Tight clustering of extracellular BP180 epitopes recognized by bullous pemphigoid autoantibodies

Bullous pemphigoid is a blistering skin disease associated with autoantibodies against the BP180 antigen, a transmembrane component of the hemidesmosome. Anti-BP180 antibodies have been demonstrated to be pathogenic in a passive transfer mouse model. One extracellular site on human BP180 (MCW-1) was previously shown to be recognized by 50-60% of bullous pemphigoid sera. To facilitate the identification of additional autoantibody-reactive epitopes, recombinant forms of the BP180 ectodomain were generated using both bacterial and mammalian expression systems. One recombinant protein, sec180e, that was expressed in COS-1 cells and that contained the entire BP180 ectodomain, provided us with a tool to detect conformational epitopes. Bullous pemphigoid sera immunoadsorbed against the major noncollagenous NC16A domain no longer reacted with sec180e, indicating that autoantibody reactivity to the BP180 ectodomain is restricted to the NC16A region. Immunoblot analysis of bullous pemphigoid sera immunoadsorbed with a series of recombinant NC16A peptides revealed the presence of three novel autoantigenic sites that, along with the MCW-1 epitope, are clustered within the N-terminal 45 amino acid stretch of NC16A. All 15 bullous pemphigoid sera tested reacted with a recombinant protein containing this BP180 segment. No disease-associated epitopes were detectable within the remaining 28 amino acids of NC16A. Thus, bullous pemphigoid patient autoantibodies react with a set of epitopes on the BP180 ectodomain that are highly clustered. This autoantibody-reactive region on human BP180 shows overlap with the corresponding murine BP180 site that is targeted by antibodies that are pathogenic in the mouse model of bullous pemphigoid. These findings suggest new directions for the development of diagnostic and therapeutic tools for this disease.