Enhancement of thymic lymphocyte proliferation by the culture supernatant of thymus epithelial cells stimulated by prolactin.

Culture supernatant of thymus epithelial cells (TECs) stimulated by prolactin (PRL) enhanced markedly DNA synthetic activity of thymic lymphocytes (TLs) as compared with the hormone-non-stimulated TECs. The supernatant, which was treated with anti-insulin-like growth factor-I (IGF-I) monoclonal antibody (MAb) (binding specifically to C region of IGF-I) has still the capacity of enhancing remarkably TL-proliferation. However, further treatment by ultrafiltration of the MAb-treated supernatant, removing the immune complex (IGF-I and anti-IGF-I MAb) from the supernatant, suppressed significantly the enhanced proliferation of TLs. It is assumed that PRL, like growth hormone (GH), promotes the release of IGF-I from TECs which induces a marked TL-proliferation. Moreover, it seems that the active site for inducing the proliferation is a region different from C, possibly the A or the B regions. TLs, present at different maturation steps, were separated into three subsets by discontinuous density gradient centrifugation and then treated with the supernatant of PRL-stimulated TECs. The least dense subset containing precursor T-cells and the most immature TLs showed the highest proliferative response to the supernatant in the comparison with other subsets and whole TLs. It is possible that the target cells to one of TL-proliferation-inducing factors (PIFs), namely IGF-I, in the supernatant exist in a greater concentration in the most immature step of TL population.