Bullous pemphigoid sera that contain antibodies to BPAg2 also contain antibodies to LABD97 that recognize epitopes distal to the NC16A domain

IgG antibodies from the sera of some patients with bullous pemphigoid (BP) react with a 180 kDa protein termed BPAg2. Antibodies in BP are directed to an extracellular noncollagenous domain of this protein termed NC16A. Our g roup has recently shown that a portion of the extracellular domain of BPAg2 is identical to LABD97 on the basis of amino acid sequencing. We evaluated sera from 33 patients with BP with circulating IgG antibodies on indirect immunofluorescence, which stained the epidermal side of split skin with tit ers ranging from 1:40 to 1:640. Immunoblotting was performed against (i) tw o preparations of proteins from epidermal extract, one containing BPAg2 and one containing LABD97, and (ii) the recombinant NC16A domain of the BPAg2 protein. Twelve sera reacted with the BPAg2 protein. Ten of these also reac ted strongly with the NC16A domain. Nine of the 12 sera also reacted with t he LABD97 antigen. Bound antibodies were eluted front the 97 kDa band and r eapplied to split skin where they bound to the epidermal side. The eluted a ntibodies also reacted to the BPAg2 protein from the epidermal extract, but did not react with the NC16A domain on immunoblot. We conclude that these nine sera react with an epitope present within BPAg2 and LABD97 but not wit hin the NC16A domain. This epitope is therefore distal to the previously de scribed epitopes in BP. In BP, epitope spreading may occur and antibodies m ay be produced that recognize the distal portion of the BPAg2 molecule iden tical to LABD97 but that do not involve the NC16A domain.