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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.

A crude thymic extract, demonstrated to possess both immunosuppressive activity on T lymphocytes in vivo, and inhibitory action on DNA synthesis in T lymphocytes in vitro, has been purified by ultra-filtration and ion exchange chromatography procedures. The immunosuppressive activity was found to be associated with a high molecular weight fraction (greater than 50,000 daltons), devoid of immunosuppressive properties, a basic peptide has been isolated which retains the in vitro inhibitory activity on DNA synthesis.

A calf thymic factor has been prepared by extraction in distilled water, followed by selective precipitation with ethanol and ammonium sulfate. The non-dialysable fraction obtained by this method was further purified by a preparative electrofocusing technique which allows the isolation of two fractions which can specifically inhibit, in vitro, without toxicity or species specificity, spontaneous DNA synthesis in hemopoietic cell suspensions. When injected in vivo in mice, 3 days after immunization with SRBC, both these fractions exhibited immunosuppressive properties. In contrast, they were without any effect on a thymus-independent antibody response to TNP-LPS, whatever the day of administration. Still more important, in vitro pretreatment of lymphoid cells by the purified fractions inhibits their ability to elicit a graft-versus-host reaction when injected to histo-incompatible recipients.