Characterization of the immediate-early 2 protein of human herpesvirus 6, a promiscuous transcriptional activator

SARS-CoV-2 Nsp2 Contributes to Inflammation by Activating NF-κB

COVID-19 is associated with robust inflammation and partially impaired antiviral responses. The modulation of inflammatory gene expression by SARS-CoV-2 is not completely understood. In this study, we characterized the inflammatory and antiviral responses mounted during SARS-CoV-2 infection. K18-hACE2 mice were infected with a Wuhan-like strain of SARS-CoV-2, and the transcriptional and translational expression interferons (IFNs), cytokines, and chemokines were analyzed in mouse lung homogenates. Our results show that the infection of mice with SARS-CoV-2 induces the expression of several pro-inflammatory CC and CXC chemokines activated through NF-κB but weakly IL1β and IL18 whose expression are more characteristic of inflammasome formation. We also observed the downregulation of several inflammasome effectors. The modulation of innate response, following expressions of non-structural protein 2 (Nsp2) and SARS-CoV-2 infection, was assessed by measuring IFNβ expression and NF-κB modulation in human pulmonary cells. A robust activation of the NF-κB p65 subunit was induced following the infection of human cells with the corresponding NF-κB-driven inflammatory signature. We identified that Nsp2 expression induced the activation of the IFNβ promoter through its NF-κB regulatory domain as well as activation of p65 subunit phosphorylation. The present studies suggest that SARS-CoV-2 skews the antiviral response in favor of an NF-κB-driven inflammatory response, a hallmark of acute COVID-19 and for which Nsp2 should be considered an important contributor.

ATF1 Restricts Human Herpesvirus 6A Replication via Beta Interferon Induction

The stimulus-induced cAMP response element (CRE)-binding protein (CREB) family of transcription factors bind to CREs to regulate diverse cellular responses, including proliferation, survival, and differentiation. Human herpesvirus 6A (HHV-6A), which belongs to the Betaherpesvirinae subfamily, is a lymphotropic herpesvirus frequently found in patients with neuroinflammatory diseases. Previous reports implicated the importance of CREs in the HHV-6A life cycle, although the effects of the binding of transcription factors to CREs in viral replication have not been fully elucidated. In this study, we analyzed the role of the CREB family of transcription factors during HHV-6A replication. We found that HHV-6A infection enhanced phosphorylation of the CREB family members CREB1 and activating transcription factor 1 (ATF1). Knockout (KO) of CREB1 or ATF1 enhanced viral gene expression and viral replication. The increase in viral yields in supernatants from ATF1-KO cells was greater than that in supernatants from CREB1-KO cells. Transcriptome sequencing (RNA-seq) analysis showed that sensors of the innate immune system were downregulated in ATF1-KO cells, and mRNAs of beta interferon (IFN-β) and IFN-regulated genes were reduced in these cells infected with HHV-6A. IFN-β treatment of ATF1-KO cells reduced progeny viral yields significantly, suggesting that the enhancement of viral replication was caused by a reduction of IFN-β. Taken together, our results suggest that ATF1 is activated during HHV-6A infection and restricts viral replication via IFN-β induction. IMPORTANCE Human herpesvirus 6A (HHV-6A) is a ubiquitous herpesvirus implicated in Alzheimer's disease, although its role in its pathogenesis has not been confirmed. Here, we showed that the transcription factor ATF1 restricts HHV-6A replication, mediated by IFN-β induction. Our study provides new insights into the role of ATF1 in innate viral immunity and reveals the importance of IFN-β for regulation of HHV-6A replication, which possibly impairs HHV-6A pathogenesis.

