Immortalization with herpesvirus saimiri modulates the cytokine secretion profile of established Th1 and Th2 human T cell clones

T blasts of six established human CD4+ T cell clones with defined Ag specificity and cytokine secretion profile (3 Th1 and 3 Th2) were immortalized with Herpesvirus saimiri (HVS) and compared with their uninfected counterparts for their ability to proliferate, produce cytokines, and express cytolytic activity. HVS-transformed Th1 and Th2 clones neither substantially changed their original surface markers nor lose their ability to proliferate in response to their specific Ag but did acquire the ability to proliferate in response to contact signals delivered by SRBC or autologous APC alone. In addition, transformation by HVS substantially enhanced the lectin-dependent cytolytic activity of Th1 clones and enabled noncytolytic Th2 clones to exert cytolytic activity. HVS-transformed Th1 clones but not their uninfected counterparts spontaneously transcribed and secreted Th1-type cytokines (IL-2, IFN-gamma, and TNF-beta) and such a production was further enhanced by stimulation with either SRBC or PMA plus anti-CD3 mAb. HVS transformed but not uninfected Th2 clones constitutively expressed both IL-4 and IL-2 mRNA and secreted IFN-gamma. Stimulation with PMA plus anti-CD3 mAb induced uninfected Th2 clones to secrete high amounts of IL-4 and IL-5 but not Th1-type cytokines, whereas the same HVS-transformed Th2 showed minimal IL-4 and IL-5 secretion with concomitant high production of IL-2, IFN-gamma, and TNF-beta. Transformation by HVS also resulted in up-regulation of TNF-alpha and IL-3 production by both Th1 and Th2 clones. The ongoing proliferation of HVS-transformed clones was partially inhibited by either anti-IL-2 or anti-IL-3 antibodies and virtually abolished by the combined addition of the two anticytokine antibodies, suggesting that both IL-2 and IL-3 can function as autocrine growth factors for HVS-transformed Th1 and Th2 clones.

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.

Herpesvirus saimiri transformed human T cell lines: a permissive system for human immunodeficiency viruses

Human peripheral blood T lymphocytes are readily transformed to continuous growth by Herpesvirus saimiri subgroup C strains. The immortalized cells express the phenotype of mature activated T cells and bear either CD4 or CD8 surface markers. Here we report that Herpesvirus saimiri transformed CD4+ cell lines are highly susceptible to infection with human immunodeficiency viruses types 1 and 2. The prototype viruses HIV-1IIIB and HIV-2ROD replicated rapidly and caused cell death within 14 days. These cell lines did also support growth of a poorly replicating HIV-2 strain (HIV-2NEP) and of primary clinical isolates. Thus H. saimiri transformed T cells represent a new system for HIV propagation and isolation, especially for HIV-2 strains with restricted cell tropism. These cells should be considered as an alternative approach in cases where conventional attempts fail.

Mouse monoclonal antibodies recognizing the activation domain of HIV-1 rev transactivator

The rev protein of human immunodeficiency virus type 1 (HIV-1), a phosphoprotein of 20 kDa apparent molecular weight, is essential to target the mRNA for virion polypeptides into the cytoplasm. So far, at least four necessary functional domains have been assigned to the HIV-1 rev protein: (1) one for RNA binding; (2) a second for nuclear/nucleolar localization that may be indistinguishable from the RNA binding motif; (3) two domains for multimerization; and (4) a putative activation domain (AD) that is suppressed in trans by dominant-negative mutant rev protein. We report three IgG1 kappa mouse monoclonal antibodies (mabs) that were independently raised against rev protein expressed in Escherichia coli. Epitopes are mapped by immunoprecipitation and Western blot screening with 40 different rev mutant peptides. Surprisingly, monoclonal antibodies from all three hybridomas recognized the activation domain of HIV-1 rev.

