Stepwise adaptation of murine cytomegalovirus to cells of a foreign host for identification of host range determinants

E2F3-dependent activation of FAM111B restricts mouse cytomegalovirus replication in primate cells

Cytomegaloviruses are highly species-specific as they replicate only in cells of their own or a closely related species. For instance, human cytomegalovirus cannot replicate in rodent cells, and mouse cytomegalovirus (MCMV) cannot replicate in human and monkey cells. However, the mechanisms underlying the host species restriction remain poorly understood. We have previously shown that passaging MCMV in human retinal pigment epithelial cells allows the virus to replicate to high titers in these cells due to the accumulation of adaptive mutations, such as loss-of-function mutations in the viral M117 gene. The M117 protein interacts with E2F transcription factors and activates E2F-dependent transcription. Here, we show that activation of E2F3 is primarily responsible for MCMV's inability to replicate in human cells. By transcriptome analysis, we identified two E2F3-induced serine proteases, FAM111A and FAM111B, as potential host restriction factors. By using shRNA-mediated gene knockdown and CRISPR/Cas9-mediated gene knockout, we demonstrated that FAM111B, but not its paralog FAM111A, suppresses MCMV replication in human and rhesus macaque cells. By immunofluorescence, we detected FAM111B predominantly in the nucleus of infected cells with enrichment in viral replication compartments, suggesting that it might play a role during viral replication. The fact that the FAM111B gene is conserved in primates but absent in rodents suggests that MCMV has not evolved to evade or counteract this restriction factor, which is not present in its natural host.

IMPORTANCE

Viruses must counteract host cell defenses to facilitate viral replication. Viruses with a narrow host range, such as the cytomegaloviruses, are unable to counteract cellular defenses in cells of a foreign species. However, little is known about the cellular host range factors restricting cytomegalovirus replication. Here, we show that mouse cytomegalovirus (MCMV) induces the expression of the FAM111 proteases and that FAM111B, but not FAM111A that has previously been shown to restrict the replication of polyomavirus and orthopoxvirus host range mutants, acts as a cellular factor suppressing MCMV replication in human and rhesus monkey cells. The identification of FAM111B as a host range factor should provide new insight into the physiological functions of this poorly characterized protein.

Cytomegalovirus immune evasion sets the functional avidity threshold for protection by CD8 T cells

Conflicting hallmarks are attributed to cytomegalovirus (CMV) infections. CMVs are viewed as being master tacticians in "immune evasion" by subverting essentially all pathways of innate and adaptive immunity. On the other hand, CMV disease is undeniably restricted to the immunologically immature or immunocompromised host, whereas an intact immune system prevents virus spread, cytopathogenic tissue infection, and thus pathological organ manifestations. Therefore, the popular term "immune evasion" is apparently incongruous with the control of CMV infections in the immunocompetent human host as well as in experimental non-human primate and rodent models. Here, we review recent work from the mouse model that resolves this obvious discrepancy for the example of the virus-specific CD8 T-cell response. Immune evasion proteins encoded by murine CMV (mCMV) interfere with the cell surface trafficking of antigenic peptide-loaded MHC class-I (pMHC-I) complexes and thereby reduce their numbers available for interaction with T-cell receptors of CD8 T cells; but this inhibition is incomplete. As a consequence, while CD8 T cells with low interaction avidity fail to receive sufficient signaling for triggering their antiviral effector function in the presence of immune evasion proteins in infected cells, a few pMHC-I complexes that escape to the cell surface are sufficient for sensitizing high-avidity CD8 T cells. It is thus proposed that the function of immune evasion proteins is to raise the avidity threshold for activation, so that in the net result, only high-avidity cells can protect. An example showing that immune evasion proteins can make the difference between life and death is the lacking control of infection in a mouse model of MHC-I histoincompatible hematopoietic cell transplantation (allogeneic-HCT). In this model, only low-avidity CD8 T cells become reconstituted by HCT and almost all infected HCT recipients die of multiple-organ CMV disease when immune evasion proteins are expressed. In contrast, lowering the avidity threshold for antigen recognition by deletion of immune evasion proteins allowed control of infection and rescued from death.

