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Natural course of epidermolysis bullosa simplex with mottled pigmentation in a Japanese family with the p.P25L mutation in KRT5

prescribed for 6 days. However, the redness and swelling worsened (Fig. 1b), and serum CRP levels further increased (10.0 mg/dL). Erosions on the lips and mucous membrane of the oral cavity also appeared (Fig. 1c). A skin punch biopsy from the right lower leg at his first visit showed subepidermal blister with inflammatory cells in the blister and papillary dermis (Fig. 1d). Indirect immunofluorescence microscopy demonstrated linear deposition of immunoglobulin G along the dermoepidermal junction (Fig. 1e). A chemiluminescent enzyme assay showed elevated levels of serum anti-BP180 antibody (84.1 U/mL; normal, <9.0). The titer of anti-desmoglein 1/3 antibody was below detection limit (<3.0 U/mL). Although pembrolizumab-induced Stevens–Johnson syndrome was a differential diagnosis, based on the clinical findings and laboratory data, we diagnosed the patient with BP (the peak Bullous Pemphigoid Disease Activity Index, 91) and started treatment with oral prednisolone (0.5 mg/kg per day) and topical steroid. Considering the possible association between pembrolizumab and BP, pembrolizumab was discontinued. After 2 weeks, skin lesions started to improve and serum levels of anti-BP180 antibody and CRP decreased. Prednisolone was gradually tapered, and BP has been controlled with daily prednisolone (9 mg/day). In most of the reported cases, the severity of BP associated with anti PD-1 antibodies is relatively mild. In addition, the clinical features are basically similar to those of common BP. In our case, the patient exhibited severe mucosal involvement and cellulitis-like lesions such as leg swelling with redness and elevated serum CRP levels. Clinicians should be aware of the possibility that BP associated with anti PD-1 antibodies may exhibit such severe and atypical clinical features.