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.

The gammaherpesvirus 68 viral cyclin facilitates expression of LANA.

Gammaherpesviruses establish life-long infections within their host and have been shown to be the causative agents of devastating malignancies. Chronic infection within the host is mediated through cycles of transcriptionally quiescent stages of latency with periods of reactivation into detectable lytic and productive infection. The mechanisms that regulate reactivation from latency remain poorly understood. Previously, we defined a critical role for the viral cyclin in promoting reactivation from latency. Disruption of the viral cyclin had no impact on the frequency of cells containing viral genome during latency, yet it remains unclear whether the viral cyclin influences latently infected cells in a qualitative manner. To define the impact of the viral cyclin on properties of latent infection, we utilized a viral cyclin deficient variant expressing a LANA-beta-lactamase fusion protein (LANA::ßla), to enumerate both the cellular distribution and frequency of LANA gene expression. Disruption of the viral cyclin did not affect the cellular distribution of latently infected cells, but did result in a significant decrease in the frequency of cells that expressed LANA::ßla across multiple tissues and in both immunocompetent and immunodeficient hosts. Strikingly, whereas the cyclin-deficient virus had a reactivation defect in bulk culture, sort purified cyclin-deficient LANA::ßla expressing cells were fully capable of reactivation. These data emphasize that the γHV68 latent reservoir is comprised of at least two distinct stages of infection characterized by differential LANA expression, and that a primary function of the viral cyclin is to promote LANA expression during latency, a state associated with ex vivo reactivation competence.

Lytic Infection with Murine Gammaherpesvirus 68 Activates Host and Viral RNA Polymerase III Promoters and Enhances Noncoding RNA Expression.

RNA polymerase III (pol III) transcribes multiple noncoding RNAs (ncRNAs) that are essential for cellular function. Pol III-dependent transcription is also engaged during certain viral infections, including those of the gammaherpesviruses (γHVs), where pol III-dependent viral ncRNAs promote pathogenesis. Additionally, several host ncRNAs are upregulated during γHV infection and play integral roles in pathogenesis by facilitating viral establishment and gene expression. Here, we sought to investigate how pol III promoters and transcripts are regulated during gammaherpesvirus infection using the murine gammaherpesvirus 68 (γHV68) system. To compare the transcription of host and viral pol III-dependent ncRNAs, we analyzed a series of pol III promoters for host and viral ncRNAs using a luciferase reporter optimized to measure pol III activity. We measured promoter activity from the reporter gene at the translation level via luciferase activity and at the transcription level via reverse transcription-quantitative PCR (RT-qPCR). We further measured endogenous ncRNA expression at single-cell resolution by flow cytometry. These studies demonstrated that lytic infection with γHV68 increased the transcription from multiple host and viral pol III promoters and further identified the ability of accessory sequences to influence both baseline and inducible promoter activity after infection. RNA flow cytometry revealed the induction of endogenous pol III-derived ncRNAs that tightly correlated with viral gene expression. These studies highlight how lytic gammaherpesvirus infection alters the transcriptional landscape of host cells to increase pol III-derived RNAs, a process that may further modify cellular function and enhance viral gene expression and pathogenesis. IMPORTANCE Gammaherpesviruses are a prime example of how viruses can alter the host transcriptional landscape to establish infection. Despite major insights into how these viruses modify RNA polymerase II-dependent generation of messenger RNAs, how these viruses influence the activity of host RNA polymerase III remains much less clear. Small noncoding RNAs produced by RNA polymerase III are increasingly recognized to play critical regulatory roles in cell biology and virus infection. Studies of RNA polymerase III-dependent transcription are complicated by multiple promoter types and diverse RNAs with variable stability and processing requirements. Here, we characterized a reporter system to directly study RNA polymerase III-dependent responses during gammaherpesvirus infection and utilized single-cell flow cytometry-based methods to reveal that gammaherpesvirus lytic replication broadly induces pol III activity to enhance host and viral noncoding RNA expression within the infected cell.

