Neural Crest–Derived Pericytes Promote Egress of Mature Thymocytes at the Corticomedullary Junction

Abstract

Thymocyte Egress A critical step during the development of the immune system is the egress of developing T lymphocytes, or thymocytes, from the thymus to peripheral organs, where they can defend against infectious microorganisms. Egress requires detection of the lipid mediator, sphingosine-1-phosphate (S1P), by thymocytes, but whether S1P detection is sufficient for egress, the source of S1P and the location where T lymphocytes exit the thymus are unknown. Zachariah and Cyster (p. 1129 , published online 22 April) found that selective expression of a transgene for the S1P receptor-1 in mouse thymocytes was sufficient for thymocyte egress. Thymocyte egress occurred at corticomedullary junctions via blood vessels, rather than via lymphatics. Pericytes, neural crest-derived cells, which ensheath blood vessels, provided the S1P for exiting thymocytes.