Hematopoietic long-term culture (hLTC) for human cytomegalovirus latency and reactivation

HCMV UL8 interaction with β-catenin and DVL2 regulates viral reactivation in CD34+ hematopoietic progenitor cells

IMPORTANCE

CD34+ hematopoietic progenitor cells (HPCs) are an important cellular reservoir for latent human cytomegalovirus (HCMV). Several HCMV genes are expressed during latency that are involved with the maintenance of the viral genome in CD34+ HPC. However, little is known about the process of viral reactivation in these cells. Here, we describe a viral protein, pUL8, and its interaction and stabilization with members of the Wnt/β-catenin pathway as an important component of viral reactivation. We further define that pUL8 and β-catenin interact with DVL2 via a PDZ-binding domain, and loss of UL8 interaction with β-catenin-DVL2 restricts viral reactivation. Our findings will be instrumental in understanding the molecular processes involved in HCMV reactivation in order to design new antiviral therapeutics.

Proximity-dependent mapping of the HCMV US28 interactome identifies RhoGEF signaling as a requirement for efficient viral reactivation

Human cytomegalovirus (HCMV) encodes multiple putative G protein-coupled receptors (GPCRs). US28 functions as a viral chemokine receptor and is expressed during both latent and lytic phases of virus infection. US28 actively promotes cellular migration, transformation, and plays a major role in mediating viral latency and reactivation; however, knowledge about the interaction partners involved in these processes is still incomplete. Herein, we utilized a proximity-dependent biotinylating enzyme (TurboID) to characterize the US28 interactome when expressed in isolation, and during both latent (CD34+ hematopoietic progenitor cells) and lytic (fibroblasts) HCMV infection. Our analyses indicate that the US28 signalosome converges with RhoA and EGFR signal transduction pathways, sharing multiple mediators that are major actors in processes such as cellular proliferation and differentiation. Integral members of the US28 signaling complex were validated in functional assays by immunoblot and small-molecule inhibitors. Importantly, we identified RhoGEFs as key US28 signaling intermediaries. In vitro latency and reactivation assays utilizing primary CD34+ hematopoietic progenitor cells (HPCs) treated with the small-molecule inhibitors Rhosin or Y16 indicated that US28 -RhoGEF interactions are required for efficient viral reactivation. These findings were recapitulated in vivo using a humanized mouse model where inhibition of RhoGEFs resulted in a failure of the virus to reactivate. Together, our data identifies multiple new proteins in the US28 interactome that play major roles in viral latency and reactivation, highlights the utility of proximity-sensor labeling to characterize protein interactomes, and provides insight into targets for the development of novel anti-HCMV therapeutics.

Hematopoietic stem cells and betaherpesvirus latency

The human betaherpesviruses including human cytomegalovirus (HCMV), human herpesvirus (HHV)-6a and HHV-6b, and HHV-7 infect and establish latency in CD34+ hematopoietic stem and progenitor cells (HPCs). The diverse repertoire of HPCs in humans and the complex interactions between these viruses and host HPCs regulate the viral lifecycle, including latency. Precise manipulation of host and viral factors contribute to preferential maintenance of the viral genome, increased host cell survival, and specific manipulation of the cellular environment including suppression of neighboring cells and immune control. The dynamic control of these processes by the virus regulate inter- and intra-host signals critical to the establishment of chronic infection. Regulation occurs through direct viral protein interactions and cellular signaling, miRNA regulation, and viral mimics of cellular receptors and ligands, all leading to control of cell proliferation, survival, and differentiation. Hematopoietic stem cells have unique biological properties and the tandem control of virus and host make this a unique environment for chronic herpesvirus infection in the bone marrow. This review highlights the elegant complexities of the betaherpesvirus latency and HPC virus-host interactions.

Epigenetic reprogramming of host and viral genes by Human Cytomegalovirus infection in Kasumi-3 myeloid progenitor cells at early times post-infection

HCMV establishes latency in myeloid cells. Using the Kasumi-3 latency model, we previously showed that lytic gene expression is activated prior to establishment of latency in these cells. The early events in infection may have a critical role in shaping establishment of latency. Here, we have used an integrative multi-omics approach to investigate dynamic changes in host and HCMV gene expression and epigenomes at early times post infection. Our results show dynamic changes in viral gene expression and viral chromatin. Analyses of Pol II, H3K27Ac and H3K27me3 occupancy of the viral genome showed that 1) Pol II occupancy was highest at the MIEP at 4 hours post infection. However, it was observed throughout the genome; 2) At 24 hours, H3K27Ac was localized to the major immediate early promoter/enhancer and to a possible second enhancer in the origin of replication OriLyt; 3) viral chromatin was broadly accessible at 24 hpi. In addition, although HCMV infection activated expression of some host genes, we observed an overall loss of de novo transcription. This was associated with loss of promoter-proximal Pol II and H3K27Ac, but not with changes in chromatin accessibility or a switch in modification of H3K27.Importance.HCMV is an important human pathogen in immunocompromised hosts and developing fetuses. Current anti-viral therapies are limited by toxicity and emergence of resistant strains. Our studies highlight emerging concepts that challenge current paradigms of regulation of HCMV gene expression in myeloid cells. In addition, our studies show that HCMV has a profound effect on de novo transcription and the cellular epigenome. These results may have implications for mechanisms of viral pathogenesis.

Human Cytomegalovirus miR-US25-1 Targets the GTPase RhoA To Inhibit CD34+ Hematopoietic Progenitor Cell Proliferation To Maintain the Latent Viral Genome

Human cytomegalovirus (HCMV) microRNAs play essential roles in latency and reactivation in CD34+ hematopoietic progenitor cells (HPCs) via regulation of viral and cellular gene expression. In the present study, we show that HCMV miR-US25-1 targets RhoA, a small GTPase required for CD34+ HPC self-renewal, proliferation, and hematopoiesis. Expression of miR-US25-1 impairs signaling through the nonmuscle myosin II light chain, which leads to a block in cytokinesis and an inhibition of proliferation. Moreover, infection with an HCMV mutant lacking miR-US25-1 resulted in increased proliferation of CD34+ HPCs and a decrease in the proportion of genome-containing cells at the end of latency culture. These observations provide a mechanism by which HCMV limits proliferation to maintain latent viral genomes in CD34+ HPCs.IMPORTANCE Each herpesvirus family establishes latency in a unique cell type. Since herpesvirus genomes are maintained as episomes, the virus needs to devise mechanisms to retain the latent genome during cell division. Alphaherpesviruses overcome this obstacle by infecting nondividing neurons, while gammaherpesviruses tether their genome to the host chromosome in dividing B cells. The betaherpesvirus human cytomegalovirus (HCMV) establishes latency in CD34+ hematopoietic progenitor cells (HPCs), but the mechanism used to maintain the viral genome is unknown. In this report, we demonstrate that HCMV miR-US25-1 downregulates expression of RhoA, a key cell cycle regulator, which results in inhibition of CD34+ HPC proliferation by blocking mitosis. Mutation of miR-US25-1 during viral infection results in enhanced cellular proliferation and a decreased frequency of genome-containing CD34+ HPCs. These results reveal a novel mechanism through which HCMV is able to regulate cell division to prevent viral genome loss during proliferation.

