Roles of phosphatidylinositol 3-kinase and NF-kappaB in human cytomegalovirus-mediated monocyte diapedesis and adhesion: strategy for viral persistence

Virion-associated US28 rapidly modulates Akt activity to suppress HCMV lytic replication in monocytes

Establishing a non-productive quiescent/silent infection within monocytes is essential for spread of human cytomegalovirus (HCMV). Yet, how HCMV establishes a quiescent infection in monocytes remains unclear. US28 is a viral G protein-coupled receptor (GPCR) essential for silent infections within cells of the myeloid lineage. We found virion-associated US28 was rapidly delivered to monocytes, while de novo synthesized US28 was delayed for several days. A recombinant mutant virus lacking US28 (US28Δ) was unable to establish a quiescent infection, resulting in a fully productive lytic replication cycle. Mechanistically, viral entry of US28Δ phosphorylated Akt at both serine 473 (S473) and threonine 308 (T308), which contrasted with the site-specific phosphorylation of Akt at S473 following WT infection. Preventing Akt bi-phosphorylation prevented lytic replication of US28Δ, and ectopic expression of a constitutively phosphorylated Akt variant triggered lytic replication of WT infection. Our data demonstrate that virion-delivered US28 fine-tunes Akt activity to permit HCMV infection to enter a quiescent state following primary infection of monocytes.

Inhibition of SIRT2 promotes death of human cytomegalovirus-infected peripheral blood monocytes via apoptosis and necroptosis

Peripheral blood monocytes are the cells predominantly responsible for systemic dissemination of human cytomegalovirus (HCMV) and a significant cause of morbidity and mortality in immunocompromised patients. HCMV establishes a silent/quiescent infection in monocytes, which is defined by the lack of viral replication and lytic gene expression. The absence of replication shields the virus within infected monocytes from the current available antiviral drugs that are designed to suppress active replication. Our previous work has shown that HCMV stimulates a noncanonical phosphorylation of Akt and the subsequent upregulation of a distinct subset of prosurvival proteins in normally short-lived monocytes. In this study, we found that SIRT2 activity is required for the unique activation profile of Akt induced within HCMV-infected monocytes. Importantly, both therapeutic and prophylactic treatment with a novel SIRT2 inhibitor, FLS-379, promoted death of infected monocytes via both the apoptotic and necroptotic cell death pathways. Mechanistically, SIRT2 inhibition reduced expression of Mcl-1, an Akt-dependent antiapoptotic Bcl-2 family member, and enhanced activation of MLKL, the executioner kinase of necroptosis. We have previously reported HCMV to block necroptosis by stimulating cellular autophagy. Here, we additionally demonstrate that inhibition of SIRT2 suppressed Akt-dependent HCMV-induced autophagy leading to necroptosis of infected monocytes. Overall, our data show that SIRT2 inhibition can simultaneously promote death of quiescently infected monocytes by two distinct death pathways, apoptosis and necroptosis, which may be vital for limiting viral dissemination to peripheral organs in immunosuppressed patients.

Overview of how HCMV manipulation of host cell intracellular trafficking networks can promote productive infection

Human cytomegalovirus (HCMV) is a significant cause of morbidity and mortality in the immunocompromised and developing fetuses. Infection has also been linked to chronic inflammatory diseases, cardiovascular disease, and the development of certain cancers. The wide range of pathologies associated with HCMV infection is attributable to the broad cellular tropism of the virus where infection affects every organ system. Like other viruses, HCMV must tailor host cells to support productive infection. In particular, HCMV dedicates many resources and various strategies to manipulate host intracellular trafficking networks to facilitate various aspects of infection across all infected cell types. The dysregulation of host intracellular trafficking networks allows the virus to translocate to the host cell nucleus for genome replication, facilitate nuclear import/export of viral proteins and immature virions, subvert the host immune response, form new organelles for progeny virion assembly, maturation and egress, and promote cellular migration and viral spread. However, due to their complex nature, many aspects of these processes are not well-studied. New research and omics-based technologies have recently begun to elucidate the extent to which HCMV dysregulates host cell trafficking machinery. Here we review the variety of strategies HCMV utilizes to dysregulate intracellular trafficking networks to promote productive infection.

The complex biology of human cytomegalovirus latency

While many viral infections are limited and eventually resolved by the host immune response or by death of the host, other viruses establish long-term relationships with the host by way of a persistent infection, that range from chronic viruses that may be eventually cleared to those that establish life-long persistent or latent infection. Viruses infecting hosts from bacteria to humans establish quiescent infections that must be reactivated to produce progeny. For mammalian viruses, most notably herpesviruses, this quiescent maintenance of viral genomes in the absence of virus replication is referred to as latency. The latent strategy allows the virus to persist quiescently within a single host until conditions indicate a need to reactivate to reach a new host or, to re-seed a reservoir within the host. Here, I review common themes in viral strategies to regulate the latent cycle and reactivate from it ranging from bacteriophage to herpesviruses with a focus on human cytomegalovirus (HCMV). Themes central to herpesvirus latency include, epigenetic repression of viral gene expression and mechanisms to regulate host signaling and survival. Critical to the success of a latent program are mechanisms by which the virus can "sense" fluctuations in host biology (within the host) or environment (outside the host) and make appropriate "decisions" to maintain latency or re-initiate the replicative program. The signals or environments that indicate the establishment of a latent state, the very nature of the latent state, as well as the signals driving reactivation have been topics of intense study from bacteriophage to human viruses, as these questions encompass the height of complexity in virus-host interactions-where the host and the virus coexist.

Human Cytomegalovirus Host Interactions: EGFR and Host Cell Signaling Is a Point of Convergence Between Viral Infection and Functional Changes in Infected Cells

Viruses have evolved diverse strategies to manipulate cellular signaling pathways in order to promote infection and/or persistence. Human cytomegalovirus (HCMV) possesses a number of unique properties that allow the virus to alter cellular events required for infection of a diverse array of host cell types and long-term persistence. Of specific importance is infection of bone marrow derived and myeloid lineage cells, such as peripheral blood monocytes and CD34⁺ hematopoietic progenitor cells (HPCs) because of their essential role in dissemination of the virus and for the establishment of latency. Viral induced signaling through the Epidermal Growth Factor Receptor (EGFR) and other receptors such as integrins are key control points for viral-induced cellular changes and productive and latent infection in host organ systems. This review will explore the current understanding of HCMV strategies utilized to hijack cellular signaling pathways, such as EGFR, to promote the wide-spread dissemination and the classic life-long herpesvirus persistence.

Alterations of monocyte NF-κB p65/RelA signaling in a cohort of older medical patients, age-matched controls, and healthy young adults

BACKGROUND

Altered monocyte NF-κB signaling is a possible cause of inflammaging and driver of aging, however, evidence from human aging studies is sparse. We assessed monocyte NF-κB signaling across different aging trajectories by comparing healthy older adults to older adults with a recent emergency department (ED) admission and to young adults.

METHODS

We used data from: 52 older (≥65 years) Patients collected upon ED admission and at follow-up 30-days after discharge; 52 age- and sex-matched Older Controls without recent hospitalization; and 60 healthy Young Controls (20-35 years). Using flow cytometry, we assessed basal NF-κB phosphorylation (pNF-κB p65/RelA; Ser529) and induction of pNF-κB following stimulation with LPS or TNF-α in monocytes. We assessed frailty (FI-OutRef), physical and cognitive function, and plasma levels of IL-6, IL-18, TNF-α, and soluble urokinase plasminogen activator receptor.

RESULTS

Patients at follow-up were frailer, had higher levels of inflammatory markers and decreased physical and cognitive function than Older Controls. Patients at follow-up had higher basal pNF-κB levels than Older Controls (median fluorescence intensity (MFI): 125, IQR: 105-153 vs. MFI: 80, IQR: 71-90, p < 0.0001), and reduced pNF-κB induction in response to LPS (mean pNF-κB MFI fold change calculated as the log10 ratio of LPS-stimulation to the PBS-control: 0.10, 95% CI: 0.08 to 0.12 vs. 0.13, 95% CI: 0.10 to 0.15, p = 0.05) and TNF-α stimulation (0.02, 95% CI: - 0.00 to 0.05 vs. 0.10, 95% CI: 0.08 to 0.12, p < 0.0001). Older Controls had higher levels of inflammatory markers than Young Controls, but basal pNF-κB MFI did not differ between Older and Young Controls (MFI: 81, IQR: 70-86; p = 0.72). Older Controls had reduced pNF-κB induction in response to LPS and TNF-α compared to Young Controls (LPS: 0.40, 95% CI: 0.35 to 0.44, p < 0.0001; and TNF-α: 0.33, 95% CI: 0.27 to 0.40, p < 0.0001). In Older Controls, basal pNF-κB MFI was associated with FI-OutRef (p = 0.02).

CONCLUSIONS

Increased basal pNF-κB activity in monocytes could be involved in the processes of frailty and accelerated aging. Furthermore, we show that monocyte NF-κB activation upon stimulation was impaired in frail older adults, which could result in reduced immune responses and vaccine effectiveness.

