Alphavirus-induced encephalomyelitis: antibody-secreting cells and viral clearance from the nervous system

Sindbis virus (SINV) infection of the central nervous system (CNS) provides a model for understanding the role of the immune response in recovery from alphavirus infection of neurons. Virus clearance occurred in three phases: clearance of infectious virus (days 3 to 7), clearance of viral RNA (days 8 to 60), and maintenance of low levels of viral RNA (>day 60). The antiviral immune response was initiated in the cervical lymph nodes with rapid extrafollicular production of plasmablasts secreting IgM, followed by germinal center production of IgG-secreting and memory B cells. The earliest inflammatory cells to enter the brain were CD8(+) T cells, followed by CD4(+) T cells and CD19(+) B cells. During the clearance of infectious virus, effector lymphocytes in the CNS were primarily CD8(+) T cells and IgM antibody-secreting cells (ASCs). During the clearance of viral RNA, there were more CD4(+) than CD8(+) T cells, and B cells included IgG and IgA ASCs. At late times after infection, ASCs in the CNS were primarily CD19(+) CD38(+) CD138(-) Blimp-1(+) plasmablasts, with few fully differentiated CD38(-) CD138(+) Blimp-1(+) plasma cells. CD19(+) CD38(+) surface Ig(+) memory B cells were also present. The level of antibody to SINV increased in the brain over time, and the proportion of SINV-specific ASCs increased from 15% of total ASCs at day 14 to 90% at 4 to 6 months, suggesting specific retention in the CNS during viral RNA persistence. B cells in the CNS continued to differentiate, as evidenced by accumulation of IgA ASCs not present in peripheral lymphoid tissue and downregulation of major histocompatibility complex (MHC) class II expression on plasmablasts. However, there was no evidence of germinal center activity or IgG avidity maturation within the CNS.     

Role of viral persistence in retaining CD8(+) T cells within the central nervous system

The continued presence of virus-specific CD8(+) T cells within the central nervous system (CNS) following resolution of acute viral encephalomyelitis implicates organ-specific retention. The role of viral persistence in locally maintaining T cells was investigated by infecting mice with either a demyelinating, paralytic (V-1) or nonpathogenic (V-2) variant of a neurotropic mouse hepatitis virus, which differ in the ability to persist within the CNS. Class I tetramer technology revealed more infiltrating virus-specific CD8(+) T cells during acute V-1 compared to V-2 infection. However, both total and virus-specific CD8(+) T cells accumulated at similar peak levels in spinal cords by day 10 postinfection (p.i.). Decreasing viral RNA levels in both brains and spinal cords following initial virus clearance coincided with an overall progressive loss of both total and virus-specific CD8(+) T cells. By 9 weeks p.i., T cells had largely disappeared from brains of both infected groups, consistent with the decline of viral RNA. T cells also completely disappeared from V-2-infected spinal cords coincident with the absence of viral RNA. By contrast, a significant number of CD8(+) T cells which contained detectable viral RNA were recovered from spinal cords of V-1-infected mice. The data indicate that residual virus from a primary CNS infection is a vital component in mediating local retention of both CD8(+) and CD4(+) T cells and that once minimal thresholds of stimuli are lost, T cells within the CNS cannot survive in an autonomous fashion.     

Chronic relapsing experimental allergic encephalomyelitis: Identification and dynamics of T and B cells within the central nervous system

Using a monoclonal antibody against guinea pig T cells and anti-guinea pig immunoglobulins, T- and B-cell dynamics were studied by immunofluorescence in situ in the central nervous system (CNS) of animals with untreated and treated chronic relapsing experimental allergic encephalomyelitis (EAE). Treated animals were given a series of injections of either myelin basic protein (MBP) in incomplete Freund's adjuvant (IFA) or MBP and galactocerebroside in IFA. Within the CNS, T and B cells showed distinct distribution patterns in untreated chronic relapsing EAE, similar to that recently described in acute EAE. T cells were predominantly localized within the CNS parenchyma and B cells were mainly found in perivascular areas. B-cell infiltrates were more extensive than in acute EAE and, although most were centered around blood vessels, some were also detectable in the parenchyma. IgG, C₃, and albumin deposits were common. These observations suggest an age-dependent difference in the immune response. In treated chronic EAE, the disease process was apparently arrested and T- and B-cell infiltrates in the white matter were negligible. Therefore, it appears that the present treatment protocol prevents lymphocytes from entering the CNS parenchyma.     

