Inducible nitric-oxide synthase plays a minimal role in lymphocytic choriomeningitis virus-induced, T cell-mediated protective immunity and immunopathology

Polarization of avian macrophages upon avian flavivirus infection

Avian Tembusu virus (TMUV) is a newly emerging avian pathogenic flavivirus that spreads rapidly, has an expanding host range and undergoes cross-species transmission. Our previous study identified avian monocytes/macrophages as the key targets of TMUV infection, since the infection of host monocytes/macrophages was crucial for the replication, transmission, and pathogenesis of TMUV. The polarization of host macrophages determines the functional phenotypes of macrophages; however, the effect of TMUV infection on macrophage polarization remains unclear. Here, we analysed the expression spectra of the marker genes of macrophage polarization upon TMUV infection in the HD11 chicken macrophage cell line and primary monocytes/macrophages isolated from the peripheral blood of specific pathogen-free (SPF) chickens and ducks. We found that viral replication mainly induced M1 marker genes and triggered nitric oxide (NO) release at different levels, suggesting that TMUV infection led mainly to host macrophages polarizing into the classically activated (M1) type. The NO that was increased upon infection did not function as an antiviral agent against TMUV, since the replication of TMUV in HD11 cells was not affected by the addition of an organic NO donor. Furthermore, upon TMUV infection, polarized HD11 cells exhibited increased migration but reduced phagocytosis, as evidenced by scratch assay and neutral red uptake assay, respectively. Our present study characterized the polarization of host monocytes/macrophages upon TMUV infection, which may lay a foundation for further research on the immune escape mechanism and pathogenic mechanism of TMUV.

Beneficial actions of melatonin in the management of viral infections: a new use for this "molecular handyman"?

Melatonin (N‐acetyl‐5‐methoxytryptamine) is a multifunctional signaling molecule that has a variety of important functions. Numerous clinical trials have examined the therapeutic usefulness of melatonin in different fields of medicine. Clinical trials have shown that melatonin is efficient in preventing cell damage under acute (sepsis, asphyxia in newborns) and chronic states (metabolic and neurodegenerative diseases, cancer, inflammation, aging). The beneficial effects of melatonin can be explained by its properties as a potent antioxidant and antioxidant enzyme inducer, a regulator of apoptosis and a stimulator of immune functions. These effects support the use of melatonin in viral infections, which are often associated with inflammatory injury and increases in oxidative stress. In fact, melatonin has been used recently to treat several viral infections, which are summarized in this review. The role of melatonin in infections is also discussed herein. Copyright © 2012 John Wiley & Sons, Ltd.

Pichinde virus induces microvascular endothelial cell permeability through the production of nitric oxide

This report is the first to demonstrate infection of human endothelial cells by Pichinde virus (PIC). PIC infection induces an upregulation of the inducible nitric oxide synthase gene; as well as an increase in detectable nitric oxide (NO). PIC induces an increase in permeability in endothelial cell monolayers which can be abrogated at all measured timepoints with the addition of a nitric oxide synthase inhibitor, indicating a role for NO in the alteration of endothelial barrier function. Because NO has shown antiviral activity against some viruses, viral titer was measured after addition of the NO synthase inhibitor and found to have no effect in altering virus load in infected EC. The NO synthase inhibition also has no effect on levels of activated caspases induced by PIC infection. Taken together, these data indicate NO production induced by Pichinde virus infection has a pathogenic effect on endothelial cell monolayer permeability.

Nitric oxide in dengue and dengue haemorrhagic fever: necessity or nuisance?

Advances in free radical research show that reactive oxygen and nitrogen oxide species, for example superoxide, nitric oxide (NO) and peroxynitrite, play an important role in the pathogenesis of different viral infections, including dengue virus. The pathogenic mechanism of dengue haemorrhagic fever (DHF) is complicated and is not clearly understood. The hallmarks of the dengue disease, the antibody-dependent enhancement, the shift from T-helper type 1 (Th1) to Th2 cytokine response and the cytokine tsunami resulting in vascular leakage can now be explained much better with the knowledge gained about NO and peroxynitrite. This paper makes an effort to present a synthesis of the current opinions to explain the pathogenesis of DHF/shock syndrome with NO on centre stage.

