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.

Developing thymocytes undergo maturation during their passage through the cortex to the medulla of the thymus before emigrating to peripheral lymphoid organs. Chemokines control T cell migration within and away from the thymus. We have previously shown that the chemokine SDF-1, produced by thymic stroma, repels mature thymocytes away from thymus via a CXCR4 mediated mechanism. We proposed that CXCR4 plays a specific role in thymic emigration. We evaluated the emigration of mature thymocytes away from thymus in a modified fetal thymic organ culture (FTOC) system in fetal thymi from CXCR4+/− (Control) and CXCR4 −/− (KO) mice at day 15.5. Fetal thymic emigrants were collected using a transwell system with a polycarbonate membrane (3 μm pore size). The immunophenotype of thymic emigrants was distinct from that of thymocytes resident in the thymus in both control and CXCR4-KO thymi. From day 9 of culture onwards, only mature SP cells were able to move away from thymus into the lower chamber of the transwell. Only SP CD4 thymocytes expressing CD62Lhigh were found among emigrants, whereas two distinct populations of SP CD8 cells expressing CD62Lhigh and CD62Llow were present in the emigrant fraction. Incubation of FTOC with pertussis toxin significantly inhibited emigration of mature SP thymocytes out of the thymus in control CXCR4 mice (p=0.002). The total number of emigrants collected from CXCR4-KO thymi was always significantly reduced compared to emigrants from control thymi. In addition, the total number of SP CD4 emigrants from CXCR4-KO thymi were strikingly reduced in comparison to control SP CD4 emigrants (p=0.035). There was no statistically significant difference in the maturation of intrathymic thymocytes from CXCR4-KO as compared to control FTOC. Analysis of CD62L expression in CXCR4-KO thymocytes showed retention of SP CD4 cells in the thymus with the immunophenotypic characteristic normally associated with recent thymic emigrants (p<0.05). Treatment of FTOC with the specific CXCR4 antagonist, AMD3100, significantly increased the total number of SP CD4 cells retained in the thymus compared to untreated FTOC (p=0.002). Treatment with AMD3100 significantly reduced the absolute number of emigrants compared to untreated FTOC (p=0.015) and again SP CD4 cells were most affected in their ability to migrate away from the thymus. Treatment of FTOC with sphingosine-1-phosphate (S1P) resulted in an increase in the emigration of SP CD4 cells from CXCR4-KO thymi (p=0.02). In conclusion these data support the view that CXCR4-mediated repulsion of mature thymocytes away from the medullary chemokine SDF-1, termed fugetaxis, is a key mechanism controlling T cell egress from the thymus. These data support the existence of a CXCR4 dependent mechanism governing thymic emigration which may itself have S1P dependent and/or independent components.