Semliki Forest virus infection leads to increased expression of adhesion molecules on splenic T-cells and on brain vascular endothelium

T cell responses in the central nervous system

T cells are required for immune surveillance of the central nervous system (CNS); however, they can also induce severe immunopathology in the context of both viral infections and autoimmunity. The mechanisms that are involved in the priming and recruitment of T cells to the CNS are only partially understood, but there has been renewed interest in this topic since the 'rediscovery' of lymphatic drainage from the CNS. Moreover, tissue-resident memory T cells have been detected in the CNS and are increasingly recognized as an autonomous line of host defence. In this Review, we highlight the main mechanisms that are involved in the priming and CNS recruitment of CD4⁺ T cells, CD8⁺ T cells and regulatory T cells. We also consider the plasticity of T cell responses in the CNS, with a focus on viral infection and autoimmunity.

Antibody-enhanced dengue disease generates a marked CNS inflammatory response in the black-tufted marmoset Callithrix penicillata

Severe dengue disease is often associated with long-term neurological impairments, but it is unclear what mechanisms are associated with neurological sequelae. Previously, we demonstrated antibody-enhanced dengue disease (ADE) dengue in an immunocompetent mouse model with a dengue virus 2 (DENV2) antibody injection followed by DENV3 virus infection. Here we migrated this ADE model to Callithrix penicillata. To mimic human multiple infections of endemic zones where abundant vectors and multiple serotypes co-exist, three animals received weekly subcutaneous injections of DENV3 (genotype III)-infected supernatant of C6/36 cell cultures, followed 24 h later by anti-DENV2 antibody for 12 weeks. There were six control animals, two of which received weekly anti-DENV2 antibodies, and four further animals received no injections. After multiple infections, brain, liver, and spleen samples were collected and tissue was immunolabeled for DENV3 antigens, ionized calcium binding adapter molecule 1, Ki-67, TNFα. There were marked morphological changes in the microglial population of ADE monkeys characterized by more highly ramified microglial processes, higher numbers of trees and larger surface areas. These changes were associated with intense TNFα-positive immunolabeling. It is unclear why ADE should generate such microglial activation given that IgG does not cross the blood-brain barrier, but this study reveals that in ADE dengue therapy targeting the CNS host response is likely to be important.

Sleep and behavior during vesicular stomatitis virus induced encephalitis in BALB/cJ and C57BL/6J mice

Intranasal application of vesicular stomatitis virus (VSV) produces a well-characterized model of viral encephalitis in mice. Within one day post-infection (PI), VSV travels to the olfactory bulb and, over the course of 7 days, it infects regions and tracts extending into the brainstem followed by clearance and recovery in most mice by PI day 14 (PI 14). Infectious diseases are commonly accompanied by excessive sleepiness; thus, sleep is considered a component of the acute phase response to infection. In this project, we studied the relationship between sleep and VSV infection using C57BL/6 (B6) and BALB/c mice. Mice were implanted with transmitters for recording EEG, activity and temperature by telemetry. After uninterrupted baseline recordings were collected for 2 days, each animal was infected intranasally with a single low dose of VSV (5×10(4) PFU). Sleep was recorded for 15 consecutive days and analyzed on PI 0, 1, 3, 5, 7, 10, and 14. Compared to baseline, amounts of non-rapid eye movement sleep (NREM) were increased in B6 mice during the dark period of PI 1-5, whereas rapid eye movement sleep (REM) was significantly reduced during the light periods of PI 0-14. In contrast, BALB/c mice showed significantly fewer changes in NREM and REM. These data demonstrate sleep architecture is differentially altered in these mouse strains and suggests that, in B6 mice, VSV can alter sleep before virus progresses into brain regions that control sleep.

Role of adhesion molecules and inflammation in Venezuelan equine encephalitis virus infected mouse brain

Background

Neuroinvasion of Venezuelan equine encephalitis virus (VEEV) and subsequent initiation of inflammation in the brain plays a crucial role in the outcome of VEEV infection in mice. Adhesion molecules expressed on microvascular endothelial cells in the brain have been implicated in the modulation of the blood brain barrier (BBB) and inflammation in brain but their role in VEEV pathogenesis is not very well understood. In this study, we evaluated the expression of extracellular matrix and adhesion molecules genes in the brain of VEEV infected mice.

Findings

Several cell to cell adhesion molecules and extracellular matrix protein genes such as ICAM-1, VCAM-1, CD44, Cadherins, integrins, MMPs and Timp1 were differentially regulated post-VEEV infection. ICAM-1 knock-out (IKO) mice infected with VEEV had markedly reduced inflammation in the brain and demonstrated a delay in the onset of clinical symptoms of disease. A differential regulation of inflammatory genes was observed in the IKO mice brain compared to their WT counterparts.

