Myeloid infection links epithelial and B cell tropisms of Murid Herpesvirus-4

Indirect CD4+ T cell protection against mouse gamma-herpesvirus infection via interferon gamma

CD4+ T cells play a key role in γ-herpesvirus infection control. However, the mechanisms involved are unclear. Murine herpesvirus type 4 (MuHV-4) allows relevant immune pathways to be dissected experimentally in mice. In the lungs, it colonizes myeloid cells, which can express MHC class II (MHCII), and type 1 alveolar epithelial cells (AEC1), which lack it. Nevertheless, CD4+ T cells can control AEC1 infection, and this control depends on MHCII expression in myeloid cells. Interferon-gamma (IFNγ) is a major component of CD4+ T cell-dependent MuHV-4 control. Here, we show that the action of IFNγ is also indirect, as CD4+ T cell-mediated control of AEC1 infection depended on IFNγ receptor (IFNγR1) expression in CD11c+ cells. Indirect control also depended on natural killer (NK) cells. Together, the data suggest that the activation of MHCII+ CD11c+ antigen-presenting cells is key to the CD4+ T cell/NK cell protection axis. By contrast, CD8+ T cell control of AEC1 infection appeared to operate independently.

IMPORTANCE

CD4+ T cells are critical for the control of gamma-herpesvirus infection; they act indirectly, by recruiting natural killer (NK) cells to attack infected target cells. Here, we report that the CD4+ T cell/NK cell axis of gamma-herpesvirus control requires interferon-γ engagement of CD11c+ dendritic cells. This mechanism of CD4+ T cell control releases the need for the direct engagement of CD4+ T cells with virus-infected cells and may be a common strategy for host control of immune-evasive pathogens.

Tissue and cellular tropism of Eptesicus fuscus gammaherpesvirus in big brown bats, potential role of pulmonary intravascular macrophages

Gammaherpesviruses (γHVs) are recognized as important pathogens in humans but their relationship with other animal hosts, especially wildlife species, is less well characterized. Our objectives were to examine natural Eptesicus fuscus gammaherpesvirus (EfHV) infections in their host, the big brown bat (Eptesicus fuscus), and determine whether infection is associated with disease. In tissue samples from 132 individual big brown bats, EfHV DNA was detected by polymerase chain reaction in 41 bats. Tissues from 59 of these cases, including 17 from bats with detectable EfHV genomes, were analyzed. An EfHV isolate was obtained from one of the cases, and electron micrographs and whole genome sequencing were used to confirm that this was a unique isolate of EfHV. Although several bats exhibited various lesions, we did not establish EfHV infection as a cause. Latent infection, defined as RNAScope probe binding to viral latency-associated nuclear antigen in the absence of viral envelope glycoprotein probe binding, was found within cells of the lymphoid tissues. These cells also had colocalization of the B-cell probe targeting CD20 mRNA. Probe binding for both latency-associated nuclear antigen and a viral glycoprotein was observed in individual cells dispersed throughout the alveolar capillaries of the lung, which had characteristics of pulmonary intravascular macrophages. Cells with a similar distribution in bat lungs expressed major histocompatibility class II, a marker for antigen presenting cells, and the existence of pulmonary intravascular macrophages in bats was confirmed with transmission electron microscopy. The importance of this cell type in γHVs infections warrants further investigation.

Macrophages drive KSHV B cell latency

Kaposi's sarcoma herpesvirus (KSHV) establishes lifelong infection and persists in latently infected B cells. Paradoxically, in vitro B cell infection is inefficient, and cells rapidly die, suggesting the absence of necessary factor(s). KSHV epidemiology unexpectedly mirrors that of malaria and certain helminthic infections, while other herpesviruses are ubiquitous. Elevated circulating monocytes are common in these parasitic infections. Here, we show that KSHV infection of monocytes or M-CSF-differentiated (M2) macrophages is highly efficient. Proteomic analyses demonstrate that infection induces macrophage production of B cell chemoattractants and activating factor. We find that KSHV acts with monocytes or M2 macrophages to stimulate B cell survival, proliferation, and plasmablast differentiation. Further, macrophages drive infected plasma cell differentiation and long-term viral latency. In Kenya, where KSHV is endemic, we find elevated monocyte levels in children with malaria. These findings demonstrate a role for mononuclear phagocytes in KSHV B cell latency and suggest that mononuclear phagocyte abundance may underlie KSHV's geographic disparity.

Murine Gammaherpesvirus 68 Efficiently Infects Myeloid Cells Resulting In An Atypical, Restricted Form Of Infection

The gammaherpesviruses (γHVs) establish a lifelong infection in their hosts, with the cellular outcome of infection intimately regulated by target cell type. Murine gammaherpesvirus 68 (MHV68), a small animal model of γHV infection, infects macrophages in vivo, resulting in a range of outcomes, from lytic replication to latent infection. Here, we have further investigated the nature of MHV68 macrophage infection using reductionist and primary in vivo infection studies. While MHV68 readily infected the J774 macrophage cell line, viral gene expression and replication were significantly impaired relative to a fully permissive fibroblast cell line. Lytic replication only occurred in a small subset of MHV68-infected J774 cells, despite the fact that these cells were fully competent to support lytic replication following pre-treatment with interleukin-4, a known potentiator of replication in macrophages. In parallel, we harvested virally-infected macrophages at 16 hours after MHV68 infection in vivo and analyzed gene expression by single cell RNA-sequencing. Among virally infected macrophages, only rare (0.25%) cells had lytic cycle gene expression, characterized by detection of multiple lytic cycle RNAs. In contrast, ~50% of virally-infected macrophages were characterized by expression of ORF75A, ORF75B and/or ORF75C, in the absence of other detectable viral RNAs. Selective transcription of the ORF75 locus also occurred in MHV68-infected J774 cells. In total, these studies indicate that MHV68 efficiently infects macrophages, with the majority of cells characterized by an atypical state of restricted viral transcription, and only rare cells undergoing lytic replication.

Lytic Replication and Reactivation from B Cells Is Not Required for Establishing or Maintaining Gammaherpesvirus Latency In Vivo

Gammaherpesviruses (GHVs) are lymphotropic tumor viruses with a biphasic infectious cycle. Lytic replication at the primary site of infection is necessary for GHVs to spread throughout the host and establish latency in distal sites. Dissemination is mediated by infected B cells that traffic hematogenously from draining lymph nodes to peripheral lymphoid organs, such as the spleen. B cells serve as the major reservoir for viral latency, and it is hypothesized that periodic reactivation from latently infected B cells contributes to maintaining long-term chronic infection. While fundamentally important to an understanding of GHV biology, aspects of B cell infection in latency establishment and maintenance are incompletely defined, especially roles for lytic replication and reactivation in this cell type. To address this knowledge gap and overcome limitations of replication-defective viruses, we generated a recombinant murine gammaherpesvirus 68 (MHV68) in which ORF50, the gene that encodes the essential immediate-early replication and transcription activator protein (RTA), was flanked by loxP sites to enable conditional ablation of lytic replication by ORF50 deletion in cells that express Cre recombinase. Following infection of mice that encode Cre in B cells with this virus, splenomegaly and viral reactivation from splenocytes were significantly reduced; however, the number of latently infected splenocytes was equivalent to WT MHV68. Despite ORF50 deletion, MHV68 latency was maintained over time in spleens of mice at levels approximating WT, reactivation-competent MHV68. Treatment of infected mice with lipopolysaccharide (LPS), which promotes B cell activation and MHV68 reactivation ex vivo, yielded equivalent increases in the number of latently infected cells for both ORF50-deleted and WT MHV68, even when mice were simultaneously treated with the antiviral drug cidofovir to prevent reactivation. Together, these data demonstrate that productive viral replication in B cells is not required for MHV68 latency establishment and support the hypothesis that B cell proliferation facilitates latency maintenance in vivo in the absence of reactivation. IMPORTANCE Gammaherpesviruses establish lifelong chronic infections in cells of the immune system and place infected hosts at risk for developing lymphomas and other diseases. It is hypothesized that gammaherpesviruses must initiate acute infection in these cells to establish and maintain long-term infection, but this has not been directly tested. We report here the use of a viral genetic system that allows for cell-type-specific deletion of a viral gene that is essential for replication and reactivation. We employ this system in an in vivo model to reveal that viral replication is not required to initiate or maintain infection within B cells.

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.

