Molecular mechanism of T-cell activation: role of protein tyrosine kinases in antigen receptor-mediated signal transduction

Cyclin-dependent kinase 5 activity is required for allogeneic T-cell responses after hematopoietic cell transplantation in mice

Molecular intermediates in T-cell activation pathways are crucial targets for the therapy and prevention of graft-versus-host disease (GVHD) following allogeneic hematopoietic cell transplantation (allo-HCT). We recently identified an essential role for cyclin-dependent kinase 5 (Cdk5) in T-cell activation and effector function, but the contribution of Cdk5 activity to the development of GVHD has not been explored. Using an established, preclinical, murine, GVHD model, we reveal that Cdk5 activity is increased in key target organs early after allo-HCT. We then generated chimeric mice (Cdk5^+/+C or Cdk5^-/-C) using hematopoietic progenitors from either embryonic day 16.5 Cdk5^+/+ or Cdk5^-/- embryos to enable analyses of the role of Cdk5 in GVHD, as germ line Cdk5 gene deletion is embryonically lethal. The immunophenotype of adult Cdk5^-/-C mice is identical to control Cdk5^+/+C mice. However, transplantation of donor Cdk5^-/-C bone marrow and T cells dramatically reduced the severity of systemic and target organ GVHD. This phenotype is attributed to decreased T-cell migration to secondary lymphoid organs (SLOs), reduced in vivo proliferation within these organs, and fewer cytokine-producing donor T cells during GVHD development. Moreover, these defects in Cdk5^-/- T-cell function are associated with altered CCR7 signaling following ligation by CCL19, a receptor:ligand interaction critical for T-cell migration into SLOs. Although Cdk5 activity in donor T cells contributed to graft-versus-tumor effects, pharmacologic inhibition of Cdk5 preserved leukemia-free survival. Collectively, our data implicate Cdk5 in allogeneic T-cell responses after HCT and as an important new target for therapeutic intervention.

Cyclin-dependent kinase 5 activity is required for T cell activation and induction of experimental autoimmune encephalomyelitis

Cyclin-dependent kinase 5 (Cdk5) is a ubiquitously expressed serine/threonine kinase. However, a requirement for Cdk5 has been demonstrated only in postmitotic neurons where there is abundant expression of its activating partners p35 and/or p39. Although hyperactivation of the Cdk5-p35 complex has been found in a variety of inflammatory neurodegenerative disorders, the potential contribution of nonneuronal Cdk5-p35 activity has not been explored in this context. We describe a previously unknown function of the Cdk5-p35 complex in T cells that is required for induction of experimental autoimmune encephalomyelitis (EAE). T cell receptor (TCR) stimulation leads to a rapid induction of Cdk5-p35 expression that is required for T lymphocyte activation. Chimeric mice lacking Cdk5 gene expression in hematopoietic tissues (Cdk5(-/-C)) are resistant to induction of EAE, and adoptive transfer of either Cdk5(-/-C) or p35(-/-) encephalitogenic lymphocytes fails to transfer disease. Moreover, our data reveal a novel mechanism involving Cdk5-mediated phosphorylation of the actin modulator coronin 1a on threonine 418. Cdk5-deficient lymphocytes lack this posttranslational modification of coronin 1a and exhibit defective TCR-induced actin polarization and reduced migration toward CCL-19. These data define a distinct role for Cdk5 in lymphocyte biology and suggest that inhibition of this kinase may be beneficial in the treatment of T cell-mediated inflammatory disorders.

