IgG3 subclass-positive primary thymic MALT lymphoma without trisomy 3 and trisomy 18: report of a case and review of literature.

The patient, a 42-year-old man, was diagnosed as having an anterior mediastinal tumor. Examination of the resected tumor showed findings consistent with a primary thymic mucosa-associated lymphoid tissue lymphoma, stage IA. Postoperative (18)F-fluorodeoxyglucose-positron emission tomography/computed tomography demonstrated fluorodeoxyglucose accumulation at the site of tumor excision. This accumulation was interpreted as representing a residual lesion, and the patient was treated with rituximab. The patient has since been in a state of complete remission for about 3 years. Sporadic mucosa-associated lymphoid tissue lymphoma cells that appeared to have a propensity for differentiating into plasma cells in this case were analyzed for IgG and IgG subclass expression by immunohistochemical staining. The mucosa-associated lymphoid tissue lymphoma cells that showed a propensity for differentiating into IgG-positive plasma cells were IgG3-positive and IgG1-, IgG2- and IgG4-negative. An increase in IgG3 or IgG1 expression in immune cells has been previously demonstrated in immune responses to continuous exposure to the same proteins or peptide antigens and most mucosa-associated lymphoid tissue lymphomas show increased IgG3 and/or IgG1 expression. It is consistent with the fact that inflammation due to stimulation by a pathogenic antigen is considered to be etiologically responsible for the development of mucosa-associated lymphoid tissue lymphoma.