Respiratory Syncytial Virus (RSV) Pulmonary Infection in Humanized Mice Induces Human Anti-RSV Immune Responses and Pathology

Humanized Mouse Systems to Study Viral Infection: A New Era in Immunology Research

For decades, scientists have relied on traditional animal models to study viral infection and the immune response. However, these models have limitations, and the search for more accurate and reliable ways to study the human-pathogen interphase has led to the development of humanized mouse systems. These revolutionary models have transformed how we understand viral infection and the human immune system's interactions with viruses to control or exacerbate disease. They are also paving the way for new treatments and therapies. In this article, we explore the history and development of humanized mouse systems and their advantages, limitations, and applications in viral immunology research. We describe the different types of humanized mouse models, including their generation and utility for studying human pathogens, with an emphasis on human-specific viruses. In addition, we discuss areas for further refinement and future applications.

Enhanced Pathogenic Consequences Induced by a Seven-Amino-Acid Extension in the G Protein of the HRSV BA9 Genotype

In a previous outbreak of the human respiratory syncytial virus (HRSV), we identified a variant strain of genotype BA9 with a seven-amino-acid extension (Q-R-L-Q-S-Y-A) at the C-terminus of the attachment protein (G). To assess the impact of this extension on the virulence of HRSV, two full-length infectious clones using the wild strain of genotype BA9 as a backbone, one containing the seven-amino-acid extension (rRSV BA9 WT), and the other deleting this extension (rRSV BA9 Δ7AA), were successfully rescued using a reverse genetics system. The biological properties and virulence of the two rescued viruses were then compared and analyzed in vitro and in vivo. Compared to the rRSV BA9 Δ7AA, the rRSV BA9 WT exhibited a larger plaque size and a more pronounced suppression of the host cell innate immune response in vitro (IFN-β levels: 154.33 pg/mL vs. 11.27 pg/mL). The rRSV BA9 WT demonstrated increased adaptability in mice, with a 10-fold higher lung viral load and a stronger inflammatory response following intranasal exposure. Our study primarily demonstrated that the C-terminal extension of the G protein of the HRSV can enhance viral virulence, underscoring the importance of virological surveillance in the prevention and treatment of severe HRSV-related disease.

Infection of nonclassic monocytes by respiratory syncytial virus induces an imbalance in the CD4+ T-cell subset response

Respiratory syncytial virus (RSV) causes lower respiratory tract infections in infants and young children, leading to a pathogenesis-associated imbalance in CD4+ T-cell subsets and monocyte subsets. To investigate whether RSV affects the imbalance of CD4+ T-cells through monocytes, we examined the effects of the RSV-infected monocyte subset on CD4+ T-cell subsets, namely, Th1, Th2, Th17, and regulatory T (Treg) subsets, on proliferation in vitro and identified key monocyte-derived cytokines. We found that RSV efficiently infects CD16+ monocytes, but not CD16- monocytes, via cocultures of CD4+ T-cells with RSV-infected CD16+ monocytes, inhibits Treg cell proliferation and increases Th2 cell frequency, suggesting that RSV causes an imbalance in the CD4+ T-cell subset by primarily infecting CD16+ monocytes. Our data also revealed that IL-1β and IL-10 are key cytokines responsible for the activities of RSV-infected CD16+ monocytes. In a mouse model, we found that high-efficiency RSV infection of mouse Ly6C- monocytes, corresponding to CD16+ monocytes in humans, and adoptive transfer of Ly6C- monocytes during RSV infection decreased the Treg frequency in the lungs and aggravated pneumonia. Our data indicate that RSV can increase its pathogenesis through infection of nonclassic monocytes, leading to a CD4+ T-cell imbalance.IMPORTANCEThis study identified a pathogenesis pathway related to the RSV-nonclassic monocyte-IL-1β/IL-10-CD4+ T-cell subset balance, which links the heterogeneity of monocytes to RSV pathogenesis and elucidates a new mechanism by which RSV infection disrupts the balance of CD4+ T cells during RSV infection. These new findings provide potential therapeutic targets for RSV infection.