Human Cytomegalovirus Mediates Unique Monocyte-to-Macrophage Differentiation through the PI3K/SHIP1/Akt Signaling Network

Blood monocytes mediate the hematogenous dissemination of human cytomegalovirus (HCMV) in the host. However, monocytes have a short 48-hour (h) lifespan and are not permissive for viral replication. We previously established that HCMV infection drives differentiation of monocytes into long-lived macrophages to mediate viral dissemination, though the mechanism was unclear. Here, we found that HCMV infection promoted monocyte polarization into distinct macrophages by inducing select M1 and M2 differentiation markers and that Akt played a central role in driving differentiation. Akt's upstream positive regulators, PI3K and SHIP1, facilitated the expression of the M1/M2 differentiation markers with p110δ being the predominant PI3K isoform inducing differentiation. Downstream of Akt, M1/M2 differentiation was mediated by caspase 3, whose activity was tightly regulated by Akt in a temporal manner. Overall, this study highlights that HCMV employs the PI3K/SHIP1/Akt pathway to regulate caspase 3 activity and drive monocyte differentiation into unique macrophages, which is critical for viral dissemination.

The role of human cytomegalovirus in atherosclerosis: a systematic review

Atherosclerosis is a progressive vascular disease with increasing morbidity and mortality year by year in modern society. Human cytomegalovirus (HCMV) infection is closely associated with the development of atherosclerosis. HCMV infection may accelerate graft atherosclerosis and the development of transplant vasculopathy in organ transplantation. However, our current understanding of HCMV-associated atherosclerosis remains limited and is mainly based on clinical observations. The underlying mechanism of the involvement of HCMV infection in atherogenesis remains unclear. Here, we summarized current knowledge regarding the multiple influences of HCMV on a diverse range of infected cells, including vascular endothelial cells, vascular smooth muscle cells, monocytes, macrophages, and T cells. In addition, we described potential HCMV-induced molecular mechanisms, such as oxidative stress, endoplasmic reticulum stress, autophagy, lipid metabolism, and miRNA regulation, which are involved in the development of HCMV-associated atherogenesis. Gaining an improved understanding of these mechanisms will facilitate the development of novel and effective therapeutic strategies for the treatment of HCMV-related cardiovascular disease.

Single cell analysis reveals human cytomegalovirus drives latently infected cells towards an anergic-like monocyte state

Human cytomegalovirus (HCMV) causes a lifelong infection through establishment of latency. Although reactivation from latency can cause life-threatening disease, our molecular understanding of HCMV latency is incomplete. Here we use single cell RNA-seq analysis to characterize latency in monocytes and hematopoietic stem and progenitor cells (HSPCs). In monocytes, we identify host cell surface markers that enable enrichment of latent cells harboring higher viral transcript levels, which can reactivate more efficiently, and are characterized by reduced intrinsic immune response that is important for viral gene expression. Significantly, in latent HSPCs, viral transcripts could be detected only in monocyte progenitors and were also associated with reduced immune-response. Overall, our work indicates that regardless of the developmental stage in which HCMV infects, HCMV drives hematopoietic cells towards a weaker immune-responsive monocyte state and that this anergic-like state is crucial for the virus ability to express its transcripts and to eventually reactivate.

Functional Profile of Human Cytomegalovirus Genes and Their Associated Diseases: A Review

The human cytomegalovirus (HCMV), whose genome is 235 ± 1.9 kbp long, is a common herpesvirus. However, the functions of many of its genes are still unknown. HCMV is closely associated with various human diseases and infects 60-90% of the global population. It can infect various human cells, including fibroblasts, epithelial cells, endothelial cells, smooth muscle cells, and monocytes. Although HCMV infection is generally asymptomatic and causes subtle clinical symptoms, it can generate a robust immune response and establish a latent infection in immunocompromised individuals, including those with AIDS, transplant recipients, and developing fetuses. Currently available antivirals approved for the treatment of HCMV-associated diseases are limited by dose-limiting toxicity and the emergence of resistance; however, vaccines and immunoglobulins are unavailable. In this review, we have summarized the recent literature on 43 newly identified HCMV genes. We have described their novel functions on the viral replication cycle, latency, and host immune evasion. Further, we have discussed HCMV-associated diseases and current therapeutic targets. Our review may provide a foundational basis for studies aiming to prevent and develop targeted therapies for HCMV-associated diseases.

The Differentiation of Human Cytomegalovirus Infected-Monocytes Is Required for Viral Replication

Viral dissemination is a key mechanism responsible for persistence and disease following human cytomegalovirus (HCMV) infection. Monocytes play a pivotal role in viral dissemination to organ tissue during primary infection and following reactivation from latency. For example, during primary infection, infected monocytes migrate into tissues and differentiate into macrophages, which then become a source of viral replication. In addition, because differentiated macrophages can survive for months to years, they provide a potential persistent infection source in various organ systems. We broadly note that there are three phases to infection and differentiation of HCMV-infected monocytes: (1) Virus enters and traffics to the nucleus through a virus receptor ligand engagement event that activates a unique signalsome that initiates the monocyte-to-macrophage differentiation process. (2) Following initial infection, HCMV undergoes a "quiescence-like state" in monocytes lasting for several weeks and promotes monocyte differentiation into macrophages. While, the initial event is triggered by the receptor-ligand engagement, the long-term cellular activation is maintained by chronic viral-mediated signaling events. (3) Once HCMV infected monocytes differentiate into macrophages, the expression of immediate early viral (IE) genes is detectable, followed by viral replication and long term infectious viral particles release. Herein, we review the detailed mechanisms of each phase during infection and differentiation into macrophages and discuss the biological significance of the differentiation of monocytes in the pathogenesis of HCMV.

HCMV modulation of cellular PI3K/AKT/mTOR signaling: New opportunities for therapeutic intervention?

Human cytomegalovirus (HCMV) remains a major public health burden domestically and abroad. Current approved therapies, including ganciclovir, are only moderately efficacious, with many transplant patients suffering from a variety of side effects. A major impediment to the efficacy of current anti-HCMV drugs is their antiviral effects are restricted to the lytic stage of viral replication. Consequently, the non-lytic stages of the viral lifecycle remain major sources of HCMV infection associated with transplant recipients and ultimately the cause of morbidity and mortality. While work continues on new antivirals that block lytic replication, the dormant stages of HCMV's unique lifecycle need to be concurrently assessed for new therapeutic interventions. In this review, we will examine the role that the PI3K/Akt/mTOR signaling axis plays during the different stages of HCMV's lifecycle, and describe the advantages of targeting this cellular pathway as an antiviral strategy. In particular, we focus on the potential of exploiting the unique modifications HCMV imparts on the PI3K/Akt/mTOR pathway during quiescent infection of monocytes, which serve an essential role in the dissemination strategy of the virus.

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.

HCMV Infection and Apoptosis: How Do Monocytes Survive HCMV Infection?

Human cytomegalovirus (HCMV) infection of peripheral blood monocytes plays a key role in the hematogenous dissemination of the virus to multiple organ systems following primary infection or reactivation of latent virus in the bone marrow. Monocytes have a short life span of 1⁻3 days in circulation; thus, HCMV must alter their survival and differentiation to utilize these cells and their differentiated counterparts-macrophages-for dissemination and long term viral persistence. Because monocytes are not initially permissive for viral gene expression and replication, HCMV must control host-derived factors early during infection to prevent apoptosis or programmed cell death prior to viral induced differentiation into naturally long-lived macrophages. This review provides a short overview of HCMV infection of monocytes and describes how HCMV has evolved to utilize host cell anti-apoptotic pathways to allow infected monocytes to bridge the 48⁻72 h viability gate so that differentiation into a long term stable mature cell can occur. Because viral gene expression is delayed in monocytes following initial infection and only occurs (begins around two to three weeks post infection in our model) following what appears to be complete differentiation into mature macrophages or dendritic cells, or both; virally-encoded anti-apoptotic gene products cannot initially control long term infected cell survival. Anti-apoptotic viral genes are discussed in the second section of this review and we argue they would play an important role in long term macrophage or dendritic cell survival following infection-induced differentiation.

