals that super-resolution optical imaging is powerful approach for studying epidermal adhesion structures in normal and diseased skin.

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.

A study of the correlation between acantholysis and apoptosis for the pathogenesis of pemphigus vulgaris

The goal of the study is to determine the particular features of pemphigus vulgaris (PV) morphogenesis and keratinocytes in the patients’ skin having morphological signs of acantholysis and apoptosis as well as changes in Langerhans cells. Materials and methods. Skin tissue samples taken from 46 PV patients were examined. The skin tissue samples underwent cytology, histopathology and immunohistochemistry examinations as well as transmission electronic microscopy. An increased number of CD1a+ (dendritic) cells and apoptotic Cpp3+ keratinocytes was revealed in the skin of the PV patients. According to the electronic microscopy data, keratinocytes with signs of apoptosis are present in fresh vesicles in the acantholysis area at early PV stages quite seldom; at the same time, Langerhans cells had morphologic signs of activation. As the number of keratinocytes with desmosome lysis signs grew, the number of apoptotic keratinocytes was increasing reliably. Conclusion. In case of PV, apoptosis has a secondary nature and develops as anoikis.

The desmosome

Desmosomes are intercellular junctions that tether intermediate filaments to the plasma membrane. Desmogleins and desmocollins, members of the cadherin superfamily, mediate adhesion at desmosomes. Cytoplasmic components of the desmosome associate with the desmosomal cadherin tails through a series of protein interactions, which serve to recruit intermediate filaments to sites of desmosome assembly. These desmosomal plaque components include plakoglobin and the plakophilins, members of the armadillo gene family. Linkage to the cytoskeleton is mediated by the intermediate filament binding protein, desmoplakin, which associates with both plakoglobin and plakophilins. Although desmosomes are critical for maintaining stable cell-cell adhesion, emerging evidence indicates that they are also dynamic structures that contribute to cellular processes beyond that of cell adhesion. This article outlines the structure and function of the major desmosomal proteins, and explores the contributions of this protein complex to tissue architecture and morphogenesis.

Pemphigus vulgaris IgG-induced desmoglein-3 endocytosis and desmosomal disassembly are mediated by a clathrin- and dynamin-independent mechanism

Pemphigus vulgaris (PV) is a life-threatening autoimmune disease characterized by oral mucosal erosions and epidermal blistering. The autoantibodies generated target the desmosomal cadherin desmoglein-3 (Dsg3). Previous studies demonstrate that upon PV IgG binding, Dsg3 is internalized and enters an endo-lysosomal pathway where it is degraded. To define the endocytic machinery involved in PV IgG-induced Dsg3 internalization, human keratinocytes were incubated with PV IgG, and various tools were used to perturb distinct endocytic pathways. The PV IgG.Dsg3 complex failed to colocalize with clathrin, and inhibitors of clathrin- and dynamin-dependent pathways had little or no effect on Dsg3 internalization. In contrast, cholesterol binding agents such as filipin and nystatin and the tyrosine kinase inhibitor genistein dramatically inhibited Dsg3 internalization. Furthermore, the Dsg3 cytoplasmic tail specified sensitivity to these inhibitors. Moreover, inhibition of Dsg3 endocytosis with genistein prevented disruption of desmosomes and loss of adhesion in the presence of PV IgG. Altogether, these results suggest that PV IgG-induced Dsg3 internalization is mediated through a clathrin- and dynamin-independent pathway and that Dsg3 endocytosis is tightly coupled to the pathogenic activity of PV IgG.

Anti-desmoglein 3 (Dsg3) Monoclonal Antibodies Deplete Desmosomes of Dsg3 and Differ in Their Dsg3-depleting Activities Related to Pathogenicity

