Independence of herpesvirus-induced T cell lymphoma from viral cyclin D homologue

Cyclin D family members are cellular protooncogenes, and their viral homologues in the Kaposi's sarcoma-associated herpesvirus (KSHV, human herpesvirus type 8 [HHV-8]) and the closely related Herpesvirus saimiri have been implicated as putative cofactors of viral transformation and pathogenesis. KSHV is regularly found in Kaposi's sarcoma and in the primary effusion B cell lymphoma and Castleman's disease associated with immunosuppression and AIDS. H. saimiri strain C488 transforms human and marmoset T cells in vitro and causes polyclonal T cell lymphoma in New World monkeys. The viral cyclins stimulate cell cycle progression of quiescent fibroblasts, and they form active cyclin-dependent kinase (CDK)6 complexes of broad substrate specificity that can resist and downregulate cellular CDK inhibitors. This study shows that the viral cyclin of H. saimiri strain C488 is not required for viral replication, T cell transformation, and pathogenicity in New World primates.

Herpesvirus saimiri vFLIP provides an antiapoptotic function but is not essential for viral replication, transformation, or pathogenicity

Apoptosis of infected cells is an important host defense mechanism, and many viruses have exploited antiapoptotic proteins that interfere with crucial cellular pathways. Viral FLICE inhibitory proteins (vFLIPs) are encoded by rhadinoviruses like herpesvirus saimiri, the related Kaposi's sarcoma-associated herpesvirus-human herpesvirus 8 (KSHV/HHV8), and the poxvirus responsible for molluscum contagiosum. The vFLIPs can block the interaction of the death receptor-adapter complex with the cellular effector FLICE (caspase-8), and this prevents the initiation of the downstream caspase cascade. KSHV/HHV8 vFLIP overexpression can confer resistance to T-cell-mediated apoptosis and acts as a tumor progression factor in a murine B-cell lymphoma model. To analyze the function of herpesvirus vFLIPs in the genetic background of the virus and in a model for viral pathogenesis, we deleted the vFLIP gene (open reading frame 71) from the genome of herpesvirus saimiri strain C488. The viral deletion mutant was viable and replicated like the wild-type virus. An antiapoptotic effect could be attributed to the vFLIP gene, but we also show that the vFLIP gene of herpesvirus saimiri is dispensable for viral transformation of T cells in vitro and for pathogenicity in cottontop tamarins in vivo.

The URNA genes of herpesvirus saimiri (strain C488) are dispensable for transformation of human T cells in vitro

The herpesvirus saimiri strain C488 genome contains five genes for small nuclear RNAs, termed herpesvirus saimiri URNAs (or HSURs). Using a cosmid-based approach, all HSURs were precisely deleted from the genome. The mutant virus replicated at levels that were similar to those of wild-type viruses in OMK cells. Although the HSURs are expressed in wild-type virus-transformed human T-cell lines, the deletion does not affect viral transformation in cell culture.

Activation of a myelin basic protein-specific human T cell clone by antigen-presenting cells from rhesus monkeys

This study addresses the capacity of peripheral blood mononuclear cells (PBMC) from rhesus monkeys (Macaca mulatta) to present myelin basic protein (MBP), a candidate auto-antigen for multiple sclerosis, to MBP-specific human CD4+ T cell clones. MHC-restriction of the human T cell clones was determined with HLA-DR-transfected L cells, and epitope specificity was established with a panel of overlapping 20-mer peptides. The MHC-DR region of the rhesus monkeys (Mamu) was characterized serologically and by sequence analysis. We identified one CD4+ HLA-DRB1*0301-restricted Th1-like human T cell clone (ES-BP8) that was activated to proliferation with human or rhesus monkey MBP, or peptide MBP 29-48 presented by PBMC from six different rhesus monkeys expressing the Mamu-DRB1*0305 or -DRB1*0306 alleles. After transformation to continuous growth with Herpesvirus saimiri, the T cell clone could still be stimulated by antigen (Ag) and Ag-presenting cells (APC) from monkeys. Two other T cell clones with the same HLA-restriction and the same peptide-specificity did not respond to MBP presented by these rhesus monkeys. The exon 2 sequences HLA-DRB1*0301, Mamu-DRB1*0305 and -DRB1*0306 differ at positions 32, 47, 67, 73 and 86. These amino acid differences are not critical for the binding of MBP 29-48 and do not abrogate recognition by the clone ES-BP8, but interfere with the recognition of the two other HLA-DRB1*0301-restricted T cell clones. In conclusion, studying Ag-presentation from rhesus monkey may provide further insight into the interaction of antigenic peptide, TCR and MHC.(ABSTRACT TRUNCATED AT 250 WORDS)

