Cytomegalovirus host entry and spread

Immune surveillance of cytomegalovirus in tissues

Cytomegalovirus (CMV), a representative member of the Betaherpesvirinae subfamily of herpesviruses, is common in the human population, but immunocompetent individuals are generally asymptomatic when infected with this virus. However, in immunocompromised individuals and immunologically immature fetuses and newborns, CMV can cause a wide range of often long-lasting morbidities and even death. CMV is not only widespread throughout the population but it is also widespread in its hosts, infecting and establishing latency in nearly all tissues and organs. Thus, understanding the pathogenesis of and immune responses to this virus is a prerequisite for developing effective prevention and treatment strategies. Multiple arms of the immune system are engaged to contain the infection, and general concepts of immune control of CMV are now reasonably well understood. Nonetheless, in recent years, tissue-specific immune responses have emerged as an essential factor for resolving CMV infection. As tissues differ in biology and function, so do immune responses to CMV and pathological processes during infection. This review discusses state-of-the-art knowledge of the immune response to CMV infection in tissues, with particular emphasis on several well-studied and most commonly affected organs.

Virological, innate, and adaptive immune profiles shaped by variation in route and age of host in murine cytomegalovirus infection

Human cytomegalovirus (hCMV) is a ubiquitous facultative pathogen, which establishes a characteristic latent and reactivating lifelong infection in immunocompetent hosts. Murine CMV (mCMV) infection is widely used as an experimental model of hCMV infection, employed to investigate the causal nature and extent of CMV's contribution to inflammatory, immunological, and health disturbances in humans. Therefore, mimicking natural human infection in mice would be advantageous to hCMV research. To assess the role of route and age at infection in modeling hCMV in mice, we infected prepubescent and young sexually mature C57BL/6 (B6) mice intranasally (i.n., a likely physiological route in humans) and intraperitoneally (i.p., a frequently used experimental route, possibly akin to transplant-mediated infection). In our hands, both routes led to comparable early viral loads and tissue spreads. However, they yielded differential profiles of innate and adaptive systemic immune activation. Specifically, the younger, prepubescent mice exhibited the strongest natural killer cell activation in the blood in response to i.p. infection. Further, the i.p. infected animals (particularly those infected at 12 weeks) exhibited larger anti-mCMV IgG and greater expansion of circulating CD8+ T cells specific for both acute (non-inflationary) and latent phase (inflationary) mCMV epitopes. By contrast, tissue immune responses were comparable between i.n. and i.p. groups. Our results illustrate a distinction in the bloodborne immune response profiles across infection routes and ages and are discussed in light of physiological parameters of interaction between CMV, immunity, inflammation, and health over the lifespan.

IMPORTANCE

The majority of experiments modeling human cytomegalovirus (hCMV) infection in mice have been carried out using intraperitoneal infection in sexually mature adult mice, which stands in contrast to the large number of humans being infected with human CMV at a young age, most likely via bodily fluids through the nasopharyngeal/oral route. This study examined the impact of the choice of age and route of infection in modeling CMV infection in mice. By comparing young, prepubescent to older sexually mature counterparts, infected either via the intranasal or intraperitoneal route, we discovered substantial differences in deployment and response intensity of different arms of the immune system in systemic control of the virus; tissue responses, by contrast, appeared similar between ages and infection routes.

Fibroblasts are a site of murine cytomegalovirus lytic replication and Stat1-dependent latent persistence in vivo

To date, no herpesvirus has been shown to latently persist in fibroblastic cells. Here, we show that murine cytomegalovirus, a β-herpesvirus, persists for the long term and across organs in PDGFRα-positive fibroblastic cells, with similar or higher genome loads than in the previously known sites of murine cytomegalovirus latency. Whereas murine cytomegalovirus gene transcription in PDGFRα-positive fibroblastic cells is almost completely silenced at 5 months post-infection, these cells give rise to reactivated virus ex vivo, arguing that they support latent murine cytomegalovirus infection. Notably, PDGFRα-positive fibroblastic cells also support productive virus replication during primary murine cytomegalovirus infection. Mechanistically, Stat1-deficiency promotes lytic infection but abolishes latent persistence of murine cytomegalovirus in PDGFRα-positive fibroblastic cells in vivo. In sum, fibroblastic cells have a dual role as a site of lytic murine cytomegalovirus replication and a reservoir of latent murine cytomegalovirus in vivo and STAT1 is required for murine cytomegalovirus latent persistence in vivo.