Pemphigus vulgaris (PV) is an autoimmune blistering disease, characterized by the loss of cell-cell adhesion between epidermal keratinocytes and the presence of autoantibody against desmoglein 3 (Dsg3), which provides adhesive integrity to desmosomes between adjacent keratinocytes. We have previously shown that PV-IgG purified from patients depletes desmosomes of Dsg3. However, PV-IgG contains not only antibodies against a variety of different epitopes of Dsg3 but also against other unknown antigens. Therefore, we examined whether the Dsg3-depleting activity of PV-IgG is generated specifically by anti-Dsg3 activity in a human squamous cell carcinoma cell line (DJM-1) and normal human keratinocytes by using four different pathogenic and nonpathogenic monoclonal antibodies against Dsg3. We demonstrate that these monoclonal antibodies deplete cells and desmosomes of Dsg3, as PV-IgG does. Individual monoclonal anti-Dsg3 antibodies display characteristic limits to their Dsg3-depleting activity, which correlates with their pathogenic activities. In combination, these antibodies exert a cumulative or synergistic effect, which may explain the potent Dsg3-depleting capability of PV-IgG, which is polyclonal. Finally, although Dsg3-depletion activity correlated with AK-monoclonal antibody pathogenicity in mouse models, the residual level of Dsg3, when below approximately 50%, does not correlate with the adhesive strength index in the present study. This may suggest that although the Dsg3 depletion is not indicative for adhesive strength, the level of Dsg3 can be used as a read-out of pathogenic changes within the cell and that the Dsg3 depletion from desmosomes plays an important role in skin fragility or susceptibility to blister formation in PV patients.

Desmoglein Endocytosis and Desmosome Disassembly Are Coordinated Responses to Pemphigus Autoantibodies

Desmosomes are adhesive intercellular junctions prominent in the skin and heart. Loss of desmosome function is associated with severe congenital and acquired disorders characterized by tissue fragility. Pemphigus vulgaris (PV) is an autoimmune disorder in which antibodies are directed against the desmosomal adhesion molecule Dsg3, resulting in severe mucosal erosions and epidermal blistering. To define the mechanisms by which Dsg3 autoantibodies disrupt keratinocyte adhesion, the fate of PV IgG and various desmosomal components was monitored in primary human keratinocytes exposed to PV patient IgG. PV IgG initially bound to keratinocyte cell surfaces and colocalized with desmosomal markers. Within 6 h after PV IgG binding to Dsg3, electron microscopy revealed that desmosomes were dramatically disrupted and keratinocyte adhesion was severely compromised. Immunofluorescence analysis indicated that PV IgG and Dsg3 were rapidly internalized from the cell surface in a complex with plakoglobin but not desmoplakin. Dsg3 internalization was associated with retraction of keratin filaments from cell-cell borders. Furthermore, the internalized PV IgG-Dsg3 complex colocalized with markers for both endosomes and lysosomes, suggesting that Dsg3 was targeted for degradation. Consistent with this possibility, biotinylation experiments demonstrated that soluble Dsg3 cell surface pools were rapidly depleted followed by loss of detergent-insoluble Dsg3. These findings demonstrate that Dsg3 endocytosis, keratin filament retraction, and the loss of keratinocyte cell-cell adhesion are coordinated responses to PV IgG.

Assembly pathway of desmoglein 3 to desmosomes and its perturbation by pemphigus vulgaris-IgG in cultured keratinocytes, as revealed by time-lapsed labeling immunoelectron microscopy

To determine the assembly pathway of desmoglein 3 (Dsg3) into desmosomes and the subsequent effects of pemphigus vulgaris immunoglobulin G (PV-IgG) on such, we employed a time-lapsed labeling for FITC/Rhodamine (Rod) double-stained immunofluorescence and 5-nm/10-nm gold double-stained immunoelectron microscopy by using PV-IgG, which was confirmed to react specifically Dsg3. Cells from a human squamous cell carcinoma cell line (DJM-1) were first treated briefly with PV-IgG (3 min), then incubated in either anti-human IgG-FITC or 5-nm gold antibody-containing medium (5 min), followed by a 60-minute chase in normal medium without antibodies. The same cells were reincubated with PV-IgG medium for 3 minutes, followed by either anti-human IgG-Rod or 10-nm gold antibodies for 5 minutes. Using this method, FITC and 5-nm gold particles show the fate of Dsg3-PV-IgG complexes during the following 60-minute chase. IgG-Rod or 10-nm gold particles, which are bound during the last 5 minutes of the chase, show Dsg3 molecules newly expressed on the cell surface during the 60-minute-chase period. Initially, Dsg3 formed two types of small clusters on the nondesmosomal plasma membrane, ie, either half-desmosome-like clusters with keratin intermediate filament (KIF) attachment or simple clusters without KIF attachment. The PV-IgG binding to Dsg3 caused the internalization of the simple clusters into endosomes, but not the half-desmosome-like clusters. After the 60-minute-chase period, both types of cell surface Dsg3 clusters were labeled with only 10-nm gold, suggesting that new Dsg3 molecules were being delivered to the cell surface. Desmosomes were labeled with both 5-nm gold and 10-nm gold, whereas the half-desmosome-like clusters were labeled with only 10-nm gold, suggesting that the desmosomes themselves were not split. These results suggest that Dsg3 first forms simple clusters, followed by KIF-attachment, and then becomes integrated into desmosomes, and that PV-IgG-induced internalization of the nondesmosomal simple clusters of Dsg3 may represent the primary effects of PV-IgG on keratinocytes.

