Signaling through the Leukocyte Integrin LFA-1 in T Cells Induces a Transient Activation of Rac-1 That Is Regulated by Vav and PI3K/Akt-1

Abstract

Integrin LFA-1 is a receptor that is able to transmit multiple intracellular signals in leukocytes. Herein we show that LFA-1 induces a potent and transient increase in the activity of the small GTPase Rac-1 in T cells. Maximal Rac-1 activity peaked 10-15 min after LFA-1 stimulation and rapidly declined to basal levels at longer times. We have identified Vav, a guanine nucleotide exchange factor for Rac-1, and PI3K/Akt, as regulators of the activation and inactivation phases of the activity of Rac-1, respectively, in the context of LFA-1 signaling based on the following experimental evidence: (i) LFA-1 induced activation of Vav and PI3K/Akt with kinetics consistent with a regulatory role for these molecules on Rac-1, (ii) overexpression of a constitutively active Vav mutant induces activation of Rac independently of LFA-1 stimulation whereas overexpression of a dominant-negative Vav mutant blocks LFA-1-mediated Rac activation, (iii) pharmacological inhibition of PI3K/Akt prevented the fall in the activity of Rac-1 after its initial activation but had no effect on Vav activity, and (iv) overexpression of a dominant-negative or a constitutively active Akt-1 induced or inhibited, respectively, Rac-1 activity. Finally, we show that T cells with a sustained Rac activity have impaired capacity to elongate onto ICAM-1. These results demonstrate that down-regulation of the activity of this GTPase is a requirement for the regulation of T cell morphology and motility and highlight the importance of temporal regulation of the signaling triggered from this integrin.