Epitopes in the Linker Subdomain Region of Envoplakin Recognized by Autoantibodies in Paraneoplastic Pemphigus Patients

Sera from paraneoplastic pemphigus (PNP) immunoprecipitate multiple antigens from human epidermal protein extract. In this study, we further characterized the autoantibodies in 12 PNP sera. Immunoblotting using recombinant linker subdomains of envoplakin, periplakin, desmoplakin, and bullous pemphigoid antigen I found that 11 of the 12 sera recognized linker subdomains of envoplakin and periplakin. We then synthesized 12 peptides covering the linker subdomain of envoplakin for ELISA. One of the peptides, peptide no. 8, was recognized by nine out of the 12 sera with a higher affinity. A method of ligand–receptor binding assay was designed and performed using this peptide labeled with fluorescence as the ligand. Peptide no. 8 bound to CD20 + cells in Castleman's tumors from the patients whose sera were positive to this peptide by ELISA. Our data suggest that the linker subdomain of plakin proteins may be one of the major areas recognized by PNP autoantibodies, and epitopes in the linker subdomain of envoplakin recognized by PNP autoantibodies with a high affinity are dispersed in several areas and are variable among PNP patients. We also demonstrate that B-lymphocyte clones specifically reacting to epidermal proteins exist in Castleman's tumors from PNP.