Inhibition of Cytomegalovirus by Pentacta pygmaea Fucosylated Chondroitin Sulfate Depends on Its Molecular Weight

Many viruses attach to host cells by first interacting with cell surface proteoglycans containing heparan sulfate (HS) glycosaminoglycan chains and then by engaging with specific receptor, resulting in virus entry. In this project, HS–virus interactions were targeted by a new fucosylated chondroitin sulfate from the sea cucumber Pentacta pygmaea (PpFucCS) in order to block human cytomegalovirus (HCMV) entry into cells. Human foreskin fibroblasts were infected with HCMV in the presence of PpFucCS and its low molecular weight (LMW) fractions and the virus yield at five days post-infection was assessed. The virus attachment and entry into the cells were visualized by labeling the purified virus particles with a self-quenching fluorophore octadecyl rhodamine B (R18). The native PpFucCS exhibited potent inhibitory activity against HCMV specifically blocking virus entry into the cell and the inhibitory activities of the LMW PpFucCS derivatives were proportional to their chain lengths. PpFucCS and the derived oligosaccharides did not exhibit any significant cytotoxicity; moreover, they protected the infected cells from virus-induced lytic cell death. In conclusion, PpFucCS inhibits the entry of HCMV into cells and the high MW of this carbohydrate is a key structural element to achieve the maximal anti-viral effect. This new marine sulfated glycan can be developed into a potential prophylactic and therapeutic antiviral agent against HCMV infection.

Therapeutic targeting of HCMV-encoded chemokine receptor US28: Progress and challenges

The pervasive human cytomegalovirus (HCMV) causes significant morbidity in immunocompromised individuals. Treatment using the current standard-of-care (SOC) is limited by severe toxic adverse effects and anti-viral resistance development. Furthermore, they only affect HCMV in its lytic phase, meaning viral disease is not preventable as latent infection cannot be treated and the viral reservoirs persist. The viral chemokine receptor (vCKR) US28 encoded by HCMV has received much attention in recent years. This broad-spectrum receptor has proven to be a desirable target for development of novel therapeutics through exploitation of its ability to internalize and its role in maintaining latency. Importantly, it is expressed on the surface of infected cells during both lytic and latent infection. US28-targeting small molecules, single-domain antibodies, and fusion toxin proteins have been developed for different treatment strategies, e.g. forcing reactivation of latent virus or using internalization of US28 as a toxin shuttle to kill infected cells. These strategies show promise for providing ways to eliminate latent viral reservoirs and prevent HCMV disease in vulnerable patients. Here, we discuss the progress and challenges of targeting US28 to treat HCMV infection and its associated diseases.

Effect of Cytomegalovirus on the Immune System: Implications for Aging and Mental Health

Human cytomegalovirus (HCMV) is a major modulator of the immune system leading to long-term changes in T-lymphocytes, macrophages, and natural killer (NK) cells among others. Perhaps because of this immunomodulatory capacity, HCMV infection has been linked with a host of deleterious effects including accelerated immune aging (premature mortality, increased expression of immunosenescence-linked markers, telomere shortening, speeding-up of epigenetic "clocks"), decreased vaccine immunogenicity, and greater vulnerability to infectious diseases (e.g., tuberculosis) or infectious disease-associated pathology (e.g., HIV). Perhaps not surprisingly given the long co-evolution between HCMV and humans, the virus has also been associated with beneficial effects, such as increased vaccine responsiveness, heterologous protection against infections, and protection against relapse in the context of leukemia. Here, we provide an overview of this literature. Ultimately, we focus on one other deleterious effect of HCMV, namely the emerging literature suggesting that HCMV plays a pathophysiological role in psychiatric illness, particularly depression and schizophrenia. We discuss this literature through the lens of psychological stress and inflammation, two well-established risk factors for psychiatric illness that are also known to predispose to reactivation of HCMV.

