(A) Serum of a representative case (patient 4) showing reactivity (green, A.a) with human embryonic kidney (HEK) cells expressing dipeptidyl-peptidase–like protein 6 (DPPX). The reactivity of a commercial antibody against DPPX (red, A.b) colocalizes with that of the patient's serum (yellow, A.c). Note that a control serum is negative (A.d–A.f). (B) Determination of immunoglobulin G (IgG) antibody subclasses in 2 cases (patients 4 and 1). Patients' antibodies bound to HEK cells expressing DPPX are demonstrated with secondary anti-human antibodies specific for the indicated subclasses. Patient 4 has DPXX antibodies of the IgG1, IgG2, and IgG4 subclasses (B.a–B.c), whereas patient 1 has IgG1 and IgG4 subclass antibodies (B.d–B.f). Nuclei counterstained with 4′, 6-diamino-2-phenylindole (A and B). Scale bars = 10 μm.