Inducible T-cell kinase (Itk) and resting lymphocyte kinase (Rlk) are downstream of TCR signaling (). They are members of the Tec family of non-receptor protein tyrosine kinases that is predominantly found in hematopoietic cells. This family consists of five members, Bruton’s tyrosine kinase (Btk), Itk, Rlk, Tec, and Bmx. These kinases are structurally similar and contain a kinase domain, a Src homology 2 (SH2) domain, an SH3 domain, a Tec homology domain consisting of a Btk homology domain and a proline-rich region, and a pleckstrin homology domain (). As an exception to this structure, Rlk has a cysteine-string motif in place of the pleckstrin homology domain, which results in Rlk being constitutively localized at the cell membrane (reviewed in 15
Role of Tec kinases in TCR signaling
Structures of the Tec family of non-receptor protein tyrosine kinases
Btk is a major component downstream of B-cell receptor (BCR) signaling, and humans deficient in Btk have defective B-cell development resulting in X-linked agammaglobulinemia (XLA). Similar, although lesser, effects are seen in xid
mice that contain mutations within Btk (reviewed in 16
). This kinase is also expressed in mast cells and is a positive regulator of FcεR1 signaling. Mast cells deficient in Btk have defects in cytokine production and activation (reviewed in 16
). In addition to Btk, mast cells also express Itk, Rlk, and Tec. Data from our laboratory indicate a negative role for Itk in mast cells signaling, in that Itk-deficient mast cells secrete increased amounts of cytokine upon stimulation and appear to have increased phospholipase Cγ1 (PLCγ1) phosphorylation (17
). These results are intriguing, since Itk has a positive role in αβ TCR signaling.
Itk, Rlk, and Tec are expressed in αβ T cells, and all three kinases appear to have a positive role in TCR signaling. In the absence of Itk or both Rlk and Itk, PLCγ activation is decreased, leading to diminished Ca2+
flux and reduced mitogen-associated protein kinase (MAPK) activation (reviewed in 15
). This, in turn, results in defective nuclear factor for activated T cells (NFAT) and activator protein-1 (AP-1) activation downstream of TCR signaling. As a consequence, Itk- and Rlk/Itk-deficient T cells produce little IL-2 and have a reduced proliferative response following activation (reviewed in 15
). Interestingly, there appears to be a hierarchy in the importance of these kinases in T cells as follows: Itk > Rlk > Tec.
Due to the defects in TCR signaling in Tec kinase-deficient mice, it was surprising that T cells developed in the absence of Itk. To determine if Itk had any role in T-cell development, we and others examined this issue in more detail. The two main stages of T-cell development where Itk might potentially have a role are β selection and positive and negative selection after α chain rearrangement. Our laboratory has recently described that the absence of Itk and Rlk/Itk affects β selection during T-cell development. We found that Itk, Rlk, and Tec are all expressed during the DN stages of T-cell development, although Itk has the highest expression during all four stages. Whereas no block during the DN1 and DN2 stages of development was observed, a modest proliferative defect during the DN to DP transition of development was seen in the absence of Itk or Rlk/Itk (18
). After β selection, thymocytes undergo multiple rounds of proliferation as they transition to the DP stage of development (7
). In Itk- and Rlk/Itk-deficient thymocytes, this proliferative burst is somewhat blunted (18
). Further, Itk-deficient thymocytes were not as effective as wildtype thymocytes at repopulating the DP and SP subsets during competitive repopulation assays (18
). Thus, although defects in the development of Itk- and Rlk/Itk-deficient T cells are more substantial at later stages, Tec kinases do appear to have a role in the transition of thymocytes from the DN to the DP stage of maturation.
The effects of the Itk and Rlk/Itk deficiency are more apparent at the DP stage of development. Although lineage commitment of conventional αβ T cells into the CD4+
subsets appears unaltered in the absence of Itk, there are alterations in positive and negative selection. For example, when the major histocompatibility complex (MHC) class I-restricted HY TCR transgenic line was crossed with Itk-, Rlk- or Rlk/Itk-deficient T cells, the positive selection HY+
T cells in transgenic female mice was inhibited in the absence of Itk or Rlk/Itk (19
). Further, when HY male mice were examined, CD8+
T cells survived negative selection in the absence of Itk and Rlk/Itk due to impaired death-inducing signals (19
). We also examined positive selection in the absence of Itk using the MHC class II-restricted TCR transgenic lines, AND, 5C.C7, and 2B4. These TCRs are specific for same MHC/peptide antigen but have different avidities for their selecting ligand and thus have different efficiencies of positive selection. Our laboratory found that 2B4 transgenic T cells, with the weakest avidity, had a reduced T-cell population in Itk-deficient mice when compared to wildtype mice (20
). However, the AND transgenic mice, which have the highest avidity TCR, had comparable T-cell populations when comparing Itk-deficient and wildtype mice (20
). Further, when examining the stages of positive selection via CD69, TCR, and heat stable antigen (HSA) expression, these studies indicated that Itk-deficient thymocytes have a delay in positive selection (20
). In addition, the marker for strength of TCR engagement, CD5, was not as highly expressed on Itk-deficient T cells (20
). Thus, it appears that a lack of Tec kinases in the TCR signaling cascade can alter the outcome of positive and negative selection processes.
In this review, we examine how Tec kinases might function in the selection of conventional αβ T cells versus innate T cells. As Itk and Rlk have been shown to influence positive and negative selection, it seemed likely that these kinases would also have a role in the lineage decisions between innate versus conventional T-cell subsets. Previously, our laboratory and others (3
) showed that Tec kinase-deficient CD8+
T cells have an innate phenotype; in addition, a small CD4+
T-cell population with similar innate characteristics has recently been described in Itk-deficient mice (23
). Our laboratory and others (24
) have also reported defects in NKT cell development and function, and more recently, we have observed that the γδ T-cell population is affected by the lack of Itk and Rlk/Itk (authors’ unpublished data). Several studies have demonstrated a role for SLAM family signaling in the development of many innate T-cell subsets (26
, reviewed in 2
). Thus, it appears that the signals controlling lineage selection and fate may be more complicated than previously thought, with a combination of different signals affecting the outcome of T-cell development. Here we describe how TCR signaling contributes to the conventional versus innate T-cell lineage decision.