Enhanced frequency of mitogen-responsive T cell precursors in old mice injected with thymosin alpha 1.

Injection of old mice with thymosin alpha 1, a synthetic peptide consisting of 28 amino acid residues and exhibiting thymic hormone-like activity, increases the splenic frequency of T cell precursors. Young (3-month-old) or old (19-20-month-old) mice received a single i.p. injection of thymosin alpha 1 or of an equimolar amount of the N14 (N-terminal amino acid residues 1-14) or C14 (C-terminal amino acid residues 15-28) synthetic fragment of the thymosin alpha 1 molecule and their spleen cells were assayed 3 days later under limiting dilution conditions to assess the frequency of mitogen-responsive and interleukin 2-producing T cells. Injection of thymosin alpha 1 or of its N14 fragment increases the frequency of responsive T lymphocytes in old, but not in young mice whereas injection of the C14 fragment has no demonstrable effect in either young or old mice. These data are consistent with our previous observation that the biological activity of thymosin alpha 1 is restricted to the N-terminal half of the molecule and suggest that this peptide amplifies the pool of mitogen-responsive and interleukin 2-producing T cells in immunodeficient old mice.