Inhibition of Csk in thymocytes reveals a requirement for actin remodeling in the initiation of full T cell receptor signaling

T cell receptor (TCR) signaling is initiated by Src-family kinases (SFKs). To understand how C-terminal Src kinase (Csk), the negative regulator of SFKs, controls the basal state and the initiation of TCR signaling, we generated mice expressing a PP1-analog inhibitor-sensitive Csk variant (Csk(AS)). Inhibition of Csk(AS) in thymocytes, without TCR engagement, induced potent SFK activation and proximal TCR signaling up to phospholipase C-γ1 (PLC-γ1). Surprisingly, increases in inositol phosphates (InsP), intracellular calcium and Erk phosphorylation were impaired. Altering the actin cytoskeleton pharmacologically or providing CD28 costimulation rescued these defects. Thus, Csk plays a critical role in preventing TCR signaling. However, our studies also revealed a requirement for actin remodeling, initiated by costimulation, for full TCR signaling.