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

Glycyrrhizin, an active component of liquorice roots, and replication of SARS-associated coronavirus

The outbreak of SARS warrants the search for antiviral compounds to treat the disease. At present, no specific treatment has been identified for SARS-associated coronavirus infection. We assessed the antiviral potential of ribavirin, 6-azauridine, pyrazofurin, mycophenolic acid, and glycyrrhizin against two clinical isolates of coronavirus (FFM-1 and FFM-2) from patients with SARS admitted to the clinical centre of Frankfurt University, Germany. Of all the compounds, glycyrrhizin was the most active in inhibiting replication of the SARS-associated virus. Our findings suggest that glycyrrhizin should be assessed for treatment of SARS.

Evasion of innate and adaptive immunity by flaviviruses

After a virus infects an animal, antiviral responses are generated that attempt to prevent dissemination. Interferons, antibody, complement, T and natural killer cells all contribute to the control and eradication of viral infections. Most flaviviruses, with the exception of some of the encephalitic viruses, cause acute disease and do not establish persistent infection. The outcome of flavivirus infection in an animal is determined by a balance between the speed of viral replication and spread, and the immune system response. Although many of the mechanistic details require further elucidation, flaviviruses have evolved specific tactics to evade the innate and adaptive immune response. A more thorough understanding of these principles could lead to improved models for viral pathogenesis and to strategies for the development of novel antiviral agents.

Nitric oxide and HSV vaginal infection in BALB/c mice

Here we study the role of nitric oxide in the vaginal infection of Balb/c mice with herpes simplex virus type 2. Inducible nitric oxide synthase (iNOS) mRNA was detected by RT-PCR in vaginal tissue and inguinal lymph nodes early postinfection. iNOS was also found to be activated in cells recovered from vaginal washings of infected animals. Animals treated with aminoguanidine (AG), an iNOS inhibitor, showed a dose-dependent increase in vaginal pathology after viral infection compared to controls. Viral titers in vaginal washings and vaginas were higher in AG-treated mice. Treated animals presented higher PMN counts in vaginal washings compared to controls. Histopathology studies revealed a profound inflammatory exudate in vaginal tissue of treated animals. Finally, RT-PCR analysis showed increased expression of the chemokines MIP-2 and RANTES in vaginal tissue and inguinal lymph nodes of these animals.

Inhibition of Japanese encephalitis virus infection by diethyldithiocarbamate is independent of its antioxidant potential

Diethyldithiocarbamate (DDTC), a low molecular weight dithiol, has been described as an immunomodulator and modifier of diverse biological actions in human and animal models, and has also been shown to be effective in several disease conditions. Therefore, we studied the therapeutic aspect of DDTC in providing inhibition of Japanese encephalitis virus (JEV) infection. DDTC tested at various doses (10-100 micromol/kg) revealed that administration at low concentration (10 micromol/kg; i.p.) on alternate days prolonged the average survival time (AST) of mice infected with lethal dose of JEV (102 LD50, i.c.) and delayed progression of the disease. The low dose also provided > 80% survival in sub-clinical (10(5) LD50, i.c.) JEV infection. Administration of DDTC to JEV-infected mice enhanced the inducible nitric oxide synthase (iNOS) activity in brain and level of serum tumour necrosis factor-alpha (TNF-alpha). We have recently demonstrated the production of nitric oxide (NO) via induction of iNOS activity is meditated by circulating macrophage-derived factor (MDF), which may be responsible for the delayed progression of the disease. DDTC-mediated inhibition of JEV is believed to involve the augmentation of protective role of MDF as evidenced by the observation that pretreatment with anti-MDF antibody significantly decreased the AST of mice and together with the inhibition of iNOS activity. Interestingly, DDTC alone did not stimulate iNOS and TNF-alpha in mock-infected normal mice. These results show that DDTC may have a possible therapeutic role during JEV infection.

Role of type I and type II interferon responses in recovery from infection with an encephalitic flavivirus

We have investigated the contribution of the interferon (IFN)-alpha/beta system, IFN-gamma and nitric oxide to recovery from infection with Murray Valley encephalitis virus, using a mouse model for flaviviral encephalitis where a small dose of virus was administered to 6-week-old wild-type and gene knockout animals by the intravenous route. We show that a defect in the IFN-alpha/beta responses results in uncontrolled extraneural virus growth, rapid virus entry into the brain and 100 % mortality. In contrast, mice deficient in IFN-gamma or nitric oxide production display an only marginally increased susceptibility to infection with the neurotropic virus.

Antibody and interleukin-12 treatment in murine models of encephalitogenic flavivirus (St. Louis encephalitis, tick-borne encephalitis) and alphavirus (Venezuelan equine encephalitis) infection

Early and sustained treatment with interleukin-12 (IL-12) ameliorated disease in a mouse model of infection with the encephalitogenic flavivirus, St. Louis encephalitis virus (SLEV, Japanese encephalitis serogroup). However, this effect was not reproduced in murine infections with either the flavivirus tick-bore encephalitis virus (TBEV) or the alphavirus Venezuelan equine encephalitis virus (VEEV). IL-12 exacerbated TBEV disease when used in conjunction with monoclonal antibody (mAb), suggesting an enhancement of immunopathology, and was without clinical effects in VEEV infection. These data confirm the need to fully understand the pathogenesis of viral infection before cytokine intervention may be employed as a broad-spectrum antiviral therapy.

Pathogenesis of flavivirus encephalitis

Within the flavivirus family, viruses that cause natural infections of the central nervous system (CNS) principally include members of the Japanese encephalitis virus (JEV) serogroup and the tick-borne encephalitis virus (TBEV) serocomplex. The pathogenesis of diseases involves complex interactions of viruses, which differ in neurovirulence potential, and a number of host factors, which govern susceptibility to infection and the capacity to mount effective antiviral immune responses both in the periphery and within the CNS. This chapter summarizes progress in the field of flavivirus neuropathogenesis. Mosquito-borne and tickborne viruses are considered together. Flavivirus neuropathogenesis involves both neuroinvasiveness (capacity to enter the CNS) and neurovirulence (replication within the CNS), both of which can be manipulated experimentally. Neuronal injury as a result of bystander effects may be a factor during flavivirus neuropathogenesis given that microglial activation and elaboration of inflammatory mediators, including IL-1β and TNF-α, occur in the CNS during these infections and may accompany the production of nitric oxide and peroxynitrite, which can cause neurotoxicity.

