Genome sequences of human cytomegalovirus strain TB40/E variants propagated in fibroblasts and epithelial cells

Inhibition of human cytomegalovirus entry into mucosal epithelial cells

Human cytomegalovirus (CMV) causes serious developmental disabilities in newborns infected in utero following oral acquisition by the mother. Thus, neutralizing antibodies in maternal saliva have potential to prevent maternal infection and, consequently, fetal transmission and disease. Based on standard cell culture models, CMV entry mediators (and hence neutralizing targets) are cell type-dependent: entry into fibroblasts requires glycoprotein B (gB) and a trimeric complex (TC) of glycoproteins H, L, and O, whereas endothelial and epithelial cell entry additionally requires a pentameric complex (PC) of glycoproteins H and L with UL128, UL130, and UL131A. However, as the mediators of mucosal cell entry and the potential impact of cellular differentiation remained unclear, the present studies utilized mutant viruses, neutralizing antibodies, and soluble TC-receptor to determine the entry mediators required for infection of mucocutaneus cell lines and primary tonsil epithelial cells. Entry into undifferentiated cells was largely PC-dependent, but PC-independent entry could be induced by differentiation. TC-independent entry was also observed and varied by cell line and differentiation. Infection of primary tonsil cells from some donors was entirely TC-independent. In contrast, an antibody to gB or disruption of virion attachment using heparin blocked entry into all cells. These findings indicate that CMV entry into the spectrum of cell types encountered in vivo is likely to be more complex than has been suggested by standard cell culture models and may be influenced by the relative abundance of virion envelope glycoprotein complexes as well as by cell type, tissue of origin, and state of differentiation.

Subconfluent ARPE-19 Cells Display Mesenchymal Cell-State Characteristics and Behave like Fibroblasts, Rather Than Epithelial Cells, in Experimental HCMV Infection Studies

Human cytomegalovirus (HCMV) has a broad cellular tropism and epithelial cells are important physiological targets during infection. The retinal pigment epithelial cell line ARPE-19 has been used to model HCMV infection in epithelial cells for decades and remains a commonly used cell type for studying viral entry, replication, and the cellular response to infection. We previously found that ARPE-19 cells, despite being derived from an epithelial cell explant, express extremely low levels of canonical epithelial proteins, such as E-cadherin and EpCAM. Here, we perform comparative studies of ARPE-19 and additional epithelial cell lines with strong epithelial characteristics. We find that ARPE-19 cells cultured under subconfluent conditions resemble mesenchymal fibroblasts, rather than epithelial cells; this is consistent with previous studies showing that ARPE-19 cultures require extended periods of high confluency culture to maintain epithelial characteristics. By reanalyzing public gene expression data and using machine learning, we find evidence that ARPE-19 cultures maintained across many labs exhibit mesenchymal characteristics and that the majority of studies employing ARPE-19 use them in a mesenchymal state. Lastly, by performing experimental HCMV infections across mesenchymal and epithelial cell lines, we find that ARPE-19 cells behave like mesenchymal fibroblasts, producing logarithmic yields of cell-free infectious progeny, while cell lines with strong epithelial character exhibit an atypical infectious cycle and naturally restrict the production of cell-free progeny. Our work highlights important characteristics of the ARPE-19 cell line and suggests that subconfluent ARPE-19 cells may not be optimal for modeling epithelial infection with HCMV or other human viruses. It also suggests that HCMV biosynthesis and/or spread may occur quite differently in epithelial cells compared to mesenchymal cells. These differences could contribute to viral persistence or pathogenesis in epithelial tissues.

Current Knowledge on the Interaction of Human Cytomegalovirus Infection, Encoded miRNAs, and Acute Aortic Syndrome

Aortic dissection is a clinicopathological entity caused by rupture of the intima, leading to a high mortality if not treated. Over time, diagnostic and investigative methods, antihypertensive therapy, and early referrals have resulted in improved outcomes according to registry data. Some data have also emerged from recent studies suggesting a link between Human Cytomegalovirus (HCMV) infection and aortic dissection. Furthermore, the use of microRNAs has also become increasingly widespread in the literature. These have been noted to play a role in aortic dissections with elevated levels noted in studies as early as 2017. This review aims to provide a broad and holistic overview of the role of miRNAs, while studying the role of HCMV infection in the context of aortic dissections. The roles of long non-coding RNAs, circular RNAs, and microRNAs are explored to identify changes in expression during aortic dissections. The use of such biomarkers may one day be translated into clinical practice to allow early detection and prognostication of outcomes and drive preventative and therapeutic options in the future.

Genetic and Functional Characterization of Congenital HCMV Clinical Strains in Ex Vivo First Trimester Placental Model

Human cytomegalovirus (HCMV) is the leading cause of congenital viral infection, leading to a variety of symptoms in the unborn child that range from asymptomatic to death in utero. Our objective was to better understand the mechanisms of placental infection by HCMV clinical strains, particularly during the first trimester of pregnancy. We thus characterized and compared the replication kinetics of various HCMV clinical strains and laboratory strains by measuring viral loads in an ex vivo model of first trimester villi and decidua, and used NGS and PCA analysis to analyze the genes involved in cell tropism and virulence factors. We observed that first trimester villi and decidua are similarly permissive to laboratory and symptomatic strains, and that asymptomatic strains poorly replicate in decidua tissue. PCA analysis allowed us to segregate our clinical strains based on their clinical characteristics, suggesting a link between gene mutations and symptoms. All these results bring forth elements that can help better understand the mechanisms that induce the appearance of symptoms or in the congenitally infected newborn.