Antibody-based protection against respiratory syncytial virus in mice and their offspring through vectored immunoprophylaxis

Respiratory syncytial virus (RSV) causes acute lower respiratory tract infections, with potential lower respiratory tract infections, which can be particularly problematic in infants and the elderly. There are no approved vaccines for RSV. The current standard of care for high-risk individuals is monthly administration of palivizumab, a humanized murine monoclonal antibody (mAb) targeting the RSV fusion protein. Adeno-associated virus (AAV)-mediated expression of mAbs has previously led to sustained expression of therapeutic concentrations of mAbs in several animal models, representing an alternative to repetitive passive administration. Intramuscular (IM) administration of AAV6.2FF expressing RSV antibodies, palivizumab or hRSV90, resulted in high concentrations of human (h)IgG1 mAbs in the serum and at various mucosal surfaces, while intranasal administration limited hIgG expression to the respiratory tract. IM administration of AAV6.2FF-hRSV90 or AAV6.2FF-palivizumab in a murine model provided sterilizing immunity against challenge with RSV A2. Evidence of maternal passive transfer of vectorized hRSV90 was detected in both murine and ovine models, with circulating mAbs providing sterilizing immunity in mouse progeny. Finally, addition of a "kill switch" comprised of LoxP sites flanking the mAb genes resulted in diminished serum hIgG after AAV-DJ-mediated delivery of Cre recombinase to the same muscle group that was originally transduced with the AAV-mAb vector. The ability of this AAV-mAb system to mediate robust, sustained mAb expression for maternal transfer to progeny in murine and ovine models emphasizes the potential of this platform for use as an alternative prophylactic vaccine for protection against neonatal infections, particularly in high-risk infants.

A pancreatic cancer mouse model with human immunity

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a tumor immune microenvironment (TIME) that promotes resistance to immunotherapy. A preclinical model system that facilitates studies of the TIME and its impact on the responsiveness of human PDAC to immunotherapies remains an unmet need. We report a novel mouse model, which develops metastatic human PDAC that becomes infiltrated by human immune cells recapitulating the TIME of human PDAC. The model may serve as a versatile platform to study the nature of human PDAC TIME and its response to various treatments.

From animal studies into clinical trials: the relevance of animal models to develop vaccines and therapies to reduce disease severity and prevent hRSV infection

INTRODUCTION

Human respiratory syncytial virus (hRSV) is an important cause of lower respiratory tract infections in the pediatric and the geriatric population worldwide. There is a substantial economic burden resulting from hRSV disease during winter. Although no vaccines have been approved for human use, prophylactic therapies are available for high-risk populations. Choosing the proper animal models to evaluate different vaccine prototypes or pharmacological treatments is essential for developing efficient therapies against hRSV.

AREAS COVERED

This article describes the relevance of using different animal models to evaluate the effect of antiviral drugs, pharmacological molecules, vaccine prototypes, and antibodies in the protection against hRSV. The animal models covered are rodents, mustelids, bovines, and nonhuman primates. Animals included were chosen based on the available literature and their role in the development of the drugs discussed in this manuscript.

EXPERT OPINION

Choosing the correct animal model is critical for exploring and testing treatments that could decrease the impact of hRSV in high-risk populations. Mice will continue to be the most used preclinical model to evaluate this. However, researchers must also explore the use of other models such as nonhuman primates, as they are more similar to humans, prior to escalating into clinical trials.

SARS-CoV-2 Variant-Specific Infectivity and Immune Profiles Are Detectable in a Humanized Lung Mouse Model

Small animal models that accurately model pathogenesis of SARS-CoV-2 variants are required for ongoing research efforts. We modified our human immune system mouse model to support replication of SARS-CoV-2 by implantation of human lung tissue into the mice to create TKO-BLT-Lung (L) mice and compared infection with two different variants in a humanized lung model. Infection of TKO-BLT-L mice with SARS-CoV-2 recapitulated the higher infectivity of the B.1.1.7 variant with more animals becoming infected and higher sustained viral loads compared to mice challenged with an early B lineage (614D) virus. Viral lesions were observed in lung organoids but no differences were detected between the viral variants as expected. Partially overlapping but distinct immune profiles were also observed between the variants with a greater Th1 profile in VIDO-01 and greater Th2 profile in B.1.1.7 infection. Overall, the TKO-BLT-L mouse supported SARS-CoV-2 infection, recapitulated key known similarities and differences in infectivity and pathogenesis as well as revealing previously unreported differences in immune responses between the two viral variants. Thus, the TKO-BLT-L model may serve as a useful animal model to study the immunopathobiology of newly emerging variants in the context of genuine human lung tissue and immune cells.

