Super-resolution imaging detects BP180 autoantigen in immunoglobulin M pemphigoid

Bullous pemphigoid is generally caused by immunoglobulin (Ig)G autoantibodies against hemidesmosomal BP180 and/or BP230. Recently, the concept of IgM pemphigoid has been proposed. A 23-year-old Japanese woman presented with a 4-month history of severely itchy papules showing subepidermal separations with mild neutrophil infiltration. Direct immunofluorescence (DIF) revealed IgM deposits at the dermoepidermal junction, but neither IgG nor IgA deposits. Indirect immunofluorescence on 1 M NaCl-split skin demonstrated deposits on the epidermal side. The optical density (OD) value of a modified IgM enzyme-linked immunosorbent assay for full-length BP180, but not for BP180-NC16A, was increased. The patient was diagnosed with IgM pemphigoid and was treated with diphenyl sulfone at 50 mg/day without recurrence. To confirm the precise autoantigen, we tried to obtain super-resolution imaging. The deposition pattern of IgM autoantibodies seemed to be oriented parallel to that of BP180. The detailed images detect DIF deposits apart from BP180-NC16A staining, but are close to type VII collagen-NC1 staining. This result suggests that the IgM autoantibodies in the patient might target the C-terminus of BP180. IgM pemphigoid is still not a widely accepted concept, and the clinical course remains unknown. We will carefully follow-up the patient. Super-resolution images may help to detect precise autoantigens in autoimmune blistering diseases.