Eosinophil cationic protein inhibits immunoglobulin production and proliferation in vitro in human plasma cells

The effect of eosinophil cationic protein (ECP) on immunoglobulin (Ig) production by and proliferation of human plasma cells was studied. ECP inhibited Ig production by and proliferation of the human plasma cell lines, IM-9 and AF-10, in a dose-dependent fashion. As little as 0.05 ng/ml ECP was found to be inhibitory, and the maximal inhibition was achieved at doses of 0.1-0.5 ng/ml ECP. This inhibition was not due to cytotoxicity, since viability was always greater than 98%. Kinetic experiments demonstrated that inhibition was observable after 24 hr of culture with ECP and that the inhibitory effect of ECP was reversible. The inhibitory effect of ECP could be blocked by anti-ECP serum, but not by control serum. Of the various cytokines tested, including interleukin (IL)-1 beta, IL-2, IL-3, IL-4, IL-5, IL-6, interferon (IFN)-alpha, IFN-gamma, granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (Epo), IL-6 reversed the inhibition, while other cytokines failed to do so. ECP also inhibited Ig (IgG1, IgG2, IgG3, IgG4, IgM, and IgA) production by and proliferation of PCA-1+ plasma cells generated in vitro with a similar dose-response pattern. This inhibition also was blocked by anti-ECP serum but not by control serum, and was restored by IL-6. These results suggest that ECP may interact with IL-6 in controlling plasma cell responses.