Cooperative activation of the human herpesvirus 6B U79/80 early gene promoter by immediate-early proteins IE1B and IE2B

The human herpesvirus 6B (HHV-6B) U79/80 gene belongs to the early gene class and appears as early as 3 hr postinfection. It is one of the most abundantly expressed transcripts and a useful diagnostic marker for viral reactivation. However, the expression mechanisms of the U79/80 gene remain unclear. To identify the viral factor(s) that activates the U79/80 promoter along with other HHV-6B core early gene promoters, p41, DNA polymerase, and U41, we examined the activities of U79/80 and other early gene promoters. In HHV-6B-infected MT-4 cells, U79/80 promoter activity was the highest among early gene promoters. In addition, we identified that HHV-6B immediate-early (IE)2B protein is one of the viral proteins involved in the activation of the U79/80 and other early gene promoters. Although the IE2B could independently activate these early gene promoters, the presence of IE1B significantly augmented the activities of early gene promoters. We also found that IE2B bound three human cytomegalovirus IE2-binding consensus, cis repression signal (CRS), within the U79/80 promoter. Moreover, the U79/80 promoter was activated by cellular factors, which are highly expressed in MT-4 cells, instead of HeLa cells because it was upregulated by mock infection and in the absence of IE2B. These results suggested that the activation mechanism of the U79/80 gene differs from other HHV-6B core early genes, apparently supporting its rapid and abundant expression. Therefore, the U79/80 early gene is an actually suitable biomarker of HHV-6B reactivation.

Role for the shelterin protein TRF2 in human herpesvirus 6A/B chromosomal integration

Human herpesviruses 6A and 6B (HHV-6A/B) are unique among human herpesviruses in their ability to integrate their genome into host chromosomes. Viral integration occurs at the ends of chromosomes within the host telomeres. The ends of the HHV-6A/B genomes contain telomeric repeats that facilitate the integration process. Here, we report that productive infections are associated with a massive increase in telomeric sequences of viral origin. The majority of the viral telomeric signals can be detected within viral replication compartments (VRC) that contain the viral DNA processivity factor p41 and the viral immediate-early 2 (IE2) protein. Components of the shelterin protein complex present at telomeres, including TRF1 and TRF2 are also recruited to VRC during infection. Biochemical, immunofluorescence coupled with in situ hybridization and chromatin immunoprecipitation demonstrated the binding of TRF2 to the HHV-6A/B telomeric repeats. In addition, approximately 60% of the viral IE2 protein localize at cellular telomeres during infection. Transient knockdown of TRF2 resulted in greatly reduced (13%) localization of IE2 at cellular telomeres (p<0.0001). Lastly, TRF2 knockdown reduced HHV-6A/B integration frequency (p<0.05), while no effect was observed on the infection efficiency. Overall, our study identified that HHV-6A/B IE2 localizes to telomeres during infection and highlight the role of TRF2 in HHV-6A/B infection and chromosomal integration.

Chromosomal Integration by Human Herpesviruses 6A and 6B

Upon infection and depending on the infected cell type, human herpesvirus 6A (HHV-6A) and 6B (HHV-6B) can replicate or enter a state of latency. HHV-6A and HHV-6B can integrate their genomes into host chromosomes as one way to establish latency. Viral integration takes place near the subtelomeric/telomeric junction of chromosomes. When HHV-6 infection and integration occur in gametes, the virus can be genetically transmitted. Inherited chromosomally integrated HHV-6 (iciHHV-6)-positive individuals carry one integrated HHV-6 copy per somatic cell. The prevalence of iciHHV-6⁺ individuals varies between 0.6% and 2%, depending on the geographical region sampled. In this chapter, the mechanisms leading to viral integration and reactivation from latency, as well as some of the biological and medical consequences associated with iciHHV-6, were discussed.

Crystal Structure of the DNA-Binding Domain of Human Herpesvirus 6A Immediate Early Protein 2

