Demonstration of antibodies to bovine desmocollin isoforms in certain pemphigus sera

Synergy among non-desmoglein antibodies contributes to the immunopathology of desmoglein antibody-negative pemphigus vulgaris

Pemphigus vulgaris (PV) is a potentially lethal mucocutaneous blistering disease characterized by IgG autoantibodies (AuAbs) binding to epidermal keratinocytes and inducing this devastating disease. Here, we observed that non-desmoglein (Dsg) AuAbs in the sera of patients with Dsg1/3 AuAb-negative acute PV are pathogenic, because IgGs from these individuals induced skin blistering in neonatal mice caused by suprabasal acantholysis. Serum levels of AuAbs to desmocollin 3 (Dsc3), M3 muscarinic acetylcholine receptor (M3AR), and secretory pathway Ca2+/Mn2+-ATPase isoform 1 (SPCA1) correlated with the disease stage of PV. Moreover, AuAb absorption on recombinant Dsc3, M3AR, or SPCA1 both prevented skin blistering in the passive transfer of AuAbs model of PV in BALB/c mice and significantly decreased the extent of acantholysis in a neonatal mouse skin explant model. Although acantholytic activities of each of these immunoaffinity-purified AuAbs could not induce a PV-like phenotype, their mixture produced a synergistic effect manifested by a positive Nikolskiy sign in the skin of neonatal mice. The downstream signaling of all pathogenic non-Dsg AuAbs involved p38 mitogen-activated protein kinase (MAPK)-mediated phosphorylation and elevation of cytochrome c release and caspase 9 activity. Anti-Dsc3 and anti-SPCA1 AuAbs also activated SRC proto-oncogene, nonreceptor tyrosine kinase (SRC). Of note, although a constellation of non-Dsg AuAbs apparently disrupted epidermal integrity, elimination of a single pathogenic AuAb could prevent keratinocyte detachment and blistering. Therefore, anti-Dsg1/3 AuAb-free PV can be a model for elucidating the roles of non-Dsg antigen-specific AuAbs in the physiological regulation of keratinocyte cell-cell adhesion and blister development.

The Evolving Story of Autoantibodies in Pemphigus Vulgaris: Development of the "Super Compensation Hypothesis"

Emerging data and innovative technologies are re-shaping our understanding of the scope and specificity of the autoimmune response in Pemphigus vulgaris (PV), a prototypical humorally mediated autoimmune skin blistering disorder. Seminal studies identified the desmosomal proteins Desmoglein 3 and 1 (Dsg3 and Dsg1), cadherin family proteins which function to maintain cell adhesion, as the primary targets of pathogenic autoAbs. Consequently, pathogenesis in PV has primarily considered to be the result of anti-Dsg autoAbs alone. However, accumulating data suggesting that anti-Dsg autoAbs by themselves cannot adequately explain the loss of cell-cell adhesion seen in PV, nor account for the disease heterogeneity exhibited across PV patients has spurred the notion that additional autoAb specificities may contribute to disease. To investigate the role of non-Dsg autoAbs in PV, an increasing number of studies have attempted to characterize additional targets of PV autoAbs. The recent advent of protein microarray technology, which allows for the rapid, highly sensitive, and multiplexed assessment of autoAb specificity has facilitated the comprehensive classification of the scope and specificity of the autoAb response in PV. Such detailed deconstruction of the autoimmune response in PV, beyond simply tracking anti-Dsg autoAbs, has provided invaluable new insights concerning disease mechanisms and enhanced disease classification which could directly translate into superior tools for prognostics and clinical management, as well as the development of novel, disease specific treatments.

Anti-desmocollin autoantibodies in nonclassical pemphigus

BACKGROUND

Despite the established pathogenic role of anti-desmoglein (Dsg) antibodies in classical pemphigus, the significance of autoantibodies to another desmosomal cadherin, desmocollin (Dsc) is at present unknown. No consistent immunoassay for immunoglobulin (Ig) G autoantibodies to Dscs has been developed.

OBJECTIVES

The aim of this study was to develop reliable assays to detect anti-Dsc autoantibodies.

