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

Melatonin decreases nitric oxide production and lipid peroxidation and increases interleukin-1 beta in the brain of mice infected by the Venezuelan equine encephalomyelitis virus

Melatonin, a potent antioxidant, has shown to be beneficial in murine Venezuelan equine encephalomyelitis (VEE) virus infection. In addition, melatonin can induces the production of interleukin-1 beta (IL-1beta), a cytokine capable of inducing increased expression of inducible nitric oxide synthase; the activity of this enzyme is increased in the brain of mice infected with VEE virus. The aim of this study was to determine the effect of VEE virus on the nitric oxide (NO) production, lipid peroxidation and IL-1beta production in the brain and serum of mice infected with VEE virus, and to investigate the modulatory role of melatonin during this viral infection. Mice were infected with 10 LD(50) of VEE virus and treated with melatonin (500 microg/kg of body weight) starting 3 days before and continuing for 5 days after virus inoculation. Mice were sacrificed on days 1, 3 and 5 postinfection and brains and blood samples were obtained. NO and IL-1beta production and lipid peroxidation levels were measured in perfused brain homogenates and serum. Increased production of brain nitrite was found on days 1, 3 and 5 postinfection and lipid peroxidation products were increased at day 5. Levels of serum nitrite were found elevated on days 3 and 5 postinfection; however, lipid peroxidation products remained similar to basal levels. Melatonin treatment decreased nitrite concentration in brain and serum of infected mice as well as the lipid peroxidation products in the brain. IL-1beta was found to be increased in the brain and serum of infected animals, and melatonin treatment induced higher levels of this cytokine (brain: about 4-fold; serum: about 8-fold). These results may be related to the beneficial effect of melatonin in the VEE experimental disease and address the possible therapeutic potential of the indoleamine in human VEE virus infection.

Melatonin Decreases Nitric oxide Production, Inducible Nitric oxide Synthase Expression and Lipid Peroxidation Induced by Venezuelan Encephalitis Equine Virus in Neuroblastoma Cell Cultures

Increased expression of inducible nitric oxide synthase has been shown in murine Venezuelan equine encephalitis (VEE) virus infection. In this experimental model, melatonin (MTL) treatment has shown to be beneficial. The aim of this study was to determine the effect of VEE virus on the nitric oxide (NO) production and lipid peroxidation in neuroblastoma cell cultures, and to investigate the role of MTL during cell-virus interaction. Neuroblastoma cells were co-cultured with VEE virus and treated with MTL at doses ranging from 0 to 1.8 mM, for 6, 12, 24 and 48 h. NO and lipid peroxidation were measured in culture supernatants and in the cellular content by nitrite concentration and thiobarbituric acid assay, respectively. Expression of inducible nitric oxide synthase (iNOS) was determined by indirect immunofluorescence. Increased production of NO and lipid peroxidation products were found in supernatants and cellular contents of VEE virus treated cultures. Both NO and lipid peroxidation were decreased by MTL treatment in a time dependent manner. Increased iNOS expression was observed in VEE virus infected cultures that was reduced by MTL treatment. These results could be related to the beneficial role of MTL in the VEE experimental disease and address the possible therapeutic potential of the hormone in human VEE virus infection.

Differential induction of antiviral effects against West Nile virus in primary mouse macrophages derived from flavivirus-susceptible and congenic resistant mice by alpha/beta interferon and poly(I-C)

A cell model of primary macrophages isolated from the peritoneal cavity of flavivirus-susceptible and congenic resistant mice has been used to study the extent and kinetics of antiviral effects against West Nile virus upon priming with alpha/beta interferon (IFN-alpha/beta) or poly(I-C) (pIC). The pattern of flavivirus resistance expressed after priming of cells in this model was in good agreement with the pattern of flavivirus resistance described in the brains of the corresponding mouse strains. While priming with either IFN-alpha/beta or pIC completely blocked flavivirus replication in macrophages from resistant mice, it only transiently reduced flavivirus replication in macrophages from susceptible mice. It was only the combined pretreatment with IFN-alpha/beta and pIC that elicited strong antiviral responses that completely prevented flavivirus replication in macrophages from susceptible mice. Primary macrophages isolated from the blood of healthy human donors expressed a similar need for double-stranded RNA (dsRNA) cofactor in developing efficient antiviral responses against West Nile virus. These findings reveal that the inefficient IFN-alpha/beta-induced antiviral effects against flaviviruses in cells from susceptible hosts could be successfully complemented by an external dsRNA factor leading to the complete eradication of the virus. This treatment appears to compensate for the lack of an inborn resistance mechanism in cells from the susceptible host. Furthermore, it may also provide useful clues for the prevention and treatment of flavivirus infections.

Genetic resistance to flaviviruses

Resistance to flavivirus-induced disease in mice was first discovered in the 1920s and was subsequently shown to be controlled by the resistant allele of a single dominant autosomal gene. While the majority of current laboratory mouse stains have a homozygous-susceptible phenotype, the resistant allele has been found to segregate in wild mouse populations in many different parts of the world. Resistance is flavivirus specific and extends to both mosquito- and tick-borne flaviviruses. Resistant animals are infected productively by flaviviruses but produce lower virus titers, especially in their brains, as compared to susceptible mice. Decreased virus production is observed in resistant animals even during a lethal infection and the times of disease onset and death are also delayed as compared to susceptible mice. An intact immune response is required to clear flaviviruses from resistant mice. The resistant phenotype is expressed constitutively and does not require interferon induction. The Flv gene was discovered using a positional cloning approach and identified as Oas1b. Susceptible mice produce a truncated Oas1b protein. A C820T transition in the fourth exon of the gene introduced a premature stop codon and was found in all susceptible mouse strains tested. Possible mechanisms by which the product of the resistant allele could confer the resistant phenotype are discussed.

Is flavivirus resistance interferon type I-independent?

Interferon type I comprises a group of major virus-inducible host antiviral factors that control infection with a great number of human and animal viruses. They are ubiquitously expressed cytokines that interfere with virus replication within different cell types by activating a number of host genes and several parallel antiviral pathways. Two major intracellular actors of IFN-I-induced antiviral states are ribonucleic acid-dependent protein kinase and 2'-5'-oligoadenylate synthetases/RNase L, both being induced by IFN-I and activated by viral double stranded ribonucleic acid. In addition, Mx proteins and ribonucleic acid-specific adenosine deaminase have also been implicated in IFN-I-induced antiviral responses to some RNA viruses. Viruses, in turn, have evolved different strategies to escape a control imposed by IFN-I and by IFN-I-induced antiviral factors. The fatal outcome of virus infection as well as the efficiency of IFN-I-based antiviral therapies in its prevention, are determined by complex interactions between viral virulence factors and cellular antiviral IFN-I inducible factors. In the light of these facts and current knowledge on IFN-I involvement in flavivirus infection, I discuss a possible role of IFN-I signalling in resistance to flavivirus infection in a model of congenic mouse strains that express different levels of susceptibility/resistance to common flaviviruses. Specifically, this review emphasizes importance of fully operative 2'-5'-oligoadenylate synthetases/RNase L pathway for the IFN-I-induced stimulation of flavivirus resistance conferred by Flv.