Immediate early proteins of human herpesvirus 6A (HHV-6A) are expressed at the outset of lytic infection and thereby regulate viral gene expression. Immediate early protein 2 (IE2) of HHV-6A is a transactivator that drives a variety of promoters. The C-terminal region of HHV-6A IE2 is shared among IE2 homologs in betaherpesviruses and is involved in dimerization, DNA binding, and transcription factor binding. In this study, the structure of the IE2 C-terminal domain (IE2-CTD) was determined by X-ray crystallography at a resolution of 2.5 Å. IE2-CTD forms a homodimer stabilized by a β-barrel core with two interchanging long loops. Unexpectedly, the core structure resembles those of the gammaherpesvirus factors EBNA1 of Epstein-Barr virus and LANA of Kaposi sarcoma-associated herpesvirus, but the interchanging loops are longer in IE2-CTD and form helix-turn-helix (HTH)-like motifs at their tips. The HTH and surrounding α-helices form a structural feature specific to the IE2 group. The apparent DNA-binding site (based on structural similarity with EBNA1 and LANA) resides on the opposite side of the HTH-like motifs, surrounded by positive electrostatic potential. Mapping analysis of conserved residues on the three-dimensional structure delineated a potential factor-binding site adjacent to the expected DNA-binding site. The predicted bi- or tripartite functional sites indicate a role for IE2-CTD as an adapter connecting the promoter and transcriptional factors that drive gene expression.IMPORTANCE Human herpesvirus 6A (HHV-6A) and HHV-6B belong to betaherpesvirus subfamily. Both viruses establish lifelong latency after primary infection, and their reactivation poses a significant risk to immunocompromised patients. Immediate early protein 2 (IE2) of HHV-6A and HHV-6B is a transactivator that triggers viral replication and contains a DNA-binding domain shared with other betaherpesviruses such as human herpesvirus 7 and human cytomegalovirus. In this study, an atomic structure of the DNA-binding domain of HHV-6A IE2 was determined and analyzed, enabling a structure-based understanding of the functions of IE2, specifically DNA recognition and interaction with transcription factors. Unexpectedly, the dimeric core resembles the DNA-binding domain of transcription regulators from gammaherpesviruses, showing structural conservation as a DNA-binding domain but with its own unique structural features. These findings facilitate further characterization of this key viral transactivator.

Laboratory and clinical aspects of human herpesvirus 6 infections

Human herpesvirus 6 (HHV-6) is a widespread betaherpesvirus which is genetically related to human cytomegalovirus (HCMV) and now encompasses two different species: HHV-6A and HHV-6B. HHV-6 exhibits a wide cell tropism in vivo and, like other herpesviruses, induces a lifelong latent infection in humans. As a noticeable difference with respect to other human herpesviruses, genomic HHV-6 DNA is covalently integrated into the subtelomeric region of cell chromosomes (ciHHV-6) in about 1% of the general population. Although it is infrequent, this may be a confounding factor for the diagnosis of active viral infection. The diagnosis of HHV-6 infection is performed by both serologic and direct methods. The most prominent technique is the quantification of viral DNA in blood, other body fluids, and organs by means of real-time PCR. Many active HHV-6 infections, corresponding to primary infections, reactivations, or exogenous reinfections, are asymptomatic. However, the virus may be the cause of serious diseases, particularly in immunocompromised individuals. As emblematic examples of HHV-6 pathogenicity, exanthema subitum, a benign disease of infancy, is associated with primary infection, whereas further virus reactivations can induce severe encephalitis cases, particularly in hematopoietic stem cell transplant recipients. Generally speaking, the formal demonstration of the causative role of HHV-6 in many acute and chronic human diseases is difficult due to the ubiquitous nature of the virus, chronicity of infection, existence of two distinct species, and limitations of current investigational tools. The antiviral compounds ganciclovir, foscarnet, and cidofovir are effective against active HHV-6 infections, but the indications for treatment, as well as the conditions of drug administration, are not formally approved to date. There are still numerous pending questions about HHV-6 which should stimulate future research works on the pathophysiology, diagnosis, and therapy of this remarkable human virus.

A human herpesvirus 6A-encoded microRNA: role in viral lytic replication

Human herpesvirus 6A (HHV-6A), a member of the betaherpesvirus family, is associated with several human diseases. Like all herpesviruses, HHV-6A establishes a lifelong, latent infection in its host. Reactivation of HHV-6A is frequent within the immunosuppressed and immunocompromised populations and results in lytic viral replication within multiple organs, often leading to severe disease. MicroRNAs (miRNAs) are key regulators of multiple cellular processes that regulate the translation of specific transcripts. miRNAs carried by herpesviruses play important roles in modulating the host cell, thereby facilitating a suitable environment for productive viral infection and/or latency. Currently, there are approximately 150 known human herpesvirus-encoded miRNAs, although an miRNA(s) encoded by HHV-6A has yet to be reported. We hypothesized that HHV-6A, like other members of the human herpesvirus family, encodes miRNAs, which function to promote viral infection. We utilized deep sequencing of small RNA species isolated from cells harboring HHV-6A to identify five novel small noncoding RNA species that originate from the viral genome, one of which has the characteristics of a viral miRNA. These RNAs are expressed during productive infection by either bacterial artificial chromosome (BAC)-derived virus in Jjhan cells or wild-type HHV-6A strain U1102 virus in HSB2 cells and are associated with the RNA induced silencing complex (RISC) machinery. Growth analyses of mutant viruses that lack each individual miRNA revealed that a viral miRNA candidate (miR-U86) targets the HHV-6A IE gene U86, thereby regulating lytic replication. The identification and biological characterization of this HHV-6A-specific miRNA is the first step to defining how the virus regulates its life cycle.