Detection of varicella zoster virus DNA and viral antigen in human cornea after herpes zoster ophthalmicus

This article describes the histopathology, immunohistochemistry, and varicella zoster virus DNA in situ hybridization of 14 corneal buttons obtained from 14 patients (average age 69.0 years) after perforating keratoplasty (four patients) or surgical enucleation (10 patients) at different times after the clinical onset of herpes zoster ophthalmicus (average 58.7 months). The main histopathologic features were intense stromal vascular scarring (12 patients) and granulomatous reaction to Descemet's membrane (nine patients). Using the peroxidase-antiperoxidase method, varicella zoster virus (VZV) antigen could be detected by immunohistochemistry in two patients within epithelial cells of the cornea and in the limbal episclera during the active phase of herpes zoster ophthalmicus. For in situ hybridization we used the 35S-labeled HindIII A and C fragment of VZV and identified viral DNA in five corneal buttons obtained 1 day to 8 years after the clinical onset of infection. Viral DNA was mainly found in mononuclear cells with eosinophilic intracytoplasmic inclusions within vascular stromal scars, in keratocytes, and in epithelial cells of the cornea. Our results show that VZV DNA is detectable in human cornea even 8 years after the clinical onset of herpes zoster ophthalmicus and may indicate VZV persistence in a latent form in corneal tissue or reactivation of the virus from an endogenous or exogenous source causing a severe and often recurrent keratitis in the progress of herpes zoster ophthalmicus.

The glycoprotein B homologue of human herpesvirus 6

The gene for the homologue of herpesvirus glycoprotein B (gB) has been identified in the genome of human herpesvirus 6 (HHV-6), strain U1102, and the nucleotide sequence was determined. The open reading frame encodes a protein of 830 amino acids (93.2K) with the characteristics of a transmembrane glycoprotein and close similarity to the gp58/116 complex of human cytomegalovirus (HCMV). Monoclonal antibodies 2D10 and 2B9 have been shown previously to react with an HHV-6 glycoprotein of apparent M(r) 112K, and its proteolytic cleavage products of M(r) 64K and 58K. We show that both monoclonal antibodies detect prokaryotically expressed carboxy-terminal fragments of the HHV-6 gB homologue. This indicates that the HHV-6 gB homologue is probably processed by proteolytic cleavage similar to its equivalents in HCMV and various other herpesviruses.

Dynamics of the immune system response in cerebrospinal fluid and blood of SIVmac-infected rhesus monkeys

Paired sera and CSF samples were collected from SIVmac-infected macaques. Animals infected with SIVmac251 maintained low gag and high env-specific antibody levels in plasma. Increasing env-specific antibody titers in CSF were associated in one animal with strong intrathecal synthesis. SIVmac239-infected monkeys revealed high antibody titers of gag and env-specificity, in one animal accompanied by weak intrathecal synthesis of virus-specific antibodies. In all animals, the CD4/CD8 ratio in CSF decreased faster compared to blood.

Lymphadenitis and lymphoproliferative lesions associated with the human herpes virus-6 (HHV-6)

A newly described herpes virus, human herpes virus 6, (HHV-6), has been linked to exanthema subitum but beyond this its pathogenetic impact remains to be determined. A large body of evidence links it to various lymphoproliferative disorders and this study was conducted to identify forms of lymphoproliferation linked to HHV-6. We studied biopsy samples from 32 patients with disorders of the lymphatic system for the presence of HHV-6, both by polymerase chain reaction (PCR) and in-situ hybridization (ISH) methods, as well as Epstein-Barr virus (EBV) viral DNA, clonal rearrangements of the antigen receptor genes and bcl-2 genes. All the specimens were studied morphologically and a clinical follow-up of up to 4 years was obtained. Seven of the 32 patients were positive for HHV-6 DNA and the remainder were negative. Two of these HHV-6 positive specimens, both from elderly persons, showed a similar distinct histological pattern diagnosed as malignant B-cell lymphoma of high grade malignancy. Two other HHV-6-positive specimens were reactive lymphadenopathies occurring in younger adults. In addition, one further specimen with evidence of EBV-involvement was from a patient who died 3 months after biopsy with fatal infectious mononucleosis (IM). These five samples had HHV-6 DNA by PCR and ISH. Two specimens without specific histologic abnormalities showed evidence of HHV-6 only by PCR but not by ISH. Both high grade malignant lymphomas showed clonal proliferations, one of monoclonal B-cells and the other of clonal T-cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Antioxidants selectively suppress activation of NF-kappa B by human T-cell leukemia virus type I Tax protein

