EphB2 and EphB3 play an important role in the lymphoid seeding of murine adult thymus

Abstract

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.

Editorial: Ephs, ephrins, and early T cell development

“…In the present study, Alfaro et al [3] describe forward signaling requirements through the EphB2 and EphB3 in blood cell progenitors and stromal cells for efficient progenitor colonization of the adult murine thymus. With the use of EphB2 2/2 , EphB3 2/2 , and EphB2LacZ mice, the authors found the percentage of ETP in all EphB mutants reduced (Fig.…”

“…When analysis of chemokine expression was restricted to mouse panendothethelial cell antigen + thymic blood vessels, reductions in chemokine expression were observed in EphB2 and EphB3 mutant thymi only. Therefore, the new results from Alfaro et al [3] indicate that Ephs and ephrins are required at early stages of T cell development. These stages can be modeled in vitro by use of RTOC or the OP9 stromal coculture system [11].…”

“…1D). As stromal cell-derived CCL21/CCL25 and P-selectin mediate thymic homing under normal physiologic conditions [10], Alfaro et al [3] reasoned that the molecular determinants of thymic homing might be reduced in EphB mutant mice. Interestingly, all EphB mutants expressed reduced thymic CCL21/CCL25, whereas only EphB2 2/2 and EphB3 2/2 mice expressed a reduced percentage of P-selectin + cells compared with control animals.…”

“…Recently, ephrins and their cognate receptors, Ephs, have emerged as potentially important mediators of thymic seeding. In this issue of the Journal of Leukocyte Biology, Alfaro et al [3] report requirements for the Eph family members EphB2 and EphB3 for efficient generation of the ETP.…”

Eph/ephrin Signaling and Biology of Mesenchymal Stromal/Stem Cells

“…Moreover, the blocking of Eph-B4/ephrin-B2 interactions with either an Eph-B4 peptide inhibitor or shRNA-mediated knockdown of Eph-B4 in MSCs, reduced their capacity to support hematopoiesis in vitro [41]. In this respect, we reported that Eph-B2-deficient mice showed reduced proportions of primitive HPSCs in bone marrow [71]. Likewise, preliminary results suggest that Eph/ephrin signal blocking with fusion proteins in cocultures of HSCs and BM-MSCs extends HSC undifferentiated condition and increases the myeloid HSC-derived colony formation.…”