A recombinant vaccinia virus encoding inducible nitric oxide synthase is attenuated in vivo

The effect of nitric oxide on vaccinia virus-encoded ribonucleotide reductase

Growth inhibition of the DNA virus vaccinia (VACV) by NO is known to occur at the level of DNA synthesis. This inhibition is partially reversed by addition of deoxyribonucleosides, suggesting that NO or NO-related species inhibit viral ribonucleotide reductase (RR). However, the effect of NO on VACV-encoded RR or other DNA-synthesizing enzymes has not been demonstrated. In order to study the effects of NO on VACV-encoded RR, DNA polymerase (DNA pol) and thymidine kinase (TK), we generated a VACV recombinant expressing murine macrophage iNOS under control of a VACV early/late promoter p7.5. Using this recombinant, we demonstrate that expression of iNOS and the resulting production of NO inhibit activity of the viral RR, but not of viral DNA pol and TK. This NO-mediated inhibition of viral RR occurred around the same time as the increase of ADP levels, while it preceded the block in VACV DNA synthesis and the decrease of ATP levels. In addition, we tested the effects of DPTA/NONOate on the growth of different VACV mutants. Fold-inhibition of the growth of VACV deletion mutant for TK was comparable to that of wild-type VACV. VACV containing amplification of the gene for the small subunit of RR appeared to be least sensitive to DPTA/NONOate, while VACV deletion mutant for the large subunit of RR was most sensitive. The results provide a direct evidence for NO-mediated inhibition of VACV-encoded RR.

Myxoma virus M141R expresses a viral CD200 (vOX-2) that is responsible for down-regulation of macrophage and T-cell activation in vivo

M141R is a myxoma virus gene that encodes a cell surface protein with significant amino acid similarity to the family of cellular CD200 (OX-2) proteins implicated in the regulation of myeloid lineage cell activation. The creation of an M141R deletion mutant myxoma virus strain (vMyx141KO) and its subsequent infection of European rabbits demonstrated that M141R is required for the full development of a lethal infection in vivo but is not required for efficient virus replication in susceptible cell lines in vitro. Minor secondary sites of infection were detected in the majority of rabbits infected with the M141R deletion mutant, demonstrating that the M141R protein is not required for the dissemination of virus within the host. When compared to wild-type myxoma virus-infected rabbits, vMyx141KO-infected rabbits showed higher activation levels of both monocytes/macrophages and lymphocytes in situ through assessments of inducible nitric oxide synthase-positive and CD25(+) infiltrating cells in infected and lymphoid tissues. Purified peripheral blood mononuclear cells from vMyx141KO-infected rabbits demonstrated an increased ability to express gamma interferon upon activation by phorbol myristate acetate plus ionomycin compared to cells purified from wild-type myxoma virus-infected rabbits. We concluded that the M141R protein is a bona fide CD200-like immunomodulator protein which is required for the full pathogenesis of myxoma virus in the European rabbit and that its loss from the virus results in increased activation levels of macrophages in infected lesions and draining lymph nodes as well as an increased activation level of circulating T lymphocytes during infection. We propose a model whereby M141R transmits inhibitory signals to tissue macrophages, and possibly resident CD200R(+) dendritic cells, that reduce their ability to antigenically prime lymphocytes and possibly provides anergic signals to T cells directly.

Myxoma virus M128L is expressed as a cell surface CD47-like virulence factor that contributes to the downregulation of macrophage activation in vivo