CD4+ T Cells Control Murine Cytomegalovirus Infection Indirectly

CD4⁺ T cells are key to controlling cytomegalovirus infections. Salivary gland infection by murine cytomegalovirus (MCMV) provides a way to identify mechanisms. CD11c⁺ dendritic cells (DC) disseminate MCMV to the salivary glands, where they transfer infection to acinar cells. Antiviral CD4⁺ T cells are often considered to be directly cytotoxic for cells expressing major histocompatibility complex class II (MHCII). However, persistently infected salivary gland acinar cells are MHCII^- and are presumably inaccessible to direct CD4 T cell recognition. Here, we show that CD4⁺ T cell depletion amplified infection of MHCII^- acinar cells but not MHCII⁺ cells. MCMV-infected mice with disrupted MHCII on CD11c⁺ cells showed increased MHCII^- acinar infection; antiviral CD4⁺ T cells were still primed, but their recruitment to the salivary glands was reduced, suggesting that engagement with local MHCII⁺ DC is important for antiviral protection. As MCMV downregulates MHCII on infected DC, the DC participating in CD4 protection may thus be uninfected. NK cells and gamma interferon (IFN-γ) may also contribute to CD4⁺ T cell-dependent virus control: CD4 T cell depletion reduced NK cell recruitment to the salivary glands, and both NK cell and IFN-γ depletion equalized infection between MHCII-disrupted and control mice. Taken together, these results suggest that CD4⁺ T cells protect indirectly against infected acinar cells in the salivary gland via DC engagement, requiring the recruitment of NK cells and the action of IFN-γ. Congruence of these results with an established CD4⁺ T cell/NK cell axis of gammaherpesvirus infection control suggests a common mode of defense against evasive viruses. IMPORTANCE Cytomegalovirus infections commonly cause problems in immunocompromised patients and in pregnancy. We lack effective vaccines. CD4⁺ T cells play an important role in normal infection control, yet how they act has been unknown. Using murine cytomegalovirus as an accessible model, we show that CD4⁺ T cells are unlikely to recognize infected cells directly. We propose that CD4⁺ T cells interact with uninfected cells that present viral antigens and recruit other immune cells to attack infected targets. These data present a new outlook on understanding how CD4⁺ T cell-directed control protects against persistent cytomegalovirus infection.

Pathogenic Exploitation of Lymphatic Vessels

Lymphatic vessels provide a critical line of communication between peripheral tissues and their draining lymph nodes, which is necessary for robust immune responses against infectious agents. At the same time, lymphatics help shape the nature and kinetics of immune responses to ensure resolution, limit tissue damage, and prevent autoimmune responses. A variety of pathogens have developed strategies to exploit these functions, from multicellular organisms like nematodes to bacteria, viruses, and prions. While lymphatic vessels serve as transport routes for the dissemination of many pathogens, their hypoxic and immune-suppressive environments can provide survival niches for others. Lymphatics can be exploited as perineural niches, for inter-organ distribution among highly motile carrier cells, as effective replicative niches, and as alternative routes in response to therapy. Recent studies have broadened our understanding of lymphatic involvement in pathogenic spread to include a wider range of pathogens, as well as new mechanisms of exploitation, which we summarize here.