Conclusions

These results improve our present understanding of VEEV induced inflammation in mouse brain.

Peripheral dendritic cells are essential for both the innate and adaptive antiviral immune responses in the central nervous system

Intranasal application of vesicular stomatitis virus (VSV) causes acute infection of the central nervous system (CNS). However, VSV encephalitis is not invariably fatal, suggesting that the CNS may contain a professional antigen-presenting cell (APC) capable of inducing or propagating a protective antiviral immune response. To examine this possibility, we first characterized the cellular elements that infiltrate the brain as well as the activation status of resident microglia in the brains of normal and transgenic mice acutely ablated of peripheral dendritic cells (DCs) in vivo. VSV encephalitis was characterized by a pronounced infiltrate of myeloid cells (CD45(high)CD11b(+)) and CD8(+) T cells containing a subset that was specific for the immunodominant VSV nuclear protein epitope. This T cell response correlated temporally with a rapid and sustained upregulation of MHC class I expression on microglia, whereas class II expression was markedly delayed. Ablation of peripheral DCs profoundly inhibited the inflammatory response as well as infiltration of virus-specific CD8(+) T cells. Unexpectedly, the VSV-induced interferon-gamma (IFN-gamma) response in the CNS remained intact in DC-deficient mice. Thus, both the inflammatory and certain components of the adaptive primary antiviral immune response in the CNS are dependent on peripheral DCs in vivo.

Early and late pathogenic events of newborn mice encephalitis experimentally induced by itacaiunas and curionópolis bracorhabdoviruses infection

In previous reports we proposed a new genus for Rhabdoviridae and described neurotropic preference and gross neuropathology in newborn albino Swiss mice after Curionopolis and Itacaiunas infections. In the present report a time-course study of experimental encephalitis induced by Itacaiunas and Curionopolis virus was conducted both in vivo and in vitro to investigate cellular targets and the sequence of neuroinvasion. We also investigate, after intranasal inoculation, clinical signs, histopathology and apoptosis in correlation with viral immunolabeling at different time points. Curionopolis and Itacaiunas viral antigens were first detected in the parenchyma of olfactory pathways at 2 and 3 days post-inoculation (dpi) and the first clinical signs were observed at 4 and 8 dpi, respectively. After Curionopolis infection, the mortality rate was 100% between 5 and 6 dpi, and 35% between 8 and 15 dpi after Itacaiunas infection. We identified CNS mice cell types both in vivo and in vitro and the temporal sequence of neuroanatomical olfactory areas infected by Itacaiunas and Curionopolis virus. Distinct virulences were reflected in the neuropathological changes including TUNEL immunolabeling and cytopathic effects, more intense and precocious after intracerebral or in vitro inoculations of Curionopolis than after Itacaiunas virus. In vitro studies revealed neuronal but not astrocyte or microglial cytopathic effects at 2 dpi, with monolayer destruction occurring at 5 and 7 dpi with Curionopolis and Itacaiunas virus, respectively. Ultrastructural changes included virus budding associated with interstitial and perivascular edema, endothelial hypertrophy, a reduced and/or collapsed small vessel luminal area, thickening of the capillary basement membrane, and presence of phagocytosed apoptotic bodies. Glial cells with viral budding similar to oligodendrocytes were infected with Itacaiunas virus but not with Curionopolis virus. Thus, Curionopolis and Itacaiunas viruses share many pathological and clinical features present in other rhabdoviruses but distinct virulence and glial targets in newborn albino Swiss mice brain.

T-cells in human encephalitis

Encephalitis literally means inflammation of the brain. In general, this inflammation can result from a viral or bacterial infection in the brain itself or alternatively from a secondary autoimmune reaction against an infection or a tumor in the rest of the body. Besides this, encephalitis is present in (believed autoimmune) diseases with unknown etiology, such as multiple sclerosis or Rasmussen encephalitis (RE). This article summarizes the existing data on the role of T-cells in the pathogenesis of three types of human encephalitis: RE, paraneoplastic encephalomyelitis, and virus encephalitis. In all of them, T-cells play a major role in disease pathogenesis, mainly mediated by major histocompatiblity complex class I-restricted CD8⁺ T-lymphocytes.

