T lymphocytes promote the development of bone marrow-derived APC in the central nervous system

Certain cells within the CNS, microglial cells and perivascular macrophages, develop from hemopoietic myelomonocytic lineage progenitors in the bone marrow (BM). Such BM-derived cells function as CNS APC during the development of T cell-mediated paralytic inflammation in diseases such as experimental autoimmune encephalomyelitis and multiple sclerosis. We used a novel, interspecies, rat-into-mouse T cell and/or BM cell-transfer method to examine the development and function of BM-derived APC in the CNS. Activated rat T cells, specific for either myelin or nonmyelin Ag, entered the SCID mouse CNS within 3-5 days of cell transfer and caused an accelerated recruitment of BM-derived APC into the CNS. Rat APC in the mouse CNS developed from transferred rat BM within an 8-day period and were entirely sufficient for induction of CNS inflammation and paralysis mediated by myelin-specific rat T cells. The results demonstrate that T cells modulate the development of BM-derived CNS APC in an Ag-independent fashion. This previously unrecognized regulatory pathway, governing the presence of functional APC in the CNS, may be relevant to pathogenesis in experimental autoimmune encephalomyelitis, multiple sclerosis, and/or other CNS diseases involving myelomonocytic lineage cells.     

Differential regulation of primary and secondary CD8+ T cells in the central nervous system

T cell accumulation and effector function following CNS infection is limited by a paucity of Ag presentation and inhibitory factors characteristic of the CNS environment. Differential susceptibilities of primary and recall CD8+ T cell responses to the inhibitory CNS environment were monitored in naive and CD8+ T cell-immune mice challenged with a neurotropic coronavirus. Accelerated virus clearance and limited spread in immunized mice was associated with a rapid and increased CNS influx of virus-specific secondary CD8+ T cells. CNS-derived secondary CD8+ T cells exhibited increased cytolytic activity and IFN-gamma expression per cell compared with primary CD8+ T cells. However, both Ag-specific primary and secondary CD8+ T cells demonstrated similar contraction rates. Thus, CNS persistence of increased numbers of secondary CD8+ T cells reflected differences in the initial pool size during peak inflammation rather than enhanced survival. Unlike primary CD8+ T cells, persisting secondary CD8+ T cells retained ex vivo cytolytic activity and expressed high levels of IFN-gamma following Ag stimulation. However, both primary and secondary CD8+ T cells exhibited reduced capacity to produce TNF-alpha, differentiating them from effector memory T cells. Activation of primary and secondary CD8+ T cells in the same host using adoptive transfers confirmed similar survival, but enhanced and prolonged effector function of secondary CD8+ T cells in the CNS. These data suggest that an instructional program intrinsic to T cell differentiation, rather than Ag load or factors in the inflamed CNS, prominently regulate CD8+ T cell function.     

Kinetics of virus-specific CD8+ -T-cell expansion and trafficking following central nervous system infection

CD8+ T cells control acute infection of the central nervous system (CNS) by neurotropic mouse hepatitis virus but do not suffice to achieve sterile immunity. To determine the lag between T-cell priming and optimal activity within the CNS, the accumulation of virus-specific CD8+ T cells in the CNS relative to that in peripheral lymphoid organs was assessed by using gamma interferon-specific ELISPOT assays and class I tetramer staining. Virus-specific CD8+ T cells were first detected in the cervical lymph nodes. Expansion in the spleen was delayed and less pronounced but also preceded accumulation in the CNS. The data further suggest peripheral acquisition of cytolytic function, thus enhancing CD8+ -T-cell effector function upon cognate antigen recognition in the CNS.     