Conserved Gammaherpesvirus Protein Kinase Counters the Antiviral Effects of Myeloid Cell-Specific STAT1 Expression To Promote the Establishment of Splenic B Cell Latency

Gammaherpesviruses establish lifelong infections and are associated with B cell lymphomas. Murine gammaherpesvirus 68 (MHV68) infects epithelial and myeloid cells during acute infection, with subsequent passage of the virus to B cells, where physiological B cell differentiation is usurped to ensure the establishment of a chronic latent reservoir. Interferons (IFNs) represent a major antiviral defense system that engages the transcriptional factor STAT1 to attenuate diverse acute and chronic viral infections, including those of gammaherpesviruses. Correspondingly, global deficiency of type I or type II IFN signaling profoundly increases the pathogenesis of acute and chronic gammaherpesvirus infection, compromises host survival, and impedes mechanistic understanding of cell type-specific role of IFN signaling. Here, we demonstrate that myeloid-specific STAT1 expression attenuates acute and persistent MHV68 replication in the lungs and suppresses viral reactivation from peritoneal cells, without any effect on the establishment of viral latent reservoir in splenic B cells. All gammaherpesviruses encode a conserved protein kinase that antagonizes type I IFN signaling in vitro. Here, we show that myeloid-specific STAT1 deficiency rescues the attenuated splenic latent reservoir of the kinase-null MHV68 mutant. However, despite having gained access to splenic B cells, the protein kinase-null MHV68 mutant fails to drive B cell differentiation. Thus, while myeloid-intrinsic STAT1 expression must be counteracted by the gammaherpesvirus protein kinase to facilitate viral passage to splenic B cells, expression of the viral protein kinase continues to be required to promote optimal B cell differentiation and viral reactivation, highlighting the multifunctional nature of this conserved viral protein during chronic infection. IMPORTANCE IFN signaling is a major antiviral system of the host that suppresses replication of diverse viruses, including acute and chronic gammaherpesvirus infection. STAT1 is a critical member and the primary antiviral effector of IFN signaling pathways. Given the significantly compromised antiviral status of global type I or type II IFN deficiency, unabated gammaherpesvirus replication and pathogenesis hinders understanding of cell type-specific antiviral effects. In this study, a mouse model of myeloid-specific STAT1 deficiency unveiled site-specific antiviral effects of STAT1 in the lungs and peritoneal cavity, but not the spleen, of chronically infected hosts. Interestingly, expression of a conserved gammaherpesvirus protein kinase was required to counteract the antiviral effects of myeloid-specific STAT1 expression to facilitate latent infection of splenic B cells, revealing a cell type-specific virus-host antagonism during the establishment of chronic gammaherpesvirus infection.

Interferon Regulatory Factor 3 Supports the Establishment of Chronic Gammaherpesvirus Infection in a Route- and Dose-Dependent Manner

Gammaherpesviruses are ubiquitous pathogens that establish lifelong infections and are associated with several malignancies, including B cell lymphomas. Uniquely, these viruses manipulate B cell differentiation to establish long-term latency in memory B cells. This study focuses on the interaction between gammaherpesviruses and interferon regulatory factor 3 (IRF-3), a ubiquitously expressed transcription factor with multiple direct target genes, including beta interferon (IFN-β), a type I IFN. IRF-3 attenuates acute replication of a plethora of viruses, including gammaherpesvirus. Furthermore, IRF-3-driven IFN-β expression is antagonized by the conserved gammaherpesvirus protein kinase during lytic virus replication in vitro In this study, we have uncovered an unexpected proviral role of IRF-3 during chronic gammaherpesvirus infection. In contrast to the antiviral activity of IRF-3 during acute infection, IRF-3 facilitated establishment of latent gammaherpesvirus infection in B cells, particularly, germinal center and activated B cells, the cell types critical for both natural infection and viral lymphomagenesis. This proviral role of IRF-3 was further modified by the route of infection and viral dose. Furthermore, using a combination of viral and host genetics, we show that IRF-3 deficiency does not rescue attenuated chronic infection of a protein kinase null gammaherpesvirus mutant, highlighting the multifunctional nature of the conserved gammaherpesvirus protein kinases in vivo In summary, this study unveils an unexpected proviral nature of the classical innate immune factor, IRF-3, during chronic virus infection.IMPORTANCE Interferon regulatory factor 3 (IRF-3) is a critical component of the innate immune response, in part due to its transactivation of beta interferon (IFN-β) expression. Similar to that observed in all acute virus infections examined to date, IRF-3 suppresses lytic viral replication during acute gammaherpesvirus infection. Because gammaherpesviruses establish lifelong infection, this study aimed to define the antiviral activity of IRF-3 during chronic infection. Surprisingly, we found that, in contrast to acute infection, IRF-3 supported the establishment of gammaherpesvirus latency in splenic B cells, revealing an unexpected proviral nature of this classical innate immune host factor.

Gammaherpesvirus Usurps Host IL-17 Signaling To Support the Establishment of Chronic Infection

Gammaherpesviruses establish lifelong infections in a majority of humans and are associated with B cell lymphomas. IL-17A is a host cytokine that plays a well-established role in the clearance of bacterial and fungal infections; however, the role of IL-17A in viral infections is poorly understood.

Gammaherpesviruses establish lifelong infection and are associated with a variety of cancers, including B cell lymphomas. These viruses manipulate the B cell differentiation process to establish lifelong infection in memory B cells. Specifically, gammaherpesviruses infect naive B cells and promote entry of both infected and uninfected naive B cells into germinal centers, where the virus usurps rapid proliferation of germinal center B cells to exponentially increase its cellular latent reservoir. In addition to facilitating the establishment of latent infection, germinal center B cells are thought to be the target of viral transformation. In this study, we have uncovered a novel proviral role of host interleukin 17A (IL-17A), a well-established antibacterial and antifungal factor. Loss of IL-17A signaling attenuated the establishment of chronic gammaherpesvirus infection and gammaherpesvirus-driven germinal center response in a route of inoculation-dependent manner. Further, IL-17A treatment directly supported gammaherpesvirus reactivation and de novo lytic infection. This study is the first demonstration of a multifaceted proviral role of IL-17 signaling.

Optimized Detection of Acute MHV68 Infection With a Reporter System Identifies Large Peritoneal Macrophages as a Dominant Target of Primary Infection

Investigating the dynamics of virus-host interactions in vivo remains an important challenge, often limited by the ability to directly identify virally infected cells. Here, we utilize a beta-lactamase activated fluorescent substrate to identify primary targets of murine gammaherpesvirus 68 (MHV68) infection in the peritoneal cavity. By optimizing substrate and detection conditions, we were able to achieve multiparameter characterization of infected cells and the ensuing host response. MHV68 infection leads to a pronounced increase in immune cells, with CD8+ T cells increasing by 3 days, and total infiltrate peaking around 8 days post-infection. MHV68 infection results in near elimination of large peritoneal macrophages (LPMs) by 8 days post-infection, and a concordant increase in small peritoneal macrophages (SPMs) and monocytes. Infection is associated with prolonged changes to myeloid cells, with a distinct population of MHC II^high LPMs emerging by 14 days. Targets of MHV68 infection could be readily detected. Between 1 and 3 days post-infection, MHV68 infects ∼5–10% of peritoneal cells, with >75% being LPMs. By 8 days post-infection, the frequency of MHV68 infection is reduced at least 10-fold, with infection primarily in SPMs, with few infected dendritic cells and B cells. Importantly, limiting dilution analysis indicates that at 3 days post-infection, the majority of MHV68-infected cells harbor latent rather than lytic virus at frequencies consistent with those identified based on reporter gene expression. Our findings demonstrate the utility of the beta-lactamase MHV68 reporter system for high throughput single-cell analysis and identify dynamic changes during primary gammaherpesvirus infection.

Deletion of Murine Gammaherpesvirus Gene M2 in Activation-Induced Cytidine Deaminase-Expressing B Cells Impairs Host Colonization and Viral Reactivation

Gammaherpesviruses (GHVs) are DNA tumor viruses that establish lifelong, chronic infections in lymphocytes of humans and other mammals. GHV infections are associated with numerous cancers, especially in immunocompromised hosts. While it is known that GHVs utilize host germinal center (GC) B cell responses during latency establishment, an understanding of how viral gene products function in specific B cell subsets to regulate this process is incomplete. Using murine gammaherpesvirus 68 (MHV68) as a small-animal model to define mechanisms of GHV pathogenesis in vivo, we generated a virus in which the M2 gene was flanked by loxP sites (M2.loxP), enabling the use of Cre-lox technology to define M2 function in specific cell types in infection and disease. The M2 gene encodes a protein that is highly expressed in GC B cells that promotes plasma cell differentiation and viral reactivation. M2 was efficiently deleted in Cre-expressing cells, and the presence of loxP sites flanking M2 did not alter viral replication or latency in mice that do not express Cre. In contrast, M2.loxP MHV68 exhibited a deficit in latency establishment and reactivation that resembled M2-null virus, following intranasal (IN) infection of mice that express Cre in all B cells (CD19-Cre). Nearly identical phenotypes were observed for M2.loxP MHV68 in mice that express Cre in germinal center (GC) B cells (AID-Cre). However, colonization of neither draining lymph nodes after IN infection nor the spleen after intraperitoneal (IP) infection required M2, although the reactivation defect was retained. Together, these data confirm that M2 function is B cell-specific and demonstrate that M2 primarily functions in AID-expressing cells to facilitate MHV68 dissemination to distal latency reservoirs within the host and reactivation from latency. Our study reveals that a viral latency gene functions within a distinct subset of cells to facilitate host colonization.IMPORTANCE Gammaherpesviruses establish lifelong chronic infections in cells of the immune system that can lead to lymphomas and other diseases. To facilitate colonization of a host, gammaherpesviruses encode gene products that manipulate processes involved in cellular proliferation and differentiation. Whether and how these viral gene products function in specific cells of the immune system is poorly defined. We report here the use of a viral genetic system that allows for deletion of specific viral genes in discrete populations of cells. We employ this system in an in vivo model to demonstrate cell-type-specific requirements for a particular viral gene. Our findings reveal that a viral gene product can function in distinct cellular subsets to direct gammaherpesvirus pathogenesis.

