Inhibition of Japanese encephalitis virus infection by nitric oxide: antiviral effect of nitric oxide on RNA virus replication.

The antiviral effects of nitric oxide (NO) on Japanese encephalitis virus (JEV), a member of the family Flaviviridae, were investigated in this study. In vitro, inhibition of replication of JEV in gamma interferon-activated RAW 264.7 murine macrophages was correlated to cellular NO production. When cocultured with infected murine neuroblastoma N18 cells, gamma interferon-activated RAW 264.7 cells also efficiently hindered JEV replication in contiguous bystanders, and this anti-JEV effect could be reversed by an NO synthase (NOS) inhibitor, N-monomethyl-L-arginine acetate. In vivo, the mortality rate increased as the NOS activity of JEV-infected mice was inhibited by its competitive inhibitor, N-nitro-L-arginine methyl ester. Moreover, when an organic donor, S-nitro-N-acetylpenicillamine (SNAP), was used, the NO-mediated antiviral effect was also observed in primarily JEV-infected N18, human neuronal NT-2, and BHK-21 cells, as well as in persistently JEV-infected C2-2 cells. These data reaffirm that NO has an effective and broad-spectrum antimicrobial activity against diversified intracellular pathogens. Interestingly, the antiviral effect of NO was not enhanced by treatment of N18 cells with SNAP prior to JEV infection, a measure which has been shown to greatly increase the antiviral effect of NO in infection by vesicular stomatitis virus. From biochemical analysis of the impact of NO on JEV replication in cell culture, NO was found to profoundly inhibit viral RNA synthesis, viral protein accumulation, and virus release from infected cells. The results herein thus suggest that NO may play a crucial role in the innate immunity of the host to restrict the initial stage of JEV infection in the central nervous system.     

Replication of Murine Cytomegalovirus in Differentiated Macrophages as a Determinant of Viral Pathogenesis

Blood monocytes or tissue macrophages play a pivotal role in the pathogenesis of murine cytomegalovirus (MCMV) infection, providing functions beneficial to both the virus and the host. In vitro and in vivo studies have indicated that differentiated macrophages support MCMV replication, are target cells for MCMV infection within tissues, and harbor latent MCMV DNA. However, this cell type presumably initiates early, antiviral immune responses as well. In addressing this paradoxical role of macrophages, we provide evidence that the proficiency of MCMV replication in macrophages positively correlates with virulence in vivo. An MCMV mutant from which the open reading frames M139, M140, and M141 had been deleted (RV10) was defective in its ability to replicate in macrophages in vitro and was highly attenuated for growth in vivo. However, depletion of splenic macrophages significantly enhanced, rather than deterred, replication of both wild-type (WT) virus and RV10 in the spleen. The ability of RV10 to replicate in intact or macrophage-depleted spleens was independent of cytokine production, as this mutant virus was a poor inducer of cytokines compared to WT virus in both intact organs and macrophage-depleted organs. Macrophages were, however, a major contributor to the production of tumor necrosis factor alpha and gamma interferon in response to WT virus infection. Thus, the data indicate that tissue macrophages serve a net protective role and may function as "filters" in protecting other highly permissive cell types from MCMV infection. The magnitude of virus replication in tissue macrophages may dictate the amount of virus accessible to the other cells. Concomitantly, infection of this cell type initiates the production of antiviral immune responses to guarantee efficient clearance of acute MCMV infection.     

The role of NK cell activity in age-dependent resistance of mice to murine cytomegalovirus infection

The resistance of mice to cell culture passaged murine cytomegalovirus (CC-MCMV) infection developed with age. In parallel with this finding, augmentation of the splenic NK cell activity in older mice was always higher than that of younger mice. The splenic NK cell activity reached the maximum level at 6 day post infection (PI) in 2-4-week-old mice while in 6-8-week-old mice it peaked at 4 days PI. When the dose of CC-MCMV was increased, the NK cell activity was potentiated accordingly. However, it was decreased on the infection with increased doses of the salivary gland passaged MCMV (SG-MCMV). NK cells augmented by MCMV infection actually inhibited in vitro replication of MCMV when they were added to mouse embryonic fibroblast (MEF) monolayers infected with CC-MCMV. Splenic and peritoneal macrophages inhibited in vitro replication of MCMV, but their activities were less potent than those of NK cells.     

