Murine cytomegalovirus paralyzes macrophages by blocking IFN gamma-induced promoter assembly

Immune Relevant and Immune Deficient Mice: Options and Opportunities in Translational Research

In 1989 ILAR published a list and description of immunodeficient rodents used in research. Since then, advances in understanding of molecular mechanisms; recognition of genetic, epigenetic microbial, and other influences on immunity; and capabilities in manipulating genomes and microbiomes have increased options and opportunities for selecting mice and designing studies to answer important mechanistic and therapeutic questions. Despite numerous scientific breakthroughs that have benefitted from research in mice, there is debate about the relevance and predictive or translational value of research in mice. Reproducibility of results obtained from mice and other research models also is a well-publicized concern. This review summarizes resources to inform the selection and use of immune relevant mouse strains and stocks, aiming to improve the utility, validity, and reproducibility of research in mice. Immune sufficient genetic variations, immune relevant spontaneous mutations, immunodeficient and autoimmune phenotypes, and selected induced conditions are emphasized.

Murine cytomegalovirus infection of mouse macrophages stimulates early expression of suppressor of cytokine signaling (SOCS)1 and SOCS3

Human cytomegalovirus (HCMV) is a species-specific β-herpesvirus that infects for life up to 80% of the world’s population and causes severe morbidity in at-risk immunocompromised populations. Suppressors of cytokine signaling (SOCS)1 and SOCS3 are host proteins that act as inducible negative feedback regulators of cytokine signaling and have been implicated in several ocular diseases and viral infections. We recently found in our mouse model of experimental cytomegalovirus retinitis that subretinally-injected murine cytomegalovirus (MCMV) stimulates ocular SOCS1 and SOCS3 during retrovirus-induced immune suppression of murine AIDS (MAIDS), and that infiltrating macrophages are prominent cellular sources of retinal SOCS1 and SOCS3 expression. Herein we investigate possible virologic mechanisms whereby MCMV infection may stimulate SOCS1 and/or SOCS3 expression in cell culture. We report that infection of IC-21 mouse macrophages with MCMV propagated through the salivary glands of BALB/c mice, but not from tissue culture in C57BL/6 fibroblasts, transiently stimulates SOCS1 and SOCS3 mRNA transcripts, but not SOCS5 mRNA. Viral tegument proteins are insufficient for this stimulation, as replication-deficient UV-inactivated MCMV fails to stimulate SOCS1 or SOCS3 in IC-21 macrophages. By contrast, infection of murine embryonic fibroblasts (MEFs) with either productive MCMV or UV-inactivated MCMV significantly stimulates SOCS1 and SOCS3 mRNA expression early after infection. Treatment of MCMV-infected IC-21 mouse macrophages with the antiviral drug ganciclovir significantly decreases MCMV-stimulated SOCS3 expression at 3 days post-infection. These data suggest cell type-specific, different roles for viral immediate early or early gene expression and/or viral tegument proteins in the early stimulation of SOCS1 and SOCS3 during MCMV infection. Furthermore, putative biphasic stimulation of SOCS3 during late MCMV infection of IC-21 mouse macrophages may occur by divergent virologic mechanisms.

The carboxy terminal region of the human cytomegalovirus immediate early 1 (IE1) protein disrupts type II inteferon signaling

Interferons (IFNs) activate the first lines of defense against viruses, and promote innate and adaptive immune responses to viruses. We report that the immediate early 1 (IE1) protein of human cytomegalovirus (HCMV) disrupts signaling by IFNγ. The carboxyl-terminal region of IE1 is required for this function. We found no defect in the initial events in IFNγ signaling or in nuclear accumulation of signal transducer and activator of transcription 1 (STAT1) in IE1-expressing cells. Moreover, we did not observe an association between disruption of IFNγ signaling and nuclear domain 10 (ND10) disruption. However, there is reduced binding of STAT1 homodimers to target gamma activated sequence (GAS) elements in the presence of IE1. Co-immunoprecipitation studies failed to support a direct interaction between IE1 and STAT1, although these studies revealed that the C-terminal region of IE1 was required for interaction with STAT2. Together, these results indicate that IE1 disrupts IFNγ signaling by interfering with signaling events in the nucleus through a novel mechanism.