Internalization of constitutive desmogleins with the subsequent induction of desmoglein 2 in pemphigus lesions

Acantholytic blisters in pemphigus vulgaris (PV) and pemphigus foliaceus (PF) are caused by a dissociation of desmosomes mediated by autoantibodies against desmoglein (Dsg) 3 and Dsg 1, respectively. The blistering occurs at the suprabasilar level in PV and at the subcorneal level in PF, which corresponds to the distribution of target antigens in the epidermis: there is a more prominent expression of Dsg 1 in the upper layer, whereas Dsg 3 is more prominent in the lower layer. To elucidate the histogenesis of acantholysis, we studied the alterations of the desmosomal components and the expression pattern of Dsg isoforms in the lesional and perilesional epidermis of pemphigus patients. The results demonstrated an internalization of the desmosomes in the lower epidermis of PV, PF and pemphigus vegetans. A similar phenomenon was induced in monolayers of keratinocytes cultured with PV sera. However, little change was observed in E-cadherin expression until acantholysis became manifest. This internalization occurred prior to overt acantholysis, and was frequently associated with the induction of Dsg 2 expression in the basilar or lower layers of the epidermis. These findings indicate an alteration of Dsg isoform expression in subclinical pemphigus lesions, which might be related to the characteristic acantholytic patterns: the suprabasilar layer in PV and the upper epidermis in PF.

Curvicircular intracytoplasmic membranous structures in keratinocytes of pemphigus foliaceus

We noticed intracytoplasmic membranous, annular, or circular structures in the lesion of pemphigus foliaceus and studied these by regular transmission electron microscopy and immunoelectron microscopy. These curvicircular bodies were observed in the preacantholytic keratinocytes of the blister wall as well as in acantholytic cells in 6 out of 6 patients with pemphigus foliaceus. They were absent in samples from 3 patients with pemphigus vulgaris. These structures were about 60-70 nm wide and consisted of 4 electron-dense layers. They were continuous with intact desmosomal structures and gap junctions in the periphery of the keratinocytes. These curvicircular membranous bodies were well labeled with immunogold particles for desmoglein, plakoglobin, connexin 43, and IgG. In contrast to pemphigus vulgaris, splitting of desmosomes through dissolution of intercellular desmoglea was seldom observed in all 6 specimens of pemphigus foliaceus. These findings suggest that in pemphigus foliaceus 1) curvicircular bodies are derived from internalized desmosomes and gap junctions, and 2) cell-to-cell adhesions are weakened by this internalization and acantholysis is initiated, while in pemphigus vulgaris the dissolution of desmoglea is the initial event. It is suggested that in pemphigus foliaceus the binding of autoantibody induces internalization of many intact desmosomes and gap junctions rather than splitting them.

Pemphigus: from immunofluorescence to molecular biology

Since the discovery of autoantibodies in patients with pemphigus, pemphigus has been intensively studied by dermatologists and cutaneous or cellular biologists by means of various techniques including immunofluorescence, immunoelectron microscopy, immunoprecipitation, immunoblotting, and molecular biology. In this article, up-dated topics on pemphigus obtained by each individual technique are reviewed. In the course of immunofluorescence studies on unusual cases of blistering diseases, a new entity characterized by immunoglobulin A (IgA)-type autoantibodies directed against keratinocyte cell surfaces has been discovered. Immunoelectron microscopy using low temperature post-embedding gold labeling enabled us to quantitate binding sites of pemphigus autoantibodies within desmosomes at different levels of epidermis. Immunoprecipitation and immunoblot analyses allowed us to characterize antigen complexes in paraneoplastic pemphigus. Finally, approaches using molecular biology not only have given us a fundamental insight that pemphigus autoantigen is a cadherin-type cell adhesion molecule both in pemphigus vulgaris and pemphigus foliaceus, but also provided tools to develop novel diagnostic and therapeutic strategies.

