Multiple myeloma with monoclonal IgG and IgD of lambda type exhibiting, under treatment, a shift from mainly IgG to mainly IgD.

A patient with multiple myeloma (MM), who initially presented with a predominant IgG lambda and a minor IgD lambda paraprotein pattern, is described. After chemotherapy, levels of the IgD lambda protein increased and the IgG lambda levels decreased. The following results were obtained when serum IgD was predominant. In the bone marrow, there were three plasma cell populations: a major one containing only delta chains, a minor one containing only gamma chains, and another minor one containing both delta and gamma chains. All these plasma cell populations contained lambda chains. Stimulation of circulating mononuclear cells with pokeweed mitogen (PWM) achieved differentiation of circulating B lymphocytes into plasma cells: 30% with only cytoplasmic delta lambda chains and 10% with only cytoplasmic gamma lambda chains. These IgG-containing plasma cells showed cytoplasmic reactivity with rabbit antiserum raised against monoclonal IgD which was shown to contain specificities recognizing both delta chains and idiotypic determinants present in both serum IgD lambda and IgG lambda. Circulating B lymphocytes were 'monoclonal': almost all expressed surface delta lambda chains, and a small proportion of them expressed both delta gamma and lambda chains. High levels of IgD were detected in the supernatants of all cultures, but high concentrations of IgG were only detected in those from PWM-stimulated cultures with very low levels of IgM and IgA. These findings suggest that plasma cells producing either IgD or IgG were derived from a common B-cell clone. Double paraproteinaemia exhibiting a shift in immunoglobulin production from IgG to IgD has not been previously described.