Memory CD8 T Cells Protect against Cytomegalovirus Disease by Formation of Nodular Inflammatory Foci Preventing Intra-Tissue Virus Spread

Cytomegaloviruses (CMVs) are controlled by innate and adaptive immune responses in an immunocompetent host while causing multiple organ diseases in an immunocompromised host. A risk group of high clinical relevance comprises transiently immunocompromised recipients of hematopoietic cell transplantation (HCT) in the “window of risk” between eradicative therapy of hematopoietic malignancies and complete reconstitution of the immune system. Cellular immunotherapy by adoptive transfer of CMV-specific CD8 T cells is an option to prevent CMV disease by controlling a primary or reactivated infection. While experimental models have revealed a viral epitope-specific antiviral function of cognate CD8 T cells, the site at which control is exerted remained unidentified. The observation that remarkably few transferred cells protect all organs may indicate an early blockade of virus dissemination from a primary site of productive infection to various target organs. Alternatively, it could indicate clonal expansion of a few transferred CD8 T cells for preventing intra-tissue virus spread after successful initial organ colonization. Our data in the mouse model of murine CMV infection provide evidence in support of the second hypothesis. We show that transferred cells vigorously proliferate to prevent virus spread, and thus viral histopathology, by confining and eventually resolving tissue infection within nodular inflammatory foci.

Construction of a mouse model of Posner-Schlossman syndrome by anterior chamber infection with cytomegalovirus

Accumulated clinical evidence has shown that Posner-Schlossman syndrome (PSS) is most likely the result of recurrent human cytomegalovirus (HCMV) infection in the anterior chamber (AC). Establishing an animal model is necessary to investigate the pathogenesis of PSS. In this study, we constructed a mouse model of (PSS) by injecting murine cytomegalovirus (MCMV) into the AC of BALB/c mice. Twenty-five BALB/c mice were divided into 5 groups. Smith strain MCMV expressing enhanced green fluorescent protein (EGFP) was passaged with mouse embryonic fibroblast (MEF). Right eyes in the 4 experiment groups received AC injection of 1 μL of virus solution with concentrations of 10³,10⁴,10⁵,10⁶ pfu/mL respectively, and the control group received only PBS. PSS-like signs (mutton-fat keratic precipitates (KP), pupil dilation, IOP elevation and corneal edema) were recorded 0-28 days post-injection (DPI). Sections of eyeballs from another 9 mice harvested on 0,10 and 28 DPI were examined to locate KP and the fluorescence signal of the virus. Reversible PSS-like signs except KP were observed in 20% and 60% mice of 10⁴ and 10⁵ groups while no PSS-like signs in the control and 10³ group; 80% in the 10⁶ group with partially unreversible signs till 28DPI. Much More fluorescent signals of virus in the iris and KP were found on 10DPI than 28 DPI, while no fluorescent signals and KP on 0DPI. The extent of PSS-like signs (pupil dilation, IOP elevation and corneal edema) was virus concentration-dependent (Spearman correlation coefficient, r = 0.830, = 0.475, = 0.662, p < 0.0001, <0.05, <0.001, respectively, n = 25). Success rate of PSS model (mice with PSS-like signs) was also virus concentration-dependent (Chi-square trend test, χ² = 6.828, df = 1, p < 0.01, n = 25). Our results indicate that AC injection of 1 μL MEF passaged MCMV (Smith strain) of 10⁴-10⁶ pfu/mL in BALB/c mice can be used to construct a mouse model of PSS. MCMV can infect iris tissue and replicate in it and then establish latency. This might account for the recurrent and self-limited nature of PSS.

P108 and T109 on E2 Glycoprotein Domain I Are Critical for the Adaptation of Classical Swine Fever Virus to Rabbits but Not for Virulence in Pigs

