Genetic analysis of innate resistance to mouse cytomegalovirus (MCMV)

Increased Viral Dissemination in the Brain and Lethality in MCMV-Infected, Dicer-Deficient Neonates

Among Herpesviruses, Human Cytomegalovirus (HCMV or HHV-5) represents a major threat during congenital or neonatal infections, which may lead to encephalitis with serious neurological consequences. However, as opposed to other less prevalent pathogens, the mechanisms and genetic susceptibility factors for CMV encephalitis are poorly understood. This lack of information considerably reduces the prognostic and/or therapeutic possibilities. To easily monitor the effects of genetic defects on brain dissemination following CMV infection we used a recently developed in vivo mouse model based on the neonatal inoculation of a MCMV genetically engineered to express Luciferase. Here, we further validate this protocol for live imaging, and demonstrate increased lethality associated with viral infection and encephalitis in mutant mice lacking Dicer activity. Our data indicate that miRNAs are important players in the control of MCMV pathogenesis and suggest that miRNA-based endothelial functions and integrity are crucial for CMV encephalitis.

Abrogation of the interferon response promotes more efficient human cytomegalovirus replication

The effect of abrogating the interferon (IFN) response on human cytomegalovirus (HCMV) replication was investigated using primary human cells engineered to block either the production of or the response to type I IFNs. In IFN-deficient cells, HCMV produced larger plaques and spread and replicated more rapidly than in parental cells. These cells demonstrate the vital role of IFNs in controlling HCMV replication and provide useful tools to investigate the IFN response to HCMV.

A chemokine-like viral protein enhances alpha interferon production by plasmacytoid dendritic cells but delays CD8+ T cell activation and impairs viral clearance

Murine cytomegalovirus encodes numerous proteins that act on a variety of pathways to modulate the innate and adaptive immune responses. Here, we demonstrate that a chemokine-like protein encoded by murine cytomegalovirus activates the early innate immune response and delays adaptive immunity, thereby impairing viral clearance. The protein, m131/129 (also known as MCK-2), is not required to establish infection in the spleen; however, a mutant virus lacking m131/129 was cleared more rapidly from this organ. In the absence of m131/129 expression, there was enhanced activation of dendritic cells (DC), and virus-specific CD8(+) T cells were recruited into the immune response earlier. Viral mutants lacking m131/129 elicited weaker production of alpha interferon (IFN-α) at 40 h postinfection, indicating that this protein exerts its effects during early rounds of viral replication in the spleen. Furthermore, while wild-type and mutant viruses activated plasmacytoid dendritic cells (pDC) equally at this time, as measured by the upregulation of costimulatory molecules, the presence of m131/129 stimulated more pDC to secrete IFN-α, accounting for the stronger IFN-α response than from the wild-type virus. These data provide evidence for a novel immunomodulatory function of a viral chemokine and expose the multifunctionality of immune evasion proteins. In addition, these results broaden our understanding of the interplay between innate and adaptive immunity.

Reversible inhibition of murine cytomegalovirus replication by gamma interferon (IFN-γ) in primary macrophages involves a primed type I IFN-signaling subnetwork for full establishment of an immediate-early antiviral state

Activated macrophages play a central role in controlling inflammatory responses to infection and are tightly regulated to rapidly mount responses to infectious challenge. Type I interferon (alpha/beta interferon [IFN-α/β]) and type II interferon (IFN-γ) play a crucial role in activating macrophages and subsequently restricting viral infections. Both types of IFNs signal through related but distinct signaling pathways, inducing a vast number of interferon-stimulated genes that are overlapping but distinguishable. The exact mechanism by which IFNs, particularly IFN-γ, inhibit DNA viruses such as cytomegalovirus (CMV) is still not fully understood. Here, we investigate the antiviral state developed in macrophages upon reversible inhibition of murine CMV by IFN-γ. On the basis of molecular profiling of the reversible inhibition, we identify a significant contribution of a restricted type I IFN subnetwork linked with IFN-γ activation. Genetic knockout of the type I-signaling pathway, in the context of IFN-γ stimulation, revealed an essential requirement for a primed type I-signaling process in developing a full refractory state in macrophages. A minimal transient induction of IFN-β upon macrophage activation with IFN-γ is also detectable. In dose and kinetic viral replication inhibition experiments with IFN-γ, the establishment of an antiviral effect is demonstrated to occur within the first hours of infection. We show that the inhibitory mechanisms at these very early times involve a blockade of the viral major immediate-early promoter activity. Altogether our results show that a primed type I IFN subnetwork contributes to an immediate-early antiviral state induced by type II IFN activation of macrophages, with a potential further amplification loop contributed by transient induction of IFN-β.