Regulation of Latency and Reactivation by Human Cytomegalovirus miRNAs

Human cytomegalovirus (HCMV) encodes 22 mature microRNAs (miRNAs), which regulate a myriad of cellular processes, including vesicular trafficking, cell cycle progression, apoptosis, and immune evasion, as well as viral gene expression. Recent evidence points to a critical role for HCMV miRNAs in mediating latency in CD34⁺ hematopoietic progenitor cells through modulation of cellular signaling pathways, including attenuation of TGFβ and EGFR signaling. Moreover, HCMV miRNAs can act in concert with, or in opposition to, viral proteins in regulating host cell functions. Here, we comprehensively review the studies of HCMV miRNAs in the context of latency and highlight the novel processes that are manipulated by the virus using these small non-coding RNAs.

CMV-encoded GPCR pUL33 activates CREB and facilitates its recruitment to the MIE locus for efficient viral reactivation

Human cytomegalovirus (HCMV) establishes life-long latent infection in hematopoietic progenitor cells and circulating monocytes in infected individuals. Myeloid differentiation coupled with immune dysregulation leads to viral reactivation, which can cause severe disease and mortality. Reactivation of latent virus requires chromatin reorganization and the removal of transcriptional repressors in exchange for transcriptional activators. While some factors involved in these processes are identified, a complete characterization of the viral and cellular factors involved in their upstream regulation remains elusive. Herein, we show the HCMV-encoded G protein-coupled receptor (GPCR), UL33, is expressed during latency. Although this viral GPCR is not required to maintain latent infection, our data reveal UL33-mediated signaling is important for efficient viral reactivation. Additionally, UL33 signaling induces cellular cyclic AMP response element binding protein (CREB1, referred to here as CREB) phosphorylation, a transcription factor that promotes reactivation when recruited to the major immediate early (MIE) enhancer/promoter. Finally, targeted pharmacological inhibition of CREB activity reverses the reactivation phenotype of the UL33 signaling-deficient mutant. In sum, our data reveal UL33-mediated signaling functions to activate CREB, resulting in successful viral reactivation.

CD34+ Hematopoietic Progenitor Cell Subsets Exhibit Differential Ability To Maintain Human Cytomegalovirus Latency and Persistence

In human cytomegalovirus (HCMV)-seropositive patients, CD34⁺ hematopoietic progenitor cells (HPCs) provide an important source of latent virus that reactivates following cellular differentiation into tissue macrophages. Multiple groups have used primary CD34⁺ HPCs to investigate mechanisms of viral latency. However, analyses of mechanisms of HCMV latency have been hampered by the genetic variability of CD34⁺ HPCs from different donors, availability of cells, and low frequency of reactivation. In addition, multiple progenitor cell types express surface CD34, and the frequencies of these populations differ depending on the tissue source of the cells and culture conditions in vitro In this study, we generated CD34⁺ progenitor cells from two different embryonic stem cell (ESC) lines, WA01 and WA09, to circumvent limitations associated with primary CD34⁺ HPCs. HCMV infection of CD34⁺ HPCs derived from either WA01 or WA09 ESCs supported HCMV latency and induced myelosuppression similar to infection of primary CD34⁺ HPCs. Analysis of HCMV-infected primary or ESC-derived CD34⁺ HPC subpopulations indicated that HCMV was able to establish latency and reactivate in CD38⁺ CD90⁺ and CD38^+/low CD90^- HPCs but persistently infected CD38^- CD90⁺ cells to produce infectious virus. These results indicate that ESC-derived CD34⁺ HPCs can be used as a model for HCMV latency and that the virus either latently or persistently infects specific subpopulations of CD34⁺ cells.IMPORTANCE Human cytomegalovirus infection is associated with severe disease in transplant patients and understanding how latency and reactivation occur in stem cell populations is essential to understand disease. CD34⁺ hematopoietic progenitor cells (HPCs) are a critical viral reservoir; however, these cells are a heterogeneous pool with donor-to-donor variation in functional, genetic, and phenotypic characteristics. We generated a novel system using embryonic stem cell lines to model HCMV latency and reactivation in HPCs with a consistent cellular background. Our study defined three key stem cell subsets with differentially regulated latent and replicative states, which provide cellular candidates for isolation and treatment of transplant-mediated disease. This work provides a direction toward developing strategies to control the switch between latency and reactivation.

Human Cytomegalovirus UL7, miR-US5-1, and miR-UL112-3p Inactivation of FOXO3a Protects CD34+ Hematopoietic Progenitor Cells from Apoptosis

Human cytomegalovirus (HCMV) infection of myeloid lineage cells, such as CD34⁺ hematopoietic progenitor cells (HPCs) or monocytes, results in the upregulation of antiapoptotic cellular proteins that protect the newly infected cells from programmed cell death. The mechanisms used by HCMV to regulate proapoptotic cellular proteins upon infection of CD34⁺ HPCs have not been fully explored. Here, we show that HCMV utilizes pUL7, a secreted protein that signals through the FLT3 receptor, and miR-US5-1 and miR-UL112-3p to reduce the abundance and activity of the proapoptotic transcription factor FOXO3a at early times after infection of CD34⁺ HPCs. Regulation of FOXO3a by pUL7, miR-US5-1, and miR-UL112 results in reduced expression of the proapoptotic BCL2L11 transcript and protection of CD34⁺ HPCs from virus-induced apoptosis. These data highlight the importance of both viral proteins and microRNAs (miRNAs) in protecting CD34⁺ HPCs from apoptosis at early times postinfection, allowing for the establishment of latency and maintenance of viral genome-containing cells.IMPORTANCE Human cytomegalovirus (HCMV) causes serious disease in immunocompromised individuals and is a significant problem during transplantation. The virus can establish a latent infection in CD34⁺ hematopoietic progenitor cells (HPCs) and periodically reactivate to cause disease in the absence of an intact immune system. What viral gene products are required for successful establishment of latency is still not fully understood. Here, we show that both a viral protein and viral miRNAs are required to prevent apoptosis after infection of CD34⁺ HPCs. HCMV pUL7 and miRNAs miR-US5-1 and miR-UL112-3p act to limit the expression and activation of the transcription factor FOXO3a, which in turn reduces expression of proapoptotic gene BCL2L11 and prevents virus-induced apoptosis in CD34⁺ HPCs.

