Poliovirus neurovirulence

Viren mit einzelsträngigem RNA-Genom in Plusstrangorientierung

Heute sind acht Virusfamilien bekannt, deren Vertreter eine einzelsträngige RNA in Plusstrangorientierung besitzen: Die Picornaviridae, Caliciviridae, Astroviridae und Hepeviren verfügen über Capside, die keine Hüllmembran aufweisen, wohingegen die Flaviviridae, Togaviridae, Arteriviridae und Coronaviridae durch membranumhüllte Partikel gekennzeichnet sind. Allen gemeinsam ist, dass sie ihre Genome als mRNA verwenden und davon ein oder mehrere Polyproteine synthetisieren, die im weiteren Verlauf durch virale oder auch zelluläre Proteasen in Einzelkomponenten gespalten werden. Die Viren verfügen über eine RNA-abhängige RNA-Polymerase, welche die Plusstrang-RNA sowie die als Zwischenprodukte der Replikation auftretenden Negativstränge übersetzt; dabei gehen die neuen genomischen RNA-Moleküle aus dem zweiten Transkriptionsschritt hervor. Die Einteilung in die unterschiedlichen Familien richtet sich nach Zahl, Größe, Lage und Orientierung der Virusgene auf der RNA, nach der Anzahl der unterschiedlichen Polyproteine, die während der Infektion synthetisiert werden, und nach dem Vorhandensein einer Hüllmembran als Teil der Virionen.

Neurovirulence tests of type 3 oral poliovirus vaccine manufactured by Lederle Laboratories, 1964-1988

Oral poliovirus vaccine (OPV) is tested for safety by evaluation of neurovirulence in rhesus and cynomolgus macaques. After intraspinal or intrathalamic injection of varying doses of vaccine, monkeys are followed for 17-21 days, killed, and a histopathological evaluation is made of the severity of poliomyelitis lesions in the spinal cord and brainstem. Each production lot of vaccine is compared with a type 1 OPV reference virus tested by the same method. Records of neurovirulence tests on production lots of type 3 OPV manufactured by Lederle Laboratories, during the period 1964-1988, have recently become available, together with the corresponding tests on type 1 reference vaccine. The cumulative data were collated, using a system under which each monkey was given a single grade according to the severity and spread of neuropathological poliomyelitis lesions. These raw data were assembled into frequency distributions ('neurovirulence profiles'), and used to compare type 3 OPV with the reference vaccine. These comparisons included monkeys injected by intraspinal injection (three vaccine dose levels) and intrathalamic injection (one vaccine dose level), and comprised independent tests conducted by the Food and Drug Administration and by the vaccine manufacturer. A total of 13 different comparisons were made, each one consisting of a pair of profiles, on type 3 OPV and reference vaccine, respectively. In total, these comparisons represented tests on more than 12,000 monkeys. Based on these neurovirulence profiles, the type 3 OPV appeared to be no more virulent than the reference vaccine.(ABSTRACT TRUNCATED AT 250 WORDS)

Viral pathogenesis and central nervous system infection

Both host defense and viral genetic factors influence the development of viral infection and disease. Due to the presence of the blood-brain barrier, infection of the central nervous system creates additional complexities in interactions between a virus and its host. Stages in viral pathogenesis defined as (1) virus entry, (2) spread, (3) tropism, (4) virulence and injury to the host, and (5) the outcome of infection are discussed for viral infections in general and those aspects unique to infections of the central nervous system. Information about neuronal physiology and function has also been revealed through studying virus infection. An increased understanding of viral pathogenetic mechanisms and host response to infection raises interesting possibilities for vaccine development and for basic studies in neurology and neurobiology.

The immune system response to viral infection of the CNS

In immunological terms the CNS is at a severe disadvantage in its ability to respond to infection by virus. First, both glial and neuronal cells normally do not express molecules of the major histocompatibility complex. Second, the most efficient cells for stimulating an immune response (leucocyte dendritic cells) are not present in the healthy CNS; and third, there is no specialized lymphatic drainage from the CNS to lymph nodes to enable the immune system to be quickly informed of the presence of an infection. Nevertheless, the immune system apparently copes with the vast majority of viral infections in the CNS. This is clearly evidenced by the reactivation of latent CNS viral infections in some immunosuppressed patients and the dramatic increase in the seventy of CNS disease in young or otherwise immunologically incompetent experimental animals infected with neurotropic viruses. The routes by which the CNS and the immune system may communicate and the varied ways in which an immune response may affect the outcome of a viral infection of the CNS are discussed.