IMPORTANCE

A majority of the human population is infected with human herpesvirus 6A (HHV-6A), a betaherpesvirus family member. Infections usually occur in young children, and upon resolution, the virus remains in a latent state within the host. Importantly, during times of weakened immune responses, the virus can reactivate and is correlated with significant disease states. Viruses encode many different types of factors that both undermine the host antiviral response and regulate viral replication, including small RNA species called microRNAs (miRNAs). Here we report that HHV-6A encodes at least one miRNA, which we named miR-U86. We have characterized the requirement of this viral miRNA and its impact on the viral life cycle and found that it functions to regulate a viral protein important for efficient viral replication. Our data suggest that viral miRNAs are important for HHV-6A and that they may serve as an important therapeutic target to inhibit the virus.

Divergent susceptibilities of human herpesvirus 6 variants to type I interferons

Two distinct human herpesvirus 6 (HHV-6) variants infect humans. HHV-6B is the etiologic agent of roseola and is associated with life-threatening neurological diseases, such as encephalitis, as well as organ transplant failure. The epidemiology and disease association for HHV-6A remain ill-defined. Specific anti-HHV-6 drugs do not exist and classic antiherpes drugs have secondary effects that are often problematic for transplant patients. Clinical trials using IFN were also performed with inconclusive results. We investigated the efficacy of type I IFN (alpha/beta) in controlling HHV-6 infection. We report that cells infected with laboratory strains and primary isolates of HHV-6B are resistant to IFN-alpha/beta antiviral actions as a result of improper IFN-stimulated gene (ISGs) expression. In contrast, HHV-6A-infected cells were responsive to IFN-alpha/beta with pronounced antiviral effects observed. Type II IFN (gamma)-signaling was unaltered in cells infected by either variant. The HHV-6B immediate-early 1 (IE1) physically interacts with STAT2 and sequestrates it to the nucleus. As a consequence, IE1B prevents the binding of ISGF3 to IFN-responsive gene promoters, resulting in ISG silencing. In comparison, HHV-6A and its associated IE1 protein displayed marginal ISG inhibitory activity relative to HHV-6B. The ISG inhibitory domain of IE1B mapped to a 41 amino acid region absent from IE1A. Transfer of this IE1B region resulted in a gain of function that conferred ISG inhibitory activity to IE1A. Our work is unique in demonstrating type I IFN signaling defects in HHV-6B-infected cells and highlights a major biological difference between HHV-6 variants.

Human herpesvirus-6B induces expression of the human endogenous retrovirus K18-encoded superantigen

BACKGROUND

The superantigen, encoded by the envelope gene (env) of the human endogenous retrovirus (HERV)-K18, may be involved in autoimmunity. Its expression is transactivated in B cells during infection with Epstein-Barr virus and in peripheral blood mononuclear cells (PBMCs) that are treated with IFN-alpha.

OBJECTIVES

We investigated whether HHV-6B infection was able to induce the expression of HERV-K18 env.

STUDY DESIGN

The expression of HERV-K18 env gene was measured by real-time quantitative PCR in HHV-6B-infected PBMC.

RESULTS

Infection of PBMCs with HHV-6B resulted in a rapid and dose-dependent induction of HERV-K18 env gene expression, predominantly in monocytes. Induction was dependent on the interaction of glycoprotein H with CD46, but did not require viral transcription or DNA synthesis. Cycloheximide inhibited both the induction and basal expression of HERV-K18 env, indicating that de novo synthesis of proteins was necessary.