Integrins are double-edged swords in pulmonary infectious diseases

Integrins are an important family of adhesion molecules that are widely distributed on immune cells in the lungs. Of note, accumulating evidences have shown that integrins are double-edged swords in pulmonary infectious diseases. On one hand, integrins promote the migration of immune cells to remove the invaded pathogens in the infected lungs. However, on the other hand, integrins also act as the targets for pathogens to escape from host immune system, which is a potential factor leading to further tissue damage. Thus, the innovative therapeutic strategies based on integrins has inspired well-founded hopes to treat pulmonary infectious diseases. In this review, we illustrate the involvement of integrins in pulmonary infectious diseases, and further discuss the innovative therapeutic targets based on integrins.

Immunopathology of RSV: An Updated Review

RSV is a leading cause of respiratory tract disease in infants and the elderly. RSV has limited therapeutic interventions and no FDA-approved vaccine. Gaps in our understanding of virus-host interactions and immunity contribute to the lack of biological countermeasures. This review updates the current understanding of RSV immunity and immunopathology with a focus on interferon responses, animal modeling, and correlates of protection.

Human respiratory syncytial virus infection in the pre-clinical calf model

Human respiratory syncytial virus (hRSV) is the most important respiratory pathogen in young children worldwide. Experimental modelling of hRSV disease by bovine RSV (bRSV) infection in calves provides an important tool for developing new strategies for prevention and treatment. Depending on the scientific hypothesis under investigation, this cognate host-virus model might have the disadvantage of using a highly related but not genetically identical virus. In this study, we aim to describe viral kinetics and (clinical) disease characteristics in calves inoculated with hRSV. Our results show that hRSV infects the upper and, to a lesser extent, the lower respiratory tract of calves. Infection causes upper airway clinical disease symptoms and neutrophilic infiltration of the lower airways. We conclude that a hRSV model in calves may aid future research involving distinct scientific questions related to hRSV disease in children.

Humanized mouse models of immunological diseases and precision medicine

With the increase in knowledge resulting from the sequencing of the human genome, the genetic basis for the underlying differences in individuals, their diseases, and how they respond to therapies is starting to be understood. This has formed the foundation for the era of precision medicine in many human diseases that is beginning to be implemented in the clinic, particularly in cancer. However, preclinical testing of therapeutic approaches based on individual biology will need to be validated in animal models prior to translation into patients. Although animal models, particularly murine models, have provided significant information on the basic biology underlying immune responses in various diseases and the response to therapy, murine and human immune systems differ markedly. These fundamental differences may be the underlying reason why many of the positive therapeutic responses observed in mice have not translated directly into the clinic. There is a critical need for preclinical animal models in which human immune responses can be investigated. For this, many investigators are using humanized mice, i.e., immunodeficient mice engrafted with functional human cells, tissues, and immune systems. We will briefly review the history of humanized mice, the remaining limitations, approaches to overcome them and how humanized mouse models are being used as a preclinical bridge in precision medicine for evaluation of human therapies prior to their implementation in the clinic.

Current Animal Models for Understanding the Pathology Caused by the Respiratory Syncytial Virus