Suppression of Japanese encephalitis virus infection by non-steroidal anti-inflammatory drugs

Japanese encephalitis virus (JEV) infection generates a rapid inflammatory response including peripheral neutrophil leucocytosis and infiltration of neutrophils into extraneural tissue. The level of inflammation correlates well with the clinical outcome in Japanese encephalitis patients. Non-steroidal anti-inflammatory drugs (NSAIDs), used medicinally for their analgesic and anti-inflammatory properties, are being considered for prevention of cardiovascular disease and cancer, as well as for treatment of human immunodeficiency virus infection. Apart from their ability to inhibit prostaglandin synthesis, the mechanisms underlying the beneficial therapeutic effects are largely unknown. We used aspirin, indomethacin and sodium salicylate to study the role of NSAIDs in JEV propagation in vitro. We found that NSAIDs suppressed JEV propagation in neuronal and non-neuronal cells. Blockade of cyclooxygenase activity by NSAIDs caused decreased production of free radicals and prostaglandins. However, these pharmacological alterations did not seem to correlate well with the antiviral effects. When cells were treated with the mitogen-activated protein kinase (MAPK) inhibitors PD 98059 and SB 203580, salicylate lost its antiviral effect. The activation of MAPK by anisomycin mimicked the action of salicylate in suppressing JEV-induced cytotoxicity. The decreased phosphorylation of extracellular signal-regulated kinase (ERK) was induced by JEV infection and the decrease in ERK was reversed by salicylate. Our data suggest that the signalling pathways of MAPK play a role in the antiviral action of salicylate.

Japanese encephalitis virus stimulates superoxide dismutase activity in rat glial cultures

Japanese encephalitis virus (JEV) infection is commonly associated with inflammatory reaction and neurological disease that occurs in the infected animals. Reactive oxygen species have been implicated as a critical mediator for inflammation and diseases. The present study investigated the change of redox potential in glial cells following JEV infection. JEV infection induced the generation of superoxide anion and nitric oxide in rat cortical glial cells. Manganese superoxide dismutase, but not copper/zinc superoxide dismutase was activated by JEV infection, and this activation was blocked by pyrrolidine dithiocarbamate. In addition, the increased superoxide dismutase activity was also apparent in JEV acutely, or persistently infected continuous cell lines. These results suggest that cellular factors regulating oxidative pathway might play roles in responding to JEV infection.

Nitric oxide in liver diseases: friend, foe, or just passerby?

Research on the free radical gas, nitric oxide (NO), during the past twenty years is one of the most rapid growing areas in biology. NO seems to play a part in almost every organ and tissue. However, there is considerable controversy and confusion in understanding its role. The liver is one organ that is clearly influenced by NO. Acute versus chronic exposure to NO has been associated with distinct patterns of liver disease. In this paper we review and discuss the involvement of NO in various liver diseases collated from observations by various researchers. Overall, the important factors in determining the beneficial versus harmful effects of NO are the amount, duration, and site of NO production. A low dose of NO serves to maximize blood perfusion, prevent platelet aggregation and thrombosis, and neutralize toxic oxygen radicals in the liver during acute sepsis and reperfusion events. NO also demonstrates antimicrobial and antiapoptosis properties during acute hepatitis infection and other inflammatory processes. However, in the setting of chronic liver inflammation, when a large sustained amount of NO is present, NO might become genotoxic and lead to the development of liver cancer. Additionally, during prolonged ischemia, high levels of NO may have cytotoxic effects leading to severe liver injury. In view of the various possible roles that NO plays, the pharmacologic modulation of NO synthesis is promising in the future treatment of liver diseases, especially with the emergence of selective NO synthase inhibitors and cell-specific NO donors.

Possible role of LTB4 in the antiviral activity of turbot (Scophthalmus maximus) leukocyte-derived supernatants against viral hemorrhagic septicemia virus (VHSV)

Turbot (Scophthalmus maximus) blood leukocyte-derived supernatants were tested for antiviral activity against viral hemorrhagic septicemia virus (VHSV). The assays were performed by quantifying the effect of the supernatants on the replication of VHSV in the rainbow trout (Oncorhynchus mykiss) cell line, RTG-2. Supernatants were obtained by incubating the leukocytes for 17 h at 18 degrees C in L-15 medium supplemented with 5% fetal calf serum (FCS). Testing of leukocyte supernatants indicated that antiviral activity against VHSV resulted in a viral titer reduction of 72.1%. After the supernatants were extracted with calcium ionophore A23187 treatment, the antiviral activity significantly increased, resulting in a viral titer reduction of 99.9%. In order to determine the nature of this antiviral activity, supernatants were produced from leukocytes treated for 17 h with inhibitors of eicosanoid biosynthesis, reactive oxygen intermediates and nitric oxide (NO) production. None of the inhibitors significantly suppressed the supernatant antiviral activity. The presence of oxygen radicals and NO was measured in the case of co-cultures of leukocytes and RTG-2 cells, but no significant differences were found in the VHSV-infected co-cultures compared to non-infected controls. Since previous work demonstrated that leukotriene B4 (LTB4) was present in turbot blood leukocyte-derived supernatants, we assessed the effect of the VHSV in vitro infection on turbot leukocyte LTB4 production by high performance liquid chromatography (HPLC). The levels of LTB4 were significantly increased in the supernatants after VHSV infection. Furthermore, exogenous LTB4 significantly inhibited VHSV replication in RTG-2 cells. These findings suggest that LTB4 may play a significant role in VHSV replication.

Induction of inducible nitric oxide synthase expression by 18beta-glycyrrhetinic acid in macrophages

Glycyrrhizin (GL), a triterpenoid saponin fraction of licorice, is reported to have anti-viral and anti-tumor activities and is metabolized to 18beta-glycyrrhetinic acid (GA) in the intestine by intestinal bacteria. However, the mechanism underlying its effects is poorly understood. To further elucidate the mechanism of GA, the aglycone of GL, we investigated the effects of GA on the release of nitric oxide (NO) and at the level of inducible NO synthase (iNOS) gene expression in mouse macrophages. We found that GA elicited a dose-dependent increase in NO production and in the level of iNOS mRNA. Since iNOS transcription has been shown to be under the control of the transcription factor nuclear factor kappaB (NF-kappaB), the effects of GA on NF-kappaB activation were examined. Transient expression assays with NF-kappaB binding sites linked to the luciferase gene revealed that the increased level of iNOS mRNA, induced by GA, was mediated by the NF-kappaB transcription factor complex. By using DNA fragments containing the NF-kappaB binding sequence, GA was shown to activate the protein/DNA binding of NF-kappaB to its cognate site, as measured by electrophoretic mobility shift assay. These results demonstrate that GA stimulates NO production and is able to up-regulate iNOS expression through NF-kappaB transactivation in macrophages.

A radical form of nitric oxide suppresses RNA synthesis of rabies virus

Hydrophobia is an incurable disease of the central nervous system. Therefore, every mode of the immune response is important to inhibit and clear infection. Innate immunity such as nitric oxide is significantly upregulated during rabies virus infection in vivo. In this report, the possible role of nitric oxide in inhibition of rabies virus replication was studied. Rabies virus infected neuroblastoma cells were treated with nitric oxide generated from SNP or SNP in the presence of ascorbate. SNP-ascorbate generates mainly NO* in culture medium while NO(+) is the major product of SNP alone. Treatment with SNP-ascorbate resulted in delay and suppression of infectious viral particle production. In contrast, treatment with SNP alone did not interfere with multiplication of this virus. The mechanism of inhibition by NO was at the level of gene expression, which was demonstrated by reduction in the level of N, G and L gene expression. The effect of SNP-ascorbate generated NO on rabies virus protein synthesis was also investigated. Synthesis of N protein in the presence of NO was suppressed which correlated to down regulation of N gene expression. We hypothesize that one of the roles of NO in the central nervous system during rabies virus infection is to limit viral dissemination by down-regulating rabies virus production through transcription inhibition.