The M128L myxoma virus gene expresses a five-membrane spanning cell surface protein with significant amino acid homology to the cellular CD47 proteins. CD47, also called integrin-associated protein (IAP), is associated with the modulation of leukocyte adhesion, motility, activation, and phagocytosis. Creation of an M128L-deletion mutant myxoma virus strain and subsequent infection of the European rabbit demonstrated that M128L is necessary for the production of a lethal infection in susceptible rabbits, while it is fully dispensable for virus replication in vitro. Secondary sites of infection developed on the majority of rabbits infected with the M128L-deletion mutant (vMyx128KO), demonstrating that the M128L protein is nonessential for the dissemination of virus within the host. Although the size and severity of the primary lesions on vMyx128KO-infected rabbits were comparable to rabbits infected with the wild-type virus at the early stages of disease progression, by day 7 the reduced virulence of the vMyx128KO virus was clearly evident and all of the animals recovered from infection by the M128L-knockout virus. Histological analysis of the tissues of vMyx128KO-infected rabbits revealed greater activation of monocyte/macrophage cells in infected and/or lymphoid tissues when compared to those of wild-type myxoma-infected rabbits. We conclude that the M128L protein is a novel CD47-like immunomodulatory gene of myxoma virus required for full pathogenesis of the virus in the European rabbit and that its loss from the virus results in increased activation of monocyte/macrophage cells during infection.

IL-15 induces IFN-beta and iNOS gene expression, and antiviral activity of murine macrophage RAW 264.7 cells

The effects of interleukine-15 (IL-15) on macrophage activation and antiviral activity have been investigated in this study. We have provided evidence that IL-15 stimulates murine macrophage RAW 264.7 cells to release nitric oxide (NO) and inhibit vaccinia virus (VV) replication in bystander human 293 cells in a dose-dependent manner. The IL-15-induced antiviral activity was partially mediated by NO, as blocking NO production by NO synthase (iNOS) inhibitor NG-monomethyl-L-arginine acetate (L-NMA) partially restored the virus replication. Interferon-gamma (IFN-gamma) was not detectable by ELISA in the cell supernatant of IL-15-activated macrophages or in the co-cultures of macrophages and infected bystander cells. Neutralizing anti-IFN-gamma, anti-IFN-gamma receptor R2, anti-TNF-alpha, or anti-IL-12 antibodies had no effect on NO production or antiviral activity. In contrast, neutralizing anti-IFN-alpha/beta antibody completely restored the VV replication and reduced the NO level to one third of that in the control. Elevated mRNA levels of IFN-beta and iNOS genes were detected in IL-15-activated RAW 264.7 cells by RT-PCR. Our data suggest that IL-15 is capable of inducing IFN-beta, which could participate in NO-mediated antiviral effect.

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.

Vaccinia virus interleukin-18-binding protein promotes virulence by reducing gamma interferon production and natural killer and T-cell activity

Interleukin-18 (IL-18) is a proinflammatory cytokine that promotes natural killer (NK) and T-cell activation. Several poxviruses, including vaccinia virus (VV), encode a soluble IL-18-binding protein (IL-18bp). The role of the VV IL-18bp (gene C12L) in vivo was studied with wild-type (vC12L), deletion mutant (vDeltaC12L), and revertant (vC12L-rev) viruses in a murine intranasal model of infection. The data show that vDeltaC12L was markedly attenuated, characterized by a mild weight loss and reduced virus titers in lungs, brain, and spleen. Three days after infection, NK cytotoxic activity was augmented in the lung, spleen, and mediastinal lymph nodes (MLNs) of vDeltaC12L-infected mice compared to controls. Seven days after infection, vDeltaC12L-infected mice displayed heightened VV-specific cytotoxic T-lymphocyte (CTL) responses in the lungs, spleen, and MLNs. Gamma interferon (IFN-gamma) levels were also dramatically elevated in lavage fluids and cells from lungs of mice infected with vDeltaC12L. Finally, we demonstrate that IL-18 is produced in vitro and in vivo after VV infection. Taken together, these data demonstrate a role for the vIL-18bp in counteracting IL-18 in both the innate and the specific immune response to VV infection and indicate that the ability of IL-18 to promote vigorous T-cell responses (cytotoxic activity and IFN-gamma production) is a critical factor in the accelerated clearance of the vDeltaC12L mutant.

