Bullous pemphigoid (BP) is a blistering skin disease in which autoantibodies develop to hemidesmosomal components of the epidermal basement membrane zone, including two major antigenic proteins of the 230-kD antigen (BPAG1) and the 180-kD antigen (BPAG2). The present study demonstrated the precise ultrastructural localization of the epitopes for autoantibodies against BPAG1 and BPAG2 in normal skin. Autoantibodies against either BPAG1 or BPAG2 were affinity-purified using nitrocellulose membrane, which was blotted with SDS-PAGE-fractionated antigens from human epidermal extract as the immunoabsorbent. Postembedding, immunogold electron microscopy was performed after skin was processed by rapid freezing and freeze substitution fixation without chemical fixatives. Purified autoantibodies against BPAG1 bound only to the intracellular domain of the hemidesmosome, and 80% of the gold labeling was within 40-140 nm from the plasma membrane (mean distance 91 nm inside). In contrast, the autoantibodies against BPAG2 bound along the plasma membrane of the hemidesmosome, and 80% of the gold labeling was within 10 nm outside to 50 nm inside the cells (mean distance 12 nm inside). These results suggest that the autoantibodies against BPAG1 and BPAG2 react with the epitopes localizing in distinct regions of the hemidesmosome complex, and may play different roles in the blister formation in patients with BP.
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