Redirecting human CD4+ T lymphocytes to the MHC class I-restricted melanoma antigen MAGE-A1 by TCR alphabeta gene transfer requires CD8alpha

Adoptive immunotherapy involving the transfer of autologous tumor or virus-reactive T lymphocytes has demonstrated its effectiveness in the eradication of cancer and virally infected cells. Clinical trails and in vitro studies have focused on CD8+ cytotoxic T-cell receptor (TCR) alphabeta lymphocytes since these cells directly kill virally infected- and tumor cells after antigen-specific recognition via their TCR alphabeta. However, increasing evidence suggests that induction of sustained immunity against cancer and viral infections depends on the presence of tumor- or virus-specific CD4+ T lymphocytes, which are restricted by MHC class II. Here, we show that these MHC class II-restricted CD4+ T lymphocytes can efficiently be redirected to MHC class I-restricted tumor cells by retroviral introduction of an HLA-A1/MAGE-A1-specific chimeric two-chain TCR ValphaCalphazeta/VbetaCbetazeta (tcTCR/zeta). However, TCR-transduced CD4+ T lymphocytes were only able to specifically bind to HLA-A1/MAGE-A1 complexes and respond to HLA-A1+/MAGE-A1+ melanoma cells when the CD8alpha gene was cointroduced. These CD4+/CD8alpha+/TCR(POS) T lymphocytes produce IFN-gamma, TNFalpha and IL-2 when specifically stimulated via the introduced TCR with immobilized HLA-A1/MAGE-A1 complexes or HLA-A1+/MAGE-A1+ melanoma cells. Furthermore, introduction of the CD8alpha gene into TCR(POS) T lymphocytes rendered these T lymphocytes cytotoxic for HLA-A1+/MAGE-A1+ melanoma cells. These results demonstrate that human CD4+ T lymphocytes when genetically grafted with an HLA-A1/MAGE-A1-specific TCR and CD8alpha are induced to kill and produce cytokines upon specific interaction with the relevant melanoma cells. Hence, CD4+ T lymphocytes, in addition to CD8+ T lymphocytes, may be critical effector cells for adoptive immuno-gene therapy to generate a sustained tumor-specific immune response in cancer patients.

Decreased binding of peptides-MHC class I (pMHC) multimeric complexes to CD8 affects their binding avidity for the TCR but does not significantly impact on pMHC/TCR dissociation rate

The CD8 coreceptor plays a crucial role in both T cell development in the thymus and in the activation of mature T cells in response to Ag-specific stimulation. In this study we used soluble peptides-MHC class I (pMHC) multimeric complexes bearing mutations in the CD8 binding site that impair their binding to the MHC, together with altered peptide ligands, to assess the impact of CD8 on pMHC binding to the TCR. Our data support a model in which CD8 promotes the binding of TCR to pMHC. However, once the pMHC/TCR complex is formed, the TCR dominates the pMHC/TCR dissociation rates. As a consequence of these molecular interactions, under physiologic conditions CD8 plays a key role in complex formation, resulting in the enhancement of CD8 T cell functions whose specificity, however, is determined by the TCR.

The effects of calpain inhibition upon IL-2 and CD25 expression in human peripheral blood mononuclear cells

Calcium is an important contributor to T cell activation; it is also the major factor in the activation of the calcium-activated neutral proteinase, calpain. For this reason, we wanted to investigate if calpain has a role in T cell activation and what aspects of this activation calpain affects. As measured by semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), calpain inhibition decreased interleukin-2 (IL-2) and CD25 mRNA expression in a dose-dependent manner, at early time points following the initial activation, and over extended periods of time in activated human peripheral blood mononuclear cells (PBMCs). Using an enzyme-linked immuno-sorbent assay (ELISA) specific for human IL-2, we found that calpain inhibition decreased IL-2 secretion in a dose-dependent manner, shortly after activation, and continuously over time. Inhibiting calpain caused a dose-dependent inhibition of CD25 cell surface expression and also inhibited expression shortly after activation and for at least 48 h. This study showed that calpain has an integral role in the synthesis of the two important T cell activation factors, IL-2 and CD25.

