The in vitro induction of differentiation of putative human stem cells by thymosin and agents that affect cyclic AMP

Presence of a peptide component of thymosin fraction-5 manifesting discrete cytostatic properties in HL-60 human promyelocytic leukemia cells

Thymosin fraction-5 (TF5), an array of small molecular weight peptides present in crude extracts of the adult bovine thymus, contains numerous constituents with demonstrable biological activity. Because TF5 generally enhances immune reactivity in a variety of settings, and additionally restricts proliferation of certain neoplasms, we examined the effects of TF5 on proliferative capacity in the human promyelocytic leukemia cell line HL-60. Vital dye-exclusion, oxidative metabolism of chromogenic dyes, and clonogenic growth profiles were monitored to assess rates of cellular proliferation; our results demonstrate that TF5 restricted HL-60 cell growth, an influence that exhibited comparable potency and efficacy among all three indices. This antiproliferative activity was labile, insofar as medium conditioned in HL-60 cells for 24 h became devoid of the initial growth-suppressive activity after 24-h culture when subsequently administered to naive cultures. Review of cytoarchitectural traits, chromatin staining by TUNEL, and fluorescent cytometric analyses demonstrated that TF5 failed to elicit apoptosis, however, suggesting that this material instead drove treated cells into growth arrest and an unanticipated cytostasis. Qualitatively similar responses were noted in the human monoblastic leukemia cell line U937. Partial purification of TF5 by FPLC yielded a component containing an antiproliferative activity associated with the approximately 1000-Da fraction. These results demonstrate that TF5 contains a sub-fraction possessing a growth-suppressive activity capable of restraining normal proliferation of human myeloid neoplasms via the apparent induction of true cytostasis.

Thymic peptide hormones: basic properties and clinical applications in cancer

The manuscript will provide an in-depth and critical review of the nomenclature, biochemistry, biological properties, and a summary of published and on-going clinical trials with all reported thymic preparations, including both partially purified thymic factors (e.g., thymosin fraction 5, thymostimulin) as well as purified and synthesized thymic peptides (e.g., thymosin alpha 1, thymulin). Particular emphasis will be placed on which thymic peptides should be categorized as true hormones. In addition, the comparative biochemistry and biological activity in animals will be summarized and contrasted for all the currently available thymic factors. The effects, in vitro of thymic factors, on peripheral blood lymphocytes isolated from normal donors and patients with primary immunodeficiency disorders, autoimmune disorders, and neoplastic disorders will also be reviewed. Finally, a detailed critical summary of the clinical trials performed with each of the thymic preparations will be presented with an emphasis on treatment of patients with cancer.

Thymosin induces helper function in OKT3-positive, E-rosette-negative human cord blood T cells

E-rosette-negative lymphocytes (TE-), which express monoclonal T-cell markers (OKT3+) were found in higher frequencies in human cord blood (CB) (30% by phenotype and 11.7% by sequential rosette isolation) than in adult peripheral blood lymphocytes (13% by phenotype and 3.7% by isolation). TE- cells had very low levels of 5'-nucleotidase, a lymphocyte maturational marker low in thymocytes. Like other CB T cells, TE- cells possessed marked suppressor capability. Irradiated TE- cells from two cord blood samples demonstrated helper activity for Ig production by adult B cells. In six cord blood samples, however, irradiated TE- cells lacked helper activity. When incubated with thymosin, these TE- cells E+ and differentiated into inducer-helper T cells. These observations confirm previous assumptions that CB T cells are not fully differentiated. Furthermore, T cells from different CB samples may be at various stages of ontogeny. We have also shown that thymosin fraction 5 is able to confer a new immunoregulatory function.

