Immunopotentiating and immunotherapeutic effects of thymic hormones and factors with special emphasis on thymic humoral factor THF-γ2

The essential role played by the thymus in the development of the immune response was well documented in many publications. These findings prompted a long series of studies devised to define the factors produced and secreted by thymus cells, which are involved in the development and nature of immunological responsiveness. First experiments done with crude thymus extracts were followed by isolation of purified products and finally by chemical characterization and synthesis of immunologically active thymus- derived peptides. In this article we review the various thymic hormones and factors described, that is, thymosin fractions 5, the thymosins, prothymosin α, thymulin (FTS-Zn), thymopoietin, thymostimulin (TP-1), Thymic humoral factor (THF), and THF-γ2. Studies demonstrating the activity of the various thymic factors in increasing the immunocompetence potential in both in vitro and in vivo conditions are discussed. The immunostimulatory potential of thymic factors was also investigated in experimental models where beneficial therapeutic effects were sought in a situation of immunological malfunction. The last part of the review is dedicated to clinical trials with thymic factors that revealed improvement in the immunocompetence potential in cases of immunodeficiencies, viral infections, and cancer and its correlation with therapeutic effectiveness. It seems that more research is required in order to better define conditions for the use of thymic factors in immunotherapy.