Activator protein-1 transactivation of the major immediate early locus is a determinant of cytomegalovirus reactivation from latency

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that latently infects hematopoietic cells and has the ability to reactivate when triggered by immunological stress. This reactivation causes significant morbidity and mortality in immune-deficient patients, who are unable to control viral dissemination. While a competent immune system helps prevent clinically detectable viremia, a portrait of the factors that induce reactivation following the proper cues remains incomplete. Our understanding of the complex molecular mechanisms underlying latency and reactivation continues to evolve. We previously showed the HCMV-encoded G protein-coupled receptor US28 is expressed during latency and facilitates latent infection by attenuating the activator protein-1 (AP-1) transcription factor subunit, c-fos, expression and activity. We now show AP-1 is a critical component for HCMV reactivation. Pharmacological inhibition of c-fos significantly attenuates viral reactivation. In agreement, infection with a virus in which we disrupted the proximal AP-1 binding site in the major immediate early (MIE) enhancer results in inefficient reactivation compared to WT. Concomitantly, AP-1 recruitment to the MIE enhancer is significantly decreased following reactivation of the mutant virus. Furthermore, AP-1 is critical for derepression of MIE-driven transcripts and downstream early and late genes, while immediate early genes from other loci remain unaffected. Our data also reveal MIE transcripts driven from the MIE promoter, the distal promoter, and the internal promoter, iP2, are dependent upon AP-1 recruitment, while iP1-driven transcripts are AP-1-independent. Collectively, our data demonstrate AP-1 binding to and activation of the MIE enhancer is a key molecular process controlling reactivation from latency.

FOXO transcription factors activate alternative major immediate early promoters to induce human cytomegalovirus reactivation

Human progenitor cells (HPCs) support human cytomegalovirus (HCMV) latency, and their differentiation along the myeloid lineage triggers cellular cues that drive reactivation. A key step during HCMV reactivation in latently infected HPCs is reexpression of viral major immediate early (MIE) genes. We recently determined that the major immediate early promoter (MIEP), which is primarily responsible for MIE gene expression during lytic replication, remains silent during reactivation. Instead, alternative promoters in the MIE locus are induced by reactivation stimuli. Here, we find that forkhead family (FOXO) transcription factors are critical for activation of alternative MIE promoters during HCMV reactivation, as mutating FOXO binding sites in alternative MIE promoters decreased HCMV IE gene expression upon reactivation and significantly decreased the production of infectious virus from latently infected primary CD34+ HPCs. These findings establish a mechanistic link by which infected cells sense environmental cues to regulate latency and reactivation, and emphasize the role of contextual activation of alternative MIE promoters as the primary drivers of reactivation.

The Role of the Human Cytomegalovirus UL133-UL138 Gene Locus in Latency and Reactivation

Human cytomegalovirus (HCMV) latency, the means by which the virus persists indefinitely in an infected individual, is a major frontier of current research efforts in the field. Towards developing a comprehensive understanding of HCMV latency and its reactivation from latency, viral determinants of latency and reactivation and their host interactions that govern the latent state and reactivation from latency have been identified. The polycistronic UL133-UL138 locus encodes determinants of both latency and reactivation. In this review, we survey the model systems used to investigate latency and new findings from these systems. Particular focus is given to the roles of the UL133, UL135, UL136 and UL138 proteins in regulating viral latency and how their known host interactions contribute to regulating host signaling pathways towards the establishment of or exit from latency. Understanding the mechanisms underlying viral latency and reactivation is important in developing strategies to block reactivation and prevent CMV disease in immunocompromised individuals, such as transplant patients.

The Requirement for US28 During Cytomegalovirus Latency Is Independent of US27 and US29 Gene Expression

The ability to establish a latent infection with periodic reactivation events ensures herpesviruses, like human cytomegalovirus (HCMV), lifelong infection, and serial passage. The host-pathogen relationship throughout HCMV latency is complex, though both cellular and viral factors influence the equilibrium between latent and lytic infection. We and others have shown one of the viral-encoded G protein-coupled receptors, US28, is required for HCMV latency. US28 potentiates signals both constitutively and in response to ligand binding, and we previously showed deletion of the ligand binding domain or mutation of the G protein-coupling domain results in the failure to maintain latency similar to deletion of the entire US28 open reading frame (ORF). Interestingly, a recent publication detailed an altered phenotype from that previously reported, showing US28 is required for viral reactivation rather than latency, suggesting the US28 ORF deletion impacts transcription of the surrounding genes. Here, we show an independently generated US28-stop mutant, like the US28 ORF deletion mutant, fails to maintain latency in hematopoietic cells. Further, we found US27 and US29 transcription in each of these mutants was comparable to their expression during wild type infection, suggesting neither US28 mutant alters mRNA levels of the surrounding genes. Finally, infection with a US28 ORF deletion virus expressed US27 protein comparable to its expression following wild type infection. In sum, our new data strongly support previous findings from our lab and others, detailing a requirement for US28 during HCMV latent infection.

Human Cytomegalovirus miRNAs Regulate TGF-β to Mediate Myelosuppression while Maintaining Viral Latency in CD34+ Hematopoietic Progenitor Cells

Infection with human cytomegalovirus (HCMV) remains a significant cause of morbidity and mortality following hematopoietic stem cell transplant (HSCT) because of various hematologic problems, including myelosuppression. Here, we demonstrate that latently expressed HCMV miR-US5-2 downregulates the transcriptional repressor NGFI-A binding protein (NAB1) to induce myelosuppression of uninfected CD34⁺ hematopoietic progenitor cells (HPCs) through an increase in TGF-β production. Infection of HPCs with an HCMVΔmiR-US5-2 mutant resulted in decreased TGF-β expression and restoration of myelopoiesis. In contrast, we show that infected HPCs are refractory to TGF-β signaling as another HCMV miRNA, miR-UL22A, downregulates SMAD3, which is required for maintenance of latency. Our data suggest that latently expressed viral miRNAs manipulate stem cell homeostasis by inducing secretion of TGF-β while protecting infected HPCs from TGF-β-mediated effects on viral latency and reactivation. These observations provide a mechanism through which HCMV induces global myelosuppression following HSCT while maintaining lifelong infection in myeloid lineage cells.