Neurotropism and neuropathological effects of selected rhabdoviruses on intranasally-infected newborn mice

Viral neurotropism is the ability of viruses to infect neuronal cells. This is well studied for herpesviruses, rabies-related viruses, and a few others, but it is poorly investigated among almost all arboviruses. In this study, we describe both the neurotropism and the neuropathological effects of Amazonian rhabdoviruses on the brains of experimentally infected-newborn mice. Suckling mice were intranasally infected with 10(-4) to 10(-8) LD50 of viruses. Animals were anaesthetized and perfused after they had become sick. Immunohistochemistry using specific anti-virus and anti-active caspase three antibodies was performed. All infected animals developed fatal encephalitis. Survival time ranged from 18 h to 15 days. Viruses presented distinct species-dependent neurotropism for CNS regions. Histopathological analysis revealed variable degrees of necrosis and apoptosis in different brain regions. These results showed that viruses belonging to the Rhabdoviridae family possess distinct tropism for CNS structures and induce different pattern of cell death depending on the CNS region.

Semliki forest virus infection of laboratory mice: a model to study the pathogenesis of viral encephalitis

Semliki Forest virus (SFV) infection of the laboratory mouse provides an experimental system to study the pathogenesis of viral encephalitis. Following extraneural inoculation the virus is efficiently neuroinvasive and crosses the blood-brain barrier to initiate perivascular foci of infection in neurons and oligodendrocytes. The outcome of infection ranges from clinically unapparent mild encephalitis to fatal panencephalitis. SFV infections of the developing nervous system are always highly destructive and are generally fatal. In contrast, SFV infections of the mature nervous system can result in persistent infection with no apparent cell loss. This dramatic difference is attributable to developmental changes in the interactions between virus and CNS cells. Antibody responses clear the systemic infection and control the CNS infection. CD8+ T-cells are required to generate the lesions of inflammatory demyelination which can be a feature of the neuropathology. This article reviews the pathogenesis of SFV encephalitis, describing the neuropathology and the mechanisms which underlie it and which may be fundamental to many viral encephalitides.

Virus demyelination

A number of viruses can initiate central nervous system (CNS) diseases that include demyelination as a major feature of neuropathology. In humans, the most prominent demyelinating diseases are progressive multifocal leukoencephalopathy, caused by JC papovirus destruction of oligodendrocytes, and subacute sclerosing panencephalitis, an invariably fatal childhood disease caused by persistent measles virus. The most common neurological disease of young adults in the developed world, multiple sclerosis, is also characterized by lesions of inflammatory demyelination; however, the etiology of this disease remains an enigma. A viral etiology is possible, because most demyelinating diseases of known etiology in both man and animals are viral. Understanding of the pathogenesis of virus-induced demyelination derives for the most part from the study of animal models. Studies with neurotropic strains of mouse hepatitis virus, Theiler's virus, and Semliki Forest virus have been at the forefront of this research. These models demonstrate how viruses enter the brain, spread, persist, and interact with immune responses. Common features are an ability to infect and persist in glial cells, generation of predominantly CD8(+) responses, which control and clear the early phase of virus replication but which fail to eradicate the infection, and lesions of inflammatory demyelination. In most cases demyelination is to a limited extent the result of direct virus destruction of oligodendrocytes, but for the most part is the consequence of immune and inflammatory responses. These models illustrate the roles of age and genetic susceptibility and establish the concept that persistent CNS infection can lead to the generation of CNS autoimmune responses.

Equid herpesvirus 1 infection of endothelial cells requires activation of putative adhesion molecules: an in vitro model

Antisera to activated equine endothelial cells, which detected surface molecules of 116 kD, 97 kD, 42 kD and 38 kD, were made to investigate the role of endothelial adhesion molecules in equid herpes virus 1 infection. These putative adhesion molecules could be induced by 17-beta oestradiol, chorionic gonadotrophin, or IL-2, as well as by LPS and PWM. In an in vitro flow system, using equine veins or arteries, equid herpesvirus 1 in leucocytes was only transferred to infect endothelial cells if both leucocytes and endothelial cells expressed these surface molecules. Blocking of the membrane molecules with polyclonal antibodies prevented transfer of virus to the endothelial cells, indicating that the adhesion molecules had a key role in effecting transfer of virus. These in vitro observations give particular insight into the reports that in the natural course of infection in horses infection of endothelial cells is restricted to certain tissues, and in a wider context the results illustrate the complexity of factors that may direct tissue tropism.