Role of reactive oxygen intermediates in Japanese encephalitis virus infection in murine neuroblastoma cells

Infection with Japanese encephalitis virus (JEV), a mosquito-borne, neurotropic flavivirus, may cause acute encephalitis in humans and induce severe cytopathic effects in various types of cultured cells. This study attempted to determine whether JEV infection induces free radical generation and whether oxidative stress contributes to virus-induced cell death in neuroblastoma cells. A rise in the intracellular level of free radicals indicated by the 2',7'-dichlorofluorescein fluorescence was observed in N18 cells following JEV infection. Cellular flavon-containing enzymes were involved in JEV-induced fluorescent change. Cells were moderately protected from JEV-induced death by diphenyleneiodonium, a flavon-containing enzyme inhibitor, whereas common antioxidants such as N-acetylcysteine, pyrrolidine dithiocarbamate, Tiron, and Trolox turned out to be ineffective. These results suggest that the direct antioxidant action is not helpful in prevention of JEV-induced neuronal cell death.

Interferon inhibits dengue virus infection by preventing translation of viral RNA through a PKR-independent mechanism

Previously, we demonstrated that pretreatment of cells with interferon (IFN) alpha + gamma or beta + gamma inhibited dengue virus (DV) replication. In this study, experiments were performed to better define the mechanism by which IFN blocks the accumulation of dengue virus (DV) RNA. Pretreatment of human hepatoma cells with IFN beta + gamma did not significantly alter virus attachment, viral entry, or nucleocapsid penetration into the cytoplasm. The inhibitory effect of IFN was retained even when naked DV RNA was transfected directly into cells, confirming that steps associated with viral entry were not the primary target of IFN action. Biosynthetic labeling experiments revealed that IFN abolished the translation of infectious viral RNA that occurred prior to RNA replication. Subcellular fractionation experiments demonstrated that IFN did not significantly alter the ability of viral RNA to attach to ribosomes. The antiviral effect of IFN appeared independent of the IFN-induced, double-stranded RNA-activated protein kinase (PKR) and RNase L, as genetically deficient PKR- RNase L- cells that were infected by DV retained sensitivity to inhibition by IFN. We conclude that IFN prevents DV infection by inhibiting translation of the infectious viral RNA through a novel, PKR-independent mechanism.

Salicylates inhibit flavivirus replication independently of blocking nuclear factor kappa B activation

Flaviviruses comprise a positive-sense RNA genome that replicates exclusively in the cytoplasm of infected cells. Whether flaviviruses require an activated nuclear factor(s) to complete their life cycle and trigger apoptosis in infected cells remains elusive. Flavivirus infections quickly activate nuclear factor kappa B (NF-kappaB), and salicylates have been shown to inhibit NF-kappaB activation. In this study, we investigated whether salicylates suppress flavivirus replication and virus-induced apoptosis in cultured cells. In a dose-dependent inhibition, we found salicylates within a range of 1 to 5 mM not only restricted flavivirus replication but also abrogated flavivirus-triggered apoptosis. However, flavivirus replication was not affected by a specific NF-kappaB peptide inhibitor, SN50, and a proteosome inhibitor, lactacystin. Flaviviruses also replicated and triggered apoptosis in cells stably expressing IkappaBalpha-DeltaN, a dominant-negative mutant that antagonizes NF-kappaB activation, as readily as in wild-type BHK-21 cells, suggesting that NF-kappaB activation is not essential for either flavivirus replication or flavivirus-induced apoptosis. Salicylates still diminished flavivirus replication and blocked apoptosis in the same IkappaBalpha-DeltaN cells. This inhibition of flaviviruses by salicylates could be partially reversed by a specific p38 mitogen-activated protein (MAP) kinase inhibitor, SB203580. Together, these results show that the mechanism by which salicylates suppress flavivirus infection may involve p38 MAP kinase activity but is independent of blocking the NF-kappaB pathway.

Highly sulfated forms of heparin sulfate are involved in japanese encephalitis virus infection

Japanese encephalitis virus (JEV) infects a broad range of cell types in vitro, though little is known about the initial events of JEV infection. In the present study, we found that highly sulfated glycosaminoglycans (GAGs) are involved in infection of both neurovirulent (RP-9) and attenuated (RP-2ms) JEV strains. Competition experiments using highly sulfated GAGs, heparin and dextran sulfate, demonstrated an inhibition of JEV's attachment and subsequent infection of BHK-21 cells. Treatment of target cells by a potent sulfation inhibitor, sodium chlorate, greatly reduced viral binding ability as well as infection, suggesting a critical role of GAGs' sulfation status on the cellular surface in JEV infection. This phenomenon was confirmed by the manifestation of a distinct binding efficiency of JEV to the wild-type CHO cell line and its mutants with defects in GAG biosynthesis. We also demonstrated the binding of JEV particles and virus envelope glycoprotein to immobilized heparin beads. Furthermore, the addition of heparin suppressed the cytopathic effects induced by JEV infection in cultured cells. Our results establish that the highly sulfated form of GAGs on cell surfaces plays a determining role in the early stage of in vitro JEV infection.

Induction of nitric oxide synthase during Japanese encephalitis virus infection: evidence of protective role

The ability of Japanese encephalitis virus (JEV) and JEV-induced macrophage-derived factor (MDF) to modulate nitric oxide synthase (NOS) activity in brain and tumor necrosis factor-alpha (TNF-alpha) and the possible antiviral role of NOS during JEV infection were investigated. NOS activity and particularly that of the inducible form of NOS (iNOS) was significantly enhanced in JEV or JEV-induced MDF-treated mice. Following JEV infection, total NOS activity in brain was gradually increased from Day 3 and reached a peak on Day 6. MDF-induced NOS activity and iNOS activity were dose dependent and maximum activity was observed at 1 h after treatment. The response was sensitive to anti-MDF antibody treatment and N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of NOS. Pretreatment of JEV-infected mice with L-NMMA increased the mortality as evident from reduced mean survival time (MST, 11.8 days) compared to placebo treated JEV-infected mice (MST, 17 days). The enhanced level of TNF-alpha observed in the early phase of JEV infection correlated well with the enhanced activity of iNOS. These observations thus provide evidence of the protective role of iNOS during JEV infection and indicate that iNOS may be a key mediator in host innate immune response to infection.