The human respiratory syncytial virus (hRSV) is the main etiologic agent of severe lower respiratory tract infections that affect young children throughout the world, associated with significant morbidity and mortality, becoming a serious public health problem globally. Up to date, no licensed vaccines are available to prevent severe hRSV-induced disease, and the generation of safe-effective vaccines has been a challenging task, requiring constant biomedical research aimed to overcome this ailment. Among the difficulties presented by the study of this pathogen, it arises the fact that there is no single animal model that resembles all aspects of the human pathology, which is due to the specificity that this pathogen has for the human host. Thus, for the study of hRSV, different animal models might be employed, depending on the goal of the study. Of all the existing models, the murine model has been the most frequent model of choice for biomedical studies worldwide and has been of great importance at contributing to the development and understanding of vaccines and therapies against hRSV. The most notable use of the murine model is that it is very useful as a first approach in the development of vaccines or therapies such as monoclonal antibodies, suggesting in this way the direction that research could have in other preclinical models that have higher maintenance costs and more complex requirements in its management. However, several additional different models for studying hRSV, such as other rodents, mustelids, ruminants, and non-human primates, have been explored, offering advantages over the murine model. In this review, we discuss the various applications of animal models to the study of hRSV-induced disease and the advantages and disadvantages of each model, highlighting the potential of each model to elucidate different features of the pathology caused by the hRSV infection.

Respiratory epithelial cells as master communicators during viral infections

PURPOSE OF REVIEW

Communication by epithelial cells during respiratory viral infections is critical in orchestrating effective anti-viral responses but also can lead to excessive inflammation. This review will evaluate studies that investigate how respiratory epithelial cells influence the behavior of immune cells and how epithelial cell/immune cell interactions contribute to antiviral responses and immunopathology outcomes.

RECENT FINDINGS

Previous studies have characterized cytokine responses of virus-infected epithelial cells. More recent studies have carefully demonstrated the effects of these cytokines on cellular behaviors within the infected lung. Infected epithelial cells release exosomes that specifically regulate responses of monocytes and neighboring epithelial cells without promoting spread of virus. In contrast, rhinovirus-infected cells induce monocytes to upregulate expression of the viral receptor, promoting spread of the virus to alternate cell types. The precise alteration of PDL expression on infected epithelial cells has been shown to switch between inhibition and activation of antiviral responses.

SUMMARY

These studies have more precisely defined the interactions between epithelial and immune cells during viral infections. This level of understanding is critical for the development of novel therapeutic strategies that promote effective antiviral responses or epithelial repair, or inhibit damaging inflammatory responses during severe respiratory viral infections.

Disruption of the airway epithelial barrier in a murine model of respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is a major cause of hospitalization for infants and young children worldwide. RSV is known to infect epithelial cells and increase the permeability of model airway epithelial monolayers in vitro. We hypothesized that RSV infection also induces airway barrier dysfunction in vivo. C57BL/6 mice were intranasally inoculated with RSV, and on day 4 post-inoculation were examined for viral replication, lung inflammation, and barrier integrity as well as the structure and molecular composition of epithelial junctions. In parallel, primary mouse tracheal epithelial cells (mTEC) were cultured for in vitro studies. RSV-infected mice lost weight and showed significant peribronchial inflammation compared with noninfected controls and UV-inactivated RSV-inoculated animals. RSV infection increased the permeability of the airway epithelial barrier and altered the molecular composition of epithelial tight junctions. The observed RSV-induced barrier disruption was accompanied by decreased expression of several tight-junction proteins and accumulation of cleaved extracellular fragments of E-cadherin in bronchoalveolar lavage and mTEC supernatants. Similarly, in vitro RSV infection of mTEC monolayers resulted in enhanced permeability and disruption of tight-junction structure. Furthermore, incubation of mTEC monolayers with a recombinant fragment of E-cadherin caused tight-junction disassembly. Taken together, these data indicate that RSV infection leads to airway barrier dysfunction in vivo, mediated by either decreased expression or cleavage of junctional proteins. Our observations provide further insights into the pathophysiology of RSV infection and provide a rationale for development of barrier-protecting agents to alleviate the pathogenic effects of RSV infection.

Improved multilineage human hematopoietic reconstitution and function in NSGS mice