Novel functions of tyrosine kinase 2 in the antiviral defense against murine cytomegalovirus

We have recently reported that tyrosine kinase 2 (Tyk2)-deficient mice have a selective defect in the in vivo defense against certain viruses. In our current study we show that Tyk2 is essential for the defense against murine CMV (MCMV). In vivo challenges with MCMV revealed impaired clearance of virus from organs and decreased survival of mice in the absence of Tyk2. Our in vitro studies demonstrate that MCMV replicates to dramatically higher titers in Tyk2-deficient macrophages compared with wild-type cells. We show an essential role of type I IFN (IFN-alphabeta) in the control of MCMV replication, with a prominent role of IFN-beta. MCMV infection leads to the activation of STAT1 and STAT2 in an IFN-alphabeta receptor 1-dependent manner. Consistent with the role of Tyk2 in IFN-alphabeta signaling, activation of STAT1 and STAT2 is reduced in Tyk2-deficient cells. However, lack of Tyk2 results in impaired MCMV-mediated gene induction of only a subset of MCMV-induced IFN-alphabeta-responsive genes. Taken together, our data demonstrate a requirement for Tyk2 in the in vitro and in vivo antiviral defense against MCMV infection. In addition to the established role of Tyk2 as an amplifier of Jak/Stat signaling upon IFN-alphabeta stimulation, we provide evidence for a novel role of Tyk2 as a modifier of host responses.     

Increased susceptibility to DNA virus infection in mice with a GCN2 mutation

The downregulation of translation through eIF2α phosphorylation is a cellular response to diverse stresses, including viral infection, and is mediated by the GCN2 kinase, protein kinase R (PKR), protein kinase-like endoplasmic reticulum kinase (PERK), and heme-regulated inhibitor kinase (HRI). Although PKR plays a major role in defense against viruses, other eIF2α kinases also may respond to viral infection and contribute to the shutdown of protein synthesis. Here we describe the recessive, loss-of-function mutation atchoum (atc) in Eif2ak4, encoding GCN2, which increased susceptibility to infection by the double-stranded DNA viruses mouse cytomegalovirus (MCMV) and human adenovirus. This mutation was identified by screening macrophages isolated from mice carrying N-ethyl-N-nitrosourea (ENU)-induced mutations. Cells from Eif2ak4(atc/atc) mice failed to phosphorylate eIF2α in response to MCMV. Importantly, homozygous Eif2ak4(atc) mice showed a modest increase in susceptibility to MCMV infection, demonstrating that translational arrest dependent on GCN2 contributes to the antiviral response in vivo.     

Cathepsin C limits acute viral infection independently of NK cell and CD8+ T-cell cytolytic function

Destruction of target cells by cytotoxic T lymphocytes (CTLs) or natural killer (NK) cells requires the coordinated action of the pore forming protein perforin (Pfp) and the granzyme (Gzm) family of serine proteases. The activation of a number of serine proteases, including GzmA and B, is predominately mediated by cathepsin C (CatC). Deficiencies in CatC-null mice were therefore expected to replicate the defects observed in GzmAB-deficient mice. We have previously determined that GzmAB-deficient mice exhibit increased susceptibility to murine cytomegalovirus (MCMV) infection. Here, we have compared the ability of CatC(-/-) mice to control MCMV infection with that of GzmAB-deficient animals. We found that CatC(-/-) mice have organ-specific defects in the ability to control MCMV replication, a phenotype that is distinct to that observed in GzmAB(-/-) mice. Significantly, the cytolytic function of CatC-deficient NK cells and CTLs elicited during infection was indistinguishable from that of wild-type cells. Hence, CatC is involved in limiting MCMV replication; however, this effect is independent of its role in promoting effector cytolytic activity. These data provide evidence for a novel and unexpected role of CatC during viral infection.     

Cat2 L-arginine transporter-deficient fibroblasts can sustain nitric oxide production.