Engagement of the CD4 receptor inhibits the interleukin-2-dependent proliferation of human T cells transformed by Herpesvirus saimiri

Infection with Herpesvirus saimiri, a tumor virus of non-human primates, transformed human CD4+ T cell clones to permanent interleukin (IL)-2-dependent growth without need for restimulation with antigen and accessory cells. The IL-2-dependent proliferation of these cells was dramatically inhibited by soluble anti-CD4 whole antibodies, F(ab')2 and Fab fragments, and also by gp 120 of human immunodeficiency virus. The inhibition was not due to cell death and could be overcome by high concentrations of exogenous IL-2. Cell surface expression of CD4, and to a lesser degree the density of the IL-2 receptor alpha chain, were reduced upon anti-CD4 treatment. After long lasting (> 12 h) incubation with anti-CD4, abundance and activity of CD4-bound p56lck were diminished while the free fraction of p56lck remained unchanged. Since IL-2 binding to its receptor activated only the CD4-bound fraction of p56lck, the IL-2-induced p56lck activity was diminished after long-term CD4 ligation. Taken together, our results suggest a cross talk between CD4- and IL-2 receptor-mediated signaling via p56lck.

Karyotypic characterization of human T-cell lines immortalized by Herpesvirus saimiri

Herpesvirus saimiri subgroup-C strains transform non-neoplastic human T lymphocytes in vitro to antigen-independent continuous growth. The karyotypes of 8 of these T-cell lines, derived from peripheral blood or bone marrow, are analyzed here. In general, these lines showed a normal diploid karyotype, although one of them had acquired a clonal chromosomal rearrangement after 21 months in culture. Chromosomally aberrant cell clones were found more frequently in cell lines derived from blood lymphocytes of leukemia patients than in cultures from healthy donors. However, the observed aberrations, some of which were defined in detail using chromosome in situ suppression hybridization (CISS) with chromosome-specific recombinant DNA libraries, did not show any relation between the various cell lines nor to the respective types of leukemia.

Transformation of human T-cell clones by Herpesvirus saimiri: intact antigen recognition by autonomously growing myelin basic protein-specific T cells

Herpesvirus saimiri has recently been shown to immortalize human T cells. It was unknown, however, whether Herpesvirus saimiri transformation affects T-cell receptor (TCR) expression and signal transduction. In the present study, we have transformed CD4+ human T-cell clones specific for human myelin basic protein. The transformed T cells were grown in interleukin 2 and divided in the absence of antigen and antigen-presenting cells. They retained the membrane phenotype of activated T cells and secreted the cytokines interferon gamma and lymphotoxin, but interleukin 4 was not detected. Further, the transformed T cells continued to express the original TCR as demonstrated by TCR variable-region-V beta-specific monoclonal antibodies and TCR sequencing. Antigen-specific recognition and signal transduction by the TCR were demonstrated by myelin-basic-protein-induced HLA-DR-restricted secretion of interferon gamma and lymphotoxin and by myelin-basic-protein-specific proliferation. Antigen specificity and reactivity have been maintained for > 1 year after transformation. Transformation with Herpesvirus saimiri now allows the production of virtually unlimited numbers of (auto)antigen-specific T cells expressing functional TCR and a stable membrane phenotype. This technology will facilitate studies of the pathogenesis of putative autoimmune diseases, such as multiple sclerosis, and may be of help in TCR-targeted immunotherapy.

