Herpesvirus saimiri replaces ZAP-70 for CD3- and CD2-mediated T cell activation

Growth transformation of human T cells by herpesvirus saimiri requires multiple Tip-Lck interaction motifs

Lymphoma induction and T-cell transformation by herpesvirus saimiri strain C488 depends on two viral oncoproteins, StpC and Tip. The major interaction partner of Tip is the protein tyrosine kinase Lck, a key regulator of T-cell activation. The Lck binding domain (LBD) of Tip comprises two interaction motifs, a proline-rich SH3 domain-binding sequence (SH3B) and a region with homology to the C terminus of Src family kinase domains (CSKH). In addition, biophysical binding analyses with purified Lck-SH2 domain suggest the phosphorylated tyrosine residue 127 of Tip (pY127) as a potential third Lck interaction site. Here, we addressed the relevance of the individual binding motifs, SH3B, CSKH, and pY127, for Tip-Lck interaction and for human T-cell transformation. Both motifs within the LBD displayed Lck binding activities and cooperated to achieve a highly efficient interaction, while pY127, the major tyrosine phosphorylation site of Tip, did not enhance Lck binding in T cells. Herpesvirus saimiri strain C488 recombinants lacking one or both LBD motifs of Tip lost their transforming potential on human cord blood lymphocytes. Recombinant virus expressing Tip with a mutation at position Y127 was still able to transform human T lymphocytes but, in contrast to wild-type virus, was strictly dependent on exogenous interleukin-2. Thus, the strong Lck binding mediated by cooperation of both LBD motifs was essential for the transformation of human T cells by herpesvirus saimiri C488. The major tyrosine phosphorylation site Y127 of Tip was particularly required for transformation in the absence of exogenous interleukin-2, suggesting its involvement in cytokine signaling pathways.

Cell transformation by Herpesvirus saimiri

Herpesvirus saimiri (Saimiriine herpesvirus-2), a gamma2-herpesvirus (rhadinovirus) of non-human primates, causes T-lymphoproliferative diseases in susceptible organisms and transforms human and non-human T lymphocytes to continuous growth in vitro in the absence of stimulation. T cells transformed by H. saimiri retain many characteristics of intact T lymphocytes, such as the sensitivity to interleukin-2 and the ability to recognize the corresponding antigens. As a result, H. saimiri is widely used in immunobiology for immortalization of various difficult-to-obtain and/or -to-maintain T cells in order to obtain useful experimental models. In particular, H. saimiri-transformed human T cells are highly susceptible to infection with HIV-1 and -2. This makes them a convenient tool for propagation of poorly replicating strains of HIV, including primary clinical isolates. Therefore, the mechanisms mediating transformation of T cells by H. saimiri are of considerable interest. A single transformation-associated protein, StpA or StpB, mediates cell transformation by H. saimiri strains of group A or B, respectively. Strains of group C, which exhibit the highest oncogenic potential, have two proteins involved in transformation-StpC and Tip. Both proteins have been shown to dramatically affect signal transduction pathways leading to the activation of crucial transcription factors. This review is focused on the biological effects and molecular mechanisms of action of proteins involved in H. saimiri-dependent transformation.

Tip, an Lck-interacting protein of Herpesvirus saimiri, causes Fas- and Lck-dependent apoptosis of T lymphocytes

Saimiriine herpesvirus-2 (Herpesvirus saimiri) transforms T lymphocytes, including human, to continuous growth in vitro. H. saimiri-induced transformation is becoming an important tool of T-cell biology, including studies of HIV replication. Two proteins of H. saimiri subgroup C, Tip and StpC, are essential for T-cell transformation. In spite of the important role of these proteins, their biological functions and the molecular mechanisms of their action remain insufficiently understood. To further elucidate the effects of Tip on T cells, we transduced T lymphocytes, using an efficient lentiviral gene transfer system, to express Tip in the absence of other H. saimiri proteins. Our results indicate that Tip specifically inhibits IL-2 production by human T lymphocytes. Furthermore, Tip promotes T-cell apoptosis, which appears to be the reason for the observed decrease in IL-2 production. Finally, the apoptotic effect of Tip in T cells is mediated by Fas and requires the presence of active Lck in the cell.

Molecular mechanisms of the effect of herpesvirus saimiri protein StpC on the signaling pathway leading to NF-kappaB activation

Herpesvirus saimiri (Saimiriine herpesvirus-2) causes lethal T lymphoproliferative diseases in the susceptible species and transforms T lymphocytes to continuous growth in vitro. H. saimiri-induced transformation of T cells is becoming an important experimental tool of biomedical research. Two proteins of H. saimiri subgroup C, Tip and StpC, are essential for T cell transformation by this virus. It has been shown previously that StpC transforms fibroblasts, activates NF-kappaB, and binds to tumor necrosis factor (TNF)-receptor-associated factor (TRAF) proteins, but the molecular mechanism of its action remains insufficiently understood. This study further characterized the effect of StpC on NF-kappaB. First, StpC activates NF-kappaB via the consensus pathway involving activation of I-kappaB kinase and subsequent phosphorylation and degradation of I-kappaB in both T lymphoid and epithelial cells. Second, triggering of this pathway by StpC in both T lymphoid and epithelial cells is dependent on the presence of functional NF-kappaB-inducing kinase (NIK). Third, StpC physically interacts with TRAF in epithelial cells, and the effect of StpC on NF-kappaB activity in these cells requires the presence of functional TRAF. Finally the effect of StpC is completely independent of TNF-alpha, a well described stimulus of NF-kappaB activity. Moreover it appears that StpC uncouples stimulation of NF-kappaB activity from TNF-alpha stimulation. Overall these results argue that the effect of StpC on NF-kappaB is similar to the effects of other viral proteins, "usurping" the TRAF/NIK/I-kappaB kinase pathway, and reinforce the notion that the role of StpC in cell transformation by H. saimiri may be mediated by signaling that results in NF-kappaB activation.