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 IIhigh 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.

High-Dimensional Characterization of IL-10 Production and IL-10-Dependent Regulation during Primary Gammaherpesvirus Infection.

IL-10 is a potent immunomodulatory cytokine produced by multiple cell types to restrain immune activation. Many herpesviruses use the IL-10 pathway to facilitate infection, but how endogenous IL-10 is regulated during primary infection in vivo remains poorly characterized. In this study, we infected mice with murine gammaherpesvirus 68 (γHV68) and analyzed the production and genetic contribution of IL-10 by mass cytometry (cytometry by time-of-flight) analysis. γHV68 infection elicited a breadth of effector CD4 T cells in the lungs of acutely infected mice, including a highly activated effector subset that coexpressed IFN-γ, TNF-α, and IL-10. By using IL-10 GFP transcriptional reporter mice, we identified that IL-10 was primarily expressed within CD4 T cells during acute infection in the lungs. IL10gfp-expressing CD4 T cells were highly proliferative and characterized by the expression of multiple coinhibitory receptors, including PD-1 and LAG-3. When we analyzed acute γHV68 infection of IL-10-deficient mice, we found that IL-10 limits the frequency of both myeloid and effector CD4 T cell subsets in the infected lung, with minimal changes at a distant mucosal site. These data emphasize the unique insights that high-dimensional analysis can afford in investigating antiviral immunity and provide new insights into the breadth, phenotype, and function of IL-10-expressing effector CD4 T cells during acute virus infection.

Multidimensional analysis of Gammaherpesvirus RNA expression reveals unexpected heterogeneity of gene expression.

Virus-host interactions are frequently studied in bulk cell populations, obscuring cell-to-cell variation. Here we investigate endogenous herpesvirus gene expression at the single-cell level, combining a sensitive and robust fluorescent in situ hybridization platform with multiparameter flow cytometry, to study the expression of gammaherpesvirus non-coding RNAs (ncRNAs) during lytic replication, latent infection and reactivation in vitro. This method allowed robust detection of viral ncRNAs of murine gammaherpesvirus 68 (γHV68), Kaposi's sarcoma associated herpesvirus and Epstein-Barr virus, revealing variable expression at the single-cell level. By quantifying the inter-relationship of viral ncRNA, viral mRNA, viral protein and host mRNA regulation during γHV68 infection, we find heterogeneous and asynchronous gene expression during latency and reactivation, with reactivation from latency identified by a distinct gene expression profile within rare cells. Further, during lytic replication with γHV68, we find many cells have limited viral gene expression, with only a fraction of cells showing robust gene expression, dynamic RNA localization, and progressive infection. Lytic viral gene expression was enhanced in primary fibroblasts and by conditions associated with enhanced viral replication, with multiple subpopulations of cells present in even highly permissive infection conditions. These findings, powered by single-cell analysis integrated with automated clustering algorithms, suggest inefficient or abortive γHV infection in many cells, and identify substantial heterogeneity in viral gene expression at the single-cell level.

Genome-wide Transcript Structure Resolution Reveals Abundant Alternate Isoform Usage from Murine Gammaherpesvirus 68.

The gammaherpesviruses, including Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and murine gammaherpesvirus 68 (MHV68, MuHV-4, γHV68), are etiologic agents of a wide range of lymphomas and non-hematological malignancies. These viruses possess large and highly dense dsDNA genomes that feature >80 bidirectionally positioned open reading frames (ORFs). The abundance of overlapping transcripts and extensive splicing throughout these genomes have until now prohibited high throughput-based resolution of transcript structures. Here, we integrate the capabilities of long-read sequencing with the accuracy of short-read platforms to globally resolve MHV68 transcript structures using the transcript resolution through integration of multi-platform data (TRIMD) pipeline. This approach reveals highly complex features, including: (1) pervasive overlapping transcript structures; (2) transcripts containing intra-gene or trans-gene splices that yield chimeric ORFs; (3) antisense and intergenic transcripts containing ORFs; and (4) noncoding transcripts. This work sheds light on the underappreciated complexity of gammaherpesvirus transcription and provides an extensively revised annotation of the MHV68 transcriptome.

