In vitro and in vivo specificities of purified thymic factors inhibiting graft-versus-host reaction.

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.