High-output nitric oxide (NO) production by nitric oxide synthase 2 (NOS2) contributes to normal cellular processes and pathophysiological conditions. The transport of L-arginine, the substrate for NOS2, is required for sustained NO production by NOS2. L-Arginine can be transported by several kinetically defined transport systems, although the majority of arginine uptake is mediated by transport system y(+), encoded by the Cat1-3 gene family. Using macrophages from Cat2-deficient mice, we previously determined that arginine uptake via CAT2 is absolutely required for sustained NO production. Because NO production by fibroblasts is important in wound healing, we sought to determine whether CAT2 is required for NO production in cytokine-stimulated Cat2-deficient and wild-type embryonic fibroblasts. Although macrophages and fibroblasts both required extracellular L-arginine for NO production, NO synthesis by activated Cat2(-/-) fibroblasts was reduced only 19%, whereas Cat2(-/-) macrophages were virtually unable to produce NO. As expected, activated Cat2(-/-) fibroblasts had reduced system y(+)-mediated arginine uptake. However, their reduced NO output was not the result of a significant difference in intracellular L-arginine levels following cytokine stimulation. Uptake experiments revealed that the L-arginine transport system y(+)L was the major cationic amino acid carrier in fibroblasts of both genotypes. We conclude that NO production in embryonic fibroblasts is only partially dependent on CAT2 and that other compensating transporters provide arginine for NOS2-mediated NO synthesis. The data demonstrate that fibroblasts and macrophages have differential dependence on CAT2-mediated L-arginine transport for NO synthesis. The important physiological implication of this finding is discussed.     

Sustained nitric oxide production in macrophages requires the arginine transporter CAT2

The aberrant production of nitric oxide (NO) contributes to the pathogenesis of diseases as diverse as cancer and arthritis. Sustained NO production via the inducible enzyme, nitric-oxide synthase 2 (NOS2), requires extracellular arginine uptake. Three closely related cationic amino acid transporter genes (Cat1-3) encode the transporters that mediate most arginine uptake in mammalian cells. Because CAT2 is induced coordinately with NOS2 in numerous cell types, we investigated a possible role for CAT2-mediated arginine transport in regulating NO production. The complexity of arginine transport systems and their biochemically similar transport properties called for a genetic approach to determine the role of CAT2. CAT2-deficient mice were generated and found to be healthy and fertile in contrast to Cat1(-/-) animals. Analysis of cytokine-activated macrophages from Cat2(-/-) mice revealed a 92% reduction in NO production and a 95% reduction in l-Arg uptake. The reduction in NO production was not due to differences in NOS2 protein expression, NOS2 activity, or intracellular l-arginine content. In conclusion, our results show that sustained abundant NO synthesis by macrophages requires arginine transport via the CAT2 transporter.     

DNA immunization confers protection against murine cytomegalovirus infection.

The murine cytomegalovirus (MCMV) immediate-early gene 1 (IE1) encodes an 89-kDa phosphoprotein (pp89) which plays a key role in protecting BALB/c mice against the lethal effects of the MCMV infection. In this report, we have addressed the question of whether "naked DNA" vaccination with a eukaryotic expression vector (pcDNA-89) that contains the MCMV IE1 gene driven by a strong enhancer/promoter can confer protection. BALB/c mice were immunized intradermally with pcDNA-89 or with the plasmid backbone pcDNAI/Amp (pcDNA) and then challenged 2 weeks later with either a lethal or a sublethal intraperitoneal dose of the K181 strain of MCMV. Variable results were obtained for the individual experiments in which mice received a lethal challenge. In four separate trials, an average of 63% of the mice immunized with pcDNA-89 survived, compared with 18% of the mice immunized with pcDNA. However, in two other trials there was no specific protection. The results of experiments in which mice were injected with a sublethal dose of MCMV were more consistent, and significant decreases in viral titer in the spleen and salivary glands of pcDNA-89-immunized mice were observed, relative to controls. At the time of peak viral replication, titers in the spleens of immunized mice were reduced 18- to >63-fold, while those in the salivary gland were reduced approximately 24- to 48-fold. Although DNA immunization elicited only a low level of seroconversion in these mice, by 7 weeks postimmunization the mice had generated a cytotoxic T-lymphocyte response against pp89. These results suggest that DNA vaccination with selected CMV genes may provide a safe and efficient means of immunizing against CMV disease.     

