In vivo effects of growth hormone on thymic cells.

ABSTRACT

Increasing evidence has placed the thymus as a target for neuroendocrine control. Herein we review the pleiotropic effects of growth hormone (GH) on this primary lymphoid organ, with emphasis on data derived from in vivo experiments. A series of results strongly indicate that GH enhances thymocyte proliferation in both rodents and humans. Moreover, in vivo treatment with GH enhances interleukin (IL)-6 production by mouse thymocytes, and ex vivo experiments show that production of other cytokines, such as IL-1 and GM-CSF, is also augmented. In a second vein, GH exerts a modulatory role on thymic hormone production, particularly the secretion of thymulin. In GH-treated animals as well as GH-transgenic mice, thymulin secretion is enhanced. In acromegalic patients we found higher levels of thymulin secretion, whereas the opposite was seen in dwarf mice and GH receptor knockout animals. Developing T cell migration is also under GH influence. Recombinant GH was found to increase human T cell engraftment in the thymus of SCID mice. Moreover, ex vivo thymocyte traffic into and out of thymic nurse cell complexes is enhanced after GH treatment. Lastly, we show that thymocyte export in vivo is modulated by GH, which favors the homing of CD4(+) recent thymic emigrants towards lymph nodes. In conclusion, the possibility that GH improves in vivo thymic functions, including thymocyte proliferation and migration, points to this molecule as a potential therapeutic adjuvant in T cell associated immunodeficiencies.