METHODS

We expressed soluble recombinant proteins (RPs) of human Dsc1-3 in mammalian cells and examined sera of various types of pemphigus, including 79 paraneoplastic pemphigus (PNP) sera, by novel enzyme-linked immunosorbent assays (ELISAs) using the RPs. We also performed ELISAs of Dsc baculoproteins and used the complementary DNA (cDNA) transfection method, and compared the results with those of mammalian ELISAs.

RESULTS

Through mammalian ELISAs, IgG autoantibodies to Dsc1, Dsc2 and Dsc3 were detected in 16.5%, 36.7% and 59.5% of PNP sera, respectively, and considerable numbers of pemphigus herpetiformis (PH) and pemphigus vegetans (PVeg) sera reacted strongly with Dsc1 and Dsc3. Mammalian ELISAs were highly specific and more sensitive than baculoprotein ELISAs or the cDNA transfection method. Several Dsc-positive sera, particularly PH sera, showed no reactivity with Dsgs. The reactivity of PNP serum and PVeg serum with Dscs was not abolished by pre-absorption with Dsg RPs.

CONCLUSIONS

The results of these novel ELISAs indicated that IgG anti-Dsc autoantibodies were frequently detected and potentially pathogenic in nonclassical pemphigus.

Pemphigus vulgaris autoantibody profiling by proteomic technique

Pemphigus vulgaris (PV) is a mucocutaneous blistering disease characterized by IgG autoantibodies against the stratified squamous epithelium. Current understanding of PV pathophysiology does not explain the mechanism of acantholysis in patients lacking desmoglein antibodies, which justifies a search for novel targets of pemphigus autoimmunity. We tested 264 pemphigus and 138 normal control sera on the multiplexed protein array platform containing 701 human genes encompassing many known keratinocyte cell-surface molecules and members of protein families targeted by organ-non-specific PV antibodies. The top 10 antigens recognized by the majority of test patients’ sera were proteins encoded by the DSC1, DSC3, ATP2C1, PKP3, CHRM3, COL21A1, ANXA8L1, CD88 and CHRNE genes. The most common combinations of target antigens included at least one of the adhesion molecules DSC1, DSC3 or PKP3 and/or the acetylcholine receptor CHRM3 or CHRNE with or without the MHC class II antigen DRA. To identify the PV antibodies most specific to the disease process, we sorted the data based on the ratio of patient to control frequencies of antigen recognition. The frequency of antigen recognition by patients that exceeded that of control by 10 and more times were the molecules encoded by the CD33, GP1BA, CHRND, SLC36A4, CD1B, CD32, CDH8, CDH9, PMP22 and HLA-E genes as well as mitochondrial proteins encoded by the NDUFS1, CYB5B, SOD2, PDHA1 and FH genes. The highest specificity to PV showed combinations of autoantibodies to the calcium pump encoded by ATP2C1 with C5a receptor plus DSC1 or DSC3 or HLA-DRA. The results identified new targets of pemphigus autoimmunity. Novel autoantibody signatures may help explain individual variations in disease severity and treatment response, and serve as sensitive and specific biomarkers for new diagnostic assays in PV patients.

Desmosomal adhesion and pemphigus vulgaris: the first half of the story

Pemphigus vulgaris (PV) is a paradigm of autoimmune disease affecting intercellular adhesion. The mechanisms that lead to cell-cell detachment (acantholysis) have crucial therapeutic implications and are currently undergoing major scrutiny. The first part of this review focuses on the classical view of the pathogenesis of PV, which is dominated by the cell adhesion molecules of the desmosome, namely desmogleins (Dsgs). Cloning of the DSG3 gene, generation DSG3 knock-out mice and isolation of monoclonal anti-Dsg3 IgG have aided to clarify the pathogenic mechanisms of PV, which are in part dependent on the fate of desmosomal molecules. These include perturbation of the desmosomal network at the transcriptional, translational, and interaction level, kinase activation, proteinase-mediated degradation, and hyper-adhesion. By the use of PV models, translational research has in turn helped shed light into the basic structure, function, and dynamics of assembly of desmosomal cadherins. The combined efforts of basic and applied research has resulted in tremendous advance into the understanding of epidermal adhesion and helped debunk old myths on the supposedly unique role of desmogleins in the mechanisms of cell-cell detachment in PV.