Host Tumor Suppressor p18INK4c Functions as a Potent Cell-Intrinsic Inhibitor of Murine Gammaherpesvirus 68 Reactivation and Pathogenesis.

Gammaherpesviruses are common viruses associated with lifelong infection and increased disease risk. Reactivation from latency aids the virus in maintaining infection throughout the life of the host and is responsible for a wide array of disease outcomes. Previously, we demonstrated that the virus-encoded cyclin (v-cyclin) of murine gammaherpesvirus 68 (γHV68) is essential for optimal reactivation from latency in normal mice but not in mice lacking the host tumor suppressor p18INK4c (p18). Whether p18 plays a cell-intrinsic or -extrinsic role in constraining reactivation remains unclear. Here, we generated recombinant viruses in which we replaced the viral cyclin with the cellular p18INK4c gene (p18KI) for targeted expression of p18, specifically within infected cells. We find that the p18KI virus is similar to the cyclin-deficient virus (cycKO) in lytic infection, establishment of latency, and infected cell reservoirs. While the cycKO virus is capable of reactivation in p18-deficient mice, expression of p18 from the p18KI virus results in a profound reactivation defect. These data demonstrate that p18 limits reactivation within latently infected cells, functioning in a cell-intrinsic manner. Further, the p18KI virus showed greater attenuation of virus-induced lethal pneumonia than the cycKO virus, indicating that p18 could further restrict γHV68 pathogenesis even in p18-sufficient mice. These studies demonstrate that host p18 imposes the requirement for the viral cyclin to reactivate from latency by functioning in latently infected cells and that p18 expression is associated with decreased disease, thereby identifying p18 as a compelling host target to limit chronic gammaherpesvirus pathogenesis.IMPORTANCE Gammaherpesviruses are ubiquitous viruses associated with multiple malignancies. The propensity to cycle between latency and reactivation results in an infection that is never cleared and often difficult to treat. Understanding the balance between latency and reactivation is integral to treating gammaherpesvirus infection and associated disease outcomes. This work characterizes the role of a novel inhibitor of reactivation, host p18INK4c, thereby bringing more clarity to a complex process with significant outcomes for infected individuals.

A Beginner's Guide to Analyzing and Visualizing Mass Cytometry Data.

Mass cytometry has revolutionized the study of cellular and phenotypic diversity, significantly expanding the number of phenotypic and functional characteristics that can be measured at the single-cell level. This high-dimensional analysis platform has necessitated the development of new data analysis approaches. Many of these algorithms circumvent traditional approaches used in flow cytometric analysis, fundamentally changing the way these data are analyzed and interpreted. For the beginner, however, the large number of algorithms that have been developed, as well as the lack of consensus on best practices for analyzing these data, raise multiple questions: Which algorithm is the best for analyzing a dataset? How do different algorithms compare? How can one move beyond data visualization to gain new biological insights? In this article, we describe our experiences as recent adopters of mass cytometry. By analyzing a single dataset using five cytometry by time-of-flight analysis platforms (viSNE, SPADE, X-shift, PhenoGraph, and Citrus), we identify important considerations and challenges that users should be aware of when using these different methods and common and unique insights that can be revealed by these different methods. By providing annotated workflow and figures, these analyses present a practical guide for investigators analyzing high-dimensional datasets. In total, these analyses emphasize the benefits of integrating multiple cytometry by time-of-flight analysis algorithms to gain complementary insights into these high-dimensional datasets.

Impaired B cell function during viral infections due to PTEN-mediated inhibition of the PI3K pathway.