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 Utilizes a Nontraditional Signal Transducer and Activator of Transcription 1 Activation Cascade via Signaling through Epidermal Growth Factor Receptor and Integrins To Efficiently Promote the Motility, Differentiation, and Polarization of Infected Monocytes

Human cytomegalovirus (HCMV) infects peripheral blood monocytes and triggers biological changes that promote viral dissemination and persistence. We have shown that HCMV induces a proinflammatory state in infected monocytes, resulting in enhanced monocyte motility and transendothelial migration, prolonged monocyte survival, and differentiation toward a long-lived M1-like macrophage phenotype. Our data indicate that HCMV triggers these changes, in the absence of de novo viral gene expression and replication, through engagement and activation of epidermal growth factor receptor (EGFR) and integrins on the surface of monocytes. We previously identified that HCMV induces the upregulation of multiple proinflammatory gene ontologies, with the interferon-associated gene ontology exhibiting the highest percentage of upregulated genes. However, the function of the HCMV-induced interferon (IFN)-stimulated genes (ISGs) in infected monocytes remained unclear. We now show that HCMV induces the enhanced expression and activation of a key ISG transcriptional regulator, signal transducer and activator of transcription (STAT1), via an IFN-independent but EGFR- and integrin-dependent signaling pathway. Furthermore, we identified a biphasic activation of STAT1 that likely promotes two distinct phases of STAT1-mediated transcriptional activity. Moreover, our data show that STAT1 is required for efficient early HCMV-induced enhanced monocyte motility and later for HCMV-induced monocyte-to-macrophage differentiation and for the regulation of macrophage polarization, suggesting that STAT1 may serve as a molecular convergence point linking the biological changes that occur at early and later times postinfection. Taken together, our results suggest that HCMV reroutes the biphasic activation of a traditionally antiviral gene product through an EGFR- and integrin-dependent pathway in order to help promote the proviral activation and polarization of infected monocytes.IMPORTANCE HCMV promotes multiple functional changes in infected monocytes that are required for viral spread and persistence, including their enhanced motility and differentiation/polarization toward a proinflammatory M1 macrophage. We now show that HCMV utilizes the traditionally IFN-associated gene product, STAT1, to promote these changes. Our data suggest that HCMV utilizes EGFR- and integrin-dependent (but IFN-independent) signaling pathways to induce STAT1 activation, which may allow the virus to specifically dictate the biological activity of STAT1 during infection. Our data indicate that HCMV utilizes two phases of STAT1 activation, which we argue molecularly links the biological changes that occur following initial binding to those that continue to occur days to weeks following infection. Furthermore, our findings may highlight a unique mechanism for how HCMV avoids the antiviral response during infection by hijacking the function of a critical component of the IFN response pathway.

Modulation of the NFκb Signalling Pathway by Human Cytomegalovirus

Many viruses trigger innate and adaptive immune responses and must circumvent the negative consequences to successfully establish infection in their hosts. Human Cytomegalovirus (HCMV) is no exception, and devotes a significant portion of its coding capacity to genes involved in immune evasion. Activation of the NFκB signalling pathway by viral binding and entry results in induction of antiviral and pro-inflammatory genes that have significant negative effects on HCMV infection. However, NFκB signalling stimulates transcription from the Major Immediate Early Promoter (MIEP) and pro-inflammatory signalling is crucial for cellular differentiation and viral reactivation from latency. Accordingly, HCMV encodes proteins that act to both stimulate and inhibit the NFκB signalling pathway. In this Review we will highlight the complex interactions between HCMV and NFκB, discussing the known agonists and antagonists encoded by the virus and suggest why manipulation of the pathway may be critical for both lytic and latent infections.

Human Cytomegalovirus Requires Epidermal Growth Factor Receptor Signaling To Enter and Initiate the Early Steps in the Establishment of Latency in CD34+ Human Progenitor Cells

The establishment of human cytomegalovirus (HCMV) latency and persistence relies on the successful infection of hematopoietic cells, which serve as sites of viral persistence and contribute to viral spread. Here, using blocking antibodies and pharmacological inhibitors, we document that HCMV activation of the epidermal growth factor receptor (EGFR) and downstream phosphatidylinositol 3-kinase (PI3K) mediates viral entry into CD34+ human progenitor cells (HPCs), resulting in distinct cellular trafficking and nuclear translocation of the virus compared to that in other immune cells, such as we have documented in monocytes. We argue that the EGFR allows HCMV to regulate the cellular functions of these replication-restricted cells via its signaling activity following viral binding. In addition to regulating HCMV entry/trafficking, EGFR signaling may also shape the early steps required for the successful establishment of viral latency in CD34+ cells, as pharmacological inhibition of EGFR increases the transcription of lytic IE1/IE2 mRNA while curbing the expression of latency-associated UL138 mRNA. EGFR signaling following infection of CD34+ HPCs may also contribute to changes in hematopoietic potential, as treatment with the EGFR kinase (EGFRK) inhibitor AG1478 alters the expression of the cellular hematopoietic cytokine interleukin 12 (IL-12) in HCMV-infected cells but not in mock-infected cells. These findings, along with our previous work with monocytes, suggest that EGFR likely serves as an important determinant of HCMV tropism for select subsets of hematopoietic cells. Moreover, our new data suggest that EGFR is a key receptor for efficient viral entry and that the ensuing signaling regulates important early events required for successful infection of CD34+ HPCs by HCMV.IMPORTANCE HCMV establishes lifelong persistence within the majority of the human population without causing overt pathogenesis in healthy individuals. Despite this, reactivation of HCMV from its latent reservoir in the bone marrow causes significant morbidity and mortality in immunologically compromised individuals, such as bone marrow and solid organ transplant patients. Lifelong persistent infection has also been linked with the development of various cardiovascular diseases in otherwise healthy individuals. Current HCMV therapeutics target lytic replication, but not the latent viral reservoir; thus, an understanding of the molecular basis for viral latency and persistence is paramount to controlling or eliminating HCMV infection. Here, we show that the viral signalosome activated by HCMV binding to its entry receptor, EGFR, in CD34+ HPCs initiates early events necessary for successful latent infection of this cell type. EGFR and associated signaling players may therefore represent promising targets for mitigating HCMV persistence.

Mcl-1 small-molecule inhibitors encapsulated into nanoparticles exhibit increased killing efficacy towards HCMV-infected monocytes

Human cytomegalovirus (HCMV) spreads and establishes a persistent infection within a host by stimulating the survival of carrier myeloid cells via the upregulation of Mcl-1, an antiapoptotic member of the Bcl-2 family of proteins. However, the lack of potent Mcl-1-specific inhibitors and a targetable delivery system has limited the ability to exploit Mcl-1 as a therapeutic strategy to eliminate HCMV-infected monocytes. In this study, we found a lead compound from a novel class of Mcl-1 small-molecule inhibitors rapidly induced death of HCMV-infected monocytes. Moreover, encapsulation of Mcl-1 antagonists into myeloid cell-targeting nanoparticles was able to selectively increase the delivery of inhibitors into HCMV-activated monocytes, thereby amplifying their potency. Our study demonstrates the potential use of nanotechnology to target Mcl-1 small-molecule inhibitors to HCMV-infected monocytes.

The Role of the Equine Herpesvirus Type 1 (EHV-1) US3-Encoded Protein Kinase in Actin Reorganization and Nuclear Egress

The serine-threonine protein kinase encoded by US3 gene (pUS3) of alphaherpesviruses was shown to modulate actin reorganization, cell-to-cell spread, and virus egress in a number of virus species. However, the role of the US3 orthologues of equine herpesvirus type 1 and 4 (EHV-1 and EHV-4) has not yet been studied. Here, we show that US3 is not essential for virus replication in vitro. However, growth rates and plaque diameters of a US3-deleted EHV-1 and a mutant in which the catalytic active site was destroyed were significantly reduced when compared with parental and revertant viruses or a virus in which EHV-1 US3 was replaced with the corresponding EHV-4 gene. The reduced plaque sizes were consistent with accumulation of primarily enveloped virions in the perinuclear space of the US3-negative EHV-1, a phenotype that was also rescued by the EHV-4 orthologue. Furthermore, actin stress fiber disassembly was significantly more pronounced in cells infected with parental EHV-1, revertant, or the recombinant EHV-1 expressing EHV-4 US3. Finally, we observed that deletion of US3 in EHV-1 did not affect the expression of adhesion molecules on the surface of infected cells.

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.

The HCMV US28 vGPCR induces potent Gαq/PLC-β signaling in monocytes leading to increased adhesion to endothelial cells

US28 transcripts have been detected in primary monocytes and in THP-1 monocytes infected with HCMV but US28 protein expression has not yet been demonstrated in these cell types. Moreover, the mechanism(s) by which US28 signals and contributes to viral pathogenesis in monocytes remains unclear. Here, we show that US28 protein is robustly expressed in HCMV infected THP-1 monocytes and that US28 can trigger Gαq dependent signaling in THP-1 cells infected with HCMV and in THP-1 cells stably expressing US28. US28 signaling in these cells is dependent on G-protein coupling, but independent of chemokine binding. Importantly, we demonstrate that this US28 signaling is functionally important as it stimulates the adhesion of monocytes to an endothelial monolayer. Our studies, which demonstrate that US28-driven Gαq signaling has profound effects on monocyte biology, suggest that US28 driven phenotypic changes in HCMV infected monocytes may play important roles in HCMV dissemination and/or pathogenesis.

Integrins as Herpesvirus Receptors and Mediators of the Host Signalosome

The repertoire of herpesvirus receptors consists of nonintegrin and integrin molecules. Integrins interact with the conserved glycoproteins gH/gL or gB. This interaction is a conserved biology across the Herpesviridae family, likely directed to promote virus entry and endocytosis. Herpesviruses exploit this interaction to execute a range of critical functions that include (a) relocation of nonintegrin receptors (e.g., herpes simplex virus nectin1 and Kaposi's sarcoma-associated herpesvirus EphA2), or association with nonintegrin receptors (i.e., human cytomegalovirus EGFR), to dictate species-specific entry pathways; (b) activation of multiple signaling pathways (e.g., Ca²⁺ release, c-Src, FAK, MAPK, and PI3K); and (c) association with Rho GTPases, tyrosine kinase receptors, Toll-like receptors, which result in cytoskeletal remodeling, differential cell type targeting, and innate responses. In turn, integrins can be modulated by viral proteins (e.g., Epstein-Barr virus LMPs) to favor spread of transformed cells. We propose that herpesviruses evolved a multipartite entry system to allow interaction with multiple receptors, including integrins, required for their sophisticated life cycle.

