Impact of the Somatotrope Growth Hormone (GH)/Insulin-Like Growth Factor 1 (IGF-1) Axis Upon Thymus Function: Pharmacological Implications in Regeneration of Immune Functions

Abstract

Abstract:The thymus is the central lymphoid structure where T-cell differentiation takes place, and a crucial organ for the maintenance of homeostasis in the immune system. Thymopoiesis includes intrathymic proliferation of T-cell precursors, selection and output of both self-tolerant and competent effector T cells, as well as of natural regulatory T cells (nTreg). In the crosstalk between the neuroendocrine and immune systems, peptide hormones have been more and more implicated in immunomodulation for the last thirty years. The somatotrope growth hormone (GH)/insulin-like growth factor-1 (IGF-1) axis in particular has been repeatedly shown to play a major regulatory role upon thymus function and T-cell development. This review will focus on the important thymotropic properties of the somatotrope GH/IGF-1 axis, and will try to discriminate these properties in function of the endocrine or paracrine/autocrine pathways involved in their mediation. Most importantly, in light of an increasing number of recent studies, GH and IGF-1 now appear as novel therapeutic agents that could be used for enhancing thymopoiesis in different cases of immune deficiencies, including aging-related immune dysfunction.Keywords: Thymus, growth hormone (GH), insulin-like growth factors (IGFs), HIV, growth hormone deficiency (GHD), Chagas disease. GENERAL INTRODUCTIONThe thymus is now considered as a crucial organ for maintenance of immune system homeostasis and the central lymphoid organ where occurs the generation of self-tolerant and competent naive T cells, as well as self-antigen specific natural Treg cells [1,2]. However, for a long time, the thymus has been regarded as an endocrine gland. In addition, the thymus now appears as a privileged site where the endocrine and immune systems intimately interact.A permanent crosstalk exists between the neuroendocrine and immune systems [3][4][5][6]. In addition to the strong modulation of immunity by glucocorticoids and sexual steroids, other hormones have been more and more involved in immunomodulation. Indeed, the somatotrope GH/IGF-1 axis, as well as prolactin and thyroid hormones [7], were shown to play an important regulatory role in T-cell development [8][9][10].GH is mainly synthesised in the anterior pituitary gland but can also be produced by immune cells [11,12]. GH has several biological actions in the immune system including regulation of thymopoiesis and T-cell development [13]. Nevertheless, it is uneasy to distinguish whether the thymotropic effects of GH are direct or mediated by IGF-1, as most *Address correspondence to this author at the University of Liege Center of Immunoendocrinology (CIL), Institute of Pathology CHU-B23, B-4000 Liege-Sart Tilman, Belgium; Tel: +32 43 66 25 50; Fax: +32 43 66 98 59; E-mail: vgeenen@ulg.ac.be of GH effects are driven by induction of IGF-1 and as IGF-1 has also been described as an endogenous factor in the thymic microenvironment [12]. The evolution of the research field has brought new insights that justify an overview update as some ancie...

Somatotrope GHRH/GH/IGF‐1 axis at the crossroads between immunosenescence and frailty

“…IGF‐1 is a growth‐promoting factor regulating cellular survival, proliferation, and differentiation . Mainly produced by the liver under the control of growth hormone (GH), IGF‐1 and GH are involved in several immune functions, especially T cell proliferation and thymic function . Interestingly, GH has been shown to increase IL‐6 production in aging animals, while high levels of IL‐6 and low levels of IGF‐1 both appear in the frail population of subjects reported in previous studies.…”