Immortalization of human T cell clones by Herpesvirus saimiri. Signal transduction analysis reveals functional CD3, CD4, and IL-2 receptors

Investigation of human activated T cells has been complicated by the need for periodic restimulation with Ag/mitogen and accessory cells and by the limited life span of most human T cell clones. To overcome these problems, we have transformed established human T cell clones to permanent growth with Herpesvirus saimiri, a lymphoma-inducing virus of nonhuman primates. Three human CD4+ T cell clones were investigated in detail. They have been growing in the presence of exogenous IL-2 but without restimulation with mitogen or feeder cells for more than 11 mo with doubling times between 2 and 4 days. In contrast, their nontransformed parent clones needed to be restimulated with PHA and feeder cells every 14 to 21 days. To compare responses of H. saimiri-transformed clones with those of their parent clones, we stimulated the cells with IL-2 or with anti-CD3 and/or anti-CD4 mAb with and without cross-linking on the cell surface. Transformed and nontransformed T cell clones were strikingly similar in parameters of early signal transduction, namely, tyrosine phosphorylation and mobilization of calcium. Ligation of their TcR/CD3 complexes by mAb or by Ag in the presence of autologous accessory cells increased the proliferation and the secretion of IFN-gamma. Taken together, we have shown that human T cell clones immortalized with H. saimiri express functional CD3, CD4, and IL-2R. They constitute a simple, stable, reproducible and accessory cell-free model system for the investigation of signal transduction events in activated human T cells.

Immortalization of human T cell clones by Herpesvirus saimiri. Signal transduction analysis reveals functional CD3, CD4, and IL-2 receptors

Investigation of human activated T cells has been complicated by the need for periodic restimulation with Ag/mitogen and accessory cells and by the limited life span of most human T cell clones. To overcome these problems, we have transformed established human T cell clones to permanent growth with Herpesvirus saimiri, a lymphoma-inducing virus of nonhuman primates. Three human CD4+ T cell clones were investigated in detail. They have been growing in the presence of exogenous IL-2 but without restimulation with mitogen or feeder cells for more than 11 mo with doubling times between 2 and 4 days. In contrast, their nontransformed parent clones needed to be restimulated with PHA and feeder cells every 14 to 21 days. To compare responses of H. saimiri-transformed clones with those of their parent clones, we stimulated the cells with IL-2 or with anti-CD3 and/or anti-CD4 mAb with and without cross-linking on the cell surface. Transformed and nontransformed T cell clones were strikingly similar in parameters of early signal transduction, namely, tyrosine phosphorylation and mobilization of calcium. Ligation of their TcR/CD3 complexes by mAb or by Ag in the presence of autologous accessory cells increased the proliferation and the secretion of IFN-gamma. Taken together, we have shown that human T cell clones immortalized with H. saimiri express functional CD3, CD4, and IL-2R. They constitute a simple, stable, reproducible and accessory cell-free model system for the investigation of signal transduction events in activated human T cells.

Herpes virus saimiri-transformed human T lymphocytes: normal functional phenotype and preserved T cell receptor signalling