Deletion of immune evasion genes provides an effective vaccine design for tumor-associated herpesviruses

Vaccines based on live attenuated viruses often induce broad, multifaceted immune responses. However, they also usually sacrifice immunogenicity for attenuation. It is particularly difficult to elicit an effective vaccine for herpesviruses due to an armament of immune evasion genes and a latent phase. Here, to overcome the limitation of attenuation, we developed a rational herpesvirus vaccine in which viral immune evasion genes were deleted to enhance immunogenicity while also attaining safety. To test this vaccine strategy, we utilized murine gammaherpesvirus-68 (MHV-68) as a proof-of-concept model for the cancer-associated human γ-herpesviruses, Epstein-Barr virus and Kaposi sarcoma-associated herpesvirus. We engineered a recombinant MHV-68 virus by targeted inactivation of viral antagonists of type I interferon (IFN-I) pathway and deletion of the latency locus responsible for persistent infection. This recombinant virus is highly attenuated with no measurable capacity for replication, latency, or persistence in immunocompetent hosts. It stimulates robust innate immunity, differentiates virus-specific memory T cells, and elicits neutralizing antibodies. A single vaccination affords durable protection that blocks the establishment of latency following challenge with the wild type MHV-68 for at least six months post-vaccination. These results provide a framework for effective vaccination against cancer-associated herpesviruses through the elimination of latency and key immune evasion mechanisms from the pathogen.

Dangerous Liaisons: Gammaherpesvirus Subversion of the Immunoglobulin Repertoire

A common biologic property of the gammaherpesviruses Epstein–Barr Virus and Kaposi sarcoma herpesvirus is their use of B lymphocytes as a reservoir of latency in healthy individuals that can undergo oncogenic transformation later in life. Gammaherpesviruses (GHVs) employ an impressive arsenal of proteins and non-coding RNAs to reprogram lymphocytes for proliferative expansion. Within lymphoid tissues, the germinal center (GC) reaction is a hub of B cell proliferation and death. The goal of a GC is to generate and then select for a pool of immunoglobulin (Ig) genes that will provide a protective humoral adaptive immune response. B cells infected with GHVs are detected in GCs and bear the hallmark signatures of the mutagenic processes of somatic hypermutation and isotype class switching of the Ig genes. However, data also supports extrafollicular B cells as a reservoir engaged by GHVs. Next-generation sequencing technologies provide unprecedented detail of the Ig sequence that informs the natural history of infection at the single cell level. Here, we review recent reports from human and murine GHV systems that identify striking differences in the immunoglobulin repertoire of infected B cells compared to their uninfected counterparts. Implications for virus biology, GHV-associated cancers, and host immune dysfunction will be discussed.

Vaccine protection against murid herpesvirus-4 is maintained when the priming virus lacks known latency genes

γ-Herpesviruses establish latent infections of lymphocytes and drive their proliferation, causing cancers and motivating a search for vaccines. Effective vaccination against murid herpesvirus-4 (MuHV-4)-driven lymphoproliferation by latency-impaired mutant viruses suggests that lytic access to the latency reservoir is a viable target for control. However, the vaccines retained the immunogenic MuHV-4 M2 latency gene. Here, a strong reduction in challenge virus load was maintained when the challenge virus lacked the main latency-associated CD8⁺ T-cell epitope of M2, or when the vaccine virus lacked M2 entirely. This protection was maintained also when the vaccine virus lacked both episome maintenance and the genomic region encompassing M1, M2, M3, M4 and ORF4. Therefore, protection did not require immunity to known MuHV-4 latency genes. As the remaining vaccine virus genes have clear homologs in human γ-herpesviruses, this approach of deleting viral latency genes could also be applied to them, to generate safe and effective vaccines against human disease.

IFN-λ Decreases Murid Herpesvirus-4 Infection of the Olfactory Epithelium but Fails to Prevent Virus Reactivation in the Vaginal Mucosa

Murid herpesvirus-4 (MuHV-4), a natural gammaherpesvirus of rodents, can infect the mouse through the nasal mucosa, where it targets sustentacular cells and olfactory neurons in the olfactory epithelium before it propagates to myeloid cells and then to B cells in lymphoid tissues. After establishment of latency in B cells, viral reactivation occurs in the genital tract in 80% of female mice, which can lead to spontaneous sexual transmission to co-housed males. Interferon-lambda (IFN-λ) is a key player of the innate immune response at mucosal surfaces and is believed to limit the transmission of numerous viruses by acting on epithelial cells. We used in vivo plasmid-mediated IFN-λ expression to assess whether IFN-λ could prophylactically limit MuHV-4 infection in the olfactory and vaginal mucosae. In vitro, IFN-λ decreased MuHV-4 infection in cells that overexpressed IFN-λ receptor 1 (IFNLR1). In vivo, prophylactic IFN-λ expression decreased infection of the olfactory epithelium but did not prevent virus propagation to downstream organs, such as the spleen where the virus establishes latency. In the olfactory epithelium, sustentacular cells readily responded to IFN-λ. In contrast, olfactory neurons did not respond to IFN-λ, thus, likely allowing viral entry. In the female genital tract, columnar epithelial cells strongly responded to IFN-λ, as did most vaginal epithelial cells, although with some variation from mouse to mouse. IFN-λ expression, however, failed to prevent virus reactivation in the vaginal mucosa. In conclusion, IFN-λ decreased MuHV-4 replication in the upper respiratory epithelium, likely by protecting the sustentacular epithelial cells, but it did not protect olfactory neurons and failed to block virus reactivation in the genital mucosa.

Immune Control of γ-Herpesviruses

Vaccination against γ-herpesviruses has been hampered by our limited understanding of their normal control. Epstein–Barr virus (EBV)-transformed B cells are killed by viral latency antigen-specific CD8⁺ T cells in vitro, but attempts to block B cell infection with antibody or to prime anti-viral CD8⁺ T cells have protected poorly in vivo. The Doherty laboratory used Murid Herpesvirus-4 (MuHV-4) to analyze γ-herpesvirus control in mice and found CD4⁺ T cell dependence, with viral evasion limiting CD8⁺ T cell function. MuHV-4 colonizes germinal center (GC) B cells via lytic transfer from myeloid cells, and CD4⁺ T cells control myeloid infection. GC colonization and protective, lytic antigen-specific CD4⁺ T cells are now evident also for EBV. Subunit vaccines have protected only transiently against MuHV-4, but whole virus vaccines give long-term protection, via CD4⁺ T cells and antibody. They block infection transfer to B cells, and need include no known viral latency gene, nor any MuHV-4-specific gene. Thus, the Doherty approach of in vivo murine analysis has led to a plausible vaccine strategy for EBV and, perhaps, some insight into what CD8⁺ T cells really do.

Glycoprotein K8.1A of Kaposi's Sarcoma-Associated Herpesvirus Is a Critical B Cell Tropism Determinant Independent of Its Heparan Sulfate Binding Activity

B lymphocytes are the major cellular reservoir in individuals infected with Kaposi's sarcoma-associated herpesvirus (KSHV), and the virus is etiologically linked to two B cell lymphoproliferative disorders. We previously described the MC116 human B cell line as a KSHV-susceptible model to overcome the paradoxical refractoriness of B cell lines to experimental KSHV infection. Here, using monoclonal antibody inhibition and a deletion mutant virus, we demonstrate that the KSHV virion glycoprotein K8.1A is critical for infection of MC116, as well as tonsillar B cells; in contrast, we confirm previous reports on the dispensability of the glycoprotein for infection of primary endothelial cells and other commonly studied non-B cell targets. Surprisingly, we found that the role of K8.1A in B cell infection is independent of its only known biochemical activity of binding to surface heparan sulfate, suggesting the possible involvement of an additional molecular interaction(s). Our finding that K8.1A is a critical determinant for KSHV B cell tropism parallels the importance of proteins encoded by positionally homologous genes for the cell tropism of other gammaherpesviruses.IMPORTANCE Elucidating the molecular mechanisms by which KSHV infects B lymphocytes is critical for understanding how the virus establishes lifelong persistence in infected people, in whom it can cause life-threatening B cell lymphoproliferative disease. Here, we show that K8.1A, a KSHV-encoded glycoprotein on the surfaces of the virus particles, is critical for infection of B cells. This finding stands in marked contrast to previous studies with non-B lymphoid cell types, for which K8.1A is known to be dispensable. We also show that the required function of K8.1A in B cell infection does not involve its binding to cell surface heparan sulfate, the only known biochemical activity of the glycoprotein. The discovery of this critical role of K8.1A in KSHV B cell tropism opens promising new avenues to unravel the complex mechanisms underlying infection and disease caused by this viral human pathogen.

Gammaherpesvirus BoHV-4 infects bovine respiratory epithelial cells mainly at the basolateral side

Bovine herpesvirus 4 (BoHV-4) is a gammaherpesvirus that is widespread in cattle. However, only a few studies about the pathogenesis of BoHV-4 primary infection have been reported. In the present study, ex vivo models with bovine nasal and tracheal mucosa explants were used to study the cellular BoHV-4-host interactions. Infection was observed in nasal but not in tracheal epithelial cells. To find a possible correlation between the integrity and restricted infection of the respiratory epithelium, both nasal mucosal and tracheal explants were treated with EGTA, a drug that disrupts the intercellular junctions, before inoculation. The infection was analyzed based on the number of plaques, plaque latitude and number of infected single cells, as determined by immunofluorescence. BoHV-4 infection in nasal mucosal explants was enhanced upon opening the tight junctions with EGTA. Infection in tracheal explants was only found after treatment with EGTA. In addition, primary bovine respiratory epithelial cells (BREC) were isolated, grown at the air–liquid interface and infected either at the apical or basolateral side by BoHV-4. The results showed that BoHV-4 preferentially bound to and entered BREC at the basolateral surfaces of both nasal and tracheal epithelial cells. The percentage of BoHV-4 infection was significantly increased both from nasal and tracheal epithelial cells after treatment with EGTA, which indicates that the BoHV-4 receptor is mainly located at the basolateral surface of these cells. Thus, our findings demonstrate that integrity of the respiratory epithelium is crucial in the host’s innate defense against primary BoHV-4 infections.

