Stromal cell-derived factor 1α and CXCR4: newly defined requirements for efficient thymic β-selection

The progressive maturation of T cells is accompanied by their migration through the thymus, with each selection stage occurring in distinct microenvironments. Many specialized receptor–ligand pairs have been defined that drive T cell differentiation, but our understanding of the complex relationship between T cells and the thymic stroma is incomplete. Recent reports have identified a role for the chemokine stromal cell-derived factor 1α and its receptor CXC chemokine receptor 4 in β-selection. This review explores these findings in detail.