Integrins on the surface of resting lymphocytes normally have low affinity and low avidity for their ligands. However, external stimuli such as chemokines and antigens generate intracellular signals that rapidly activate integrins, converting them to a clustered high affinity/high avidity state that can bind adhesion molecules on other cells or ECM components (1, 12). The nature of this "inside-out" signaling that promotes integrin activation is not completely understood. Many signaling pathways contribute to integrin activation, including the pathways that involve phosphatidylinositol 3-kinase, protein kinase C, Fyb/ADAP (adhesion and degranulation adaptor protein), and the Rho GTPase (12-15).Once activated, integrins can bind their ligands and initiate intracellular signaling pathways that regulate many aspects of cell behavior, including cell survival, proliferation, and differentiation and reorganization of the actin cytoskeleton (16). The reorganization of a cell's actin cytoskeleton and the resulting * This work was supported by a grant from the Cancer Research Society of Canada (to M. R. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.