Transplantation and cytogenetic studies of Herpesvirus saimiri-induced disease in Marmoset monkeys

Herpesvirus saimiri

Herpesvirus saimiri (saimiriine herpesvirus 2) is the classical prototype of the gamma(2)-herpesviruses or rhadinoviruses, which also contains a human member, the Kaposi's sarcoma-associated herpesvirus. The T-lymphotropic Herpesvirus saimiri establishes specific replicative and persistent conditions in different primate host species. Virtually all squirrel monkeys (Saimiri sciureus) are persistently infected with this virus. In its natural host, the virus does not cause disease, whereas it induces fatal acute T-cell lymphoma in other monkey species after experimental infection. The virus can be isolated by cocultivation of permissive epithelial cells with peripheral blood cells from naturally infected squirrel monkeys and from susceptible New World monkeys during the virus-induced disease. Tumour-derived and in vitro-transformed T-cell lines from New World monkeys release virus particles. Herpesvirus ateles is a closely related virus of spider monkeys (Ateles spp.) and has similar pathogenic properties to Herpesvirus saimiri in other New World primate species. Similar to other rhadinoviruses, the genome of Herpesvirus saimiri harbours a series of virus genes with pronounced homology to cellular counterparts including a D-type cyclin, a G-protein-coupled receptor, an interleukin-17, a superantigen homologue, and several inhibitors of the complement cascade and of different apoptosis pathways. Preserved function has been demonstrated for most of the homologues of cellular proteins. These viral functions are mostly dispensable for the transforming and pathogenic capability of the virus. However, they are considered relevant for the apathogenic persistence of Herpesvirus saimiri in its natural host. A terminal region of the non-repetitive coding part of the virus genome is essential for pathogenicity and T-cell transformation. Based on the pathogenic phenotypes and the different alleles of this variable region, the virus strains have been assigned to three subgroups, termed A, B and C. In the highly oncogenic subgroup C strains, the two virus genes stpC and tip are transcribed from one bicistronic mRNA and are essential for transformation and leukaemia induction. stpC fulfils the typical criteria of an oncogene; its product interacts with Ras and tumour necrosis factor-associated factors and induces mitogen-activated protein kinase and nuclear factor kappa B activation. Tip interacts with the RNA transport factor Tap, with signal transduction and activation of transcription factors, and with the T-cellular tyrosine kinase Lck, which is activated by this interaction and phosphorylates Tip as a substrate. It is of particular interest that certain subgroup C virus strains such as C488 are capable of transforming human T lymphocytes to stable growth in culture. The transformed human T cells harbour multiple copies of the viral genome in the form of stable, non-integrated episomes. The cells express only a few virus genes and do not produce virus particles. The transformed cells maintain the antigen specificity and many other essential functions of their parental T-cell clones. Based on the preserved functional phenotype of the transformed T cells, Herpesvirus saimiri provides useful tools for T-cell immunology, for gene transfer and possibly also for experimental adoptive immunotherapy.

Regulation of the herpesvirus saimiri oncogene stpC, similar to that of T-cell activation genes, in growth-transformed human T lymphocytes

Herpesvirus saimiri strain C488, a T-cell tumor virus of New World primates, transforms human T lymphocytes to stable interleukin-2-dependent growth without need for further stimulation by antigen or mitogen. The transformed cell lines show the phenotype of activated mature T cells and retain many essential features of the primary parental cells, e.g., antigen specificity. In contrast to transformed New World monkey T cells, the human lines do not support lytic growth of the virus, even after chemical stimulation. Here we show that many viral genes remain silent during episomal persistence. However, the viral oncogene stpC is predominantly transcribed and translated to a stable cytoplasmic protein of 20 kDa that is heterogeneously expressed in individual cells. This 1.7-kb mRNA is bicistronic, encoding also Tip, a viral protein interacting with the T-cell-specific tyrosine kinase Lck. stpC/tip transcripts are heavily induced upon stimulation by mitogen or phorbol ester. Block of protein synthesis does not abolish transcription: treatment with cycloheximide greatly induces stpC/tip mRNA levels. Thus, this gene complex is regulated similarly to early T-cell activation genes. Constitutive and induced expression engage different transcription start sites. The T-cell regulation of the viral genes stpC and tip may contribute to the T-cell tropism of growth transformation by herpesvirus saimiri.