CONCLUSIONS

HHV-6B induced transcriptional activation of the endogenous superantigen HERV-K18 independently of virus replication, which may have consequences for the development of autoimmunity.

Inhibition of transcription of the beta interferon gene by the human herpesvirus 6 immediate-early 1 protein

Human herpesviruses (HHV) are stealth pathogens possessing several decoy or immune system evasion mechanisms favoring their persistence within the infected host. Of these viruses, HHV-6 is among the most successful human parasites, establishing lifelong infections in nearly 100% of individuals around the world. To better understand this host-pathogen relationship, we determined whether HHV-6 could interfere with the development of the innate antiviral response by affecting interferon (IFN) biosynthesis. Using inducible cell lines and transient transfection assays, we have identified the immediate-early 1 (IE1) protein as a potent inhibitor of IFN-beta gene expression. IE1 proteins from both HHV-6 variants were capable of suppressing IFN-beta gene induction. IE1 prevents IFN-beta gene expression triggered by Sendai virus infection, double-stranded RNA (dsRNA) and dsDNA transfection, or the ectopic expression of IFN-beta gene activators such as retinoic inducible gene I protein, mitochondrial antiviral signaling protein, TBK-1, IkappaB kinase epsilon (IKKepsilon), and IFN regulatory factor 3 (IRF3). While the stability of IFN-beta mRNA is not affected, IE1-expressing cells have reduced levels of dimerized IRF3 and nucleus-translocated IRF3 in response to activation by TBK-1 or IKKepsilon. Using nuclear extracts and gel shift experiments, we could demonstrate that in the presence of IE1, IRF3 does not bind efficiently to the IFN-beta promoter sequence. Overall, these results indicate that the IE1 protein of HHV-6, one of the first viral proteins synthesized upon viral entry, is a potent suppressor of IFN-beta gene induction and likely contributes to favor the establishment of and successful infection of cells with this virus.

Epitope mapping of a monoclonal antibody specific for human herpesvirus 6 variant A immediate-early 2 protein

BACKGROUND

Human herpesvirus 6 (HHV-6) variants A and B are distinct viruses that differ in their biological properties and association to disease. Diagnostic tools able to discriminate between these two variants and between active or latent HHV-6 infection are much needed. In our effort to develop variant-specific antibodies against HHV-6 immediate-early (IE) proteins, we had previously generated P6H8, a monoclonal antibody (mAb) reacting specifically with the immediate-early 2 (IE2) proteins from HHV-6A.

OBJECTIVES

To characterize the P6H8 HHV-6 variant specific mAb and evaluate its potential as part of a variant-specific diagnostic tool for HHV-6A infection. Consequently, our objective was to map the epitope recognized by P6H8.

STUDY DESIGN

In order to map P6H8 reactivity by Western blotting, we generated deletion mutants of IE2 protein as well as various GST-IE2 fusion proteins. HHV-6A infected cells were used to demonstrate P6H8 reactivity against native IE2. A synthetic peptide corresponding to the P6H8 epitope was used to confirm our results and block P6H8 reactivity.

RESULTS

We mapped the P6H8 epitope to amino acids 1078-1089 of HHV-6A IE2. A peptide (FTPFYYQSSRTR) recreating this epitope was effective in blocking the recognition of both native and recombinant IE2 by P6H8.

CONCLUSIONS

Our work provides a precise characterization of the P6H8 mAb and its specificity toward the IE2 protein of HHV-6 variant A which could prove useful for the differential diagnostic of active infection by HHV-6.

Functional interaction between human herpesvirus 6 immediate-early 2 protein and ubiquitin-conjugating enzyme 9 in the absence of sumoylation