Oxygen radical scavengers, such as dithiocarbamates and cysteine derivatives, inhibit activation of the ubiquitous transcription factor nuclear factor kappa B (NF-kappa B) after treatment of cells with tumor necrosis factor, phorbol ester, and interleukin-1. An involvement of oxygen radicals was more directly evident from the induction of NF-kappa B by low concentrations of H2O2 and the demonstration that cells stimulated with various NF-kappa B inducers release H2O2 and superoxide. In this study, we used the antioxidant pyrrolidine dithiocarbamate (PDTC) to investigate whether the activation of NF-kappa B by the viral transactivator Tax from human T-cell leukemia virus type I also depends on the production of reactive oxygen intermediates. The Tax-induced activation of the DNA-binding activity of NF-kappa B in Jurkat T cells was potently suppressed by micromolar concentrations of PDTC. Within the same concentration range, PDTC and two other dithiocarbamates also strongly interfered with transactivation of the long terminal repeat (LTR) of human immunodeficiency virus type 1 by Tax but had no effect on transactivation of the same LTR by Tat. Transactivation of the human T-cell leukemia virus type I LTR by Tax was also barely influenced. Tax seems to activate NF-kappa B by a mechanism shared with all other inducers of NF-kappa B tested so far. It appears that one of the pleiotropic activities of Tax leads to an enhanced production of oxygen radicals that are required for activation of NF-kappa B.

Genetic relationships between bovine herpesvirus 4 and the gammaherpesviruses Epstein-Barr virus and herpesvirus saimiri

The overall arrangement of genes in the unique central part of the bovine herpesvirus type 4 (BHV-4) genome has been deduced by analysis of short DNA sequences. Twenty-three genes conserved in at least one of the completely sequenced herpesviruses have been identified and localized. All of these genes encoded amino acid sequences with higher similarity to proteins of the gammaherpesviruses Epstein-Barr virus (EBV) and herpesvirus saimiri (HVS) than to the homologous products of the alphaherpesviruses varicella-zoster virus and herpes simplex virus type 1 or the betaherpesvirus human cytomegalovirus. The genome organization of BHV-4 had also an overall colinearity with that of the gammaherpesviruses EBV and HVS. Furthermore, the BHV-4 genes content and arrangement were more similar to those of HVS than to those of EBV, suggesting that BHV-4 and HVS are evolutionarily more closely related to each other than either are to EBV. BHV-4 DNA sequences were generally deficient in CpG dinucleotide. This CpG deficiency is characteristic of gammaherpesvirus genomes and suggests that the BHV-4 latent genome is extensively methylated. Despite several biological features similar to those of betaherpesviruses, BHV-4 displays the molecular characteristics of the representative members of the gammaherpesvirinae subfamily.

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.

Herpesvirus saimiri has a gene specifying a homologue of the cellular membrane glycoprotein CD59

Herpesvirus saimiri (HSV) is a T-lymphotropic tumor virus that causes fulminant lymphomas and leukemias in various New World primates other than its natural host, the squirrel monkey (Saimiri sciureus). In the course of completing the nucleotide sequence of its genome, we identified an open reading frame of 363 nucleotides, designated HVS-15, that has no detectable homology to any other viral sequences to date. HVS-15 encodes a 121-amino-acid protein which shows significant similarities to human CD59, a phosphatidyl-inositol-glycan-anchored glycoprotein involved in T-cell activation and restriction of complement-mediated lysis. The predicted HVS-15 gene product is more similar to human CD59 than to the related murine Ly-6 antigens. A nucleotide sequence identity of 64% was found between HVS-15 and the CD59 reading frame, and a 48% identity exists between the corresponding protein sequences. The comparison of the amino acid sequences revealed a number of conserved structural features such as a similar pattern of hydrophobic termini and an identical cysteine skeleton.

