Mitogen-induced switching of immunoglobulin heavy-chain class secretion in chronic B-lymphocytic leukaemia and immunocytoma cell populations.

Lymphocytes from peripheral blood, bone marrow, and lymph nodes from six patients with chronic B-lymphoproliferative disease were studied. Two of the patients had a polymorphic immunocytoma with surface membrane immunoglobulin phenotype mu kappa. Four patients had chronic B-lymphocytic leukaemia (CLL) with the phenotypes mu kappa, mu delta kappa, mu lambda, and mu delta lambda, respectively. In no case did the monoclonal cell populations express gamma chains on the surface, and none produced monoclonal immunoglobulin in vivo. The malignant B lymphocytes were cultured in the presence of polyclonal B-cell activators. Immunoglobulin secretion was detected with the protein A plaque assay and with a modification of the reversed haemolysis-in-agar plaque assay. Immunoglobulin secretion was induced in four of the cell populations. In both the immunocytomas and in one of the CLL cell clones the heat-inactivated Staphylococcus aureus strain Cowan I induced secretion of IgG, as well as IgM, associated only with the same light chain as was expressed on the surface of the unstimulated cells. Thus, mitogens can induce immunoglobulin secretion in vitro in malignant B lymphocytes, and Cowan I bacteria also have the ability to induce a switch of immunoglobulin production from IgM to IgG in a fraction of the leukaemic cell populations.