Production of Immunoglobulin Isotypes by Peripheral Blood Mononuclear Cells in Multiple Myeloma.

The immunoglobulin (Ig) isotype (IgG, IgA, IgM) production of peripheral blood mononuclear cells from 28 patients having multiple myeloma (MM) was analyzed. The total Ig secreting capacity of the cells, as measured by ELISA from the cell culture medium, was not found to be significantly reduced in MM (1,118 +/- 1,394 micrograms/l) as compared to the values of 9 controls (898 +/- 520 micrograms/l), but a significant isotype switching towards the tumor paraprotein type was observed in the patients with active MM (p < 0.001). The percentage of IgG in the active IgG-MM was 88 +/- 11% and that of IgA in the active IgA-MM 83 +/- 13%, the control values being 44 +/- 11% for IgG and 44 +/- 13% for IgA. The proportions of isotypes resembled those of the controls in the inactive phase of the disease. Despite this dominating paraprotein class isotype production, no evidence of Ig gene clonal rearrangements was found in cells studied by either Southern blotting or the more sensitive polymerase chain reaction method, which suggests that polyclonal rather than monoclonal PB B cells are responsible for the Ig production observed.