Enhanced frequency of mitogen‐responsive T cell precursors in old mice injected with thymosin α₁

Injection of old mice with thymosin α₁, 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 α₁ 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 α₁ 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 α₁ 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 α₁ 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.