Sources and signals regulating type I interferon production: lessons learned from cytomegalovirus

Type I interferons (IFN-αβ) are pleiotropic cytokines critical for antiviral host defense, and the timing and magnitude of their production involve a complex interplay between host and pathogen factors. Mouse cytomegalovirus (a β-herpesvirus) is a persistent virus that induces a biphasic IFN-αβ response during the first days of infection. The cell types and molecular mechanisms governing these 2 phases are unique, with splenic stromal cells being a major source of initial IFN-αβ, requiring communication with B cells expressing lymphotoxin, a tumor necrosis factor family cytokine. Here we review the factors that regulate this lymphotoxin-IFN-αβ "axis" during cytomegalovirus infection, highlight how stroma-derived IFN-αβ contributes in other models, and discuss how deregulation of this axis can lead to pathology in some settings.

Modulation of host innate and adaptive immune defenses by cytomegalovirus: timing is everything

Human cytomegalovirus (HCMV) (HHV-5, a beta-herpesvirus) causes the vast majority of infection-related congenital birth defects, and can trigger severe disease in immune suppressed individuals. The high prevalence of societal infection, the establishment of lifelong persistence and the growing number of immune-related diseases where HCMV is touted as a potential promoter is slowly heightening public awareness to this virus. The millions of years of co-evolution between CMV and the immune system of its host provides for a unique opportunity to study immune defense strategies, and pathogen counterstrategies. Dissecting the timing of the cellular and molecular processes that regulate innate and adaptive immunity to this persistent virus has revealed a complex defense network that is shaped by CMV immune modulation, resulting in a finely tuned host-pathogen relationship.

NK gene complex and chromosome 19 loci enhance MHC resistance to murine cytomegalovirus infection

An H-2(k) MHC locus is critical for murine cytomegalovirus (MCMV) resistance in MA/My mice and virus control is abolished if H-2(k) is replaced with H-2(b) MHC genes from MCMV-susceptible C57L mice. Yet, H-2(k) resistance varies with genetic background; thus, modifiers of virus resistance must exist. To identify non-MHC resistance loci, spleen and liver MCMV levels and genome-wide genotypes were assessed in (C57L x MA/My) and (MA/My x C57L) F(2) offspring (representing 550 meioses). Significantly, a non-Mendelian frequency of MHC genotypes was observed for offspring of the latter cross. Quantitative trait loci (QTL) and their interaction potential in MCMV resistance were assessed in R/qtl; QTL on chromosomes 17, 6, and 19 affected MCMV levels in infected animals. A chromosome 6 QTL was linked with the NK gene complex and acted in an additive fashion with an H-2(k) MHC QTL to mitigate spleen MCMV levels. We provide biological confirmation that this chromosome 6 QTL provided MCMV control independent of H-2(k) via NK cells. Importantly, both chromosome 6 and 19 QTLs contribute to virus control independent of H-2(k). Altogether, MHC and non-MHC MCMV-resistance QTL contribute in early resistance to MCMV infection in this genetic system.

Identification of Mouse Cytomegalovirus Resistance Loci by ENU Mutagenesis

Host resistance to infection depends on the efficiency with which innate immune responses keep the infectious agent in check. Innate immunity encompasses components with sensing, signaling and effector properties. These elements with non-redundant functions are encoded by a set of host genes, the resistome. Here, we review our findings concerning the resistome. We have screened randomly mutagenized mice for susceptibility to a natural opportunistic pathogen, the mouse cytomegalovirus. We found that some genes with initially no obvious functions in innate immunity may be critical for host survival to infections, falling into a newly defined category of genes of the resistome.

