Innate immunity: the missing link in neuroprotection and neurodegeneration?

Innate immunity was previously thought to be a nonspecific immunological programme that was engaged by peripheral organs to maintain homeostasis after stress and injury. Emerging evidence indicates that this highly organized response also takes place in the central nervous system. Through the recognition of neuronal fingerprints, the long-term induction of the innate immune response and its transition to an adaptive form might be central to the pathophysiology and aetiology of neurodegenerative disorders. Paradoxically, this response also protects neurons by favouring remyelination and trophic support afforded by glial cells.     

Constitutive expression of TGF-bêta1, interleukin-6 and interleukin-8 by tumor cells as a major component of immune escape in human ovarian carcinoma

Tumors could use several mechanisms to coexist with the host's immune system or to protect themselves from an immune response. Thus, insufficient expression of cell surface molecules on tumor cells, which are important for T cell recognition or activation, could lead to induction of a state of tolerance. Tumor cells could also produce cytokines that would inhibit the immune response and allow tumor progression. Here, we studied, in vitro, the cell surface expression of immunologically important molecules in seven ovarian carcinoma (OVCA) cell lines and the constitutive expression of cytokines. All OVCA cell lines expressed MHC class I molecules, ICAM-1 and LFA-3 adhesion molecules, necessary to induce a specific cytotoxic T-cell response, as well as the CD40 costimulatory molecules. Conversely, the lack of the dominant costimulatory molecules, CD80 (B7.1) and CD86 (B7.2) could be a possible explanation of poor immunogenicity of OVCA tumors. Immunosuppressive TGF-beta1 was detected at the mRNA level in all cell lines but was weakly secreted in supernatants. By contrast, IL-10 was never found. Most of them constitutively produced IL-8 and IL-6, two cytokines known as tumor promoting factors whereas the proinflammatory cytokines TNF-alpha, IL-1beta and GM-CSF were rarely produced. Data from this study could be useful for designing new strategies of immunotherapy to improve immunogenicity and/or limit protumor cytokine production.     

In vivo polarization of IFN-gamma at Kupfer and non-Kupfer immunological synapses during the clearance of virally infected brain cells

Kupfer-type immunological synapses are thought to mediate intercellular communication between antiviral T cells and virally infected target Ag-presenting brain cells in vivo during an antiviral brain immune response. This hypothesis predicts that formation of Kupfer-type immunological synapses is necessary for polarized distribution of effector molecules, and their directed secretion toward the target cells. However, no studies have been published testing the hypothesis that cytokines can only form polarized clusters at Kupfer-type immunological synapses. Here, we show that IFN-gamma and granzyme-B cluster in a polarized fashion at contacts between T cells and infected astrocytes in vivo. In some cases these clusters were found in Kupfer-type immunological synapses between T cells and infected astrocytes, but we also detected polarized IFN-gamma at synaptic immunological contacts which did not form Kupfer-type immunological synaptic junctions, i.e., in the absence of polarization of TCR or LFA-1. This indicates that TCR signaling, which leads to the production, polarization, and eventual directed secretion of effector molecules such as IFN-gamma, occurs following the formation of both Kupfer-type and non-Kupfer type immunological synaptic junctions between T cells and virally infected target astrocytes in vivo.     

IgA plasma cells express the negative regulatory co-stimulatory molecule programmed cell death 1 ligand and have a potential tolerogenic role in the intestine

To maintain immune homeostasis in the intestine, the intestinal immune system has evolved several tolerogenic mechanisms toward intestinal microflora and food antigens. Although programmed cell death-1 (PD-1) protein has been implicated in immunological tolerance in the intestine and gut-associated lymphoid tissues (GALTs), distribution of its ligands PD-L1 and PD-L2 in the small intestine lamina propria (LP) are unknown. We investigated PD-L1 expression in intestinal LP and found that IgA plasma cells (PCs) were major PD-L1 expressing cells. PD-L1 expression levels on IgA PCs were higher than that on IgG PCs in peripheral lymphoid tissues. IgA PCs expressed antigen-presenting molecule MHC class II and co-stimulatory molecules CD80, CD86, and PD-L2. IgA PCs isolated from intestinal LP exhibited antigen presentation activity, and in the presence of TGF-β induced FoxP3⁺ regulatory T cells, but not IFN-γ⁺ Th1 cells, from naïve T cells. Thus, IgA PCs in the intestine may be involved in an immune regulatory role in the intestinal immune system.     

