A CXCR4-Dependent Chemorepellent Signal Contributes to the Emigration of Mature Single-Positive CD4 Cells from the Fetal Thymus

Abstract

Developing thymocytes undergo maturation while migrating through the thymus and ultimately emigrate from the organ to populate peripheral lymphoid tissues. The process of thymic emigration is controlled in part via receptor-ligand interactions between the chemokine stromal-derived factor (SDF)-1, and its cognate receptor CXCR4, and sphingosine 1-phosphate (S1P) and its receptor S1PR. The precise mechanism by which S1P/S1PR and CXCR4/SDF-1 contribute to thymic emigration remains unclear. We proposed that S1P-dependent and -independent mechanisms might coexist and involve both S1P-induced chemoattraction and SDF-1-mediated chemorepulsion or fugetaxis of mature thymocytes. We examined thymocyte emigration in thymi from CXCR4-deficient C57BL/6 embryos in a modified assay, which allows the collection of CD62Lhigh and CD69low recent thymic emigrants. We demonstrated that single-positive (SP) CD4 thymocytes, with the characteristics of recent thymic emigrants, failed to move away from CXCR4-deficient fetal thymus in vitro. We found that the defect in SP CD4 cell emigration that occurred in the absence of CXCR4 signaling was only partially overcome by the addition of the extrathymic chemoattractant S1P and was not associated with abnormalities in thymocyte maturation and proliferative capacity or integrin expression. Blockade of the CXCR4 receptor in normal thymocytes by AMD3100 led to the retention of mature T cells in the thymus in vitro and in vivo. The addition of extrathymic SDF-1 inhibited emigration of wild-type SP cells out of the thymus by nullifying the chemokine gradient. SDF-1 was also shown to elicit a CXCR4-dependent chemorepellent response from fetal SP thymocytes. These novel findings support the thesis that the CXCR4-mediated chemorepellent activity of intrathymic SDF-1 contributes to SP thymocyte egress from the fetal thymus.

A Cell-Intrinsic Role for Mst1 in Regulating Thymocyte Egress

“…However, S1P-mediated chemotactic migration of mature thymocytes remained intact, indicating that Mst1 is unlikely to be involved in supporting S1P-induced thymic egress. These results support the notion that thymic emigration is regulated by both S1P-dependent and -independent mechanisms (41) and that Mst1 plays a particularly important role in S1P-independent chemotactic responses.…”

Opposing chemokine gradients control human thymocyte migration in situ

“…These data -in agreement with previous reports for postnatal thymocytes (11,36) -indicate that positive selection signals at the DP stage are accompanied by a change in responsiveness to both CCL21 and CXCL12. Interestingly, chemotaxis toward CXCL12 was regained in the most mature CD8 + SP thymocytes ( Figure 4D), consistent with murine and human studies that suggest a role for CXCR4 in thymic egress (37,38). We also examined responses of human fetal thymocytes to another cortical chemokine, CCL25, and found that all subsets exhibited robust migration toward CCL25, with no apparent difference between CD69 -and CD69 + DP thymocytes ( Figure 4E).…”

DLGH1 Is a Negative Regulator of T-Lymphocyte Proliferation

“…As an alternative approach, we also generated and analyzed mice harboring a conditional ("floxed") Dlgh1 allele, in which DLGH1 expression can be ablated in T-lineage cells in the presence of lck-Cre transgenes (see We also addressed the possibility that DLGH1 may be required for T-cell responses to chemokine gradients, such as stromal cell-derived factor 1␣ (SDF-1␣), which have been implicated in the regulation of normal thymic egress (30). In this context, previous studies implicated Scribble, a DLGH1-related protein, in SDF-1␣-mediated migration of T cells (16).…”