Role of nitric oxide synthase type 2 in acute infection with murine cytomegalovirus

Whether or not NO plays a critical role in murine CMV (MCMV) infection has yet to be elucidated. In this study, we examined the role of NO in acute infection with MCMV using NO synthase type 2 (NOS2)-deficient mice. NOS2(-/-) mice were more susceptible to lethal infection with MCMV than NOS2(+/+) mice and generated a much higher peak virus titer in the salivary gland after acute infection. A moderate increase in the MCMV titer was also observed in other organs of NOS2(-/-) mice such as the spleen, lung, and liver. The immune responses to MCMV infection including NK cell cytotoxicity and CTL response in NOS2(-/-) mice were comparable with those of NOS2(+/+) mice. Moreover, the ability to produce IFN-gamma is not impaired in NOS2(-/-) mice after MCMV infection. The peritoneal macrophages from NOS2(-/-) mice, however, exhibited a lower antiviral activity than those from NOS2(+/+) mice, resulting in an enhanced viral replication in macrophages themselves. Treatment of these cells from NOS2(+/+) mice with a selective NOS2 inhibitor decreased the antiviral activity to a level below that obtained with NOS2(-/-) mice. In addition, the absence of NOS2 and NOS2-mediated antiviral activity of macrophages resulted in not only an enhanced MCMV replication and a high mortality but also a consequent risk of the latency. It was thus concluded that the NOS2-mediated antiviral activity of macrophages via NO plays a protective role against MCMV infection at an early and late stage of the infection.

Induction of arginases I and II in cornea during herpes simplex virus infection

Induction of inducible nitric oxide synthase (iNOS) following corneal infection with herpes simplex virus type-1 (HSV-1) generates nitric oxide (NO), an important player in the defense against viral infection. Changes in arginine metabolism during infection are not limited to effects of iNOS but can also involve arginases, which can modulate NO synthesis and produce ornithine for the generation of polyamines and proline. The latter are important molecules involved in tissue damage and repair during inflammation. In this study we determined the responses of arginase I and II in a murine model of HSV-1-induced stromal keratitis (HSK). In the cornea iNOS and arginase II mRNA were co-induced as the initial inflammation developed at 2 days postinfection (p.i.). As stromal keratitis progressed (days 8-15 p.i.) arginase I mRNA was induced tenfold, in contrast to a moderate decrease in arginase II and a loss of iNOS expression. These results suggest that elevated expression of arginase I and II in the cornea at late stages of ocular HSV-1 infection may play a role in lesion expression in HSK.

Innate resistance to flavivirus infection in mice controlled by Flv is nitric oxide-independent

Innate resistance to flaviviruses in mice is active in the brain where it restricts virus replication. This resistance is controlled by a single genetic locus, FLV, located on mouse chromosome 5 near the locus encoding the neuronal form of nitric oxide synthase (Nos1). Since nitric oxide (NO) has been implicated in antiviral activity, its involvement in natural resistance to flaviviruses has been hypothesized. Here we present data on NO production before and during flavivirus infection in both brain tissue and peritoneal macrophages from two flavivirus-resistant (FLV(r)) and one congenic susceptible (FLV(s)) mouse strains. This study provides evidence that NO is not involved in the expression of flavivirus resistance controlled by FLV since: (a) there is no difference in brain tissue NO levels between susceptible and resistant mice, and (b) lipopolysaccharide-induced NO does not abrogate the difference in flavivirus replication in peritoneal macrophages from susceptible and resistant mice.

Preconditioning regulation of bcl-2 and p66shc by human NOS1 enhances tolerance to oxidative stress

Preconditioning stress induced by a transient ischemia may increase brain tolerance to oxidative stress, and the underlying neuroprotective mechanisms are not well understood. In a series of experiments, we found that endogenous nitric oxide (NO), S-nitrosoglutathione (GSNO), and antioxidants blocked serum deprivation-induced oxidative stress and apoptosis in human neuroblastoma cells. Similar to nuclear redox factor-1 (Ref-1), mRNA of human neuronal nitric oxide synthase (hNOS1) was maximally up-regulated within 2 h after oxidative stress and down-regulated by NO/GSNO and hydroxyl radical (OH) scavenger. A brief preconditioning stress induced by serum deprivation for 2 h caused a delayed increase in the expression of hNOS1 protein and the associated formation of NO and cGMP, which in turn decreased OH generation and stress-related cell death. In addition to inhibiting caspase-3 through a dithiothreitol-sensitive S-nitrosylation process, preconditioning stress concomitantly up-regulated the expression of the anti-apoptotic bcl-2 protein and down-regulated the p66shc adaptor protein. This beneficial cytoprotective process of preconditioning stress is mediated by newly synthesized NO because it can be suppressed by the inhibition of hNOS1 and guanylyl cyclase. Therefore, the constitutive isoform of hNOS1 is dynamically redox-regulated to meet both functional and compensatory demands of NO for gene regulation, antioxidant defense, and tolerance to oxidative stress.

Hematopoietic cells from CD155-transgenic mice express CD155 and support poliovirus replication ex vivo

Despite identification of the poliovirus (PV) receptor (CD155), mechanisms by which this molecule mediates paralytic disease remain obscure. Unanswered questions include CD155 localization in human tissues, the nature of cells supporting the first round of replication, identity of nonneural replication sites, and route of entry into the CNS. In earlier work, we showed that CD155 is expressed on primary human monocytes and that these cells support low, but statistically significant, levels of PV replication ex vivo without prior culturing. We hypothesize that monocytes support PV replication in vivo and that they contribute to pathogenesis. In the current study, we tested whether CD155-transgenic mouse hematopoietic cells express cell surface CD155 and whether these cells support PV replication. We found that the majority of monocyte/macrophages from peritoneal washes express CD155. In addition, 26-32% of CD155-transgenic bone marrow and spleen cells express CD155 on monocyte/macrophages, T cells and hematopoietic precursor cells. Various tissues supported PV replication without pre-culturing, however, pre-culturing or pre-treatment of mice with thioglycollate increased virus yield. These results are consistent with those from human cells and suggest that the CD155 transgenic mouse model is useful to help understand the role of hematopoietic cells in PV pathogenesis.

Catecholamines inhibit microglial nitric oxide production

Viral infection in the central nervous system can induce nitric oxide production, which serves as a major host defense against viral infection. Under stress, catecholamine secretion is enhanced and immune responses are diminished in animals. Using N9 microglial cells, this study tested the effect of catecholamines on microglial nitric oxide production. Results indicated that each member of the catecholamine family (dopamine, norepinephrine and epinephrine) was a potent inhibitor of the microglial nitric oxide production. In contrast, dopa, the immediate precursor of the catecholamine biosynthesis pathway, was a weak inhibitor, except at very high concentrations. The inhibitory effect of catecholamines was mimicked by an alpha-adrenergic receptor agonist (phenylephrine) and by a beta-adrenergic receptor agonist (isoproterenol), but not by forskolin or analogs of cyclic adenosine monophosphate. Western blot analysis indicated that catecholamines caused a slight decrease in the formation of inducible nitric oxide synthase. These results suggest that catecholamines have the ability to block nitric oxide production by microglia, which could partially explain the impaired immune protection against viral infection in the central nervous system in stressed animals.

Bing de ling, a Chinese herbal formula, stimulates multifaceted immunologic responses in mice

Bing de ling is a Chinese herbal formula most commonly used in complementary medical settings against viral disorders. We have found that bing de ling potentiates upregulation of immune activity when administered to mice in dosages proportional to those used clinically. These mice demonstrated significant elevation of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) production in splenocytes and enhancement of macrophage, natural killer cell, and lymphokine-activated killer cell cytotoxicity. These data are consistent with bing de ling's clinically observed efficacy against viruses and identify the formula as a promising candidate for clinical trials against diverse diseases that may respond to increased immunologic activity.