Interplay between CMVs and interferon signaling: implications for pathogenesis and therapeutic intervention

Most human individuals are latently infected with human CMV, a prototypic β-herpesvirus, frequently acquired during early childhood. In the absence of adequate immune control, the otherwise asymptomatic infection causes life-threatening disease. To enable efficient replication and to maintain lifelong latency in immunocompetent hosts, CMVs have evolved numerous molecules mediating immune evasive properties, targeting both innate and adaptive immune responses. Upon infection, cells secrete interferons (IFNs), which initiate an extremely fast signal transduction cascade upon binding to their cognate receptors, culminating in a pronounced change in the cellular gene expression profile. This response leads to the establishment of an intracellular antimicrobial state and to the recruitment, as well as stimulation, of the adaptive immune system. Unfortunately, CMVs impede the IFN system by interfering with its induction, signaling and downstream effector functions. This review aims to present our current understanding of such cytomegaloviral IFN-evasive properties, their pathogenic implications and potential for therapeutic exploitation.

Hypomorphic mutation in the site-1 protease Mbtps1 endows resistance to persistent viral infection in a cell-specific manner

The prototypic arenavirus lymphocytic choriomeningitis virus (LCMV), which naturally persists in rodents, represents a model for HIV, HBV, and HCV. Cleavage of the viral glycoprotein precursor by membrane-bound transcription factor peptidase, site 1 (Mbtps1 or site-1 protease), is crucial for the life cycle of arenaviruses and therefore represents a potential target for therapy. Recently, we reported a viable hypomorphic allele of Mbtps1 (woodrat) encoding a protease with diminished enzymatic activity. Using the woodrat allele, we examine the role of Mbtps1 during persistent LCMV infection. Surprisingly, Mbtps1 inhibition limits persistent but not acute viral infection and is associated with an organ/cell type-specific decrease in viral titers. Analysis of bone marrow-derived dendritic cells from woodrat mice supports their specific role in resolving persistent viral infection. These results support in vivo targeting of Mbtps1 in the treatment of arenavirus infections and demonstrate a critical role for dendritic cells in persistent viral infections.

Absence of vasoactive intestinal peptide expression in hematopoietic cells enhances Th1 polarization and antiviral immunity in mice

Vasoactive intestinal peptide (VIP) induces regulatory dendritic cells (DC) in vitro that inhibit cellular immune responses. We tested the role of physiological levels of VIP on immune responses to murine CMV (mCMV) using VIP-knockout (VIP-KO) mice and radiation chimeras engrafted with syngenic VIP-KO hematopoietic cells. VIP-KO mice had less weight loss and better survival following mCMV infection compared with wild-type (WT) littermates. mCMV-infected VIP-KO mice had lower viral loads, faster clearance of virus, with increased numbers of IFN-γ(+) NK and NKT cells, and enhanced cytolytic activity of NK cells. Adaptive antiviral cellular immunity was increased in mCMV-infected VIP-KO mice compared with WT mice, with more Th1/Tc1-polarized T cells, fewer IL-10(+) T cells, and more mCMV-M45 epitope peptide MHC class I tetramer(+) CD8(+) T cells (tetramer(+) CD8 T cells). mCMV-immune VIP-KO mice had enhanced ability to clear mCMV peptide-pulsed target cells in vivo. Enhanced antiviral immunity was also seen in WT transplant recipients engrafted with VIP-KO hematopoietic cells, indicating that VIP synthesized by neuronal cells did not suppress immune responses. Following mCMV infection there was a marked upregulation of MHC-II and CD80 costimulatory molecule expression on DC from VIP-KO mice compared with DC from WT mice, whereas programmed death-1 and programmed death ligand-1 expression were upregulated in activated CD8(+) T cells and DC, respectively, in WT mice, but not in VIP-KO mice. Because the absence of VIP in immune cells increased innate and adaptive antiviral immunity by altering costimulatory and coinhibitory pathways, selective targeting of VIP signaling represents an attractive therapeutic target to enhance antiviral immunity.

Glioma-associated cytomegalovirus mediates subversion of the monocyte lineage to a tumor propagating phenotype

PURPOSE

Cytomegalovirus (CMV) has been ubiquitously detected within high-grade gliomas, but its role in gliomagenesis has not been fully elicited.

EXPERIMENTAL DESIGN

Glioblastoma multiforme (GBM) tumors were analyzed by flow cytometry to determine CMV antigen expression within various glioma-associated immune populations. The glioma cancer stem cell (gCSC) CMV interleukin (IL)-10 production was determined by ELISA. Human monocytes were stimulated with recombinant CMV IL-10 and levels of expression of p-STAT3, VEGF (vascular endothelial growth factor), TGF-β, viral IE1, and pp65 were determined by flow cytometry. The influence of CMV IL-10-treated monocytes on gCSC biology was ascertained by functional assays.

