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Adult thymuses lacking either ephrin type B receptor 2 (EphB2) or EphB3, or expressing a truncated form of EphB2, the forward signal-deficient EphB2LacZ, have low numbers of early thymic progenitors (ETPs) and are colonized in vivo by reduced numbers of injected bone marrow (BM) lineage-negative (Lin(-)) cells. Hematopoietic progenitors from these EphB mutants showed decreased capacities to colonize wild type (WT) thymuses compared with WT precursors, with EphB2(-/-) cells exhibiting the greatest reduction. WT BM Lin(-) cells also showed decreased colonizing capacity into mutant thymuses. The reduction was also more severe in EphB2(-/-) host thymuses, with a less severe phenotype in the EphB2LacZ thymus. These results suggest a major function for forward signaling through EphB2 and, to a lesser extent, EphB3, in either colonizing progenitor cells or thymic stromal cells, for in vivo adult thymus recruitment. Furthermore, the altered expression of the molecules involved in thymic colonization that occurs in the mutant thymus correlates with the observed colonizing capacities of different mutant mice. Reduced production of CCL21 and CCL25 occurred in the thymus of the 3 EphB-deficient mice, but their expression, similar to that of P-selectin, on blood vessels, the method of entry of progenitor cells into the vascular thymus, only showed a significant reduction in EphB2(-/-) and EphB3(-/-) thymuses. Decreased migration into the EphB2(-/-) thymuses correlated also with reduced expression of both ephrinB1 and ephrinB2, without changes in the EphB2LacZ thymuses. In the EphB3(-/-) thymuses, only ephrinB1 expression appeared significantly diminished, confirming the relevance of forward signals mediated by the EphB2-ephrinB1 pair in cell recruitment into the adult thymus., (© Society for Leukocyte Biology.)

Trypanosoma cruzi acute infection leads to thymic atrophy, largely as a result of death of immature DP T cells. In a second vein, the glucocorticoid hormone imbalance promotes DP T cell apoptosis in infected mice. Herein, we assessed the involvement of caspase signaling in thymocyte death during T. cruzi acute infection. BALB/c mice were infected i.p. with 10(2) trypomastigote forms of T. cruzi and analyzed from 7 to 19 dpi. Thymocyte apoptosis was observed in early stages of infection, increasing along with time postinfection. Immature DN and DP as well as CD4(+) and CD8(+) thymocytes from infected mice showed increased activation of caspase-8, -9, and -3. In vitro treatment of thymocytes from infected mice with a general caspase inhibitor or the combination of caspase-8- and caspase-9-specific inhibitors increased the number of living thymocytes. Intrathymic injection of the general caspase inhibitor, but not caspase-8 or -9 inhibitors individually, prevented thymic atrophy and thymocyte depletion in infected mice. Moreover, blockade of glucocorticoid receptor activity with RU486 prevented DP thymocyte apoptosis, together with caspase-8 and -9 activation. These findings indicate that DP T cell apoptosis following experimental T. cruzi acute infection is dependent on glucocorticoid stimulation, promoting caspase-8 and -9 activation.

It is well demonstrated that the thymus gland is under neuroendocrine control. Thymic endocrine function can be modulated by a variety of hormones including those secreted by the thyroid gland. This prompted us to investigate putative influences of T3 in further aspects of thymus physiology. We showed that T3-treated animals exhibited an increase in thymus weight, cellularity and cycling cells. Moreover, Thy1+ thymocytes as well as CD4-CD8 defined subsets were augmented in absolute numbers, whereas PgP.1+ cells increased in both absolute and percentage values. In parallel, the total numbers of thymic nurse cells were also increased. Regarding the expression of extracellular matrix components (ECM) by microenvironmental cells, we observed an enhancement in the intrathymic ECM upon T3 in vivo treatment. Similar effects were found in vitro by treating a thymic epithelial cell line or thymic nurse cell-derived epithelial cultures with T3. This treatment also increased the expression of ECM receptors by thymic epithelial cultures. Interestingly, an enhancement in thymocyte/thymic epithelial cell adhesion ratio was observed after T3 treatment of epithelial cells. Our data suggest that T3 exerts a pleiotropic effect upon thymus physiology, stimulating thymocyte differentiation, not only by modulating epithelial cell hormonal secretion but also their production of ECM proteins and respective receptors.