The Tec family tyrosine kinases, Itk and Rlk, are expressed in thymocytes and peripheral T cells and regulate thresholds of T cell receptor signaling. Yet little is known about the specific role of Itk- and Rlk-dependent signals in CD8(+) T cell maturation. We show here that Itk(-/-) and Rlk(-/-)Itk(-/-) mice were nearly devoid of conventional CD8(+) T cells and, instead, contained a large population of CD8(+) T cells that bear striking similarity to lineages of innate lymphocytes. Itk(-/-) and Rlk(-/-)Itk(-/-) CD8(+) thymocytes and T cells were CD44(hi), CD122(+), and NK1.1(+); were able to produce interferon-gamma directly ex vivo; and were dependent on interleukin-15. Itk(-/-) and Rlk(-/-)Itk(-/-) CD8(+) thymocytes expressed abundant transcripts for the T box transcription factor, eomesodermin, correlating with their phenotype and function. These data indicate a critical role for Itk and Rlk in conventional CD8(+) T cell development in the thymus.

Three members of the Tec family kinases, Itk, Rlk and Tec, have been implicated in signaling downstream of the T cell receptor (TCR). The activity of these kinases in T cells has been shown to be important for the full activation of phospholipase C-gamma1 (PLC-gamma1). Disruption of Tec family signaling in Itk-/- and Rlk-/-Itk-/- mice has multiple effects on T cell development, cytokine production and T-helper cell differentiation. Furthermore, mice possessing mutations in signaling molecules upstream of PLC-gamma1, such as Src homology 2 (SH2) domain-containing phosphoprotein of 76 kDa (SLP-76), linker for activation of T cells (LAT) and Vav1, or in members of the nuclear factor for activated T cells (NFAT) family of transcription factors, which are downstream of PLC-gamma1, have been found to have similar phenotypes to Tec family-deficient mice, emphasizing the importance of this pathway in regulating T cell activation, differentiation and homeostasis.

The Tec family tyrosine kinase Itk is critical for efficient signaling downstream of the TCR. Biochemically, Itk is directly phosphorylated and activated by Lck. Subsequently, Itk activates phospholipase C-γ1, leading to calcium mobilization and extracellular signal-regulated kinase/mitogen-activated protein kinase activation. These observations suggested that Itk might play an important role in positive selection and CD4/CD8 lineage commitment during T cell development in the thymus. To test this, we crossed Itk-deficient mice to three lines of TCR transgenics and analyzed progeny on three different MHC backgrounds. Analysis of these mice revealed that fewer TCR transgenic T cells develop in the absence of Itk. In addition, examination of multiple T cell development markers indicates that multiple stages of positive selection are affected by the absence of Itk, but the T cells that do develop appear normal. In contrast to the defects in positive selection, CD4/CD8 lineage commitment seems to be intact in all the TCR transgenic itk−/− lines tested. Overall, these data indicate that altering TCR signals by the removal of Itk does not affect the appropriate differentiation of thymocytes based on their MHC specificity, but does impact the efficiency with which thymocytes complete their maturation process.

Itk and Rlk are members of the Tec kinase family of nonreceptor protein tyrosine kinases that are expressed in T cells, NK cells, and mast cells. These proteins are involved in the regulation of signaling processes downstream of the TCR in CD4+ T cells, particularly in the phosphorylation of phospholipase C-γ1 after TCR activation; furthermore, both Itk and Rlk are important in CD4+ T cell development, differentiation, function, and homeostasis. However, few studies have addressed the roles of these kinases in CD8+ T cell signaling and function. Using Itk−/− and Itk−/−Rlk−/− mice, we examined the roles of these Tec family kinases in CD8+ T cells, both in vitro and in vivo. These studies demonstrate that the loss of Itk and Rlk impairs TCR-dependent signaling, causing defects in phospholipase C-γ1, p38, and ERK activation as well as defects in calcium flux and cytokine production in vitro and expansion and effector cytokine production by CD8+ T cells in response to viral infection. These defects cannot be rescued by providing virus-specific CD4+ T cell help, thereby substantiating the important role of Tec kinases in CD8+ T cell signaling.

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