Microglia and Microglia-Like Cell Differentiated from DC Inhibit CD4 T Cell Proliferation

The central nervous system (CNS) is generally regarded as a site of immune privilege, whether the antigen presenting cells (APCs) are involved in the immune homeostasis of the CNS is largely unknown. Microglia and DCs are major APCs in physiological and pathological conditions, respectively. In this work, primary microglia and microglia-like cells obtained by co-culturing mature dendritic cells with CNS endothelial cells in vitro were functional evaluated. We found that microglia not only cannot prime CD4 T cells but also inhibit mature DCs (maDCs) initiated CD4 T cells proliferation. More importantly, endothelia from the CNS can differentiate maDCs into microglia-like cells (MLCs), which possess similar phenotype and immune inhibitory function as microglia. Soluble factors including NO lie behind the suppression of CD4 T cell proliferation induced by both microglia and MLCs. All the data indicate that under physiological conditions, microglia play important roles in maintaining immune homeostasis of the CNS, whereas in a pathological situation, the infiltrated DCs can be educated by the local microenvironment and differentiate into MLCs with inhibitory function.     

Mechanisms of central nervous system viral persistence: the critical role of antibody and B cells.

Contributions of humoral and cellular immunity in controlling neurotropic mouse hepatitis virus persistence within the CNS were determined in B cell-deficient J(H)D and syngeneic H-2(d) B cell+ Ab-deficient mice. Virus clearance followed similar kinetics in all mice, confirming initial control of virus replication by cellular immunity. Nevertheless, virus reemerged within the CNS of all Ab-deficient mice. In contrast to diminished T cell responses in H-2(b) B cell-deficient muMT mice, the absence of B cells or Ab in the H-2(d) mice did not compromise expansion, recruitment into the CNS, or function of virus-specific CD4+ and CD8+ T cells. The lack of B cells and lymphoid architecture thus appears to manifest itself on T cell responses in a genetically biased manner. Increasing viral load did not enhance frequencies or effector function of virus-specific T cells within the CNS, indicating down-regulation of T cell responses. Although an Ab-independent antiviral function of B cells was not evident during acute infection, the presence of B cells altered CNS cellular tropism during viral recrudescence. Reemerging virus localized almost exclusively to oligodendroglia in B cell+ Ab-deficient mice, whereas it also replicated in astrocytes in B cell-deficient mice. Altered tropism coincided with distinct regulation of CNS virus-specific CD4+ T cells. These data conclusively demonstrate that the Ab component of humoral immunity is critical in preventing virus reactivation within CNS glial cells. B cells themselves may also play a subtle role in modulating pathogenesis by influencing tropism.     

Glial epigenetics in neuroinflammation and neurodegeneration

Epigenetic regulation shapes the differentiation and response to stimuli of all tissues and cells beyond what genetics would dictate. Epigenetic regulation acts through covalent modifications of DNA and histones while leaving the nucleotide code intact. However, these chromatin modifications are known to be vital components of the regulation of cell fate and response. With regards to the central nervous system (CNS), little is known about how epigenetic regulation shapes the function of neural cell types. The focus of research so far has been on epigenetic regulation of neuronal function and the role of epigenetics in tumorigenesis. However, the glial cell compartment, which makes up 90 % of all CNS cells, has so far received scant attention as to how epigenetics shape their differentiation and function. Here, we highlight current knowledge about epigenetic changes in glial cells occurring during CNS injury, neuroinflammatory conditions and neurodegenerative disease. This review offers an overview of the current understanding of epigenetic regulation in glial cells in CNS 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.     

Defining the NG2-expressing cell of the adult CNS

The NG2 proteoglycan is believed to be an in vivomarker for oligodendrocyte progenitors found in the developing brain. The prevalence of NG2-expressing cells that remain in the adult CNS following the end of gliogenesis is significant. Current research is focused on how this cell participates in the normal function of the adult CNS and whether it may be activated by injury and/or contribute to repair. Despite substantial evidence for a sub-population of NG2-expressing cells playing a glial progenitor role in the adult CNS, there is much to be learned. Specifically, the heterogeneity of this population has not been adequately addressed for the adult CNS and while NG2 cells continue to divide in the adult CNS it is not clear what function they serve once myelination is complete. Future studies should elucidate the functional importance of NG2 in a variety of cell functions and shed light on the role NG2-expressing cells play in the intact and diseasedCNS.     