Olfactory function in congenital cytomegalovirus infection: a prospective study

Congenital cytomegalovirus (CMV) infection leads to olfactory bulb lesions in the fetus, yet little is known about its impact on olfaction after birth. Here, we have assessed in a prospective study conducted on children in two French hospitals from 2016 to 2019, infection severity and olfactory performance after congenital CMV infection. Children with congenital CMV infection aged 3 to 10 years and healthy controls (CTL) matched for age and sex to CMV children symptomatic at birth (sCMV) were enrolled. Olfactory discrimination was assessed using mono-odorants and binary mixtures. Data were analyzed for 54 children with PCR-confirmed congenital CMV infection, including 34 sCMV (median [IQR] age, 6 [5-8] years; 19 [55.9%] male), and 20 CMV asymptomatic at birth (aCMV, median [IQR] age, 4 [3-6] years; 12 [60.0%] male). sCMV were compared to 34 CTL children. Olfactory scores in CMV-infected children were independent from vestibular deficit and hearing loss. The olfactory score was efficient to discriminate between CTL and sCMV for children > 6 years (area under the receiver-operating characteristic curve (AUC, 0.85; P = 0.0006), but not for children < 7 years. For children > 6 years, the proportion of children with total olfactory score < 4 differed between sCMV and CTL groups (91.2% and 18.7%, P < 0.001), but not between aCMV and age-matched healthy control groups.   Conclusion: Congenital CMV infection is associated with reduced olfactory performance in children with infection symptoms at birth.   Clinical trial registration: NCT02782988 (registration date: May 26, 2016). What is Known: •Congenital cytomegalovirus infection leads to olfactory bulb lesions in the fetus, yet little is known about its impact on olfaction after birth. •Depending on neonatal clinical presentation, children are either categorized as having a symptomatic or asymptomatic infection at birth. What is New: •Congenital cytomegalovirus infection is associated with reduced olfactory performance in children with infection symptoms at birth.

Olfactory Entry Promotes Herpesvirus Recombination

Herpesvirus genomes show abundant evidence of past recombination. Its functional importance is unknown. A key question is whether recombinant viruses can outpace the immunity induced by their parents to reach higher loads. We tested this by coinfecting mice with attenuated mutants of murid herpesvirus 4 (MuHV-4). Infection by the natural olfactory route routinely allowed mutant viruses to reconstitute wild-type genotypes and reach normal viral loads. Lung coinfections rescued much less well. Attenuated murine cytomegalovirus mutants similarly showed recombinational rescue via the nose but not the lungs. These infections spread similarly, so route-specific rescue implied that recombination occurred close to the olfactory entry site. Rescue of replication-deficient MuHV-4 confirmed this, showing that coinfection occurred in the first encountered olfactory cells. This worked even with asynchronous inoculation, implying that a defective virus can wait here for later rescue. Virions entering the nose get caught on respiratory mucus, which the respiratory epithelial cilia push back toward the olfactory surface. Early infection was correspondingly focused on the anterior olfactory edge. Thus, by concentrating incoming infection into a small area, olfactory entry seems to promote functionally significant recombination. IMPORTANCE All organisms depend on genetic diversity to cope with environmental change. Small viruses rely on frequent point mutations. This is harder for herpesviruses because they have larger genomes. Recombination provides another means of genetic optimization. Human herpesviruses often coinfect, and they show evidence of past recombination, but whether this is rare and incidental or functionally important is unknown. We showed that herpesviruses entering mice via the natural olfactory route meet reliably enough for recombination routinely to repair crippling mutations and restore normal viral loads. It appeared to occur in the first encountered olfactory cells and reflected a concentration of infection at the anterior olfactory edge. Thus, natural host entry incorporates a significant capacity for herpesvirus recombination.

Rat Cytomegalovirus Virion-Associated Proteins R131 and R129 Are Necessary for Infection of Macrophages and Dendritic Cells

Cytomegalovirus (CMV) establishes persistent, latent infection in hosts, causing diseases in immunocompromised patients, transplant recipients, and neonates. CMV infection modifies the host chemokine axis by modulating chemokine and chemokine receptor expression and by encoding putative chemokine and chemokine receptor homologues. The viral proteins have roles in cellular signaling, migration, and transformation, as well as viral dissemination, tropism, latency and reactivation. Herein, we review the contribution of CMV-encoded chemokines and chemokine receptors to these processes, and further elucidate the viral tropism role of rat CMV (RCMV) R129 and R131. These homologues of the human CMV (HCMV)-encoded chemokines UL128 and UL130 are of particular interest because of their dual role as chemokines and members of the pentameric entry complex, which is required for entry into cell types that are essential for viral transmission and dissemination. The contributions of UL128 and UL130 to acceleration of solid organ transplant chronic rejection are poorly understood, and are in need of an effective in vivo model system to elucidate the phenomenon. We demonstrated similar molecular entry requirements for R129 and R131 in the rat cells, as observed for HCMV, and provided evidence that R129 and R131 are part of the viral entry complex required for entry into macrophages, dendritic cells, and bone marrow cells.