Human Cytomegalovirus Induces an Atypical Activation of Akt To Stimulate the Survival of Short-Lived Monocytes

Human cytomegalovirus (HCMV) is a pervasive herpesvirus responsible for significant morbidity and mortality among immunodeficient/naive hosts. Following a primary HCMV infection, circulating blood monocytes mediate the systemic spread of the virus. Extending the short 48-h life span of monocytes is critical to the viral dissemination process, as these blood-borne cells are nonpermissive for virus replication until they are fully differentiated into macrophages. Here, we show that HCMV glycoprotein gB binding to cellular epidermal growth factor receptor (EGFR) during HCMV entry initiated a rapid (within 15 min) activation of the apoptosis suppressor Akt, which was maintained through 72 h. The virus-induced activation of Akt was more robust than that with the normal myeloid growth factor macrophage colony-stimulating factor (M-CSF) and was essential for infected monocytes to bypass the 48-h viability checkpoint. Activation of phosphoinositide 3-kinase (PI3K) following EGFR engagement by HCMV mediated the phosphorylation of Akt. Moreover, HCMV entry drove a switch away from the PI3K p110δ isoform, which was required for the viability of uninfected monocytes, to the p110β isoform in order to facilitate the Akt-dependent prosurvival state within infected cells. Simultaneously, in contrast to M-CSF, HCMV promoted a rapid increase in SH2 domain-containing inositol 5-phosphatase 1 (SHIP1) expression, leading to signaling through a noncanonical Akt activation pathway. To ensure maximum Akt activity, HCMV also induced an early phosphorylation-dependent inactivation of the negative regulator phosphatase and tensin homolog. Overall, our data indicate that HCMV hijacks the upstream Akt signaling network to induce a nontraditional activation of Akt and subsequently a prosurvival decision at the 48-h cell fate checkpoint, a vital step for HCMV's dissemination and persistence strategy.

IMPORTANCE

HCMV is found throughout the world with a prevalence of 55 to 100% within the human population. HCMV infection is generally asymptomatic in immunocompetent or naive individuals but is a significant cause of morbidity and mortality among the immunocompromised. Widespread organ inflammation is associated with symptomatic infections, which is a direct consequence of the viral dissemination strategy. Inflammatory peripheral blood monocytes facilitate the spread of HCMV. However, HCMV must subvert the naturally short life span of monocytes. In this work, we demonstrate that HCMV induces the activation of Akt, an antiapoptotic protein, in a manner distinct from that of normal myeloid growth factors. Moreover, we decipher how HCMV dysregulates the upstream Akt signaling network during viral entry to promote an Akt-dependent prosurvival state following infection. Delineation of the virus-specific mechanisms that regulate cellular prosurvival pathways in order to drive the survival of HCMV-infected monocytes is important to identifying new anti-HCMV therapeutic targets.

Human Cytomegalovirus Promotes Survival of Infected Monocytes via a Distinct Temporal Regulation of Cellular Bcl-2 Family Proteins

Monocytes play a key role in the hematogenous dissemination of human cytomegalovirus (HCMV) to target organ systems. To infect monocytes and reprogram them to deliver infectious virus, HCMV must overcome biological obstacles, including the short life span of monocytes and their antiviral proapoptotic response to infection. We have shown that virally induced upregulation of cellular Mcl-1 promotes early survival of HCMV-infected monocytes, allowing cells to overcome an early apoptotic checkpoint at around 48 h postinfection (hpi). Here, we demonstrate an HCMV-dependent shift from Mcl-1 as the primary antiapoptotic player to the related protein, Bcl-2, later during infection. Bcl-2 was upregulated in HCMV-infected monocytes beginning at 48 hpi. Treatment with the Bcl-2 antagonist ABT-199 only reduced the prosurvival effects of HCMV in target monocytes beginning at 48 hpi, suggesting that Mcl-1 controls survival prior to 48 hpi, while Bcl-2 promotes survival after 48 hpi. Although Bcl-2 was upregulated following viral binding/signaling through cellular integrins (compared to Mcl-1, which is upregulated through binding/activation of epidermal growth factor receptor [EGFR]), it functioned similarly to Mcl-1, adopting the early role of Mcl-1 in preventing caspase-3 cleavage/activation. This distinct, HCMV-induced shift from Mcl-1 to Bcl-2 occurs in response to a cellular upregulation of proapoptotic Bax, as small interfering RNA (siRNA)-mediated knockdown of Bax reduced the upregulation of Bcl-2 in infected monocytes and rescued the cells from the apoptotic effects of Bcl-2 inhibition. Our data demonstrate a distinct survival strategy whereby HCMV induces a biphasic regulation of cellular Bcl-2 proteins to promote host cell survival, leading to viral dissemination and the establishment of persistent HCMV infection.

IMPORTANCE

Hematogenous dissemination of HCMV via infected monocytes is a crucial component of the viral survival strategy and is required for the establishment of persistent infection and for viral spread to additional hosts. Our system of infected primary human blood monocytes provides us with an opportunity to answer specific questions about viral spread and persistence in in vivo-relevant myeloid cells that cannot be addressed with the more traditionally used replication-permissive cells. Our goal in examining the mechanisms whereby HCMV reprograms infected monocytes to promote viral dissemination is to uncover new targets for therapeutic intervention that would disrupt key viral survival and persistence strategies. Because of this important role in maintaining survival of HCMV-infected monocytes, our new data on the role of Bcl-2 regulation during viral infection represents a promising molecular target for mitigating viral spread and persistence.

Equine Herpesvirus Type 1 Enhances Viral Replication in CD172a+ Monocytic Cells upon Adhesion to Endothelial Cells

Equine herpesvirus type 1 (EHV-1) is a main cause of respiratory disease, abortion, and encephalomyelopathy in horses. Monocytic cells (CD172a(+)) are the main carrier cells of EHV-1 during primary infection and are proposed to serve as a "Trojan horse" to facilitate the dissemination of EHV-1 to target organs. However, the mechanism by which EHV-1 is transferred from CD172a(+) cells to endothelial cells (EC) remains unclear. The aim of this study was to investigate EHV-1 transmission between these two cell types. We hypothesized that EHV-1 employs specific strategies to promote the adhesion of infected CD172a(+) cells to EC to facilitate EHV-1 spread. Here, we demonstrated that EHV-1 infection of CD172a(+) cells resulted in a 3- to 5-fold increase in adhesion to EC. Antibody blocking experiments indicated that α4β1, αLβ2, and αVβ3 integrins mediated adhesion of infected CD172a(+) cells to EC. We showed that integrin-mediated phosphatidylinositol 3-kinase (PI3K) and ERK/MAPK signaling pathways were involved in EHV-1-induced CD172a(+) cell adhesion at early times of infection. EHV-1 replication was enhanced in adherent CD172a(+) cells, which correlates with the production of tumor necrosis factor alpha (TNF-α). In the presence of neutralizing antibodies, approximately 20% of infected CD172a(+) cells transferred cytoplasmic material to uninfected EC and 0.01% of infected CD172a(+) cells transmitted infectious virus to neighboring cells. Our results demonstrated that EHV-1 infection induces adhesion of CD172a(+) cells to EC, which enhances viral replication, but that transfer of viral material from CD172a(+) cells to EC is a very specific and rare event. These findings give new insights into the complex pathogenesis of EHV-1.

IMPORTANCE

Equine herpesvirus type 1 (EHV-1) is a highly prevalent pathogen worldwide, causing frequent outbreaks of abortion and myeloencephalopathy, even in vaccinated horses. After primary replication in the respiratory tract, EHV-1 disseminates via cell-associated viremia in peripheral blood mononuclear cells (PBMC) and subsequently infects the endothelial cells (EC) of the pregnant uterus or central nervous system, leading in some cases to abortion and/or neurological disorders. Recently, we demonstrated that CD172a(+) monocytic carrier cells serve as a "Trojan horse" to facilitate EHV-1 spread from blood to target organs. Here, we investigated the mechanism underlying the transmission of EHV-1 from CD172a(+) cells to EC. We demonstrated that EHV-1 infection induces cellular changes in CD172a(+) cells, promoting their adhesion to EC. We found that both cell-to-cell contacts and the secretion of soluble factors by EC activate EHV-1 replication in CD172a(+) cells. This facilitates transfer of cytoplasmic viral material to EC, resulting mainly in a nonproductive infection. Our findings give new insights into how EHV-1 may spread to EC of target organs in vaccinated horses.

