The immune system response to viral infection of the CNS

Protective Humoral Immunity in the Central Nervous System Requires Peripheral CD19-Dependent Germinal Center Formation following Coronavirus Encephalomyelitis

B cell subsets with phenotypes characteristic of naive, non-isotype-switched, memory (B_mem) cells and antibody-secreting cells (ASC) accumulate in various models of central nervous system (CNS) inflammation, including viral encephalomyelitis. During neurotropic coronavirus JHMV infection, infiltration of protective ASC occurs after T cell-mediated viral control and is preceded by accumulation of non-isotype-switched IgD⁺ and IgM⁺ B cells. However, the contribution of peripheral activation events in cervical lymph nodes (CLN) to driving humoral immune responses in the infected CNS is poorly defined. CD19, a signaling component of the B cell receptor complex, is one of multiple regulators driving B cell differentiation and germinal center (GC) formation by lowering the threshold of antigen-driven activation. JHMV-infected CD19^-/- mice were thus used to determine how CD19 affects CNS recruitment of B cell subsets. Early polyclonal ASC expansion, GC formation, and virus-specific ASC were all significantly impaired in CLN of CD19^-/- mice compared to wild-type (WT) mice, consistent with lower and unsustained virus-specific serum antibody (Ab). ASC were also significantly reduced in the CNS, resulting in increased infectious virus during persistence. Nevertheless, CD19 deficiency did not affect early CNS IgD⁺ B cell accumulation. The results support the notion that CD19-independent factors drive early B cell mobilization and recruitment to the infected CNS, while delayed accumulation of virus-specific, isotype-switched ASC requires CD19-dependent GC formation in CLN. CD19 is thus essential for both sustained serum Ab and protective local Ab within the CNS following JHMV encephalomyelitis.IMPORTANCE CD19 activation is known to promote GC formation and to sustain serum Ab responses following antigen immunization and viral infections. However, the contribution of CD19 in the context of CNS infections has not been evaluated. This study demonstrates that antiviral protective ASC in the CNS are dependent on CD19 activation and peripheral GC formation, while accumulation of early-recruited IgD⁺ B cells is CD19 independent. This indicates that IgD⁺ B cells commonly found early in the CNS do not give rise to local ASC differentiation and that only antigen-primed, peripheral GC-derived ASC infiltrate the CNS, thereby limiting potentially harmful nonspecific Ab secretion. Expanding our understanding of activation signals driving CNS migration of distinct B cell subsets during neuroinflammatory insults is critical for preventing and managing acute encephalitic infections, as well as preempting reactivation of persistent viruses during immune-suppressive therapies targeting B cells in multiple sclerosis (MS), such as rituximab and ocrelizumab.

Protection of CD3 delta knockout mice from lymphocytic choriomeningitis virus-induced immunopathology: implications for viral neuroinvasion

For a virus to establish a neuronal infection, it must spread from its primary site of infection to the central nervous system (CNS) before immune-mediated clearance occurs. Lymphocytic choriomeningitis virus (LCMV) is a murine pathogen that can result in persistent neuronal infection in newborn mice and in adults that lack CD8(+) T cells. To determine the neuroinvasive capacity of LCMV in the presence of an existent, but compromised, cytotoxic T lymphocyte response, the course of LCMV infection was examined in mice that possess 10% of the normal complement of T lymphocytes, due to the lack of the CD3 delta (delta) subunit of the T cell receptor complex (CD3 delta KO mice). Unlike immunocompetent mice that produced a massive immune response that caused death by 6-7 days postinfection, CD3 delta KO mice mounted a weak response and survived. The presence of viral antigen gradually shifted from the class I MHC-positive meninges and ependyma to class I MHC-deficient CNS neurons 10-30 days postinoculation. The infected CD3 delta KO mice developed a delayed T cell response that suppressed virus replication in peripheral tissues but not in the CNS; subsequent adoptive transfer experiments supported the hypothesis that the lack of clearance from neurons was due to sequestration of LCMV in an immune-privileged cell type. Based on these results, we propose that a critical parameter in the pathogenesis of neurotropic viruses is the rate of immune activation; individuals with impaired T cell responses may be more vulnerable to persisting CNS infections.

