Low molecular weight inhibitors of lymphocyte transformation.

A variety of factors isolated from bovine thymus have been found to inhibit the transformation of human and mouse lymphocytes. The majority of this activity fractionates as low molecular weight material by ultrafiltration or column chromatography. Three distinct fractions of low molecular weight have been isolated. One fraction contains the polyamines spermine and spermidine. A second fraction contains thymidine or thymidine-like nucleotides. The third fraction appears to be polypeptide in nature, and gives an estimated molecular weight of 500--600, is heat and pH stable, and is easily extracted by solutions containing organic solvents. Preliminary steps in the isolation of this inhibitor are presented, and its relationship to other immunosuppressive and anti-mitotic agents is discussed.