CXCR3-dependent plasma blast migration to the central nervous system during viral encephalomyelitis

Immunoglobulin in cerebral spinal fluid and antibody secreting cells (ASC) within the central nervous system (CNS) parenchyma are common hallmarks of microbial infections and autoimmune disorders. However, the signals directing ASC migration into the inflamed CNS are poorly characterized. This study demonstrates that CXCR3 mediates CNS accumulation of ASC during neurotropic coronavirus-induced encephalomyelitis. Expansion of CXCR3-expressing ASC in draining lymph nodes prior to accumulation within the CNS was consistent with their recruitment by sustained expression of CXCR3 ligands during viral persistence. Both total and virus-specific ASC were reduced greater than 80% in the CNS of infected CXCR3(-/-) mice. Similar T cell CNS recruitment and local T cell-dependent antiviral activity further indicated that the ASC migration defect was T cell independent. Furthermore, in contrast to the reduction of ASC in the CNS, neither virus-specific ASC trafficking to bone marrow nor antiviral serum antibody was reduced relative to levels in control mice. Impaired ASC recruitment into the CNS of infected CXCR3(-/-) mice coincided with elevated levels of persisting viral RNA, sustained infectious virus, increased clinical disease, and mortality. These results demonstrate that CXCR3 ligands are indispensable for recruitment of activated ASC into the inflamed CNS and highlight their local protective role during persistent infection.     

Recruitment of endogenous neural progenitor cells by malignant neoplasms of the central nervous system

It is proposed here that malignancies of the central nervous system (CNS) are capable of recruiting non-malignant CNS precursor cells and that doing so worsens the course of the disease. In particular, the argument is put forward that such tumors can activate resident neural stem cells, attract them or their progeny to the tumor site, and induce them to proliferate. What begins as a normal wound repair response by the recruited cells can eventually result in augmentation of the tumor. In support of this hypothesis, evidence consistent with the ideas proposed is presented. Since these recruited cells are non-malignant, it should be possible to interfere with this process. This would not necessarily remove the threat posed by the cancer, but could beneficially impact patients by slowing progression. Interfering with recruitment could simultaneously serve to block autocrine stimulation by tumor cells. In contrast, introducing exogenous stem cells could exacerbate the recruitment process unless measures are taken to preclude this possibility. Finally, it is worth noting that the situation described in the current hypothesis might apply to a variety of other stem and precursor cell-containing systems throughout the body.     

CD24 on the resident cells of the central nervous system enhances experimental autoimmune encephalomyelitis

CD24 is a cell surface glycoprotein that is expressed on both immune cells and cells of the CNS. We have previously shown that CD24 is required for the induction of experimental autoimmune encephalomyelitis (EAE), an experimental model for the human disease multiple sclerosis (MS). The development of EAE requires CD24 expression on both T cells and non-T host cells in the CNS. To understand the role of CD24 on the resident cells in the CNS during EAE development, we created CD24 bone marrow chimeras and transgenic mice in which CD24 expression was under the control of a glial fibrillary acidic protein promotor (AstroCD24TG mice). We showed that mice lacking CD24 expression on the CNS resident cells developed a mild form of EAE; in contrast, mice with overexpression of CD24 in the CNS developed severe EAE. Compared with nontransgenic mice, the CNS of AstroCD24TG mice had higher expression of cytokine genes such as IL-17 and demyelination-associated marker P8; the CNS of AstroCD24TG mice accumulated higher numbers of Th17 and total CD4+ T cells, whereas CD4+ T cells underwent more proliferation during EAE development. Expression of CD24 in CD24-deficient astrocytes also enhanced their costimulatory activity to myelin oligodendrocyte glycoprotein-specific, TCR-transgenic 2D2 T cells. Thus, CD24 on the resident cells in the CNS enhances EAE development via costimulation of encephalitogenic T cells. Because CD24 is increased drastically on resident cells in the CNS during EAE, our data have important implications for CD24-targeted therapy of MS.     