The classical swine fever virus (CSFV) live attenuated vaccine C-strain is adaptive to rabbits and attenuated in pigs, in contrast with the highly virulent CSFV Shimen strain. Previously, we demonstrated that P108 and T109 on the E2 glycoprotein (E2^P108-T109) in domain I (E2^DomainI) rather than R132, S133, and D191 in domain II (E2^DomainII) determine C-strain's adaptation to rabbits (ATR) (Y. Li, L. Xie, L. Zhang, X. Wang, C. Li, et al., Virology 519:197-206, 2018). However, it remains elusive whether these critical amino acids affect the ATR of the Shimen strain and virulence in pigs. In this study, three chimeric viruses harboring E2^P108-T109, E2^DomainI, or E2^DomainII of C-strain based on the non-rabbit-adaptive Shimen mutant vSM-HCLVE^rns carrying the E^rns glycoprotein of C-strain were generated and evaluated. We found that E2^P108-T109 or E2^DomainI but not E2^DomainII of C-strain renders vSM-HCLVE^rns adaptive to rabbits, suggesting that E2^P108-T109 in combination with the E^rns glycoprotein (E2^P108-T109-E^rns) confers ATR on the Shimen strain, creating new rabbit-adaptive CSFVs. Mechanistically, E2^P108-T109-E^rns of C-strain mediates viral entry during infection in rabbit spleen lymphocytes, which are target cells of C-strain. Notably, pig experiments showed that E2^P108-T109-E^rns of C-strain does not affect virulence compared with the Shimen strain. Conversely, the substitution of E2^DomainII and E^rns of C-strain attenuates the Shimen strain in pigs, indicating that the molecular basis of the CSFV ATR and that of virulence in pigs do not overlap. Our findings provide new insights into the mechanism of adaptation of CSFV to rabbits and the molecular basis of CSFV adaptation and attenuation.IMPORTANCE Historically, live attenuated vaccines produced by blind passage usually undergo adaptation in cell cultures or nonsusceptible hosts and attenuation in natural hosts, with a classical example being the classical swine fever virus (CSFV) lapinized vaccine C-strain, which was developed by hundreds of passages in rabbits. However, the mechanism of viral adaptation to nonsusceptible hosts and the molecular basis for viral adaptation and attenuation remain largely unknown. In this study, we demonstrated that P108 and T109 on the E2 glycoprotein together with the E^rns glycoprotein of the rabbit-adaptive C-strain confer adaptation to rabbits on the highly virulent CSFV Shimen strain by affecting viral entry during infection but do not attenuate the Shimen strain in pigs. Our results provide vital information on the different molecular bases of CSFV adaptation to rabbits and attenuation in pigs.

Vaccine vectors: the bright side of cytomegalovirus

Cytomegaloviruses (CMVs) present singular features that are particularly advantageous for human vaccine development, a current medical need. Vaccines that induce neutralizing antibodies are among the most successful and efficacious available. However, chronic and persistent human infections, pathogens with high variability of exposed proteins, as well as tumors, highlight the need for developing novel vaccines inducing strong and long-lasting cellular immune responses mediated by effector or effector memory CD8+ cytotoxic T lymphocytes. CMVs induce the most potent CD8+ T lymphocyte response to a pathogen known in each of their hosts, maintain and even increase it for life for selected antigens, in what is known as the ever growing inflationary memory, and maintain an effector memory status due to recent and repeated antigen stimulation that endows these inflationary T lymphocytes with superior and faster protective potency. In addition to these CMV singularities, this family of viruses has two more common favorable features: they can superinfect an already infected host, which is needed in face of the high CMV prevalence, and they can harbor very large segments of foreign DNA at many different genomic sites. All these properties endow CMVs with a singular potential to be used as human vaccine vectors. Current developments with most of the recombinant CMV-based vaccine candidates that have been tested in animal models against clinically relevant viral and bacterial infections and for their use in tumor immunotherapy are reviewed herein. Since CMV vectors should be designed to avoid the risk of disease in immunocompromised individuals, special attention is also paid to attenuated vectors. Taken together, the results support the future use of CMV-based vaccine vectors to induce protective CD8+ T lymphocyte responses in humans, mainly against viral infections and as anti-tumor vaccines.

The Susceptibility of Primary Dermis Fibroblasts from the Chinese Tree Shrew to Human Cytomegalovirus Infection

As a universal pathogen leading to neonatal defects and transplant failure, human cytomegalovirus (HCMV) has strict species specificity and this has prevented the development of a suitable animal model for the pathogenesis study. The mechanism of cross-species barrier remains elusive and there are so far no non-human cell culture models that support HCMV replication. The Chinese tree shrew (Tupaia belangeri chinensis) is a small laboratory animal and evolutionary closely related with primates. We investigated the susceptibility of primary tree shrew dermis fibroblasts (TSDF) to HCMV infection. Infection with a GFP-expressing HCMV virus resulted in green fluorescence in infected cells with the expression of IE1, UL44 and pp28. The titers of cell-free viruses reached 10³ PFU/mL at 96 hpi, compared to titers of 10⁴ PFU/mL observed in primary human foreskin fibroblasts. Our results suggested that TSDF was semi-permissive for HCMV infection. The TSDF model could be further used to investigate key factors influencing cross-species multiplication of HCMV.