The jing and ras1 pathways are functionally related during CNS midline and tracheal development

The Drosophila jing gene encodes a zinc finger protein required for the differentiation and survival of embryonic CNS midline and tracheal cells. We show that there is a functional relationship between jing and the Egfr pathway in the developing CNS midline and trachea. jing function is required for Egfr pathway gene expression and MAPK activity in both the CNS midline and trachea. jing over-expression effects phenocopy those of the Egfr pathway and require Egfr pathway function. Activation of the Egfr pathway in loss-of-function jing mutants partially rescues midline cell loss. Egfr pathway genes and jing show dominant genetic interactions in the trachea and CNS midline. Together, these results show that jing regulates signal transduction in developing midline and tracheal cells.     

Mechanism of stem cells in the central nervous system

Injury to the central nervous system (CNS) can result in severe functional impairment. The brain and spinal cord, which constitute the CNS, have been viewed for decades as having a very limited capacity for regeneration. However, over the last several years, the body of evidence supporting the concept of regeneration and continuous renewal of neurons in specific regions of the CNS has increased. This evidence has significantly altered our perception of the CNS and has offered new hope for possible cell therapy strategies to repair lost function. Transplantation of stem cells or the recruitment of endogenous stem cells to repair specific regions of the brain or spinal cord is the next exciting research challenge. However, our understanding of the existing stem cell pool in the adult CNS remains limited. This review will discuss the identification and characterization of CNS stem cells in the adult brain and spinal cord.     

CD4 T cells promote CD8 T cell immunity at the priming and effector site during viral encephalitis

CD4 T cell activation during peripheral infections not only is essential in inducing protective CD8 T cell memory but also promotes CD8 T cell function and survival. However, the contributions of CD4 T cell help to antiviral CD8 T cell immunity during central nervous system (CNS) infection are not well established. Encephalitis induced by the sublethal coronavirus JHMV was used to identify when CD4 T cells regulate CD8 T cell responses following CNS infection. Peripheral expansion of virus-specific CD8 T cells was impaired when CD4 T cells were ablated prior to infection but not at 4 days postinfection. Delayed CD4 T cell depletion abrogated CD4 T cell recruitment to the CNS but only slightly diminished CD8 T cell recruitment. Nevertheless, the absence of CNS CD4 T cells was associated with reduced gamma interferon (IFN-γ) and granzyme B expression by infiltrating CD8 T cells, increased CD8 T cell apoptosis, and impaired control of infectious virus. CD4 T cell depletion subsequent to CD4 T cell CNS migration restored CD8 T cell activity and virus control. Analysis of γc-dependent cytokine expression indicated interleukin-21 (IL-21) as a primary candidate optimizing CD8 T cell activity within the CNS. These results demonstrate that CD4 T cells play critical roles in both enhancing peripheral activation of CD8 T cells and prolonging their antiviral function within the CNS. The data highlight the necessity for temporally and spatially distinct CD4 T cell helper functions in sustaining CD8 T cell activity during CNS infection.     

Gatekeeper role of brain antigen‐presenting CD11c+ cells in neuroinflammation

Multiple sclerosis is the most frequent chronic inflammatory disease of the CNS. The entry and survival of pathogenic T cells in the CNS are crucial for the initiation and persistence of autoimmune neuroinflammation. In this respect, contradictory evidence exists on the role of the most potent type of antigen‐presenting cells, dendritic cells. Applying intravital two‐photon microscopy, we demonstrate the gatekeeper function of CNS professional antigen‐presenting CD11c⁺ cells, which preferentially interact with Th17 cells. IL‐17 expression correlates with expression of GM‐CSF by T cells and with accumulation of CNS CD11c⁺ cells. These CD11c⁺ cells are organized in perivascular clusters, targeted by T cells, and strongly express the inflammatory chemokines Ccl5, Cxcl9, and Cxcl10. Our findings demonstrate a fundamental role of CNS CD11c⁺ cells in the attraction of pathogenic T cells into and their survival within the CNS. Depletion of CD11c⁺ cells markedly reduced disease severity due to impaired enrichment of pathogenic T cells within the CNS.     

