Mott cells in a patient with a lymphoproliferative disorder. Differentiation of a clone of B lymphocytes into Mott cells.

An unusual lymphoproliferative disorder characterized by diffuse infiltration of the bone marrow by lymphocytes and Mott cells without other organ involvement is described. The clinical course was remarkable for a spontaneous remission. The cells infiltrating the bone marrow were predominantly lymphocytes and Mott cells. The lymphocytes stained exclusively for membrane IgM-kappa by immunofluorescence, and the Mott cells contained typical Russell bodies staining in a rim pattern for IgM-kappa. These findings suggested that the lymphocytes were differentiating into Mott cells. This was demonstrated in vitro by stimulating peripheral blood lymphocytes with mitogens. Differentiation of B lymphocytes into Mott cells containing IgM-kappa was observed. These appeared to be identical to the Mott cells found in the marrow. The Mott cells were probably nonsecretory because of the absence of a monoclonal serum immunoglobulin. Electron microscopic studies showed that the electron-dense cytoplasmic inclusions resembled Russell bodies. This case probably represents an expanded clone of B lymphocytes with demonstrable in vivo and in vitro differentiation into Mott cells.