Isolation of an immunosuppressive peptide fraction from the serum of cancer patients.

An immunosuppressive peptide fraction was isolated by means of gel filtration, membrane partition, and ion-exchange chromatography from the sera of patients hospitalized for cancer. The resulting peptide fraction, which was heterogeneous as judged by high-voltage electrophoresis, was found to suppress both phytohemagglutinin-induced proliferation of lymphocytes in vitro and the in vivo induction of splenic plaque-forming cells in mice. The specific activity of the peptide fraction, which was isolated from the sera of cancer patients, was significantly increased over that of the unfractionated starting material. Moreover, in control experiments, when the sera of normals or non-cancer-bearing hospitalized individuals were subjected to the same chromatographic techniques, no active peptide fraction could be obtained.