Pemphigus vulgaris and pemphigus foliaceus sera show an inversely graded binding pattern to extracellular regions of desmosomes in different layers of human epidermis

We analyzed the location of binding sites for pemphigus vulgaris (PV) antigen and pemphigus foliaceus (PF) antigen in the human epidermis using serum samples obtained from three patients with PV and three patients with PF. Confocal laser scanning microscopy, immunofluorescent examination of ultrathin cryosections, and immunoperoxidase electron microscopy demonstrated discontinuous dots along the epidermal cell surfaces. Immunogold electron microscopy of ultrathin cryosections showed specific binding of PV and PF autoantibodies only to desmosomes. Post-embedding immunogold electron microscopy using cryofixation and cryosubstitution enabled the whole depth of the epidermis to be examined and the binding of PV and PF autoantibodies to be quantitated by counting gold particles. Both PV and PF autoantibodies bound to all desmosomes in the epidermis, but not to the surface of the non-desmosomal keratinocytes. The majority of auto-antibody binding occurred in the extracellular domain (PV, 62%; PF, 69%). The statistical analysis of two-way analysis of variance regarding the number of gold particles labeling a single desmosome confirmed a significant interaction between subtypes of pemphigus (PV and PF) and the different epidermal cell layers (p < 0.044). The results indicate that the number of gold particles bound to individual desmosomes with PV sera was significantly higher in the lower epidermis than in the upper epidermis, and that of PF sera showed reciprocal pattern. This inversely graded binding pattern suggests heterogeneity of the composition of the desmosomes, which may explain the differences in level of acantholysis between PV and PF.

Adhesion molecules. I: Keratinocyte-keratinocyte interactions; cadherins and pemphigus

During the last few years, considerable progress has been made in our understanding of the structure and function of cadherins and of the pathophysiology of pemphigus. Cadherins are a multiple gene family of Ca(++)-dependent cell adhesion molecules with a typical single-spanning transmembrane structure. Cadherins have two major subfamilies, classic cadherin and desmosomal cadherin. Classic cadherins, including E-, P-, and N-cadherins, are characterized by a homophilic binding specificity. They localize at adherens junctions and mediate physiologic interaction with the involvement of cytoplasmic anchoring molecules, catenins, and the actin-based cytoskeleton network. Desmosomal cadherins, the desmocollins and desmogleins, localize at desmosomes and are linked to the intermediate keratin filaments network via plakoglobin and desmoplakin. Molecular cloning has demonstrated that the autoantigens of both pemphigus vulgaris and pemphigus foliaceus are members of the desmoglein subfamily of the cadherin supergene family. Thus, pemphigus is characterized as an anti-cadherin autoimmune disease. Furthermore, a baculovirus recombinant protein of pemphigus vulgaris antigen was capable of absorbing out the pathogenic autoantibodies from patients' sera, providing a possibility of antigen-specific therapeutic strategies for pemphigus.

Immunofluorescence and immunoelectron microscopy analyses of a human monoclonal anti-epithelial cell surface antibody that recognizes a 185-kD polypeptide: a component of the paraneoplastic pemphigus antigen complex?

We recently reported the production of a human monoclonal antibody (MoAb) derived from a patient with pemphigus vulgaris (PV) that binds to the keratinocyte membrane and reacts with a 185-kD polypeptide by immunoblot analysis. We have since examined the tissue specificity of that MoAb, F12. By indirect immunofluorescence (IIF), F12 stained both the cell membrane and the basement membrane zone of stratified squamous epithelia. Moreover, MoAb F12 stained other epithelial tissues, such as urinary bladder, small bowel, thymus, and liver, and non-epithelial tissues, such as myocardium. Indirect immunoelectron microscopy (IIEM) analysis showed that MoAb F12 bound to a component common to desmosomal and hemidesmosomal plaques and to zona adherens-type junctions between hepatocytes and bile duct cells. Inhibition experiments were then performed with sera from patients with pemphigus vulgaris, pemphigus foliaceus, paraneoplastic pemphigus, or bullous pemphigoid. Three sera blocked F12 reactivity; two were from paraneoplastic pemphigus patients and the other was from the pemphigus vulgaris patient whose peripheral blood lymphocytes were used to make F12. All these sera recognized a 185-kD band that co-migrated with the polypeptide labeled by MoAb F12 on immunoblots. In addition, the IIF and IIEM staining patterns of MoAb F12 were similar to those observed with sera from two patients with paraneoplastic pemphigus. These observations suggest a relationship between MoAb F12 and the autoimmune response characterizing paraneoplastic pemphigus patients' sera.