Lymphocyte maturation in the human thymus. Relevance of purine nucleotide metabolism for intrathymic T cell function

The combination of centrifugal elutriation as an efficient and reproducible method to separate thymocytes by size, micromethods to assess purine interconversion enzymes, and assessment of purine (deoxy)nucleoside inhibition of mitogen responses enabled us to study purine metabolism at the intrathymic level. Out of six fractions, four (nos. 3-6), containing medium- and large-sized lymphocytes, showed a proliferative response after stimulation with phytohaemagglutinin (PHA). In fractions 1-6 the number of cells with an immature immunological phenotype gradually decreased, and cells with the phenotype of mature cells gradually increased. The enzyme activity ratio of adenosine deaminase to purine nucleoside phosphorylase gradually decreased from 21 in fraction 1 to 7 in the last fraction (blood T-cell value, 0.7). We conclude that this enzyme activity ratio is a useful marker for intrathymic T-cell maturation stages. In PHA-responsive cell fractions (3-6), the sensitivity to inhibition of the PHA response by (deoxy)adenosine and deoxyguanosine was inversely related to the enzyme activity ratio of ecto-5'-nucleotidase to deoxycytidine kinase. These findings are compatible with the hypothesis that intracellular concentrations of phosphorylated (deoxy)nucleosides are related to this inhibition. We conclude that the differences in purine metabolism among the various (mitogen-responsive) human thymocyte fractions are related to lymphoid cell function. Since the number of cells contributing to the enzyme activities and the number of cells contributing to the proliferative response (about 15% of unseparated cells) differ considerably, it is not possible to evaluate enzyme activities in unseparated thymocytes in terms of relationships between purine metabolism and lymphocyte function.

Changes in purine nucleoside phosphorylase activity during thymosin-induced human null cell differentiation

Purine nucleoside phosphorylase (PNP) is a purine salvage pathway enzyme which we have found to be 8-10 times more active (per cell) in human peripheral blood null lymphocytes than in T lymphocytes. To test the hypothesis that null cells are, in part, pre-T lymphocytes we have defined an in vitro system for null cell differentiation into T cells and examined PNP activity during this differentiation process. We found that about 10% of human null cells could be driven to differentiate into T cells using thymosin fraction 5 (TF5) an extract of bovine thymus glands. The response to TF5 was dose related to up to 250 micrograms/ml with a maximum response occurring by 42-46 hr incubation. Exposure to TF5 was necessary for more than 4 hr but no more than 8 hr in order to obtain a maximum response. Both OKT4 and OKT8 positive cells were present in the newly differentiated T cell population but OKT8 positive cells appeared to predominate (OKT4/OKT8 = 0.698 +/- 0.30, mean +/- 1 SD). The differentiation process did not involve DNA synthesis but was inhibited at 4 degrees C. In the newly differentiated T cells PNP activity per cell was 8- to 10-fold lower (36 +/- 23 nm/hr/106 cells) than in null cells (311 +/- 136), and was at a level similar to mature T cells (56 +/- 7). Thus, human peripheral blood null cells can be induced to differentiate into T lymphocytes which can be characterized by both surface markers and biochemical parameters. Future studies will look at the function of TF5-induced T cells and the regulation of PNP activity during the differentiation process.

Increased thymic hormone responsive suppressor T lymphocyte function in chronic active hepatitis

Mitogen-induced suppressor T lymphocyte function was evaluated in patients with chronic active hepatitis (CAH). The in vitro effect of the biological response modifier, thymosin fraction 5, on the suppressive activity of peripheral blood mononuclear cells (PBM) was also assessed. Suppressor cell activity was significantly decreased in patients with CAH when compared to controls (P less than 0.001). In the absence of the inducing mitogen, thymosin-treated PBM from both patients and controls promoted enhancement of tritiated thymidine uptake by cocultured allogeneic lymphocytes. When thymosin-treated mononuclear cells were mitogen-activated; patients, but not the controls, showed a marked increase in suppressor activity (P less than 0.001). These results indicate that the polypeptides contained in thymosin fraction 5 can promote a helper effect in patients and controls. Furthermore, PBM from patients with CAH contain a subset of lymphocytes that can express a suppressive function following thymosin treatment. We conclude that thymosin fraction 5 can promote an in vitro restoration of suppressor T cell function in patients with CAH.

