INVIVO STIMULATION OF THYMOCYTE PROLIFERATION BY THYMOCYTE GROWTH PEPTIDE (TGP)

Chromatographically pure thymocyte growth Peptide (TGP) was injected into one of the two thymus lobes of guinea pigs. Pulse labelling of DNA-synthesizing cells was obtained by an intracardial injection of bromodeoxyuridine (BrdUrd). After 24 and 48 h, the frequency of labelled cells was increased in the TGP-treated lobe in comparison with the control lobe. At 24 h, the increase was found in non-rosetting, large, low-density cells, the population containing the highest frequency of DNA-synthesizing cells. At 48 h, labelled rosette-forming, small, high-density cells appeared in an increased frequency in the TGP-treated lobe. Serum thymic factor (FTS), which is structurally related to TGP, did not produce similar effects. The results show that TGP is active in vivo and stimulates proliferation of cycling thymic precursor cells which then are transformed into small thymocytes.