Figure 1.

Endothelial cells regulate T-cell progenitor entry. Initial steps in thymocyte development involve homing to, and colonizing of, the thymus by bone marrow-derived blood-borne progenitors via blood vessels at the corticomedullary junction (orange dotted line). Entry of these progenitors is controlled by thymic endothelial cells (EC) which regulate this process through a number of different mechanisms detailed in the inset figure (black dotted line/box). ECs express P-selectin, the ligand for which (P-selectin glycoprotein ligand 1, PSGL-1) is expressed by lymphoid progenitor cells. P-selectin-PSGL1 interactions are essential in slowing down progenitor movement, allowing them to interact with ICAM-1 and VCAM-1 expressed by ECs. In adult thymus, CCL25 (ligand for CCR9) which is produced by both TEC and ECs has also been shown to be essential for progenitor homing to the thymus. Additionally, CCR7 is involved in thymus colonization, although its precise ligand requirements (CCL19 and/or CCL21) is not known.