Suppression of polyclonal B cell proliferation mediated by supernatants from human myeloma bone marrow cell cultures.

For functional characterization and semi-quantitative estimation of soluble regulator factors influencing polyclonal B cell proliferation and differentiation, we established two assays. One of the assays measures enhancement or inhibition of proliferation from purified human spleen B lymphocytes, and the other one the effect of soluble factors on CESS-cell differentiation. We found no difference concerning regulator factors for B cell differentiation between bone marrow cell culture supernatants from multiple myeloma (MM) patients and from controls, whereas significantly higher suppressor activity on polyclonal B cell proliferation could be detected in the former group of supernatants. The extent of such determined suppressor activity in vitro correlated with the amount of polyclonal serum IgM of the corresponding patients. These results indicate that one or several soluble suppressor factors may be involved in immunoregulatory mechanisms responsible for the humoral immunodeficiency observed in MM patients.