Host signaling and EGR1 transcriptional control of human cytomegalovirus replication and latency

Sustained phosphotinositide3-kinase (PI3K) signaling is critical to the maintenance of alpha and beta herpesvirus latency. We have previously shown that the beta-herpesvirus, human cytomegalovirus (CMV), regulates epidermal growth factor receptor (EGFR), upstream of PI3K, to control states of latency and reactivation. How signaling downstream of EGFR is regulated and how this impacts CMV infection and latency is not fully understood. We demonstrate that CMV downregulates EGFR early in the productive infection, which blunts the activation of EGFR and its downstream pathways in response to stimuli. However, CMV infection sustains basal levels of EGFR and downstream pathway activity in the context of latency in CD34+ hematopoietic progenitor cells (HPCs). Inhibition of MEK/ERK, STAT or PI3K/AKT pathways downstream of EGFR increases viral reactivation from latently infected CD34⁺ HPCs, defining a role for these pathways in latency. We hypothesized that CMV modulation of EGFR signaling might impact viral transcription important to latency. Indeed, EGF-stimulation increased expression of the UL138 latency gene, but not immediate early or early viral genes, suggesting that EGFR signaling promotes latent gene expression. The early growth response-1 (EGR1) transcription factor is induced downstream of EGFR signaling through the MEK/ERK pathway and is important for the maintenance of hematopoietic stemness. We demonstrate that EGR1 binds the viral genome upstream of UL138 and is sufficient to promote UL138 expression. Further, disruption of EGR1 binding upstream of UL138 prevents the establishment of latency in CD34⁺ HPCs. Our results indicate a model whereby UL138 modulation of EGFR signaling feeds back to promote UL138 gene expression and suppression of replication for latency. By this mechanism, the virus has hardwired itself into host cell biology to sense and respond to changes in homeostatic host cell signaling.

Host signaling is important for regulating states of cytomegalovirus (CMV) replication and latency. We have shown that human cytomegalovirus regulates EGFR levels and trafficking and that sustained EGFR or downstream PI3K signaling is a requirement for viral latency. Changes in host signaling have the ability to alter viral and host gene expression to impact the outcome of infection. Here we show that EGFR signaling through MEK/ERK pathway induces the host EGR1 transcription factor that is highly expressed in hematopoietic stem cells and necessary for the maintenance of hematopoietic stemness. Downregulation of EGR1 promotes stem cell mobilization and differentiation, known stimuli for CMV reactivation. We identified functional EGR1 binding sites upstream of the UL138 CMV latency gene and EGR1 stimulated UL138 expression to reinforce the latent infection. Mutant viruses where the regulation of UL138 by EGR1 is disrupted are unable to establish latency in CD34⁺ HPCs. This study advances our understanding of how host signaling impacts decisions to enter into or exit from latency. The regulation of viral gene expression by host signaling allows the virus to sense and respond to changes in host stress or differentiation.

HCMV miR-US22 down-regulation of EGR-1 regulates CD34+ hematopoietic progenitor cell proliferation and viral reactivation

Reactivation of latent Human Cytomegalovirus (HCMV) in CD34+ hematopoietic progenitor cells (HPCs) is closely linked to hematopoiesis. Viral latency requires maintenance of the progenitor cell quiescence, while reactivation initiates following mobilization of HPCs to the periphery and differentiation into CD14+ macrophages. Early growth response gene 1 (EGR-1) is a transcription factor activated by Epidermal growth factor receptor (EGFR) signaling that is essential for the maintenance of CD34+ HPC self-renewal in the bone marrow niche. Down-regulation of EGR-1 results in mobilization and differentiation of CD34+ HPC from the bone marrow to the periphery. In the current study we demonstrate that the transcription factor EGR-1 is directly targeted for down-regulation by HCMV miR-US22 that results in decreased proliferation of CD34+ HPCs and a decrease in total hematopoietic colony formation. We also show that an HCMV miR-US22 mutant fails to reactivate in CD34+ HPCs, indicating that expression of EGR-1 inhibits viral reactivation. Since EGR-1 promotes CD34+ HPC self-renewal in the bone marrow niche, HCMV miR-US22 down-regulation of EGR-1 is a necessary step to block HPC self-renewal and proliferation to induce a cellular differentiation pathway necessary to promote reactivation of virus.

Alternative promoters drive human cytomegalovirus reactivation from latency

Reactivation from latency requires reinitiation of viral gene expression and culminates in the production of infectious progeny. The major immediate early promoter (MIEP) of human cytomegalovirus (HCMV) drives the expression of crucial lytic cycle transactivators but is silenced during latency in hematopoietic progenitor cells (HPCs). Because the MIEP has poor activity in HPCs, it is unclear how viral transactivators are expressed during reactivation. It has been presumed that viral gene expression is reinitiated via de-repression of the MIEP. We demonstrate that immediate early transcripts arising from reactivation originate predominantly from alternative promoters within the canonical major immediate early locus. Disruption of these intronic promoters results in striking defects in re-expression of viral genes and viral genome replication in the THP-1 latency model. Furthermore, we show that these promoters are necessary for efficient reactivation in primary CD34+ HPCs. Our findings shift the paradigm for HCMV reactivation by demonstrating that promoter switching governs reactivation from viral latency in a context-specific manner.