Dynamics of viral and proviral loads of feline immunodeficiency virus within the feline central nervous system during the acute phase following intravenous infection

Animal models of human immunodeficiency virus 1, such as feline immunodeficiency virus (FIV), provide the opportunities to dissect the mechanisms of early interactions of the virus with the central nervous system (CNS). The aims of the present study were to evaluate viral loads within CNS, cerebrospinal fluid (CSF), ocular fluid, and the plasma of cats in the first 23 weeks after intravenous inoculation with FIV(GL8). Proviral loads were also determined within peripheral blood mononuclear cells (PBMCs) and brain tissue. In this acute phase of infection, virus entered the brain in the majority of animals. Virus distribution was initially in a random fashion, with more diffuse brain involvement as infection progressed. Virus in the CSF was predictive of brain parenchymal infection. While the peak of virus production in blood coincided with proliferation within brain, more sustained production appeared to continue in brain tissue. In contrast, proviral loads in the brain decreased to undetectable levels in the presence of a strengthening PBMC load. A final observation in this study was that there was no direct correlation between viral loads in regions of brain or ocular tissue and the presence of histopathology.     

Natural recovery and protection from autoimmune encephalomyelitis: contribution of CD4+CD25+ regulatory cells within the central nervous system

Immune regulation of autoimmune disease can function at two sites: at the secondary lymphoid organs or in the target organ itself. In this study, we investigated the natural resolution of autoimmune pathology within the CNS using murine experimental autoimmune encephalomyelitis (EAE). Recovery correlates with the accumulation of IL-10-producing CD4+CD25+ T cells within the CNS. These CD4+CD25+ cells represent as many as one in three of CD4+ cells in the CNS during recovery, they are FoxP3+ and express other markers associated with regulatory cells (CTLA-4, GITR, and alpha(E)beta7), and they have regulatory function ex vivo. Depletion of CD25+ cells inhibits the natural recovery from EAE. Also, depletion of CD25+ cells after recovery removes the resistance to reinduction of EAE observed in this model. Furthermore, passive transfer of CNS-derived CD4+CD25+ cells in low numbers provides protection from EAE in recipient mice. These are the first data demonstrating the direct involvement of CD4+CD25+ regulatory T cells in the natural resolution of autoimmune disease within the target organ.     

Shifting hierarchies of interleukin-10-producing T cell populations in the central nervous system during acute and persistent viral encephalomyelitis

Interleukin-10 (IL-10) mRNA is rapidly upregulated in the central nervous system (CNS) following infection with neurotropic coronavirus and remains elevated during persistent infection. Infection of transgenic IL-10/green fluorescent protein (GFP) reporter mice revealed that CNS-infiltrating T cells were the major source of IL-10, with minimal IL-10 production by macrophages and resident microglia. The proportions of IL-10-producing cells were initially similar in CD8(+) and CD4(+) T cells but diminished rapidly in CD8(+) T cells as the virus was controlled. Overall, the majority of IL-10-producing CD8(+) T cells were specific for the immunodominant major histocompatibility complex (MHC) class I epitope. Unlike CD8(+) T cells, a large proportion of CD4(+) T cells within the CNS retained IL-10 production throughout persistence. Furthermore, elevated frequencies of IL-10-producing CD4(+) T cells in the spinal cord supported preferential maintenance of IL-10 production at the site of viral persistence and tissue damage. IL-10 was produced primarily by the CD25(+) CD4(+) T cell subset during acute infection but prevailed in CD25(-) CD4(+) T cells during the transition to persistent infection and thereafter. Overall, these data demonstrate significant fluidity in the T-cell-mediated IL-10 response during viral encephalitis and persistence. While IL-10 production by CD8(+) T cells was limited primarily to the time of acute effector function, CD4(+) T cells continued to produce IL-10 throughout infection. Moreover, a shift from predominant IL-10 production by CD25(+) CD4(+) T cells to CD25(-) CD4(+) T cells suggests that a transition to nonclassical regulatory T cells precedes and is retained during CNS viral persistence.     

