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The herpes simplex virus viion contains a function that mediates the shutoff of host-protein synthesis and the degradation of host mRNA. Viral mutants affected in this function (vhs mutants) have previously been derived. Cells infected with these mutants exhibit a more stable synthesis of host as well as the immediate early (a)-viral proteins. We now show that a function associated with purified virions of the wild-type virus reduces the half-life of host and a mRNAs, whereas purified vhs-I mutant virions lack this activity. The functional half-life of many early ((3)-and late (y)-viral mRNAs is also prolonged in mutant virus infections. These studies suggest that the wild-type virion brings into cells a function that indiscrininately reduces the half-life of both host and viral transcripts and that the early translational shutoff of the host is a consequence of this function. This function may facilitate rapid transitions in the expression of groups of genes that are transcriptionally turned on at different times after infection.Cells infected with herpes simplex viruses 1 and 2 proceed through the sequential transcription of several coordinately regulated groups of viral genes, including the a (immediate early), A3 (delayed early), and yj and y2 (late) genes (1-3). As suggested originally by Honess and Roizman (1), this transcriptional regulation must also be accompanied by a negative translational-regulatory scheme. Thus, the translation of host mRNA is turned off soon after virus entry into the cells, and the synthesis of a-and (3-viral polypeptides decreases at later intervals after infection.The early shutoff of host-protein synthesis (4) (15). Surprisingly, the vhs mutants are also altered with respect to a function that is present in the wild-type (wt) virus inoculum, which decreases the synthesis of a proteins from preexisting a mRNA (15). These findings suggest that a single virion-associated function might limit the translation of both host and a mRNAs.In this paper, we present data concerning the specificity of the host shutofffunction. We show that purified virions of the wt virus, but not those of the vhs-i mutant virus, carry a function that reduces the half-life of host and a mRNAs.Furthermore, the functional half-life of many (-and y-viral mRNAs is also prolonged in vhs-i-infected cells. These findings suggest that wt virions contain a function that indiscriminately reduces the half-life of the majority of infected-cell mRNAs. EXPERIMENTAL PROCEDURESCells and Viruses. Mouse Ltk-cells were obtained from B. Roizman (University of Chicago) and Vero monkey cells were obtained from S. Bachenheimer (University of North Carolina). The derivation of the vhs-i mutant was described previously (15). Virus stocks representing cell lysates were prepared by three cycles of freeze-thawing of infected cells (16), and virions were purified in dextran T 10 gradients (17,18).Drug Treatment and Analyses of Proteins. For cycloheximide reversal in the presence of actinomycin D (1), the cells were incubated fo...

A new human herpesvirus has been isolated from CD4' T cells purified from peripheral blood mononuclear cells of a healthy individual (RK), following incubation of the cells under conditions promoting T-cell activation. The virus could not be recovered from nonactivated cells. Cultures of lymphocytes infected with the RK virus exhibited a cytopathic effect, and electron microscopic analyses revealed a characteristic herpesvirus structure. RK virus DNA did not hybridize with large probes derived from herpes simplex virus, EpsteinBarr virus, varicella-zoster virus, and human cytomegalovirus. The genetic relatedness of the RK virus to the recently identified T-lymphotropic human herpesvirus 6 (HHV-6) was investigated by restriction enzyme analyses using 21 different enzymes and by blot hybridization analyses using 11 probes derived from two strains of HHV-6 (Z29 and U1102 MATERIALS AND METHODSPurification of CD4+ T Cells and T-Cell Activation. CD4+ T cells were isolated from peripheral blood lymphocytes (PBLs) by negative selection using immunoadsorption with goat anti-mouse immunoglobulin-coated magnetic particles, as previously described (10, 11). The cells were >99% CD2+ and >96% CD4+, as determined by flow cytometry. Monocytes were <0.1% as determined by staining with nonspecific esterase. For T-cell activation the cultures were incubated for 2 days with plastic-immobilized CD3 monoclonal antibody (mAb) G19-4 (11). To maintain cell proliferation, the cells were further cultured with interleukin 2 (IL-2; Calbiochem) at 30 units/ml or with CD28 mAb 9.3 (12). Cultures were restimulated at weekly intervals with plastic-immobilized CD3 mAb. The cells were cryopreserved in 7.5% dimethyl sulfoxide.Virus Propagation. The Z29 strain of HHV-6 (6) was obtained from C. Lopez (Centers for Disease Control, Atlanta). The U1102 strain (5) was obtained from R. W. Honess (National Institute of Medical Research, London). HHV-6 (Z29) was propagated in PBLs as described (13). Briefly, PBLs were precultured for 3 days in RPMI-10% medium [RPMI 1640 medium plus gentamicin (50 jig/ml) with 10% heat-inactivated fetal bovine serum] containing phytohemagglutinin (PHA; Difco) at 10 ,ug/ml. Infection was done in RPMI-10% medium with PHA at 5 ,ug/ml. HHV-6 (U1102) and HHV-7 (RK) were similarly propagated in PHApretreated PBLs. However, the infection was done in RPMI-10% medium.Electron tTo whom reprint requests should be addressed. 748The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Shortly after the discovery of human herpesvirus 6 (HHV-6), two distinct variants, HHV-6A and HHV-6B, were identified. In 2012, the International Committee on Taxonomy of Viruses (ICTV) classified HHV-6A and HHV-6B as separate viruses. This review outlines several of the documented epidemiological, biological, and immunological distinctions between HHV-6A and HHV-6B, which support the ICTV classification. The utilization of virus-specific clinical and laboratory assays for distinguishing HHV-6A and HHV-6B is now required for further classification. For clarity in biological and clinical distinctions between HHV-6A and HHV-6B, scientists and physicians are herein urged, where possible, to differentiate carefully between HHV-6A and HHV-6B in all future publications.