Herpes virus saimiri (HVS), a primate herpes virus, transforms human CD4+ and CD8+ T lymphocytes to continuous growth in vitro. We have previously shown that HVS-transformed human T cells (HVS-T cells) respond to stimulation via CD2 with autocrine growth. In the present study we have investigated the functional characteristics of HVS-T cells. We describe that these cells can perform all the functions of normal T cells, i.e. cytokine secretion, cytotoxicity, and exocytosis of granule esterases. All these activities can be triggered via CD2 by binding to its natural ligand or via the TCR, e.g. by anti-TCR antibodies, by recognition of a bacterial superantigen and by MHC-restricted recognition of specific antigen. The pattern of tyrosine-phosphorylated proteins after TCR triggering was identical in HVS-T cells and normal T cells. We conclude that HVS-T cells can respond to TCR-mediated signals with the functions of normal T lymphocytes. Furthermore, HVS-T cells are the only transformed human T cells that can be specifically triggered by cytotoxicity and esterase release. The finding that the TCR functions normally in these cells will make HVS a convenient means to immortalize antigen-specific human T lymphocytes.

CD2-mediated autocrine growth of herpes virus saimiri-transformed human T lymphocytes

Herpes virus saimiri (HVS) immortalizes T lymphocytes from a variety of primates and causes acute T cell lymphomas and leukemias in nonnatural primate hosts. Here we have analyzed the requirements for growth of three HVS-transformed human T cell lines. The cells expressed the phenotype of activated T cells: two were CD4+, and one was CD8+. All three cells responded to all allogeneic human cell lines tested with enhanced proliferation, production of interleukin 2 (IL-2), and increased expression of the IL-2 receptor. Binding of CD2 to its ligand CD58 was the critical event mediating stimulation because: (a) monoclonal antibodies (mAbs) to CD2 and to CD58, but not to a variety of other surface structures, blocked induced and spontaneous proliferation and IL-2 production; (b) only anti-CD2 mAbs were stimulatory if crosslinked; (c) a nonstimulatory cell was rendered stimulatory by CD58 transfection; and (d) the cells responded specifically to CD58 on sheep red blood cells. Growth of the cells required activation because cyclosporin A and FK506 blocked stimulator cell-induced IL-2 production and proliferation as well as the spontaneous growth of the lines. Antibodies to the IL-2 receptor reduced proliferation of the cells and blocked IL-2 utilization. Taken together, these results show that HVS-transformed T cells proliferate in response to CD2-mediated contact with stimulator cells or with each other in an IL-2-dependent fashion. They suggest that HVS transforms human T cells to an activation-dependent autocrine growth.

Stable growth transformation of human T lymphocytes by herpesvirus saimiri

Herpesvirus saimiri induces T-cell lymphomas in various species of New World monkeys and in rabbits, and it is able to immortalize monkey T lymphocytes in vitro. Sequences responsible for these effects have been localized to a region of the genome that varies significantly among the virus subgroups A, B, and C. We now report that infection of human blood lymphocytes and thymocytes with strains of subgroup C, in contrast to viruses of the other subgroups, yields continuously proliferating T-cell lines with the phenotype of mature CD4- or CD8-positive cells. Infection with strains of Herpes-virus saimiri subgroup C can thus be used to generate human T-cell lines for a variety of immunological and developmental studies.

Transformation to continuous growth of primary human T lymphocytes by human T-cell leukemia virus type I X-region genes transduced by a Herpesvirus saimiri vector

The role of the X region of the genome of the human T-cell leukemia virus type I (HTLV-I) in the immortalization of lymphocytes has been difficult to distinguish from its role in viral replication as this region encodes at least two genes, tax and rex, required for replication and the expression of viral proteins. To determine whether the X region does encode immortalizing functions, a fragment of the HTLV-I provirus capable of expressing known X-region proteins was inserted into the genome of a transformation-defective, replication-competent Herpesvirus saimiri. Infection of fresh mitogen-activated human cord blood and thymocytes yielded immortal T-cell lines that had the same phenotype (CD4+, CD5+, HLA class II+, interleukin 2 receptor alpha-chain +) as lymphocytes transformed by cocultivation with HTLV-I. These experiments demonstrate that the X region encodes the functions of HTLV-I that immortalize a distinct subpopulation of human T cells. The experiments also demonstrate the utility of the H. saimiri vector for the transduction of heterologous genes into human T cells.