Murine Cytomegalovirus Glycoprotein O Promotes Epithelial Cell Infection In Vivo

Cytomegaloviruses (CMVs) establish systemic infections across diverse cell types. Glycoproteins that alter tropism can potentially guide their spread. Glycoprotein O (gO) is a nonessential fusion complex component of both human CMV (HCMV) and murine CMV (MCMV). We tested its contribution to MCMV spread from the respiratory tract. In vitro, MCMV lacking gO poorly infected fibroblasts and epithelial cells. Cell binding was intact, but penetration was delayed. In contrast, myeloid infection was preserved, and in the lungs, where myeloid and type 2 alveolar epithelial cells are the main viral targets, MCMV lacking gO showed a marked preference for myeloid infection. Its poor epithelial cell infection was associated with poor primary virus production and reduced virulence. Systemic spread, which proceeds via infected CD11c⁺ myeloid cells, was initially intact but then diminished, because less epithelial infection led ultimately to less myeloid infection. Thus, the tight linkage between peripheral and systemic MCMV infections gave gO-dependent infection a central role in host colonization.IMPORTANCE Human cytomegalovirus is a leading cause of congenital disease. This reflects its capacity for systemic spread. A vaccine is needed, but the best viral targets are unclear. Attention has focused on the virion membrane fusion complex. It has 2 forms, so we need to know what each contributes to host colonization. One includes the virion glycoprotein O. We used murine cytomegalovirus, which has equivalent fusion complexes, to determine the importance of glycoprotein O after mucosal infection. We show that it drives local virus replication in epithelial cells. It was not required to infect myeloid cells, which establish systemic infection, but poor local replication reduced systemic spread as a secondary effect. Therefore, targeting glycoprotein O of human cytomegalovirus has the potential to reduce both local and systemic infections.

Gammaherpesvirus Colonization of the Spleen Requires Lytic Replication in B Cells

Gammaherpesviruses infect lymphocytes and cause lymphocytic cancers. Murid herpesvirus-4 (MuHV-4), Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus all infect B cells. Latent infection can spread by B cell recirculation and proliferation, but whether this alone achieves systemic infection is unclear. To test the need of MuHV-4 for lytic infection in B cells, we flanked its essential ORF50 lytic transactivator with loxP sites and then infected mice expressing B cell-specific Cre (CD19-Cre). The floxed virus replicated normally in Cre^- mice. In CD19-Cre mice, nasal and lymph node infections were maintained; but there was little splenomegaly, and splenic virus loads remained low. Cre-mediated removal of other essential lytic genes gave a similar phenotype. CD19-Cre spleen infection by intraperitoneal virus was also impaired. Therefore, MuHV-4 had to emerge lytically from B cells to colonize the spleen. An important role for B cell lytic infection in host colonization is consistent with the large CD8⁺ T cell responses made to gammaherpesvirus lytic antigens during infectious mononucleosis and suggests that vaccine-induced immunity capable of suppressing B cell lytic infection might reduce long-term virus loads.IMPORTANCE Gammaherpesviruses cause B cell cancers. Most models of host colonization derive from cell cultures with continuous, virus-driven B cell proliferation. However, vaccines based on these models have worked poorly. To test whether proliferating B cells suffice for host colonization, we inactivated the capacity of MuHV-4, a gammaherpesvirus of mice, to reemerge from B cells. The modified virus was able to colonize a first wave of B cells in lymph nodes but spread poorly to B cells in secondary sites such as the spleen. Consequently, viral loads remained low. These results were consistent with virus-driven B cell proliferation exploiting normal host pathways and thus having to transfer lytically to new B cells for new proliferation. We conclude that viral lytic infection is a potential target to reduce B cell proliferation.

Viral FGARAT ORF75A promotes early events in lytic infection and gammaherpesvirus pathogenesis in mice

Gammaherpesviruses encode proteins with homology to the cellular purine metabolic enzyme formyl-glycinamide-phosphoribosyl-amidotransferase (FGARAT), but the role of these viral FGARATs (vFGARATs) in the pathogenesis of a natural host has not been investigated. We report a novel role for the ORF75A vFGARAT of murine gammaherpesvirus 68 (MHV68) in infectious virion production and colonization of mice. MHV68 mutants with premature stop codons in orf75A exhibited a log reduction in acute replication in the lungs after intranasal infection, which preceded a defect in colonization of multiple host reservoirs including the mediastinal lymph nodes, peripheral blood mononuclear cells, and the spleen. Intraperitoneal infection rescued splenic latency, but not reactivation. The 75A.stop virus also exhibited defective replication in primary fibroblast and macrophage cells. Viruses produced in the absence of ORF75A were characterized by an increase in the ratio of particles to PFU. In the next round of infection this led to the alteration of early events in lytic replication including the deposition of the ORF75C tegument protein, the accelerated kinetics of viral gene expression, and induction of TNFα release and cell death. Infecting cells to deliver equivalent genomes revealed that ORF75A was required for initiating early events in infection. In contrast with the numerous phenotypes observed in the absence of ORF75A, ORF75B was dispensable for replication and pathogenesis. These studies reveal that murine rhadinovirus vFGARAT family members ORF75A and ORF75C have evolved to perform divergent functions that promote replication and colonization of the host.

Gammaherpesviruses are infectious agents that cause cancer. The study of viral genes unique to this subfamily may offer insight into the strategies that these viruses use to persist in the host and drive disease. The vFGARATs are a family of viral proteins found only in gammaherpesviruses, and are critical for replication in cell culture. Here we report that a rhadinovirus of rodents requires a previously uncharacterized vFGARAT family member, ORF75A, to support viral growth and persistence in mice. In addition, viruses lacking ORF75A are defective in the production of infectious viral particles. Thus, duplications and functional divergence of the various vFGARATs in the rhadinovirus lineage have likely been driven by selective pressures to disseminate within and colonize the host. Identification of the shared host processes that are targeted by the diverse family of vFGARATs may reveal novel targets for therapeutic agents to prevent life-long infections by these oncogenic viruses.

Conditional mutagenesis in vivo reveals cell type- and infection stage-specific requirements for LANA in chronic MHV68 infection

Gammaherpesvirus (GHV) pathogenesis is a complex process that involves productive viral replication, dissemination to tissues that harbor lifelong latent infection, and reactivation from latency back into a productive replication cycle. Traditional loss-of-function mutagenesis approaches in mice using murine gammaherpesvirus 68 (MHV68), a model that allows for examination of GHV pathogenesis in vivo, have been invaluable for defining requirements for specific viral gene products in GHV infection. But these approaches are insufficient to fully reveal how viral gene products contribute when the encoded protein facilitates multiple processes in the infectious cycle and when these functions vary over time and from one host tissue to another. To address this complexity, we developed an MHV68 genetic platform that enables cell-type-specific and inducible viral gene deletion in vivo. We employed this system to re-evaluate functions of the MHV68 latency-associated nuclear antigen (mLANA), a protein with roles in both viral replication and latency. Cre-mediated deletion in mice of loxP-flanked ORF73 demonstrated the necessity of mLANA in B cells for MHV68 latency establishment. Impaired latency during the transition from draining lymph nodes to blood following mLANA deletion also was observed, supporting the hypothesis that B cells are a major conduit for viral dissemination. Ablation of mLANA in infected germinal center (GC) B cells severely impaired viral latency, indicating the importance of viral passage through the GC for latency establishment. Finally, induced ablation of mLANA during latency resulted in complete loss of affected viral genomes, indicating that mLANA is critically important for maintenance of viral genomes during stable latency. Collectively, these experiments provide new insights into LANA homolog functions in GHV colonization of the host and highlight the potential of a new MHV68 genetic platform to foster a more complete understanding of viral gene functions at discrete stages of GHV pathogenesis.

Gammaherpesviruses (GHVs), including the human pathogens Epstein-Barr virus and Kaposi sarcoma-associated herpesvirus, establish lifelong infections that can lead to cancer. Defining the functions of viral gene products in acute replication and chronic infection is important for understanding how these viruses cause disease. Infection of mice with the related GHV, murine gammaherpesvirus 68 (MHV68), provides a tractable small animal model for defining how viral gene products function in chronic infection and understanding how host factors limit disease. Here we describe the development of a new viral genetic platform that enables the targeted deletion of specific genes from the viral genome in discrete host cells or at distinct times during infection. We utilize this system to better define requirements for the conserved latency-associated nuclear antigen in MHV68 lytic replication and latency in mice. This work highlights the utility of this MHV68 genetic platform for defining mechanisms of GHV infection and disease.