Urban legends: pemphigus vulgaris

Pemphigus vulgaris (PV) is the most common type of pemphigus. PV pathogenesis is still debated, and treatment remains challenging. We investigated five controversial topics: (1) What are the target antigens in PV? (2) Do desmogleins adequately address PV pathophysiology? (3) How does acantholysis occur in PV? (4) Is PV still a lethal disease? (5) What is the role of rituximab (RTX) in PV treatment? Results from extensive literature searches suggested the following: (1) Target antigens of PV include a variety of molecules and receptors that are not physically compartmentalized within the epidermis. (2) PV is caused by a variety of autoantibodies to keratinocyte self-antigens, which concur to cause blistering by acting synergistically. (3) The concept of apoptolysis distinguishes the unique mechanism of autoantibody-induced keratinocyte damage in PV from other known forms of cell death. (4) PV remains potentially life-threatening largely because of treatment side effects, but it is uncertain which therapies carry the highest likelihood of lethal risk. (5) RTX is a very promising treatment option in patients with widespread recalcitrant or life-threatening PV. RTX's cost is an issue, its long-term side effects are still unknown, and randomized controlled trials are needed to establish the optimal dosing regimen.

Pemphigus autoimmunity: hypotheses and realities

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

IgG autoantibodies against desmocollin 3 in pemphigus sera induce loss of keratinocyte adhesion

Pemphigus is considered an autoimmune bullous skin disorder associated with IgG against the desmosomal components, desmoglein 3 (Dsg3) and desmoglein 1 (Dsg1). This concept is supported by the in vitro and in vivo pathogenicity of anti-Dsg3/Dsg1 IgG and the mucosal blistering phenotype of mice with a genetic deficiency of Dsg3. Mice deficient for another desmosomal adhesion molecule, desmocollin 3 (Dsc3), show a similar pemphigus phenotype, and we investigated the pathogenicity of Dsc3-reactive IgG autoantibodies that were identified previously in a subset of patients with atypical pemphigus. We here demonstrate that IgG against Dsc3 causes loss of adhesion of epidermal keratinocytes. Specifically, IgG against Dsc3 was purified from Dsc3-reactive pemphigus sera by affinity column chromatography using recombinant human Dsc3. Affinity purified IgG was functionally active and did not only react with recombinant Dsc3 but also with epidermis and cultured human keratinocytes. Moreover, Dsc3-reactive IgG induced loss of adhesion of epidermal keratinocytes in a dispase-based keratinocyte dissociation assay that was reversed on pre-adsorption with human Dsc3 but not Dsg3. These findings demonstrate that IgG autoantibodies against an additional component of the desmosomes, Dsc3, induce loss of keratinocyte adhesion and thus may contribute to blister formation in pemphigus.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Desmosome structure, composition and function

Desmosomes are intercellular junctions of epithelia and cardiac muscle. They resist mechanical stress because they adopt a strongly adhesive state in which they are said to be hyper-adhesive and which distinguishes them from other intercellular junctions; desmosomes are specialised for strong adhesion and their failure can result in diseases of the skin and heart. They are also dynamic structures whose adhesiveness can switch between high and low affinity adhesive states during processes such as embryonic development and wound healing, the switching being signalled by protein kinase C. Desmosomes may also act as signalling centres, regulating the availability of signalling molecules and thereby participating in fundamental processes such as cell proliferation, differentiation and morphogenesis. Here we consider the structure, composition and function of desmosomes, and their role in embryonic development and disease.

