CD99 (E2) up-regulates alpha4beta1-dependent T cell adhesion to inflamed vascular endothelium under flow conditions.

CD99/E2 is an integral transmembrane protein which forms, together with Xga, a distinct family whose genes are located in the pseudoautosomal region. The number of T cells that firmly bound to vascular endothelial cells under physiological shear stress increased 2-14-fold upon CD99 stimulation, and bound cells became much more resistant to detachment forces and spread. T cell arrest occurred within 1 min and was dependent on the alpha4beta1-VCAM-1 pathway. In contrast, the alphaLbeta2-ICAM-1 pathway remained unactivated. This was observed with T cell lines and with activated peripheral blood lymphocytes, and was limited within the resting peripheral CD4+ T cells to the memory subset, while virgin cells were unaffected. This discloses a stepwise regulation of the T cell extravasation cascade.