Transient suppression of B cell function often accompanies acute viral infection. However, the molecular signaling circuitry that enforces this hyporesponsiveness is undefined. In this study, experiments identify up-regulation of the inositol phosphatase PTEN (phosphatase and tensin homolog) as primarily responsible for defects in B lymphocyte migration and antibody responses that accompany acute viral infection. B cells from mice acutely infected with gammaherpesvirus 68 are defective in BCR- and CXCR4-mediated activation of the PI3K pathway, and this, we show, is associated with increased PTEN expression. This viral infection-induced PTEN overexpression appears responsible for the suppression of antibody responses observed in infected mice because PTEN deficiency or expression of a constitutively active PI3K rescued function of B cells in infected mice. Conversely, induced overexpression of PTEN in B cells in uninfected mice led to suppression of antibody responses. Finally, we demonstrate that PTEN up-regulation is a common mechanism by which infection induces suppression of antibody responses. Collectively, these findings identify a novel role for PTEN during infection and identify regulation of the PI3K pathway, a mechanism previously shown to silence autoreactive B cells, as a key physiological target to control antibody responses.

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

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.

A Conserved Gammaherpesvirus Cyclin Specifically Bypasses Host p18(INK4c) To Promote Reactivation from Latency.

UNLABELLED: Gammaherpesviruses (GHVs) carry homologs of cellular genes, including those encoding a viral cyclin that promotes reactivation from latent infection. The viral cyclin has reduced sensitivity to host cyclin-dependent kinase inhibitors in vitro; however, the in vivo significance of this is unclear. Here, we tested the genetic requirement for the viral cyclin in mice that lack the host inhibitors p27(Kip1) and p18(INK4c), two cyclin-dependent kinase inhibitors known to be important in regulating B cell proliferation and differentiation. While the viral cyclin was essential for reactivation in wild-type mice, strikingly, it was dispensable for reactivation in mice lacking p27(Kip1) and p18(INK4c). Further analysis revealed that genetic ablation of only p18(INK4c) alleviated the requirement for the viral cyclin for reactivation from latency. p18(INK4c) regulated reactivation in a dose-dependent manner so that the viral cyclin was dispensable in p18(INK4c) heterozygous mice. Finally, treatment of wild-type cells with the cytokine BAFF, a known attenuator of p18(INK4c) function in B lymphocytes, was also able to bypass the requirement for the viral cyclin in reactivation. These data show that the gammaherpesvirus viral cyclin functions specifically to bypass the cyclin-dependent kinase inhibitor p18(INK4c), revealing an unanticipated specificity between a GHV cyclin and a single cyclin-dependent kinase inhibitor. IMPORTANCE: The gammaherpesviruses (GHVs) cause lifelong infection and can cause chronic inflammatory diseases and cancer, especially in immunosuppressed individuals. Many GHVs encode a conserved viral cyclin that is required for infection and disease. While a common property of the viral cyclins is that they resist inhibition by normal cellular mechanisms, it remains unclear how important it is that the GHVs resist this inhibition. We used a mouse GHV that either contained or lacked a viral cyclin to test whether the viral cyclin lost importance when these inhibitory pathways were removed. These studies revealed that the viral cyclin was required for optimal function in normal mice but that it was no longer required following removal or reduced function of a single cellular inhibitor. These data define a very specific role for the viral cyclin in bypassing one cellular inhibitor and point to new methods to intervene with viral cyclins.

Trehalose-mediated autophagy impairs the anti-viral function of human primary airway epithelial cells.

