Thymic mucosa-associated lymphoid tissue lymphoma with immunoglobulin-storing histiocytosis in Sjögren's syndrome.

Mucosa-associated lymphoid tissue (MALT) lymphoma arising from the thymus is extremely rare. Only 33 cases of thymic MALT lymphoma have been reported to date. We present the case of a 53-year-old Japanese woman with Sjögren's syndrome who was diagnosed with thymic MALT lymphoma. In addition, the patient had the characteristic clinical and pathological features of thymic MALT lymphoma, as found in most of the 33 previous cases, except that there was an immunoglobulin G (IgG) phenotype, i.e. Sjögren's syndrome, epithelial cysts, lymphoepithelial lesions, and marked plasmacytic differentiation. The serum IgA levels were also elevated with IgA kappa M protein. This hypergammaglobulinemia remained unchanged after operation. The serological abnormalities may not arise from MALT lymphoma itself and may arise from the immune system hyper-reactivity evoked by Sjögren's syndrome. Of further interest were marked accumulations of CD68-positive histiocytes containing abundant eosinophilic globular inclusions in their cytoplasm. These inclusions were immunopositive for IgG-kappa, suggesting immunoglobulin inclusion bodies. The globular immunoglobulin inclusion bodies have been reported in non-crystallized immunoglobulin-storing histiocytosis in only one patient with multiple myeloma. To our knowledge, this is the first case of thymic MALT lymphoma with marked accumulation of histiocytes with immunoglobulin inclusions in a patient with Sjögren's syndrome.