A Virus Genetic System to Analyze the Fusogenicity of Human Cytomegalovirus Glycoprotein B Variants

Viruses can induce the fusion of infected and neighboring cells, leading to the formation of syncytia. Cell-cell fusion is mediated by viral fusion proteins on the plasma membrane of infected cells that interact with cellular receptors on neighboring cells. Viruses use this mechanism to spread rapidly to adjacent cells or escape host immunity. For some viruses, syncytium formation is a hallmark of infection and a known pathogenicity factor. For others, the role of syncytium formation in viral dissemination and pathogenicity remains poorly understood. Human cytomegalovirus (HCMV) is an important cause of morbidity and mortality in transplant patients and the leading cause of congenital infections. Clinical HCMV isolates have broad cell tropism but differ in their ability to induce cell-cell fusions, and little is known about the molecular determinants. We developed a system to analyze HCMV glycoprotein B (gB) variants in a defined genetic background. HCMV strains TB40/E and TR were used as vectors to compare the fusogenicity of six gB variants from congenitally infected fetuses with those from three laboratory strains. Five of them conferred the ability to induce the fusion of MRC-5 human embryonic lung fibroblasts to one or both backbone strains, as determined by a split GFP-luciferase reporter system. The same gB variants were not sufficient to induce syncytia in infected ARPE-19 epithelial cells, suggesting that additional factors are involved. The system described here allows a systematic comparison of the fusogenicity of viral envelope glycoproteins and may help to clarify whether fusion-promoting variants are associated with increased pathogenicity.

Strain-Dependent Restriction of Human Cytomegalovirus by Zinc Finger Antiviral Proteins

Cellular antiviral factors that recognize viral nucleic acid can inhibit virus replication. These include the zinc finger antiviral protein (ZAP), which recognizes high CpG dinucleotide content in viral RNA. Here, we investigated the ability of ZAP to inhibit the replication of human cytomegalovirus (HCMV). Depletion of ZAP or its cofactor KHNYN increased the titer of the high-passage HCMV strain AD169 but had little effect on the titer of the low-passage strain Merlin. We found no obvious difference in expression of several viral proteins between AD169 and Merlin in ZAP knockdown cells, but observed a larger increase in infectious virus in AD169 compared to Merlin in the absence of ZAP, suggesting that ZAP inhibited events late in AD169 replication. In addition, there was no clear difference in the CpG abundance of AD169 and Merlin RNAs, indicating that genomic content of the two virus strains was unlikely to be responsible for differences in their sensitivity to ZAP. Instead, we observed less ZAP expression in Merlin-infected cells late in replication compared to AD169-infected cells, which may be related to different abilities of the two virus strains to regulate interferon signaling. Therefore, there are strain-dependent differences in the sensitivity of HCMV to ZAP, and the ability of low-passage HCMV strain Merlin to evade inhibition by ZAP is likely related to its ability to regulate interferon signaling, not the CpG content of RNAs produced from its genome. IMPORTANCE Determining the function of cellular antiviral factors can inform our understanding of virus replication. The zinc finger antiviral protein (ZAP) can inhibit the replication of diverse viruses. Here, we examined ZAP interaction with the DNA virus human cytomegalovirus (HCMV). We found HCMV strain-dependent differences in the ability of ZAP to influence HCMV replication, which may be related to the interaction of HCMV strains with the type I interferon system. These observations affect our current understanding of how ZAP restricts HCMV and how HCMV interacts with the type I interferon system.

Fibroblast, Epithelial and Endothelial Cell-Derived Human Cytomegalovirus Strains Display Distinct Neutralizing Antibody Responses and Varying Levels of gH/gL Complexes

In sequential sera from pregnant women with HCMV primary infection (PI), the serum neutralizing activity is higher against virions produced in epithelial and endothelial cells than in fibroblasts. Immunoblotting shows that the pentamer complex/trimer complex (PC/TC) ratio varies according to the producer cell culture type used for the virus preparation to be employed in the neutralizing antibody (NAb) assay, and is lower in fibroblasts and higher in epithelial, and especially endothelial cells. The blocking activity of TC- and PC-specific inhibitors varies according to the PC/TC ratio of virus preparations. The rapid reversion of the virus phenotype following its back passage to the original cell culture (fibroblasts) potentially argues in favor of a producer cell effect on virus phenotype. However, the role of genetic factors cannot be overlooked. In addition to the producer cell type, the PC/TC ratio may differ in single HCMV strains. In conclusion, the NAb activity not only varies with different HCMV strains, but is a dynamic parameter changing according to virus strain, type of target and producer cells, and number of cell culture passages. These findings may have some important implications for the development of both therapeutic antibodies and subunit vaccines.

Human Cytomegalovirus Replication and Infection-Induced Syncytia Formation in Labial, Foreskin, and Fetal Lung Fibroblasts

Only a handful of cell types, including fibroblasts, epithelial, and endothelial cells, can support human cytomegalovirus (CMV) replication in vitro, in striking contrast to the situation in vivo. While the susceptibility of epithelial and endothelial cells to CMV infection is strongly modulated by their anatomical site of origin, multiple CMV strains have been successfully isolated and propagated on fibroblasts derived from different organs. As oral mucosal cells are likely involved in CMV acquisition, we sought to evaluate the ability of infant labial fibroblasts to support CMV replication, compared to that of commonly used foreskin and fetal lung fibroblasts. No differences were found in the proportion of cells initiating infection, or in the amounts of viral progeny produced after exposure to the fibroblast-adapted CMV strain AD169 or to the endothelial cell-adapted strain TB40/E. Syncytia formation was, however, significantly enhanced in infected labial and lung fibroblasts compared to foreskin-derived cells, and did not occur after infection with AD169. Together, these data indicate that fibroblast populations derived from different tissues are uniformly permissive to CMV infection but retain phenotypic differences of potential importance for infection-induced cell-cell fusion, and ensuing viral spread and pathogenesis in different organs.