Isolation of an Immunosuppressive Peptide Fraction from Human Plasma

An immunosuppressive peptide fraction was isolated from Cohn fraction IV of normal human plasma by employing the low temperature-low salt-ethanol procedure of Cohn, fractional precipitation with ammonium sulfate, dialysis and ultrafiltration. The dissociation of the active peptide from an enriched protein fraction was found to be dependent upon high ionic strength and low pH. The resulting peptide fraction was electrophoretically heterogeneous although it appeared monodisperse by ultracentrifugation sedimenting with a coefficient of 0.6S. The peptide fraction was found to suppress both phytohemagglutinin-induced lymphocyte proliferation in vitro and the in vivo induction of splenic plaque-forming cells in mice immunized with sheep erythrocytes. The specific activity of this fraction was increased approximately 10-fold over that of Cohn fraction IV.