Neuron-derived IgG Protects Neurons from Complement-dependent Cytotoxicity

Passive immunity of the nervous system has traditionally been thought to be predominantly due to the blood-brain barrier. This concept must now be revisited based on the existence of neuron-derived IgG. The conventional concept is that IgG is produced solely by mature B lymphocytes, but it has now been found to be synthesized by murine and human neurons. However, the function of this endogenous IgG is poorly understood. In this study, we confirm IgG production by rat cortical neurons at the protein and mRNA levels, with 69.0 ± 5.8% of cortical neurons IgG-positive. Injury to primary-culture neurons was induced by complement leading to increases in IgG production. Blockage of neuron-derived IgG resulted in more neuronal death and early apoptosis in the presence of complement. In addition, FcγRI was found in microglia and astrocytes. Expression of FcγR I in microglia was increased by exposure to neuron-derived IgG. Release of NO from microglia triggered by complement was attenuated by neuron-derived IgG, and this attenuation could be reversed by IgG neutralization. These data demonstrate that neuron-derived IgG is protective of neurons against injury induced by complement and microglial activation. IgG appears to play an important role in maintaining the stability of the nervous system.     

Lipopolysaccharide-evoked activation of p38 and JNK leads to an increase in ICAM-1 expression in Schwann cells of sciatic nerves

Lipopolysaccharide is a major constituent of the outer membrane of Gram-negative bacteria. It activates monocytes and macrophages to produce cytokines such as tumor necrosis factor-alpha and interleukins IL-1beta and IL-6. These cytokines appear to be responsible for the neurotoxicity observed in peripheral nervous system inflammatory disease. It has been reported that, in the central nervous system, the expression level of intercellular adhesion molecule-1 (ICAM-1) was dramatically upregulated in response to LPS, as well as many inflammatory cytokines. ICAM-1 contributes to multiple processes seen in central nervous system inflammatory disease, for example migration of leukocytes to inflammatory sites, and adhesion of polymorphonuclear cells and monocytes to central nervous system cells. In the present study, we found that lipopolysacharide evoked ICAM-1 mRNA and protein expression early at 1 h post-injection, and the most significant increase was seen at 4 h. Immunofluorescence double-labeling suggested that most of the ICAM-1-positive staining was located in Schwann cells. Using Schwann cell cultures, we demonstrated that ICAM-1 expression in Schwann cells is regulated by mitogen-activated protein kinases, especially the p38 and stress-activated protein kinase/c-Jun N-terminal kinase pathways. Thus, it is thought that upregulation of ICAM-1 expression in Schwann cells may be important for host defenses after peripheral nervous system injury, and reducing the biosynthesis of ICAM-1 and other cytokines by blocking the cell signal pathway might provide a new strategy against inflammatory and immune reaction after peripheral nerve injury.     

Control of immune responses by trafficking cell surface proteins, vesicles and lipid rafts to and from the immunological synapse

Supramolecular clusters at the immunological synapse provide a mechanism for structuring complex communication networks between cells of the immune system. Regulating intra- and intercellular trafficking of proteins and lipids to and from the immunological synapse provides an additional level of complexity in determining the functional outcome of immune cell interactions. An emergent principle is that molecules requiring tightly regulated cell surface expression, e.g. negative regulators of cell activation or molecules promoting cytotoxicity, are trafficked to the immunological synapse from intracellular secretory as required lysosomes. Many molecules required for the early stages of the intercellular communication are already present at the cell surface, sometimes in lipid rafts, and are rapidly translocated laterally to the intercellular contact. Our understanding of these events critically depends on utilizing appropriate technologies for probing supramolecular recognition in live cells. Thus, we also present here a critical discussion of the technologies used to study lipid rafts and, more broadly, a map of the spatial and temporal dimensions covered by current live cell physical techniques, highlighting where advances are needed to exceed current spatial and temporal boundaries.     

Immunology of pregnancy: cellular mechanisms allowing fetal survival within the maternal uterus

Pregnancy success remains a fascinating phenomenon to immunologists as it defies the immunological rules of rejection. Although it was previously thought that the maternal immune system does not see the fetus, it is now well documented that fetal cells reach the maternal body and encounter host immune cells. Natural tolerance mechanisms following this interaction remain to be fully elucidated. This article reviews the current literature on mechanisms of adaptive immunity, with emphasis on regulatory T cells and heme oxygenase 1 (HO-1). We propose a scenario in which regulatory T cells create a tolerant microenvironment at the fetal-maternal interface characterised by the presence of tolerance-associated molecules such as HO-1, which has been shown to be of vital importance for fetal survival.     

Toll信号通路在中枢神经系统损伤中的作用

Toll样受体(Toll-like receptors,TLR)是先天性免疫反应识别病原体的一个重要分子,在免疫系统中发挥关键作用.其家族各种成员的主要功能是识别入侵病原体表面的各种不同分子模式,随后启动免疫反应,达到保护机体作用.在大脑中,小胶质细胞可以作为抗原提呈细胞,参与脑内免疫反应,也可以通过分泌各种促炎症因子启动或促进免疫反应,而TLR家族在中枢神经免疫系统的作用仍存在争议,它既可以通过促进神经免疫反应枢纽因子的表达来增强免疫,也可因免疫过度而损伤神经细胞.总之,Toll信号通路对中枢神经系统疾病有一定的调控作用.     

