Preparation and properties of thymostatin, a new thymic inhibitor of DNA and RNA synthesis

Current status of thymosin research: evidence for the existence of a family of thymic factors that control T-cell maturation

Thymosin fraction 5 contains several distinct hormonal-like factors which are effective in partially or fully inducing and maintaining immune function. Several of the peptide components of fraction 5 have been purified, sequenced and studied in assay systems designed to measure T-cell differentiation and function. These studied indicate that a number of the purified peptides act on different subpopulations of T-cells (see Figure 1). Thymosin beta 3 and beta 4 peptides act on terminal deoxynucleotidyl transferase (TdT) negative precursor T-cells to induce TdT positive cells. Thymosin alpha 1 induces the formation of functional helper cells and conversion of Lyt- cells to Lyt 1+, 2+, 3+ cells. Thymosin alpha 7 induces the formation of functional suppressor T-cells and also converts Lyt- cells to Lyt 1+, 2+, 3+ cells. These studies have provided further evidence that the thymus secretes a family of distinct peptides which act at various sites of the maturation sequence of T-cells to induce and maintain immune function. Phase I and Phase II clinical studied with thymosin in the treatment of primary immunodeficiency diseases, autoimmune diseases, and cancer point to a major role of the endocrine thymus in the maintenance of immune balance and in the treatment of diseases characterized by thymic malfunction. It is becoming increasingly clear that immunological maturation is a process involving a complex number of steps and that a single factor initiating a single cellular event might not be reflected in any meaningful immune reconstitution unless it is the only peptide lacking. Given the complexity of the maturation sequence of T-cells and the increasing numbers of T-cell subpopulations that are being identified, it would be surprising if a single thymic factor could control all of the steps and populations involved. Rather, it would appear that the control of T-cell maturation and function involves a complex number of thymic-specific factors and other molecules that rigidly control the intermediary steps in the differentiation process.

Low-molecular-weight peptide inhibits RNA synthesis in human leukemic and phytohemagglutinin-stimulated leukocytes and globin mRNA transcription in differentiating Friend cells

The RNA synthesis of human leukemic leukocytes and phytohemagglutinin-stimulated lymphocytes is markedly reduced by administration of a low-molecular-weight nonhistone peptide factor from calf thymus. Treatment with the factor strongly inhibits hemoglobin production and globin mRNA transcription in dimethyl sulfoxide-stimulated Friend cells without appreciably modifying the rate of cell growth. Evidence for specificity of these effects is provided by the lack of action of the factor on both growth rate and RNA synthesis of a number of nondifferentiating cell lines from various animal species. After removal of the compound, both human lymphocytes and Friend cells can be stimulated by phytohemagglutinin and by dimethyl sulfoxide, respectively, ruling out any toxic effect.

Transformation of human lymphocytes: inhibition by homologous alpha globulin

An alpha globulin fraction prepared from normal human plasma by column chromatography prevents homologous lymphocyte transformation and the stimulation of DNA, and also protein synthesis induced by phytohemagglutinin and specific antigens. These observations support the concept of a normal circulating immunosuppressant factor which prevents lymphoid cell proliferation.

Potentiation of lymph node antibody synthesis by normal thymus in vitro

Antibody formation during the secondary response in vitro was enhanced when normal thymus fragments from 1- to 2-week-old rabbits were coincubated with immune popliteal lymph node fragments. Although the addition of three thymus fragments to three lymph node isolates produced a significant increase in antibody synthesis, six or nine thymus fragments stimulated the response still further; normal spleen or normal lymph node did not stimulate. Radioactive amino acid incorporation into antibody confirmed that de novo antibody synthesis was potentiated by the thymus. In addition, increased synthesis of total globulins was quantitatively measured. The thymus seemed to enhance formation of only 7S immunoglobulin as determined by sucrose gradient centrifugation. It is not known whether thymus cells, humoral factors, or both participate in this response, or what intracellular metabolic events are being stimulated when the thymus is added to the cultures.