Summary Neurodegeneration, the slow and progressive dysfunction and loss of neurons and axons in the central nervous system, is the primary pathological feature of acute and chronic neurodegenerative conditions such as Alzheimer’s disease and Parkinson’s disease, neurotropic viral infections, stroke, paraneoplastic disorders, traumatic brain injury and multiple sclerosis. Despite different triggering events, a common feature is chronic immune activation, in particular of microglia, the resident macrophages of the central nervous system. Apart from the pathogenic role of immune responses, emerging evidence indicates that immune responses are also critical for neuroregeneration. Here, we review the impact of innate and adaptive immune responses on the central nervous system in autoimmune, viral and other neurodegenerative disorders, and discuss their contribution to either damage or repair. We also discuss potential therapies aimed at the immune responses within the central nervous system. A better understanding of the interaction between the immune and nervous systems will be crucial to either target pathogenic responses, or augment the beneficial effects of immune responses as a strategy to intervene in chronic neurodegenerative diseases.

IntroductionRegulatory T cells are the central element for the maintenance of peripheral tolerance. [1][2][3][4][5] Upon activation, they switch into an active suppressor mode that allows them to neutralize or to inactivate other effector cells, such as B cells or T cells. Several types of regulatory T cells have been identified so far, of which most belong to the CD4 ϩ lineage. Based on the expression of the interleukin 2 (IL-2) receptor-␣ chain (CD25), they can be subdivided into 2 major subsets, CD25 Ϫ CD4 ϩ T cells (Tr1) 2 and CD25 ϩ CD4 ϩ T cells (Treg cells). 6 Whereas Tr1 cells seem to acquire their suppressor status in the periphery by polarization of naive T cells, Treg cells are described as "born" suppressors. Although some recent data indicate that at least to some extent Treg cells can derive also from the CD25 Ϫ CD4 ϩ T-cell subset, 7 there is little doubt that regulatory CD25 ϩ CD4 ϩ T cells are mainly produced intrathymically, where they are specifically trained for recognition of autoantigens. 8,9 To carry out their suppressor function, regulatory T cells, as all other lymphocytes, have to migrate to lymphoid organs and to sites of inflammation. Homing and trafficking are facilitated by the expression of distinct sets of chemokine receptors 10 of which 2, CCR8 and CCR4, have already been reported to be important for regulatory human CD25 ϩ CD4 ϩ T cells. 11 Although their expression is not exclusively restricted to regulatory cells, 12 CCR4 and even more CCR8 are apparently expressed by a large fraction of regulatory CD25 ϩ T cells. Another chemokine receptor that has not been considered yet is CCR6. The ligands, CCL20 and ␤-defensin, are produced mainly in inflamed sites and attract CCR6 ϩ cells by inducing migration 13,14 and attachment to endothelial cells. 15 CCR6 is a marker of certain dendritic cell (DC), B-cell, and memory T-cell subsets 13,16 and its expression on Langerhans cells and differentiated monocytes can be induced by It is also present on the surface of "immature" DCs, 18 known to induce tolerance, 19 as well as on a subset of suppressive DCs that overexpress the enzyme indoleamine 2,3-dioxygenase (IDO). 20 Importantly, CD4 ϩ T cells express CCR6 only as antigenexperienced memory cells. 13 In this study we show that CCR6 is present on a substantial fraction of regulatory CD25 ϩ CD4 ϩ T cells. These cells exhibit a phenotype of activation, expansion, and memory typical for effector-memory T cells. Because they seem to control immune responses directly at inflamed sites, these regulatory effector/memory-like T cells appear to function as a natural counterbalance to "regular" effector-memory T cells. Materials and methods Antibodies and reagentsFluorescein isothiocyanate (FITC)-, phycoerythrin (PE)-, or allophycocyanin (APC)-conjugated streptavidin, secondary antibodies, and isotype controls were purchased from BD PharMingen (San Diego, CA), R&D Systems (Minneapolis, MN), Coulter (Fullerton, CA), or Caltag (Burlingame, CA). ␣CD3, ␣CD4, ␣CD5, ␣CD8a, ␣CD11a, ␣CD11b, ␣CD25, ␣CD44, ␣...