Pemphigus vulgaris antigen, a desmoglein type of cadherin, is localized within keratinocyte desmosomes

Pemphigus vulgaris antigen (PVA) is a member of the desmoglein subfamily of cadherin cell adhesion molecules. Because autoantibodies in this disease cause blisters due to loss of epidermal cell adhesion, and because desmoglein is found in the desmosome cell adhesion junction, we wanted to determine if PVA is also found in desmosomes. By immunofluorescence, PV IgG bound, in a dotted pattern, to the cell surface of cultured human keratinocytes induced to differentiate with calcium, suggesting junctional staining. However, by preembedding, immunogold electron microscopic studies only slight labeling could be detected in desmosomes, presumably because of difficulty in gold penetration of intact desmosomes. We therefore treated the keratinocytes with 0.01% trypsin in 1 mM calcium, conditions known to preserve cadherin antigenicity but that caused slight separation of desmosomes, before immunogold staining. In this case there was extensive labeling of the extracellular part of desmosomes but not of the interdesmosomal cell membrane which was stained with anti-beta 2-microglobulin antibodies. To confirm the specificity of this binding we showed that antibodies raised in rabbits against the extracellular portions of PVA also bound desmosomes in these cultures. In intact mouse epidermis we could also show slight, but specific, immunogold desmosomal labeling with PV IgG. Furthermore, neonatal mice injected with PV IgG affinity purified on PVA showed desmosomal separation with the IgG localized to desmosomal cores. These results indicate that PVA is organized and concentrated within the desmosome where it presumably functions to maintain the integrity of stratifying epithelia.

Dynamic expression of pemphigus and desmosomal antigens by cultured keratinocytes

Dynamic expression of pemphigus antigens by cultured human and mouse keratinocytes was compared with that of desmosome-associated molecules and cellular markers relating to epidermal differentiation. Plakoglobin was detected in localized areas of keratinocyte sheets in low Ca2+ (0.15 mM) KGM medium. In minimum essential medium (MEM) containing 1.8 mM Ca2+, plakoglobin was expressed in the intercellular spaces (ICS) throughout the keratinocyte sheet. Desmoplakin I and II, which were present in the cytoplasm of keratinocytes in the low Ca2+ medium, moved to the cell surface after the medium was changed to MEM. Desmoglein 1 and pemphigus vulgaris (PV) antigens were observed in the ICS of both the monolayers and stratified areas in the MEM. Pemphigus foliaceus (PF) antigens, frequently together with desmoglein 1, involucrin and keratins specific for the upper layer of the epidermis, were expressed by stratified keratinocytes but not the cells in the monolayers. The Western blotting study of the cultured keratinocyte extract showed 160- and 130-kDa bands positive for desmoglein 1 antigens and a 130-kDa band stained with PV sera. These findings suggest that although desmoglein 1 molecules bear PF antigenic sites, their expression pattern by cultured keratinocytes is closely related to that of PV rather than PF antigens. The PF antigenic sites may be formed on desmoglein 1 during epidermal differentiation.

Ultrastructural localization of pemphigus vulgaris and pemphigus foliaceus antigens in cultured human squamous carcinoma cells

The ultrastructural localization of the pemphigus vulgaris (PV) and pemphigus foliaceus (PF) antigens in cultured human squamous carcinoma cells was observed using immunogold electron microscopy. Both the PV and PF autoantibodies bound only to the extracellular portion of the desmosomal structures. After incubation at 37 degrees C, the PV antigen-antibody complexes were observed within the cultured cells. PV and PF antigen expression was markedly reduced when the cells were cultured in medium with a low Ca2+ concentration.

Ultrastructural binding site of pemphigus foliaceus autoantibodies: comparison with pemphigus vulgaris

We studied in vivo binding sites of pemphigus foliaceus (PF) autoantibodies by immuno-gold labelling technique, and compared them with those of pemphigus vulgaris (PV). In early acantholytic lesions of PF, the bound antibodies indicated by 5 nm protein A-colloidal gold particles were observed on the surface of keratinocytes, with particular affinity for desmosomes and separated attachment plaques. Nondesmosomal cell surfaces were sparsely labeled with the gold particles. A similar binding pattern was seen in the epidermal sheets obtained from a PV patient utilizing the Nikolsky phenomenon. These findings indicate that both PF and PV antigen-antibody complexes are densely located on the desmosomal areas in early pemphigus lesions, suggesting the pathogenic importance of functional impairment of desmosomes by the autoantibodies.