The Autoantibodies to α6β4 Integrin of Patients Affected by Ocular Cicatricial Pemphigoid Recognize Predominantly Epitopes Within the Large Cytoplasmic Domain of Human β4

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Abstract
This study was undertaken to characterize the antigenic determinants recognized by the autoantibodies of patients with ocular cicatricial pemphigoid (OCP). OCP is a subepithelial, blistering, autoimmune disease that mainly affects the conjunctiva and other mucous membranes. We previously demonstrated that a cDNA clone, isolated from a keratinocyte expression library by using immunoaffinity-purified OCP autoantibody, encoded the cytoplasmic domain of β4 integrin subunit. Our subsequent studies showed that sera from all the OCP patients that were tested recognize the human β4 integrin subunit. To identify the prevalent epitopes of the anti-β4 autoantibodies of OCP, we have used cell lines transfected with vectors encoding a wild-type β4 subunit, a tailless β4 subunit, or a β4 subunit lacking the extracellular domain. Nontransfected cell lines were used as controls. Lysates from these cell lines were analyzed with OCP sera, IgG fractions from OCP sera, and immunoaffinity-purified OCP autoantibodies. Abs to extracellular and cytoplasmic domains of human β4 integrin were used as positive controls, whereas normal human sera and normal human IgG fractions were used as negative controls. The reactivity of OCP Abs was determined by using immunoblotting, immunoprecipitation, and FACS analysis. The results of this study indicate that OCP sera, OCP IgG fractions, and immunoaffinity-purified OCP autoantibodies react with the intracellular and not the extracellular domain of human β4 integrin subunit. In vitro cell culture experiments demonstrated that OCP autoantibody binds to the cytoplasm of the cells. The relevance of these findings to the pathogenesis of OCP is discussed.

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