Herpes simplex virus in mice: electron microscopy of neural spread

The role of primary infection of Schwann cells in the aetiology of infective inflammatory neuropathies

Numerous different pathogens are responsible for infective peripheral neuropathies and this is generally the result of the indirect effects of pathogen infection, namely anti pathogen antibodies cross reacting with epitopes on peripheral nerve, auto reactive T cells attacking myelin, circulating immune complexes and complement fixation. Primary infection of Schwann cells (SC) associated with peripheral nerve inflammation is rare requiring pathogens to cross the Blood Peripheral Nerve Barrier (BPNB) evade anti-pathogen innate immune pathways and invade the SC. Spirochetes Borrelia bourgdorferi and Trepomema pallidum are highly invasive, express surface lipo proteins, but despite this SC are rarely infected. However, Trypanosoma cruzi (Chaga's disease) and Mycobacterium leprae. Leprosy are two important causes of peripheral nerve infection and both demonstrate primary infection of SC. This is due to two novel strategies; T. cruzi express a trans-silalidase that mimics host neurotrophic factors and infects SC via tyrosine kinase receptors. M. leprae demonstrates multi receptor SC tropism and subsequent infection promotes nuclear reprogramming and dedifferentiation of host SC into progenitor stem like cells (pSLC) that are vulnerable to M. leprae infection. These two novel pathogen evasion strategies, involving stem cells and receptor mimicry, provide potential therapeutic targets relevant to the prevention of peripheral nerve inflammation by inhibiting primary SC infection.

Pathogenesis of virus-induced demyelination

Demyelination is a component of several viral diseases of humans. The best known of these are subacute sclerosing panencephalitis (SSPE) and progressive multifocal leukoencephalopathy (PML). There are a number of naturally occurring virus infections of animals that involve demyelination and many of these serve as instructive models for human demyelinating diseases. In addition to the naturally occurring diseases, many viruses have been shown to be capable of producing demyelination in experimental situations. In discussing virus-associated demyelinating disease, the chapter reviews the architecture and functional organization of the CNS and considers what is known of the interaction of viruses with CNS cells. It also discusses the immunology of the CNS that differs in several important aspects from that of the rest of the body. Experimental models of viral-induced demyelination have also been considered. Viruses capable of producing demyelinating disease have no common taxonomic features; they include both DNA and RNA viruses, enveloped and nonenveloped viruses. The chapter attempts to summarize the important factors influencing viral demyelination, their common features, and possible mechanisms.

Spread of herpes simplex virus type-1 (Miyama +GC strain) to the central nervous system after intraperitoneal inoculation: the role of the myenteric plexus of the gut

The pathways taken by the HSV-1 virus after intraperitoneal (i.p.) inoculation were studied in 5-week old male C3H/HeN mice injected with 1 x 10(4) PFU (100 LD50) or 5 x 10(5) PFU (5000 LD50) of HSV-1 (Miyama +GC strain). At the higher dosage (5 x 10(5) PFU), HSV-1 began replicating in the adrenal from the first day, then in the gut and thoracic portion of the spinal cord by the third day, and in the brainstem by the fourth day, as shown by the titers of the virus in these organs. By immunoperoxidase staining HSV-1 was localized in a necrotic area of the adrenal, the myenteric plexus of the gut, the intermediolateral columns of the thoracic cord, and the vagus nerve nuclei of the medulla oblongata. In the low dose mice (1 x 10(4) PFU), HSV-1 was not isolated from the adrenal or thoracic segment of the spinal cord from the time of inoculation until the time of death. It was, however, isolated from the gut on days 4-6 and from the brainstem by day 5. HSV-1 was never isolated from the blood of either group at any time. The localizations of viral replication suggest that in the mice inoculated with 1 x 10(4) PFU, HSV-1 spreads to the brainstem via the vagal nerves after replication in the myenteric plexus of the gut. In the mice given the higher dose, localizations suggest not only the above route, but also that the virus spread to the intermediolateral columns of the spinal cord after replicating in the adrenal.