Cholinergic control of epidermal cohesion

The non-neuronal cholinergic system of human epidermis includes the keratinocyte (KC) acetylcholine (ACh) axis composed of the enzymes mediating ACh synthesis and degradation, and two classes of ACh receptors, the nicotinic and muscarinic ACh receptors, mediating biological effects of the cutaneous cytotransmitter ACh. Regulation of KC cell-cell and cell-matrix adhesion is one of the important biological functions of cutaneous ACh. The downstream targets of ACh effects mediated by distinct ACh receptor subtypes include both the intercellular adhesion molecules, such as classical and desmosomal cadherins, and integrins mediating KC adhesion to a substrate. The signaling pathways include activation or inhibition of kinase cascades resulting in either up- or down-regulation of the expression of cell adhesion molecules or changes in their phosphorylation status, or both. The components of the KC ACh axis are involved in cutaneous blistering in patients with autoimmune pemphigus, junctional and dystrophic forms of epidermolysis bullosa, thermal burns, and mustard-induced vesication. Recent progress with the development of antiacantholytic therapies of patients with pemphigus using cholinomimetics indicates that cholinergic drugs may be a promising approach for other cutaneous blistering disorders.

The profile of fogo selvagem (endemic pemphigus foliaceus) at the University Hospital of Brasilia-Brazil. Epidemiological and clinical considerations

BACKGROUND

Endemic pemphigus foliaceus (EPF) or fogo selvagem (FS) is an endemic autoimmune disease, characterized by flaccid bullae induced by IgG(4) subclass antibodies. The authors demonstrate the epidemiological and clinical status of patients who have been followed at the University Hospital of Brasilia (HUB) for more than 15 years.

METHODS

One hundred and ninety-six patients with FS took part in the project. In setting up this study a historical descriptive cohort of patients was put together. In order to collect data, the authors used a questionnaire where the patient indicated the sex, age at the onset, occupation, origin, clinical status, including scalp compromise, evolution, cofactors influencing clinical worsening and the treatment compliance. In order to minimize loss to follow up, the authors used the statistical method of incidence density (patients/years).

RESULTS

The disease occurred in 58.4% of the young patients in the 11-30-year age bracket, and 52% came from urban areas. These patients included students and teachers. Localized disease predominated as compared with the generalized presentation of this condition. Fifty-nine percent of the patients evolved to the recurrent form. Those patients with the evolutional form, in remission for more than 1 year (94%), had been followed for more than 5 years. Even patients with the less active forms of the disease had scalp lesions. Thus scalp lesions are not an indicator of bad prognosis.

CONCLUSIONS

In the present study, the disease affected patients from a higher sociocultural class than previously described. Furthermore, in contrast to other reports, a substantial number of the patients lived in urban areas, although often spending some time in a rural setting for leisure or professional activities. This study suggests that the longer the follow up, the higher the likelihood that the disease would progress to a more controlled clinical presentation. Scalp lesions were not related to adverse prognosis. Sun, heat and infections act as triggering factors for the immunological imbalance, worsening the disease.

The detection of IgG and IgA autoantibodies to desmocollins 1-3 by enzyme-linked immunosorbent assays using baculovirus-expressed proteins, in atypical pemphigus but not in typical pemphigus

BACKGROUND

We have shown previously that human desmocollin (Dsc) 1 is recognized by IgA autoantibodies of subcorneal pustular dermatosis (SPD) type IgA pemphigus. However, the presence of IgG anti-Dsc autoantibodies is still controversial, and antibodies to Dsc2 and Dsc3 have not been clearly identified.

OBJECTIVES

To investigate this by producing recombinant proteins consisting of the entire extracellular domains of human Dsc1, 2 and 3 in baculovirus, and to use them to establish an enzyme-linked immunosorbent assay (ELISA).

METHODS

By this ELISA, we examined in total 165 cases of various types of autoimmune bullous diseases, as well as 23 normal controls.