Understanding human γδ T cell biology toward a better management of cytomegalovirus infection

Cytomegalovirus (CMV) infection is responsible for significant morbidity and mortality in immunocompromised patients, namely solid organ and hematopoietic cell transplant recipients, and can induce congenital infection in neonates. There is currently an unmet need for new management and treatment strategies. Establishment of an anti-CMV immune response is critical in order to control CMV infection. The two main human T cells involved in HCMV-specific response are αβ and non-Vγ9Vδ2 T cells that belong to γδ T cell compartment. CMV-induced non-Vγ9Vδ2 T cells harbor a specific clonal expansion and a phenotypic signature, and display effector functions against CMV. So far, only two main molecular mechanisms underlying CMV sensing have been identified. Non-Vγ9Vδ2 T cells can be activated either by stress-induced surface expression of the γδT cell receptor (TCR) ligand annexin A2, or by a multimolecular stress signature composed of the γδTCR ligand endothelial protein C receptor and co-stimulatory signals such as the ICAM-1-LFA-1 axis. All this basic knowledge can be harnessed to improve the clinical management of CMV infection in at-risk patients. In particular, non-Vγ9Vδ2 T cell monitoring could help better stratify the risk of infection and move forward a personalized medicine. Moreover, recent advances in cell therapy protocols open the way for a non-Vγ9Vδ2 T cell therapy in immunocompromised patients.

Involvement of heparan sulfate during mouse cytomegalovirus infection in murine-derived immortalized neuronal cell line

Cytomegalovirus infection cause of severe developmental disorders of the CNS. Aim: In this study, we utilized a differentiated mouse-derived hippocampal cell line (dHT22) to understand mouse CMV (MCMV) infection. Results: The expression of immediate early genes ( IE) 1 and 3 confirmed the time-dependent susceptibility of dHT22 cells to MCMV infection. MCMV infection alters the cellular distribution of heparan sulfate (HS). In addition, pretreatment with heparinase significantly reduces virus infectivity. Conclusion: The compartmentalization of HS in MCMV infected cells suggests multiple roles of HS in virus life cycle ranging from viral entry to viral transport and cellular remodeling. An enzymatic heparinase assay confirmed that HS is critical for viral entry and trafficking.

Inhibition of Human Cytomegalovirus Entry into Host Cells Through a Pleiotropic Small Molecule

Human cytomegalovirus (HCMV) infections are wide-spread among the general population with manifestations ranging from asymptomatic to severe developmental disabilities in newborns and life-threatening illnesses in individuals with a compromised immune system. Nearly all current drugs suffer from one or more limitations, which emphasizes the critical need to develop new approaches and new molecules. We reasoned that a ‘poly-pharmacy’ approach relying on simultaneous binding to multiple receptors involved in HCMV entry into host cells could pave the way to a more effective therapeutic outcome. This work presents the study of a synthetic, small molecule displaying pleiotropicity of interactions as a competitive antagonist of viral or cell surface receptors including heparan sulfate proteoglycans and heparan sulfate-binding proteins, which play important roles in HCMV entry and spread. Sulfated pentagalloylglucoside (SPGG), a functional mimetic of heparan sulfate, inhibits HCMV entry into human foreskin fibroblasts and neuroepithelioma cells with high potency. At the same time, SPGG exhibits no toxicity at levels as high as 50-fold more than its inhibition potency. Interestingly, cell-ELISA assays showed downregulation in HCMV immediate-early gene 1 and 2 (IE 1&2) expression in presence of SPGG further supporting inhibition of viral entry. Finally, HCMV foci were observed to decrease significantly in the presence of SPGG suggesting impact on viral spread too. Overall, this work offers the first evidence that pleiotropicity, such as demonstrated by SPGG, may offer a new poly-therapeutic approach toward effective inhibition of HCMV.