The immediate-early 2 (IE2) protein of human herpesvirus 6 is a potent transactivator of cellular and viral promoters. To better understand the biology of IE2, we generated a LexA-IE2 fusion protein and screened, using the yeast two-hybrid system, a Jurkat T-cell cDNA library for proteins that could interact with IE2. The most frequently isolated IE2-interacting protein was the human ubiquitin-conjugating enzyme 9 (Ubc9), a protein involved in the small ubiquitin-like modifier (SUMO) conjugation pathway. Using deletion mutants of IE2, we mapped the IE2-Ubc9-interacting region to residues 989 to 1037 of IE2. The interaction was found to be of functional significance to IE2, as Ubc9 overexpression significantly repressed promoter activation by IE2. The C93S Ubc9 mutant exhibited a similar effect on IE2, indicating that the E2 SUMO-conjugating function of Ubc9 is not required for its repressive action on IE2. No consensus sumoylation sites or evidence of IE2 conjugation to SUMO could be demonstrated under in vivo or in vitro conditions. Moreover, expression levels and nuclear localization of IE2 were not altered by Ubc9 overexpression, suggesting that Ubc9's repressive function likely occurs at the transcriptional complex level. Overall, our results indicate that Ubc9 influences IE2's function and provide new information on the complex interactions that occur between herpesviruses and the sumoylation pathway.

Mapping of human herpesvirus 6 immediate-early 2 protein transactivation domains

The immediate-early 2 (IE2) protein of human herpesvirus 6 (HHV-6) is a potent transactivator of multiple cellular and viral promoters. Deletion mutants of HHV-6 variant A IE2 allowed us to map functional transactivation domains acting on complex and minimal promoter sequences. This mapping showed that both the N-terminal and C-terminal domains of IE2 are required for efficient transactivation, and that deletion of the C-terminal (1397-1466) tail of IE2 drastically reduces both transactivation and the intranuclear distribution of IE2. Moreover, we determined that the ATF/CRE binding site within the HHV-6A polymerase promoter is not required for efficient transactivation by IE2, whereas the R3 repeat region of the putative immediate-early promoter of HHV-6A is responsive to and positively regulated by IE2. These results contrast sharply to that of human cytomegalovirus (HCMV) IE2, which down-regulates its promoter. Our characterization of HHV-6 IE2 transactivating activity provides a better understanding of the complex interactions of this protein with the viral and cellular transcription machinery and highlights significant differences with the IE2 protein of HCMV.

Reciprocal transactivation between HIV-1 and other human viruses

A variety of rare clinical syndromes are seen with strikingly increased prevalence in HIV-1-infected individuals, many with underlying viral etiologies. The emergence of these diseases in AIDS reflects a reduction in the ability of the immune system to mount an adequate defense against viruses in general due to the damage inflicted to the immune system by HIV-1 infection. However, in many cases, it has been found that HIV-1 can enhance the level of expression and hence the life cycle of other viruses independently of immunosuppression through specific interactions with the viruses. This can occur either directly by HIV-1 proteins such as Tat enhancing the activity of heterologous viral promoters, and/or indirectly by HIV-1 inducing the expression of cytokines and activation of their downstream signaling that eventually promotes the multiplication of the other virus. In a reciprocal manner, the effects of other viruses can enhance the pathogenicity of HIV-1 infection in individuals with AIDS through stimulation of the HIV-1 promoter activity and genome expression. The purpose of this review is to examine the cross-interactions between these viruses and HIV-1.

Phenotypic alterations and survival of monocytes following infection by human herpesvirus-6

Freshly isolated monocytes rapidly undergo physiological changes in vitro, resulting in programmed cell death (apoptosis). Activation of monocytes, which promotes differentiation into macrophages, is known to inhibit apoptotic processes. In the present study, we report that human herpesvirus-6 (HHV-6) prevents monocytes from undergoing spontaneous apoptosis during the first 72 hours of culture. Furthermore, significant alterations in cell-surface phenotype were observed after 72 hours of infection with HHV-6. HHV-6-infected monocyte cultures have considerably reduced levels of CD14, CD64 (FcgammaRI) and HLA-DR antigen on their surface, while CD32 (FcgammaRII) expression is unaffected. On the basis of these results, we hypothesize that HHV-6 promotes monocytes survival and causes phenotypic modifications that could favor immune evasion and ensure its persistence within the infected host.

