NBxFO factor--a novel suppressor factor produced by fusion of neonatal spleen and a nonsecreting myeloma.

Spleens from 1-day-old DBA/2J mice were fused to the nonsecreting myeloma, FO. Dialyzed supernates of these cells were found to suppress the antigen-specific proliferative response of cloned helper or alloreactive T cells at a final concentration of less than or equal to 5%. The same supernate-containing factor did not suppress the response to IL-2 of an IL-2-addicted T cell line. The factor was found not to suppress the production of either IL-2 or antibody, following stimulation of spleen cells with LPS. Absorption analysis revealed that the target of the factor was the accessory cell population. Further experiments indicated that the factor blocked the proliferation of thymocytes due to IL-1. Biochemical analysis revealed a molecular weight for the factor of about 90,000 and a pI of approximately 4.5.