Rac Regulates Integrin-Mediated Spreading and Increased Adhesion of T Lymphocytes

ABSTRACT

Leukocyte adhesion to the extracellular matrix (ECM) is tightly controlled and is vital for the immune response. Circulating lymphocytes leave the bloodstream and adhere to ECM components at sites of inflammation and lymphoid tissues. Mechanisms for regulating T-lymphocyte–ECM adhesion include (i) an alteration in the affinity of cell surface integrin receptors for their extracellular ligands and (ii) an alteration of events following postreceptor occupancy (e.g., cell spreading). Whereas H-Ras and R-Ras were previously shown to affect T-cell adhesion by altering the affinity state of the integrin receptors, no signaling molecule has been identified for the second mechanism. In this study, we demonstrated that expression of an activated mutant of Rac triggered dramatic spreading of T cells and their increased adhesion on immobilized fibronectin in an integrin-dependent manner. This effect was not mimicked by expression of activated mutant forms of Rho, Cdc42, H-Ras, or ARF6, indicating the unique role of Rac in this event. The Rac-induced spreading was accompanied by specific cytoskeletal rearrangements. Also, a clustering of integrins at sites of cell adhesion and at the peripheral edges of spread cells was observed. We demonstrate that expression of RacV12 did not alter the level of expression of cell surface integrins or the affinity state of the integrin receptors. Moreover, our results indicate that Rac plays a role in the regulation of T-cell adhesion by a mechanism involving cell spreading, rather than by altering the level of expression or the affinity of the integrin receptors. Furthermore, we show that the Rac-mediated signaling pathway leading to spreading of T lymphocytes did not require activation of c-Jun kinase, serum response factor, or pp70^{S6 kinase} but appeared to involve a phospholipid kinase.

ACKNOWLEDGMENTS

We thank P. D. Stahl for generous support; E. J. Brown, J. Schorey, and H. Shenoi for reagents, helpful discussions, and critical reading of the manuscript; A. Iafrate and J. Skowronski for cells, protocols, and advice for T-cell electroporation; R. Packer and M. Coronesi for technical assistance; and N. Novach and T. A. Yednock for providing antibodies.

This work was supported by grants of the National Cancer Institute (1R01CA 72982-OIAI) and an award from the Sidney Kimmel Foundation for Cancer Research and The V Foundation to L.V.A. and by grants from the Leukemia Research Foundation and the American Cancer Society (ACS-IRG 36-39) to C.D.-S. C.D.-S. is a Leukemia Society of America Special Fellow.