Nonclassical pemphigus with exclusively IgG anti‐desmocollin 3‐specific antibodies

Significance of anti-desmocollin autoantibodies in pemphigus

The major autoantigens for pemphigus are desmogleins (Dsgs), cell-cell adhesive structure proteins, one of the desmosomal cadherins. Recent progress in molecular biology has revealed that IgG autoantibodies of classical pemphigus react with Dsg1 or Dsg3. Desmocollins (Dscs) also belong to the cadherin supergene family that provides structure to the desmosomes and play an important role in cell-to-cell adhesion. In addition to the presence of four desmosomal Dsg isoforms, i.e. Dsg1-4, Dsc1, 2 and 3, all of which are derived from different genes, Dsc1 has been previously identified as the target antigen of IgA autoantibodies in the subcorneal pustular dermatosis (SPD)-type of intercellular IgA dermatosis. In addition to the IgA anti-Dsc1 autoantiboides, the presence of IgG anti-Dsc autoantibodies is described in patients of some autoimmune bullous diseases. In particular, the current pemphigus detecting autoantibodies to Dscs has shown a tendency in atypical variants of pemphigus. Therefore, autoantibodies against Dscs alone may cause detachment of cell-cell adhesion in the epidermis in some pemphigus. However, except for the findings of a few in vitro and in vivo studies, there is currently no clear evidence for the pathogenicity of anti-Dsc autoantibodies in pemphigus, whereas significance of anti-Dsg autoantibodies is well established. This article describes the structure and function of the Dscs, and explores the evidence regarding the pathogenic role of anti-Dsc autoantibodies in pemphigus.

Anti-Desmocollin Autoantibodies in Autoimmune Blistering Diseases

The presence of anti-desmocollin (Dsc) antibodies is rarely described in autoimmune blistering diseases patients. Moreover, several clinical phenotypes of pemphigus may be associated with these antibodies. In this review we analyze clinicopathological, immunologic and outcome features of anti-Dsc autoimmune blistering diseases patients, to improve their diagnosis and management. We conducted a systematic search of PubMed and Embase (1990-present) for studies reporting cases of autoimmune blistering diseases with anti-Dsc antibodies. We classified the selected patients as patients with exclusively anti-Dsc autoantibodies, and patients with anti-Dsc and other autoantibodies. Of 93 cases with anti-Dsc autoantibodies included, 38 (41%) had exclusively these antibodies. Only 18% of patients presented with the typical clinicopathological phenotype of pemphigus vulgaris or pemphigus foliaceous. Mucosal involvement was seen in approximately half of the patients. Up to 18% of cases were associated with neoplasms. Acantholysis was described in 54% of cases with histopathological information. Treatments and outcomes vary in the different clinical phenotypes. The presence of anti-Dsc antibodies must be suspected mainly in those patients with either atypical pemphigus, in special with clinical pustules, or in cases showing intraepithelial or dermal neutrophilic/eosinophilic infiltrate on histological examination and dual pattern by direct immunofluorescence examination.

Human desmocollin 3-specific IgG antibodies are pathogenic in a humanized HLA-class II transgenic mouse model of pemphigus

Pemphigus is a potentially lethal autoimmune bullous skin disorder, which is associated with IgG autoantibodies against desmoglein 3 (Dsg3) and Dsg1. Notably, a subset of pemphigus patients presents with a similar clinical phenotype in the absence of anti-Dsg IgG, suggesting the presence of serum IgG reactive with desmosomal components other than Dsg1 or Dsg3. We and others have previously shown that such patients have serum IgG autoantibodies against desmocollin 3 (Dsc3), a component of desmosomes, that induce loss of keratinocyte adhesion ex vivo. Moreover, Dsc3 hypomorphic mice show a severe blistering phenotype of the mucous membrane which is highly characteristic in pemphigus. These findings prompted us to study induction and regulation of anti-human Dsc3 IgG in humanized mice transgenic for HLA-DRB1*04:02, which is a highly prevalent haplotype in pemphigus. We show that IgG from sera of immunized mice induce acantholysis in a dispase-based keratinocyte dissociation assay via the activation of p38 mitogen-activated protein kinases and epidermal growth factor receptor. Passive IgG transfer from mice immunized with recombinant human Dsc3 into neonates did not induce intraepidermal loss of adhesion presumably due to the lack of homology between human and mouse Dsc3. Ex vivo stimulation of splenocytes from Dsc3-immunized mice with human Dsc3 leads to a significant proliferative interferon-γ and interleukin 4 T cell response, which is restricted by HLA-DR/DQ. These findings suggest that induction of pathogenic anti-Dsc3 IgG is associated with Dsc3-specific T cells that recognize Dsc3 in association with HLA-DRB1*04:02.