Detection of binding sites for spiroperidol on leukemic cells: its value for the phenotype characterization of lymphoid leukemias

The specific binding of the dopamine antagonist spiroperidol was studied in leukemic cell samples of various phenotypes. Among these only B-cell samples from chronic lymphocytic leukemias (7/7) and some "null" cell samples from acute lymphoblastic leukemias (2/7) showed specific binding. B cells from a prolymphocytic leukemia were negative as were also T-lymphoïd and non-lymphoïd leukemic cells at different stages of maturation. This pattern can be clearly correlated with the previous results obtained with normal blood cells and on cell lines. Moreover, it suggests that the detection of spiroperidol binding sites could provide a new means of distinguishing different phenotypes among B cells and early lymphoïd cells. Our results open the way to further studies which might show a correlation between spiroperidol binding sites and the new immunological markers defining subsets among non-T lymphoïd cells, as well as defining their physiological meaning.

Resynthesis of trypsinized sheep red blood cell receptors on human lymphocytes: comparison of the effects of immunopotentiators of biological and synthetic origin in vitro

Five immunostimulants--thymosin, dialyzable leukocyte extracts containing transfer factor (DLE), isoprinosine, BM 12 531 (azimexon), and levamisole--were compared separately and in various combinations for their ability to increase the binding of sheep erythrocytes by trypsinized human peripheral blood lymphocytes in vitro. Levamisole produced the greatest enhancement when used alone, DLE produced a smaller increase, and isoprinosine, thymosin, and BM 12 531 were less effective. When combining two of these agents, the greatest increase was seen when one of the components was thymosin and the other either DLE, isoprinosine, or BM 12 531. These results indicate possible synergistic mechanisms and the possible advantage of combined therapeutic use of selected combinations of immunostimulants. In the majority of the combinations, however, inhibition rather than stimulation of rosette formation was observed.

Effect of thymosin on cellular immunity in old age

Cell-mediated immunity (CMI) in a population of 28 well-nourished, disease-free old-age individuals (65--103 years) was compared to that of healthy young adult controls (22--41 years) and the effects in vitro of thymosin fraction 5 (TF5) evaluated. The number of T cells (T cell rosettes, TCR) in the peripheral blood was the same as controls. However, there was a significant depression in lymphocyte response to phytohemagglutinin (PHA) (p Less Than 0.005). Limiting dilution of the PHA response revealed an inherent defect in the function of those cells that are capable of responding to PHA. The formation of TCR from old-age subjects was similar to controls when incubated with TF5, i.e. there was no enhancement. When TF5 was incubated with old-age and normal control lymphocytes in the mixed lymphocyte reaction (MLR), there were significant increases in the responses in both (71 +/- 39% and 64 +/- 52%, respectively; p Greater Than 0.2). There was also a mitogenic effect of TF5 on resting lymphocytes which was significantly greater in the control than in the old-age population (p Less Than 0.002). Our results suggest that in old age: (1) there is no loss in absolute numbers of T lymphocytes; (2) there is a defect in the ability of individual lymphocytes to function; and (3) the response of lymphocytes in vitro to TF5 in forming TCR and in the MLR is statistically similar to young adult controls, and suggests that the decline in CMI with age is not due to a loss of thymosin-responsive cells.

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.

In vitro induction of E-rosette formation in human bone marrow cells by the thymic proteins LSHr and LSHh

Two thymic factors, LSHh and LSHr, were found to be active in the induction of E-rosette formation. In vitro incubation of human bone marrow cells, isolated by Ficoll gradient centrifugation, with subnanogram quantities of either of these LSH proteins increased the number of E-rosette-forming cells in the bone marrow cultures by a factor of two. Kidney extract and bovine serum albumin did not show significant activity when tested as controls. Commercial thymopoietin II peptide was found to be active in the assay at comparable concentrations. The data allow a comparison of LHS proteins with other preparations tested in this manner.