Impaired T cell immunity in B cell-deficient mice following viral central nervous system infection

CD8(+) T cells are required to control acute viral replication in the CNS following infection with neurotropic coronavirus. By contrast, studies in B cell-deficient (muMT) mice revealed Abs as key effectors in suppressing virus recrudescence. The apparent loss of initial T cell-mediated immune control in the absence of B cells was investigated by comparing T cell populations in CNS mononuclear cells from infected muMT and wild-type mice. Following viral recrudescence in muMT mice, total CD8(+) T cell numbers were similar to those of wild-type mice that had cleared infectious virus; however, virus-specific T cells were reduced at least 3-fold by class I tetramer and IFN-gamma ELISPOT analysis. Although overall T cell recruitment into the CNS of muMT mice was not impaired, discrepancies in frequencies of virus-specific CD8(+) T cells were most severe during acute infection. Impaired ex vivo cytolytic activity of muMT CNS mononuclear cells, concomitant with reduced frequencies, implicated IFN-gamma as the primary anti viral factor early in infection. Reduced virus-specific CD8(+) T cell responses in the CNS coincided with poor peripheral expansion and diminished CD4(+) T cell help. Thus, in addition to the lack of Ab, limited CD8(+) and CD4(+) T cell responses in muMT mice contribute to the ultimate loss of control of CNS infection. Using a model of virus infection restricted to the CNS, the results provide novel evidence for a role of B cells in regulating T cell expansion and differentiation into effector cells.     

Mast cells exert effects outside the central nervous system to influence experimental allergic encephalomyelitis disease course

Previous studies using mast cell-deficient mice (W/W(v)) revealed that mast cells influence disease onset and severity of experimental allergic/autoimmune encephalomyelitis (EAE), the murine model for multiple sclerosis. The mast cell populations of these mice can be restored by transferring bone marrow-derived mast cells (BMMCs). Studies using the W/W(v) reconstitution model have lead to major advances in our understanding of mast cell roles in vivo. However, despite its common use, details regarding the sites and kinetics of mast cell repopulation have remained largely uncharacterized. In this study, we examined the kinetics and tissue distribution of green fluorescent protein(+) BMMCs in reconstituted W/W(v) mice to identify sites of mast cell influence in EAE. Reconstitution of naive animals with BMMCs does not restore mast cell populations to all organs, notably the brain, spinal cord, lymph nodes, and heart. Despite the absence of mast cells in the CNS, reconstituted mice exhibit an EAE disease course equivalent to that induced in wild-type mice. Mast cells are found adjacent to T cell-rich areas of the spleen and can migrate to the draining lymph node after disease induction. These data reveal that mast cells can act outside the CNS to influence EAE, perhaps by affecting the function of autoreactive lymphocytes.     

Interleukin-6, produced by resident cells of the central nervous system and infiltrating cells, contributes to the development of seizures following viral infection

Cells that can participate in an innate immune response within the central nervous system (CNS) include infiltrating cells (polymorphonuclear leukocytes [PMNs], macrophages, and natural killer [NK] cells) and resident cells (microglia and sometimes astrocytes). The proinflammatory cytokine interleukin-6 (IL-6) is produced by all of these cells and has been implicated in the development of behavioral seizures in the Theiler's murine encephalomyelitis virus (TMEV)-induced seizure model. The assessment, via PCR arrays, of the mRNA expression levels of a large number of chemokines (ligands and receptors) in TMEV-infected and mock-infected C57BL/6 mice both with and without seizures did not clearly demonstrate the involvement of PMNs, monocytes/macrophages, or NK cells in the development of seizures, possibly due to overlapping function of the chemokines. Additionally, C57BL/6 mice unable to recruit or depleted of infiltrating PMNs and NK cells had seizure rates comparable to those of controls following TMEV infection, and therefore PMNs and NK cells do not significantly contribute to seizure development. In contrast, C57BL/6 mice treated with minocycline, which affects monocytes/macrophages, microglial cells, and PMNs, had significantly fewer seizures than controls following TMEV infection, indicating monocytes/macrophages and resident microglial cells are important in seizure development. Irradiated bone marrow chimeric mice that were either IL-6-deficient mice reconstituted with wild-type bone marrow cells or wild-type mice reconstituted with IL-6-deficient bone marrow cells developed significantly fewer behavioral seizures following TMEV infection. Therefore, both resident CNS cells and infiltrating cells are necessary for seizure development.     