Murine cytomegalovirus disseminates independently of CX3CR1, CCL2 or its m131/m129 chemokine homologue

Cytomegaloviruses (CMVs) use myeloid cells to move within their hosts. Murine CMV (MCMV) colonizes the salivary glands for long-term shedding, and reaches them via CD11c⁺ infected cells. A need to recruit patrolling monocytes for systemic spread has been proposed, based on poor salivary gland infection in fractalkine receptor (CX3CR1)-deficient mice. We found no significant CX3CR1 dependence of salivary gland infection. CCL2 and the viral m131/m129 chemokine homologue were also redundant for acute MCMV spread, arguing against a need for inflammation or infection to recruit additional monocytes to the entry site. M131/m129 promoted salivary gland infection, but only after the initial seeding of infected cells to this site. Our data support the idea that MCMV disseminates by infecting and mobilizing tissue-resident dendritic cells.

Murine Cytomegalovirus Spread Depends on the Infected Myeloid Cell Type

Cytomegaloviruses (CMVs) colonize blood-borne myeloid cells. Murine CMV (MCMV) spreads from the lungs via infected CD11c⁺ cells, consistent with an important role for dendritic cells (DC). We show here that MCMV entering via the olfactory epithelium, a natural transmission portal, also spreads via infected DC. They reached lymph nodes, entered the blood via high endothelial venules, and then entered the salivary glands, driven by constitutive signaling of the viral M33 G protein-coupled receptor (GPCR). Intraperitoneal infection also delivered MCMV to the salivary glands via DC. However, it also seeded F4/80⁺ infected macrophages to the blood; they did not enter the salivary glands or require M33 for extravasation. Instead, they seeded infection to a range of other sites, including brown adipose tissue (BAT). Peritoneal cells infected ex vivo then adoptively transferred showed similar cell type-dependent differences in distribution, with abundant F4/80⁺ cells in BAT and CD11c⁺ cells in the salivary glands. BAT colonization by CMV-infected cells was insensitive to pertussis toxin inhibition of the GPCR signaling through G_i/o substrate, whereas salivary gland colonization was sensitive. Since salivary gland infection required both M33 and G_i/o-coupled signaling, whereas BAT infection required neither, these migrations were mechanistically distinct. MCMV spread from the lungs or nose depended on DC, controlled by M33. Infecting other monocyte populations resulted in unpredictable new infections.IMPORTANCE Cytomegaloviruses (CMVs) spread through the blood by infecting monocytes, and this can lead to disease. With murine CMV (MCMV) we can track infected myeloid cells and so understand how CMVs spread. Previous experiments have injected MCMV into the peritoneal cavity. MCMV normally enters mice via the olfactory epithelium. We show that olfactory infection spreads via dendritic cells, which MCMV directs to the salivary glands. Peritoneal infection similarly reached the salivary glands via dendritic cells. However, it also infected other monocyte types, and they spread infection to other tissues. Thus, infecting the "wrong" monocytes altered virus spread, with potential to cause disease. These results provide a basis for understanding how the monocyte types infected by human CMV might promote different infection outcomes.

Emerging roles of cytomegalovirus-encoded G protein-coupled receptors during lytic and latent infection

Cytomegaloviruses (CMVs) have developed multiple diverse strategies to ensure their replicative success and to evade immune recognition. Given the fact that G protein-coupled receptors (GPCRs) are key regulators of numerous cellular processes and modify a variety of signaling pathways, it is not surprising that CMVs and other herpesviruses have hijacked mammalian GPCRs during their coevolution. Human cytomegalovirus (HCMV) encodes for four viral GPCR homologues (vGPCRs), termed US27, US28, UL33, and UL78. Although HCMV-encoded GPCRs were first described in 1990, the pivotal functions of these viral receptor proteins were detected only recently. Here, we summarize seminal knowledge on the functions of herpesviral vGPCRs with a focus on novel roles of cytomegalovirus-encoded vGPCRs for viral spread and the regulation of latency.