Primary replication and invasion of the bovine gammaherpesvirus BoHV-4 in the genital mucosae

Bovine herpesvirus 4 (BoHV-4) is a gammaherpesvirus that is widespread in cattle. Ex vivo models with bovine genital tract mucosa explants were set up to study molecular/cellular BoHV-4-host interactions. Bovine posterior vagina, cervix and uterus body were collected from cows at two stages of the reproductive cycle for making mucosa explants. The BoHV-4 replication kinetics and characteristics within the three different mucosae of animals in the follicular and luteal phase were assessed by virus titration. The number of plaques, plaque latitude and number of infected cells were determined by immunofluorescence. BoHV-4 replicated in a productive way in all genital mucosal tissues. It infected single individual cells in both epithelium and lamina propria of the genital mucosae at 24 hours post-inoculation (hpi). Later, small BoHV-4 epithelial plaques were formed that did not spread through the basement membrane. 50% of the number of BoHV-4 infected cells were identified as cytokeratin⁺ and CD172a⁺ cells in the three parts of the genital tract at 24 hpi. Upon a direct injection of genital explants with BoHV-4, fibrocytes became infected, indicating that the unidentified 50% of the infected cells are most probably fibrocytes. In this study, in vivo-related in vitro genital tract models were successfully established and the early stage of the pathogenesis of a genital infection was clarified: BoHV-4 starts with a productive infection of epithelial cells in the reproductive tract, forming small foci followed by a non-productive infection of surveilling monocytic cells which help BoHV-4 to invade into deeper tissues.

Type I Interferon Signaling to Dendritic Cells Limits Murid Herpesvirus 4 Spread from the Olfactory Epithelium

Murid herpesvirus 4 (MuHV-4) is a B cell-tropic gammaherpesvirus that can be studied in vivo Despite viral evasion, type I interferons (IFN-I) limit its spread. After MuHV-4 inoculation into footpads, IFN-I protect lymph node subcapsular sinus macrophages (SSM) against productive infection; after peritoneal inoculation, they protect splenic marginal zone macrophages, and they limit MuHV-4 replication in the lungs. While invasive infections can be used to test specific aspects of host colonization, it is also important to understand natural infection. MuHV-4 taken up spontaneously by alert mice enters them via olfactory neurons. We determined how IFN-I act in this context. Blocking IFN-I signaling did not increase neuronal infection but allowed the virus to spread to the adjacent respiratory epithelium. In lymph nodes, a complete IFN-I signaling block increased MuHV-4 lytic infection in SSM and increased the number of dendritic cells (DC) expressing viral green fluorescent protein (GFP) independently of lytic infection. A CD11c⁺ cell-directed signaling block increased infection of DC only. However, this was sufficient to increase downstream infection, consistent with DC providing the main viral route to B cells. The capacity of IFN-I to limit DC infection indicated that viral IFN-I evasion was only partly effective. Therefore, DC are a possible target for IFN-I-based interventions to reduce host colonization.IMPORTANCE Human gammaherpesviruses infect B cells and cause B cell cancers. Interventions to block virus binding to B cells have not stopped their infection. Therefore, we must identify other control points that are relevant to natural infection. Human infections are difficult to analyze. However, gammaherpesviruses colonize all mammals. A related gammaherpesvirus of mice reaches B cells not directly but via infected dendritic cells. We show that type I interferons, an important general antiviral defense, limit gammaherpesvirus B cell infection by acting on dendritic cells. Therefore, dendritic cell infection is a potential point of interferon-based therapeutic intervention.

CD8+ T cell evasion mandates CD4+ T cell control of chronic gamma-herpesvirus infection

Gamma-herpesvirus infections are regulated by both CD4⁺ and CD8⁺ T cells. However clinical disease occurs mainly in CD4⁺ T cell-deficient hosts. In CD4⁺ T cell-deficient mice, CD8⁺ T cells control acute but not chronic lung infection by Murid Herpesvirus-4 (MuHV-4). We show that acute and chronic lung infections differ in distribution: most acute infection was epithelial, whereas most chronic infection was in myeloid cells. CD8⁺ T cells controlled epithelial infection, but CD4⁺ T cells and IFNγ were required to control myeloid cell infection. Disrupting the MuHV-4 K3, which degrades MHC class I heavy chains, increased viral epitope presentation by infected lung alveolar macrophages and allowed CD8⁺ T cells to prevent disease. Thus, viral CD8⁺ T cell evasion led to niche-specific immune control, and an essential role for CD4⁺ T cells in limiting chronic infection.

Gamma-herpesviruses chronically infect most people. While infection is usually asymptomatic, disease occurs if the immune system is weakened. Understanding how immune control normally works should provide a basis for preventing disease. In mice, CD8⁺ T cells can control acute gamma-herpesvirus infection but not chronic infection. We show that acute and chronic infections involve different cell types. CD8⁺ T cells controlled epithelial cell infection, which predominated acutely, but they could not control chronic macrophage infection unless viral immune evasion was disabled. Instead CD4⁺ T cells were required. Thus, viral evasion made host defence cell type-specific: CD8⁺ T cells controlled epithelial cell infection; CD4⁺ T cells controlled macrophage infection; and comprehensive control required both T cell subsets.

Type I Interferons and NK Cells Restrict Gammaherpesvirus Lymph Node Infection

Gammaherpesviruses establish persistent, systemic infections and cause cancers. Murid herpesvirus 4 (MuHV-4) provides a unique window into the early events of host colonization. It spreads via lymph nodes. While dendritic cells (DC) pass MuHV-4 to lymph node B cells, subcapsular sinus macrophages (SSM), which capture virions from the afferent lymph, restrict its spread. Understanding how this restriction works offers potential clues to a more comprehensive defense. Type I interferon (IFN-I) blocked SSM lytic infection and reduced lytic cycle-independent viral reporter gene expression. Plasmacytoid DC were not required, but neither were SSM the only source of IFN-I, as IFN-I blockade increased infection in both intact and SSM-depleted mice. NK cells restricted lytic SSM infection independently of IFN-I, and SSM-derived virions spread to the spleen only when both IFN-I responses and NK cells were lacking. Thus, multiple innate defenses allowed SSM to adsorb virions from the afferent lymph with relative impunity. Enhancing IFN-I and NK cell recruitment could potentially also restrict DC infection and thus improve infection control.

IMPORTANCE

Human gammaherpesviruses cause cancers by infecting B cells. However, vaccines designed to block virus binding to B cells have not stopped infection. Using a related gammaherpesvirus of mice, we have shown that B cells are infected not via cell-free virus but via infected myeloid cells. This suggests a different strategy to stop B cell infection: stop virus production by myeloid cells. Not all myeloid infection is productive. We show that subcapsular sinus macrophages, which do not pass infection to B cells, restrict gammaherpesvirus production by recruiting type I interferons and natural killer cells. Therefore, a vaccine that speeds the recruitment of these defenses might stop B cell infection.

Herpes Simplex Virus 1 Interaction with Myeloid Cells In Vivo

Herpes simplex virus 1 (HSV-1) enters mice via olfactory epithelial cells and then colonizes the trigeminal ganglia (TG). Most TG nerve endings are subepithelial, so this colonization implies subepithelial viral spread, where myeloid cells provide an important line of defense. The outcome of infection of myeloid cells by HSV-1 in vitro depends on their differentiation state; the outcome in vivo is unknown. Epithelial HSV-1 commonly infected myeloid cells, and Cre-Lox virus marking showed nose and lung infections passing through LysM-positive (LysM(+)) and CD11c(+) cells. In contrast, subcapsular sinus macrophages (SSMs) exposed to lymph-borne HSV-1 were permissive only when type I interferon (IFN-I) signaling was blocked; normally, their infection was suppressed. Thus, the outcome of myeloid cell infection helped to determine the HSV-1 distribution: subepithelial myeloid cells provided a route of spread from the olfactory epithelium to TG neurons, while SSMs blocked systemic spread.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) infects most people and can cause severe disease. This reflects its persistence in nerve cells that connect to the mouth, nose, eye, and face. Established infection seems impossible to clear. Therefore, we must understand how it starts. This is difficult in humans, but mice show HSV-1 entry via the nose and then spread to its preferred nerve cells. We show that this spread proceeds in part via myeloid cells, which normally function in host defense. Myeloid infection was productive in some settings but was efficiently suppressed by interferon in others. Therefore, interferon acting on myeloid cells can stop HSV-1 spread, and enhancing this defense offers a way to improve infection control.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

Type I Interferons Direct Gammaherpesvirus Host Colonization

Gamma-herpesviruses colonise lymphocytes. Murid Herpesvirus-4 (MuHV-4) infects B cells via epithelial to myeloid to lymphoid transfer. This indirect route entails exposure to host defences, and type I interferons (IFN-I) limit infection while viral evasion promotes it. To understand how IFN-I and its evasion both control infection outcomes, we used Mx1-cre mice to tag floxed viral genomes in IFN-I responding cells. Epithelial-derived MuHV-4 showed low IFN-I exposure, and neither disrupting viral evasion nor blocking IFN-I signalling markedly affected acute viral replication in the lungs. Maximising IFN-I induction with poly(I:C) increased virus tagging in lung macrophages, but the tagged virus spread poorly. Lymphoid-derived MuHV-4 showed contrastingly high IFN-I exposure. This occurred mainly in B cells. IFN-I induction increased tagging without reducing viral loads; disrupting viral evasion caused marked attenuation; and blocking IFN-I signalling opened up new lytic spread between macrophages. Thus, the impact of IFN-I on viral replication was strongly cell type-dependent: epithelial infection induced little response; IFN-I largely suppressed macrophage infection; and viral evasion allowed passage through B cells despite IFN-I responses. As a result, IFN-I and its evasion promoted a switch in infection from acutely lytic in myeloid cells to chronically latent in B cells. Murine cytomegalovirus also showed a capacity to pass through IFN-I-responding cells, arguing that this is a core feature of herpesvirus host colonization.

