Intimate interactions between thymic epithelial cells (TECs) and thymocytes (T) have been repeatedly reported as essential for performing intrathymic T-cell education. Nevertheless, it has been described that animals exhibiting defects in these interactions were capable of a proper positive and negative T-cell selection. In the current review, we first examined distinct types of TECs and their possible role in the immune surveillance. However, EphB-deficient thymi that exhibit profound thymic epithelial (TE) alterations do not exhibit important immunological defects. Eph and their ligands, the ephrins, are implicated in cell attachment/detachment and govern, therefore, TEC–T interactions. On this basis, we hypothesized that a few normal TE areas could be enough for a proper phenotypical and functional maturation of T lymphocytes. Then, we evaluated in vivo how many TECs would be necessary for supporting a normal T-cell differentiation, concluding that a significantly low number of TEC are still capable of supporting normal T lymphocyte maturation, whereas with fewer numbers, T-cell maturation is not possible.

Numerous studies emphasize the relevance of thymocyte–thymic epithelial cell (TECs) interactions for the functional maturation of intrathymic T lymphocytes. The tyrosine kinase receptors, Ephs (erythropoietin-producing hepatocyte kinases) and their ligands, ephrins (Eph receptor interaction proteins), are molecules known to be involved in the regulation of numerous biological systems in which cell-to-cell interactions are particularly relevant. In the last years, we and other authors have demonstrated the importance of these molecules in the thymic functions and the T-cell development. In the present report, we review data on the effects of Ephs and ephrins in the functional maturation of both thymic epithelial microenvironment and thymocyte maturation as well as on their role in the lymphoid progenitor recruitment into the thymus.

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.

In the present study, the relevance of EphB2 and EphB3 tyrosine kinase receptors for the maturation of medullary thymic epithelial cells (TECs) is analyzed. The absence of both molecules, but particularly that of EphB2, courses with altered maturation of medullary Cld3,4^hiSSEA1⁺ epithelial progenitor cells, mature medulla epithelial cells, defined by the expression of specific cell markers, including UEA1, MHCII, CD40, CD80, and AIRE, and reduced expansion of medullary islets. In vivo assays demonstrate that these changes are a consequence of the absence of EphBs in both TECs and thymocytes. On the other hand, the changes, that remains in the adult thymus, correlated well with reduced proportions of E15.5 Vγ5⁺RANKL⁺ cells in EphB-deficient thymi that could result in decreased stimulation of RANK⁺ medullary TECs to mature, a fact that was confirmed by recovering of proportions of both CD40^hiCD80⁺ and MHCII^hiUEA1⁺ mature medullary TECs of mutant E14.5 alymphoid thymic lobes by agonist anti-RANK antibody treatment. Accordingly, the effects of EphB deficiency on medullary TECs maturation are recovered by RANK stimulation.