RESULTS

CMV showed a tropism for macrophages (MΦ)/microglia and CD133+ gCSCs within GBMs. The gCSCs produce CMV IL-10, which induces human monocytes (the precursor to the central nervous system MΦs/microglia) to assume an M2 immunosuppressive phenotype (as manifested by downmodulation of the major histocompatibility complex and costimulatory molecules) while upregulating immunoinhibitory B7-H1. CMV IL-10 also induces expression of viral IE1, a modulator of viral replication and transcription in the monocytes. Finally, the CMV IL-10-treated monocytes produced angiogenic VEGF, immunosuppressive TGF-β, and enhanced migration of gCSCs.

CONCLUSIONS

CMV triggers a feedforward mechanism of gliomagenesis by inducing tumor-supportive monocytes.

Human cytomegalovirus decreases constitutive transcription of MHC class II genes in mature Langerhans cells by reducing CIITA transcript levels

Human cytomegalovirus (HCMV) productively infects CD34(+) progenitor-derived, mature Langerhans-type dendritic cells (matLC) and reduces surface expression of MHC class II complexes (MHC II) by increasing intracellular retention of these molecules. To determine whether HCMV also inhibits MHC II expression by other mechanisms, we assessed mRNA levels of the class II transcriptional regulator, CIITA, and several of its target genes in infected matLC. Levels of CIITA, HLA-DRA (DRA) and DRB transcripts, and new DR protein synthesis were compared in mock-infected and HCMV-infected cells by quantitative PCR and pulse-chase immunoprecipitation analyses, respectively. CIITA mRNA levels were significantly lower in HCMV-infected matLC as compared to mock-infected cells. When assessed in the presence of Actinomycin D, the stability of CIITA transcripts was not diminished by HCMV. Analysis of promoter-specific CIITA isoforms revealed that types I, III and IV all were decreased by HCMV, a result that differs from changes after incubation of these cells with lipopolysaccharide (LPS). Exposure to UV-inactivated virus failed to reduce CIITA mRNA levels, implicating de novo viral gene expression in this effect. HCMV-infected matLC also expressed lower levels of DR transcripts and reduced DR protein synthesis rates compared to mock-infected matLC. In summary, we demonstrate that HCMV infection of a human dendritic cell subset inhibits constitutive CIITA expression, most likely at the transcriptional level, resulting in reduced MHC II biosynthesis. We suggest this represents a new mechanism of modulation of mature LC by HCMV.

TLR2-dependent inhibition of macrophage responses to IFN-gamma is mediated by distinct, gene-specific mechanisms

Mycobacterium tuberculosis uses multiple mechanisms to avoid elimination by the immune system. We have previously shown that M. tuberculosis can inhibit selected macrophage responses to IFN-γ through TLR2-dependent and -independent mechanisms. To specifically address the role of TLR2 signaling in mediating this inhibition, we stimulated macrophages with the specific TLR2/1 ligand Pam₃CSK₄ and assayed responses to IFN-γ. Pam₃CSK₄ stimulation prior to IFN-γ inhibited transcription of the unrelated IFN-γ-inducible genes, CIITA and CXCL11. Surface expression of MHC class II and secretion of CXCL11 were greatly reduced as well, indicating that the reduction in transcripts had downstream effects. Inhibition of both genes required new protein synthesis. Using chromatin immunoprecipitation, we found that TLR2 stimulation inhibited IFN-γ-induced RNA polymerase II binding to the CIITA and CXCL11 promoters. Furthermore, TATA binding protein was unable to bind the TATA box of the CXCL11 promoter, suggesting that assembly of transcriptional machinery was disrupted. However, TLR2 stimulation affected chromatin modifications differently at each of the inhibited promoters. Histone H3 and H4 acetylation was reduced at the CIITA promoter but unaffected at the CXCL11 promoter. In addition, NF-κB signaling was required for inhibition of CXCL11 transcription, but not for inhibition of CIITA. Taken together, these results indicate that TLR2-dependent inhibition of IFN-γ-induced gene expression is mediated by distinct, gene-specific mechanisms that disrupt binding of the transcriptional machinery to the promoters.