Immune serological diagnosis of pemphigus

Pemphigus is a rare autoimmune blistering disease which manifests with painful erosions and blisters of the skin and mucosa. This disorder is caused by autoantibodies attacking desmosomal proteins, necessary for cell-cell contact stability and epidermal integrity. Desmoglein (Dsg) 1 and Dsg3 are the two major target antigens in pemphigus. Yet, many other target proteins, which have been described over the years, seem to be involved in the loss of epidermal integrity. Clinical examination, combined to serological advances and detection of targeted antigens, permitted to differentiate among several pemphigus subtypes, in which pemphigus vulgaris and pemphigus foliaceus are the most common. Nowadays, serological analysis in pemphigus is a fundamental step of the diagnostic algorithm. This is based on analysis of clinical symptoms, histopathological examination of lesional skin, detection of tissue bound and circulating antibodies by direct and indirect immunofluorescence, and determination of target antigens either by enzyme-linked immunosorbent essay (ELISA) or by western blot analysis. A correct and exhaustive diagnostic algorithm is fundamental to characterize pemphigus subtypes, which lastly permits to adopt a correct treatment approach. Moreover, quality and quantity of circulating antibodies in patient's sera deliver important information regarding clinical course, disease severity and treatment response; thus, relevantly affecting physician's decision. To facilitate this process, "easy-to-perform" diagnostic kits with high sensitivity and specificity are being commercialized. In this review, we focus on available methods and established assays to correctly detect circulating autoantibodies in pemphigus. Moreover, we discuss subtype specific serological peculiarities in the five most relevant subtypes (pemphigus vulgaris, pemphigus foliaceus, pemphigus vegetans, paraneoplastic pemphigus and intercellular IgA dermatosis (also called as IgA pemphigus).

Development of a Desmocollin-3 Active Mouse Model Recapitulating Human Atypical Pemphigus

Pemphigus vulgaris (PV) is a life-threatening mucocutaneous autoimmune blistering disease. It is often associated with autoantibodies to the desmosomal adhesion proteins Desmoglein 3 (DSG3) and Desmoglein 1 (DSG1). Recently, auto-antigens, such as desmocollins and others have been described in PV and in atypical pemphigus forms such as Pemphigus Herpetiformis (PH), Pemphigus Vegetans (PVeg), and Paraneoplastic Pemphigus (PP). Desmocollins belong to a cadherin subfamily that provides structure to the desmosomes and play an important role in cell-to-cell adhesion. In order to verify the pathogenic activity of anti-Desmocollin 3 (DSC3) antibodies, we developed an active disease model of pemphigus expressing anti-DSC3 autoantibodies or anti-DSC3 and anti-DSG3 antibodies. This approach included the adoptive transfer of DSC3 and/or DSG3 lymphocytes to Rag2^-/- immunodeficient mice that express DSC3 and DSG3. Our results show that the presence of anti-DSC3 auto-antibodies is sufficient to determine the appearance of a pathological phenotype relatable to pemphigus, but with features not completely super-imposable to those observed in the DSG3 active model, suggesting that the DSC3 active model might mimic the atypical pemphigus. Moreover, the presence of both anti-DSC3 and anti-DSG3 antibodies determines a more severe phenotype and a slower response to prednisolone. In conclusion, we have developed an adult DSC3 pemphigus mouse model that differs from the DSG3 model and supports the concept that antigens other than desmogleins may be responsible for different phenotypes in human pemphigus.