Role of arginine metabolism in immunity and immunopathology

A heterogeneous set of cells that are commonly grouped as "myeloid cells", interacts in a complex landscape of physiological and pathological situations. In this review we attempt to trace a profile of the "myeloid connection" through different normal and pathological states, by analyzing common metabolic pathways of the amino acid l-arginine. Myeloid cells exert various, often divergent, actions on the immune response through mechanisms that exploit mediators of this peculiar metabolic pathway, ranging from l-arginine itself to its downstream metabolites, like nitric oxide and polyamines. Various pathological situations, including neoplastic and autoimmune diseases, as well as injury repair and infections are discussed here, showing how l-arginine metabolism is able to play a dual role, both as an active protector and a possible threat to the organism.

Nonspecific antiviral immunity by formalin-fixed Coxiella burnetii is enhanced in the absence of nitric oxide

Mice treated with a single injection of formalin-fixed Coxiella burnetii showed a significant increase in resistance to vaccinia virus (VV) infection compared to untreated mice. C. burnetii stimulated dramatically high levels of nitric oxide (NO) in the serum of treated mice, suggesting that NO might play a role in resistance to virus infection. To test this hypothesis, the effect of C. burnetii treatment on VV replication was examined in NOS2-/- and wild-type mice. C. burnetii treatment inhibited VV replication in both the knockout and wild-type mice but the effect was significantly greater in the NOS2-/- mice. Experiments in IFNgamma receptor knockout mice indicated that the nonspecific antiviral immunity induced by C. burnetii was dependent on IFNgamma and not NO. In the absence of NO, indoleamine 2,3-dioxygenase (IDO) was increased in C. burnetii-treated mice and this may contribute to the accelerated virus clearance in NOS2-/- mice.

Theiler's virus infection: a model for multiple sclerosis

Both genetic background and environmental factors, very probably viruses, appear to play a role in the etiology of multiple sclerosis (MS). Lessons from viral experimental models suggest that many different viruses may trigger inflammatory demyelinating diseases resembling MS. Theiler's virus, a picornavirus, induces in susceptible strains of mice early acute disease resembling encephalomyelitis followed by late chronic demyelinating disease, which is one of the best, if not the best, animal model for MS. During early acute disease the virus replicates in gray matter of the central nervous system but is eliminated to very low titers 2 weeks postinfection. Late chronic demyelinating disease becomes clinically apparent approximately 2 weeks later and is characterized by extensive demyelinating lesions and mononuclear cell infiltrates, progressive spinal cord atrophy, and axonal loss. Myelin damage is immunologically mediated, but it is not clear whether it is due to molecular mimicry or epitope spreading. Cytokines, nitric oxide/reactive nitrogen species, and costimulatory molecules are involved in the pathogenesis of both diseases. Close similarities between Theiler's virus-induced demyelinating disease in mice and MS in humans, include the following: major histocompatibility complex-dependent susceptibility; substantial similarities in neuropathology, including axonal damage and remyelination; and paucity of T-cell apoptosis in demyelinating disease. Both diseases are immunologically mediated. These common features emphasize the close similarities of Theiler's virus-induced demyelinating disease in mice and MS in humans.

Blunted stress responses in delayed type hypersensitivity in mice lacking the neuronal isoform of nitric oxide synthase

Nitric oxide (NO) is implicated in inflammation and hypothalamic-pituitary responses to immune stimuli; however, the specific role of NO from neurons during stress-induced immune responses remains unspecified. We measured antigen-specific delayed-type-hypersensitivity (DTH) responses in the skin of wild-type (WT) and neuronal nitric oxide synthase knockout (nNOS(-/-)) mice at baseline and after 2 h of restraint. Baseline corticosterone concentrations were higher in nNOS(-/-) than WT mice. However, stress-induced increases in corticosterone were dampened in nNOS(-/-) mice, and restraint suppressed DTH only in WT animals. Furthermore, WT mice lost more body mass after stress, and exhibited more anxiety-like behavior in the open field, than nNOS(-/-) mice. Neuronal NO appears to be involved in the neuroendocrine-immune response to stress, perhaps via glucocorticoid regulation.