Genetic manipulation of NOD/SCID (NS) mice has yielded numerous sub-strains with specific traits useful for the study of human hematopoietic xenografts, each with unique characteristics. Here, we have compared the engraftment and output of umbilical cord blood (UCB) CD34+ cells in four immune-deficient strains: NS, NS with additional IL2RG knockout (NSG), NS with transgenic expression of human myeloid promoting cytokines SCF, GM-CSF, and IL-3 (NSS), and NS with both IL2RG knockout and transgenic cytokine expression (NSGS). Overall engraftment of human hematopoietic cells was highest in the IL2RG knockout strains (NSG and NSGS), while myeloid cell output was notably enhanced in the two strains with transgenic cytokine expression (NSS and NSGS). In further comparisons of NSG and NSGS mice, several additional differences were noted. NSGS mice were found to have a more rapid reconstitution of T cells, improved B cell differentiation, increased levels of NK cells, reduced platelets, and reduced maintenance of primitive CD34+ cells in the bone marrow. NSGS were superior hosts for secondary engraftment and both strains were equally suitable for experiments of graft versus host disease. Increased levels of human cytokines as well as human IgG and IgM were detected in the serum of humanized NSGS mice. Furthermore, immunization of humanized NSGS mice provided evidence of a functional response to repeated antigen exposure, implying a more complete hematopoietic graft was generated in these mice. These results highlight the important role that myeloid cells and myeloid-supportive cytokines play in the formation of a more functional xenograft immune system in humanized mice.

Human Immune System Mice for the Study of Human Immunodeficiency Virus-Type 1 Infection of the Central Nervous System

Immunodeficient mice transplanted with human cell populations or tissues, also known as human immune system (HIS) mice, have emerged as an important and versatile tool for the in vivo study of human immunodeficiency virus-type 1 (HIV-1) pathogenesis, treatment, and persistence in various biological compartments. Recent work in HIS mice has demonstrated their ability to recapitulate critical aspects of human immune responses to HIV-1 infection, and such studies have informed our knowledge of HIV-1 persistence and latency in the context of combination antiretroviral therapy. The central nervous system (CNS) is a unique, immunologically privileged compartment susceptible to HIV-1 infection, replication, and immune-mediated damage. The unique, neural, and glia-rich cellular composition of this compartment, as well as the important role of infiltrating cells of the myeloid lineage in HIV-1 seeding and replication makes its study of paramount importance, particularly in the context of HIV-1 cure research. Current work on the replication and persistence of HIV-1 in the CNS, as well as cells of the myeloid lineage thought to be important in HIV-1 infection of this compartment, has been aided by the expanded use of these HIS mouse models. In this review, we describe the major HIS mouse models currently in use for the study of HIV-1 neuropathogenesis, recent insights from the field, limitations of the available models, and promising advances in HIS mouse model development.

A Hitchhiker's guide to humanized mice: new pathways to studying viral infections

Humanized mice are increasingly appreciated as an incredibly powerful platform for infectious disease research. The often very narrow species tropism of many viral infections, coupled with the sometimes misleading results from preclinical studies in animal models further emphasize the need for more predictive model systems based on human cells rather than surrogates. Humanized mice represent such a model and have been greatly enhanced with regards to their immune system reconstitution as well as immune functionality in the past years, resulting in their recommendation as a preclinical model by the US Food and Drug Administration. This review aims to give a detailed summary of the generation of human peripheral blood lymphocyte-, CD34⁺ haematopoietic stem cell- and bone marrow/liver/thymus-reconstituted mice and available improved models (e.g. myeloid- or T-cell-only mice, MISTRG, NSG-SGM3). Additionally, we summarize human-tropic viral infections, for which humanized mice offer a novel approach for the study of disease pathogenesis as well as future perspectives for their use in biomedical, drug and vaccine research.

Protection induced by virus-like particle vaccine containing tandem repeat gene of respiratory syncytial virus G protein

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract illness in infants, young children and the elderly. However, there is no licensed vaccine available against RSV infection. In this study, we generated virus-like particle (VLP) vaccine and investigated the vaccine efficacy in a mouse model. For VLP vaccines, tandem gene (1–780 bp) for V1 VLPs and tandem repeat gene (repeated 450–780 bp) for V5 VLPs were constructed in pFastBac^TM vectors, respectively. Influenza matrix protein 1 (M1) was used as a core protein in the VLPs. Notably, upon challenge infection, significantly lower virus loads were measured in the lung of mice immunized with V1 or V5 VLPs compared to those of naïve mice and formalin-inactivated RSV immunized control mice. In particular, V5 VLPs immunization showed significantly lower virus titers than V1 VLPs immunization. Furthermore, V5 VLPs immunization elicited increased memory B cells responses in the spleen. These results indicated that V5 VLP vaccine containing tandem repeat gene protein provided better protection than V1 VLPs with significantly decreased inflammation in the lungs. Thus, V5 VLPs could be a potential vaccine candidate against RSV.

