Identification of Desmocollin-1 as a Major Target Autoantigen in Canine Pemphigus Foliaceus

Canine pemphigus foliaceus (cPF) is the most common autoimmune skin disease in this species. Although several important questions about the pathogenesis have been addressed over the last three decades, for example the pathogenicity of circulating anti-keratinocyte IgG autoantibodies and their desmosomal targeting, the identity of the antigen(s) against which the majority of affected dogs produce autoantibodies remains unknown.;In our first set of experiment, we postulated that the candidate autoantigen(s) in cPF should be involved in keratinocyte adhesion, and, therefore, would likely be an adhesion molecule. To test this hypothesis, we first identified indirect immunofluorescence (IIF) patterns of selected desmosomal and non-desmosomal adhesion proteins in normal canine footpad, haired skin and buccal mucosal epithelia. Secondly, we identified the most common IIF staining pattern among cPF sera, which was then compared to those of the desmosomal and non-desmosomal adhesion proteins. Out of the 66 cPF sera, 53 (80%) contained anti-keratinocyte IgG that only stained canine footpad epithelium but not buccal mucosa. A comparison of this common pattern to those of desmosomal and non-desmosomal adhesion proteins revealed a close match with that of desmocollin-1 (DSC1). This observation led to the hypothesis that DSC1 is a major autoantigen in cPF.;To address this hypothesis, canine DSC1 (cDSC1) was cloned and expressed by human kidney epithelial cells (293T). Using a live-cell IIF, 85 cPF sera were screened for the presence of anti-DSC1 IgG. Sera from 35 healthy dogs, eight from exfoliative superficial pyoderma (ESP)-affected dogs and 21 dogs with non-PF autoimmune blistering skin diseases (AIBSD) served as controls. All sera were tested concurrently by IIF on canine DSG1-transfected as well as nontransfected cells. Using this approach, we were able to demonstrate that 83%, 70% and 24% of IIFpos cPF and PTPF (Promeris-triggered PF), IIFneg cPF and non-PF AISBD, respectively, contained IgG antibodies binding to the ectopically expressed cDSC1 in its native conformation. A concurrent reactivity to both DSC1 and DSG1 was found in five cPF sera. None of the sera from healthy dogs or dogs with ESP reacted on any of the transfected cells. Immunoabsorption with purified human DSC1 was able to markedly reduce or abolish the subsequent IIF staining on footpad epithelium, thereby confirming that most of the anti-keratinocyte IgG autoantibodies were directed against this molecule. Finally, in seven out of ten dogs where serial serum samples were obtained before and one to two months after initiating treatment, a reduction in anti-DSC1 IgG titers was seen that matched the improvement of clinical signs.;Subsequent experiments using immunoprecipitation-immunoblotting revealed DSC1-binding autoantibodies in sera from cPF patients and also in sera from healthy controls. The latter were undetectable by indirect immunofluorescence. Further studies are needed to identify the precise specificity of these antibodies in sera from apparently normal dogs.;Altogether, our data suggest that DSC1 is the major autoantigen in cPF. The identification of this major cPF autoantigen will allow future research to decipher the mechanism of lesion formation in this disease and to develop novel diagnostic and therapeutic strategies for dogs with this common autoimmune blistering skin disease.