The c-Abl tyrosine kinase regulates actin remodeling at the immune synapse.

Actin dynamics during T-cell activation are controlled by the coordinate action of multiple actin regulatory proteins, functioning downstream of a complex network of kinases and other signaling molecules. The c-Abl nonreceptor tyrosine kinase regulates actin responses in nonhematopoietic cells, but its function in T cells is poorly understood. Using kinase inhibitors, RNAi, and conditional knockout mice, we investigated the role of c-Abl in controlling the T-cell actin response. We find that c-Abl is required for normal actin polymerization and lamellipodial spreading at the immune synapse, and for downstream events leading to efficient interleukin-2 production. c-Abl also plays a key role in signaling chemokine-induced T-cell migration. c-Abl is required for the appropriate function of 2 proteins known to be important for controlling actin responses to T-cell receptor (TCR) engagement, the actin-stabilizing adapter protein HS1, and the Rac1-dependent actin polymerizing protein WAVE2. c-Abl binds to phospho-HS1 via its SH2 domains and is required for full tyrosine phosphorylation of HS1 during T-cell activation. In addition, c-Abl is required for normal localization of WAVE2 to the immune synapse (IS). These studies identify c-Abl as a key player in the signaling cascade, leading to actin reorganization during T-cell activation.

HS1 functions as an essential actin-regulatory adaptor protein at the immune synapse.

HS1, the leukocyte-specific homolog of cortactin, regulates F-actin in vitro and is phosphorylated in response to TCR ligation, but its role in lymphocyte activation has not been addressed. We demonstrate that HS1-deficient T cells fail to accumulate F-actin at the immune synapse (IS) and, upon TCR ligation, form actin-rich structures that are disordered and unstable. Early TCR activation events are intact in these cells, but Ca2+ influx and IL-2 gene transcription are defective. Importantly, HS1 tyrosine phosphorylation is required for its targeting to the IS and for its function in regulating actin dynamics and IL-2 promoter activity. Phosphorylation also links HS1 to multiple signaling proteins, including Lck, PLCgamma1, and Vav1, and is essential for the stable recruitment of Vav1 to the IS. Taken together, our studies show that HS1 is indispensable for signaling events leading to actin assembly and IL-2 production during T cell activation.