Human IgG repertoire of malaria antigen-immunized human immune system (HIS) mice

Humanized mouse models present an important tool for preclinical evaluation of new vaccines and therapeutics. Here we show the human variable repertoire of antibody sequences cloned from a previously described human immune system (HIS) mouse model that possesses functional human CD4+ T cells and B cells, namely HIS-CD4/B mice. We sequenced variable IgG genes from single memory B-cell and plasma-cell sorted from splenocytes or whole blood lymphocytes of HIS-CD4/B mice that were vaccinated with a human plasmodial antigen, a recombinant Plasmodium falciparum circumsporozoite protein (rPfCSP). We demonstrate that rPfCSP immunization triggers a diverse B-cell IgG repertoire composed of various human VH family genes and distinct V(D)J recombinations that constitute diverse CDR3 sequences similar to humans, although low hypermutated sequences were generated. These results demonstrate the substantial genetic diversity of responding human B cells of HIS-CD4/B mice and their capacity to mount human IgG class-switched antibody response upon vaccination.

Animal models of respiratory syncytial virus infection

Human respiratory syncytial virus (hRSV) is a major cause of respiratory disease and hospitalisation of infants, worldwide, and is also responsible for significant morbidity in adults and excess deaths in the elderly. There is no licensed hRSV vaccine or effective therapeutic agent. However, there are a growing number of hRSV vaccine candidates that have been developed targeting different populations at risk of hRSV infection. Animal models of hRSV play an important role in the preclinical testing of hRSV vaccine candidates and although many have shown efficacy in preclinical studies, few have progressed to clinical trials or they have had only limited success. This is, at least in part, due to the lack of animal models that fully recapitulate the pathogenesis of hRSV infection in humans. This review summarises the strengths and limitations of animal models of hRSV, which include those in which hRSV is used to infect non-human mammalian hosts, and those in which non-human pneumoviruses, such as bovine (b)RSV and pneumonia virus of mice (PVM) are studied in their natural host. Apart from chimpanzees, other non-human primates (NHP) are only semi-permissive for hRSV replication and experimental infection with large doses of virus result in little or no clinical signs of disease, and generally only mild pulmonary pathology. Other animal models such as cotton rats, mice, ferrets, guinea pigs, hamsters, chinchillas, and neonatal lambs are also only semi-permissive for hRSV. Nevertheless, mice and cotton rats have been of value in the development of monoclonal antibody prophylaxis for infants at high risk of severe hRSV infection and have provided insights into mechanisms of immunity to and pathogenesis of hRSV. However, the extent to which they predict hRSV vaccine efficacy and safety is unclear and several hRSV vaccine candidates that are completely protective in rodent models are poorly effective in chimpanzees and other NHP, such as African Green monkeys. Furthermore, interpretation of findings from many rodent and NHP models of vaccine-enhanced hRSV disease has been confounded by sensitisation to non-viral antigens present in the vaccine and challenge virus. Studies of non-human pneumoviruses in their native hosts are more likely to reflect the pathogenesis of natural hRSV infection, and experimental infection of calves with bRSV and of mice with PVM result in clinical disease and extensive pulmonary pathology. These animal models have not only been of value in studies on mechanisms of immunity to and the pathogenesis of pneumovirus infections but have also been used to evaluate hRSV vaccine concepts. Furthermore, the similarities between the epidemiology of bRSV in calves and hRSV in infants and the high level of genetic and antigenic similarity between bRSV and hRSV, make the calf model of bRSV infection a relevant model for preclinical evaluation of hRSV vaccine candidates which contain proteins that are conserved between hRSV and bRSV.