Antibody Prevents Virus Reactivation Within the Central Nervous System

The neurotropic JHM strain of mouse hepatitis virus (JHMV) produces an acute CNS infection characterized by encephalomyelitis and demyelination. The immune response cannot completely eliminate virus, resulting in persistence associated with chronic ongoing CNS demyelination. The contribution of humoral immunity to viral clearance and persistent infection was investigated in mice homozygous for disruption of the Ig μ gene (IgM−/−). Acute disease developed with equal kinetics and severity in IgM−/− and syngeneic C57BL/6 (wt) mice. However, clinical disease progressed in IgM−/− mice, while wt mice recovered. Viral clearance during acute infection was similar in both groups, supporting a primary role of cell-mediated immunity in viral clearance. In contrast to wt mice, in which infectious virus was reduced to below detection following acute infection, increasing infectious virus was recovered from the CNS of the IgM−/− mice following initial clearance. No evidence was obtained for selection of variant viruses nor was there an apparent loss of cell-mediated immunity in the absence of Ab. Passive transfer of anti-JHMV Ab following initial clearance prevented reactivation of infectious virus within the CNS of IgM−/− mice. These data demonstrate the clearance of infectious virus during acute disease by cell-mediated immunity. However, immunologic control is not maintained in the absence of anti-viral Ab, resulting in recrudescence of infectious virus. These data suggest that humoral immunity plays no role in controlling virus during acute infection, but plays an important role in establishing and maintaining CNS viral persistence.

IFN-γ Is Required for Viral Clearance from Central Nervous System Oligodendroglia

Infection of the central nervous system (CNS) by the JHM strain of mouse hepatitis virus (JHMV) is a rodent model of the human demyelinating disease multiple sclerosis. The inability of effective host immune responses to eliminate virus from the CNS results in a chronic infection associated with ongoing recurrent demyelination. JHMV infects a variety of CNS cell types during the acute phase of infection including ependymal cells, astrocytes, microglia, oligodendroglia, and rarely in neurons. Replication within the majority of CNS cell types is controlled by perforin-dependent virus-specific CTL. However, inhibition of viral replication in oligodendroglia occurs via a perforin-independent mechanism(s). The potential role for IFN-γ as mediator controlling JHMV replication in oligodendroglia was examined in mice deficient in IFN-γ secretion (IFN-γ0/0 mice). IFN-γ0/0 mice exhibited increased clinical symptoms and mortality associated with persistent virus, demonstrating an inability to control replication. Neither antiviral Ab nor CTL responses were diminished in the absence of IFN-γ, although increased IgG1 was detected in IFN-γ0/0 mice. Increased virus Ag in the absence of IFN-γ localized almost exclusively to oligodendroglia and was associated with increased CD8+ T cells localized within white matter. These data suggest that although perforin-dependent CTL control virus replication within astrocytes and microglia, which constitute the majority of infected CNS cells, IFN-γ is critical for control of viral replication in oligodendroglia. Therefore, different mechanisms are used by the host defenses to control virus replication within the CNS, dependent upon the phenotype of the targets of virus replication.

Identification of CD4+ and CD8+ T cell subsets and B cells in the brain of dogs with spontaneous acute, subacute-, and chronic-demyelinating distemper encephalitis

CD4 and CD8 antigen expression of T cells as well as B cell and canine distemper virus (CDV) antigen distribution were immunohistologically examined in the cerebellum of dogs with spontaneous distemper encephalitis. Cellular and viral antigen expression were evaluated at intralesional and extralesional sites and in the perivascular space. Histologically, acute and subacute non-inflammatory encephalitis and subacute inflammatory and chronic plaques were distinguished. Demyelination was a feature of all subacute and chronic lesions, although the majority of plaques exhibited no or only a low level of active demyelination as demonstrated by single macrophages with luxol fast blue positive material in their cytoplasm. CDV antigen expression, observed in all distemper brains, was reduced in chronic plaques. CD4+, CD8+, and B cells were absent in controls and in some brains with acute encephalitis. A mild infiltration of CD8+ cells was noticed in the neuropil of the remaining brains with acute and all brains with subacute non-inflammatory encephalitis. Single CD4+ cells were found in two brains with acute and in all brains with subacute non-inflammatory encephalitis. Numerous CD8+ and CD4+ cells and few B cells, with a preponderance of CD8+ cells, were detected in subacute inflammatory and chronic lesions. In contrast, in perivascular infiltrates (PVI) of subacute and chronic lesions a dominance of CD4+ cells was detected. The dominating CD8+ cells in acute and subacute non-inflammatory encephalitis might be involved in viral clearance or contribute as antibody-independent cytotoxic T cells to early lesion development. In subacute inflammatory and chronic lesions CD8+ cells may function as cytotoxic effector cells and CD4+ cells by initiating a delayed-type hypersensitivity reaction. The simultaneous occurrence of perivascular B and CD4+ cells indicated that an antibody-mediated cytotoxicity could synergistically enhance demyelination. Summarized, temporal and spatial distribution of CD4+, CD8+ and B cells and virus antigen in early and late lesions support the hypothesis of a heterogeneous in part immune-mediated plaque pathogenesis in distemper demyelination.

