Endogenous Thymic Factors Regulating Cell Proliferation and Analysis of Their Mechanism of Action

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Abstract
Endogenous factors inhibiting the proliferation of T[thymus-derived]-lymphocytes were investigated which may function as modulators of T-lymphocyte production within the thymus. An extract from calf thymus (T4) enriched in lymphocyte chalone arrested rat thymocytes at the G1 .fwdarw. S boundary and in the S phase of the cell cycle in short-term cultures. It also inhibited the proliferative response of human peripheral blood lymphocytes to PHA-P [phytohemagglutinin-P] in a time-dependent manner, as well as the spontaneous proliferation of in vitro cultured human chronic leukemic lymphoblasts. This crude extract contained 2 active moities which were isolated by molecular filtration on Sephadex G-75 column. A species non-specific, cell line selectivity inhibitory effect was characteristic of the high MW fraction MW > 40,000). This activity was resistant to moderate heat treatment and trypsin but was sensitive to milk alkaline hydrolysis and to RNase A digestion. About 10 protein components and a toluidine blue positive substance were detected by analytical polyacrylamide gel electrophoresis. The active inhibitor, a proposed protein-RNA complex, might be identical with the chalone. The low MW, non-dialysable factor (T4-4) inhibited [3H]thymidine incorporation into acid insoluble DNA in a cell non-specific manner. A possible relationship between the 2 activities was discussed.