Difference in binding sites of autoantibodies against 230- and 170-kD bullous pemphigoid antigens on salt-split skin

The purpose of this study is to clarify a relationship between bullous pemphigoid autoantibodies against two major BP antigens, 230 kilodalton (kD) (BPAG1) and 180 kD (BPAG2) and their binding sites, using immunoblot analysis and indirect immunofluorescence on salt-split skin. Of the 135 sera obtained from patients in whom bullous pemphigoid had previously been diagnosed by clinical and immunopathologic criteria, all 52 sera recognizing only BPAG1 stained only the epidermal side of split skin (epidermal pattern). Of 24 sera recognizing only BPAG2, 20 showed the epidermal pattern and four stained both the epidermal and dermal sides (combined pattern). Of 42 recognizing both BPAG1 and BPAG2, 35 showed the epidermal pattern and seven showed the combined pattern. Of 17 that reacted with neither antigen, nine showed the epidermal pattern, four showed the combined pattern, and four stained only the dermal side (dermal pattern). Two of the four cases that showed a dermal pattern were retrospectively identified as epidermolysis bullosa acquisita. Immunoelectron microscopy confirmed that a serum with combined pattern bound to both intracellular and extracellular sites of hemidesmosomes. Our results suggest that autoantibodies that react solely with BPAG1 bind exclusively to the epidermal side of salt-split skin and never show either a combined or a dermal pattern, and that most antibodies against BPAG2 bind to the epidermal side. The combined pattern suggests the presence of autoantibodies against the extracellular epitopes of BPAG2 that are separated from the epidermis during the salt-splitting process.