Antibody-mediated enhancement of the rate, magnitude, and responsiveness of vesicular stomatitis virus induced alpha interferon production

A majority of adults without evidence of exposure to vesicular stomatitis virus (VSV) have serum IgG antibodies that interact with pro-inflammatory TLR7 in the presence of VSV, and enhance several aspects of VSV-induced IFN-alpha production. Enhancing IgG antibody enables human PBMC to make IFN-alpha more rapidly and in higher titers in response to a broad range of VSV-concentrations that include those too low to independently stimulate IFN-alpha production. These antibody-mediated functions compensate for the inherent delay in virus-induced IFN-alpha production in vitro, and have the potential to improve the in vivo IFN-alpha response and effectively terminate infection before the occurrence of clinically apparent disease. The frequent presence of enhancing antibody in persons without predictable VSV exposure has implications for naturally occurring infections with this and other viruses, and for the use of viruses as vaccine vectors and oncolytic agents.

Chemical mutagenesis: a new strategy against the global threat of infectious diseases

The perpetual evolution of drug-resistant microbes, the overwhelming burden of acquired immune suppression due to HIV, the emergence or re-emergence of various pathogens (West Nile virus, pandemic influenza, Creutzfeld-Jacob disease), and increased fears of bioterrorism has drawn a great deal of new attention to infectious diseases. The pathogenesis of infection is characterized by complex interactions of potentially virulent microorganisms with host genetic and acquired factors. Chemical mutagenesis of the mouse genome provides a robust method to unravel this challenging problem. To deepen our understanding of the natural host response to pathogens, our team and others are interrogating the mouse genome to define genes that are crucial to the defense against infectious diseases (pathogen recognition, viral defense, bacterial defense, prion infection). In this review we highlight the current progress of these efforts and propose a toolbox for other groups that are interested in this endeavor.

Forward genetic dissection of immunity to infection in the mouse

Forward genetics is an experimental approach in which gene mapping and positional cloning are used to elucidate the molecular mechanisms underlying phenotypic differences between two individuals for a given trait. This strategy has been highly successful for the study of inbred mouse strains that show differences in innate susceptibility to bacterial, parasitic, fungal, and viral infections. Over the past 20 years, these studies have led to the identification of a number of cell populations and critical biochemical pathways and proteins that are essential for the early detection of and response to invading pathogens. Strikingly, the macrophage is the point of convergence for many of these genetic studies. This has led to the identification of diverse pathways involved in extracellular and intracellular pathogen recognition, modification of the properties and content of phagosomes, transcriptional response, and signal transduction for activation of adaptive immune mechanisms. In models of viral infections, elegant genetic studies highlighted the pivotal role of natural killer cells in the detection and destruction of infected cells.

Lymphotoxin-mediated crosstalk between B cells and splenic stroma promotes the initial type I interferon response to cytomegalovirus

Toll-like receptor (TLR)-dependent pathways control the production of IFNalphabeta, a key cytokine in innate immune control of viruses including mouse cytomegalovirus (MCMV). The lymphotoxin (LT) alphabeta-LTbeta receptor signaling pathway is also critical for defense against MCMV and thought to aid in the IFNbeta response. We find that upon MCMV infection, mice deficient for lymphotoxin (LT)alphabeta signaling cannot mount the initial part of a biphasic IFNalphabeta response, but show normal levels of IFNalphabeta during the sustained phase of infection. Significantly, the LTalphabeta-dependent, IFNalphabeta response is independent of TLR signaling. B, but not T, cells expressing LTbeta are essential for promoting the initial IFNalphabeta response. LTbetaR expression is required strictly in splenic stromal cells for initial IFNalphabeta production to MCMV and is dependent upon the NF-kappaB-inducing kinase (NIK). These results reveal a TLR-independent innate host defense strategy directed by B cells in communication with stromal cells via the LTalphabeta cytokine system.