Cmv1-independent antiviral role of NK cells revealed in murine cytomegalovirus-infected New Zealand White mice

Ly49H(+) NK cells play a critical role in innate antiviral immune responses to murine CMV (MCMV). Ly49H(b6) recognition of MCMV-encoded m157 on infected cells activates natural killing required for host resistance. We show that mAb 3D10 (anti-Ly49H) recognizes comparable subsets of NK cells from New Zealand White (NZW), New Zealand Black (NZB), and C57BL/6 spleens. However, virus levels in the spleens of MCMV-infected NZW and NZB mice differed greatly. We found that MCMV replication in infected NZW spleens was limited through NK cells. Alternately, NZB mice were profoundly susceptible to MCMV infection. Although 3D10 mAb injections given before infection interfere with Cmv1-type resistance in C57BL/6 mice, similar mAb injections did not affect NZW resistance, likely because NZW NK cell receptors did not bind MCMV-encoded m157. Instead, anti-MCMV host defenses in hybrid NZ offspring were associated with multiple chromosome locations including several putative quantitative trait loci that did not overlap with H-2 or NK gene complex loci. This study revealed a novel pathway used by NK cells to defend against MCMV infection. Thus, the importance of Ly49H in MCMV infection may be shaped by other additional background genes.     

Absence of involvement of nitric oxide in LP-BM5-induced immunodeficiency syndrome

Abstract To examine the role of nitric oxide (NO) in murine AIDS (MAIDS) pathogenesis, we determined NO production and inducible NOS (iNOS) mRNA expression in the macrophages of LP-BM5-infected mice, together with the in vivo effects of l -NAME, a competitive inhibitor of NO synthase. LP-BM5 infection induced neither spontaneous nitrite production nor iNOS mRNA expression. No differences in IFNγ + LPS-induced nitrite production or iNOS mRNA expression were observed in macrophages from non-infected or infected mice. Spleen weight, ecotropic MuLV replication, the blood lymphocyte phenotype and proliferative response of splenocytes were not modified by l -NAME. LP-BM5 infection did not increase macrophage NO production and NO production did not appear to protect against LP-BM5-induced immunodeficiency.     

Pathogen-specific TLR2 protein activation programs macrophages to induce Wnt-beta-catenin signaling

Innate immunity recognizes and resists various pathogens; however, the mechanisms regulating pathogen versus nonpathogen discrimination are still imprecisely understood. Here, we demonstrate that pathogen-specific activation of TLR2 upon infection with Mycobacterium bovis BCG, in comparison with other pathogenic microbes, including Salmonella typhimurium and Staphylococcus aureus, programs macrophages for robust up-regulation of signaling cohorts of Wnt-β-catenin signaling. Signaling perturbations or genetic approaches suggest that infection-mediated stimulation of Wnt-β-catenin is vital for activation of Notch1 signaling. Interestingly, inducible NOS (iNOS) activity is pivotal for TLR2-mediated activation of Wnt-β-catenin signaling as iNOS(-/-) mice demonstrated compromised ability to trigger activation of Wnt-β-catenin signaling as well as Notch1-mediated cellular responses. Intriguingly, TLR2-driven integration of iNOS/NO, Wnt-β-catenin, and Notch1 signaling contributes to its capacity to regulate the battery of genes associated with T(Reg) cell lineage commitment. These findings reveal a role for differential stimulation of TLR2 in deciding the strength of Wnt-β-catenin signaling, which together with signals from Notch1 contributes toward the modulation of a defined set of effector functions in macrophages and thus establishes a conceptual framework for the development of novel therapeutics.     