Junin virus-induced astrocytosis is impaired by iNOS inhibition

Because Junin virus (JV) experimental encephalitis of mice and rats is characterized by mild histopathological changes that do not seem to justify per se lethality after intracerebral infection, such a murine model seems adequate to investigate the potential role of inducible nitric oxide synthase (iNOS) as a pathogenic factor. Concomitant with a predominant astrocyte reaction, increased immunoperoxidase expression of iNOS, mitochondrial superoxide dismutase (SODm) and glutathione peroxidase (GPX) was disclosed in brain of mice infected with JV strain #44. When specific inhibition of iNOS was achieved by intraperitoneal administration of amino guanidine (AG), significantly greater mortality was observed in treated animals (70% vs. 40%), together with similar infective titers ( approximately 10(7) PFU/g) but lower astrocytosis, as shown by glial fibrillary acidic (GFAP) labeling. As regards SODm and GPX immunochemical expression in neurons, no differences were found between mice with or without AG treatment. The present results suggest that the apparent protective role of nitric oxide (NO), when synthesized by iNOS, is unrelated to reduced viral replication but rather to enhanced astrocyte activation behaving as a beneficial cell response to virus-induced CNS damage.

Alterations in NF-kappaB and RBP-Jkappa by arenavirus infection of macrophages in vitro and in vivo

Pichinde virus is an arenavirus that infects guinea pigs and serves as an animal model for human Lassa fever. An attenuated Pichinde virus variant (P2) and a virulent variant (P18) are being used to delineate pathogenic mechanisms that culminate in shock. In guinea pigs, the infection has been shown to begin in peritoneal macrophages following intraperitoneal inoculation and then spreads to the spleen and other reticuloendothelial organs. We show here that infection of the murine monocytic cell line P388D1 with either Pichinde virus variant resulted in the induction of inflammatory cytokines and effectors, including interleukin-6 and tumor necrosis factor alpha. Since these genes are regulated in part by the cellular transcription factors NF-kappaB and RBP-Jkappa, we compared the activities of NF-kappaB and RBP-Jkappa in P388D1 cells following infection with Pichinde virus. The attenuated P2 virus inhibited NF-kappaB activation and caused a shift in the size of the RBP-Jkappa complex. The virulent P18 virus showed less inhibition of NF-kappaB and failed to alter the size of the RBP-Jkappa complex. Peritoneal cells from P2-infected guinea pigs showed induction of NF-kappaB RelA/p50 heterodimer and p50/p50 homodimer and manifested an increase in the size of RBP-Jkappa. By contrast, P18 induced large amounts of the NF-kappaB p50/p50 dimer but failed to induce RelA/p50 or to cause an increase in the RBP-Jkappa size. Taken together, these changes suggest that the attenuated viral strain induces an "activation" of macrophages, while the virulent form of the virus does not.

Nitric oxide and the immune response

During the past two decades, nitric oxide (NO) has been recognized as one of the most versatile players in the immune system. It is involved in the pathogenesis and control of infectious diseases, tumors, autoimmune processes and chronic degenerative diseases. Because of its variety of reaction partners (DNA, proteins, low-molecular weight thiols, prosthetic groups, reactive oxygen intermediates), its widespread production (by three different NO synthases (NOS) and the fact that its activity is strongly influenced by its concentration, NO continues to surprise and perplex immunologists. Today, there is no simple, uniform picture of the function of NO in the immune system. Protective and toxic effects of NO are frequently seen in parallel. Its striking inter- and intracellular signaling capacity makes it extremely difficult to predict the effect of NOS inhibitors and NO donors, which still hampers therapeutic applications.

