IgG4 plasma cell myeloma: Clinicopathological characteristics and diagnosis

Plasma cell myeloma (PCM) is usually associated with the presence of M‐protein in the serum and urine of patients, and about half of the PCM cases exhibit the IgG M‐protein and increased gamma‐globulin fraction on membrane electrophoresis. The IgG4 subclass is located in the beta‐2 fraction on membrane electrophoresis. The aim of this study was to develop a method to evaluate IgG4‐producing PCM (IgG4‐PCM) and its clinicopathological characteristics. We found three cases of IgG4‐PCM among 80 cases of IgG‐producing PCM by membrane electrophoresis, which were confirmed by IgG4 immunostaining. None of the cases had a clinical history of IgG4‐related disease, although they exhibited high levels of serum IgG4. A bone marrow aspiration specimen had an increased number of plasma cells with a relatively mature morphology. No cases exhibited lymphoplasmacytic inflammation, obliterative phlebitis or fibrosis. Immunohistochemistry revealed that tumor cells expressed CD138 and IgG4 and showed monoclonal expression of kappa. We revealed that IgG4‐PCM might not be associated with IgG4‐related disease and that the detection of M‐protein with beta‐globulin fraction by electrophoresis may be useful for screening IgG4‐PCM.