5-Azacytidine treatment of a murine cytotoxic T cell line alters interferon-gamma gene induction by interleukin 2

Genetic susceptibility to multiple sclerosis (MS) in Caucasians was previously shown to be correlated to the presence of given alleles at the HLA-DR and Gm loci. We now demonstrate that the humoral immune response in MS central nervous system (CNS) is modulated by both loci: the levels of IgG1 subclass and IgG1 allotypes in cerebrospinal fluid of MS patients depend on both their Gm genotype and their HLA-DR2 or HLA-DR7 phenotype. That HLA-DR molecules may either participate in a preferential recruitment of IgG1 allotype-producing B cells in MS CNS or act after such a selective homing is discussed. These results demonstrate that both HLA and Gm loci are synergistically involved in the modulation of the humoral immune response.     

LFA-3 plasmid DNA enhances Ag-specific humoral- and cellular-mediated protective immunity against herpes simplex virus-2 in vivo: involvement of CD4+ T cells in protection

Adhesion molecules are important for cell trafficking and delivery of secondary signals for stimulation of T cells and antigen-presenting cells (APCs) in a variety of immune and inflammatory responses. Adhesion molecules lymphocyte function-associated antigen (LFA)-1 and CD2 on T cells recognize intercellular adhesion molecule (ICAM)-1 and LFA-3 on APCs, respectively. Recent studies have suggested that these molecules might play a regulatory role in antigen-specific immune responses. To investigate specific roles of adhesion molecules in immune induction we coimmunized LFA-3 and ICAM-1 cDNAs with a gD plasmid vaccine and then analyzed immune modulatory effects and protection against lethal herpes simplex virus (HSV)-2 challenge. We observed that gD-specific IgG production was enhanced by LFA-3 coinjection. However, little change in IgG production was observed by ICAM-1 coinjection. Furthermore, both Th1 and Th2 IgG isotype production was driven by LFA-3. LFA-3 also enhanced Th cell proliferative responses and production of interleukin (IL)-2, interferon-gamma, IL-4, and IL-10 from splenocytes. In contrast, ICAM-1 showed slightly increasing effects on T-cell proliferation responses and cytokine production. beta-Chemokine production (RANTES, MIP-1alpha, and MCP-1) was also influenced by LFA-3 or ICAM-1. When animals were challenged with a lethal dose of HSV-2, LFA-3-coimmunized animals exhibited an enhanced survival rate, as compared to animals given ICAM-1 or gD DNA vaccine alone. This enhanced protection appears to be mediated by CD4+ T cells, as determined by in vitro and in vivo T-cell subset deletion. These studies demonstrate that adhesion molecule LFA-3 can play an important role in generating protective antigen-specific immunity in the HSV model system through increased induction of CD4+ Th1 T-cell subset.     

Interferon induces astrocyte maturation causing an increase in cholinergic properties of cultured human spinal cord cells

It has been well established that interferon-gamma (IFN-gamma) can modify the immune status of cells in the central nervous system (CNS) by inducing major histocompatibility antigens. Furthermore, it has been shown that endogenous IFN can be produced in the brain following viral infection and a form of IFN-alpha/beta can be produced by astrocytes in culture. Here we show that IFN can induce astrocyte maturation and alter neurotransmitter properties in cultured CNS neurons at a given developmental stage. IFN causes a dose-dependent increase in choline acetyltransferase activity and glial fibrillary acidic protein (GFAP) immunoreactivity in cultures of human embryonic spinal cord neurons. The GABAergic activity and the Thy1 immunoreactivity remain unchanged. IFN-gamma does not act directly on the neurons but via the nonneuronal cells, probably the astrocytes, which in turn stimulate the cholinergic traits. These studies could be important for demonstrating an action of the immune system on glial cell maturation and on the neurotransmitter phenotype expression in CNS neurons.     

Depression and stimulation of immunologic memory by typhoid lipopolysaccharide and polysaccharide]

The effect of typhoid lipopolysaccharide (LPS) and polysaccharide (PS) on immunological memory in the system of IgM and IgG synthesis and rosette-forming cells was studied. When introduced into animals previously immunized with SRBC, PS stimulated, under certain conditions, immunological memory in the system of IgM synthesis and rosette-forming cells, while the injection of LPS induced only an insignificant stimulation of immunological memory. No stimulation in the system of IgG synthesis was observed after the injection of both LPS and PS. The suppression of immunological memory was noted in the animals receiving LPS as well as in those receiving PS the immunosuppressing effect produced by LPS was more pronounced.     