Early adrenal infection by herpes simplex virus type-1 (Miyama + GC strain): special reference to inoculation dose and spread from the adrenal to the central nervous system

Male C3H/HeN mice, aged 5 weeks, were inoculated intraperitoneally (i.p.) with different doses (1 x 10(3), 1 x 10(5), 5 x 10(5), 1 x 10(6) pfu) of the herpes simplex virus type-1 (HSV-1) (Miyama + GC strain). The LD50 of this virus was 10(2) pfu (i.p.) per mouse. All the mice in each group died 12 days after inoculation. Adrenal necrosis was found to be dose-dependent, the threshold dose being 5 x 10(5) pfu. In addition, encephalitis and inflammatory cell infiltration in abdominal ganglia appeared in 3-4 days after inoculation. By the plaque method, HSV-1 was detected first in the adrenal glands, then in neurons in the spinal cord and the brain. These findings suggest that in mice inoculated with doses of virus sufficient to infect the adrenal gland, HSV-1 spreads to the central nervous system through peripheral nerves after replication in the adrenal.

Herpes simplex encephalitis. Immunohistological demonstration of spread of virus via olfactory pathways in mice

Six-week-old Balb/c mice were inoculated intranasally with a suspension of HSV1 virus and the distribution of viral antigen in the brain 3-7 days later was surveyed using the immunoperoxidase technique. Virus was first detectable in the brain 4 days later at 2 distinct sites: the trigeminal root entry zone in the brain stem and the olfactory bulbs. On succeeding days virus spread from the trigeminal focus to many other brain stem nuclei and, in some mice, to the thalamus and the cerebellum. From the olfactory bulbs, in a proportion of mice, virus spread to anterior olfactory nucleus, lateral olfactory tract, septal nuclei, temporal lobe, hippocampus and cingulate cortex. Infection of olfactory bulbs was found to occur following intracorneal as well as intranasal inoculation of virus. The relevance of this model to human herpes simplex encephalitis is discussed.

Histological study of the progression of herpes simplex virus in mice

The progress of an experimental infection with Herpesvirus hominis type 1 was studied in newborn mice inoculated into the foot pad of the hind leg. To trace the viral antigen, the unlabeled antibody enzyme PAP (peroxidase/antiperoxidase) method was employed. The virus antigen appeared first in the epidermal and connective tissue cells of the inoculation site, and then progressed along the sciatic nerve. This nerve was studied by electron microscopy and showed active multiplication within the Schwann cells, with the production of virions, some of which were found in the intercellular spaces. No intra-axonal particles were observed. The infection then spread to the spinal ganglia and to the spinal cord. In this progression, the pia mater appeared to play an important role. From the spinal cord, the infection spread to the encephalon. The present study supports a mixed route for the neural transport of herpes simplex virus: a) by cell-to-cell transmission (Schwann and connective tissue cells in the sciatic nerve; meningeal cells, neurons and glial cells in the CNS); b) by a passive motion of the virions along the intercellular spaces. The inoculated virus also gave rise to viremia with viral multiplication in several viscera.

Progression of herpes encephalitis in rabbits following the intra-ocular injection of type I virus

The injection of HSV type I into the vitreous body of the eye in the 18-day-old albino rabbits consistently induces herpes encephalitis with 90% survival. The longterm observations up to 64 days post-inculation indicate that HSV travels slowly by cell-to-cell infection of neuroglia. The lesions follow a defined anatomical pathway producing a progressive disease not dissimilar to the natural human disease. Semiserial (1 micron) sections show that as the infection progresses tissue repair follows. It appears that HSV does not become latent, repeated episodes of viral activity cause further damage with which the repair does not keep pace. By the sixteenth day virus particles and lesions are found in contralateral side of the chiasma, lateral geniculate body and both optic nerves, but not in the ipsilateral geniculate body. This suggests that the virus does not follow the axonal route.