Human rhinovirus (HRV) is the most common cause of acute exacerbations of chronic lung diseases including asthma. Impaired anti-viral IFN-λ1 production and increased HRV replication in human asthmatic airway epithelial cells may be one of the underlying mechanisms leading to asthma exacerbations. Increased autophagy has been shown in asthmatic airway epithelium, but the role of autophagy in anti-HRV response remains uncertain. Trehalose, a natural glucose disaccharide, has been recognized as an effective autophagy inducer in mammalian cells. In the current study, we used trehalose to induce autophagy in normal human primary airway epithelial cells in order to determine if autophagy directly regulates the anti-viral response against HRV. We found that trehalose-induced autophagy significantly impaired IFN-λ1 expression and increased HRV-16 load. Inhibition of autophagy via knockdown of autophagy-related gene 5 (ATG5) effectively rescued the impaired IFN-λ1 expression by trehalose and subsequently reduced HRV-16 load. Mechanistically, ATG5 protein interacted with retinoic acid-inducible gene I (RIG-I) and IFN-ß promoter stimulator 1 (IPS-1), two critical molecules involved in the expression of anti-viral interferons. Our results suggest that induction of autophagy in human primary airway epithelial cells inhibits the anti-viral IFN-λ1 expression and facilitates HRV infection. Intervention of excessive autophagy in chronic lung diseases may provide a novel approach to attenuate viral infections and associated disease exacerbations.

Gammaherpesvirus small noncoding RNAs are bifunctional elements that regulate infection and contribute to virulence in vivo.

UNLABELLED: Many viruses express noncoding RNAs (ncRNAs). The gammaherpesviruses (γHVs), including Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and murine γHV68, each contain multiple ncRNA genes, including microRNAs (miRNAs). While these ncRNAs can regulate multiple host and viral processes in vitro, the genetic contribution of these RNAs to infection and pathogenesis remains largely unknown. To study the functional contribution of these RNAs to γHV infection, we have used γHV68, a small-animal model of γHV pathogenesis. γHV68 encodes eight small hybrid ncRNAs that contain both tRNA-like elements and functional miRNAs. These genes are transcribed by RNA polymerase III and are referred to as the γHV68 TMERs (tRNA-miRNA-encoded RNAs). To determine the total concerted genetic contribution of these ncRNAs to γHV acute infection and pathogenesis, we generated and characterized a recombinant γHV68 strain devoid of all eight TMERs. TMER-deficient γHV68 has wild-type levels of lytic replication in vitro and normal establishment of latency in B cells early following acute infection in vivo. In contrast, during acute infection of immunodeficient mice, TMER-deficient γHV68 has reduced virulence in a model of viral pneumonia, despite having an enhanced frequency of virus-infected cells. Strikingly, expression of a single viral tRNA-like molecule, in the absence of all other virus-encoded TMERs and miRNAs, reverses both attenuation in virulence and enhanced frequency of infected cells. These data show that γHV ncRNAs play critical roles in acute infection and virulence in immunocompromised hosts and identify these RNAs as a new potential target to modulate γHV-induced infection and pathogenesis. IMPORTANCE: The gammaherpesviruses (γHVs) are a subfamily of viruses associated with chronic inflammatory diseases and cancer, particularly in immunocompromised individuals. These viruses uniformly encode multiple types of noncoding RNAs (ncRNAs) that are not translated into proteins. It remains unclear how virus-expressed ncRNAs influence the course and outcome of infection in vivo. Here, we generated a mouse γHV that lacks the expression of multiple ncRNAs. Notably, this mutant virus is critically impaired in the ability to cause disease in immunocompromised hosts yet shows a paradoxical increase in infected cells early during infection in these hosts. While the original mouse virus encodes multiple ncRNAs, the expression of a single domain of one ncRNA can partially reverse the defects of the mutant virus. These studies demonstrate that γHV ncRNAs can directly contribute to virus-induced disease in vivo and that these RNAs may be multifunctional, allowing the opportunity to specifically interfere with different functional domains of these RNAs.

CD4 T cells specific for a latency-associated γ-herpesvirus epitope are polyfunctional and cytotoxic.

The oncogenic γ-herpesviruses EBV and Kaposi sarcoma-associated herpesvirus are ubiquitous human pathogens that establish lifelong latent infections maintained by intermittent viral reactivation and reinfection. Effector CD4 T cells are critical for control of viral latency and in immune therapies for virus-associated tumors. In this study, we exploited γHV68 infection of mice to enhance our understanding of the CD4 T cell response during γ-herpesvirus infection. Using a consensus prediction approach, we identified 16 new CD4 epitope-specific responses that arise during lytic infection. An additional epitope encoded by the M2 protein induced uniquely latency-associated CD4 T cells, which were not detected at the peak of lytic infection but only during latency and were not induced postinfection with a latency-deficient virus. M2-specific CD4 T cells were selectively cytotoxic, produced multiple antiviral cytokines, and sustained IL-2 production. Identification of latency-associated cytolytic CD4 T cells will aid in dissecting mechanisms of CD4 immune control of γ-herpesvirus latency and the development of therapeutic approaches to control viral reactivation and pathology.