Japanese Encephalitis Virus Induce Immuno-Competency in Neural Stem/Progenitor Cells

Background

The low immunogenicity of neural stem/progenitor cells (NSPCs) coupled with negligible expression of MHC antigens has popularized their use in transplantation medicine. However, in an inflammatory environment, the NSPCs express costimulatory molecules and MHC antigens, and also exhibit certain immunomodulatory functions. Since NSPCs are the cellular targets in a number of virus infections both during postnatal and adult stages, we wanted to investigate the immunological properties of these stem cells in response to viral pathogen.

Methodology/Principal Findings

We utilized both in vivo mouse model and in vitro neurosphere model of Japanese encephalitis virus (JEV) infection for the study. The NSPCs residing in the subventricular zone of the infected brains showed prominent expression of MHC-I and costimulatory molecules CD40, CD80, and CD86. Using Flow cytometry and fluorescence microscopy, we observed increased surface expression of co-stimulatory molecule and MHC class I antigen in NSPCs upon progressive JEV infection in vitro. Moreover, significant production of pro-inflammatory cyto/chemokines was detected in JEV infected NSPCs by Cytokine Bead Array analysis. Interestingly, NSPCs were capable of providing functional costimulation to allogenic T cells and JEV infection resulted in increased proliferation of allogenic T cells, as detected by Mixed Lymphocyte reaction and CFSE experiments. We also report IL-2 production by NSPCs upon JEV infection, which possibly provides mitogenic signals to T cells and trigger their proliferation.

Conclusion/Significance

The in vivo and in vitro findings clearly indicate the development of immunogenicity in NSPCs following progressive JEV infection, in our case, JEV infection. Following a neurotropic virus infection, NSPCs possibly behave as immunogenic cells and contribute to both the innate and adaptive immune axes. The newly discovered immunological properties of NSPCs may have implications in assigning a new role of these cells as non-professional antigen presenting cells in the central nervous system.     

Astrocytes as antigen-presenting cells. I. Induction of Ia antigen expression on astrocytes by T cells via immune interferon and its effect on antigen presentation

Ia antigens seem to control immune responses on at least two levels. First, they influence the antigen recognition repertoire of the T cells. Second, their variable expression on certain antigen-presenting cells is a powerful regulatory mechanism for the local immune reaction. This is particularly important in the central nervous system (CNS) in which no Ia antigens are normally expressed. Recent experiments in this context have shown that astrocytes are able to express Ia antigens during interaction with T cells, and that they function as antigen-presenting cells. The Ia-inducing activity is produced by activated T cells, and can be replaced by immune interferon (IFN-gamma). In this study we report on the functional and kinetic relationship between Ia antigen expression on astrocytes and the immune-specific activation of T cells by astrocytes. Normal resting astrocytes were found to be negative for Ia antigens by immunofluorescence and by biochemical criteria. Moreover, they are only able to stimulate T cells after they have been induced to express Ia antigens by a signal from the T cells, which is probably mediated by IFN-gamma. In conclusion, the immune-specific interaction between astrocytes and T lymphocytes is a sensitively controlled system that might be pivotal to the development of immune responses in the brain. Malfunction of the system could be an important factor in the pathogenesis of aberrant immune reactions in the CNS, e.g., in multiple sclerosis.     

Fc-glycosylation of IgG1 is modulated by B-cell stimuli

We have recently shown that IgG1 directed against antigens thought to be involved in the pathogenesis of rheumatoid arthritis harbor different glycan moieties on their Fc-tail, as compared with total sera IgG1. Given the crucial roles of Fc-linked N-glycans for the structure and biological activity of IgG, Fc-glycosylation of antibodies is receiving considerable interest. However, so far little is known about the signals and factors that could influence the composition of these carbohydrate structures on secreted IgG produced by B lymphocytes. Here we show that both "environmental" factors, such as all-trans retinoic acid (a natural metabolite of vitamin A), as well as factors stimulating the innate immune system (i.e. CpG oligodeoxynucleotide, a ligand for toll-like receptor 9) or coming from the adaptive immune system (i.e. interleukin-21, a T-cell derived cytokine) can modulate IgG1 Fc-glycosylation. These factors affect Fc-glycan profiles in different ways. CpG oligodeoxynucleotide and interleukin-21 increase Fc-linked galactosylation and reduce bisecting N-acetylglucosamine levels, whereas all-trans retinoic acid significantly decreases galactosylation and sialylation levels. Moreover, these effects appeared to be stable and specific for secreted IgG1 as no parallel changes of the corresponding glycans in the cellular glycan pool were observed. Interestingly, several other cytokines and molecules known to affect B-cell biology and antibody production did not have an impact on IgG1 Fc-coupled glycan profiles. Together, these data indicate that different stimuli received by B cells during their activation and differentiation can modulate the Fc-linked glycosylation of secreted IgG1 without affecting the general cellular glycosylation machinery. Our study, therefore, furthers our understanding of the regulation of IgG1 glycosylation at the cellular level.