Equine Herpesvirus Type 1 Enhances Viral Replication in CD172a+ Monocytic Cells upon Adhesion to Endothelial Cells

Equine herpesvirus type 1 (EHV-1) is a main cause of respiratory disease, abortion, and encephalomyelopathy in horses. Monocytic cells (CD172a(+)) are the main carrier cells of EHV-1 during primary infection and are proposed to serve as a "Trojan horse" to facilitate the dissemination of EHV-1 to target organs. However, the mechanism by which EHV-1 is transferred from CD172a(+) cells to endothelial cells (EC) remains unclear. The aim of this study was to investigate EHV-1 transmission between these two cell types. We hypothesized that EHV-1 employs specific strategies to promote the adhesion of infected CD172a(+) cells to EC to facilitate EHV-1 spread. Here, we demonstrated that EHV-1 infection of CD172a(+) cells resulted in a 3- to 5-fold increase in adhesion to EC. Antibody blocking experiments indicated that α4β1, αLβ2, and αVβ3 integrins mediated adhesion of infected CD172a(+) cells to EC. We showed that integrin-mediated phosphatidylinositol 3-kinase (PI3K) and ERK/MAPK signaling pathways were involved in EHV-1-induced CD172a(+) cell adhesion at early times of infection. EHV-1 replication was enhanced in adherent CD172a(+) cells, which correlates with the production of tumor necrosis factor alpha (TNF-α). In the presence of neutralizing antibodies, approximately 20% of infected CD172a(+) cells transferred cytoplasmic material to uninfected EC and 0.01% of infected CD172a(+) cells transmitted infectious virus to neighboring cells. Our results demonstrated that EHV-1 infection induces adhesion of CD172a(+) cells to EC, which enhances viral replication, but that transfer of viral material from CD172a(+) cells to EC is a very specific and rare event. These findings give new insights into the complex pathogenesis of EHV-1.

IMPORTANCE

Equine herpesvirus type 1 (EHV-1) is a highly prevalent pathogen worldwide, causing frequent outbreaks of abortion and myeloencephalopathy, even in vaccinated horses. After primary replication in the respiratory tract, EHV-1 disseminates via cell-associated viremia in peripheral blood mononuclear cells (PBMC) and subsequently infects the endothelial cells (EC) of the pregnant uterus or central nervous system, leading in some cases to abortion and/or neurological disorders. Recently, we demonstrated that CD172a(+) monocytic carrier cells serve as a "Trojan horse" to facilitate EHV-1 spread from blood to target organs. Here, we investigated the mechanism underlying the transmission of EHV-1 from CD172a(+) cells to EC. We demonstrated that EHV-1 infection induces cellular changes in CD172a(+) cells, promoting their adhesion to EC. We found that both cell-to-cell contacts and the secretion of soluble factors by EC activate EHV-1 replication in CD172a(+) cells. This facilitates transfer of cytoplasmic viral material to EC, resulting mainly in a nonproductive infection. Our findings give new insights into how EHV-1 may spread to EC of target organs in vaccinated horses.

Cytomegalovirus enhances macrophage TLR expression and MyD88-mediated signal transduction to potentiate inducible inflammatory responses

Circulating monocytes carrying human CMV (HCMV) migrate into tissues, where they differentiate into HCMV-infected resident macrophages that upon interaction with bacterial products may potentiate tissue inflammation. In this study, we investigated the mechanism by which HCMV promotes macrophage-orchestrated inflammation using a clinical isolate of HCMV (TR) and macrophages derived from primary human monocytes. HCMV infection of the macrophages, which was associated with viral DNA replication, significantly enhanced TNF-α, IL-6, and IL-8 gene expression and protein production in response to TLR4 ligand (LPS) stimulation compared with mock-infected LPS-stimulated macrophages during a 6-d in vitro infection. HCMV infection also potentiated TLR5 ligand-stimulated cytokine production. To elucidate the mechanism by which HCMV infection potentiated inducible macrophage responses, we show that infection by HCMV promoted the maintenance of surface CD14 and TLR4 and TLR5, which declined over time in mock-infected macrophages, and enhanced both the intracellular expression of adaptor protein MyD88 and the inducible phosphorylation of IκBα and NF-κB. These findings provide additional information toward elucidating the mechanism by which HCMV potentiates bacteria-induced NF-κB-mediated macrophage inflammatory responses, thereby enhancing organ inflammation in HCMV-infected tissues.

Human cytomegalovirus modulates monocyte-mediated innate immune responses during short-term experimental latency in vitro

The ability of human cytomegalovirus (HCMV) to establish lifelong persistence and reactivate from latency is critical to its success as a pathogen. Here we describe a short-term in vitro model representing the events surrounding HCMV latency and reactivation in circulating peripheral blood monocytes that was developed in order to study the immunological consequence of latent virus carriage. Infection of human CD14(+) monocytes by HCMV resulted in the immediate establishment of latency, as evidenced by the absence of particular lytic gene expression, the transcription of latency-associated mRNAs, and the maintenance of viral genomes. Latent HCMV induced cellular differentiation to a macrophage lineage, causing production of selective proinflammatory cytokines and myeloid-cell chemoattractants that most likely play a role in virus dissemination in the host. Analysis of global cellular gene expression revealed activation of innate immune responses and the modulation of protein and lipid synthesis to accommodate latent HCMV infection. Remarkably, monocytes harboring latent virus exhibited selective responses to secondary stimuli known to induce an antiviral state. Furthermore, when challenged with type I and II interferon, latently infected cells demonstrated a blockade of signaling at the level of STAT1 phosphorylation. The data demonstrate that HCMV reprograms specific cellular pathways in monocytes, most notably innate immune responses, which may play a role in the establishment of, maintenance of, and reactivation from latency. The modulation of innate immune responses is likely a viral evasion strategy contributing to viral dissemination and pathogenesis in the host.

IMPORTANCE

HCMV has the ability to establish a lifelong infection within the host, a phenomenon termed latency. We have established a short-term model system in human peripheral blood monocytes to study the immunological relevance of latent virus carriage. Infection of CD14(+) monocytes by HCMV results in the generation of latency-specific transcripts, maintenance of viral genomes, and the capacity to reenter the lytic cycle. During short-term latency in monocytes the virus initiates a program of differentiation to inflammatory macrophages that coincides with the modulation of cytokine secretion and specific cellular processes. HCMV-infected monocytes are hindered in their capacity to exert normal immunoprotective mechanisms. Additionally, latent virus disrupts type I and II interferon signaling at the level of STAT1 phosphorylation. This in vitro model system can significantly contribute to our understanding of the molecular and inflammatory factors that initiate HCMV reactivation in the host and allow the development of strategies to eradicate virus persistence.

Regulatory T cells protect fine particulate matter-induced inflammatory responses in human umbilical vein endothelial cells

OBJECTIVE

To investigate the role of CD4(+)CD25(+) T cells (Tregs) in protecting fine particulate matter (PM-) induced inflammatory responses, and its potential mechanisms.

METHODS

Human umbilical vein endothelial cells (HUVECs) were treated with graded concentrations (2, 5, 10, 20, and 40 µg/cm(2)) of suspension of fine particles for 24h. For coculture experiment, HUVECs were incubated alone, with CD4(+)CD25(-) T cells (Teff), or with Tregs in the presence of anti-CD3 monoclonal antibodies for 48 hours, and then were stimulated with or without suspension of fine particles for 24 hours. The expression of adhesion molecules and inflammatory cytokines was examined.

RESULTS

Adhesion molecules, including vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1), and inflammatory cytokines, such as interleukin (IL-) 6 and IL-8, were increased in a concentration-dependent manner. Moreover, the adhesion of human acute monocytic leukemia cells (THP-1) to endothelial cells was increased and NF- κ B activity was upregulated in HUVECs after treatment with fine particles. However, after Tregs treatment, fine particles-induced inflammatory responses and NF- κ B activation were significantly alleviated. Transwell experiments showed that Treg-mediated suppression of HUVECs inflammatory responses impaired by fine particles required cell contact and soluble factors.

CONCLUSIONS

Tregs could attenuate fine particles-induced inflammatory responses and NF- κ B activation in HUVECs.

HCMV reprogramming of infected monocyte survival and differentiation: a Goldilocks phenomenon

The wide range of disease pathologies seen in multiple organ sites associated with human cytomegalovirus (HCMV) infection results from the systemic hematogenous dissemination of the virus, which is mediated predominately by infected monocytes. In addition to their role in viral spread, infected monocytes are also known to play a key role in viral latency and life-long persistence. However, in order to utilize infected monocytes for viral spread and persistence, HCMV must overcome a number of monocyte biological hurdles, including their naturally short lifespan and their inability to support viral gene expression and replication. Our laboratory has shown that HCMV is able to manipulate the biology of infected monocytes in order to overcome these biological hurdles by inducing the survival and differentiation of infected monocytes into long-lived macrophages capable of supporting viral gene expression and replication. In this current review, we describe the unique aspects of how HCMV promotes monocyte survival and differentiation by inducing a “finely-tuned” macrophage cell type following infection. Specifically, we describe the induction of a uniquely polarized macrophage subset from infected monocytes, which we argue is the ideal cellular environment for the initiation of viral gene expression and replication and, ultimately, viral spread and persistence within the infected host.

