Prolongation of murine skin grafts by FTS and its synthetic analogues.

Synthetic serum thymic factor (FTS) and several of its analogues have been examined for immunoregulatory properties in a murine skin graft rejection model, including a syngeneic male-to-female system and an allogeneic system. In the syngeneic system, control animals with and without placebo had a mean graft survival time of 29 +/- 2 and 24 +/- 2 days respectively. Administration of FTS [10 ng bound to a carboxymethylcellulose (CMC) vehicle three times weekly until rejection] significantly delayed rejection (49 +/- 3 days). Similar although less striking results were obtained across the H-2 barrier. Several FTS analogues with retarded degradation were tested in the same system without CMC. All of them were as immunosuppressive as FTS. Interestingly, an analogue accelerated or delayed rejection depending on dosage, indicating the multiplicity of action of thymic peptides on the various T cell subsets.