A kinetic analysis of immune mediators in the lungs of mice infected with vaccinia virus and comparison with intradermal infection

The early inflammatory response to a virus may be critical in restricting infection and in shaping the subsequent adaptive immune response. In this study we have examined the early inflammatory response of mice following infection with vaccinia virus (VV) strain Western Reserve (WR). Respiratory challenge of BALB/c mice with VV led to early virus replication in the lung and upper respiratory tract followed by dissemination of virus to other visceral organs and to the brain. The number of inflammatory cells, largely macrophages and T lymphocytes, recovered from bronchoalveolar lavage (BAL) fluid increased markedly during infection and coincided with the expression of CC chemokine ligands (CCL) 3, 2 and 11 and CXC chemokine ligands (CXCL) 1 and 2/3 in BAL. The peak of the inflammatory response occurred around day 10 and declined thereafter. The antiviral cytokines IFN-gamma and TNF-alpha, and the reactive nitrogen intermediate nitric oxide (NO), were also detected in BAL from VV-infected mice. A markedly different inflammatory response was observed after intradermal inoculation of WR into the ear pinnae of mice. Intradermal challenge was followed by highly localized virus replication and by a cellular influx, consisting largely of neutrophils and T lymphocytes, into the dermal compartment of the infected ear. Together these findings highlight differences in the pathogenesis and in the cellular inflammatory response to WR following intranasal and intradermal inoculation of mice.

IL-4 and IL-10 antagonize IL-12-mediated protection against acute vaccinia virus infection with a limited role of IFN-gamma and nitric oxide synthetase 2

Resistance or susceptibility to most infectious diseases is strongly determined by the balance of type 1 vs type 2 cytokines produced during infection. However, for viruses, this scheme may be applicable only to infections with some cytopathic viruses, where IFN-gamma is considered as mandatory for host defense with little if any participation of type 2 responses. We studied the role of signature Th1 (IL-12, IFN-gamma) and Th2 (IL-4, IL-10) cytokines for immune responses against vaccinia virus (VV). IL-12-/- mice were far more susceptible than IFN-gamma-/- mice, and primary CTL responses against VV were absent in IL-12-/- mice but remained intact in IFN-gamma-/- mice. Both CD4+ and CD8+ T cells from IL-12-/- mice were unimpaired in IFN-gamma production, although CD4+ T cells showed elevated Th2 cytokine responses. Virus replication was impaired in IL-4-/- mice and, even more strikingly, in IL-10-/- mice, which both produced elevated levels of the proinflammatory cytokines IL-1alpha and IL-6. Thus, IL-4 produced by Th2 cells and IL-10 produced by Th2 cells and probably also by macrophages counteract efficient anti-viral host defense. Surprisingly, NO production, which is considered as a major type 1 effector pathway inhibited by type 2 cytokines, appears to play a limited role against VV, because NO sythetase 2-deficient mice did not show increased viral replication. Thus, our results identify a new role for IL-12 in defense beyond the induction of IFN-gamma and show that IL-4 and IL-10 modulate host protective responses to VV.

Identification of nitric oxide synthase 2 as an innate resistance locus against ectromelia virus infection

To assess whether nitric oxide synthase 2 (NOS2) fulfills the criteria of an innate resistance locus against an acute viral infection, we inoculated genetically deficient NOS2-/- mice with virulent ectromelia virus (EV), the causative agent of mousepox. NOS2-/- mice proved highly susceptible to EV yet showed no diminution in other well-characterized anti-EV immune responses, i.e. , gamma interferon secretion and NK cell and EV-specific cytotoxic T lymphocyte activities. Thus, the NOS2 locus can be considered a critical monogenic determinant of EV resistance that contributes to host survival.

