Effects of growth hormone on thymocyte development from progenitor cells in the bone marrow

The effect of growth hormone (GH) on T cell differentiation was studied in young and old mice, employing in vivo and in vitro experimental approaches. Injections of GH during a period of 3 months to young and old mice resulted in a significant increase in the cell number and the percentage of CD3+ cells in the thymus of the old, but not in the young mice. Treatment of intact fetal thymus (FT) lobes with recombinant human GH (hGH) had no significant effect on cell numbers or on the values of CD4/CD8 thymocyte subsets. When partially depleted FT (10 Gy) were colonized with bone marrow (BM) cells and subsequently cultivated on monolayers of GH3, a GH-secreting cell line, the values of T cells deriving from the donor BM cells were elevated. Treatment with hGH to cocultures of lymphoid-depleted FT (dGUA) with BM lent further support to the idea that GH affects the newly emigrating BM cells, rather than the resident thymocytes. The results suggest that GH affects the thymocyte progenitors in the BM at the early stage of their development in the thymus.