Recruitment kinetics and composition of antibody-secreting cells within the central nervous system following viral encephalomyelitis

Infection by the neurotropic JHM strain of mouse hepatitis virus produces an acute demyelinating encephalomyelitis. While cellular immunity initially eliminates infectious virus, CNS viral persistence is predominantly controlled by humoral immunity. To better understand the distinct phases of immune control within the CNS, the kinetics of humoral immune responses were determined in infected mice. Early during clearance of the JHM strain of mouse hepatitis virus, only few virus-specific Ab-secreting cells (ASC) were detected in the periphery or CNS, although mature B cells and ASC without viral specificity were recruited into the CNS concomitant with T cells. Serum antiviral Ab and CNS virus-specific ASC became prominent only during final elimination of infectious virus. Virus-specific ASC peaked in lymphoid organs before the CNS, suggesting peripheral B cell priming and maturation. Following elimination of infectious virus, virus-specific ASC continued to increase within the CNS and then remained stable during persistence, in contrast to declining T cell numbers. These data comprise three novel findings. Rapid recruitment of B cells in the absence of specific Ab secretion supports a potential Ab-independent effector function involving lysis of virus-infected cells. Delayed recruitment relative to viral clearance and subsequent maintenance of a stable CNS ASC population demonstrate differential regulation of T and B lymphocytes within the infected CNS. This supports a critical role of humoral immunity in regulating viral CNS persistence. Lastly, altered antiviral ASC specificities following clearance of infectious virus suggest ongoing recruitment of peripheral memory cells and/or local B cell differentiation.     

Inverted immunodominance and impaired cytolytic function of CD8+ T cells during viral persistence in the central nervous system.

Mice infected with the neurotropic JHM strain of mouse hepatitis virus (JHMV) clear infectious virus; nevertheless, virus persists in the CNS as noninfectious RNA, resulting in ongoing primary demyelination. Phenotypic and functional analysis of CNS infiltrating cells during acute infection revealed a potent regional CD8+ T cell response comprising up to 50% virus-specific T cells. The high prevalence of virus-specific T cells correlated with ex vivo cytolytic activity and efficient reduction in viral titers. Progressive viral clearance coincided with the loss of cytolytic activity, but retention of IFN-gamma secretion and increased expression of the early activation marker CD69, indicating differential regulation of effector function. Although the total number of infiltrating T cells declined following clearance of infectious virus, CD8+ T cells, both specific for the dominant viral epitopes and of unknown specificity, were retained within the CNS, suggesting an ongoing T cell response during persistent CNS infection involving a virus-independent component. Reversed immunodominance within the virus-specific CD8+ T cell population further indicated epitope-specific regulation, supporting ongoing T cell activation. Even in the absence of infectious virus, the CNS thus provides an environment that maintains both unspecific and Ag-specific CD8+ T cells with restricted effector function. Chronic T cell stimulation may thus play a role in preventing viral recrudescence, while increasing the risk of pathological conditions, such as demyelination.     

The role of astrocytes in oxidative stress of central nervous system: A mixed blessing

Central nervous system (CNS) maintains a high level of metabolism, which leads to the generation of large amounts of free radicals, and it is also one of the most vulnerable organs to oxidative stress. Emerging evidences have shown that, as the key homeostatic cells in CNS, astrocytes are deeply involved in multiple aspects of CNS function including oxidative stress regulation. Besides, the redox level in CNS can in turn affect astrocytes in morphology and function. The complex and multiple roles of astrocytes indicate that their correct performance is crucial for the normal functioning of the CNS, and its dysfunction may result in the occurrence and progression of various neurological disorders. To date, the influence of astrocytes in CNS oxidative stress is rarely reviewed. Therefore, in this review we sum up the roles of astrocytes in redox regulation and the corresponding mechanisms under both normal and different pathological conditions.     