The pathogenesis and pathway into the central nervous system after intraocular infection of herpes simplex virus type I in rabbits

Herpes simplex virus (HSV) type I was injected into the right eye of 18-day-old New Zealand albino rabbits and the animals were killed on the fourth and eighth days after inoculation. Longitudinal section of the optic nerves and chiasma showed that both myelinated axons and neuroglial cells crossed at the chiasma. Semi-serial (1 micrometer) and ultrathin sections showed the presence of HSV in both astrocytes and oligodendrocytes, although no particles were seen in the myelinated axons; the infected cells were confined to the medial side of the right optic nerve. HSV travels centropetally along the optic pathway and slowly spreads laterally by cell-to-cell infection. The virus does not appear to kill the astrocytes and oligodendrocytes, and also does not directly damage the myelin sheath.

Ultrastructure and function in sympathetic ganglia isolated from rats infected with pseudorabies virus

(1) After inoculation of the pseudorabies virus in the anterior chamber of the eye of the rat, virions can be found only in the neurons of the superior cervical sympathetic ganglion and in the sensory ganglion of the fifth nerve on the inoculated side. Other nervous structures--central or peripheral--are not infected. (2) It is shown that the retrograde axonal flow carries the virus from the eye to the sympathetic neurons. (3) The ultrastructure of the infected neuron has been studied at various intervals after inoculation and at different stages of the viral replication. (4) Excised infected ganglia in vitro show a spontaneous electrophysiological activity that can be recorded on both the post- and preganglionic nerve. Such an activity has never been seen in normal excised ganglion of rat. (5) The shape and frequency of the electrophysiological discharges recorded on the postganglionic nerve have been analyzed at various intervals after inoculation. (6) Correlations established between the ultrastructure, the effect of various drugs and the electrophysiological activity permit the proposal of various hypothesis about the abnormal activity of the infected neurons.

Retrograde axonal transport of horseradish peroxidase from cornea to trigeminal ganglion

Horseradish peroxidase (HRP) was dripped on the scarified left cornea of adult mice. Twenty-four hours later the animals were fixed by vascular perfusion and frozen sections cut from both trigeminal ganglia. After incubation for peroxidase activity labelled nerve cells were restricted to the medial ophthalmic part of the ganglion ipsilateral to HRP administration. If the scarification was omitted no neuronal labelling was observed. This labelling of the neurons is most probably the result from axonal uptake and subsequent retrograde axonal transport of the tracer. The similarity in distribution of peroxidase labelled nerve cells and the first ganglionic lesions occurring after instillation of herpes simplex virus in the cornea is pointed out.

Activation of latent herpesvirus hominis in explants of rabbit trigeminal ganglia: the influence of immune serum

More than fifty albino rabbits were inoculated into the right scarified cornea with 10(7) PFU of the Kupka strain of human herpes virus type 1 (HHV-1). At intervals ranging from 4--280 days post infection (p.i.), both gasserian ganglia, both trigeminal nerve trunks and pieces from brain stem and from both corneas were explanted. Activation of the latent HHV-1 was found mainly in the homolateral ganglion tissue, but also in explants originating from the opposite ganglia. Within 24--72 hours, prior to the release of virus into the medium, one infectious unit of HHV was recovered from 10(4)--10(5) cells of the ganglion explant. In addition, a few neurons and satellite cells revealed the presence of virus-specific antigens when the explants were examined by immunofluorescence in serial sections. If the gangia were explanted in the presence of immune serum, the virus recovery rate was at least twice lower as compared to the virus activation in explants kept in the absence of immune serum.