Murine Cytomegalovirus Exploits Olfaction To Enter New Hosts

Viruses transmit via the environmental and social interactions of their hosts. Herpesviruses have colonized mammals since their earliest origins, suggesting that they exploit ancient, common pathways. Cytomegaloviruses (CMVs) are assumed to enter new hosts orally, but no site has been identified. We show by live imaging that murine CMV (MCMV) infects nasally rather than orally, both after experimental virus uptake and during natural transmission. Replication-deficient virions revealed the primary target as olfactory neurons. Local, nasal replication by wild-type MCMV was not extensive, but there was rapid systemic spread, associated with macrophage infection. A long-term, transmissible infection was then maintained in the salivary glands. The viral m131/m129 chemokine homolog, which influences tropism, promoted salivary gland colonization after nasal entry but was not required for entry per se. The capacity of MCMV to transmit via olfaction, together with previous demonstrations of experimental olfactory infection by murid herpesvirus 4 (MuHV-4) and herpes simplex virus 1 (HSV-1), suggest that this is a common, conserved route of mammalian herpesvirus entry.

Cytomegaloviruses (CMVs) infect most mammals. Human CMV (HCMV) harms people with poor immune function and can damage the unborn fetus. It infects approximately 1% of live births. We lack a good vaccine. One problem is that how CMVs first enter new hosts remains unclear. Oral entry is often assumed, but the evidence is indirect, and no infection site is known. The difficulty of analyzing HCMV makes related animal viruses an important source of insights. Murine CMV (MCMV) infected not orally but nasally. Specifically, it targeted olfactory neurons. Viral transmission was also a nasal infection. Like HCMV, MCMV infected cells by binding to heparan, and olfactory surfaces display heparan to incoming viruses, whereas most other mucosal surfaces do not. These data establish a new understanding of CMV infections and a basis for infection control.

A Gammaherpesvirus Noncoding RNA Is Essential for Hematogenous Dissemination and Establishment of Peripheral Latency

Noncoding RNAs (ncRNAs) represent an intriguing and diverse class of molecules that are now recognized for their participation in a wide array of cellular processes. Viruses from multiple families have evolved to encode their own such regulatory RNAs; however, the specific in vivo functions of these ncRNAs are largely unknown. Epstein-Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV) are ubiquitous human pathogens that are associated with the development of numerous malignancies. Like EBV and KSHV, murine gammaherpesvirus 68 (MHV68) establishes lifelong latency in B cells and is associated with lymphomagenesis. The work described here reveals that the MHV68 ncRNA TMER4 acts at a critical bottleneck in local lymph nodes to facilitate hematogenous dissemination of the virus and establishment of latency at peripheral sites.

Recent intense investigations have uncovered important functions for a diverse array of novel noncoding RNA (ncRNA) species, including microRNAs (miRNAs) and long noncoding RNAs. Not surprisingly, viruses from multiple families have evolved to encode their own regulatory RNAs; however, the specific in vivo functions of these ncRNAs are largely unknown. The human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV) are highly ubiquitous pathogens that are associated with the development of a wide range of malignancies, including Burkitt’s lymphoma, Hodgkin’s lymphoma, nasopharyngeal carcinoma, and Kaposi’s sarcoma. Like EBV and KSHV, murine gammaherpesvirus 68 (MHV68) establishes lifelong latency in B cells and is associated with lymphoproliferative disease and lymphoma. Similar to the EBV-encoded small RNA (EBER)-1 and -2, MHV68 encodes eight 200- to 250-nucleotide polymerase III-transcribed ncRNAs called TMERs (tRNA-miRNA-encoded RNAs), which are highly expressed in latently infected cells and lymphoproliferative disease. To define the in vivo contribution of TMERs to MHV68 biology, we generated a panel of individual TMER mutant viruses. Through comprehensive in vivo analyses, we identified TMER4 as a key mediator of virus dissemination. The TMER4 mutant virus replicated normally in lungs and spread with normal kinetics and distribution to lung-draining lymph nodes, but it was significantly attenuated for infection of circulating blood cells and for latency establishment at peripheral sites. Notably, TMER4 stem-loops but not miRNAs were essential for wild-type TMER4 activity. Thus, these findings revealed a crucial miRNA-independent function of the TMER4 ncRNA in MHV68 hematogenous dissemination and latency establishment.

IMPORTANCE Noncoding RNAs (ncRNAs) represent an intriguing and diverse class of molecules that are now recognized for their participation in a wide array of cellular processes. Viruses from multiple families have evolved to encode their own such regulatory RNAs; however, the specific in vivo functions of these ncRNAs are largely unknown. Epstein-Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV) are ubiquitous human pathogens that are associated with the development of numerous malignancies. Like EBV and KSHV, murine gammaherpesvirus 68 (MHV68) establishes lifelong latency in B cells and is associated with lymphomagenesis. The work described here reveals that the MHV68 ncRNA TMER4 acts at a critical bottleneck in local lymph nodes to facilitate hematogenous dissemination of the virus and establishment of latency at peripheral sites.

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Alveolar Macrophages Are a Prominent but Nonessential Target for Murine Cytomegalovirus Infecting the Lungs

Cytomegaloviruses (CMVs) infect the lungs and cause pathological damage there in immunocompromised hosts. How lung infection starts is unknown. Inhaled murine CMV (MCMV) directly infected alveolar macrophages (AMs) and type 2 alveolar epithelial cells (AEC2s) but not type 1 alveolar epithelial cells (AEC1s). In contrast, herpes simplex virus 1 infected AEC1s and murid herpesvirus 4 (MuHV-4) infected AEC1s via AMs. MCMV-infected AMs prominently expressed viral reporter genes from a human CMV IE1 promoter; but most IE1-positive cells were AEC2s, and CD11c-cre mice, which express cre in AMs, switched the fluorochrome expression of <5% of floxed MCMV in the lungs. In contrast, CD11C-cre mice exhibited fluorochrome switching in >90% of floxed MuHV-4 in the lungs and 50% of floxed MCMV in the blood. AM depletion increased MCMV titers in the lung during the acute phase of infection. Thus, the influence of AMs was more restrictive than permissive. Circulating monocytes entered infected lungs in large numbers and became infected, but not directly; infection occurred mainly via AEC2s. Mice infected with an MCMV mutant lacking its m131/m129 chemokine homolog, which promotes macrophage infection, showed levels of lung infection equivalent to those of wild-type MCMV-infected mice. The level of lung infiltration by Gr-1-positive cells infected with the MCMV m131/m129-null mutant was modestly different from that for wild-type MCMV-infected lungs. These results are consistent with myeloid cells mainly disseminating MCMV from the lungs, whereas AEC2s provide local amplification.

IMPORTANCE

Cytomegaloviruses (CMVs) chronically and systemically infect most mammals. Human CMV infection is usually asymptomatic but causes lung disease in people with poor immune function. As human infection is hard to analyze, studies with related animal viruses provide important insights. We show that murine CMV has two targets in the lungs: macrophages and surfactant-secreting epithelial cells. Acute virus replication occurred largely in epithelial cells. Macrophages had an important defensive role, as their removal increased the level of infection. These results establish the dual nature of lung infection, with local virus replication occurring in epithelial cells and spread occurring via quiescently infected macrophages. Distinct therapies may be needed to target these contrasting events.

Deletion of Murid Herpesvirus 4 ORF63 Affects the Trafficking of Incoming Capsids toward the Nucleus

Gammaherpesviruses are important human and animal pathogens. Despite the fact that they display the classical architecture of herpesviruses, the function of most of their structural proteins is still poorly defined. This is especially true for tegument proteins. Interestingly, a potential role in immune evasion has recently been proposed for the tegument protein encoded by Kaposi's sarcoma-associated herpesvirus open reading frame 63 (ORF63). To gain insight about the roles of ORF63 in the life cycle of a gammaherpesvirus, we generated null mutations in the ORF63 gene of murid herpesvirus 4 (MuHV-4). We showed that disruption of ORF63 was associated with a severe MuHV-4 growth deficit both in vitro and in vivo. The latter deficit was mainly associated with a defect of replication in the lung but did not affect the establishment of latency in the spleen. From a functional point of view, inhibition of caspase-1 or the inflammasome did not restore the growth of the ORF63-deficient mutant, suggesting that the observed deficit was not associated with the immune evasion mechanism identified previously. Moreover, this growth deficit was also not associated with a defect in virion egress from the infected cells. In contrast, it appeared that MuHV-4 ORF63-deficient mutants failed to address most of their capsids to the nucleus during entry into the host cell, suggesting that ORF63 plays a role in capsid movement. In the future, ORF63 could therefore be considered a target to block gammaherpesvirus infection at a very early stage of the infection.

IMPORTANCE

The important diseases caused by gammaherpesviruses in human and animal populations justify a better understanding of their life cycle. In particular, the role of most of their tegument proteins is still largely unknown. In this study, we used murid herpesvirus 4, a gammaherpesvirus infecting mice, to decipher the role of the protein encoded by the viral ORF63 gene. We showed that the absence of this protein is associated with a severe growth deficit both in vitro and in vivo that was mainly due to impaired migration of viral capsids toward the nucleus during entry. Together, our results provide new insights about the life cycle of gammaherpesviruses and could allow the development of new antiviral strategies aimed at blocking gammaherpesvirus infection at the very early stages.