Study of the persistence of Aujeszky's disease (pseudorabies) virus in peripheral blood mononuclear cells and tissues of experimentally infected pigs

The presence of Aujeszky's disease virus (ADV) in peripheral blood mononuclear cells (PBMC) and tissues of experimentally infected pigs was studied. Vaccinated and unvaccinated pigs were inoculated with different doses of Aujeszky's disease NIA-3 strain. Pigs were periodically bled and PBMC were used for virus isolation and PCR detection of virus. Tissues were obtained at the time of death (8 weeks post-inoculation) and used for ADV genome detection by PCR. ADV genome was amplified from PBMC during the acute phase of infection and, in some experimental groups, up to 38 days post-inoculation (PI). The virus was sporadically detected by virus isolation performed from PBMC. In neural tissues, ADV was constantly amplified from the trigeminal ganglia and the olfactory bulb of persistently infected pigs (euthanized 8 weeks PI). In other tissues, the viral genome was rarely detected in lymph nodes and tonsils, and occasionally, in the bone marrow. Our results indicated that PBMC are not an appropriate source for detecting ADV persistence, since inconsistent results were obtained throughout the experiments. In neural tissues, the olfactory bulb turned out to be as important a target for ADV persistence as the trigeminal ganglia. Viral genome detection in the bone marrow indicated that this tissue may play a role in the establishment of a persistent infection.

Collaboration of antibody and inflammation in clearance of rabies virus from the central nervous system

To investigate the involvement of various cellular and humoral aspects of immunity in the clearance of rabies virus from the central nervous system, (CNS), we studied the development of clinical signs and virus clearance from the CNS in knockout mice lacking either B and T cells, CD8+ cytotoxic T cells, B cells, alpha/beta interferon (IFN-alpha/beta) receptors, IFN-gamma receptors, or complement components C3 and C4. Following intranasal infection with the attenuated rabies virus CVS-F3, normal adult mice of different genetic backgrounds developed a transient disease characterized by loss of body weight and appetite depression which peaked at 13 days postinfection (p.i.). While these animals had completely recovered by day 21 p.i., mice lacking either B and T cells or B cells alone developed a progressive disease and succumbed to infection. Mice lacking either CD8+ T cells, IFN receptors, or complement components C3 and C4 showed no significant differences in the development of clinical signs by comparison with intact counterparts having the same genetic background. However, while infectious virus and viral RNA could be detected in normal control mice only until day 8 p.i., in all of the gene knockout mice studied except those lacking C3 and C4, virus infection persisted through day 21 p.i. Analysis of rabies virus-specific antibody production together with histological assessment of brain inflammation in infected animals revealed that clearance of CVS-F3 by 21 days p.i. correlated with both a strong inflammatory response in the CNS early in the infection (day 8 p.i.), and the rapid (day 10 p.i.) production of significant levels of virus-neutralizing antibody (VNA). These studies confirm that rabies VNA is an absolute requirement for clearance of an established rabies virus infection. However, for the latter to occur in a timely fashion, collaboration between VNA and inflammatory mechanisms is necessary.

Interferon regulatory factor element and interferon regulatory factor 1 in the induction of major histocompatibility complex class I genes in neural cells

The role of the MHC-IRF-E and interferon regulatory factor 1 (IRF-1) in the regulation of MHC class I genes in astrocytes was analyzed. Transcriptional activation of MHC class I genes after treatment of astrocytes with various inducers occurred over a period of hours and correlated with cell surface expression. Functional analysis of the MHC class I gene promoter region confirmed that induction was controlled by a restricted region of 88 base pairs containing two well-defined inducible enhancers, the MHC-CRE and the MHC-IRF-E. Further analysis showed that potential MHC-CRE enhancer activity was silent. Therefore, the MHC-IRF-E, rather than the MHC-CRE, appeared responsible for enhancement of the MHC class I gene and was supported by three findings: (1) site-directed mutation of the MHC-IRF-E-abrogated induction, (2) promoter constructs containing IRF-Es as the sole enhancers were highly inducible in astrocytes, and (3) the expression of transcription factor IRF-1, which acts through the MHC-IRF-E to induce MHC class I genes, was induced to high levels in parallel with that of MHC class I induction. The induction of the IRF-1 gene correlated with the prior induction of the gamma-activated factor (GAF) or NF-kappa B, depending on the inducer, indicating that both gamma activation sites (GAS) and kappa B sites in the IRF-1 promoter are important.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuron-specific regulation of major histocompatibility complex class I, interferon-beta, and anti-viral state genes