Activation of natural killer (NK) T cells during murine cytomegalovirus infection enhances the antiviral response mediated by NK cells

NK1.1+ T (NKT) cells are efficient regulators of early host responses which have been shown to play a role in tumor surveillance. The relevance of NKT cells in immune surveillance of viral infections, however, is not well understood. In this study, we investigated the functional relevance of NKT cells in controlling herpesvirus infections by using challenge with murine cytomegalovirus (MCMV) as the study model. This model has proven to be one of the best systems for evaluating the role of NK cells during virus infection. Using gene-targeted mice and alpha-galactosylceramide (alpha-GalCer) as an exogenous stimulator of NKT cells, we have analyzed the role of these cells in the immune surveillance of MCMV infection. Our studies in NKT-cell-deficient, T-cell receptor Jalpha281 gene-targeted mice have established that classical NKT cells do not play a critical role in the early clearance of MCMV infection. Importantly, however, activation of NKT cells by alpha-GalCer resulted in reduced viral replication in visceral organs. Depletion studies, coupled with analysis of gene-targeted mice lacking perforin and gamma interferon (IFN-gamma), have revealed that the antiviral effects of alpha-GalCer involve NK cells and have clearly demonstrated that the antiviral activity of alpha-GalCer, unlike the antitumor one, is critically dependent on both perforin and IFN-gamma.     

Endothelial nitric oxide synthase is predominantly involved in angiotensin II modulation of renal vascular resistance and norepinephrine release

Nitric oxide (NO) is mainly generated by endothelial NO synthase (eNOS) or neuronal NOS (nNOS). Recent studies indicate that angiotensin II generates NO release, which modulates renal vascular resistance and sympathetic neurotransmission. Experiments in wild-type [eNOS(+/+) and nNOS(+/+)], eNOS-deficient [eNOS(-/-)], and nNOS-deficient [nNOS(-/-)] mice were performed to determine which NOS isoform is involved. Isolated mice kidneys were perfused with Krebs-Henseleit solution. Endogenous norepinephrine release was measured by HPLC. Angiotensin II dose dependently increased renal vascular resistance in all mice species. EC(50) and maximal pressor responses to angiotensin II were greater in eNOS(-/-) than in nNOS(-/-) and smaller in wild-type mice. The nonselective NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 0.3 mM) enhanced angiotensin II-induced pressor responses in nNOS(-/-) and wild-type mice but not in eNOS(-/-) mice. In nNOS(+/+) mice, 7-nitroindazole monosodium salt (7-NINA; 0.3 mM), a selective nNOS inhibitor, enhanced angiotensin II-induced pressor responses slightly. Angiotensin II-enhanced renal nerve stimulation induced norepinephrine release in all species. L-NAME (0.3 mM) reduced angiotensin II-mediated facilitation of norepinephrine release in nNOS(-/-) and wild-type mice but not in eNOS(-/-) mice. 7-NINA failed to modulate norepinephrine release in nNOS(+/+) mice. (4-Chlorophrnylthio)guanosine-3', 5'-cyclic monophosphate (0.1 nM) increased norepinephrine release. mRNA expression of eNOS, nNOS, and inducible NOS did not differ between mice strains. In conclusion, angiotensin II-mediated effects on renal vascular resistance and sympathetic neurotransmission are modulated by NO in mice. These effects are mediated by eNOS and nNOS, but NO derived from eNOS dominates. Only NO derived from eNOS seems to modulate angiotensin II-mediated renal norepinephrine release.     

Role of phosphatidylinositol-3-kinase-gamma (PI3Kgamma)-mediated pathway in 17beta-estradiol-induced killing of L. mexicana in macrophages from C57BL/6 mice