Human Cytomegalovirus US28 Ligand Binding Activity Is Required for Latency in CD34+ Hematopoietic Progenitor Cells and Humanized NSG Mice

Human cytomegalovirus (HCMV) infection of CD34+ hematopoietic progenitor cells (CD34+ HPCs) provides a critical reservoir of virus in stem cell transplant patients, and viral reactivation remains a significant cause of morbidity and mortality. The HCMV chemokine receptor US28 is implicated in the regulation of viral latency and reactivation. To explore the role of US28 signaling in latency and reactivation, we analyzed protein tyrosine kinase signaling in CD34+ HPCs expressing US28. US28-ligand signaling in CD34+ HPCs induced changes in key regulators of cellular activation and differentiation. In vitro latency and reactivation assays utilizing CD34+ HPCs indicated that US28 was required for viral reactivation but not latency establishment or maintenance. Similarly, humanized NSG mice (huNSG) infected with TB40E-GFP-US28stop failed to reactivate upon treatment with granulocyte-colony-stimulating factor, but viral genome levels were maintained. Interestingly, HCMV-mediated changes in hematopoiesis during latency in vivo and in vitro was also dependent upon US28, as US28 directly promoted differentiation toward the myeloid lineage. To determine whether US28 constitutive activity and/or ligand-binding activity were required for latency and reactivation, we infected both huNSG mice and CD34+ HPCs in vitro with HCMV TB40E-GFP containing the US28-R129A mutation (no CA) or Y16F mutation (no ligand binding). TB40E-GFP-US28-R129A was maintained during latency and exhibited normal reactivation kinetics. In contrast, TB40E-GFP-US28-Y16F exhibited high levels of viral genome during latency and reactivation, indicating that the virus did not establish latency. These data indicate that US28 is necessary for viral reactivation and ligand binding activity is required for viral latency, highlighting the complex role of US28 during HCMV latency and reactivation.IMPORTANCE Human cytomegalovirus (HCMV) can establish latency following infection of CD34+ hematopoietic progenitor cells (HPCs), and reactivation from latency is a significant cause of viral disease and accelerated graft failure in bone marrow and solid-organ transplant patients. The precise molecular mechanisms of HCMV infection in HPCs are not well defined; however, select viral gene products are known to regulate aspects of latency and reactivation. The HCMV-encoded chemokine receptor US28, which binds multiple CC chemokines as well as CX3CR1, is expressed both during latent and lytic phases of the virus life cycle and plays a role in latency and reactivation. However, the specific timing of US28 expression and the role of ligand binding in these processes are not well defined. In this report, we determined that US28 is required for reactivation but not for maintaining latency. However, when present during latency, US28 ligand binding activity is critical to maintaining the virus in a quiescent state. We attribute the regulation of both latency and reactivation to the role of US28 in promoting myeloid lineage cell differentiation. These data highlight the dynamic and multifunctional nature of US28 during HCMV latency and reactivation.

Human cytomegalovirus G protein-coupled receptor US28 promotes latency by attenuating c-fos

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that undergoes latency in cells of the hematopoietic compartment, although the mechanisms underlying establishment and maintenance of latency remain elusive. We previously reported that the HCMV-encoded G protein-coupled receptor (GPCR) homolog US28 is required for successful latent infection. We now show that US28 protein (pUS28) provided in trans complements the US28Δ lytic phenotype in myeloid cells, suggesting that sustained US28 expression is necessary for long-term latency. Furthermore, expression of pUS28 at the time of infection represses transcription from the major immediate early promoter (MIEP) within 24 h. However, this repression is only maintained in the presence of continual pUS28 expression provided in trans Our data also reveal that pUS28-mediated signaling attenuates both expression and phosphorylation of cellular fos (c-fos), an AP-1 transcription factor subunit, to repress MIEP-driven transcription. AP-1 binds to the MIEP and promotes lytic replication, and in line with this we find that US28Δ infection results in an increase in AP-1 binding to the MIEP, compared with WT latent infection. Pharmacological inhibition of c-fos represses the MIEP during US28Δ infection to levels similar to those we observe during WT latent infection. Together, our data reveal that US28 is required for both establishment and long-term maintenance of HCMV latency, which is modulated, at least in part, by repressing functional AP-1 binding to the MIEP.

Human Cytomegalovirus UL135 Interacts with Host Adaptor Proteins To Regulate Epidermal Growth Factor Receptor and Reactivation from Latency

Human cytomegalovirus, HCMV, is a betaherpesvirus that establishes a lifelong latent infection in its host that is marked by recurrent episodes of reactivation. The molecular mechanisms by which the virus and host regulate entry into and exit from latency remain poorly understood. We have previously reported that UL135 is critical for reactivation, functioning in part by overcoming suppressive effects of the latency determinant UL138 We have demonstrated a role for UL135 in diminishing cell surface levels and targeting epidermal growth factor receptor (EGFR) for turnover. The attenuation of EGFR signaling promotes HCMV reactivation in combination with cellular differentiation. In this study, we sought to define the mechanisms by which UL135 functions in regulating EGFR turnover and viral reactivation. Screens to identify proteins interacting with pUL135 identified two host adaptor proteins, CIN85 and Abi-1, with overlapping activities in regulating EGFR levels in the cell. We mapped the amino acids in pUL135 necessary for interaction with Abi-1 and CIN85 and generated recombinant viruses expressing variants of pUL135 that do not interact with CIN85 or Abi-1. These recombinant viruses replicate in fibroblasts but are defective for reactivation in an experimental model for latency using primary CD34+ hematopoietic progenitor cells (HPCs). These UL135 variants have altered trafficking of EGFR and are defective in targeting EGFR for turnover. These studies demonstrate a requirement for pUL135 interactions with Abi-1 and CIN85 for regulation of EGFR and mechanistically link the regulation of EGFR to reactivation.IMPORTANCE Human cytomegalovirus (HCMV) establishes a lifelong latent infection in the human host. While the infection is typically asymptomatic in healthy individuals, HCMV infection poses life-threatening disease risk in immunocompromised individuals and is the leading cause of birth defects. Understanding how HCMV controls the lifelong latent infection and reactivation of replication from latency is critical to developing strategies to control HCMV disease. Here, we identify the host factors targeted by a viral protein that is required for reactivation. We define the importance of this virus-host interaction in reactivation from latency, providing new insights into the molecular underpinnings of HCMV latency and reactivation.

