Effects of a nonapeptide FTS on lymphocyte differentiations in vitro

Abstract

BACH et al.1,2 have reported the amino acid sequence of a putative thymic hormone isolated from pig serum3, a nonapeptide called FTS (facteur thymique serique). Natural and synthetic FTS showed high activity in the rosette test used as a bioassay1,2. Bioassays for thymic hormones4, however, can be spuriously triggered by many substances of non-thymic origin5, a finding probably related to mediation by a cyclic AMP second signal6. Early steps in B-cell differentiation are also mediated by cyclic AMP (refs 6–9) and comparison of induction of T cells and B cells from committed precursor cells in vitro (the dual induction assay) enables a distinction to be made between selective and non-selective inducing agents7,8. This is exemplified by induction studies in the mouse with thymopoietin10,11 and ubiquitin7,12,13. Bovine thymopoietin selectively induces T-cell differentiation and actually inhibits B-cell differentiation9. By contrast, ubiquitin is non-selective, inducing both T- and B-cell differentiation7. Ubiquitin at high concentrations does not induce differentiation whereas these high concentrations do not impair induction by thymopoietin or other agents8,14. Another feature of induction by ubiquitin is its inhibition by the β-adrenoceptor-blocking drug propranolol, which is without effect on induction by thymopoietin7. We have evaluated synthetic FTS in the dual induction system in the chicken to determine whether it showed inductive selectivity appropriate to a thymic hormone, and have found that it induced non-selective differentiation of both T cells and B cells and closely resembled ubiquitin in its activity.