Inhibition of microglial nitric oxide production by hydrocortisone and glucocorticoid precursors

Viral and bacterial infection in the central nervous system can induce nitric oxide production, which serves as a major host defense against invading microorganisms. Glucocorticoids secretion is enhanced and immune responses are diminished in stressed animals or in patients suffering depression. Using N9 microglial cells, this study tested the hypothesis that glucocorticoids and their precursors caused an impaired immune defense in animals because these compounds could inhibit microglial nitric oxide production. Results indicated that both hydrocortisone and the synthetic glucocorticoid, dexamethasone, were potent inhibitors of the microglial nitric oxide production. While glucocorticoid precursors were not as potent as hydrocortisone, the potency of these precursors increased linearly as they advanced on the biosynthesis pathway. Northern and Western blot analyses indicated that hydrocortisone and dexamethasone might interfere with the inducible nitric oxide synthase at either the transcription or at the post-translational level, depending on the concentrations used. These results suggest that glucocorticoids have the ability to block nitric oxide production by microgila, which could partially explain the impaired immune protection against infection in the central nervous system in stressed animals.

Modulation of Dengue virus infection in human cells by alpha, beta, and gamma interferons

A role for interferon (IFN) in modulating infection by dengue virus (DV) has been suggested by studies in DV-infected patients and IFN receptor-deficient mice. To address how IFN modulates DV type 2 infection, we have assayed IFN-alpha, -beta, and -gamma for the ability to enhance or diminish antibody-independent and antibody-dependent cell infection using a competitive, asymmetric reverse transcriptase-mediated PCR (RT-PCR) assay that quantitates positive and negative strands of viral RNA, a flow cytometric assay that measures viral antigen, and a plaque assay that analyzes virion production. Our data suggest that IFN-alpha and -beta protect cells against DV infection in vitro. Treatment of hepatoma cells with IFN-alpha or -beta decreases viral RNA levels greater than 1, 000-fold, the percentage of cells infected 90 to 95%, and the amount of infectious virus secreted 150- to 100,000-fold. These results have been reproduced with several cell types and viral strains, including low-passage isolates. In contrast, IFN-gamma has a more variable effect depending on the cell type and pathway of infection. Quantitative RT-PCR experiments indicate that IFN inhibits DV infection by preventing the accumulation of negative-strand viral RNA.

Antiviral effect of nitric oxide during Japanese encephalitis virus infection

The ability of Japanese encephalitis virus (JEV) and JEV-induced macrophage derived neutrophil chemotactic factor (MDF) to produce nitric oxide (NO), and the possible antiviral effect of NO during JEV infection, was investigated. Splenic macrophages of JEV infected mice produced maximum NO in vivo at day 7 post infection, and in vitro at 24 h after JEV stimulation. MDF-induced NO production was dose dependent and maximal at 60 min after MDF treatment. The response was sensitive to anti-MDF antibody treatment and the nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA). Pretreatment of mice with L-NMMA increased the mortality to 100% in JEV infected mice in vivo and inhibited NO production in vitro, while MDF stimulated macrophages inhibited virus replication with high levels of NO production. MDF treatment increased the survival rate of JEV infected mice. The findings thus demonstrate that MDF induces production of NO during JEV infection, which has an antiviral effect. This may be one of the important mechanisms of natural immunity in controlling the initial stages of JEV infection.

Neuroprotective strategies in Parkinson's disease: protection against progressive nigral damage induced by free radicals

Brain undergoes neurodegeneration when excess free radicals overwhelm antioxidative defense systems during senescence, head trauma and/or neurotoxic insults. A site-specific accumulation of ferrous citrate-iron complexes in the substantia nigra dopaminergic neurons could lead to exaggerated dopamine turnover, dopamine auto-oxidation, free radical generation, and oxidant stress. Eventually, this iron-catalyzed dopamine auto-oxidation results in the accumulation of neuromelanin, a progressive loss of nigral neurons, and the development of Parkinson's disease when brain dopamine depletion is greater than 80%. Emerging evidence indicates that free radicals such as hydroxyl radicals ((.-)OH) and nitric oxide ((.-)NO) may play opposite role in cell and animal models of parkinsonism. (.-)OH is a cytotoxic oxidant whereas oNO is an atypical neuroprotective antioxidant. (.-)NO and S-nitrosoglutathione (GSNO) protect nigral neurons against oxidative stress caused by 1-methyl-4-phenylpyridinium (MPP(+)), dopamine, ferrous citrate, hemoglobin, sodium nitroprusside and peroxynitrite. MPP(+), the toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), increases the nigral uptake of iron complexes and dopamine overflow leading to the generation of (.-)OH, protein oxidation, lipid peroxidation, and associated retrograde degeneration. In addition to GSNO, MPP(+)-induced oxidative neurotoxicity can be prevented by antioxidants including selegiline, 7-nitroindazole, 17beta-estradiol, melatonin, alpha-phenyl-tert-butylnitrone and U78517F. Similar to selegiline, 7-nitroindazole is a MAO-B inhibitor, which blocks the bio-activation of MPTP and oxidative stress. Freshly prepared but not light exposed, (.-)NO-exhausted GSNO is about 100 times more potent than the classic antioxidant glutathione. Via S-nitrosylation, GSNO also inhibits proteolysis and cytotoxicity caused by caspases and HIV-1 protease. Furthermore, in addition to protection against serum deprivation stress, the induction of neuronal NOS1 in human cells increases tolerance to MPP(+)-induced neuro-toxicity since newly synthesized (.-)NO prevents apoptosis possibly through up-regulation of bcl-2 and down regulation of p66(shc). In conclusion, reactive oxygen species are unavoidable by-products of iron-catalyzed dopamine auto-oxidation, which can initiate lipid peroxidation, protein oxidation, DNA damage, and nigral loss, all of which can be prevented by endogenous and exogenous (.-)NO. Natural and man-made antioxidants can be employed as part of preventative or neuroprotective treatments in Parkinson's disease and perhaps dementia complexes as well. For achieving neuroprotection and neuro-rescue in early clinical parkinsonian stages, a cocktail therapy of multiple neuroprotective agents may be more effective than the current treatment with extremely high doses of a single antioxidative agent.

Role of nitric oxide on the replication of viral haemorrhagic septicemia virus (VHSV), a fish rhabdovirus

In the present work, we have studied the role of nitric oxide (NO) on the replication of viral haemorrhagic septicemia virus (VHSV), a virus which produces high mortalities in fish aquaculture worldwide and that is known to replicate in turbot (Scophthalmus maximus) head kidney macrophages. Viral infection of turbot kidney macrophages in vitro induced an up-regulation of NO production and we have tested whether this endogenous NO production induced by VHSV on macrophages had an antiviral effect using the NO synthase inhibitor, N-omega-nitro-L-arginine (L-NAME). When L-NAME was added to the VHSV-infected cultures, no increase on VHSV titer was observed, even though the inhibitor was capable of decreasing NO production. When exogenous NO was apported by the nitric oxide donor, glycerin trinitrate (GTN) an antiviral effect on VHSV was observed. The NO donor significantly inhibited VHSV replication on a turbot fibroblast cell line (TV-1) and on turbot kidney macrophages.