Selective 4-Thiouracil Labeling of RNA Transcripts within Latently Infected Cells after Infection with Human Cytomegalovirus Expressing Functional Uracil Phosphoribosyltransferase

Infections with human cytomegalovirus (HCMV) are highly prevalent in the general population as the virus has evolved the capacity to undergo distinct replication strategies resulting in lytic, persistent, and latent infections. During the latent life cycle, HCMV resides in subsets of cells within the hematopoietic cell compartment, including hematopoietic progenitor cells (HPCs) and peripheral blood monocytes. Since only a small fraction of these cell types harbor viral genomes during natural latency, identification and analysis of distinct changes mediated by viral infection are difficult to assess. In order to characterize latent infections of HPCs, we used an approach that involves complementation of deficiencies within the human pyrimidine salvage pathway, thus allowing for conversion of labeled uracil into rUTP. Here, we report the development of a recombinant HCMV that complements the defective human pyrimidine salvage pathway, allowing incorporation of thiol containing UTP into all RNA species that are synthesized within an infected cell. This virus grows to wild-type kinetics and can establish a latent infection within two distinct culture models of HCMV latency. Using this recombinant HCMV, we report the specific labeling of transcripts only within infected cells. These transcripts reveal a transcriptional landscape during HCMV latency that is distinct from uninfected cells. The utility of this labeling system allows for the identification of distinct changes within host transcripts and will shed light on characterizing how HCMV establishes and maintains latency.IMPORTANCE HCMV is a significant pathogen that accounts for a substantial amount of complications within the immunosuppressed and immunocompromised. Of particular significance is the capacity of HCMV to reactivate within solid tissue and bone marrow transplant recipients. While it is known that HCMV latency resides within a fraction of HPCs and monocytes, the exact subset of cells that harbor latent viral genomes during natural infections remain uncharacterized. The capacity to identify changes within the host transcriptome during latent infections is critical for developing approaches that therapeutically or physically eliminate latent viral genome containing cells and will represent a major breakthrough for reducing complications due to HCMV reactivation posttransplant. In this report, we describe the generation and use of a recombinant HCMV that allows specific and distinct labeling of RNA species that are produced within virally infected cells. This is a critical first step in identifying how HCMV affects the host cell during latency and more importantly, allows one to characterize cells that harbor latent HCMV.

Tumor Necrosis Factor Alpha Induces Reactivation of Human Cytomegalovirus Independently of Myeloid Cell Differentiation following Posttranscriptional Establishment of Latency

HCMV is an important human pathogen that establishes lifelong latent infection in myeloid progenitor cells and reactivates frequently to cause significant disease in immunocompromised people. Our observation that viral gene expression is first turned on and then turned off to establish latency suggests that there is a host defense, which may be myeloid cell specific, responsible for transcriptional silencing of viral gene expression. Our observation that TNF-α induces reactivation independently of differentiation provides insight into molecular mechanisms that control reactivation.

We used the Kasumi-3 model to study human cytomegalovirus (HCMV) latency and reactivation in myeloid progenitor cells. Kasumi-3 cells were infected with HCMV strain TB40/Ewt-GFP, flow sorted for green fluorescent protein-positive (GFP⁺) cells, and cultured for various times to monitor establishment of latency, as judged by repression of viral gene expression (RNA/DNA ratio) and loss of virus production. We found that, in the vast majority of cells, latency was established posttranscriptionally in the GFP⁺ infected cells: transcription was initially turned on and then turned off. We also found that some of the GFP⁻ cells were infected, suggesting that latency might be established in these cells at the outset of infection. We were not able to test this hypothesis because some GFP⁻ cells expressed lytic genes and thus it was not possible to separate them from GFP⁻ quiescent cells. In addition, we found that the pattern of expression of lytic genes that have been associated with latency, including UL138, US28, and RNA2.7, was the same as that of other lytic genes, indicating that there was no preferential expression of these genes once latency was established. We confirmed previous studies showing that tumor necrosis factor alpha (TNF-α) induced reactivation of infectious virus, and by analyzing expression of the progenitor cell marker CD34 as well as myeloid cell differentiation markers in IE⁺ cells after treatment with TNF-α, we showed that TNF-α induced transcriptional reactivation of IE gene expression independently of differentiation. TNF-α-mediated reactivation in Kasumi-3 cells was correlated with activation of NF-κB, KAP-1, and ATM.

Molecular Determinants and the Regulation of Human Cytomegalovirus Latency and Reactivation

Human cytomegalovirus (HCMV) is a beta herpesvirus that establishes a life-long persistence in the host, like all herpesviruses, by way of a latent infection. During latency, viral genomes are maintained in a quieted state. Virus replication can be reactivated from latency in response to changes in cellular signaling caused by stress or differentiation. The past decade has brought great insights into the molecular basis of HCMV latency. Here, we review the complex persistence of HCMV with consideration of latent reservoirs, viral determinants and their host interactions, and host signaling and the control of cellular and viral gene expression that contributes to the establishment of and reactivation from latency.

Human Cytomegalovirus Encodes a Novel FLT3 Receptor Ligand Necessary for Hematopoietic Cell Differentiation and Viral Reactivation

The ability of human cytomegalovirus (HCMV) to reactivate from latent infection of hematopoietic progenitor cells (HPCs) is intimately linked to cellular differentiation. HCMV encodes UL7 that our group has shown is secreted from infected cells and induces angiogenesis. In this study, we show that UL7 is a ligand for Fms-like tyrosine kinase 3 receptor (Flt-3R), a well-known critical factor in HPC differentiation. We observed that UL7 directly binds Flt-3R and induces downstream signaling cascades, including phosphatidylinositol 3-kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathways. Importantly, we show that UL7 protein induces differentiation of both CD34⁺ HPCs and CD14⁺ monocytes. Last, we show that an HCMV mutant lacking UL7 fails to reactivate in CD34⁺ HPCs in vitro as well as in humanized mice. These observations define the first virally encoded differentiation factor with significant implications not only for HCMV reactivation but also for alteration of the hematopoietic compartment in transplant patients.IMPORTANCE Human cytomegalovirus (HCMV) remains a significant cause of morbidity and mortality in allogeneic hematopoietic stem cell transplant recipients. CD34⁺ hematopoietic progenitor cells (HPCs) represent a critical reservoir of latent HCMV in the transplant population, thereby providing a source of virus for dissemination to visceral organs. HCMV reactivation has been linked to HPC/myeloid cellular differentiation; however, the mechanisms involved in these events are poorly understood at the molecular level. In this study, we show that a viral protein is a ligand for Fms-like tyrosine kinase 3 receptor (Flt-3R) and that the binding of HCMV UL7 to the Flt-3R triggers HPC and monocyte differentiation. Moreover, the loss of UL7 prevents viral reactivation in HPCs in vitro as well as in humanized mice. These observations define the first virally encoded differentiation factor with significant implications not only for HCMV reactivation but also for alteration of the hematopoietic compartment in transplant patients.