Inhibition of complement-mediated cytolysis by the terminal complement inhibitor of herpesvirus saimiri

Herpesvirus saimiri (HVS) is a lymphotropic herpesvirus that induces T-cell transformation in vitro and causes lymphomas and leukemias in New World primates other than its natural host, the squirrel monkey. Nucleotide sequence analysis of the HVS genome revealed two open reading frames with significant homology to genes for human complement regulatory molecules. One of these genes encodes a predicted protein (designated HVSCD59) with 48% amino acid sequence identity to the human terminal complement regulatory protein CD59 (HuCD59). The CD59 homolog from squirrel monkey (SMCD59) was cloned, and the corresponding amino acid sequence showed 69% identity with HVSCD59. BALB/3T3 cells stably expressing HVSCD59, SMCD59, or HuCD59 were equally protected from complement-mediated lysis by human serum. However, only HVSCD59-expressing cells were effectively protected from complement-mediated lysis when challenged with rat serum, suggesting that HVSCD59 was less species restrictive. The complement regulatory activity of HVSCD59 and SMCD59 occurred after C3b deposition, indicating terminal complement inhibition. Treatment of BALB/3T3 stable transfectants with phosphatidylinositol-specific phospholipase C prior to complement attack decreased the complement regulatory function of HVSCD59, suggesting cell surface attachment via a glycosyl-phosphatidylinositol anchor. Cells expressing HVSCD59 effectively inhibited complement-mediated lysis by squirrel monkey serum in comparison with SMCD59-expressing cells. Finally HVSCD59-specific transcripts were detected in owl monkey cells permissive for lytic HVS replication but not in T cells transformed by HVS, which failed to produce virions. These data are the first to demonstrate a functional, virally encoded terminal complement inhibitor and suggest that HVSCD59 represents a humoral immune evasion mechanism supporting the lytic life cycle of HVS.

Monoclonal antibodies EBU-141 (CDw75) and EBU-65 allow reliable distinction between mature and pre-B-cell tumors in suspension and on tissue sections

Two new monoclonal antibodies, EBU-65 and EBU-141, were raised by immunization with plasma cell line U-266. Both antibodies strongly react with B lymphocytes in immunofluorescent staining as well as on paraffin-embedded sections. More than 200 leukemias and lymphomas were tested, and for both antibodies reactivity was found only with "mature" B-cell tumors but not with precursor B-cell leukemias. None of the non-B-lineage hematolymphatic tumors tested was stained by EBU-141 or EBU-65. A subpopulation of T lymphocytes particularly present in nonmalignant pleural effusions was detected by EBU-65 additionally. Although EBU-141 was clustered as CDw75 and EBU-65 as "unique," a close relationship of the staining pattern was found and both antibodies react with a sialyltransferase. In particular, CDw75 antibody EBU-141 was demonstrated to be very useful for immunophenotyping of B-cell neoplasias, while EBU-65 reacted with most multiple myelomas and a subgroup of "activated"-appearing T cells.

Herpesvirus saimiri-induced lymphoblastoid rabbit cell line: growth characteristics, virus persistence, and oncogenic properties