Activation of E2F-dependent transcription by the mouse cytomegalovirus M117 protein affects the viral host range

Cytomegaloviruses (CMVs) have a highly restricted host range as they replicate only in cells of their own or closely related species. To date, the molecular mechanisms underlying the CMV host restriction remain poorly understood. However, it has been shown that mouse cytomegalovirus (MCMV) can be adapted to human cells and that adaptation goes along with adaptive mutations in several viral genes. In this study, we identify MCMV M117 as a novel host range determinant. Mutations in this gene enable the virus to cross the species barrier and replicate in human RPE-1 cells. We show that the M117 protein is expressed with early kinetics, localizes to viral replication compartments, and contributes to the inhibition of cellular DNA synthesis. Mechanistically, M117 interacts with members of the E2F transcription factor family and induces E2F target gene expression in murine and human cells. While the N-terminal part of M117 mediates E2F interaction, the C-terminal part mediates self-interaction. Both parts are required for the activation of E2F-dependent transcription. We further show that M117 is dispensable for viral replication in cultured mouse fibroblasts and endothelial cells, but is required for colonization of mouse salivary glands in vivo. Conversely, inactivation of M117 or pharmacological inhibition of E2F facilitates MCMV replication in human RPE-1 cells, whereas replacement of M117 by adenovirus E4orf6/7, a known E2F activator, prevents it. These results indicate that E2F activation is detrimental for MCMV replication in human cells. In summary, this study identifies MCMV M117 as a novel E2F activator that functions as a host range determinant by precluding MCMV replication in human cells.

Mouse Model of Cytomegalovirus Disease and Immunotherapy in the Immunocompromised Host: Predictions for Medical Translation that Survived the "Test of Time"

Human Cytomegalovirus (hCMV), which is the prototype member of the β-subfamily of the herpesvirus family, is a pathogen of high clinical relevance in recipients of hematopoietic cell transplantation (HCT). hCMV causes multiple-organ disease and interstitial pneumonia in particular upon infection during the immunocompromised period before hematopoietic reconstitution restores antiviral immunity. Clinical investigation of pathomechanisms and of strategies for an immune intervention aimed at restoring antiviral immunity earlier than by hematopoietic reconstitution are limited in patients to observational studies mainly because of ethical issues including the imperative medical indication for chemotherapy with antivirals. Aimed experimental studies into mechanisms, thus, require animal models that match the human disease as close as possible. Any model for hCMV disease is, however, constrained by the strict host-species specificity of CMVs that prevents the study of hCMV in any animal model including non-human primates. During eons of co-speciation, CMVs each have evolved a set of "private genes" in adaptation to their specific mammalian host including genes that have no homolog in the CMV virus species of any other host species. With a focus on the mouse model of CD8 T cell-based immunotherapy of CMV disease after experimental HCT and infection with murine CMV (mCMV), we review data in support of the phenomenon of "biological convergence" in virus-host adaptation. This includes shared fundamental principles of immune control and immune evasion, which allows us to at least make reasoned predictions from the animal model as an experimental "proof of concept." The aim of a model primarily is to define questions to be addressed by clinical investigation for verification, falsification, or modification and the results can then give feedback to refine the experimental model for research from "bedside to bench".

Reduction of the survival time of pig xenotransplants by porcine cytomegalovirus

Background

Xenotransplantation using pig cells, tissues and organs may help to overcome the shortage of human tissues and organs for the treatment of tissue and organ failure. Progress in the prevention of immunological rejection using genetically modified pigs and new, more effective, immunosuppression regimens will allow clinical application of xenotransplantation in near future. However, xenotransplantation may be associated with the transmission of potentially zoonotic porcine microorganisms. Until now the only xenotransplantation-associated transmission was the transmission of the porcine cytomegalovirus (PCMV) into non-human primates. PCMV caused a significant reduction of the survival time of the pig transplant.

