Figure 1. Plasma IGF-1 concentrations and sjTREC frequency in PBMCs from patients with GH deficiency and on GH treatment. The interruption of GH-treatment for 1 month induced a very significant decrease in blood IGF-1 and sjTREC levels (A). Both parameters were restored at initial levels one month after GH resumption (B). ***P,0.001 (by Wilcoxon’s signed rank test, N = 22). As shown in C, there is a significant positive correlation between blood IGF-1 levels and sjTREC frequencies (R = 0.61, P,0.01 by Spearman’s analysis). doi:10.1371/journal.pone.0005668.g001 Thymus Function in AGHD Relationship between age and sjTREC frequency (Fig. 2) There was no significant correlation between age and sjTREC levels in patients with AGHD under GH treatment since at least 2 years. After GH withdrawal during one month, age and sjTREC were negatively correlated (P,0.02), as well as one month after GH resumption (P,0.01). Quantification of sj/b TREC ratio Mean DJbTREC frequency was neither affected by GH withdrawal, nor by GH resumption (Fig. 3A). However, a significant decrease of the sj/b TREC ratio was associated with interruption of GH treatment (P,0.01). One month after GH reintroduction, there was a tendency for sj/b ratio increase, but this did not reach the level measured before GH withdrawal (Fig. 3B). Discussion Until now, thymopoiesis has never been investigated in humans with AGHD, and only one study has investigated the impact of GH deficiency upon immune parameters in humans [31]. Our results show that, in patients with well-documented GH deficiency, both the frequency of circulating sjTRECs (marker of thymic T cell output) in PBMCs and the sj/bTREC ratio (marker of intrathymic precursor T cell proliferation) are significantly decreased one month after GH withdrawal. Resumption of GH treatment for one month is sufficient to increase thymic T cell output to a level similar or close to the one measured before GH interruption. There was a significant negative correlation between age and sjTREC frequency one month after GH withdrawal, as well as one month after GH resumption. This negative correlation was not observed in AGHD patients treated by GH for at least two years. The age-related decline in thymopoiesis as reflected by a marked decrease in sjTREC in elderly is a common observation. The reason for the discrepancy observed in this study is unknown. It could be due to the rather small number of patients or to a time length of GH resumption insufficient to return to the situation observed before GH withdrawal. These results are not surprising given the vast amount of literature about the thymotropic effects and immunomodulating properties of GH and IGF-1 (extensive review in [17]). Immune defects in hypophysectomised or in Igf12/ 2 deficient animals are partially or totally reversed by GH administration. While GH and IGF-1 are not essential for lymphopoiesis or lymphocyte effector function, they clearly appear to be immunostimulatory. Intrathymic T cell proliferation was also stimulated by GH treatment, but was not completely restored after one month of treatment. Although there is clear evidence that the magnitude of thymic output depends on the intrathymic proliferation of precursor T cells [7,8], our data show that GH treatment restores the frequency of recent thymic emigrants before the level of intrathymic T cell proliferation, suggesting that limited thymic output is sufficient to repopulate these subsets. Moreover, it is possible that changes in peripheral survival capacities in these subsets also participate to their reconstitution. Therefore, in patients with AGHD, GH treatment significantly affects thymus function and particularly the peripheral level of recent thymic emigrants. The close positive correlation observed between plasma IGF-1 concentrations and the frequency of sjTRECs in PBMCs argues for an important role of IGF-1 in mediating GH impact upon human thymic function as already suggested by others [29]. Several groups have investigated the components of the IGF axis, including IGF binding proteins (IGFBPs), in the human thymus. PLoS ONE | www.plosone.org 4 May 2009 | Volume 4 | Issue 5 | e5668