How do immune cells overcome the blood–brain barrier in multiple sclerosis?

The presence of the blood-brain barrier (BBB) restricts the movement of soluble mediators and leukocytes from the periphery to the central nervous system (CNS). Leukocyte entry into the CNS is nonetheless an early event in multiple sclerosis (MS), an inflammatory disorder of the CNS. Whether BBB dysfunction precedes immune cell infiltration or is the consequence of perivascular leukocyte accumulation remains enigmatic, but leukocyte migration modifies BBB permeability. Immune cells of MS subjects express inflammatory cytokines, reactive oxygen species (ROS) and enzymes that can facilitate their migration to the CNS by influencing BBB function, either directly or indirectly. In this review, we describe how immune cells from the peripheral blood overcome the BBB and promote CNS inflammation in MS through BBB disruption.     

Diverse functions of distal regulatory elements at the IFNG locus

Previous studies have identified multiple conserved noncoding sequences (CNS) at the mouse Ifng locus sufficient for enhancer activity in cell-based assays. These studies do not directly address biology of the human IFNG locus in a genomic setting. IFNG enhancers may be functionally redundant or each may be functionally unique. We test the hypothesis that each IFNG enhancer has a unique necessary function using a bacterial artificial chromosome transgenic model. We find that CNS-30, CNS-4, and CNS+20 are required at distinct stages of Th1 differentiation, whereas CNS-16 has a repressive role in Th1 and Th2 cells. CNS+20 is required for IFN-γ expression by memory Th1 cells and NKT cells. CNS-4 is required for IFN-γ expression by effector Th1 cells. In contrast, CNS-16, CNS-4, and CNS+20 are each partially required for human IFN-γ expression by NK cells. Thus, IFNG CNS enhancers have redundant necessary functions in NK cells but unique necessary functions in Th cells. These results also demonstrate that distinct CNSs are required to transcribe IFNG at each stage of the Th1 differentiation pathway.     

Perforin-mediated effector function within the central nervous system requires IFN-gamma-mediated MHC up-regulation

CD8(+) T cells infiltrating the CNS control infection by the neurotropic JHM strain of mouse hepatitis virus. Differential susceptibility of infected cell types to clearance by perforin or IFN-gamma uncovered distinct, nonredundant roles for these antiviral mechanisms. To separately evaluate each effector function specifically in the context of CD8(+) T cells, pathogenesis was analyzed in mice deficient in both perforin and IFN-gamma (PKO/GKO) or selectively reconstituted for each function by transfer of CD8(+) T cells. Untreated PKO/GKO mice were unable to control the infection and died of lethal encephalomyelitis within 16 days, despite substantially higher CD8(+) T cell accumulation in the CNS compared with controls. Uncontrolled infection was associated with limited MHC class I up-regulation and an absence of class II expression on microglia, coinciding with decreased CD4(+) T cells in CNS infiltrates. CD8(+) T cells from perforin-deficient and wild-type donors reduced virus replication in PKO/GKO recipients. By contrast, IFN-gamma-deficient donor CD8(+) T cells did not affect virus replication. The inability of perforin-mediated mechanisms to control virus in the absence of IFN-gamma coincided with reduced class I expression. These data not only confirm direct antiviral activity of IFN-gamma within the CNS but also demonstrate IFN-gamma-dependent MHC surface expression to guarantee local T cell effector function in tissues inherently low in MHC expression. The data further imply that IFN-gamma plays a crucial role in pathogenesis by regulating the balance between virus replication in oligodendrocytes, CD8(+) T cell effector function, and demyelination.