Human cytomegalovirus interferes with signal transducer and activator of transcription (STAT) 2 protein stability and tyrosine phosphorylation

We have investigated the role of signal transducer and activator of transcription (STAT) 2 during human cytomegalovirus (HCMV) replication and found that protein levels of STAT2 are downregulated. STAT2 downregulation was observed in HCMV clinical isolates and laboratory strains with the exception of strain Towne. The HCMV-induced loss of STAT2 protein occurred despite an increased accumulation of STAT2 mRNA; it required HCMV early gene expression. The decrease in STAT2 was sensitive to proteasome inhibition, suggesting degradation of STAT2 via the ubiquitin proteasome pathway. Notably, pUL27, the HCMV homologue of the mouse CMV pM27 protein, which mediates the selective proteolysis of STAT2, did not induce STAT2 downregulation. Moreover, preceding STAT2 degradation, alpha/beta interferon (IFN)-receptor-mediated tyrosine phosphorylation of STAT2 was inhibited in HCMV-infected cells. This effect was paralleled by impaired tyrosine activation of STAT1 and STAT3. Accordingly, IFNs affected the replication efficiency of STAT2 degrading and non-degrading HCMV strains to a similar degree. In summary, HCMV abrogates IFN receptor signalling at multiple checkpoints by independent mechanisms including UL27-independent degradation of STAT2 and a preceding blockade of STAT2 phosphorylation.

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.

GPX-Macrophage Expression Atlas: a database for expression profiles of macrophages challenged with a variety of pro-inflammatory, anti-inflammatory, benign and pathogen insults

BACKGROUND

Macrophages play an integral role in the host immune system, bridging innate and adaptive immunity. As such, they are finely attuned to extracellular and intracellular stimuli and respond by rapidly initiating multiple signalling cascades with diverse effector functions. The macrophage cell is therefore an experimentally and clinically amenable biological system for the mapping of biological pathways. The goal of the macrophage expression atlas is to systematically investigate the pathway biology and interaction network of macrophages challenged with a variety of insults, in particular via infection and activation with key inflammatory mediators. As an important first step towards this we present a single searchable database resource containing high-throughput macrophage gene expression studies.

DESCRIPTION

The GPX Macrophage Expression Atlas (GPX-MEA) is an online resource for gene expression based studies of a range of macrophage cell types following treatment with pathogens and immune modulators. GPX-MEA follows the MIAME standard and includes an objective quality score with each experiment. It places special emphasis on rigorously capturing the experimental design and enables the searching of expression data from different microarray experiments. Studies may be queried on the basis of experimental parameters, sample information and quality assessment score. The ability to compare the expression values of individual genes across multiple experiments is provided. In addition, the database offers access to experimental annotation and analysis files and includes experiments and raw data previously unavailable to the research community.

CONCLUSION

GPX-MEA is the first example of a quality scored gene expression database focussed on a macrophage cellular system that allows efficient identification of transcriptional patterns. The resource will provide novel insights into the phenotypic response of macrophages to a variety of benign, inflammatory, and pathogen insults. GPX-MEA is available through the GPX website at http://www.gti.ed.ac.uk/GPX.

The Effects of Allitridin on the Expression of Transcription Factors T-bet and GATA-3 in Mice Infected by Murine Cytomegalovirus

This study was designed to investigate the effects of allitridin on the expression of transcription factors T-bet and GATA-3 in mice infected by murine cytomegalovirus (MCMV). A BALB/c mouse model system of MCMV infection was established. Twenty mice were allocated randomly into an allitridin-treated group (n = 10) and a placebo control group (n = 10). The same dose (25 mg/kg/day) and regimen of allitridin were used in the treated group in the 24 hours after virus infection; the same volume of saline solution was injected in placebo control mice. In an additional blank control group (n = 10), the same volume of saline solution was injected. The expression levels of the transcription factors T-bet and GATA-3 were measured by reverse transcription-polymerase chain reaction. The expression levels of the T helper (Th) 1 cytokine interferon-gamma (IFN-gamma) and the Th2 cytokine interleukin (IL)-10 in supernatant of spleen cell culture were measured by enzyme-linked immunosorbent assay. MCMV infection markedly down-modulated the expression of IFN-gamma and T-bet and significantly up-modulated the expression of IL-10 and GATA-3. Allitridin induced significantly (P < .01) increased expression of the transcription factor T-bet and the Th1 cytokine IFN-gamma and markedly (P < .01) decreased expression of the transcription factor GATA-3 and the Th2 cytokine IL-10. Thus MCMV infection could lead to disequilibrium of Th1/Th2 cytokine expression: The level of the Th1 cytokine IFN-gamma was decreased significantly, and Th2 cytokine IL-10 was overexpressed markedly. Allitridin could up-regulate the expression of T-bet and IFN-gamma and inhibit the expression of GATA-3 and IL-10 in MCMV-infected mice, indicating a Th1 dominant state, which should enhance the specific cellular immune reactions against cytomegalovirus (CMV) and be helpful for clearance of CMV from the host.