RESULTS

None of 45 sera of classical pemphigus showed either IgG or IgA antibodies to any Dsc. In contrast, one atypical pemphigus serum showed both IgG and IgA antibodies to Dsc1, which were adsorbed by incubation with Dsc1 baculoprotein. Furthermore, this ELISA detected both IgA and IgG anti-Dsc3 antibodies in one atypical case, and IgA antibodies to both Dsc2 and Dsc3 in another. This reactivity was confirmed by positive IgA immunofluorescence with Dsc2 and Dsc3 expressed on COS-7 cells. These results show that both IgG and IgA autoantibodies against all of Dsc1-3 are present in the sera of particular cases of nonclassical pemphigus, except for IgG antibodies to Dsc2, but that they are not detected in classical pemphigus. Unexpectedly, although IgA antibodies of all of eight SPD type IgA pemphigus sera reacted with Dsc1 expressed on COS-7 cells, only one serum was positive in Dsc1 ELISA for IgA.

CONCLUSIONS

This result indicates either that Dscs expressed by baculovirus may not adopt the correct conformation or that Dscs may need association with other molecules to express all the epitopes for autoantibodies.

Novel human alpha9 acetylcholine receptor regulating keratinocyte adhesion is targeted by Pemphigus vulgaris autoimmunity

Pemphigus vulgaris (PV) is a potentially fatal autoimmune mucocutaneous blistering disease. It was assumed that PV is caused by anti-desmoglein (Dsg) 3 autoimmunity because absorption of PV sera with a chimeric baculoprotein containing the Dsg 3 and IgG1 portions, rDsg3-Ig-His, eliminated disease-causing antibodies. In this study we demonstrate that rDsg3-Ig-His adsorbs out autoantibodies to different keratinocyte antigens, including a non-Dsg 3 130-kd polypeptide. Because the pool of disease-causing PV IgGs contains antibodies against the keratinocyte acetylcholine receptor (AChR), we sought to identify the targeted receptor(s). Preincubation of monkey esophagus with PV antibodies blocked specific staining of the keratinocyte cell membrane with rabbit monoepitopic antibody to alpha9 AChR, indicating that this first of its kind AChR with dual, muscarinic and nicotinic pharmacology is targeted by PV autoimmunity. Anti-alpha9 antibody stained keratinocytes in a fishnet-like intercellular pattern, and visualized a single band at approximately 50 kd in Western blots of keratinocyte membrane proteins. Using step-by-step reverse transcription polymerase chain reactions with primers based on known alpha9 sequence regions, we identified the complete reading frame of human alpha9. Its amino acid sequence showed 85% similarity with rat alpha9. Treatment of keratinocyte monolayers with anti-alpha9 antibody induced pemphigus-like acantholysis, which could be reversed either spontaneously or by using the cholinergic agonist carbachol. We conclude that alpha9 is coupled to physiological regulation of keratinocyte adhesion, and its interaction with PV IgG may lead to blister development.

Human desmocollin 1a transiently expressed in COS-7 cells and NIH 3T3-3 cells is reacted by IgG4 antibodies in a pemphigus foliaceus serum

Human desmocollin (Dsc) 1 is an autoantigen in the subcorneal pustular dermatosis type of IgA pemphigus. Moreover, Dscs, particularly bovine Dscs, are recognized by IgG antibodies in certain sera of various types of pemphigus by immunoblotting. In this study, 10 pemphigus sera were examined by immunofluorescence for IgG antibodies to human Dsc1a, Dsc2a and Dsc3a transiently expressed in COS-7 and NIH 3T3-3 cells using two different transfection methods. IgG antibodies in a number of sera showed a strong background staining with untransfected COS-7 and NIH 3T3-3 cells. Using COS-7 cells, a pemphigus foliaceus serum, which did not stain untransfected COS-7 cells, was found to contain IgG antibodies reactive exclusively with Dsc1a. This serum showed the same reactivity in studies using NIH 3T3-3 cells. Moreover, this pemphigus foliaceus serum contained IgG4, but not IgG1 antibodies, to Dscla expressed in the NIH 3T3-3 cells. These results indicate that autoantibody response in pemphigus foliaceus might be more heterogenous than hitherto supposed.