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.

Role of free radicals in viral pathogenesis and mutation

Oxygen radicals and nitric oxide (NO) are generated in excess in a diverse array of microbial infections. Emerging concepts in free radical biology are now shedding light on the pathogenesis of various diseases. Free-radical induced pathogenicity in virus infections is of great importance, because evidence suggests that NO and oxygen radicals such as superoxide are key molecules in the pathogenesis of various infectious diseases. Although oxygen radicals and NO have an antimicrobial effect on bacteria and protozoa, they have opposing effects in virus infections such as influenza virus pneumonia and several other neurotropic virus infections. A high output of NO from inducible NO synthase, occurring in a variety of virus infections, produces highly reactive nitrogen oxide species, such as peroxynitrite, via interaction with oxygen radicals and reactive oxygen intermediates. The production of these various reactive species confers the diverse biological functions of NO. The reactive nitrogen species cause oxidative tissue injury and mutagenesis through oxidation and nitration of various biomolecules. The unique biological properties of free radicals are further illustrated by recent evidence showing accelerated viral mutation by NO-induced oxidative stress. NO appears to affect a host's immune response, with immunopathological consequences. For example, NO is reported to suppress type 1 helper T cell-dependent immune responses during infections, leading to type 2 helper T cell-biased immunological host responses. NO-induced immunosuppression may thus contribute to the pathogenesis of virus infections and help expansion of quasispecies population of viral pathogens. This review describes the pathophysiological roles of free radicals in the pathogenesis of viral disease and in viral mutation as related to both nonspecific inflammatory responses and immunological host reactions modulated by NO.

Nitric oxide and virus infection

Nitric oxide (NO) has complex and diverse functions in physiological and pathophysiological phenomena. The mechanisms of many events induced by NO are now well defined, so that a fundamental understanding of NO biology is almost established. Accumulated evidence suggests that NO and oxygen radicals such as superoxide are key molecules in the pathogenesis of various infectious diseases. NO biosynthesis, particularly through expression of an inducible NO synthase (iNOS), occurs in a variety of microbial infections. Although antimicrobial activity of NO is appreciated for bacteria and protozoa, NO has opposing effects in virus infections such as influenza virus pneumonia and certain other neurotropic virus infections. iNOS produces an excessive amount of NO for long periods, which allows generation of a highly reactive nitrogen oxide species, peroxynitrite, via a radical coupling reaction of NO with superoxide. Thus, peroxynitrite causes oxidative tissue injury through potent oxidation and nitration reactions of various biomolecules. NO also appears to affect a host's immune response, with immunopathological consequences. For example, overproduction of NO in virus infections in mice is reported to suppress type 1 helper T-cell-dependent immune responses, leading to type 2 helper T-cell-biased immunological host responses. Thus, NO may be a host response modulator rather than a simple antiviral agent. The unique biological properties of NO are further illustrated by our recent data suggesting that viral mutation and evolution may be accelerated by NO-induced oxidative stress. Here, we discuss these multiple roles of NO in pathogenesis of virus infections as related to both non-specific inflammatory responses and immunological host reactions modulated by NO during infections in vivo.

Partial impairment of cytokine responses in Tyk2-deficient mice

To assess the role of the Janus kinase (Jak) family member Tyk2, we have generated Tyk2-/- mice. In contrast to other Jaks, where inactivation leads to a complete loss of the respective cytokine receptor signal, Tyk2-/- mice display reduced responses to IFNalpha/beta and IL-12 and a selective deficiency in Stat3 activation in these pathways. Unexpectedly, IFNgamma signaling is also impaired in Tyk2-/- mice. Tyk2-/- macrophages fail to produce nitric oxide upon lipopolysaccharide induction. Tyk2-/- mice are unable to clear vaccinia virus and show a reduced T cell response after LCMV challenge. These data imply a selective contribution of Tyk2 to the signals triggered by various biological stimuli and cytokine receptors.