Cutting edge: central nervous system plasmacytoid dendritic cells regulate the severity of relapsing experimental autoimmune encephalomyelitis

Plasmacytoid dendritic cells (pDCs) have both stimulatory and regulatory effects on T cells. pDCs are a major CNS-infiltrating dendritic cell population during experimental autoimmune encephalomyelitis but, unlike myeloid dendritic cells, have a minor role in T cell activation and epitope spreading. We show that depletion of pDCs during either the acute or relapse phases of experimental autoimmune encephalomyelitis resulted in exacerbation of disease severity. pDC depletion significantly enhanced CNS but not peripheral CD4(+) T cell activation, as well as IL-17 and IFN-gamma production. Moreover, CNS pDCs suppressed CNS myeloid dendritic cell-driven production of IL-17, IFN-gamma, and IL-10 in an IDO-independent manner. The data demonstrate that pDCs play a critical regulatory role in negatively regulating pathogenic CNS CD4(+) T cell responses, highlighting a new role for pDCs in inflammatory autoimmune disease.     

Nonmyelin-specific T cells accelerate development of central nervous system APC and increase susceptibility to experimental autoimmune encephalomyelitis

Previously we demonstrated that both myelin-specific and nonmyelin-specific rat T cells were capable of accelerating the development of transplanted rat BM-derived APC in the CNS of SCID C.B-17/scid (SCID) mice. This suggested that nonmyelin-specific T cells might be capable of increasing susceptibility to EAE by increasing the number and function of APC in the CNS before disease induction. To assess this possibility, we evaluated disease incidence, day of onset, duration, mean peak severity, cumulative disease index, and histopathology in the presence or absence of nonmyelin-specific T cells. The results demonstrate an association between T cell responses to nonmyelin Ags, accelerated development of BM-derived CNS APC before disease induction, and heightened susceptibility to CNS inflammation mediated by myelin-specific T cells. This suggests that T cell responses to nonmyelin Ags can potentiate CNS inflammation by elevating the functional presence of CNS APC.     

Transmission of prions within the gut and towards the central nervous system

The prion protein is a glycoprotein characterized by a folded ?-helical structure that, under pathological conditions, misfolds and aggregates into its infectious isoform as ?-sheet rich amyloidic deposits. The accumulation of the abnormal protein is responsible for a group of progressive and fatal disorders characterized by vacuolation, gliosis, and spongiform degeneration. Prion disorders are characterized by a triple aetiology: familial, sporadic or acquired, although most cases are sporadic. The mechanisms underlying prion neurotoxicity remain controversial, while novel findings lead to hypothesize intriguing pathways responsible for prion spreading. The present review aims to examine the involvement of the gastrointestinal tract and hypothesizes the potential mechanisms underlying cell-to-cell transmission of the prion protein. In particular, a special emphasis is posed on the mechanisms of prion transmission within the gut and towards the central nervous system. The glycation of prion protein to form advanced glycation end-products (AGE) interacting with specific receptors placed on neighboring cells (RAGE) represents the key hypothesis to be discussed.     

Transmission of prions within the gut and toward the central nervous system

The prion protein is a glycoprotein characterized by a folded α-helical structure that, under pathological conditions, misfolds and aggregates into its infectious isoform as β-sheet rich amyloidic deposits. The accumulation of the abnormal protein is responsible for a group of progressive and fatal disorders characterized by vacuolation, gliosis and spongiform degeneration. Prion disorders are characterized by a triple aetiology: familial, sporadic or acquired, although most cases are sporadic. The mechanisms underlying prion neurotoxicity remain controversial, while novel findings lead to hypothesize intriguing pathways responsible for prion spreading.The present review aims to examine the involvement of the gastrointestinal tract and hypothesizes the potential mechanisms underlying cell-to-cell transmission of the prion protein. In particular, a special emphasis is posed on the mechanisms of prion transmission within the gut and towards the central nervous system. The glycation of prion protein to form advanced glycation end-products (AGE) interacting with specific receptors placed on neighboring cells (RAGE) represents the key hypothesis to be discussed.Key words: prion disease, prion protein, gastrointestinal tract, autonomic nervous system, neurodegenerative diseases, AGE, RAGE