97 kDa linear IgA bullous dermatosis antigen localizes in the lamina lucida between the NC16A and carboxyl terminal domains of the 180 kDa bullous pemphigoid antigen

Linear IgA bullous dermatosis is an autoimmune blistering disease characterized by circulating IgA antibasement membrane autoantibodies, A 97 kDa protein (97-LAD), which localizes at the basement membrane zone of normal human skin, is one of the major autoantigens associated with this disease and possesses multiple regions of amino acid identity with the extracellular domain of the 180 kDa bullous pemphigoid antigen, BPAG2. To investigate further the relationship between 97-LAD and BPAG2, immunogold electron microscopy was performed on cryo-ultrathin sections of normal human skin using a series of polyclonal and monoclonal antibodies. Gold particles immunolabeling two newly developed monoclonal antibodies against 97-LAD were localized to the lamina lucida, This immunolabeling pattern was associated with hemidesmosomes and localized at a mean distance of 28 nm beneath the plasma membrane of basal keratinocytes. In contrast, polyclonal antibodies against a fusion protein containing the NC16A domain of BPAG2 immunolabeled the plasma membrane of the hemidesmosomal complex, whereas polyclonal antibodies against the carboxyl terminus mainly immunolabeled the lower lamina lucida with a mean distance of 42 nm beneath the plasma membrane. By double immunolabeling, 97-LAD was localized as if being sandwiched between the NC16A and the carboxyl terminal domains of BPAG2, These results clearly demonstrated the co-localization of 97-LAD and the extracellular portion of BPAG2 in the lamina lucida, and suggested that 97-LAD is closely related to, and/or forms a complex with, the extracellular domain of BPAG2.