The role of T-cell-mediated mechanisms in virus infections of the nervous system

T lymphocytes play a decisive role in the course and clinical outcome of viral CNS infection. Summarizing the information presented in this review, the following sequence of events might occur during acute virus infection: After invasion of the host and a few initial rounds of replication, the virus reaches the CNS in most cases by hematogeneous spread. After passage through the BBB, CNS cells are infected and replication of virus in brain cells causes activation of the surrounding microglia population. Moreover, local production of IFN-alpha/beta induces expression of MHC antigens on CNS cells, and microglial cells start to phagocytose cellular debris, which accumulates as a result of virus-induced cytopathogenic effects. Upon phagocytosis, microglia becomes more activated; they up-regulate MHC molecules, acquire antigen presentation capabilities and secrete chemokines. This will initiate up-regulation of adhesion molecules on adjacent endothelial cells of the BBB. Transmigration of activated T lymphocytes through the BBB is followed by interaction with APC, presenting the appropriate peptides in the context of MHC antigens. It appears that CD8+ T lymphocytes are amongst the first mononuclear cells to arrive at the infected tissue. Without a doubt, their induction and attraction is deeply influenced by natural killer cells, which, after virus infection, secrete IFN-gamma, a cytokine that stimulates CD8+ T cells and diverts the immune response to a TH1-type CD4+ T cell-dominated response. Following the CD8+ T lymphocytes, tissue-penetrating, TH1 CD4+ T cells contact local APC. This results in a tremendous up-regulation of MHC molecules and secretion of more chemotactic and toxic substances. Consequently an increasing number of inflammatory cells, including macrophages/microglia and finally antibody-secreting plasma cells, are attracted to the site of virus infection. All trapped cells are mainly terminally differentiated cells that are going to enter apoptosis during or shortly after exerting their effector functions. The clinical consequences and the influence of the effector phase on the further course of the infection depends on the balance and fine-tuning of the contributing lymphoid cell populations. Generally, any delay in the recruitment of effector lymphocytes to the tissue or an unbalanced combination of lymphocyte subsets allows the virus to spread in the CNS, which in turn will cause severe immune-mediated tissue effects as well as disease. If either too late or partially deficient, the immune system response may contribute to a lethal outcome or cause autosensitization to brain-specific antigens by epitope spreading to the antigen-presenting system in peripheral lymphoid tissue. This could form the basis for subsequent booster reactions of autosensitized CD4+ T cells--a process that finally will end in an inflammatory autoimmune reaction, which in humans we call multiple sclerosis. In contrast, a rapid and specific local response in the brain tissue will result in efficient limitation of viral spread and thereby a subclinical immune system-mediated termination of the infection. After clearance of virus-infected cells, downsizing of the local response probably occurs via self-elimination of the contributing T cell populations and/or by so far unidentified signal pathways. However, much of this is highly speculative, and more data have to be collected to make decisive conclusions regarding this matter. Several strategies have been developed by viruses to escape T cell-mediated eradication, including interference with the MHC class I presentation pathway of the host cell or "hiding" in cells which lack MHC class I expression. This may result in life-long persistence of the virus in the brain, a state which probably is actively controlled by T lymphocytes. Under severe immunosuppression, however, reactivation of viral replication can occur, which is a lethal threat to the host.

A role for alpha4-integrin in the pathology following Semliki Forest virus infection

Migration of cells into the central nervous system (CNS) is a pivotal step in the pathogenesis of immune-mediated diseases such as multiple sclerosis (MS), experimental allergic encephalomyelitis (EAE) and virus-induced demyelinating diseases. Such migration is dependent on expression of adhesion molecules. The expression of adhesion molecules in the CNS was studied in Biozzi ABH mice infected with Semliki Forest virus (SFV) A7(74) - an important demyelinating model of MS. Expression of LFA-1alpha/CD11a, LFA-1beta/CD18 and ICAM-1/CD56 were rapidly elevated and remained high whereas MAC-1, CD44 and VCAM-1/CD106 were less widely expressed. The alpha4-integrin VLA-4/CD49d was more specifically associated with CNS lesions. To identify the importance of VLA-4, CD44, ICAM-1 and MAC-1 in the pathogenesis of SFV infection, monoclonal antibodies that block these adhesion molecules were administered in vivo during infection. Anti-VLA-4 treatment dramatically reduced the cellular infiltrates and demyelination within the CNS but did not affect the clearance of virus while antibodies to CD44, ICAM and MAC-1 antibody treatment had no effect. This study demonstrates that SFV infection induces the expression of adhesion molecules within the CNS and that VLA-4 plays an important role in the development of inflammation and demyelination in the CNS following SFV infection.

Leukocyte traffic in the central nervous system: the participants and their roles

While the central nervous system has long been considered immunologically privileged, over the past decade it has become evident that a wide variety of leukocyte types traffic through the nervous system. It is also apparent that the rules governing the trafficking of these disparate cell types are different for each. Some arrive, and probably depart, continuously as part of normal physiology. Others only appear to seek a specific antigen or in response to tissue damage. In this review the nature and function of individual cell types are discussed and our current knowledge regarding the parameters governing their entry into the CNS is examined.