Gerald A. Cole

Immunosuppression and experimental virus infection of the nervous system.

Publisher Summary This chapter describes the current views of the pathogenesis of virus infections of the nervous system, with particular attention to certain aspects of virus-host interactions. Following invasion of the central nervous system, infection can follow a variety of patterns, as to number and distribution of neuronal and non-neuronal cells involved. There is a corresponding diversity in the pathological lesions of the central nervous system (CNS) produced by acute virus infection. Infection can be pictured as a race between virus and host defenses, where many factors, acting through different mechanisms, can influence the outcome. Outcome is always determined by multiple virus and host variables, although single variables can be independently studied under experimentally controlled conditions in the laboratory. The chapter demonstrates that in many virus-host combinations, the immune response plays an important role in recovery from primary infections. It mentions that an immunopathological process mediates the disease which follows certain CNS virus infections.

Experimental encephalitis following peripheral inoculation of West Nile virus in mice of different ages.

Experimental arbovirus infections of mice provide a convenient model to study factors which determine the occurrence or severity of encephalitis following extraneural infection with certain neurotropic viruses. Varying doses of West Nile or Powassan viruses were inoculated by intraperitoneal or intramuscular routes into mice of varing ages; individual variables were manipulated to influence the outcome of infection. Three patterns of pathogenesis were delineated: (1) Fatal encephalitis, preceded by early viraemia, and invasion of the central nervous system. (2) Inapparent infection, with no detectable viraemia and no evidence of central nervous system invasion. (3) Subclinical encephalitis, usually preceded by trace viraemia, with minimal transient levels of virus in the brain. In this latter type of subclinical infection with a potentially lethal virus, the immune response probably plays an important role in recovery.

Cerebellar Hypoplasia in Neonatal Rats Caused by Lymphocytic Choriomeningitis Virus

Lymphocytic choriomeningitis virus, strain E-350, when inoculated intracerebrally in rats 1 to 7 days old, produces an acute destructive infection of the cerebellar cortex resulting in permanent cerebellar hypoplasia and ataxia. Several other arenoviruses may produce a similar lesion in neonatal rodents.

PATHOGENESIS OF CEREBELLAR HYPOPLASIA PRODUCED BY LYMPHOCYTIC CHORIOMENINGITIS VIRUS INFECTION OF NEONATAL RATS: 1. EVOLUTION OF DISEASE FOLLOWING INFECTION AT 4 DAYS OF AGE

Intracerebral inoculation of 4-day-old rats with LCM virus (Armstrong E-350 strain) resulted in a severe cerebellar necrosis and consequent ataxia. The pathogenesis of this disease was studied using virological, serological, and histological methods. Cerebellar necrosis occurred concomitantly with an acute and severe transitory choriomeningitis. While neural structures involved in postnatal replication were heavily infected, only the cerebellum consistently showed pathological changes. Necrosis was first evident in the granule cells, with secondary loss of Purkinje cells and white matter, but with sparing of the deep cerebellar nuclei. Brain virus reached high titers early after infection and gradually decreased finally to disappear 5 months after inoculation. Viremia was minimal and transitory, occurring within two weeks of infection. Although complement fixing antibodies developed, their relation to the cerebellar lesion is questionable since the pathology was well established prior to the occurrence of detectable levels of antibody. In addition, the acute phase of infection was associated with a retardation in growth rate, the consequence of which was a permanent impairment in development reflected by reduced brain and body weights.

Potentiation of Experimental Arbovirus Encephalitis by Immunosuppressive Doses of Cyclophosphamide

THE role of the immune response in recovery from primary viral infection has been debated for many years, in part because of the inconclusive nature of the relevant evidence1,2. The ability of children with defective immunoglobulin synthesis to recover uneventfully from certain viral infections3, and the recognition of non-immune host defences such as interferon4, have suggested that the immune response plays a small part. The recent development of an increasing variety of techniques for the production of immunosuppression permits a re-examination of the question. Using the potent alkylating agent, cyclophosphamide5,6, we have found that administration of immunosuppressive doses of the drug converts a silent abortive arbovirus infection of the rodent central nervous system (CNS) into a lethal encephalitis.

VIRUS-INDUCED CELL-MEDIATED IMMUNOPATHOLOGICAL DISEASE

Publisher Summary This chapter explains virus-induced cell-mediated immunopathological disease. There are several important observations that provide a foundation for understanding virus-induced cell-mediated immunopathology. Several classes of viruses mature by budding through cellular membranes, thereby, acquiring their outer envelope. The principal antigens of such envelopes are virus-specified proteins and are therefore foreign to the host. Many enveloped RNA viruses are relatively or absolutely noncytocidal, so that infected cells may function and divide in an apparently normal manner while persistently carrying virus-coded neoantigens on their membranes. The immune response to enveloped viruses can result in the production of antibodies which, with complement, can lyse either intact virus or viable virus-infected cells. T cell-mediated cytolysis can be used as the basis for an in vitro assay of cell-mediated immunity. The occurrence of virus-induced immunopathological disease is rendered plausible by these considerations. However, the interaction of virus and immune response in vivo introduces a variety of additional complexities that are at present only partially understood.