A cytomegaloviral protein reveals a dual role for STAT2 in IFN-{gamma} signaling and antiviral responses

A mouse cytomegalovirus (MCMV) gene conferring interferon (IFN) resistance was identified. This gene, M27, encodes a 79-kD protein that selectively binds and down-regulates for signal transducer and activator of transcription (STAT)-2, but it has no effect on STAT1 activation and signaling. The absence of pM27 conferred MCMV susceptibility to type I IFNs (alpha/beta), but it had a much more dramatic effect on type II IFNs (gamma) in vitro and in vivo. A comparative analysis of M27(+) and M27(-) MCMV revealed that the antiviral efficiency of IFN-gamma was partially dependent on the synergistic action of type I IFNs that required STAT2. Moreover, STAT2 was directly activated by IFN-gamma. This effect required IFN receptor expression and was independent of type I IFNs. IFN-gamma induced increasing levels of tyrosine-phosphorylated STAT2 in M27(-) MCMV-infected cells that were essential for the antiviral potency of IFN-gamma. pM27 represents a new strategy for simultaneous evasions from types I and II IFNs, and it documents an unknown biological significance for STAT2 in antiviral IFN-gamma responses.

Cell-type-specific regulation of distinct sets of gene targets by Pax3 and Pax3/FKHR

The oncogenic fusion protein, Pax3/FKHR, is a more potent transcription factor relative to its normal counterpart, Pax3. Since Pax3 induced a mesenchymal to epithelial transition (MET) in human SaOS-2 osteosarcomas, we hypothesized that Pax3/FKHR would also induce a morphological change in SaOS-2 cells. We demonstrate here that Pax3/FKHR more potently induces a MET in SaOS-2 cells than Pax3. This greater potency was further evident where Pax3/FKHR, but not Pax3, induced a morphological alteration in U2-OS osteosarcoma cells. By microarray analysis, we determined that Pax3/FKHR altered the expression of gene targets in a manner quantitatively and qualitatively distinct from Pax3. Three classes of genes were identified: (i) genes induced or repressed by Pax3 and Pax3/FKHR, (ii) genes induced or repressed by Pax3/FKHR but not Pax3 and (iii) genes induced by Pax3/FKHR but repressed by Pax3. Chromatin immunoprecipitations confirmed the direct binding of Pax3/FKHR to the promoter region of several factors including cannabinoid receptor-1, EPHA2 and EPHA4. Verification of the microarray data also revealed coordinate alteration in the expression of factors involved in BMP4 signalling. Regulation of gene expression by Pax3 and Pax3/FKHR is, however, cell-type specific. BMP4 expression, for example, was repressed by both Pax3 and Pax3/FKHR in SaOS-2 cells, while in the rhabdomyosarcoma, RD, Pax3/FKHR, but not Pax3, induced BMP4 expression. Thus, our data reveal that Pax3/FKHR regulates a distinct but overlapping set of genes relative to Pax3 and that the global set of Pax3 and Pax3/FKHR gene targets is cell-type specific.

Identification of a mouse cytomegalovirus gene selectively targeting CD86 expression on antigen-presenting cells

We and others have shown that infection of dendritic cells with murine cytomegalovirus (MCMV) leads to severe functional impairment of these antigen-presenting cells (D. M. Andrews, C. E. Andoniou, F. Granucci, P. Ricciardi-Castagnoli, and M. A. Degli-Esposti, Nat. Immunol. 2:1077-1084, 2001; S. Mathys, T. Schroeder, J. Ellwart, U. H. Koszinowski, M. Messerle, and U. Just, J. Infect. Dis. 187:988-999, 2003). Phenotypically, reduced surface expression of costimulatory molecules and major histocompatibility complex molecules was detected. In order to identify the molecular basis for the observed effects, we generated MCMV mutants with large deletions of nonessential genes. The study was facilitated by the finding that a monocyte-macrophage cell line displayed similar phenotypic alterations after MCMV infection. By analyzing the expression of cell surface molecules on infected cells, we identified a mutant virus which is no longer able to downmodulate the expression of the costimulatory molecule CD86. Additional mutants with smaller deletions allowed us to pin down the responsible gene to a certain genomic region. RNA analysis led to the identification of the spliced gene m147.5, encoding a protein with 145 amino acids. Experiments with an m147.5 mutant revealed that the protein affects CD86 expression only, suggesting that additional MCMV genes are responsible for downmodulation of the other surface molecules. Identification of viral gene products interfering with functionally important proteins of antigen-presenting cells will provide the basis to dissect the complex interaction of CMV with these important cells and to evaluate the biological importance of these viral genes in vivo.