The objective of this thesis was to investigate the effects of the
somatotrope GH/IGF-1 axis upon the thymus. This work included two parts:
1. Translational research study:
Thymus function in adult GH deficiency (AGHD) with and without GH treatment
Background: Despite age-related adipose involution, T cell generation in the thymus (thymopoiesis) is maintained beyond puberty in adults. In rodents, growth hormone (GH), insulin-like growth factor-1 (IGF-1), and GH secretagogues reverse agerelated changes in thymus cytoarchitecture and increase thymopoiesis. GH administration also enhances thymic mass and function in HIV-infected patients. Until now, thymic function has not been investigated in adult GH deficiency (AGHD). The objective of this clinical study was to evaluate thymic function in AGHD, as well as the repercussion upon thymopoiesis of
GH treatment for restoration of GH/IGF-1 physiological levels.
Methodology/Principal Findings: Twenty-two patients with documented AGHD were enrolled in this study. The following parameters were measured: plasma IGF-1 concentrations, signal-joint T-cell receptor excision circle (sjTREC) frequency, and
sj/b TREC ratio. Analyses were performed at three time points: firstly on GH treatment at maintenance dose, secondly one month after GH withdrawal, and thirdly one month after GH resumption. After 1-month interruption of GH treatment, both
plasma IGF-1 concentrations and sjTREC frequency were decreased (p,0.001). Decreases in IGF-1 and sjTREC levels were correlated (r = 0.61, p,0.01). There was also a decrease in intrathymic T cell proliferation as indicated by the reduced sj/b
TREC ratio (p,0.01). One month after reintroduction of GH treatment, IGF-1 concentration and sjTREC frequency regained a level equivalent to the one before GH withdrawal. The sj/b TREC ratio also increased with GH resumption, but did not return to the level measured before GH withdrawal.
Conclusions: In patients with AGHD under GH treatment, GH withdrawal decreases thymic T cell output, as well as intrathymic T cell proliferation. These parameters of thymus function are completely or partially restored one month after GH resumption. These data indicate that the functional integrity of the somatotrope GH/IGF-1 axis is important for the maintenance of a normal thymus function in human adults.
2. Fundamental study:
intrathymic expression of members of the GH/IGF-1 axis and effects of GH on T-cell differentiation in murine fetalthymic organ cultures (FTOC).
We here address the question of expression and role of GH/IGF axis in the thymus.
Methods: Using RT-qPCR, the expression profile of various components of the somatotrope
GH/IGF axis was measured in different thymic cell types and during thymus embryogenesis in Balb/c mice. Effect of GH on T-cell differentiation was explored through thymic organotypic culture.
Results: Transcription of Gh, Igf1, Igf2 and their related receptors predominantly occurred in
thymic epithelial cells (TEC), while a low level of Gh and Igf1r transcription was also evidenced in thymic T cells (thymocytes). Gh, Ghr, Ins2, Igf1, Igf2, and Igfr1, displayed distinct expression profiles depending on the developmental stage. The protein concentration of IGF-1 and IGF-2 were in accordance with the profile of their gene expression. In fetal thymus organ cultures (FTOC) derived from Balb/c mice, treatment with exogenous GH resulted in a significant increase of double negative CD4-CD8- T cells and CD4+ T cells, with a concomitant decrease in double positive CD4+CD8+ T cells. These changes were inhibited by concomitant treatment with GH and GHR antagonist pegvisomant. However, GH treatment also induced a significant decrease in FTOC Gh, Ghr and Igf1 expression.
Conclusion: These data show that the thymotropic properties of the somatotrope GH/IGF-1 axis involve an interaction between exogenous GH and GHR expressed by TEC. Since thymic IGF-1 is not increased by GH treatment, the effects of GH upon T-cell differentiation could implicate a different local growth factor or cytokine.