Bovine Herpesvirus 4 Modulates Its β-1,6-N-Acetylglucosaminyltransferase Activity through Alternative Splicing

Carbohydrates play major roles in host-virus interactions. It is therefore not surprising that, during coevolution with their hosts, viruses have developed sophisticated mechanisms to hijack for their profit different pathways of glycan synthesis. Thus, the Bo17 gene of Bovine herpesvirus 4 (BoHV-4) encodes a homologue of the cellular core 2 protein β-1,6-N-acetylglucosaminyltransferase-mucin type (C2GnT-M), which is a key player for the synthesis of complex O-glycans. Surprisingly, we show in this study that, as opposed to what is observed for the cellular enzyme, two different mRNAs are encoded by the Bo17 gene of all available BoHV-4 strains. While the first one corresponds to the entire coding sequence of the Bo17 gene, the second results from the splicing of a 138-bp intron encoding critical residues of the enzyme. Antibodies generated against the Bo17 C terminus showed that the two forms of Bo17 are expressed in BoHV-4 infected cells, but enzymatic assays revealed that the spliced form is not active. In order to reveal the function of these two forms, we then generated recombinant strains expressing only the long or the short form of Bo17. Although we did not highlight replication differences between these strains, glycomic analyses and lectin neutralization assays confirmed that the splicing of the Bo17 gene gives the potential to BoHV-4 to fine-tune the global level of core 2 branching activity in the infected cell. Altogether, these results suggest the existence of new mechanisms to regulate the activity of glycosyltransferases from the Golgi apparatus.

IMPORTANCE

Viruses are masters of adaptation that hijack cellular pathways to allow their growth. Glycans play a central role in many biological processes, and several studies have highlighted mechanisms by which viruses can affect glycosylation. Glycan synthesis is a nontemplate process regulated by the availability of key glycosyltransferases. Interestingly, bovine herpesvirus 4 encodes one such enzyme which is a key enzyme for the synthesis of complex O-glycans. In this study, we show that, in contrast to cellular homologues, this virus has evolved to alternatively express two proteins from this gene. While the first one is enzymatically active, the second results from the alternative splicing of the region encoding the catalytic site of the enzyme. We postulate that this regulatory mechanism could allow the virus to modulate the synthesis of some particular glycans for function at the location and/or the moment of infection.

Host entry by gamma-herpesviruses--lessons from animal viruses?

The oncogenicity of gamma-herpesviruses (γHVs) motivates efforts to control them and their persistence makes early events key targets for intervention. Human γHVs are often assumed to enter naive hosts orally and infect B cells directly. However, neither assumption is supported by direct evidence, and vaccination with the Epstein-Barr virus (EBV) gp350, to block virion binding to B cells, failed to reduce infection rates. Thus, there is a need to re-evaluate assumptions about γHV host entry. Given the difficulty of analysing early human infections, potentially much can be learned from animal models. Genomic comparisons argue that γHVs colonized mammals long before humans speciation, and so that human γHVs are unlikely to differ dramatically in behaviour from those of other mammals. Murid Herpesvirus-4 (MuHV-4), which like EBV and the Kaposi's Sarcoma-associated Herpesvirus (KSHV) persists in memory B cells, enters new hosts via olfactory neurons and exploits myeloid cells to spread. Integrating these data with existing knowledge of human and veterinary γHVs suggests a new model of host entry, with potentially important implications for infection control.

B-cell-independent lymphoid tissue infection by a B-cell-tropic rhadinovirus

Lymphocytes provide gammaherpesviruses with a self-renewing substrate for persistent infection and with transport to mucosal sites for host exit. Their role in the initial colonization of new hosts is less clear. Murid herpesvirus 4 (MuHV-4), an experimentally accessible, B-cell-tropic rhadinovirus (gamma-2 herpesvirus), persistently infects both immunocompetent and B-cell-deficient mice. A lack of B-cells did not compromise MuHV-4 entry into lymphoid tissue, which involved myeloid cell infection. However, it impaired infection amplification and MuHV-4 exit from lymphoid tissue, which involved myeloid to B-cell transfer.

Subcapsular sinus macrophages limit acute gammaherpesvirus dissemination

Lymphocyte proliferation, mobility and longevity make them prime targets for virus infection. Myeloid cells that process and present environmental antigens to lymphocytes are consequently an important line of defence. Subcapsular sinus macrophages (SSMs) filter the afferent lymph and communicate with B-cells. How they interact with B-cell-tropic viruses is unknown. We analysed their encounter with murid herpesvirus-4 (MuHV-4), an experimentally accessible gammaherpesvirus related to Kaposi's sarcoma-associated herpesvirus. MuHV-4 disseminated via lymph nodes, and intranasally or subcutaneously inoculated virions readily infected SSMs. However, this infection was poorly productive. SSM depletion with clodronate-loaded liposomes or with diphtheria toxin in CD169–diphtheria toxin receptor transgenic mice increased B-cell infection and hastened virus spread to the spleen. Dendritic cells provided the main route to B-cells, and SSMs slowed host colonization, apparently by absorbing virions non-productively from the afferent lymph.

Rhadinovirus host entry by co-operative infection

Rhadinoviruses establish chronic infections of clinical and economic importance. Several show respiratory transmission and cause lung pathologies. We used Murid Herpesvirus-4 (MuHV-4) to understand how rhadinovirus lung infection might work. A primary epithelial or B cell infection often is assumed. MuHV-4 targeted instead alveolar macrophages, and their depletion reduced markedly host entry. While host entry was efficient, alveolar macrophages lacked heparan - an important rhadinovirus binding target - and were infected poorly ex vivo. In situ analysis revealed that virions bound initially not to macrophages but to heparan⁺ type 1 alveolar epithelial cells (AECs). Although epithelial cell lines endocytose MuHV-4 readily in vitro, AECs did not. Rather bound virions were acquired by macrophages; epithelial infection occurred only later. Thus, host entry was co-operative - virion binding to epithelial cells licensed macrophage infection, and this in turn licensed AEC infection. An antibody block of epithelial cell binding failed to block host entry: opsonization provided merely another route to macrophages. By contrast an antibody block of membrane fusion was effective. Therefore co-operative infection extended viral tropism beyond the normal paradigm of a target cell infected readily in vitro; and macrophage involvement in host entry required neutralization to act down-stream of cell binding.

All viral infections start with host entry. Entry into cells is studied widely in isolated cultures; entry into live hosts is more complicated and less well understood: our tissues have specific anatomical structures and our cells differ markedly from most cultured cells in size, shape and behaviour. The respiratory tract is a common site of virus infection. Size dictates where inhaled particles come to rest, and virus-sized particles can reach the lungs. Rhadinoviruses chronically infect both humans and economically important animals, and cause lung disease. We used a well-characterized murine example to determine how a rhadinovirus enters the lungs. At its peak, infection was prominent in epithelial cells lining the lung air spaces. However it started in macrophages, which normally clear the lungs of inhaled debris. Only epithelial cells expressed the molecules required for virus binding, but only macrophages internalized virus particles after binding; infection involved interaction between these different cell types. Blocking epithelial infection with an antibody did not stop host entry because attached antibodies increase virus uptake by lung macrophages; but an antibody that blocks macrophage infection was effective. Thus, understanding how rhadinovirus infections work in normal tissues provided important information for their control.

A murid gamma-herpesviruses exploits normal splenic immune communication routes for systemic spread

Gamma-herpesviruses (γHVs) are widespread oncogenic pathogens that chronically infect circulating lymphocytes. How they subvert the immune check-point function of the spleen to promote persistent infection is not clear. We show that Murid Herpesvirus-4 (MuHV-4) enters the spleen by infecting marginal zone (MZ) macrophages, which provided a conduit to MZ B cells. Relocation of MZ B cells to the white pulp allowed virus transfer to follicular dendritic cells. From here the virus reached germinal center B cells to establish persistent infection. Mice lacking MZ B cells, or treated with a sphingosine-1-phosphate receptor agonist to dislocate them, were protected against MuHV-4 colonization. MuHV-4 lacking ORF27, which encodes a glycoprotein necessary for efficient intercellular spread, could infect MZ macrophages but was impaired in long-term infection. Thus, MuHV-4, a γHV, exploits normal immune communication routes to spread by serial lymphoid/myeloid exchange.

B cell response to herpesvirus infection of the olfactory neuroepithelium

Viruses commonly infect the respiratory tract. Analyses of host defense have focused on the lungs and the respiratory epithelium. Spontaneously inhaled murid herpesvirus 4 (MuHV-4) and herpes simplex virus 1 (HSV-1) instead infect the olfactory epithelium, where neuronal cilia are exposed to environmental antigens and provide a route across the epithelial mucus. We used MuHV-4 to define how B cells respond to virus replication in this less well-characterized site. Olfactory infection elicited generally weaker acute responses than lung infection, particularly in the spleen, reflecting slower viral replication and spread. Few virus-specific antibody-forming cells (AFCs) were found in the nasal-associated lymphoid tissue (NALT), a prominent response site for respiratory epithelial infection. Instead, they appeared first in the superficial cervical lymph nodes. The focus of the AFC response then moved to the spleen, matching the geography of virus dissemination. Little virus-specific IgA response was detected until later in the bone marrow. Neuroepithelial HSV-1 infection also elicited no significant AFC response in the NALT and a weak IgA response. Thus, olfactory herpesvirus infection differed immunologically from an infection of the adjacent respiratory epithelium. Poor IgA induction may help herpesviruses to transmit via long-term mucosal shedding.