Pemphigus with features of both vulgaris and foliaceus variants, associated with antibodies to 160 and 130 kDa antigens

We report an unusual variant of pemphigus in a 44-year-old man. He presented with scaly and crusted erosions associated with pruritic vesicles and erythema mainly on the chest, abdomen, back and face. Histology showed acantholysis with neutrophilic spongiosis in the granular layer and subcorneal region of the epidermis. Intercellular IgG in the epidermis was positive on direct immunofluorescence. Indirect immunofluorescence showed intercellular antibodies at a titre of 1 : 2 in the suprabasal epidermis. Circulating autoantibodies to 130 kDa and 160 kDa antigens were detected in the patient's serum by immunoblot analysis using epidermal extracts. These two antibodies eluted from individual nitrocellulose membranes reacted with the intercellular space in the epidermis on indirect immunofluorescence. This observation suggests that these antibodies correspond to desmogleins 3 and 1, respectively. The clinical symptoms almost completely disappeared after 28 days treatment with oral prednisolone (30 mg/day), leaving brown pigmented flecks on the lesional sites. These findings suggest that this patient's pemphigus has features of both the vulgaris and foliaceus variants, with antibodies against desmogleins 3 and 1.

A YAC contig joining the desmocollin and desmoglein loci on human chromosome 18 and ordering of the desmocollin genes

The desmocollins and desmogleins are members of the cadherin family of adhesive proteins present in the desmosome type of cell-cell junction. All of the known desmoglein and desmocollin isoforms, which have differing tissue and developmental distributions, are coded by very closely linked genes at 18q12.1. We have previously described YAC clones carrying all three known desmoglein (DSG) genes. We have now isolated YAC clones that carry all three known desmocollin genes (DSC1, 2, and 3) from two libraries and also isolated clones that join the DSC locus to the DSG locus, forming a complete contig for the region. Absence of chimeric ends for some of the YACs was confirmed by isolating Vectorette PCR products for the YAC ends and mapping the derived DNA sequences back to other YACs from CEPH. The whole DSC/DSG gene complex occupies no more than about 700 kb, and the genes are arranged in the order cen-3'-DSC3-DSC2-DSC1-5'-5'-DSG1-DSG3-D SG2-3'-tel, so that the two gene clusters are transcribed outward from the interlocus region. A P1 clone carrying part of DSC2 and DSC3 confirmed the relative orientation of transcription of these two genes. The conservation of close genetic linkage may be of trivial importance related to the recent duplication of these genes or may be because there is a region within the locus that is involved in coordinating the expression of the desmoglein and desmocollin genes.

Immunological identification and characterization of the desmosomal cadherin Dsg2 in coupled and uncoupled epithelial cells and in human tissues

Cells of epithelia, but also of certain other tissues such as myocardium and the dendritic reticulum of lymph node follicles, are interconnected by numerous intercellular junctions termed desmosomes. These are characterized by a set of transmembrane glycoproteins, i.e. the desmosomal cadherins, desmoglein(s) and desmocollin(s). Using cDNA-derived hybridization probes, we have previously shown that different desmogleins exist (Dsg1-3) and that only one Dsg isoform, Dsg2, is found in diverse kinds of tissues, tumors and cultured cell lines whereas the synthesis of Dsg1 and Dsg3 is much more restricted, primarily to stratified epithelia [51]. We now report immunocytochemical results obtained with a series of monoclonal and polyclonal antibodies specific for either the aminoterminal extracellular portion or the carboxyterminal cytoplasmic segment of Dsg2. These antibodies detect Dsg2 in all tissues possessing desmosomes, including human stratified and single-layered polar epithelia, as well as non-epithelial tissues such as myocardium and lymph node follices. They also react with the desmosomes of carcinomas and of diverse cultured epithelium-derived cell lines. Moreover, antibodies specific for extracellular domain regions of Dsg2 react with the "half-desmosomes" present on the surfaces of uncoupled intact epithelial cells. Remarkably, in stratified squamous epithelia the Dsg2-reaction is not homogeneous, as this glycoprotein is detected only in the basal cell layer and appears to be absent from suprabasal strata. The potential value of Dsg2-specific antibodies in histology and in tumor diagnosis as well as in studies of the mechanisms desmosomal cell coupling is discussed.