Inhibition of the kinase Csk in thymocytes reveals a requirement for actin remodeling in the initiation of full TCR signaling

Signaling via the T cell antigen receptor (TCR) is initiated by Src-family kinases (SFKs). To understand how the kinase Csk, a negative regulator of SFKs, controls the basal state and the initiation of TCR signaling, we generated mice that express a Csk variant sensitive to an analog of the common kinase inhibitor PP]. (Csk(As)). Inhibition of Csk(As) in thymocytes, without engagement of the TCR, induced potent activation of SFKs and proximal TCR signaling up to phospholipase C-gamma 1 (PLC-gamma 1). Unexpectedly, increases in inositol phosphates, intracellular calcium and phosphorylation of the kinase Erk were impaired. Altering the actin cytoskeleton pharmacologically or providing costimulation via CD28 'rescued' those defects. Thus, Csk has a critical role in preventing TCR signaling. However, our studies also revealed a requirement for actin remodeling, initiated by costimulation, for full TCR signaling.