CD44 mediated hyaluronan adhesion of Toxoplasma gondii-infected leukocytes

Toxoplasma gondii is an obligate intracellular apicomplexan parasite that infects humans and animals. Ingested parasites cross the intestinal epithelium, invade leukocytes and are then disseminated to peripheral organs. However, the mechanism of extravasation of the infected leukocytes remains poorly understood. In this study, we demonstrate that T. gondii-invaded human and mouse leukocytes express higher level of CD44, a ligand of hyaluronan (HA), and its expression on myeloid and non-myeloid leukocytes causes T. gondii-invaded human and mouse leukocyte to adhere to HA more effectively than non-invaded leukocytes. The specific adherence of parasite-invaded leukocytes was inhibited by anti CD44 antibody. Leukocytes of CD44 knockout mice did not show parasite-invaded leukocyte specific adhesion. Our results indicate that parasite-invaded leukocytes, regardless of whether myeloid or not, gain higher ability to adhere to HA than non-invaded leukocytes, via upregulation of CD44 expression and/or selective invasion to CD44 highly expressing cells. The difference in ability to adhere to HA between parasite-invaded cells and non-invaded neighboring cells might facilitate effective delivery of parasite-invaded leukocytes to the HA-producing endothelial cell surface and/or HA-rich extra cellular matrix.

The HCMV gH/gL/UL128-131 complex triggers the specific cellular activation required for efficient viral internalization into target monocytes

We have established that HCMV acts as a specific ligand engaging and activating cellular integrins on monocytes. As a result, integrin signaling via Src activation leads to the functional activation of paxillin required for efficient viral entry and for the biological changes in monocytes needed for viral dissemination. These biological/molecular changes allow HCMV to use monocytes as "vehicles" for systemic spread and the establishment of lifelong persistence. However, it remains unresolved how HCMV specifically induces this observed monocyte activation. It was previously demonstrated that the HCMV gH/gL/UL128-131 glycoprotein complex facilitates viral entry into biologically relevant cell types. Nevertheless, the mechanism by which the gH/gL/UL128-131 complex promotes this process is unknown. We now show that only HCMV virions possessing the gH/gL/UL128-131 complex are capable of activating integrin/Src/paxillin-signaling in monocytes. In fibroblasts, this signaling is reversed, such that virus lacking the gH/gL/UL128-131 complex is the only virus able to induce the paxillin activation cascade. The presence of the gH/gL/UL128-131 complex also may have an inhibitory effect on integrin-mediated signaling pathway in fibroblasts. Furthermore, we demonstrate that the presence of the gH/gL/UL128-131 complex on the viral envelope, through its activation of the integrin/Src/paxillin pathway, is necessary for efficient HCMV internalization into monocytes and that appropriate actin and dynamin regulation is critical for this entry process. Importantly, productive infection in monocyte-derived macrophages was seen only in cells exposed to HCMV expressing the gH/gL/UL128-131 complex. From our data, the HCMV gH/gL/U128-131 complex emerges as the specific ligand driving the activation of the receptor-mediated signaling required for the regulation of the actin cytoskeleton and, consequently, for efficient and productive internalization of HCMV into monocytes. To our knowledge, our studies demonstrate a possible molecular mechanism for why the gH/gL/UL128-131 complex dictates HCMV tropism and why the complex is lost as clinical isolates are passaged in the laboratory.

Functional genomics approaches to understand cytomegalovirus replication, latency and pathogenesis

Cytomegalovirus (CMV) is a species-specific herpesvirus that is ubiquitous in the population and has the potential to cause significant disease in immunocompromised individuals as well as in congenitally infected infants. CMV establishes latency in cells of the myeloid lineage following primary infection. High-throughput functional genomics approaches have provided insight into the mechanisms of CMV replication, but although CMV latency cell models have been useful in elucidating the mechanisms of viral latency and reactivation, omics approaches have proven challenging in these cell systems. This review will summarize the current state of knowledge concerning the use of functional genomics technologies to understand mechanisms of CMV replication, latency and pathogenesis.

Human cytomegalovirus infection enhances NF-κB/p65 signaling in inflammatory breast cancer patients

Human Cytomegalovirus (HCMV) is an endemic herpes virus that re-emerges in cancer patients enhancing oncogenic potential. Recent studies have shown that HCMV infection is associated with certain types of cancer morbidity such as glioblastoma. Although HCMV has been detected in breast cancer tissues, its role, if any, in the etiology of specific forms of breast cancer has not been investigated. In the present study we investigated the presence of HCMV infection in inflammatory breast cancer (IBC), a rapidly progressing form of breast cancer characterized by specific molecular signature. We screened for anti-CMV IgG antibodies in peripheral blood of 49 non-IBC invasive ductal carcinoma (IDC) and 28 IBC patients. In addition, we screened for HCMV-DNA in postsurgical cancer and non-cancer breast tissues of non-IBC and IBC patients. We also tested whether HCMV infection can modulate the expression and activation of transcriptional factor NF-κB/p65, a hallmark of IBC. Our results reveal that IBC patients are characterized by a statistically significant increase in HCMV IgG antibody titers compared to non-IBC patients. HCMV-DNA was significantly detected in cancer tissues than in the adjacent non-carcinoma tissues of IBC and IDC, and IBC cancer tissues were significantly more infected with HCMV-DNA compared to IDC. Further, HCMV sequence analysis detected different HCMV strains in IBC patients tissues, but not in the IDC specimens. Moreover, HCMV-infected IBC cancer tissues were found to be enhanced in NF-κB/p65 signaling compared to non-IBC patients. The present results demonstrated a correlation between HCMV infection and IBC. Etiology and causality of HCMV infection with IBC now needs to be rigorously examined.

A quantitative evaluation of cell migration by the phagokinetic track motility assay

Cellular motility is an important biological process for both unicellular and multicellular organisms. It is essential for movement of unicellular organisms towards a source of nutrients or away from unsuitable conditions, as well as in multicellular organisms for tissue development, immune surveillance and wound healing, just to mention a few roles(1,2,3). Deregulation of this process can lead to serious neurological, cardiovascular and immunological diseases, as well as exacerbated tumor formation and spread(4,5). Molecularly, actin polymerization and receptor recycling have been shown to play important roles in creating cellular extensions (lamellipodia), that drive the forward movement of the cell(6,7,8). However, many biological questions about cell migration remain unanswered. The central role for cellular motility in human health and disease underlines the importance of understanding the specific mechanisms involved in this process and makes accurate methods for evaluating cell motility particularly important. Microscopes are usually used to visualize the movement of cells. However, cells move rather slowly, making the quantitative measurement of cell migration a resource-consuming process requiring expensive cameras and software to create quantitative time-lapsed movies of motile cells. Therefore, the ability to perform a quantitative measurement of cell migration that is cost-effective, non-laborious, and that utilizes common laboratory equipment is a great need for many researchers. The phagokinetic track motility assay utilizes the ability of a moving cell to clear gold particles from its path to create a measurable track on a colloidal gold-coated glass coverslip(9,10). With the use of freely available software, multiple tracks can be evaluated for each treatment to accomplish statistical requirements. The assay can be utilized to assess motility of many cell types, such as cancer cells(11,12), fibroblasts(9), neutrophils(13), skeletal muscle cells(14), keratinocytes(15), trophoblasts(16), endothelial cells(17), and monocytes(10,18-22). The protocol involves the creation of slides coated with gold nanoparticles (Au°) that are generated by a reduction of chloroauric acid (Au(3+)) by sodium citrate. This method was developed by Turkevich et al. in 1951(23) and then improved in the 1970s by Frens et al.(24,25). As a result of this chemical reduction step, gold particles (10-20 nm in diameter) precipitate from the reaction mixture and can be applied to glass coverslips, which are then ready for use in cellular migration analyses(9,26,27). In general, the phagokinetic track motility assay is a quick, quantitative and easy measure of cellular motility. In addition, it can be utilized as a simple high-throughput assay, for use with cell types that are not amenable to time-lapsed imaging, as well as other uses depending on the needs of the researcher. Together, the ability to quantitatively measure cellular motility of multiple cell types without the need for expensive microscopes and software, along with the use of common laboratory equipment and chemicals, make the phagokinetic track motility assay a solid choice for scientists with an interest in understanding cellular motility.

Human cytomegalovirus stimulates monocyte-to-macrophage differentiation via the temporal regulation of caspase 3

Monocytes are primary targets for human cytomegalovirus (HCMV) infection and are proposed to be responsible for hematogenous dissemination of the virus. Biologically, monocytes have a short life span of 48 h in the circulation, a period of time during which monocytes must make a cell fate decision on whether to undergo apoptosis or differentiate into a macrophage. We have previously shown that HCMV infection stimulates monocyte-to-macrophage differentiation; however, the mechanism(s) by which HCMV-infected monocytes simultaneously navigate the 48-h "viability gate" and undergo macrophagic differentiation has remained elusive. Studies have demonstrated that the level of caspase 3 and 8 activities in monocytes may mediate the delicate balance between apoptosis and macrophage colony-stimulating factor (M-CSF)-induced myeloid differentiation. Here, we show that HCMV infection, unlike M-CSF treatment, does not induce caspase 8 activity to promote myeloid differentiation. However, HCMV infection does induce a temporal activation of caspase 3, with only a low level of active caspase 3 being observed after the 48-h viability checkpoint. Consistent with the role of a time-dependent activation of caspase 3 in promoting myeloid differentiation, the inhibition of caspase 3 blocked HCMV-induced monocyte-to-macrophage differentiation. Temporal transcriptome and functional analyses identified heat shock protein 27 (HSP27) and Mcl-1, two known regulators of caspase 3 activation, as being upregulated prior to the 48-h viability gate following HCMV infection. Using small interfering RNAs (siRNAs), we demonstrate that HCMV targets the rapid induction of HSP27 and Mcl-1, which cooperatively function to precisely control caspase 3 activity in order to allow for HCMV-infected monocytes to successfully traverse the 48-h cell fate decision checkpoint and commence macrophage maturation. Overall, this study highlights a unique regulatory mechanism employed by HCMV to tightly modulate the caspase 3 activity needed to promote myeloid differentiation, a key process in the viral dissemination and persistence strategy.

