Autonomous Maturation of α/β T Lineage Cells in the Absence of Cooh-Terminal Src Kinase (Csk)

The deletion of COOH-terminal Src kinase (Csk), a negative regulator of Src family protein tyrosine kinases (PTKs), in immature thymocytes results in the development of α/β T lineage cells in T cell receptor (TCR) β-deficient or recombination activating gene (rag)-1–deficient mice. The function of Csk as a repressor of Lck and Fyn activity suggests activation of these PTKs is solely responsible for the phenotype observed in csk-deficient T lineage cells. We provide genetic evidence for this notion as α/β T cell development is blocked in lck−/−fyn−/− csk-deficient mice. It remains unclear whether activation of Lck and Fyn in the absence of Csk uncouples α/β T cell development entirely from engagement of surface-expressed receptors. We show that in mice expressing the α/β TCR on csk-deficient thymocytes, positive selection is biased towards the CD4 lineage and does not require the presence of major histocompatibility complex (MHC) class I and II. Furthermore, the introduction of an MHC class I–restricted transgenic TCR into a csk-deficient background results in the development of mainly CD4 T cells carrying the transgenic TCR both in selecting and nonselecting MHC background. Thus, T