Role of the Src homology 2 domains and interdomain regions in ZAP-70 phosphorylation and enzymatic activity

The protein tyrosine kinase ZAP-70, which mediates T-cell antigen receptor (TCR) signalling, contains three distinct functional modules, two tandemly arranged SH2 domains, a kinase domain and a linker region (interdomain B) that connects them. ZAP-70 enzymatic activation is strictly dependent on the binding, via its SH2 domains, to the triggered TCR and on tyrosine phosphorylation. Here we utilized recombinant ZAP-70 and carried out a mutational analysis to understand the structural requirements for its activation. We show that deletion of both SH2 domains corresponding to the first 254 residues moderately increases ZAP-70 enzymatic activity on an exogenous substrate in vitro, results in increased tyrosine phosphorylation and produces subtle conformational changes, as judged by altered SDS/PAGE migration. Mutation of Tyr292, 315 and 319 to Phe in the interdomain B region, which constitute the major phosphorylation sites both in vitro and in vivo, did not affect ZAP-70 enzymatic activity. Moreover, deletion analysis of the interdomain B region established residues 320-619 as a minimal region endowed with full kinase activity. We propose that binding of ZAP-70 to the TCR promotes, through conformational changes, its extensive phosphorylation on tyrosine. However, Tyr292, 315 and 319 do not affect ZAP-70 enzymatic activity and may influence ZAP-70 signalling only indirectly by mediating its association with intracellular transducers.

Autoimmune enteropathy with distinct mucosal features in T-cell activation deficiency: the contribution of T cells to the mucosal lesion

BACKGROUND

Autoimmune enteropathy is normally characterised by crypt hyperplastic villous atrophy with enterocyte autoantibodies, activation of mucosal lymphocytes and increased epithelial HLA-DR. This case involved a severely affected Portuguese infant who was found to have lymphocyte activation deficiency and demonstrated correspondingly distinct mucosal features.

METHODS

A female infant of nonconsanguineous parents was treated for vomiting and diarrhoea, first with milk exclusion and then with parenteral nutrition. Lymphocyte subsets and immunoglobulin concentrations were normal, but in vitro testing showed no activation in response to phytohaemagglutinin, Candida, or purified protein derivative, although the response to interleukin (IL)-2 was intact. Interleukin-2 deficiency was excluded. Analysis of jejunal biopsy specimens revealed only mild villous blunting with absent goblet cells, normal epithelial proliferation, and no crypt hyperplasia. The dense infiltrate of CD8+ and CD4+ T lymphocytes showed normal CD2 and CD3 expression but no activation or proliferation markers. HLA-DR was not increased on epithelium or lymphocytes. Thus, in addition to in vitro evidence for lymphocyte activation deficiency, the mucosal specimens showed no evidence of in situ T-cell activation.

RESULTS

After development of overwhelming septicaemia, the patient died at 18 months, just before a planned bone marrow transplant.

CONCLUSIONS

These findings confirm significant heterogeneity within autoimmune enteropathy. Formal immune function testing should be performed in all affected infants to identify T-cell activation deficiencies. The distinct mucosal findings suggest that activated T cells usually induce the crypt hyperplastic villous atrophy characteristic of classic autoimmune enteropathy.

Activation of STAT transcription factors by herpesvirus Saimiri Tip-484 requires p56lck

Signal transducers and activators of transcription (STATs) relay signals from activated cell surface receptors directly to the nucleus. Previously, a protein required for T-cell transformation by the DNA tumor virus herpesvirus saimiri (HVS) and designated tyrosine kinase interacting protein (Tip-484) was shown to interact with and dramatically upregulate the activity of p56lck. p56lck is a nonreceptor tyrosine kinase that is essential for signaling by the T-cell receptor and also interacts with the CD4, CD8, and interleukin-2 receptors. The present data show activation of STAT1 and -3 by Tip-484. STAT1 and -3 were also found to complex with glutathione S-transferase-Tip-484 only in the presence of p56lck, and STAT3 was shown to be phosphorylated by the Tip-484-p56lck multiprotein complex in vitro. Infection of T cells with HVS or expression of recombinant Tip-484 significantly increased the DNA-binding activity of the STAT1 and STAT3 transcription factors in nuclear extracts and also increased the phosphorylation of STAT3 in vivo. This is the first report of STAT activation by a DNA tumor virus protein. Moreover, these studies demonstrate that p56lck is required for STAT activation by Tip-484.