A high proportion of early response genes are constitutively activated in T cells by HTLV-I

Immortalization of T cells by HTLV-I is mediated by the X region of the virus and probably involves transactivation of cellular genes. We show that T cells transformed by HTLV-I constitutively express a high proportion of early response genes that are normally transiently induced following antigenic or mitogenic activation of T cells. Thus, HTLV-I-infected T cells display an 'early activation' phenotype that is distinct from the gene expression pattern of continuously dividing T cells. Ten early response genes representing a diverse array of functional categories were assayed. Four DNA-binding proteins/transcription factors including the p50 subunit of NF-kappa B were evaluated. A protein(s) encoded by the X region of HTLV-I appeared to contribute to up-regulated expression of most, if not all, of the early response genes. For those genes that could be assayed, increased transcriptional rates, but not substantial changes in mRNA half-life, were demonstrated in the presence of pX-encoded proteins, suggesting that the transcriptional transactivator, Tax, affects the induction or maintenance of transcription for these mitogen-inducible genes. Therefore, Tax may mimic or interact with a component(s) of the signal transduction pathway activated by antigen or mitogen treatment. These data demonstrate that early response genes, some of which probably play roles in initiating or maintaining cellular proliferation, are frequent targets of HTLV-I activation.

Primary structure of the herpesvirus saimiri genome

This report describes the complete nucleotide sequence of the genome of herpesvirus saimiri, the prototype of gammaherpesvirus subgroup 2 (rhadinoviruses). The unique low-G + C-content DNA region has 112,930 bp with an average base composition of 34.5% G + C and is flanked by about 35 noncoding high-G + C-content DNA repeats of 1,444 bp (70.8% G + C) in tandem orientation. We identified 76 major open reading frames and a set of seven U-RNA genes for a total of 83 potential genes. The genes are closely arranged, with only a few regions of sizable noncoding sequences. For 60 of the predicted proteins, homologous sequences are found in other herpesviruses. Genes conserved between herpesvirus saimiri and Epstein-Barr virus (gammaherpesvirus subgroup 1) show that their genomes are generally collinear, although conserved gene blocks are separated by unique genes that appear to determine the particular phenotype of these viruses. Several deduced protein sequences of herpesvirus saimiri without counterparts in most of the other sequenced herpesviruses exhibited significant homology with cellular proteins of known function. These include thymidylate synthase, dihydrofolate reductase, complement control proteins, the cell surface antigen CD59, cyclins, and G protein-coupled receptors. Searching for functional protein motifs revealed that the virus may encode a cytosine-specific methylase and a tyrosine-specific protein kinase. Several herpesvirus saimiri genes are potential candidates to cooperate with the gene for saimiri transformation-associated protein of subgroup A (STP-A) in T-lymphocyte growth stimulation.

Role of human T-cell leukemia virus type 1 X region proteins in immortalization of primary human lymphocytes in culture

Human T-cell leukemia virus type 1 (HTLV-1) immortalizes human CD4+ T lymphocytes in culture. Previous studies show that in the context of a herpesvirus saimiri vector, the sequence of the X region at the 3' end of the HTLV-1 genome is also capable of immortalizing CD4+ lymphocytes in the absence of HTLV-1 structural proteins. The X region of HTLV-1 encodes two trans-acting viral proteins, the 42-kDa Tax protein and the 27-kDa Rex protein. Infection of human cord blood cells with herpesvirus saimiri recombinants which contain HTLV-1 X region sequences defective for expression of tax, rex, or both tax and rex demonstrates that tax function is necessary and sufficient for immortalization of primary human CD4+ cord blood lymphocytes in culture in the context of the herpesvirus saimiri vector.