Main body of the abstract

Here the available publications were analysed in order to establish the mechanism how PCMV shortened the survival time of xenotransplants. PCMV is a herpesvirus related to the human cytomegalovirus and the human herpesviruses 6 and 7. These three human herpesviruses can cause serious disease among immunocompromised human individuals, including transplant recipients. It was shown that PCMV predominantly contributes to the reduction of transplant survival in non-human primates by disruption of the coagulation system and by suppression and exhaustion of the immune system.

Conclusion

Although it is still unknown whether PCMV infects primate cells including human cells, indirect mechanism of the virus infection may cause reduction of the xenotransplant survival in future clinical trials and therefore PCMV has to be eliminated from donor pigs.

Antagonism of the Protein Kinase R Pathway in Human Cells by Rhesus Cytomegalovirus

While cytomegalovirus (CMV) infections are often limited in host range by lengthy coevolution with a single host species, a few CMVs are known to deviate from this rule. For example, rhesus macaque CMV (RhCMV), a model for human CMV (HCMV) pathogenesis and vaccine development, can replicate in human cells, as well as in rhesus cells. Both HCMV and RhCMV encode species-specific antagonists of the broadly acting host cell restriction factor protein kinase R (PKR). Although the RhCMV antagonist of PKR, rTRS1, has very limited activity against human PKR, here, we show it is essential for RhCMV replication in human cells because it prevents human PKR from phosphorylating the translation initiation factor eIF2α, thereby allowing continued translation and viral replication. Although rTRS1 is necessary for RhCMV replication, it is not sufficient to rescue replication of HCMV lacking its own PKR antagonists in human fibroblasts. However, overexpression of rTRS1 in human fibroblasts enabled HCMV expressing rTRS1 to replicate, indicating that elevated levels or early expression of a weak antagonist can counteract a resistant restriction factor like human PKR. Exploring potential mechanisms that might allow RhCMV to replicate in human cells revealed that RhCMV makes no less double-stranded RNA than HCMV. Rather, in human cells, RhCMV expresses rTRS1 at levels 2 to 3 times higher than those of the HCMV-encoded PKR antagonists during HCMV infection. These data suggest that even a modest increase in expression of this weak PKR antagonist is sufficient to enable RhCMV replication in human cells.IMPORTANCE Rhesus macaque cytomegalovirus (RhCMV) offers a valuable model for studying congenital human cytomegalovirus (HCMV) pathogenesis and vaccine development. Therefore, it is critical to understand variations in how each virus infects and affects its host species to be able to apply insights gained from the RhCMV model to HCMV. While HCMV is capable only of infecting cells from humans and very closely related species, RhCMV displays a wider host range, including human as well as rhesus cells. RhCMV expresses an antagonist of a broadly acting antiviral factor present in all mammalian cells, and its ability to counter both the rhesus and human versions of this host factor is a key component of RhCMV's ability to cross species barriers. Here, we examine the molecular mechanisms that allow this RhCMV antagonist to function against a human restriction factor.

Refining human T-cell immunotherapy of cytomegalovirus disease: a mouse model with 'humanized' antigen presentation as a new preclinical study tool

With the cover headline 'T cells on the attack,' the journal Science celebrated individualized cancer immunotherapy by adoptive transfer of T cells as the 'Breakthrough of the Year' 2013 (J. Couzin-Frankel in Science 342:1432-1433, 2013). It is less well recognized and appreciated that individualized T cell immunotherapy of cytomegalovirus (CMV) infection is approaching clinical application for preventing CMV organ manifestations, interstitial CMV pneumonia in particular. This coincident medical development is particularly interesting as reactivated CMV infection is a major viral complication in the state of transient immunodeficiency after the therapy of hematopoietic malignancies by hematopoietic cell transplantation (HCT). It may thus be attractive to combine T cell immunotherapy of 'minimal residual disease/leukemia (MRD)' and CMV-specific T cell immunotherapy to combat both risks in HCT recipients simultaneously, and ideally with T cells derived from the respective HLA-matched HCT donor. Although clinical trials of human CMV-specific T cell immunotherapy were promising in that the incidence of virus reactivation and disease was found to be reduced with statistical significance, animal models are still instrumental for providing 'proof of concept' by directly documenting the prevention of viral multiple-organ histopathology and organ failure under controlled conditions of the absence versus presence of the therapy, which obviously is not feasible in an individual human patient. Further, animal models can make predictions regarding parameters that determine the efficacy of T cell immunotherapy for improved study design in clinical investigations, and they allow for manipulating host and virus genetics. The latter is of particular value as it opens the possibility for epitope specificity controls that are inherently missing in clinical trials. Here, we review a recently developed new mouse model that is more approximated to human CMV-specific T cell immunotherapy by 'humanizing' antigen presentation using antigenically chimeric CMV and HLA-transgenic mice to allow for an in vivo testing of the antiviral function of human CMV-specific T cells. As an important new message, this model predicts that T cell immunotherapy is most efficient if CD4 T cells are equipped with a transduced TCR directed against an epitope presented by MHC/HLA class-I for local delivery of 'cognate' help to CD8 effector T cells at infected MHC/HLA class-II-negative host tissue cells.