The regulation of major histocompatibility complex (MHC) class I, interferon (IFN)-beta, and anti-viral state expression in neurons was analyzed. Treatment of neurons with either double-stranded RNA (poly I: poly C) or virus, but not IFNs, induced high levels of IFN-beta, but not MHC class I genes. However, neurons treated with IFN-beta established an anti-viral state. Transfection of neurons with IFN-beta constructs showed that a region containing PRDI (IRF-E site) and PRDII (kappa B site) mediated induction, but closely related sites in a MHC class I construct did not. Gel mobility shift assays indicated that transcription factors containing the RelA (p65) component of NF-kappa B, but not p50, bound to PRDII. PRDI, however, bound to transcriptional antagonist IRF-2. Unique selective induction of these transcription factors is likely to mediate non-coordinate expression of IFN-beta, MHC class I, and anti-viral state genes in neurons.

Penetration of solutes, viruses, and cells across the blood-brain barrier

The aspects presented here of how solutes, viruses and cells are able to cross the BBB indicate that there must be an active interaction of endothelium with viruses and immune system cells before they can penetrate the brain and spinal cord. The axoplasmic pathway taken by lectin-solute conjugates is similar but not identical to that followed by viral particles during their retrograde or anterograde transit through the axoplasm. Both the conjugates and virus are transferred to other neurons transsynaptically but the receptor mediated transfer utilized by viruses is far more specific. Cranial nerves are involved in both the entry and egress of antigens into and out of the brain. Antigen, generated within the CNS, may be able to escape from the brain to lymphoid tissue by passing into the fluid around a cranial nerve, thence via the lymph into lymph nodes to initiate an immune response involving the CNS.

Cervical lymphoid tissue but not the central nervous system supports proliferation of virus-specific T lymphocytes during coronavirus-induced encephalitis in rats

The CD4+ T lymphocyte response in the central nervous system (CNS) and cervical lymph nodes (CLNs) of rats with different susceptibility to coronavirus-induced encephalitis was investigated. The majority of CD4+ T lymphocytes entering the virus-infected CNS in the course of the infection are primed cells that neither proliferate ex vivo nor can be stimulated to proliferation by viral antigens or mitogen in vitro. In contrast, T lymphocytes taken from CLNs of the same animals revealed a strong proliferative response. Restimulation of CLN lymphocytes by viral antigens disclosed a striking difference between the disease-resistant rat strain Brown Norway (BN) and the susceptible Lewis (LEW) strain. Whereas BN lymphocytes responded as early as 5 days post infection, it took more than 11 days until a comparable proliferation was detectable in LEW lymphocytes. From these data we postulate that the majority of T lymphocytes entering the virus-infected brain after sensitisation and expansion in cervical lymph nodes is unresponsive to further proliferation signals and that the kinetics and magnitude of T lymphocyte stimulation in CLNs play an important role in the clinical course of the infection.

Antibody-mediated clearance of viruses from the mammalian central nervous system

The novel role of antibody in clearing virus from the central nervous system without the help of other immune effectors is an important phenomenon that has only recently been documented. Possible routes for antibodies across the blood-brain barrier and how they work in the CNS are discussed here.

Augmentation of major histocompatibility complex class I and ICAM-1 expression on glial cells following measles virus infection: evidence for the role of type-1 interferon

An intracellular staining procedure for the cytoskeletal marker, glial fibrillary acidic protein of astrocytes, has been developed which allows flow cytometric phenotyping of astrocytes within complex mixtures of glial cells. Employing this technique, we show here that measles virus infection of rat mixed glial cell cultures results in a rapid augmentation of major histocompatibility complex (MHC) class I and ICAM-1 on the majority of astrocytes in culture. MHC class I levels are increased on macrophages/microglia but ICAM-1 expression is not normally affected on this cell type. Some MHC class II induction is also observed after virus infection but only on astrocytes. A type-I interferon (IFN)-inducible protein, Mx, was identified in cultured glial cells after infection. Qualitatively comparable MHC class I and ICAM-1 enhancement after addition of type-I IFN, supports the conclusion that this cytokine(s) released as a result of virus infection, is responsible for alterations in the expression of molecules on glial cells, that are involved in T cell recognition. Astrocytes after viral infection were more susceptible to alloantigen-specific cytotoxic T lymphocytes and cytotoxic T lymphocyte activity was substantially reduced in the presence of mAb specific for MHC class I, ICAM-1 and LFA-1 but not MHC class II. The relevance of these findings to T cell recognition of virus-infected cells in the central nervous system is discussed.

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.