A nonproducer lymphoblastoid cell line (7710) containing the herpesvirus saimiri (HVS) genome was established from the HVS-positive spleen of a male, inbred New Zealand White rabbit (III/J strain) which had developed a well-differentiated lymphoma after inoculation of HVS and 12-O-tetradecanoylphorbol-13-acetate (TPA). Antibodies to HVS early and late antigens were detected in the serum of rabbit 7710 by indirect immunofluorescence and immunoprecipitation. The cell line was of T-cell origin, did not produce HVS, and could not be superinfected with HVS. However, HVS early antigens could be induced in the cells with n-butyric acid and TPA or TPA alone. On the other hand, late antigens were never observed, and infectious virus could not be rescued by cocultivation of 7710 cell with OMK cells. The 7710 cells were T-cell growth factor dependent, even after many in vitro passages. The 7710 cell line contained multiple copies of a nonintegrated, covalently closed circular HVS genome. As is characteristic of some other HVS-transformed nonproducer lymphoid cell lines, a large segment of unique light (L) DNA was missing in the persistent circular viral DNA present in 7710 cells. This deletion spanned at least 42.5 kilobases, corresponding to the segment between 12.3 and 50.7 map units of full-length, infectious virion L-DNA. The 7710 cells failed to induce tumors in athymic nude mice, but inbred rabbits inoculated with as few as 100 of these cells developed fatal lymphomas. Chromosomal analysis showed that tumors were due to the growth of donor cells. Cells recovered from one of the rabbits inoculated with 7710 cells also contained HVS DNA and, after in vitro culture, induced the same type of lymphoma when inoculated into two other III/J-strain rabbits. None of the previously described HVS-transformed cell lines have been able to induce tumors in either their host species or nude mice. Thus, our demonstration that the 7710 cell line is readily transplantable in syngeneic rabbits represents the first available model which allows analysis of many biological and molecular aspects of the in vivo oncogenicity of HVS.

Persistence of virulent canine distemper virus in lymphoblastoid cell lines

Persistent infection with virulent canine distemper virus (CDV-SH) was established in 2 human lymphoblastoid B cell lines (Wi-L2 and Raji), and one human (HSB), one simian (1670) and one canine (CT-45-S) lymphoblastoid T cell line. Cell free virus from persistently infected T cell lines was avirulent for dogs but virulence was maintained during 31 cell passages in persistently infected B cell lines.

Virus-specific transcription in a Herpesvirus saimiri-transformed lymphoid tumor cell line

Herpesvirus saimiri-transformed lymphoid tumor cell lines contain nonintegrated covalently closed circular viral DNA molecules in high multiplicity. One of those cell lines, 1670, carries large viral DNA circles (202 kilobase pairs) with two stretches of repetitive DNA (70.8% G + C) that are interspersed between two segments of unique DNA (36% G + C). Since it was not known if there is any viral gene expression in H. saimiri-transformed cells, we initiated a study of transcription in cell line 1670. cDNA was generated by reverse transcription of cellular RNA and hybridized with cloned virion DNA fragments. The experiments indicated that appreciable transcription is restricted to a single segment of unique DNA. This sequence is present once only in the circular viral DNA and corresponds to unique DNA between map units 0.89 and 0.93 of virion DNA. By Northern blot hybridizations with labeled cloned probes of virion unique DNA, one predominant virus-specific polyadenylated transcript of, at most, 2.7 kilo-bases could be detected in tumor cell line 1670. The direction of transcription was determined by hybridization with randomly primed cDNA and, in parallel, with oligodeoxythymidylate-primed cDNA probes. Apparently, the patterns of virus-specific RNA synthesis in the H. saimiri-transformed cells are clearly distinct from the transcription program in other herpesvirus transformation systems analyzed before.

Specifically unmethylated cytidylic-guanylate sites in Herpesvirus saimiri DNA in tumor cells

The restriction endonucleases MspI (CCGG), HpaII (CCGG), FnuDII (CGCG), and HaeIII (GGCC) were used to study the methylation of Herpesvirus saimiri DNA in tumor cells taken directly from tumor-bearing animals. No evidence was found for methylation of the 5' terminal C in the sequence CCGG or of the internal C in the sequence GGCC, but extensive methylation of CG was detected. Fifteen HpaII sites and 17 FnuDII sites were detected in the unique DNA region of the H. saimiri strain used. Twenty-eight of the 32 sites were methylated in greater than 90% of the viral DNA molecules in tumor cells, but the remaining 4 sites were unmethylated in greater than 95% of the viral DNA molecules in tumor cells. The locations of the four specifically unmethylated sites were mapped and appeared to be identical in the four different induced leukemias examined (one owl monkey and three white-lipped marmosets). The nonproducer 1670 tumor cell line, in continuous passage for over 7 years, contained four similar specifically unmethylated sites. Possibilities for the physiological significance of the unmethylated sites are discussed.

