Suppressive effect of thymosin fraction 5 on proliferation of cultured human T lymphocytes

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.

Immunomodulatory Effect and Acetylcholine Receptor Binding of a Thymopeptide (Tp4) on Human Peripheral Blood Lymphocytes

The effect of a synthetic thymopeptide, Tp4 (RGH 0206) on nAChR binding capacity and stimulatory responses of normal human peripheral lymphocytes were studied. The present study is the first to demostrate that Tp4 binds to the nAChR on PBLs and this binding is resulted in rapid modulatory effect on the activation signal and affects also the proliferative response. The inhibitory effect of Tp4 on 125Iα-Btx or 3H-nicotine binding was in correlation with a rapid increase of cGMP-level. Another activation parameter; AChE activity, showing the same concentration dependency, was declined reaching the maximal effect in the same, 6–60 nM, range. After a 72h incubation with suboptimal dose of PHA, both proliferation rate and AChE activity were inhibited. The results are in good agreement with earlier observations on the suppressive nature of nAChR on PBLs and on binding of thymopeptides to nAChR from electric tissue, as well as with the reported inhibiting effect of thymus hormones on IL-2 production. The present data might have relevance to the pathogenesis and diagnostics of myasthenia gravis, because the binding of overproduced thymopeptides to nAChRs in various organs could lead to different effects, i.e. to altered cholinergic transmission at the neuromuscular junctions and to the alteration of lymphocyte functions on PBLs and on thymocytes too.

Inhibition of in vitro proliferative response of cultured T lymphocytes to interleukin-2 by gold sodium thiomalate

Gold sodium thiomalate (GST), in concentrations attainable during chrysotherapy for rheumatoid arthritis, significantly inhibited the proliferative responses of cultured T cells stimulated by interleukin-2 (IL-2). The observed suppression was not related to altered kinetics, cell death, or interference with the binding of IL-2 to cell surface receptors. It appeared that GST affected an early step in the proliferative process, since maximum inhibition was obtained by the addition of GST within 4 hours of stimulation; progressive reduction of suppression was observed when GST was added later. Significant inhibition occurred when cultured T cells were preincubated for 24 hours with GST and washed prior to IL-2 stimulation, although the degree of suppression was decreased. Thus, inhibitory activity was not dependent on the continued presence of GST throughout culture. These findings suggest that there is a direct inhibition of T lymphocytes by GST, which may be important in immunomodulation by gold compounds.