Human cytomegalovirus induction of a unique signalsome during viral entry into monocytes mediates distinct functional changes: a strategy for viral dissemination

HCMV pathogenesis is a direct consequence of the hematogenous dissemination of the virus to multiple host organ sites. The presence of infected monocytes in the peripheral blood and organs of individuals exhibiting primary HCMV infection have long suggested that these blood sentinels are responsible for mediating viral spread. Despite monocytes being "at the right place at the right time", their short lifespan and the lack of productive viral infection in these cells complicate this scenario of a monocyte-driven approach to viral dissemination by HCMV. However, our laboratory has provided evidence that HCMV infection is able to induce a highly controlled polarization of monocytes toward a unique and long-lived proinflammatory macrophage, which we have demonstrated to be permissive for viral replication. These observations suggest that HCMV has evolved as a distinct mechanism to induce select proinflammatory characteristics that provide infected monocytes with the necessary tools to mediate viral spread following a primary infection. In the absence of viral gene products during the early stages of infection, the process by which HCMV "tunes" the inflammatory response in infected monocytes to promote viral spread and subsequently, viral persistence remains unclear. In this current review, we focus on the viral entry process of HCMV and the potential role of receptor-ligand interactions in modulating monocyte biology. Specifically, we examine the signaling pathways initiated by the distinct combination of cellular receptors simultaneously engaged and activated by HCMV during viral entry and how the acquisition of this distinct signalsome results in a nontraditional activation of monocytes leading to the induction of the unique, functional attributes observed in monocytes following HCMV infection.

Granulocyte-colony stimulating factor reactivates human cytomegalovirus in a latently infected humanized mouse model

Human cytomegalovirus (HCMV) is a significant cause of morbidity and mortality in organ transplant recipients. The use of granulocyte-colony stimulating factor (G-CSF)-mobilized stem cells from HCMV seropositive donors is suggested to double the risk of late-onset HCMV disease and chronic graft-versus-host disease in recipients when compared to conventional bone marrow transplantation with HCMV seropositive donors, although the etiology of the increased risk is unknown. To understand mechanisms of HCMV transmission in patients receiving G-CSF-mobilized blood products, we generated a NOD-scid IL2Rγ(c)(null)-humanized mouse model in which HCMV establishes latent infection in human hematopoietic cells. In this model, G-CSF induces the reactivation of latent HCMV in monocytes/macrophages that have migrated into organ tissues. In addition to establishing a humanized mouse model for systemic and latent HCMV infection, these results suggest that the use of G-CSF mobilized blood products from seropositive donors pose an elevated risk for HCMV transmission to recipients.

Human cytomegalovirus-regulated paxillin in monocytes links cellular pathogenic motility to the process of viral entry

We have established that human cytomegalovirus (HCMV) infection modulates the biology of target primary peripheral blood monocytes, allowing HCMV to use monocytes as "vehicles" for its systemic spread. HCMV infection of monocytes results in rapid induction of phosphatidylinositol-3-kinase [PI(3)K] and NF-κB activities. Integrins, which are upstream of the PI(3)K and NF-κB pathways, were shown to be involved in HCMV binding to and entry into fibroblasts, suggesting that receptor ligand-mediated signaling following viral binding to integrins on monocytes could trigger the functional changes seen in infected monocytes. We now show that integrin engagement and the activation of the integrin/Src signaling pathway are essential for the induction of HCMV-infected monocyte motility. To investigate how integrin engagement by HCMV triggers monocyte motility, we examined the infected-monocyte transcriptome and found that the integrin/Src signaling pathway regulates the expression of paxillin, which is an important signal transducer in the regulation of actin rearrangement during cell adhesion and movement. Functionally, we observed that paxillin is activated via the integrin/Src signaling pathway and is required for monocyte motility. Because motility is intimately connected to cellular cytoskeletal organization, a process that is also important in viral entry, we investigated the role paxillin regulation plays in the process of viral entry into monocytes. New results confirmed that HCMV entry into target monocytes was significantly reduced in cells deficient in paxillin expression or the integrin/Src/paxillin signaling pathway. From our data, HCMV-cell interactions emerge as an essential trigger for the cellular changes that allow for HCMV entry and hematogenous dissemination.

CD4+CD25+Foxp3+ regulatory T cells protect the proinflammatory activation of human umbilical vein endothelial cells

OBJECTIVE

To investigate the role of CD4(+)CD25(+)forkhead box P 3 (Foxp3)(+) T-regulatory cells (Tregs) in protecting the activation and function of human umbilical vein endothelial cells (HUVECs) induced by proinflammatory stimulus and the mechanisms of it.

METHODS AND RESULTS

ECs play a major role in atherogenic initiation, changing their quiescence into activated phenotypes to support every phase of the inflammatory process. HUVECs were incubated alone, with Tregs or CD4(+)CD25(-) T cells in the presence of anti-CD3 monoclonal antibodies for 48 hours, and then were stimulated with or without oxidized low-density lipoprotein/lipopolysaccharide for an additional 24 hours. Tregs are able to induce alternative expression of immune phenotypic markers of activated HUVECs by down modulating CD86 and to inhibit the adhesion molecule, such as vascular cell adhesion molecule-1 (VCAM-1) and proinflammatory cytokine (eg, monocyte chemoattractant protein-1 and interleukin 6), response of HUVECs to oxidized low-density lipoprotein/lipopolysaccharide. Moreover, Tregs downregulate proinflammatory factor nuclear factor-κB activation and induce resistance to suppression of anti-inflammatory factor Kruppellike factor 2 in HUVECs induced by a proinflammatory stimulus. Mechanism studies reveal that Treg-mediated suppression of HUVEC proinflammatory cytokines and adhesion molecule expression impaired by oxidized low-density lipoprotein/lipopolysaccharide require cell contact by cytotoxic T-lymphocyte antigen-4 and CD86 and by soluble factors (mainly interleukin 10 and transforming growth factor [TGF]-β).

CONCLUSIONS

Tregs may exert their protective effects against atherogenesis in part through inducing an immune-inhibitory phenotype of ECs involving cytotoxic T-lymphocyte antigen-4-dependent cell-to-cell contact and also requiring soluble factors (mainly interleukin 10 and TGF-β).

Cytomegalovirus infection leads to microvascular dysfunction and exacerbates hypercholesterolemia-induced responses

Cytomegalovirus (CMV) persistently infects more than 60% of the worldwide population. In immunocompetent hosts, it has been implicated in several diseases, including cardiovascular disease, possibly through the induction of inflammatory pathways. Cardiovascular risk factors promote an inflammatory phenotype in the microvasculature long before clinical disease is evident. This study determined whether CMV also impairs microvascular homeostasis and synergizes with hypercholesterolemia to exaggerate these responses. Intravital microscopy was used to assess endothelium-dependent and -independent arteriolar vasodilation and venular leukocyte and platelet adhesion in mice after injection with either mock inoculum or murine CMV (mCMV). Mice were fed a normal (ND) or high-cholesterol (HC) diet beginning at 5 weeks postinfection (p.i.), or a HC diet for the final 4 weeks of infection. mCMV-ND mice exhibited impaired endothelium-dependent vasodilation versus mock-ND at 9 and 12 weeks and endothelium-independent arteriolar dysfunction by 24 weeks. Transient mild leukocyte adhesion occurred in mCMV-ND venules at 7 and 21 weeks p.i. HC alone caused temporary arteriolar dysfunction and venular leukocyte and platelet recruitment, which were exaggerated and prolonged by mCMV infection. The time of introduction of HC after mCMV infection determined whether mCMV+HC led to worse venular inflammation than either factor alone. These findings reveal a proinflammatory influence of persistent mCMV on the microvasculature, and suggest that mCMV infection enhances microvasculature susceptibility to both inflammatory and thrombogenic responses caused by hypercholesterolemia.

PI3K-dependent upregulation of Mcl-1 by human cytomegalovirus is mediated by epidermal growth factor receptor and inhibits apoptosis in short-lived monocytes

Monocytes are a primary target for human CMV (HCMV) infection and are a key cell type responsible for hematogenous dissemination of the virus. Biologically, these cells have a short lifespan of 1-3 d in the circulation, yet infected cells remain viable for weeks despite the lack of viral antiapoptotic gene expression during this period. To understand the mechanism by which HCMV inhibits the initial phase of monocyte apoptosis, we focused on the viral modulation of early prosurvival cell signaling events after infection. We demonstrate in this study that the viral upregulation of the PI3K pathway promotes an early block in apoptosis after infection. Temporal transcriptome and protein analyses revealed Mcl-1, a member of the Bcl-2 family, was transiently induced in a PI3K-dependent manner during the early stages of HCMV infection. In accord with the survival studies, virally induced levels of Mcl-1 expression dissipated to mock levels by 72 h postinfection. Through the use of Mcl-1-specific small interfering RNA, we confirmed the functional role that Mcl-1 plays as a key early regulator of apoptosis in monocytes. Lastly, we showed that HCMV engagement and activation of the epidermal growth factor receptor during viral binding triggered the upregulation of Mcl-1. Overall, our data indicates that activation of the epidermal growth factor receptor/PI3K signaling pathway, via the PI3K-dependent upregulation of Mcl-1, is required to circumvent apoptosis in naturally short-lived monocytes during the early stages of HCMV infection, thus ensuring the early steps in the viral persistence strategy.