Mutual Interference between Cytomegalovirus and Reconstitution of Protective Immunity after Hematopoietic Cell Transplantation

Hematopoietic cell transplantation (HCT) is a therapy option for aggressive forms of hematopoietic malignancies that are resistant to standard antitumoral therapies. Hematoablative treatment preceding HCT, however, opens a “window of opportunity” for latent Cytomegalovirus (CMV) by releasing it from immune control with the consequence of reactivation of productive viral gene expression and recurrence of infectious virus. A “window of opportunity” for the virus represents a “window of risk” for the patient. In the interim between HCT and reconstitution of antiviral immunity, primarily mediated by CD8⁺ T cells, initially low amounts of reactivated virus can expand exponentially, disseminate to essentially all organs, and cause multiple organ CMV disease, with interstitial pneumonia (CMV-IP) representing the most severe clinical manifestation. Here, I will review predictions originally made in the mouse model of experimental HCT and murine CMV infection, some of which have already paved the way to translational preclinical research and promising clinical trials of a preemptive cytoimmunotherapy of human CMV disease. Specifically, the mouse model has been pivotal in providing “proof of concept” for preventing CMV disease after HCT by adoptive transfer of preselected, virus epitope-specific effector and memory CD8⁺ T cells bridging the critical interim. However, CMV is not a “passive antigen” but is a pathogen that actively interferes with the reconstitution of protective immunity by infecting bone marrow (BM) stromal cells that otherwise form niches for hematopoiesis by providing the structural microenvironment and by producing hematopoietically active cytokines, the hemopoietins. Depending on the precise conditions of HCT, reduced homing of transplanted hematopoietic stem- and progenitor cells to infected BM stroma and impaired colony growth and lineage differentiation can lead to “graft failure.” In consequence, uncontrolled virus spread causes morbidity and mortality. In the race between viral BM pathology and reconstitution of antiviral immunity following HCT, exogenous reconstitution of virus-specific CD8⁺ T cells by adoptive cell transfer as an interventional strategy can turn the balance toward control of CMV.

Non-cognate bystander cytolysis by clonal epitope-specific CTL lines through CD28-CD80 interaction inhibits antibody production: A potential caveat to CD8 T-cell immunotherapy

Adoptive transfer of virus epitope-specific CD8 T cells is an immunotherapy option to control cytomegalovirus (CMV) infection and prevent CMV organ disease in immunocompromised solid organ transplantation (SOT) and hematopoietic cell transplantation (HCT) recipients. The therapy aims at an early, selective recognition and cytolysis of infected cells for preventing viral spread in tissues with no adverse immunopathogenic side-effects by attack of uninfected bystander cells. Here we describe that virus epitope-specific, cloned T-cell lines lyse target cells that present the cognate antigenic peptide to the TCR, but simultaneously have the potential to lyse uninfected cells expressing the CD28 ligand CD80 (B7-1). While TCR-mediated cytolysis requires co-receptor CD8 and depends on perforin, the TCR-independent and viral epitope-independent cytolysis through CD28-CD80 signaling does not require CD8 on the effector cells and is perforin-independent. Importantly, this non-cognate cytolysis pathway leads to bystander cytolysis of CD80-expressing B-cell blasts and thereby inhibits pan-specific antibody production.