We recently demonstrated that 17beta-estradiol (E2) enhances killing of Leishmania mexicana in macrophages from both male and female DBA/2 mouse by increasing nitric oxide (NO) production. Here, we analyzed the effect of E2 on leishmanicidal activity and cytokine production by bone marrow-derived macrophages (BMDMs) from male and female C57BL/6 mice in vitro, specifically examining the role of phosphatidylinositol-3-kinase-gamma (PI3Kgamma) in E2-induced parasite killing. Unlike its effect on macrophages from both male and female DBA/2 mice, E2 only increased leishmanicidal activity in macrophages from female C57BL/6 mice, which was evident by a significant reduction in both infection rates and infection levels compared to sham controls. E2-treated BMDMs from female C57BL/6 mice expressed higher levels of interferon-gammaRalpha, and also produced more interleukin (IL)-12, IL-6 and NO than both the sham controls and E2-treated male-derived macrophages. Sham-treated BMDMs from female PI3Kgamma-/- C57BL/6 mice displayed lower infection rates and infection levels compared to sham-treated wild-type (WT) macrophages. However E2, unlike its effect on macrophages from female WT C57BL/6 mice, failed to reduce infection rates and infection levels in BMDMs from female PI3Kgamma-/- mice. Interestingly, E2-treated BMDMs from female C57BL/6 mice produced significant amounts of inflammatory cytokines and NO in levels comparable to those observed in sham-treated PI3Kgamma-deficient macrophages as well as E2-treated macrophages from WT mice. These findings show that E2 exerts a distinct effect on leishmanicidal activity of macrophages from male versus female C57BL/6 mice. In addition, they suggest that PI3Kgamma is not required for E2-induced cytokine and NO production in L. mexicana-infected macrophages from female C57BL/6 mice but it may be involved in parasite clearance from these cells.     

Neuronal nitric oxide synthase is necessary for elimination of Giardia lamblia infections in mice

NO produced by inducible NO synthase (NOS2) is important for the control of numerous infections. In vitro, NO inhibits replication and differentiation of the intestinal protozoan parasite Giardia lamblia. However, the role of NO against this parasite has not been tested in vivo. IL-6-deficient mice fail to control Giardia infections, and these mice have reduced levels of NOS2 mRNA in the small intestine after infection compared with wild-type mice. However, NOS2 gene-targeted mice and wild-type mice treated with the NOS2 inhibitor N-iminoethyl-L-lysine eliminated parasites as well as control mice. In contrast, neuronal NOS (NOS1)-deficient mice and wild-type mice treated with the nonspecific NOS inhibitor NG-nitro-L-arginine methyl ester and the NOS1-specific inhibitor 7-nitroindazole all had delayed parasite clearance. Finally, Giardia infection increased gastrointestinal motility in wild-type mice, but not in SCID mice. Furthermore, treatment of wild-type mice with NG-nitro-L-arginine methyl ester or loperamide prevented both the increased motility and the elimination of parasites. Together, these data show that NOS1, but not NOS2, is necessary for clearance of Giardia infection. They also suggest that increased gastrointestinal motility contributes to elimination of the parasite and may also contribute to parasite-induced diarrhea. Importantly, this is the first example of NOS1 being involved in the elimination of an infection.     

Novel role for calcium-independent phospholipase A(2) in the macrophage antiviral response of inducible nitric-oxide synthase expression

The double-stranded (ds) RNA-dependent protein kinase (PKR) is a primary regulator of antiviral responses; however, the ability of dsRNA to activate nuclear factor-kappa B (NF-kappa B) and dsRNA + interferon gamma (IFN-gamma) to stimulate inducible nitric-oxide synthase (iNOS) expression by macrophages isolated from PKR(-/-) mice suggests that signaling pathways in addition to PKR participate in antiviral activities. We have identified a novel phospholipid-signaling cascade that mediates macrophage activation by dsRNA and viral infection. Bromoenol lactone (BEL), a selective inhibitor of the calcium-independent phospholipase A(2) (iPLA(2)), prevents dsRNA- and virus-induced iNOS expression by RAW 264.7 cells and mouse macrophages. BEL does not modulate dsRNA-induced interleukin 1 expression, nor does it affect dsRNA-induced NF-kappa B activation. Protein kinase A (PKA) and the cAMP response element binding protein (CREB) are downstream targets of iPLA(2), because selective PKA inhibition prevents dsRNA-induced iNOS expression, and the inhibitory actions of BEL on dsRNA-induced iNOS expression are overcome by the direct activation of PKA. In addition, BEL inhibits dsRNA-induced CREB phosphorylation and CRE reporter activation. PKR does not participate in iPLA(2) activation or iNOS expression, because dsRNA stimulates iPLA(2) activity and dsRNA + IFN-gamma induces iNOS expression and nitric oxide production to similar levels by macrophages isolated from PKR(+/+) and PKR(-/-) mice. These findings support a PKR-independent signaling role for iPLA(2) in the antiviral response of macrophages.