Activation of Innate and Adaptive Immunity by a Recombinant Human Cytomegalovirus Strain Expressing an NKG2D Ligand

Development of an effective vaccine against human cytomegalovirus (HCMV) is a need of utmost medical importance. Generally, it is believed that a live attenuated vaccine would best provide protective immunity against this tenacious pathogen. Here, we propose a strategy for an HCMV vaccine that aims at the simultaneous activation of innate and adaptive immune responses. An HCMV strain expressing the host ligand ULBP2 for the NKG2D receptor was found to be susceptible to control by natural killer (NK) cells, and preserved the ability to stimulate HCMV-specific T cells. Infection with the ULBP2-expressing HCMV strain caused diminished cell surface levels of MHC class I molecules. While expression of the NKG2D ligand increased the cytolytic activity of NK cells, NKG2D engagement in CD8+ T cells provided co-stimulation and compensated for lower MHC class I expression. Altogether, our data indicate that triggering of both arms of the immune system is a promising approach applicable to the generation of a live attenuated HCMV vaccine.

Human cytomegalovirus (CMV) is a major cause of morbidity and mortality in congenitally infected newborns and immunocompromised individuals, indicating an utmost need for a vaccine to protect these vulnerable groups. Recent experimental studies in animal models, including non-human primates, have shown that attenuated CMVs trigger a potent immune response and are attractive vaccine candidates. However, an effective CMV vaccine is still not available. Here, we demonstrate that rational engineering of a live attenuated human CMV vaccine candidate is feasible. We equipped a CMV strain with an immunostimulatory molecule that is a ligand for an activating receptor present on both Natural Killer cells and CD8+ T cells. Moreover, we deleted several immunoevasins involved in downregulation of MHC class I molecules and of a ligand for Natural Killer cells in order to elicit stronger immune responses. In vitro assays using human immune cells and a first assessment in a humanized mouse model in vivo suggest that the generated CMV strain is attenuated and has the ability to induce a virus-specific immune response. Our study proposes this novel approach for the development of a rationally engineered CMV vaccine.

Capsid-CPSF6 Interaction Is Dispensable for HIV-1 Replication in Primary Cells but Is Selected during Virus Passage In Vivo

Cleavage and polyadenylation specificity factor subunit 6 (CPSF6), a host factor that interacts with the HIV-1 capsid (CA) protein, is implicated in diverse functions during the early part of the HIV-1 life cycle, including uncoating, nuclear entry, and integration targeting. Preservation of CA binding to CPSF6 in vivo suggests that this interaction is fine-tuned for efficient HIV-1 replication in physiologically relevant settings. Nevertheless, this possibility has not been formally examined. To assess the requirement for optimal CPSF6-CA binding during infection of primary cells and in vivo, we utilized a novel CA mutation, A77V, that significantly reduced CA binding to CPSF6. The A77V mutation rendered HIV-1 largely independent from TNPO3, NUP358, and NUP153 for infection and altered the integration site preference of HIV-1 without any discernible effects during the late steps of the virus life cycle. Surprisingly, the A77V mutant virus maintained the ability to replicate in monocyte-derived macrophages, primary CD4(+) T cells, and humanized mice at a level comparable to that for the wild-type (WT) virus. Nonetheless, revertant viruses that restored the WT CA sequence and hence CA binding to CPSF6 emerged in three out of four A77V-infected animals. These results suggest that the optimal interaction of CA with CPSF6, though not absolutely essential for HIV-1 replication in physiologically relevant settings, confers a significant fitness advantage to the virus and thus is strictly conserved among naturally circulating HIV-1 strains.

IMPORTANCE

CPSF6 interacts with the HIV-1 capsid (CA) protein and has been implicated in nuclear entry and integration targeting. Preservation of CPSF6-CA binding across various HIV-1 strains suggested that the optimal interaction between CA and CPSF6 is critical during HIV-1 replication in vivo Here, we identified a novel HIV-1 capsid mutant that reduces binding to CPSF6, is largely independent from the known cofactors for nuclear entry, and alters integration site preference. Despite these changes, virus carrying this mutation replicated in humanized mice at levels indistinguishable from those of the wild-type virus. However, in the majority of the animals, the mutant virus reverted back to the wild-type sequence, hence restoring the wild-type level of CA-CPSF6 interactions. These results suggest that optimal binding of CA to CPSF6 is not absolutely essential for HIV-1 replication in vivo but provides a fitness advantage that leads to the widespread usage of CPSF6 by HIV-1 in vivo.