Human Cytomegalovirus Induces Cellular and Humoral Virus-specific Immune Responses in Humanized BLT Mice

The strict species specificity of Human Cytomegalovirus (HCMV) has impeded our understanding of antiviral adaptive immune responses in the context of a human immune system. We have previously shown that HCMV infection of human hematopoietic progenitor cells engrafted in immune deficient mice (huNSG) results in viral latency that can be reactivated following G-CSF treatment. In this study, we characterized the functional human adaptive immune responses in HCMV latently-infected huBLT (humanized Bone marrow-Liver-Thymus) mice. Following infection, huBLT mice generate human effector and central memory CD4+ and CD8+ T-cell responses reactive to peptides corresponding to both IE and pp65 proteins. Additionally, both HCMV specific IgM and IgG B-cell responses with the ability to neutralize virus were detected. These results indicate that the HCMV huBLT mouse model may provide a valuable tool to study viral latency and reactivation as well as evaluate HCMV vaccines and immune responses in the context of a functional human immune system.

Human Cytomegalovirus miR-UL148D Facilitates Latent Viral Infection by Targeting Host Cell Immediate Early Response Gene 5

The mechanisms underlying human cytomegalovirus (HCMV) latency remain incompletely understood. Here, we showed that a HCMV-encoded miRNA, miR-UL148D, robustly accumulates during late stages of experimental latent HCMV infection in host cells and promotes HCMV latency by modulating the immediate early response gene 5 (IER5)-cell division cycle 25B (CDC25B) axis in host cells. miR-UL148D inhibited IER5 expression by directly targeting the three-prime untranslated region(3'UTR) of IER5 mRNA and thus rescued CDC25B expression during the establishment of viral latency. Infection with NR-1ΔmiR-UL148D, a derivative of the HCMV clinical strain NR-1 with a miR-UL148D knockout mutation, resulted in sustained induction of IER5 expression but decreased CDC25B expression in host cells. Mechanistically, we further showed that CDC25B plays an important role in suppressing HCMV IE1 and lytic gene transcription by activating cyclin-dependent kinase 1 (CDK-1). Both gain-of-function and lose-of-function assays demonstrated that miR-UL148D promotes HCMV latency by helping maintain CDC25B activity in host cells. These results provide a novel mechanism through which a HCMV miRNA regulates viral latency.

Human Cytomegalovirus Latency: Approaching the Gordian Knot

Herpesviruses have evolved exquisite virus-host interactions that co-opt or evade a number of host pathways to enable the viruses to persist. Persistence of human cytomegalovirus (CMV), the prototypical betaherpesvirus, is particularly complex in the host organism. Depending on host physiology and the cell types infected, CMV persistence comprises latent, chronic, and productive states that may occur concurrently. Viral latency is a central strategy by which herpesviruses ensure their lifelong persistence. Although much remains to be defined about the virus-host interactions important to CMV latency, it is clear that checkpoints composed of viral and cellular factors exist to either maintain a latent state or initiate productive replication in response to host cues. CMV offers a rich platform for defining the virus-host interactions and understanding the host biology important to viral latency. This review describes current understanding of the virus-host interactions that contribute to viral latency and reactivation.

Long and Short Isoforms of the Human Cytomegalovirus UL138 Protein Silence IE Transcription and Promote Latency

The UL133-138 locus present in clinical strains of human cytomegalovirus (HCMV) encodes proteins required for latency and reactivation in CD34(+) hematopoietic progenitor cells and virion maturation in endothelial cells. The encoded proteins form multiple homo- and hetero-interactions and localize within secretory membranes. One of these genes, UL136 gene, is expressed as at least five different protein isoforms with overlapping and unique functions. Here we show that another gene from this locus, the UL138 gene, also generates more than one protein isoform. A long form of UL138 (pUL138-L) initiates translation from codon 1, possesses an amino-terminal signal sequence, and is a type one integral membrane protein. Here we identify a short protein isoform (pUL138-S) initiating from codon 16 that displays a subcellular localization similar to that of pUL138-L. Reporter, short-term transcription, and long-term virus production assays revealed that both pUL138-L and pUL138-S are able to suppress major immediate early (IE) gene transcription and the generation of infectious virions in cells in which HCMV latency is studied. The long form appears to be more potent at silencing IE transcription shortly after infection, while the short form seems more potent at restricting progeny virion production at later times, indicating that both isoforms of UL138 likely cooperate to promote HCMV latency.

IMPORTANCE

Latency allows herpesviruses to persist for the lives of their hosts in the face of effective immune control measures for productively infected cells. Controlling latent reservoirs is an attractive antiviral approach complicated by knowledge deficits for how latently infected cells are established, maintained, and reactivated. This is especially true for betaherpesviruses. The functional consequences of HCMV UL138 protein expression during latency include repression of viral IE1 transcription and suppression of virus replication. Here we show that short and long isoforms of UL138 exist and can themselves support latency but may do so in temporally distinct manners. Understanding the complexity of gene expression and its impact on latency is important for considering potential antivirals targeting latent reservoirs.

Inhibition of the FACT Complex Reduces Transcription from the Human Cytomegalovirus Major Immediate Early Promoter in Models of Lytic and Latent Replication

The successful colonization of the majority of the population by human cytomegalovirus is a direct result of the virus's ability to establish and, more specifically, reactivate from latency. The underlying cellular factors involved in viral reactivation remain unknown. Here, we show that the host complexfacilitateschromatintranscription (FACT) binds to the major immediate early promoter (MIEP) and that inhibition of this complex reduces MIEP transactivation, thus inhibiting viral reactivation.

Complex Interplay of the UL136 Isoforms Balances Cytomegalovirus Replication and Latency

Human cytomegalovirus (HCMV), a betaherpesvirus, persists indefinitely in the human host through poorly understood mechanisms. The UL136 gene is carried within a genetic locus important to HCMV latency termed the UL133/8 locus, which also carries UL133, UL135, and UL138. Previously, we demonstrated that UL136 is expressed as five protein isoforms ranging from 33-kDa to 19-kDa, arising from alternative transcription and, likely, translation initiation mechanisms. We previously showed that the UL136 isoforms are largely dispensable for virus infection in fibroblasts, a model for productive virus replication. In our current work, UL136 has emerged as a complex regulator of HCMV infection in multiple contexts of infection relevant to HCMV persistence: in an endothelial cell (EC) model of chronic infection, in a CD34⁺ hematopoietic progenitor cell (HPC) model of latency, and in an in vivo NOD-scid IL2Rγ_c^null humanized (huNSG) mouse model for latency. The 33- and 26-kDa isoforms promote replication, while the 23- and 19-kDa isoforms suppress replication in ECs, in CD34⁺ HPCs, and in huNSG mice. The role of the 25-kDa isoform is context dependent and influences the activity of the other isoforms. These isoforms localize throughout the secretory pathway, and loss of the 33- and 26-kDa UL136 isoforms results in virus maturation defects in ECs. This work reveals an intriguing functional interplay between protein isoforms that impacts virus replication, latency, and dissemination, contributing to the overall role of the UL133/8 locus in HCMV infection.