In Vitro Correlates of LCM Virus-Induced Immune Response

LCM virus-specific immune responses of virus-immune and virus carrier BALB/c mice were studied by an in vitro assay involving the lysis of 51Cr-labeled virus-infected “target” L cells by either splenic lymphocytes or antibody in the presence of complement. Lysis by immune lymphocytes was inhibited by anti-θ serum but could not be blocked by anti-viral complement-fixing antibody. The susceptibility of target cells to lymphocyte-mediated lysis was closely related to the density of immunofluorescent staining virus-specific antigen at their surfaces. The number of cells with demonstrable surface antigen decreased with continued passage in culture without a concomitant decrease in intracytoplasmic viral antigen.

MARINE INVERTEBRATE ORIGIN OF A REACTANT TO MAMMALIAN T CELLS

Molecular components of invertebrate phagocytic cells could well prove to be a fruitful source for the investigation of the evolutionary progenitors of those proteins that are concerned with immunological defense functions in the vertebrate 2 We have recently described a protein isolated from coelomocytes of the sea star (Asterias forbesi) that has many of the biological properties of lymphokines produced by specifically immune mammalian thymus-derived lymphocytes (T cells) after contact with homologous antigen.3 The subcutaneous injection of microgram quantities of this sea star factor (SSF) produces a lesion that is temporally, physically, and cytologically indistinguishable from a delayed tuberculin reaction. This delayed inflammatory response was observed in all mammalian species tested, including man, monkeys, rabbits, sheep, fetal lambs, rats, and guinea pigs. SSF was also shown to be a migration-inhibitory factor (MIF) for normal guinea pig peritoneal macrophages in vitro.s Macrophage activation, as judged by increase in cell size and the uptake of euchrysine in lysosomes, was readily produced in normal mouse peritoneal macrophages 48 hours after the injection of 10 pg SSF.4q The finding of a protein mediator from an invertebrate source with remarkable effects on cells concerned in mammalian immune phenomena prompted us to examine the direct effect of SSF on T cells in a number of in vivo and in vitro systems. The present study examines the effect of SSF on the response of mice to both T-dependent and T-independent antigens, its effect on the development and function of cytolytically active T cells, and its effect on T lymphocytes in culture; the latter was determined by the measurement of alloantigen stimulation in the mixed lymphocyte response (MLR) and of the mitogenesis induced by the lectin concanavalin A.

Pathogenesis of LCM Disease in the Rat

Intracerebral inoculation of neonatal rats with LCM virus resulted in varying extents of central nervous system parenchymal pathology dependent upon host age at infection and upon virus strain. Lesions were confined to brain areas undergoing postnatal proliferation and migration, with the most striking involvement being a profound necrosis of most of the cerebellar cortex following infection of rats at 4 days of age. The immunopathological basis of this LCM virus-induced cerebellar pathology has been demonstrated both by preventing it with immunosuppression of normally susceptible animals and by eliciting it by adoptive immunization, with LCM virus-immune spleen cells, of infected animals not normally susceptible.

LYMPHOCYTIC CHORIOMENINGITIS VIRUS‐INDUCED CEREBELLAR HYPOPLASIA IN NEONATAL RATS *

Lymphocytic choriomeningitis (LCM) virus infections of mice following intracerebral inoculation have been extensively 9-11 The outcome of such infections has been shown to be age-de~endent.~ Neonatal mice have the ability to sustain an asymptomatic, life-long infection unaccompanied by detectable histologic changes in neuronal elements of the brain, but nevertheless carry high virus levels in this organ and other tissues. In contrast, older animals usually die following an acute convulsive diathesis that is thought to be mediated by a component of the host’s virus-specific immune response acting upon cells of the meninges, choroid plexus, and ependyma whose membranous surfaces have been altered by the infectious process.s Histologically there is a severe inflammatory reaction that is largely confined to these areas without morphological alteration of the cerebral parenchyma. Relatively few data exist on LCM virus infections of rats beyond what has already been reported,*. :’ which suggests that a detailed study of infection in this host might present an interesting comparison with previous studies employing mice. We now report the occurrence of an acute destructive process of the cerebellum following intracerebral inoculation of LCM virus into the neonatal rat. This striking pathological lesion, to our knowledge, has not been previously described.