Erratic behavior of nitric oxide within the immune system: illustrative review of conflicting data and their immunopharmacological aspects

The literature data assembled in this article document the variation of immunobiological effects of nitric oxide (NO). A number of factors are obviously responsible for the diversity, ranging from inactivity, alleviation, but not rarely to exacerbation of certain pathogenetic processes. A better understanding of NO interactions with the immune system can only be reached if more complex experimental designs to study the effects of reactive nitrogen species are adopted in the future. They should integrate major participating variables and take into account pharmacodynamic/kinetic aspects of NO production in triggering the ultimate effects. If manipulation of NO in the organism by means of recently developed NO inhibitors and NO donors is to become a rational tool of immunopharmacological strategies, detailed knowledge of their pharmacologies and toxicologies is urgently needed in order to differentiate between the effects of NO and other side effects. Hopefully, this approach could improve the predictability of the clinical outcomes of NO manipulation.

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.

Importance of the cysteine-rich carboxyl-terminal half of V protein for Sendai virus pathogenesis

The Sendai virus V protein is a nonstructural trans-frame protein whose cysteine-rich C-terminal half is fused to the acidic N-terminal half of the P protein via mRNA editing. We recently created a mutant by disrupting the editing motif, which is devoid of mRNA editing and hence unable to produce the V protein, and demonstrated that this V(-) virus replicated normally or even faster with augmented gene expression and cytopathogenicity in cells in vitro, but was strongly attenuated in pathogenicity for mice (A. Kato, K. Kiyotani, Y. Sakai, T. Yoshida, and Y. Nagai, EMBO J. 16:578-587, 1997). Thus, although categorized as a nonessential protein, the V protein appeared to encode a luxury function required for the viral in vivo pathogenesis. Here, we created another version of a V-deficient mutant, VdeltaC, encoding only the N-terminal half but not the V-specific C-terminal half, by introducing a stop codon in the trans-V frame, and then we compared its in vitro and in vivo phenotypes with those of the V(-) and wild-type viruses. The VdeltaC virus was found to be similar to the wild-type virus in vitro with no augmented gene expression and cytopathogenicity, but in vivo, it resembled the V(-) virus, displaying a similarly attenuated phenotype. Thus, the pathogenicity determinant in the V protein was mapped to the C-terminal half. The N-terminal half was likely sufficient to confer normal (wild-type) in vitro phenotypes.

Cytokines and immunity to viral infections

In this review, we discuss two broad approaches we have taken to study the role of cytokines and chemokines in antiviral immunity. Firstly, recombinant vaccinia viruses were engineered to express genes encoding cytokines and chemokines of interest. Potent antiviral activity was mediated by many of these encoded factors, including IL-2, IL-12, IFN-gamma, TNF-alpha, CD40L, Mig and Crg-2. In some cases, host defense mechanisms were induced (IL-2, IL-12, Mig and Crg-2), whilst for others, a direct antiviral effect was demonstrated (IFN-gamma, TNF-alpha and CD40L). In sharp contrast, vector-directed expression of IL-4, a type 2 factor, greatly increased virus virulence, due to a downregulation of host type 1 immune responses. Our second experimental approach involved the use of strains of mice deficient for the production of particular cytokines or their receptors, often in combination with our engineered viruses. Mice deficient in either IFN-gamma, IFN-gamma R, IFN-alpha/beta R, TNFRs, CD40 or IL-6 were, in general, highly susceptible to poxvirus infection. Surprisingly, not only the TNFR1, but also the TNFR2, was able to mediate the antiviral effects of TNF-alpha in vivo, whilst the antiviral activity observed following CD40-CD40L interaction is a newly defined function which may involve apoptosis of infected cells. Through the use of perforin-deficient mice, we were able to demonstrate a requirement for this molecule in the clearance of some viruses, such as ectromelia virus, whilst for others, such as vaccinia virus, perforin was less important but IFN-gamma was essential.

Recombinant viruses as vaccines and immunological tools

Recombinant viruses have been investigated as candidate vaccines, and have also been used extensively as immunological tools. Recent advances in this area include the following: the construction and testing of a recombinant simian immunodeficiency virus encoding human interferon-gamma; the development of new vectors such as recombinant poliovirus; and the generation of polyepitope vaccines. Basic immunological research has benefited from the use of recombinant viruses to further understand the role of molecules such as CD40 ligand, nitric oxide and interleukin-4.