The study shows that the peptide thymosin‑alpha‑1 can boost certain immune cells in a lab test, while another peptide, thymosin‑beta‑4, can calm the immune response by activating suppressor cells. These effects were seen only in test‑tube experiments with human blood cells, not in real people.

Thymosin alpha 1 and thymosin beta 4, two peptides isolated from preparations of calf thymus fraction 5, were tested in the human mixed lymphocyte reaction (MLR). Thymosin alpha 1 was found capable of enhancing both the allogeneic and autologous MLR. On the contrary, thymosin beta 4 suppressed MLR proliferative responses. Study of the responses of the T cell subpopulations revealed that T4+ (helper/inducer) cells but not T8+ (suppressor/cytotoxic) are responsible for the enhanced, proliferative response to allo- and autoantigens in the presence of thymosin alpha 1. Both the autologous and the allogeneic proliferative responses of either T4+ cells or T8+ cells were not influenced by the addition of thymosin beta 4 in the cultures. However, when T4+ and T8+ subsets were cocultured, thymosin beta 4 was capable of activating T8+ cells to suppress the allogeneic and the autologous proliferative response of T4+ cells. These studies show that thymosin fraction 5 peptides exert immunoregulatory effects on the human MLR proliferative responses in vitro.