Bullous pemphigoid (BP) and linear IgA disease (LAD) are autoimmune subepidermal blistering skin disease associated with autoantibodies against the transmembrane hemidesmosomal protein BP180/type XVII collagen. It has been demonstrated previously that BP is characterized predominantly by IgG autoantibodies, while autoantibodies in LAD mainly belong to the IgA isotype. The aim of the present study was to investigate the hypothesis that there is a significant overlap in the autoantibody isotype associated with these two disease. Several new recombinant forms of BP180 were generated in the baculovirus expression system, including the full-length protein. IgG autoantibodies to BP 180 were detectable in 39 of 40 (98%) of BP sera; interestingly, 88% of BP sera also contained IgA anti-BP180 autoantibodies. Similarly, anit-BP180 reactivity in LAD sera (n=22) was also attributed to both an IgA (68%) and an IgG (76%) autoanitbody response. IgA and IgG autoantibodies to the intracellular portion of BP180 were found in 14% and 28% of BP sera, respectively, and in 8% of LAD sera (same percentage for both isotypes). Our findings clearly demonstrate that both BP and LAD patients have dual IgA and IgG autoimmune response to BP180 which is directed not only to the ectodomain, but also to the intracellular portion of this protein. (C) 2000 Academic Press.