Nitric oxide and macrophage antiviral extrinsic activity

In this study we evaluated the relationship between nitric oxide (NO) and macrophage antiviral extrinsic activity. Macrophages activated by intraperitoneal injection of herpes simplex virus-2 (HSV-2), showed both extrinsic antiviral activity and high nitrite production in contrast to non-activated, resident macrophages. The extrinsic antiviral activity was observed in cultures of Vero cells infected with HSV-1 and HSV-2. The NO inhibitor N-monomethyl-l-arginine acetate (l-NMA) impaired the antiviral activity of HSV-elicited macrophages. The effect was dose dependent and correlated with a reduction of nitrite in the culture media. The effect of l-NMA was reversed by the addition of l-arginine. These data indicate that NO could be responsible for the described activity. Furthermore, l-NMA treatment resulted in the aggravation of HSV-1-induced keratitis in the mouse model, supporting a defensive role of NO in the pathogenesis of HSV-1 corneal infection.

Inhibition of influenza virus replication by nitric oxide

Nitric oxide (NO) has been shown to contribute to the pathogenesis of influenza virus-induced pneumonia in mouse models. Here we show that replication of influenza A and B viruses in Mabin Darby canine kidney cells is severely impaired by the NO donor, S-nitroso-N-acetylpenicillamine. Reduction of productively infected cells and virus production proved to correlate with inhibition of viral RNA synthesis, indicating that NO affects an early step in the replication cycle of influenza viruses.

Nitric oxide modulates HIV-1 replication

Although nitric oxide (NO) production is increased in HIV-1-infected patients, and NO is known to inhibit the replication of several viruses, very little is known about the effects of NO on HIV-1 replication. In the present studies, we find that S-nitrosothiols (RSNOs), a class of NO donor compounds present in the human circulatory system, inhibit HIV-1 replication in acutely infected human peripheral blood mononuclear cells (PBMCs) and have an additive inhibitory effect on HIV-1 replication in combination with 3'-azido-3'-deoxythymidylate (AZT). RSNOs inhibit HIV-1 replication in acutely infected PBMCs at a step in the viral replicative cycle after reverse transcription, but before or during viral protein expression through a cGMP-independent mechanism. In the latently infected U1 cell line, NO donor compounds and intracellular NO production stimulate HIV-1 reactivation. These studies suggest that NO both inhibits HIV-1 replication in acutely infected cells and stimulates HIV-1 reactivation in chronically infected cells. Thus, NO may have a physiologic role in HIV-1 replication, and NO donor compounds, which have been used for decades in the treatment of coronary artery disease with limited toxicity, might be useful in the treatment of HIV-1 disease by inhibiting acute infection, reactivating latent virus, or both.

Membrane permeabilization by small hydrophobic nonstructural proteins of Japanese encephalitis virus

Infection with Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, may cause acute encephalitis in humans and induce severe cytopathic effects in various types of cultured cells. We observed that JEV replication rendered infected baby hamster kidney (BHK-21) cells sensitive to the translational inhibitor hygromycin B or alpha-sarcine, to which mock-infected cells were insensitive. However, little is known about whether any JEV nonstructural (NS) proteins contribute to virus-induced changes in membrane permeability. Using an inducible Escherichia coli system, we investigated which parts of JEV NS1 to NS4 are capable of modifying membrane penetrability. We found that overexpression of NS2B-NS3, the JEV protease, permeabilized bacterial cells to hygromycin B whereas NS1 expression failed to do so. When expressed separately, NS2B alone, but not NS3, was sufficient to alter bacterial membrane permeability. Similarly, expression of NS4A or NS4B also rendered bacteria susceptible to hygromycin B inhibition. Examination of the effect of NS1 to NS4 expression on bacterial growth rate showed that NS2B exhibited the greatest inhibitory capability, followed by a modest repression from NS2A and NS4A, whereas NS1, NS3, and NS4B had only trivial influence with respect to the vector control. Furthermore, when cotransfected with a reporter gene luciferase or beta-galactosidase, transient expression of NS2A, NS2B, and NS4B markedly reduced the reporter activity in BHK-21 cells. Together, our results suggest that upon JEV infection, these four small hydrophobic NS proteins have various modification effects on host cell membrane permeability, thereby contributing in part to virus-induced cytopathic effects in infected cells.

Mechanisms of cytokine-mediated inhibition of viral replication

In this report, the role of nitric oxide synthase (NOS) and IL-12 administration in inhibition of vesicular stomatitis virus (VSV) from infected neuroblastoma cells was examined. We previously have shown that cytokine treatment of cells results in the induction of NOS-1, and this is associated with a 2 log inhibition of VSV production. We performed these studies to examine the mechanism by which viral replication is suppressed. Neuroblastoma cells (NB41A3) were treated with either IL-12 or medium and subsequently infected with VSV. Viral protein and mRNA were isolated from these cells, and their levels were measured by Western or Northern blots, respectively. mRNA levels were decreased modestly, but viral proteins were decreased substantially in cells pretreated with IL-12, suggesting that the inhibitory effect of NO is working at the translational level. Cytokine treatment of cells was not associated with oxidative stress. The viral proteins also were nitrosylated. These data suggest that the mechanism of NO inhibition of viral replication occurs through translational interference and posttranslational modifications of viral components.

Nitric oxide inhibits HIV tat-induced NF-kappaB activation

To evaluate the roles of nitric oxide (NO) on human immunodeficiency virus (HIV) Tat-induced transactivation of HIV long terminal repeat (HIV-LTR), we examined the effect of NO in the regulation of nuclear factor (NF)-kappaB, a key transcription factor involved in HIV gene expression and viral replication. In the present study, we demonstrate that HIV Tat activates NF-kappaB and that this activation can be attenuated by endogenous or exogenous NO. Inhibition of endogenous NO production with the NO synthase (NOS) inhibitor L-NMMA causes a significant increase in Tat-induced NF-kappaB activity. In addition, NO attenuates signal-initiated degradation of IkappaBalpha, an intracellular inhibitor of NF-kappaB, and blocks the DNA binding activity of the NF-kappaB p50/p50 homodimer and p50/p65 heterodimer. To determine how NO is induced by HIV Tat, reverse transcription polymerase chain reaction was used to demonstrate the induction of NOS-2 and NOS-3 mRNA by Tat. Although a putative NF-kappaB binding site was identified in the -74 GGAGAGCCCCC -64 region of the NOS-3 gene promoter, gel mobility shift assays and site-directed mutation analyses suggest that the putative NF-kappaB site is not of primary importance. Rather, several Sp-1 sites adjoining the putative NF-kappaB binding site in the promoter region of NOS-3 gene are required for the induction of NOS-3 gene expression by Tat.