Update on human herpesvirus 6 biology, clinical features, and therapy

Human herpesvirus 6 (HHV-6) is a betaherpesvirus that is closely related to human cytomegalovirus. It was discovered in 1986, and HHV-6 literature has expanded considerably in the past 10 years. We here present an up-to-date and complete overview of the recent developments concerning HHV-6 biological features, clinical associations, and therapeutic approaches. HHV-6 gene expression regulation and gene products have been systematically characterized, and the multiple interactions between HHV-6 and the host immune system have been explored. Moreover, the discovery of the cellular receptor for HHV-6, CD46, has shed a new light on HHV-6 cell tropism. Furthermore, the in vitro interactions between HHV-6 and other viruses, particularly human immunodeficiency virus, and their relevance for the in vivo situation are discussed, as well as the transactivating capacities of several HHV-6 proteins. The insight into the clinical spectrum of HHV-6 is still evolving and, apart from being recognized as a major pathogen in transplant recipients (as exemplified by the rising number of prospective clinical studies), its role in central nervous system disease has become increasingly apparent. Finally, we present an overview of therapeutic options for HHV-6 therapy (including modes of action and resistance mechanisms).

Human herpesvirus 6 immediate-early 2 protein interacts with heterogeneous ribonucleoprotein K and casein kinase 2

Human herpesvirus 6 (HHV-6) immediate-early (IE) 2 protein (IE2) may play important but incompletely defined roles during infection. We used yeast two-hybrid screening to detect proteins interacting with HHV-6 IE2, and found heterogeneous nuclear ribonucleoprotein K (hnRNP K) and the beta subunit of casein kinase 2 (CK2beta) specifically interacted with HHV-6 IE2. The interactions were confirmed by GST pull-down assay, coimmunoprecipitation, and colocalization studies. These findings indicate that the HHV-6 IE2 protein interacts with hnRNP K and CK2, and these interactions may affect viral and cellular RNA transcription and translation in viral replication.

Characterization of human herpesvirus 6 variant B immediate-early 1 protein modifications by small ubiquitin-related modifiers

The human herpesvirus 6 (HHV-6) immediate-early (IE) 1 protein undergoes SUMOylation events during the infectious process. In the present work, we report that Lys-802 (K-802) of IE1 from HHV-6 variant B is the only target residue capable of conjugation to SUMO-1/SMT3C/Sentrin-1, SUMO-2/SMT3A/Sentrin-3 or SUMO-3/SMT3B/Sentrin-2 as determined by transfection and in vitro SUMOylation experiments. PolySUMOylated forms of IE1 were also observed, suggesting that SUMO branching occurs at the K-802 residue. Overexpression of SUMO-1, -2 and -3 led to an overall increase in IE1 levels, irrespective of K-802. The SUMO residues could be efficiently removed by incubating SUMOylated IE1 with SENP1, a recently identified SUMO peptidase. SUMOylation-deficient mutants of IE1 co-localized with nuclear promyelocytic leukaemia protein (PML) oncogenic domains (PODs) as efficiently as WT IE1, indicating that POD targeting is independent of IE1 SUMOylation status. However, in contrast to infection, PODs did not aggregate in IE1B-transfected cells, suggesting that other viral proteins are involved in the process. Transactivation studies indicated that IE1, in combination with IE2, could efficiently transactivate diverse promoters, independent of its SUMOylation status. Overall, the results presented provide a detailed biochemical characterization of post-translational modifications of the HHV-6 IE1 protein by SUMO peptides, contributing to our understanding of the complex interactions between herpesviruses and the SUMO-conjugation pathway.

Generation and characterization of a monoclonal antibody specific for human herpesvirus 6 variant A immediate-early 2 protein

Two variants (A and B) of human herpesvirus 6 (HHV-6) can be isolated from humans, with each variant having unique biological properties. HHV-6 variant typing is mainly done following amplification of viral genomic DNA followed by restriction endonuclease digestion. Our objective was to generate a monoclonal antibody (mAb) that would allow us to discriminate between variants A and B of HHV-6. BALB/c mice were immunized with a recombinant glutathione-S-transferase protein fused to the immediate-early (IE) 2 protein from HHV-6 variant A. Following splenocytes fusion, one IgG1 kappa light chain mAb (P6H8) was isolated and found to react specifically with variant A IE2 protein in immunofluorescence and western blot assays. The P6H8 antibody represents a useful tool for both fundamental research and clinical applications allowing for the discrimination of infections caused by HHV-6 variants A or B.