New member of the multigene family of complement control proteins in herpesvirus saimiri

A number of glycoproteins are regulators of the complement cascade and prevent damage to cells by inappropriate activation of complement. In humans, all of them are encoded by a multigene family on chromosome I and share a characteristic structural feature, the short consensus repeats of about 61 amino acids with a constant framework of cysteine, proline, and tryptophan. We found the gene for glycoproteins of analogous structure in herpesvirus saimiri, a T-lymphotropic tumor virus of New World primates. Unspliced transcripts code for a membrane-bound 65- to 75-kDa virion surface component, while spliced mRNA instructs a secreted glycoprotein of 47 to 53 kDa. Expression of complement control proteins suggests a novel mechanism of counteracting host immune defense to prevent elimination of a virus that is capable of persisting in circulating lymphocytes.

Gene for the major antigenic structural protein (p100) of human herpesvirus 6

A human herpesvirus 6 (HHV-6) structural protein of 100 kDa (p100) is the polypeptide most frequently and intensively reactive in immunoblotting analyses with human sera on HHV-6-infected cells or partially purified virions. The gene for p100 was identified by screening a bacteriophage lambda library with monospecific rabbit antisera. The gene codes for a polypeptide of 870 amino acids with a calculated molecular size of 97 kDa. Its amino-terminal third is weakly homologous to the immunogenic basic matrix phosphoprotein pp150 of human cytomegalovirus. Five fragments representing more than 93% of HHV-6 p100 were prokaryotically expressed. The antigenic epitopes of p100 were preliminary mapped by immunoblotting with human sera. They are located within the carboxy-terminal part which is neither homologous nor cross-reactive to pp150 of human cytomegalovirus. Availability of the gene for the immunodominant structural protein should provide tools for studies of pathogenesis by HHV-6.

Nucleotide sequence of HSUR 6 and HSUR 7, two small RNAs of herpesvirus saimiri

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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.

Alpha-lipoic acid is an effective inhibitor of human immuno-deficiency virus (HIV-1) replication

Alpha-lipoic acid, a naturally occurring disulfide-compound that acts as a cellular coenzyme, inhibits replication of HIV-1 in cultured lymphoid T-cells. Alpha-lipoic acid was added 16 hours after infection of the T-cell lines Jurkat, SupT1 and Molt-4 with HTLV IIIB and HIV-1 Wal (a wild type HIV-1 isolate). We observed a dose dependent inhibition of HIV-1-replication in CPE (Cytopathic effect) formation, reverse transcriptase activity and plaque formation on CD4-transformed HeLa-cells. An over 90% reduction of reverse transcriptase activity could be achieved with 70 micrograms alpha-lipoic acid/ml, a complete reduction of plaque-forming units at concentrations of greater than or equal to 35 micrograms alpha-lipoic acid/ml. An augmentation of the antiviral activity was seen by combination of zidovudine and low dose of alpha-lipoic acid (7 micrograms/ml). Trypan blue staining revealed no toxic effects of alpha-lipoic acids on peripheral blood mono-nuclear cells and T-cell lines even in concentrations of greater than or equal to 70 micrograms/ml. Therefore, we propose the inclusion of alpha-lipoic acid into chemotherapy trials in combination with zidovudine.

The unique region of the human herpesvirus 6 genome is essentially collinear with the UL segment of human cytomegalovirus

The entire genome of human herpesvirus 6 (HHV-6) strain U1102 was cloned as overlapping fragments in cosmid and plasmid vectors. Cleavage maps were constructed for the restriction endonucleases BamHI, EcoRI, NotI and SmaI. The genome of HHV-6 U1102 is a linear dsDNA of 163 kbp, consisting of a long unique 142 kbp region flanked by direct terminal repeats of 10.5 kbp. Short stretches (290 to 470 nucleotides) of DNA, four from the terminal repeats and 55 from the UL region, were sequenced and compared by computer with the known herpesvirus amino acid sequences. Homologies were found for 10 open reading frames that are scattered over the UL region of HHV-6. Their relative positions and orientations indicate that the unique region of HHV-6 is essentially collinear with the UL region of human cytomegalovirus (HCMV), but is not collinear with the other human herpesviruses. It confirms and extends earlier observations that HHV-6 is more closely related to the beta-herpesvirus HCMV, suggesting that HHV-6 may be considered as the prototype of a new beta 2-herpesvirus subgroup.