γ-Herpes virus-68, but not Pseudomonas aeruginosa or influenza A (H1N1), exacerbates established murine lung fibrosis.

Patients with idiopathic pulmonary fibrosis (IPF) often do worse following infection, but the cause of the decline is not fully understood. We previously demonstrated that infection with a murine gamma herpes virus (γHV-68) could exacerbate established lung fibrosis following administration of fluorescein isothiocyanate (McMillan et al. Am J Respir Crit Care Med 177: 771-780, 2008). In the present study, we anesthetized mice and injected saline or bleomycin intratracheally on day 0. On day 14, mice were anesthetized again and infected with either a Gram-negative bacteria (Pseudomonas aeruginosa), or with H1N1 or γHV-68 viruses. Measurements were then made on days 15, 21, or 35. We demonstrate that infection with P. aeruginosa does not exacerbate extracellular matrix deposition post-bleomycin. Furthermore, fibrotic mice are effectively able to clear P. aeruginosa infection. In contrast, bleomycin-treated mice develop worse lung fibrosis when infected with γHV-68, but not when infected with H1N1. The differential ability of γHV-68 to cause increased collagen deposition could not be explained by differences in inflammatory cell recruitment or whole lung chemokine and cytokine responses. Alveolar epithelial cells from γHV-68-infected mice displayed increased expression of TGFß receptor 1, increased SMAD3 phosphorylation, and evidence of apoptosis measured by cleaved poly-ADP ribose polymerase (PARP). The ability of γHV-68 to augment fibrosis required the ability of the virus to reactivate from latency. This property appears unique to γHV-68, as the ß-herpes virus, cytomegalovirus, did not have the same effect.

Virus-encoded microRNAs facilitate gammaherpesvirus latency and pathogenesis in vivo.

UNLABELLED: Gammaherpesviruses, including Epstein-Barr virus (EBV), Kaposi sarcoma-associated herpesvirus (KSHV, or HHV-8), and murine gammaherpesvirus 68 (MHV68, γHV68, or MuHV-4), are B cell-tropic pathogens that each encode at least 12 microRNAs (miRNAs). It is predicted that these regulatory RNAs facilitate infection by suppressing host target genes involved in a wide range of key cellular pathways. However, the precise contribution that gammaherpesvirus miRNAs make to viral life cycle and pathogenesis in vivo is unknown. MHV68 infection of mice provides a highly useful system to dissect the function of specific viral elements in the context of both asymptomatic infection and disease. Here, we report (i) analysis of in vitro and in vivo MHV68 miRNA expression, (ii) generation of an MHV68 miRNA mutant with reduced expression of all 14 pre-miRNA stem-loops, and (iii) comprehensive phenotypic characterization of the miRNA mutant virus in vivo. The profile of MHV68 miRNAs detected in infected cell lines varied with cell type and did not fully recapitulate the profile from cells latently infected in vivo. The miRNA mutant virus, MHV68.Zt6, underwent normal lytic replication in vitro and in vivo, demonstrating that the MHV68 miRNAs are dispensable for acute replication. During chronic infection, MHV68.Zt6 was attenuated for latency establishment, including a specific defect in memory B cells. Finally, MHV68.Zt6 displayed a striking attenuation in the development of lethal pneumonia in mice deficient in IFN-γ. These data indicate that the MHV68 miRNAs may facilitate virus-driven maturation of infected B cells and implicate the miRNAs as a critical determinant of gammaherpesvirus-associated disease. IMPORTANCE: Gammaherpesviruses such as EBV and KSHV are widespread pathogens that establish lifelong infections and are associated with the development of numerous types of diseases, including cancer. Gammaherpesviruses encode many small noncoding RNAs called microRNAs (miRNAs). It is predicted that gammaherpesvirus miRNAs facilitate infection and disease by suppressing host target transcripts involved in a wide range of key cellular pathways; however, the precise contribution that these regulatory RNAs make to in vivo virus infection and pathogenesis is unknown. Here, we generated a mutated form of murine gammaherpesvirus (MHV68) to dissect the function of gammaherpesvirus miRNAs in vivo. We demonstrate that the MHV68 miRNAs were dispensable for short-term virus replication but were important for establishment of lifelong infection in the key virus reservoir of memory B cells. Moreover, the MHV68 miRNAs were essential for the development of virus-associated pneumonia, implicating them as a critical component of gammaherpesvirus-associated disease.