The persistence of DNA viruses, and particularly of herpesviruses, remains an enigma because we have not completely defined the viral and host factors important to persistence. Human cytomegalovirus, a herpesvirus, persists in the absence of disease in immunocompetent individuals but poses a serious disease threat to transplant patients and the developing fetus. There is no vaccine, and current therapies do not target latent reservoirs. In an effort to define the viral factors important to persistence, we have studied viral genes with no known viral replication function in contexts important to HCMV persistence. Using models relevant to viral persistence, we demonstrate opposing roles of protein isoforms encoded by the UL136 gene in regulating latent and replicative states of infection. Our findings reveal an intriguing interplay between UL136 protein isoforms and define UL136 as an important regulator of HCMV persistence.

Human Cytomegalovirus US28 Is Important for Latent Infection of Hematopoietic Progenitor Cells

Human cytomegalovirus (HCMV) resides latently in hematopoietic progenitor cells (HPCs). During latency, only a subset of HCMV genes is transcribed, including one of the four virus-encoded G protein-coupled receptors (GPCRs), US28. Although US28 is a multifunctional lytic protein, its function during latency has remained undefined. We generated a panel of US28 recombinant viruses in the bacterial artificial chromosome (BAC)-derived clinical HCMV strain TB40/E-mCherry. We deleted the entire US28 open reading frame (ORF), deleted all four of the viral GPCR ORFs, or deleted three of the HCMV GPCRs but not the US28 wild-type protein. Using these recombinant viruses, we assessed the requirement for US28 during latency in the Kasumi-3 in vitro latency model system and in primary ex vivo-cultured CD34(+) HPCs. Our data suggest that US28 is required for latency as infection with viruses lacking the US28 ORF alone or in combination with the remaining HCMV-encoded GPCR results in transcription from the major immediate early promoter, the production of extracellular virions, and the production of infectious virus capable of infecting naive fibroblasts. The other HCMV GPCRs are not required for this phenotype as a virus expressing only US28 but not the remaining virus-encoded GPCRs is phenotypically similar to that of wild-type latent infection. Finally, we found that US28 copurifies with mature virions and is expressed in HPCs upon virus entry although its expression at the time of infection does not complement the US28 deletion latency phenotype. This work suggests that US28 protein functions to promote a latent state within hematopoietic progenitor cells.

IMPORTANCE

Human cytomegalovirus (HCMV) is a widespread pathogen that, once acquired, remains with its host for life. HCMV remains latent, or quiescent, in cells of the hematopoietic compartment and upon immune challenge can reactivate to cause disease. HCMV-encoded US28 is one of several genes expressed during latency although its biological function during this phase of infection has remained undefined. Here, we show that US28 aids in promoting experimental latency in tissue culture.

Epigenetic regulation of human cytomegalovirus latency: an update

Human cytomegalovirus (HCMV) is a ubiquitous virus which infects 50-90% of the population worldwide. In immunocompetent hosts, HCMV either remains unnoticed or causes mild symptoms. Upon primary infection it establishes latent infection in a few cells. However, in certain situations where immunity is either immature or compromised, HCMV may reactivate and cause mortality and morbidity. Therefore, it is utmost important to understand how HCMV establishes latent infection and associated mechanisms responsible for its reactivation. Several mechanisms are involved in the regulation of latency including chromatin remodeling by an array of enzymes and microRNAs. Here we will describe the epigenetic regulation of HCMV latency. Further we will discuss the unique HCMV latency signature and patho-physiological relevance of latent HCMV infection.

Complex expression of the UL136 gene of human cytomegalovirus results in multiple protein isoforms with unique roles in replication

Human cytomegalovirus (HCMV) is a complex DNA virus with a 230-kb genome encoding 170 and up to 750 proteins. The upper limit of this coding capacity suggests the evolution of complex mechanisms to substantially increase the coding potential from the 230-kb genome. Our work examines the complexity of one gene, UL136, encoded within the ULb' region of the genome that is lost during serial passage of HCMV in cultured fibroblasts. UL136 is expressed as five protein isoforms. We mapped these isoforms and demonstrate that they originate from both a complex transcriptional profile and, possibly, the usage of multiple translation initiation sites. Intriguingly, the pUL136 isoforms exhibited distinct subcellular distributions with varying association with the Golgi apparatus. The subcellular localization of membrane-bound isoforms of UL136 differed between when they were expressed exogenously and when they were expressed in the context of viral infection, suggesting that the trafficking of these isoforms is mediated by infection-specific factors. While UL136, like most ULb' genes, was dispensable for replication in fibroblasts, the soluble 23- and 19-kDa isoforms suppressed virus replication. In CD34(+) hematopoietic progenitor cells (HPCs) infected in vitro, disruption of the 23- and 19-kDa isoforms resulted in increased replication and a loss of the latency phenotype, similar to the effects of the UL138 latency determinant encoded within the same genetic locus. Our work suggests a complex interplay between the UL136 isoforms which balances viral replication in multiple cell types and likely contributes to the cell type-dependent phenotypes of the UL133/8 locus and the outcome of HCMV infection.

IMPORTANCE

HCMV is a significant cause of morbidity in immunocompromised individuals, including transplant patients. The lifelong persistence of the virus results in a high seroprevalence worldwide and may contribute to age-related pathologies, such as atherosclerosis. The mechanisms of viral persistence are poorly understood; however, understanding the molecular basis of persistence is imperative for the development of new treatments. In this work, we characterize a complex HCMV gene, UL136, which is expressed as five protein isoforms. These isoforms arise predominantly from complex transcriptional mechanisms, which contribute to an increased coding capacity of the virus. Further, the UL136 isoforms oppose the activity of one another to balance HCMV replication in multiple cell types. We identify soluble isoforms of UL136 that function to suppress virus replication in fibroblasts and in CD34(+) HPCs for latency.