The capsule supports survival but not traversal of Escherichia coli K1 across the blood-brain barrier

The vast majority of cases of gram-negative meningitis in neonates are caused by K1-encapsulated Escherichia coli. The role of the K1 capsule in the pathogenesis of E. coli meningitis was examined with an in vivo model of experimental hematogenous E. coli K1 meningitis and an in vitro model of the blood-brain barrier. Bacteremia was induced in neonatal rats with the E. coli K1 strain C5 (O18:K1) or its K1(-) derivative, C5ME. Subsequently, blood and cerebrospinal fluid (CSF) were obtained for culture. Viable bacteria were recovered from the CSF of animals infected with E. coli K1 strains only; none of the animals infected with K1(-) strains had positive CSF cultures. However, despite the fact that their cultures were sterile, the presence of O18 E. coli was demonstrated immunocytochemically in the brains of animals infected with K1(-) strains and was seen by staining of CSF samples. In vitro, brain microvascular endothelial cells (BMEC) were incubated with K1(+) and K1(-) E. coli strains. The recovery of viable intracellular organisms of the K1(+) strain was significantly higher than that for the K1(-) strain (P = 0.0005). The recovery of viable intracellular K1(-) E. coli bacteria was increased by cycloheximide treatment of BMEC (P = 0.0059) but was not affected by nitric oxide synthase inhibitors or oxygen radical scavengers. We conclude that the K1 capsule is not necessary for the invasion of bacteria into brain endothelial cells but is responsible for helping to maintain bacterial viability during invasion of the blood-brain barrier.

Neuronal expression of NOS-1 is required for host recovery from viral encephalitis

The role of nitric oxide synthase (NOS) in host defense and clearance of vesicular stomatitis virus (VSV) from the central nervous system (CNS) was examined. NOS-1, NOS-2, and NOS-3 knockout mice were infected with VSV and were treated with either IL-12 or medium. IL-12 treatment resulted in substantially decreased VSV titers in wildtype and NOS-3 knockout mice, but had a marginal effect in the NOS-1 and NOS-2 knockout mice. NOS-1 expression in neurons was associated with survival from VSV infection. The data indicate that the enzyme activity is local, since NOS-2 expression in microglia and inflammatory macrophages and NOS-3 expression in astrocytes, endothelial cells, and ependymal cells did not compensate.

Human CD46 enhances nitric oxide production in mouse macrophages in response to measles virus infection in the presence of gamma interferon: dependence on the CD46 cytoplasmic domains

CD46 is a transmembrane complement regulatory protein widely expressed on nucleated human cells. Laboratory-adapted strains of measles virus (MV) bind to the extracellular domains of CD46 to enter human cells. The cytoplasmic portion of CD46 consists of a common juxtamembrane region and different distal sequences called Cyt1 and Cyt2. The biological functions of these cytoplasmic sequences are unknown. In this study, we show that expression of human CD46 with the Cyt1 cytoplasmic domain in mouse macrophages enhances production of nitric oxide (NO) in response to MV infection in the presence of gamma interferon (IFN-gamma). Human CD46 does not increase the basal levels of NO production in mouse macrophages and does not augment NO production induced by double-stranded polyribonucleotides. Replacing the cytoplasmic domain of human CD46 with Cyt2 reduces MV and IFN-gamma-induced NO production in mouse macrophages. Deleting the entire cytoplasmic domains of human CD46 does not prevent MV infection but markedly attenuates NO production in response to MV and IFN-gamma. Mouse macrophages expressing a tailless human CD46 mutant are more susceptible to MV infection and produce 2 to 3 orders of magnitude more infectious virus than mouse macrophages expressing human CD46 with intact cytoplasmic domains. These results reveal a novel function of CD46 dependent on the cytoplasmic domains (especially Cyt1), which augments NO production in macrophages. These findings may have significant implications for roles of CD46 in innate immunity and MV pathogenesis.

Infection of primary cultures of human Kupffer cells by Dengue virus: no viral progeny synthesis, but cytokine production is evident

We investigated the ability of dengue virus to invade human primary Kupffer cells and to complete its life cycle. The virus effectively penetrated Kupffer cells, but the infection did not result in any viral progeny. Dengue virus-replicating Kupffer cells underwent apoptosis and were cleared by phagocytosis. Infected Kupffer cells produced soluble mediators that could intervene in dengue virus pathogenesis.

In vitro effects of recombinant chicken interferon-gamma on immune cells

We used the recombinant chicken interferon-gamma (ChIFN-gamma) to determine its in vitro effects on chicken immune cells. We found that ChIFN-gamma induced nitric oxide (NO) production, upregulated Ia expression on the cell surface, and inhibited the replication of Newcastle disease virus in NCSU and HD11 cells (chicken macrophage cell lines). In addition, ChIFN-gamma had an antiproliferative effect on RP9 cells, a chicken B cell line. Finally, ChIFN-gamma inhibited mitogenic proliferation of normal chicken spleen cells and induced the cells to generate NO. Inhibition of viral replication and mitogenic proliferation of normal cells were correlated with NO production. We conclude that recombinant chicken ChIFN-gamma modulates chicken immune cells.

Human Immunodeficiency Virus-1–Infected Macrophages Induce Inducible Nitric Oxide Synthase and Nitric Oxide (NO) Production in Astrocytes: Astrocytic NO as a Possible Mediator of Neural Damage in Acquired Immunodeficiency Syndrome

Nitric oxide (NO) plays an important role in normal neural cell function. Dysregulated or overexpression of NO contributes to neurologic damage associated with various pathologies, including human immunodeficiency virus (HIV)-associated neurological disease. Previous studies suggest that HIV-infected monocyte-derived macrophages (MDM) produce low levels of NO in vitro and that inducible nitric oxide synthase (iNOS) is expressed in the brain of patients with neurologic disease. However, the levels of NO could not account for the degree of neural toxicity observed. In this study, we found that induction of iNOS with concomitant production of NO occurred in primary human astrocytes, but not in MDM, when astrocytes were cocultured with HIV-1–infected MDM. This coincided with decreased HIV replication in infected MDM. Supernatants from cocultures of infected MDM and astrocytes also stimulated iNOS/NO expression in astrocytes, but cytokines known to induce iNOS expression (interferon-γ, interleukin-1β, and tumor necrosis factor-) were not detected. In addition, the recombinant HIV-1 envelope protein gp41, but not rgp120, induced iNOS in cocultures of uninfected MDM and astrocytes. This suggests that astrocytes may be an important source of NO production due to dysregulated iNOS expression and may constitute one arm of the host response resulting in suppression of HIV-1 replication in the brain. It also leads us to speculate that neurologic damage observed in HIV disease may ensue from prolonged, high level production of NO.

