We describe a DNA vaccine strategy that allows antigens to be produced in vivo in the context of an alphaviral replicon. Mice immunized with such vectors developed humoral and cellular immune responses at higher levels than mice that received a conventional DNA vaccine vector. Immunized animals acquired protective immunity to lethal influenza challenge. Compared with traditional DNA vaccine strategies in which vectors are persistent and the expression constitutive, the expression mediated by the alphaviral vector was transient and lytic. As a result, biosafety risks such as chromosomal integration, and the induction of immunological tolerance, could be circumvented.

Enhancing immune responses using suicidal DNA vaccines

Among arthropod-borne and related viruses, the Flaviviridae are medically the most important group and biologically one of the most intriguing. Elucidation of perplexingly complex virus- and host-specified factors that underlie virulence and pathogenesis has lagged behind other areas of virology, and the available information is largely descriptive. Ultimately, flavivirus biology and pathogenesis will be understood in terms of viral gene expression, virus receptor—host cell membrane interactions, biochemical alterations in host cells, physiological responses, and immune and nonimmune mechanisms that control virus replication and virus spread, subjects to which other chapters in this book are devoted. There will remain, however, a need to understand and synthesize this information with observations relating to infection at the level of the intact organism (virus, vector, and host) and of populations of organisms in nature. It is at these levels that the phenomena that require explanation first present themselves.

/pdf/pathobiology-of-the-flaviviruses-3kg47tio7y.pdf

Pathobiology of the Flaviviruses

IP: 54.70.40.11 On: Fri, 07 Dec 2018 19:28:21 Journal of General Virology (1999), 80, 2287–2297. Printed in Great Britain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The molecular pathogenesis of Semliki Forest virus: a model virus made useful?

We have investigated the induction of cell death in cultured cells by the virulent SFV4 and avirulent A7 strains of Semliki Forest virus (SFV). In BHK cells, death occurred by a typical apoptotic mechanism, as did the death of oligodendrocytes in glial cell cultures. For cerebellar neuron cultures, virus-induced death was due to necrosis. Although the SFV4 and A7 strains did not differ in the mechanism of induction of cell death, the virulent SFV4 strain did multiply to a higher titre in cultured neurons than the avirulent A7 strain. This is consistent with previous animal studies which indicate that the virulence of SFV strains is controlled by rapidity of multiplication in the CNS, leading to a lethal threshold of damage, rather than differential cell tropism or cell death mechanisms. The immune-mediated demyelination induced by avirulent strains may be triggered by apoptosis of oligodendrocytes, the consequences of which are obscured by death for virulent strains.

Death mechanisms in cultured cells infected by Semliki Forest virus

The replicase protein nsP2 of Semliki Forest virus (SFV) has a 648RRR nuclear localization signal and is transported to the nucleus. SFV-RDR has a single amino acid change which disrupts this sequence and nsP2 nuclear transport. In BHK cells, SFV4 and SFV-RDR replicate to high titers, but SFV-RDR is less virulent in mice. We compared the replication of SFV4 and SFV-RDR in adult mouse brain. Both SFV4 and SFV-RDR were neuroinvasive following intraperitoneal inoculation. SFV4 spread rapidly throughout the brain, whereas SFV-RDR infection was confined to small foci of cells. Both viruses infected neurons and oligodendrocytes. Both viruses induced apoptosis in cultured BHK cells but not in the cells of the adult mouse brain. SFV-RDR infection of mice lacking alpha/beta interferon receptors resulted in widespread virus distribution in the brain. Thus, a component of the viral replicase plays an important role in the neuropathogenesis of SFV.

A single amino acid change in the nuclear localization sequence of the nsP2 protein affects the neurovirulence of Semliki Forest virus.

Summary
The role of immune mechanisms in the pathogenicity of the M9 mutant of Semliki Forest virus (SFV) has been examined by the use of immune-deficient and immune-suppressed mice. In immune-competent BALB/c and C57BL/6 mice, the lesions in the central nervous system (CNS) were characterized by acute demyelinating meningo-encephalomyelitis. Myelin vacuolation and demyelination were more severe in BALB/c mice than in mice with a C57BL background. The mortality was 12% and 29% respectively. Treatment with cyclophosphamide or sodium aurothiomalate did not greatly alter the type of lesion produced, although mortality was increased. Lesions were less severe in nude (T-lymphocyte-deficient) mice and more severe in beige (natural killer cell-deficient) mice than in most immune-competent mice. Mortality was marginally increased in nude mice but not in beige mice. Demyelination in nude mice was followed rapidly by remyelination. Immune modification did not significantly alter the titres of virus in the brain at 7 days post-infection and infectious virus had been cleared from the brain by 14 days in all cases. Degenerating oligodendrocytes were detected in the CNS of all immune-modified mice examined at day 7. This study therefore suggests that both immune mechanisms and destruction of oligodendrocytes play a role in the production of demyelination by M9.

The Pathogenicity of the M9 Mutant of Semliki Forest Virus in Immune-compromised Mice

Summary
The multiplication of the M9, A7 and L10 strains of Semliki Forest virus (SFV), both in weanling mice and primary mouse brain cell cultures, was compared. Following both intraperitoneal (i.p.) and intracerebral (i.c.) injection, the virulent L10 strain multiplied to higher titre in the mouse central nervous system (CNS) than did the less virulent M9 and A7 strains, whereas M9 multiplied to higher titre than A7. By the i.c. route, all three virus strains multiplied to higher titre than following i.p. injection. Multiplication of A7 and M9 in oligodendrocytes, but not neurons, was detected following i.c. injection. All three virus strains showed a tropism for cultured mouse glial cells rather than neurons. The L10 strain multiplied better in neurons than did A7 or M9. It is concluded that the mechanism of acute demyelination induced by the M9 and A7 strains is similar. Based on this and previous studies, it is proposed that infection of glial cells triggers immune-mediated demyelination. The virulence of the L10 strain is due to its ability to exceed a lethal threshold of damage to neurons before immune intervention can occur.

The pathogenicity of the A7, M9 and L10 strains of Semliki Forest virus for weanling mice and primary mouse brain cell cultures.

Intraperitoneal inoculation with the M9 mutant of Semliki Forest virus caused focal demyelinating encephalomyelitis in weanling BALB/c and C57BL/6 mice. Demyelination was more severe in BALB/c than in C57BL/6 mice. Virus particles were seen in oligodendrocytes in areas of myelin vacuolation 5 and 7 days post inoculation (DPI). Oligodendrocytes containing virus in BALB/c mice showed hypertrophy and vacuolar degeneration. There was a mononuclear cell infiltrate and lymphocytes and necrotic cells were present in vacuoles in myelin sheaths. Demyelinating plaques containing macrophages laden with myelin debris were most prominent 14 DPI when virus was cleared from the brain. Remyelination of the central type occurred 28 DPI in BALB/c mice. These findings indicate that direct virus-induced injury to oligodendrocytes has a major role in the initiation of inflammation and demyelination in this model system.

/pdf/oligodendrocyte-infection-and-demyelination-produced-in-mice-142koglgwq.pdf

Oligodendrocyte infection and demyelination produced in mice by the M9 mutant of Semliki Forest virus.

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The Pathogenicity of the M9 Mutant of Semliki Forest Virus in Immune-compromised Mice

The pathogenicity of the A7, M9 and L10 strains of Semliki Forest virus for weanling mice and primary mouse brain cell cultures.

Oligodendrocyte infection and demyelination produced in mice by the M9 mutant of Semliki Forest virus.