Multiple sclerosis (MS) is widely considered to be the result of an aggressive autoreactive T cell attack on myelin. How these autoimmune responses arise in MS is unclear, but they could result from virus infections. Thus, viral and autoimmune diseases in animals have been used to investigate the possible pathogenic mechanisms operating in MS. The autoimmune model, experimental autoimmune encephalomyelitis, is the most widely-used animal model and has greatly influenced therapeutic approaches targeting autoimmune responses. To investigate demyelination and remyelination in the absence of the adaptive immune response, toxin-induced demyelination models are used. These include using cuprizone, ethidium bromide and lysolecithin to induce myelin damage, which rapidly lead to remyelination when the toxins are withdrawn. The virus models include natural and experimental infections such as canine distemper, visna infection of sheep, and infection of non-human primates. The most commonly used viral models in rodents are Semliki Forest virus and Theiler's murine encephalomyelitis virus. The viral and experimental autoimmune encephalomyelitis models have been instrumental in the understanding of how viruses trigger inflammation, demyelination and neurodegeneration in the central nervous system. However, due to complexity of the animal models, pathological mechanisms are also examined in central nervous system cell culture systems including co-cultures, aggregate cultures and brain slice cultures. Here we critically review in vitro and in vivo models used to investigate MS. Since knowledge gained from these models forms the basis for the development of new therapeutic approaches for MS, we address the applicability of the models. Finally, we provide guidance for using and reporting animal studies with the aim of improving translational studies to the clinic.

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The immune system is inextricably linked with many neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), a devastating neuromuscular disorder affecting motor cell function with an average survival of 3 years from symptoms onset. In ALS, there is a dynamic interplay between the resident innate immune cells, that is, microglia and astrocytes, which may become progressively harmful to motor neurons. Although innate and adaptive immune responses are associated with progressive neurodegeneration, in the early stages of ALS immune activation pathways are primarily considered to be beneficial promoting neuronal repair of the damaged tissues, though a harmful effect of T cells at this stage of disease has also been observed. In addition, although auto-antibodies against neuronal antigens are present in ALS, it is unclear whether these arise as a primary or secondary event to neuronal damage, and whether the auto-antibodies are indeed pathogenic. Understanding how the immune system contributes to the fate of motor cells in ALS may shed light on the triggers of disease as well as on the mechanisms contributing to the propagation of the pathology. Immune markers may also act as biomarkers while pathways involved in immune action may be targets of new therapeutic strategies. Here, we review the modalities by which the immune system senses the core pathological process in motor neuron disorders, focusing on tissue-specific immune responses in the neuromuscular junction and in the neuroaxis observed in affected individuals and in animal models of ALS. We elaborate on existing data on the immunological fingerprint of ALS that could be used to identify clues on the disease origin and patterns of progression.

Neuroaxonal degeneration is a pathological hallmark of multiple sclerosis (MS) contributing to irreversible neurological disability. Pathological mechanisms leading to axonal damage include autoimmunity to neuronal antigens. In actively demyelinating lesions, myelin is phagocytosed by microglia and blood-borne macrophages, whereas the fate of degenerating or damaged axons is unclear. Phagocytosis is essential for clearing neuronal debris to allow repair and regeneration. However, phagocytosis may lead to antigen presentation and autoimmunity, as has been described for neuroaxonal antigens. Despite this notion, it is unknown whether phagocytosis of neuronal antigens occurs in MS. Here, we show using novel, well-characterized antibodies to axonal antigens, that axonal damage is associated with HLA-DR expressing microglia/macrophages engulfing axonal bulbs, indicative of axonal damage. Neuronal proteins were frequently observed inside HLA-DR 1 cells in areas of axonal damage. In vitro, phagocytosis of neurofilament light (NF-L), present in white and gray matter, was observed in human microglia. The number of NF-L or myelin basic protein (MBP) positive cells was quantified using the mouse macrophage cell line J774.2. Intracellular colocalization of NF-L with the lysosomal membrane protein LAMP1 was observed using confocal microscopy confirming that NF-L is taken up and degraded by the cell. In vivo, NF-L and MBP was observed in cerebrospinal fluid cells from patients with MS, suggesting neuronal debris is drained by this route after axonal damage. In summary, neuroaxonal debris is engulfed, phagocytosed, and degraded by HLA-DR 1 cells. Although uptake is essential for clearing neuronal debris, phagocytic cells could also play a role in augmenting autoimmunity to neuronal antigens. V

