Alloantigen-activated mouse T cells secrete a factor which binds to the Fc fragment of IgG and blocks complement (C) activation by IgG (immunoglobulin-binding factor, IBF). IBF was found to suppress the direct plaque-forming cell (PFC) response of mouse spleen cell cultures to sheep erythrocytes and to dinitrophenylated aminoethyldextran (T-independent antigen). Purification of IBF by affinity chromatography on IgG-coated Sepharose columns led to an increase of the suppressive capacity with IgG, IgM, or Fab2 from IgG) the factor responsible for inhibiting the PFC response could not be dissociated from that responsible for the inhibitory activity of IBF on C-dependent hemolysis. No effect was seen when cultures were pretreated for 6 h, or when IBF was added at 72 h. These data are compatible with the view that IBF is a soluable mediator of suppressor T cells which may interfere with terminal differentiation of antibody-forming cell precursors.

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