Role of IFN-γ-Induced Indoleamine 2,3 Dioxygenase and Inducible Nitric Oxide Synthase in the Replication of Human Cytomegalovirus in Retinal Pigment Epithelial Cells

An in vitro model of human CMV infection of primary retinal pigment epithelial (RPE) cells was used to study the effects of cytokines on CMV replication in these cells, which are targets of CMV infection in vivo. IFN-γ and IFN-β were potent inhibitors of CMV replication in RPE cells, while TNF-α, IL-1β, or TGF-β2 did not affect viral replication. Inhibition by IFN-γ, and to a lesser extent IFN-β, was almost completely reversed by addition of l-tryptophan to the culture medium, strongly implicating the indoleamine 2,3 dioxygenase (IDO) pathway. Polyadenylated IDO mRNA accumulation was detected as early as 2 h after IFN stimulation. Furthermore, CMV blocked the production of nitric oxide by the inducible form of nitric oxide synthase. This inhibition depended on a functional viral genome. However, exogenous nitric oxide significantly inhibited viral protein expression in RPE cells. Thus, CMV infection blocks the inducible nitric oxide synthase pathway activated by IFN-γ and IL-1β, but cannot counteract the IFN-induced IDO pathway, which ultimately controls its replication in primary human RPE cells.

Nitric oxide inhibits HIV-1 replication in human astrocytoma cells

Astroglial cells represent a target for HIV infection in the central nervous system. In astrocytes, HIV infection is poorly productive, being characterized by a persistent state of viral latency. However, activation of the nuclear factor NF-kappaB and its binding to HIV long terminal repeat (LTR) can induce HIV replication. Moreover, nitric oxide (NO) can affect NF-kappaB activation in glial cells. Therefore, we hypothesize that NO may reduce HIV replication in human astroglial cells by inhibiting HIV-1 LTR transcriptional activity. In this respect, we show that NO donors reduce viral replication in HIV-1-infected human astrocytoma T67 cells, taken as an astroglial model. Furthermore, using transfected T67 cells, we demonstrate that NO donors inhibit HIV-1 LTR transcriptional activity. These results suggest that the use of NO-releasing drugs may represent a potential, novel approach in inhibiting HIV replication in the central nervous system.

Mosquito feeding modulates Th1 and Th2 cytokines in flavivirus susceptible mice: an effect mimicked by injection of sialokinins, but not demonstrated in flavivirus resistant mice

Culex pipiens and Aedes aegypti mosquitoes were fed on C3H/HeJ mice and systemic cytokine production was quantified from stimulated lymphocytes harvested four to ten days after feeding. Mosquito feeding on C3H/HeJ mice significantly down regulated IFN gamma production seven to ten days post feeding by Cx. pipiens and seven days after Ae aegypti feeding. Th2 cytokines, IL-4 and IL-10, were significantly up regulated 4-7 days after Cx. pipiens and Ae. aegypti feeding. The immunosuppressive effect of Cx. pipiens feeding on systemic cytokine production was not evident in congenic flavivirus resistant (C3H/RV) mice, as systemic IFN gamma and IL-2 were significantly up regulated at days 7 and 10, correlating with a significant decrease in IL-4 10 days after feeding by Cx. pipiens mosquitoes. Inoculation of 5-1000 ng of sialokinin-I into C3H/HeJ mice mimicked the effect of Ae. aegypti feeding by down regulating Th1 cytokines and significantly up regulating Th2 cytokines four days post inoculation. Injections of sialokinin-II resulted in only moderate effects on IFN gamma and IL-4 production seven and ten days after injection. Thus natural feeding by two arbovirus vectors had a profound T cell modulatory effect in vivo in virus susceptible animals which was not demonstrated in the flavivirus resistant host. Moreover, sialokinin-I and sialokinin-II mimicked the effect of mosquito feeding by modulating the host T cell response. These results may lend new insight into specific aspects of the role of the mosquito vector in potentiating virus transmission in the mammalian host.

Antiapoptotic but not antiviral function of human bcl-2 assists establishment of Japanese encephalitis virus persistence in cultured cells

Upon infection of Japanese encephalitis virus (JEV), baby hamster kidney (BHK-21) and Chinese hamster ovary (CHO) cells were killed by a mechanism involved in apoptosis. While readily established in a variety of cell lines, JEV persistence has never been successfully instituted in BHK-21 and CHO cells. Since stable expression of human bcl-2 in BHK-21 cells has been shown to delay JEV-induced apoptosis, in this study we investigated whether JEV persistence could be established in such cells. When constitutively expressing bcl-2, but not its closest homolog, bcl-XL, following a primary lytic infection, approximately 5 to 10% of BHK-21 and CHO cells became persistently JEV infected during a long-term culture. From the persistent bulks, several independent clones were selected and expanded to form stable cell lines that continuously produced infectious virus without marked cytopathic effects (CPE). Among these stable cell lines, the truncated nonstructural protein 1 (NS1) was also detected and was indistinguishable from the NS1 truncations previously observed in JEV-persistent murine neuroblastoma N18 cells. However, the stable expression of NS1 alone, regardless of whether it was truncated or full length, failed to render the engineered cells persistently infected by JEV, implying that aberrant NS1 proteins were likely a consequence of, rather than a cause for, the viral persistence. Enforced bcl-2 expression, which did not affect virus replication and spread during the early phase of cytolytic infection, appeared to attain JEV persistence by restriction of virus-induced CPE. Our results suggest that it is the antiapoptotic, rather than the antiviral, effect of cellular bcl-2 which plays a role in the establishment of JEV persistence.

Study of Dengue virus infection in SCID mice engrafted with human K562 cells

Here we report that severe combined immunodeficient (SCID) mice engrafted with human K562 cells (K562-SCID mice) can be used as an animal model to study dengue virus (DEN) infection. After intratumor injection into K562 cell masses of PL046, a Taiwanese DEN-2 human isolate, the K562-SCID mice showed neurological signs of paralysis and died at approximately 2 weeks postinfection. In addition to being detected in the tumor masses, high virus titers were detected in the peripheral blood and the brain tissues, indicating that DEN had replicated in the infected K562-SCID mice. In contrast, the SCID mice were resistant to DEN infection and the mock-infected K562-SCID mice survived for over 3 months. These data illustrate that DEN infection contributed directly to the deaths of the infected K562-SCID mice. Other serotypes of DEN were also used to infect the K562-SCID mice, and the mortality rates of the infected mice varied with different challenge strains, suggesting the existence of diverse degrees of virulence among DENs. To determine whether a neutralizing antibody against DEN in vitro was also protective in vivo, the K562-SCID mice were challenged with DEN-2 and received antibody administration at the same time or 1 day earlier. Our results revealed that the antibody-treated mice exhibited a reduction in mortality and a delay of paralysis onset after DEN infection. In contrast to K562-SCID, the persistently DEN-infected K562 cells generated in vitro invariably failed to be implanted in the mice. It seems that in the early stage of implantation, a gamma interferon activated, nitric oxide-mediated anti-DEN effect might play a role in the innate immunity against DEN-infected cells. The system described herein offers an opportunity to explore DEN replication in vivo and to test various antiviral protocols in infected hosts.

Effect of nitric oxide on poliovirus infection of two human cell lines

The role of nitric oxide after poliovirus infection of the human HeLa (carcinoma) and U937 (promonocytic) cell lines has been analyzed. Both types of cells produced detectable levels of nitric oxide after poliovirus infection. However, this production was not sufficient to limit viral productivity. On the other hand, pretreatment with the nitric oxide donor glycerine trinitrate lengthened the course of poliovirus infection.