A conserved RNA polymerase III promoter required for gammaherpesvirus TMER transcription and microRNA processing.

Canonical RNA polymerase III (pol III) type 2 promoters contain a single A and B box and are well documented for their role in tRNA and SINE transcription in eukaryotic cells. The genome of Murid herpesvirus 4 (MuHV-4) contains eight polycistronic tRNA-microRNA encoded RNA (TMER) genes that are transcribed from a RNA pol III type 2-like promoter containing triplicated A box elements. Here, we demonstrate that the triplicated A box sequences are required in their entirety to produce functional MuHV-4 miRNAs. We also identify that these RNA pol III type 2-like promoters are conserved in eukaryotic genomes. Human and mouse predicted tRNA genes containing these promoters also show enrichment of alternative RNA pol III transcription termination sequences and are predicted to give rise to longer tRNA primary transcripts.

Interleukin-1 receptor-associated kinase M (IRAK-M) promotes human rhinovirus infection in lung epithelial cells via the autophagic pathway.

Human rhinovirus (HRV) is the most common viral etiology in acute exacerbations of asthma. However, the exact mechanisms underlying HRV infection in allergic airways are poorly understood. IL-13 increases interleukin-1 receptor associated kinase M (IRAK-M) and subsequently inhibits airway innate immunity against bacteria. However, the role of IRAK-M in lung HRV infection remains unclear. Here, we provide the first evidence that IRAK-M over-expression promotes lung epithelial HRV-16 replication and autophagy, but inhibits HRV-16-induced IFN-ß and IFN-λ1 expression. Inhibiting autophagy reduces HRV-16 replication. Exogenous IFN-ß and IFN-λ1 inhibit autophagy and HRV-16 replication. Our data indicate the enhancing effect of IRAK-M on epithelial HRV-16 infection, which is partly through the autophagic pathway. Impaired anti-viral interferon production may serve as a direct or an indirect (e.g., autophagy) mechanism of enhanced HRV-16 infection by IRAK-M over-expression. Targeting autophagic pathway or administrating anti-viral interferons may prevent or attenuate viral (e.g., HRV-16) infections in allergic airways.

T-box transcription factor T-bet, a key player in a unique type of B-cell activation essential for effective viral clearance.

IgG2a is known to be the most efficient antibody isotype for viral clearance. Here, we demonstrate a unique pathway of B-cell activation, leading to IgG2a production, and involving synergistic stimulation via B-cell antigen receptors, toll-like receptor 7 (TLR7), and IFNγ receptors on B cells. This synergistic stimulation leads to induction of T-box transcription factor T-bet expression in B cells, which, in turn, drives expression of CD11b and CD11c on B cells. T-bet/CD11b/CD11c positive B cells appear during antiviral responses and produce high titers of antiviral IgG2a antibodies that are critical for efficient viral clearance. The results thus demonstrate a previously unknown role for T-bet expression in B cells during viral infections. Moreover, the appearance of T-bet(+) B cells during antiviral responses and during autoimmunity suggests a possible link between these two processes.