A 52-year-old man with a 4-week history of recurrent painful erosions on the buccal mucosa and gingiva presents with scattered erosions on the chest and back. Histopathology shows intraepidermal acantholysis with a "tombstone" appearance and suprabasal clefting. Direct immunofluorescence reveals IgG and C3 deposits in an intercellular pattern. Which of the following antibodies is MOST likely to be detected in the serum of this patient?