The regulation of leucocyte transendothelial migration by endothelial signalling events

Leucocytes use sophisticated mechanisms to cross the endothelium lining the vasculature. This is initiated by chemokine- and adhesion molecule-induced intracellular signalling that controls adhesion, spreading, and motility. At the same time, adherent leucocytes trigger the endothelium, manipulating the barrier to promote their transmigration into the underlying tissues. Over the past years, our insights in the associated signalling events within the endothelium have increased considerably, albeit the order of events, their crosstalk, and the consequences for endothelial cells and leucocytes are only partially resolved. Here, we briefly review endothelial signalling that is initiated at the apical endothelial membrane, where the first contact with the leucocytes takes place and signal transduction is induced. In addition, we discuss subsequent events at endothelial cell-cell junctions insofar as they have been linked to transendothelial migration. Finally, we briefly touch upon the modulation of endothelial signalling by infectious pathogens, since these have developed additional, elegant ways to manipulate the endothelium and transendothelial migration that may provide new, relevant insights into this process.

Activation of EGFR on monocytes is required for human cytomegalovirus entry and mediates cellular motility

Human cytomegalovirus (HCMV) rapidly induces a mobile and functionally unique proinflammatory monocyte following infection that is proposed to mediate viral spread. The cellular pathways used by HCMV to initiate these biological changes remain unknown. Here, we document the expression of the epidermal growth factor receptor (EGFR) on the surface of human peripheral blood monocytes but not on other blood leukocyte populations. Inhibition of EGFR signaling abrogated viral entry into monocytes, indicating that EGFR can serve as a cellular tropism receptor. Moreover, HCMV-activated EGFR was required for the induction of monocyte motility and transendothelial migration, two biological events required for monocyte extravasation into peripheral tissue, and thus viral spread. Transcriptome analysis revealed that HCMV-mediated EGFR signaling up-regulated neural Wiskott-Aldrich syndrome protein (N-WASP), an actin nucleator whose expression and function are normally limited in leukocytes. Knockdown of N-WASP expression blocked HCMV-induced but not phorbol 12-myristate 13-acetate (PMA)-induced monocyte motility, suggesting that a switch to and/or the distinct use of a new actin nucleator controlling motility occurs during HCMV infection of monocytes. Together, these data provide evidence that EGFR plays an essential role in the immunopathobiology of HCMV by mediating viral entry into monocytes and stimulating the aberrant biological activity that promotes hematogenous dissemination.

Up-regulation of hyaluronan receptors in Toxoplasma gondii-infected monocytic cells

The apicomplexan, obligate intracellular parasite Toxoplasma gondii orally infects humans and animals. The parasites cross the intestinal epithelium, invade leukocytes in the general circulation and then disseminate into the peripheral organs. The mechanism of extravasation of the infected leukocytes, however, remains poorly understood. It is known that adhesion of leukocytes to extracellular matrix (ECM) is an important factor in extravasation, and CD44 and ICAM-1 on the leukocyte surface are known receptors for hyaluronan (HA), an ECM component. In this study, we demonstrated up-regulation of CD44 and ICAM-1 expression on the surface of T. gondii-infected human monocytic THP-1 cells and fresh isolated human monocyte. T. gondii-infected THP-1 cells adhered more efficiently to immobilized HA than did non-infected cells. T. gondii-infected monocytes in the general circulation might preferentially adhere to the ECM and migrate out from blood vessels, so transporting parasites into the peripheral organs.

NF-kappaB and phosphatidylinositol 3-kinase activity mediates the HCMV-induced atypical M1/M2 polarization of monocytes

Human cytomegalovirus infection of monocytes stimulates a unique monocyte differentiation reprogramming resulting in polarization towards an M1 pro-inflammatory macrophage that simultaneously exhibits characteristics of an M2 anti-inflammatory macrophage. Our laboratory has previously shown that HCMV infection stimulates monocyte NF-kappaB and PI(3)K activities and now provides evidence that these cellular factors are essential for the HCMV-induced polarization of infected monocytes/macrophages. We find that the induction of NF-kappaB and PI(3)K activities following HCMV infection was required for the initiation of monocyte-to-macrophage differentiation. HCMV-infected monocytes treated with Bay11-7802 (an inhibitor of NF-kappaB activity) or LY294002 [an inhibitor of PI(3)K activity] prior to infection exhibited a small, round and monocyte-like undifferentiated morphology and the lack of CD68 upregulation (a macrophage differentiation marker). Detailed transcriptome analysis revealed 48%, 7% and 31% of HCMV-induced M1-associated genes were dependent on NF-kappaB, PI(3)K or both activities, respectively; while 100% of HCMV-induced M2-associated genes required both NF-kappaB and PI(3)K activities. Functionally, we demonstrated that NF-kappaB and PI(3)K activities were critical for the production of M1- and M2-associated cytokines/chemokines, in HCMV-induced differentiating monocytes. Supernatant from HCMV-infected monocytes pretreated with Bay11-7802 or LY294002 exhibited an 80% and 67% reduction in cell motility-inducing activity. Overall, these data show that HCMV usurps monocyte NF-kappaB and PI(3)K signal transduction pathways to induce the unique polarization of HCMV-infected monocytes needed for the earliest steps in the viral dissemination and persistence strategy.

Bcl-3-regulated transcription from major immediate-early promoter of human cytomegalovirus in monocyte-derived macrophages

Monocytes/macrophages are key cells in the pathogenesis of human CMV (HCMV) infection, but the in vitro rate of viral production in primary human monocyte-derived macrophages (MDM) is considerably lower than in fibroblasts. Considering that the NF-kappaB signaling pathway is potentially involved in the replication strategy of HCMV through efficient transactivation of the major immediate-early promoter (MIEP), efficient viral replication, and late gene expression, we investigated the composition of the NF-kappaB complex in HCMV-infected MDMs and fibroblasts. Preliminary studies showed that HCMV could grow in primary MDM culture but that the viral titer in culture supernatants was lower than that observed in the supernatants of more permissive MRC5 fibroblasts. EMSA and microwell colorimetric NF-kappaB assay demonstrated that HCMV infection of MDMs increased p52 binding activity without activating the canonical p50/p65 complex. Moreover, Bcl-3 was up-regulated and was demonstrated to associate with p52, indicating p52/Bcl-3 complexes as the major component of the NF-kappaB complex in MDMs. Luciferase assays in promonocytic U937 cells transfected with an MIEP-luciferase reporter construct demonstrated MIEP activation in response to p52 and Bcl-3 overexpression. Chromatin immunoprecipitation assay demonstrated that p52 and Bcl-3 bind the MIEP in acutely HCMV-infected MDMs. In contrast, HCMV infection of MRC5 fibroblasts resulted in activation of p50/p65 heterodimers. Thus, activation of p52/Bcl-3 complexes in MDMs and p50/p65 heterodimers in fibroblasts in response to HCMV infection might explain the low-level growth of the virus in MDMs vs efficient growth in fibroblasts.

Activation of intracellular signaling pathways by the murine cytomegalovirus G protein-coupled receptor M33 occurs via PLC-{beta}/PKC-dependent and -independent mechanisms

The presence of numerous G protein-coupled receptor (GPCR) homologs within the herpesvirus genomes suggests an essential role for these genes in viral replication in the infected host. Such is the case for murine cytomegalovirus (MCMV), where deletion of the M33 GPCR or replacement of M33 with a signaling defective mutant has been shown to severely attenuate replication in vivo. In the present study we utilized a genetically altered version of M33 (termed R131A) in combination with pharmacological inhibitors to further characterize the mechanisms by which M33 activates downstream signaling pathways. This R131A mutant of M33 fails to support salivary gland replication in vivo and, as such, is an important tool that can be used to examine the signaling activities of M33. We show that M33 stimulates the transcription factor CREB via heterotrimeric G(q/11) proteins and not through promiscuous coupling of M33 to the G(s) pathway. Using inhibitors of signaling molecules downstream of G(q/11), we demonstrate that M33 stimulates CREB transcriptional activity in a phospholipase C-beta and protein kinase C (PKC)-dependent manner. Finally, utilizing wild-type and R131A versions of M33, we show that M33-mediated activation of other signaling nodes, including the mitogen-activated protein kinase family member p38alpha and transcription factor NF-kappaB, occurs in the absence of G(q/11) and PKC signaling. The results from the present study indicate that M33 utilizes multiple mechanisms to modulate intracellular signaling cascades and suggest that signaling through PLC-beta and PKC plays a central role in MCMV pathogenesis in vivo.