Vascular and neuroglial changes in experimental herpes simplex encephalitis: ultrastructural study

Generalized vascular changes and diffused proliferation of reactive microglia were observed in an experimental model of HSV encephalitis of mice. The wide spread of these changes contrasted with the localized character of virus replication and the confined areas of damaged nervous tissue. The vascular and microglial changes were precocious in animals inoculated with concentrated virus suspension (10(5.5)LD50) while they appeared late in mice inoculated with diluted virus suspension (100 LD50). After inoculation with U.V. inactivated virus no changes were seen. The results obtained in this study suggest that the vascular and microglial modifications are not related to a direct cytopathic effect of the virus but dependent on the amount of virus present in the central nervous system and linked to the virus DNA.

Herpes genitalis in guinea-pigs. I. Kinetic study in infection with Herpesvirus hominis type 2

The kinetics of virus replication after vaginal infection of guinea-pigs with HVH 2/Angelotti were studied in relation to the appearance of local and general symptoms. Most virus was isolated from the genital tract 24-48 hours post infection. Virus was first isolated from the spinal cord 48-72 hours post infection. Penetration into the brain only occurred occasionally, and later. Under the experimental conditions employed, no virus was found in the blood, spleen, kidneys, adrenals or inguinal lymph nodes. The local symptoms (typical genital herpes) and the general symptoms (paralysis and death) started after maximum virus replication had been reached and seemed to be a consequence of neural, rather than of haematogenic or lymphogenic spread.

Herpes genitalis in guinea-pigs. II. Morphological studies in female guinea-pigs infected with Herpesvirus hominis type 2

Gross and microscopial morphological changes developing in female guinea-pigs after vaginal infection with HVH 2/Angelotti were studied. In the mucosa of the external genital tract there were inflammatory changes with formation of intra-epithelial vesicles, erosions and ulcerations. In the late stages of the infection signs of inflammatory dysplasia were also observed. The infection spread into the nervous system and produced characteristic inflammatory changes. The inflammation began as a bilateral posterior myelitis and ascended in the course of infection through the upper spinal-cord towards the brain-stem. The morphological changes were preceded by increased virus replication in the respective tissues and were correlated in time with clinical symptoms. The morphological changes seen at the site of inoculation in the external genital tract of the guinea-pig bore a certain resemblance to those seen in some cases of human infection with the same type of virus.

Viral disease of the labyrinth. II. An experimental model using mouse cytomegalovirus

Investigation of the routes of infection of viral labyrinthitis was undertaken using mouse cytomegalovirus (CMV) and CMV-free Swiss-Webster mice. Intraperitoneal and intranasal inoculation caused no CMV infection of the ears or central nervous system. Intraperitoneal inoculation of pregnant mice yielded no evidence of fetal infection. Intracerebral inoculation of newborn mice resulted in viral infection of the ear by extension: 1) from the arachnoid into cochlear periotic connective tissue via the cochlear aqueduct and 2) along the perineurium of the acoustic nerve into the modiolus. There were neither changes in the stria vascularis nor endolymphatic labyrinth in spite of vasculitis and viremia. This experimental labyrinthitis resembles that found in human temporal bones infected with herpes zoster rather than the endolymphatic labyrinthitis seen in rubella, rubeola, mumps, and cytomegalovirus. The chronic effects of the mouse CMV labyrinthitis are under study.

Pathogenesis of herpetic neuritis and ganglionitis in mice: evidence for intra-axonal transport of infection

The pathogenesis of acute herpetic infection in the nervous system has been studied following rear footpad inoculation of mice. Viral assays performed on appropriate tissues at various time intervals indicated that the infection progressed sequentially from peripheral to the central nervous system, with infectious virus reaching the sacrosciatic spinal ganglia in 20 to 24 hr. The infection also progressed to ganglia in mice given high levels of anti-viral antibody. Immunofluorescent techniques demonstrated that both neurons and supporting cells produced virus-specific antigens. By electron microscopy, neurons were found to produce morphologically complete virions, but supporting cells replicated principally nucleocapsids. These results are discussed in the context of possible mechanisms by which herpes simplex virus might travel in nerve trunks. They are considered to offer strong support for centripetal transport in axons.

Immunosuppression and experimental virus infection of the nervous system

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.

Rabiesvirus neuronitis

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