Further purification and chemical characterization of the lymphocyte-inhibiting-factor extracted from thymus (LIFT).

A thymic extract which was previously demonstrated to contain a lymphocyte-inhibiting-factor (LIFT) based on its immunosuppressive activity in vivo and its antiproliferative properties on lymphocytes in vitro, has been purified using ultrafiltration procedures. Most of the activity measured by the ability to inhibit DNA synthesis in short term cultures of mouse thymocytes, was recovered in the 10,000-50,000 m. wt fraction (I fraction). In contrast, similar extracts from non-lymphoid organs were always ineffective in decreasing DNA synthesis in thymocytes. The I fraction was also inhibitory of DNA synthesis in mouse spleen cells stimulated by PHA whatever the time at which the fraction was added after the mitogen stimulation. This fraction, as well as the crude thymic extract, was ineffective in decreasing DNA synthesis in non-lymphoid target cells. The I fraction was further purified by Sephadex G50 filtration and DEAE Sephadex chromatography. The active molecule seemed to be a heat resistant basic peptide probably bound to a ribonucleotide moiety.