Hydrogen bonds (H-bonds) are crucial for the stability of the peptide-major histocompatibility complex (MHC) complex. In particular, the H-bonds formed between the peptide ligand and the MHC class II binding site appear to have a great influence on the half-life of the complex. Here we show that functional groups with the capacity to disrupt hydrogen bonds (e.g. -OH) can efficiently catalyze ligand exchange reactions on HLA-DR molecules. In conjunction with simple carrier molecules (such as propyl or benzyl residues), they trigger the release of low affinity ligands, which permits the rapid binding of peptides with higher affinity. Similar to HLA-DM, these compounds are able to influence the MHC class II ligand repertoire. In contrast to HLA-DM, however, these simple small molecules are still active at neutral pH. Under physiological conditions, they increase the number of "peptide-receptive" MHC class II molecules and facilitate exogenous peptide loading of dendritic cells. The drastic acceleration of the ligand exchange on these antigen presenting cells suggests that, in general, availability of H-bond donors in the extracellular milieu controls the rate of MHC class II ligand exchange reactions on the cell surface. These molecules may therefore be extremely useful for the loading of antigens onto dendritic cells for therapeutic purposes.Peptide ligands bind to the peptide-binding groove of MHC 1 class II molecules by an array of intermolecular hydrogen bonds (H-bonds). These hydrogen bonds are mostly formed between the backbone of the peptide and conserved residues of the MHC class II molecule. Some of these H-bonds are particularly crucial for the stability of the ligand complex (1). It has been shown for a murine MHC class II molecule that the elimination of H-bonds between the ligand and residues His-81 or Asp-82 of the I-A d ␤-chain results in a rapid loss of the bound peptide (2). Detailed kinetic studies with these mutated MHC molecules revealed peptide dissociation rates that were increased up to 200-fold (3). This increase was in the same range observed after addition of HLA-DM to the peptide complex of the nonmutated I-A d molecule. It was therefore proposed that HLA-DM-mediated ligand release (4, 5) is also accomplished by the disruption of H-bonds (6), a hypothesis also introduced when the crystal structure of HLA-DM was published (7).Because H-bonds appear to be fundamental in maintaining the stability of the MHC class II peptide complexes, we started to investigate small molecules capable of disrupting H-bonds with the goal of achieving an HLA-DM-like catalytic effect on the kinetics of peptide binding. H-bonds require a hydrogen donor and an acceptor group, which provides a free electron pair. Some of the functional groups that can fulfill this function are hydroxyl or amino groups. They are present in a variety of natural and synthetic molecules, such as lipids, metabolites, amino acids, and pharmaceutical drugs. One example is ethanol, where the well known physiological effects appear to resul...

Neurological dysfunction and motor neuron degeneration in amyotrophic lateral sclerosis (ALS) is strongly associated with neuroinflammation reflected by activated microglia and astrocytes in the CNS. In ALS endogenous triggers in the CNS such as aggregated protein and misfolded proteins activate a pathogenic response by innate immune cells. However, there is also strong evidence for a neuroprotective immune response in ALS. Emerging evidence also reveals changes in the peripheral adaptive immune responses as well as alterations in the blood brain barrier that may aid traffic of lymphocytes and antibodies into the CNS. Understanding the triggers of neuroinflammation is key to controlling neuronal loss. Here, we review the current knowledge regarding the roles of non-neuronal cells as well as the innate and adaptive immune responses in ALS. Existing ALS animal models, in particular genetic rodent models, are very useful to study the underlying pathogenic mechanisms of motor neuron degeneration. We also discuss the approaches used to target the pathogenic immune responses and boost the neuroprotective immune pathways as novel immunotherapies for ALS.