Herpesvirus saimiri DNA in tumor cells--deleted sequences and sequence rearrangements

Herpesvirus saimiri DNA in continuous lymphoblastoid cell lines obtained from viral induced tumors in marmosets has been analyzed by gel electrophoresis of restricted DNA. Southern transfer to nitrocellulose filters, and hybridization to 32P-labeled viral DNA or DNA fragments. The viral DNA fragments EcoRI-G, -H, -D, and -I, KpnI-A, and BamHI-D and -E were not detected in Southern transfers of DNA from the nonproducing 1670 cell line. For each restriction endonuclease, a new fragment appeared, consistent with a 13.0-megadalton deletion of viral DNA sequences. This deletion encompassed 35 to 48 +/- 0.6 megadaltons from the left end of the unique DNA region. A sequence arrangement map is presented for the major population of H. saimiri DNA sequences in the 1670 cell line. Although H. saimiri DNA in the nonproducing 70N2 cell line can be distinguished from viral DNA in the 1670 cell line by several criteria, the same sequences were found to be deleted in the major population of viral DNA molecules. Unlike 1670 and 70N2 cells, restricted DNA from the virus-producing cell lines 77/5 and 1926 contained all of the DNA fragments present in the parental virion DNA. DNA from 1670, 70N2, and 77/5 cells contained additional viral DNA fragments that did not comigrate with any virion DNA fragments. Most of these unexplained fragments were confined to or highly enriched in partially purified circular or linear DNA fractions. DNA from tumor cells taken directly from a tumor-bearing animal contained viral DNA indistinguishable from the parental virion DNA by the assay conditions used. These results indicate that viral DNA sequence rearrangements can occur upon cultivation of tumor cells in vitro and that excision of DNA sequences from the viral genome may play a role in establishing the nonproducing state of some tumor cell lines.

Infection of multiple T-cell subsets and changes in lymphocyte functions associated with Herpesvirus saimiri infection of owl monkeys

We examined the association of Herpesvirus saimiri with lymphocyte subsets and the functional integrity and distribution of these populations in owl monkeys with chronic, disease-free infections, in uninoculated, control animals, and in one monkey with H. saimiri-induced lymphoma. The lymphocyte subpopulations examined included total T cells, T cells with receptors for the Fc portion of immunoglobulin G (the Tgamma cells), T cells lacking this receptor (the Tgamma(-) population), and non-T cells. These studies showed that in chronically infected monkeys, H. saimiri was found in both Tgamma and Tgamma(-) populations and that the relative distribution of lymphocyte subsets was not different than the relative distribution in normal animals. The peripheral blood of the one leukemic animal studied showed an increase in total T cells, and both Tgamma and Tgamma(-) cells were increased in number and contained recoverable H. saimiri. In animals with chronic infections, which previously were thought to be immunologically normal, we showed that the Tgamma cells had lost the ability to respond to phytohemagglutinin. When the level of nonspecific cytotoxic activity was examined, we found that the lymphocytes from infected animals were as active as those from uninfected monkeys and that this activity was maintained at normal levels during disease. In the leukemic blood there was a relative increase in the cytotoxic activity of the Tgamma(-) cells. The Tgamma(-) cells obtained from leukemic blood lacked the ability to respond to phytohemagglutinin and could suppress the phytohemagglutinin response of normal cells. This suppressor cell activity was resistant to 3,000 rads of X irradiation. We also found that cells reactive to H. saimiri antigens could be demonstrated in the lymph nodes but not in the peripheral circulation of the lymphomatous monkey.