IMPORTANCE

Herpesviruses are widespread, persistent pathogens against which vaccines have had limited success. We need to understand better how they interact with host immunity. MuHV-4 and HSV-1 inhaled by alert mice infect the olfactory neuroepithelium, suggesting that this is a natural entry route. Its immunology is almost completely unknown. The antibody response to neuroepithelial herpesvirus infection started in the cervical lymph nodes, and unlike respiratory influenza virus infection, did not significantly involve the nasal-associated lymphoid tissue. MuHV-4 and HSV-1 infections also elicited little virus-specific IgA. Therefore, vaccine-induced IgA might provide a defense that herpesviruses are ill-equipped to meet.

Establishment of murine gammaherpesvirus latency in B cells is not a stochastic event

Murid γ-herpesvirus-4 (MuHV-4) promotes polyclonal B cell activation and establishes latency in memory B cells via unclear mechanisms. We aimed at exploring whether B cell receptor specificity plays a role in B cell susceptibility to viral latency and how this is related to B cell activation. We first observed that MuHV-4-specific B cells represent a minority of the latent population, and to better understand the influence of the virus on non-MuHV-4 specific B cells we used the SW_HEL mouse model, which produce hen egg lysozyme (HEL)-specific B cells. By tracking HEL⁺ and HEL⁻ B cells, we showed that in vivo latency was restricted to HEL⁻ B cells while the two populations were equally sensitive to the virus in vitro. Moreover, MuHV-4 induced two waves of B cell activation. While the first wave was characterized by a general B cell activation, as shown by HEL⁺ and HEL⁻ B cells expansion and upregulation of CD69 expression, the second wave was restricted to the HEL⁻ population, which acquired germinal center (GC) and plasma cell phenotypes. Antigenic stimulation of HEL⁺ B cells led to the development of HEL⁺ GC B cells where latent infection remained undetectable, indicating that MuHV-4 does not benefit from acute B cell responses to establish latency in non-virus specific B cells but relies on other mechanisms of the humoral response. These data support a model in which the establishment of latency in B cells by γ-herpesviruses is not stochastic in terms of BCR specificity and is tightly linked to the formation of GCs.

Murid γ-herpesvirus-4 (MuHV-4) is a good model to study infectious mononucleosis in mice, in which the virus ultimately establishes life-long latency in B cells. Whereas several viral proteins have been shown to modulate B cell behavior, in the present study we aimed at clarifying the parameters that dictate the establishment of viral latency from the B cell perspective. Indeed, the B cell repertoire is highly diverse and it remains unknown whether latency takes place randomly in B cells. To study this question, we isolated latently infected B cells in which we observed a low frequency of virus-specific B cells, suggesting that viral latency is not restricted to this population. To better understand MuHV-4 influence on non-virus specific B cells, we then followed the fate of B cells specific for a foreign antigen, hen egg lysozyme (HEL). While in vitro experiments showed that HEL-specific B cells could be acutely infected by MuHV-4, these cells were resistant to MuHV-4 latent infection in vivo. These results suggest that while establishment of γ-herpesvirus latency is not restricted to virus-specific B cells, it does not take place randomly in B cells and relies on mechanisms that remain to be identified.

BAFF receptor deficiency limits gammaherpesvirus infection

Lymphocyte colonization by gammaherpesviruses (γHVs) is an important target for cancer prevention. However, how it works is not clear. Epstein-Barr virus drives autonomous B cell proliferation in vitro but in vivo may more subtly exploit the proliferative pathways provided by lymphoid germinal centers (GCs). Murid herpesvirus 4 (MuHV-4), which realistically infects inbred mice, provides a useful tool with which to understand further how a γHV colonizes B cells in vivo. Not all γHVs necessarily behave the same, but common events can with MuHV-4 be assigned an importance for host colonization and so a potential as therapeutic targets. MuHV-4-driven B cell proliferation depends quantitatively on CD4(+) T cell help. Here we show that it also depends on T cell-independent survival signals provided by the B cell-activating factor (BAFF) receptor (BAFF-R). B cells could be infected in BAFF-R(-/-) mice, but virus loads remained low. This corresponded to a BAFF-R-dependent defect in GC colonization. The close parallels between normal, antigen-driven B cell responses and virus-infected B cell proliferation argue that in vivo, γHVs mostly induce infected B cells into normal GC reactions rather than generating large numbers of autonomously proliferating blasts.

IMPORTANCE

γHVs cause cancers by driving the proliferation of infected cells. B cells are a particular target. Thus, we need to know how virus-driven B cell proliferation works. Controversy exists as to whether viral genes drive it directly or less directly orchestrate the engagement of normal, host-driven pathways. Here we show that the B cell proliferation driven by a murid γHV requires BAFF-R. This supports the idea that γHVs exploit host proliferation pathways and suggests that interfering with BAFF-R could more generally reduce γHV-associated B cell proliferation.

A gammaherpesvirus uses alternative splicing to regulate its tropism and its sensitivity to neutralization

Human gammaherpesviruses are associated with the development of lymphomas and epithelial malignancies. The heterogeneity of these tumors reflects the ability of these viruses to route infection to different cell types at various stages of their lifecycle. While the Epstein Barr virus uses gp42 – human leukocyte antigen class II interaction as a switch of cell tropism, the molecular mechanism that orientates tropism of rhadinoviruses is still poorly defined. Here, we used bovine herpesvirus 4 (BoHV-4) to further elucidate how rhadinoviruses regulate their infectivity. In the absence of any gp42 homolog, BoHV-4 exploits the alternative splicing of its Bo10 gene to produce distinct viral populations that behave differently based on the originating cell. While epithelial cells produce virions with high levels of the accessory envelope protein gp180, encoded by a Bo10 spliced product, myeloid cells express reduced levels of gp180. As a consequence, virions grown in epithelial cells are hardly infectious for CD14+ circulating cells, but are relatively resistant to antibody neutralization due to the shielding property of gp180 for vulnerable entry epitopes. In contrast, myeloid virions readily infect CD14+ circulating cells but are easily neutralized. This molecular switch could therefore allow BoHV-4 to promote either, on the one hand, its dissemination into the organism, or, on the other hand, its transmission between hosts.

Gammaherpesviruses are highly prevalent human and animal pathogens. These viruses display sophisticated entry mechanisms, allowing them to infect different cell types inside a host but also to transmit between hosts in the presence of neutralizing antibodies. Here, we used bovine herpesvirus 4 (BoHV-4) to decipher how some gammaherpesviruses manage to do this. We found that, as function of the originating cell types, BoHV-4 is able to modify its tropism as well as its sensitivity to antibody neutralization just by controlling the alternative splicing of one of its genes. This virus therefore exploits post-transcriptional events to generate viral populations with distinct phenotypes.

Glycoprotein B cleavage is important for murid herpesvirus 4 to infect myeloid cells

Glycoprotein B (gB) is a conserved herpesvirus virion component implicated in membrane fusion. As with many—but not all—herpesviruses, the gB of murid herpesvirus 4 (MuHV-4) is cleaved into disulfide-linked subunits, apparently by furin. Preventing gB cleavage for some herpesviruses causes minor infection deficits in vitro, but what the cleavage contributes to host colonization has been unclear. To address this, we mutated the furin cleavage site (R-R-K-R) of the MuHV-4 gB. Abolishing gB cleavage did not affect its expression levels, glycosylation, or antigenic conformation. In vitro, mutant viruses entered fibroblasts and epithelial cells normally but had a significant entry deficit in myeloid cells such as macrophages and bone marrow-derived dendritic cells. The deficit in myeloid cells was not due to reduced virion binding or endocytosis, suggesting that gB cleavage promotes infection at a postendocytic entry step, presumably viral membrane fusion. In vivo, viruses lacking gB cleavage showed reduced lytic spread in the lungs. Alveolar epithelial cell infection was normal, but alveolar macrophage infection was significantly reduced. Normal long-term latency in lymphoid tissue was established nonetheless.

Systemic and local infection routes govern different cellular dissemination pathways during gammaherpesvirus infection in vivo

Human gammaherpesviruses cause morbidity and mortality associated with infection and transformation of lymphoid and endothelial cells. Knowledge of cell types involved in virus dissemination from primary virus entry to virus latency is fundamental for the understanding of gammaherpesvirus pathogenesis. However, the inability to directly trace cell types with respect to virus dissemination pathways has prevented definitive conclusions regarding the relative contribution of individual cell types. Here, we describe that the route of infection affects gammaherpesvirus dissemination pathways. We constructed a recombinant murine gammaherpesvirus 68 (MHV-68) variant harboring a cassette which switches fluorescent markers in a Cre-dependent manner. Since the recombinant virus which was constructed on the wild-type background was attenuated, in this study we used an M1-deleted version, which infected mice with normal kinetics. Infection of Cre-transgenic mice with this convertible virus was used to estimate the quantitative contribution of defined cell types to virus productivity and dissemination during the acute phase of MHV-68 infection. In systemic infection, we found splenic vascular endothelial cells (EC) among the first and main cells to produce virus. After local infection, the contribution of EC to splenic virus production did not represent such early kinetics. However, at later time points, B cell-derived viruses dominated splenic productivity independently of systemic or local infection. Systemic versus local infection also governed the cell types involved in loading peritoneal exudate cells, leading to latency in F4/80- and CD11b-positive target cells. Systemic infection supported EC-driven dissemination, whereas local infection supported B cell-driven dissemination.