Human neutrophil integrin alpha9beta1: up-regulation by cell activation and synergy with beta2 integrins during adhesion to endothelium under flow.

Neutrophil beta1 integrin expression and contribution to cell adhesion were revisited in this study. alpha9beta1 and alpha5beta1 appeared here as the main beta1 integrins expressed on the membrane of resting platelet-depleted neutrophils-alpha6beta1 representing <15% and alpha2beta1 undetectable. Neutrophil activation slightly enhanced alpha5 expression, did not change alpha6, but resulted in a two- to threefold increase of alpha9beta1, which then became the major beta1 integrin of the neutrophil membrane. alpha9beta1 was the only beta1 integrin to be up-regulated after transendothelial migration across TNF-alpha-activated HUVECs. As alpha9beta1 binds VCAM-1, we analyzed its participation to neutrophil adhesion to TNF-alpha-activated endothelial cells. Blocking anti-alpha9 mAb had little effect on neutrophil static adhesion, contrasting with the strong inhibition by anti-beta2 mAb. Under flow conditions, the anti-alpha9 mAb had no effect by itself on neutrophil adhesion to activated HUVECs but enhanced the blocking effect of anti-beta2 antibodies significantly and further enhanced the velocity of beta2-blocked rolling neutrophils. In conclusion, we describe here for the first time a nearly exclusive up-regulation of alpha9beta1 expression among all beta1 integrins during neutrophil activation and transendothelial migration and a possibly important synergy between alpha9beta1 and beta2 integrins in stabilizing neutrophil adhesion to endothelium under flow conditions.