Enhancement of DNA synthetic activity of thymic lymphocytes by the culture supernatant of thymus epithelial cells stimulated by growth hormone.

The authors examined the effect of the culture supernatant of growth hormone (GH)-stimulated thymus epithelial cells (TECs) on DNA synthetic activity of thymic lymphocytes (TLs) and then examined TL proliferation-inducing factors released from the TECs. TEC line, IT-45R1 derived from Wistar strain rat, was used. It was revealed that the supernatant from TECs treated with GH enhanced significantly DNA synthetic activity of TLs and that the activity of the least dense subset of TLs, containing undifferentiated lymphoid cells and the most immature TLs, was significantly increased by the supernatant as compared with other subsets. Anti-insulin like growth factor-I (IGF-I) monoclonal antibody (MAb) binding specifically to C region of IGF-I molecule was added to the culture supernatant from the GH-treated TECs, and then the supernatant was treated with ultrafiltration (MW cutting off; more than 50 kDa). When TLs were incubated with the ultrafiltered supernatant, the enhancement of TL proliferation induced by the supernatant of GH-treated TECs was significantly suppressed. However, the suppression did not descend to the level of TL-proliferative response observed in the supernatant of GH non-stimulated TECs. These results suggested that IGF-I released into the supernatant from GH-stimulated TECs enhances markedly the DNA synthetic activity of TLs and that the TL-proliferation-inducing factors (PIFs) other than IGF-I possibly exist in the supernatant of GH-stimulated TECs.