Microglia induce myelin basic protein-specific T cell anergy or T cell activation, according to their state of activation

Strategies to target the central nervous system HIV reservoir

PURPOSE OF THE REVIEW

The central nervous system (CNS) is an hotspot for HIV persistence and may be a major obstacle to overcome for curative strategies. The peculiar anatomical, tissular and cellular characteristics of the HIV reservoir in the CNS may need to be specifically addressed to achieve a long-term HIV control without ART. In this review, we will discuss the critical challenges that currently explored curative strategies may face in crossing the blood-brain barrier (BBB), targeting latent HIV in brain-resident myeloid reservoirs, and eliminating the virus without eliciting dangerous neurological adverse events.

RECENT FINDINGS

Latency reversing agents (LRA), broadly neutralizing monoclonal antibodies (bNabs), chimeric antigen receptor (CAR) T-cells, and adeno-associated virus 9-vectored gene-therapies cross the BBB with varying efficiency. Although brain penetration is poor for bNAbs, viral vectors for in vivo gene-editing, certain LRAs, and CAR T-cells may reach the cerebral compartment more efficiently. All these approaches, however, may encounter difficulties in eliminating HIV-infected perivascular macrophages and microglia. Safety, including local neurological adverse effects, may also be a concern, especially if high doses are required to achieve optimal brain penetration and efficient brain cell targeting.

SUMMARY

Targeting the CNS remains a potential problem for the currently investigated HIV curing strategies. In vivo evidence on CNS effectiveness is limited for most of the investigated strategies, and additional studies should be focused on evaluating the interplay between the cerebral HIV reservoir and treatment aiming to achieve an ART-free cure.

Microglial Inflammatory Responses to SARS-CoV-2 Infection: A Comprehensive Review

Coronavirus disease 2019 (COVID-19) is primarily a respiratory disease causing a worldwide pandemic in the year of 2019. SARS-CoV-2 is an enveloped, positive-stranded RNA virus that could invade the host through spike protein and exhibits multi-organ effects. The Brain was considered to be a potential target for SARS-CoV-2 infection. Although neuropsychiatric symptoms and cognitive impairments were observed in COVID-19 patients even after recovery the mechanism of action is not well documented. In this review, the contribution of microglia in response to SARS-CoV-2 infection was discussed aiming to design a therapeutic regimen for the management of neuroinflammation and psycho-behavioral alterations. Priming of microglia facilitates the hyper-activation state when it interacts with SARS-CoV-2 known as the 'second hit'. Moreover, the microgliosis produces reactive free radicals and pro-inflammatory cytokines like IL-1β, IFN-γ, and IL-6 which ultimately contribute to a 'cytokine storm', thereby increasing the occurrence of cognitive and neurological dysfunction. It was reported that elevated CCL11 may be responsible for psychiatric disorders and ROS/RNS-induced oxidative stress could promote major depressive disorder (MDD) and phenotypic switching. Additionally, during SARS-CoV-2 infection microglia-CD8+ T cell interaction may have a significant role in neuronal cell death. This cytokine-mediated cellular cross-talking plays a crucial role in pro-inflammatory and anti-inflammatory balance within the COVID-19 patient's brain. Therefore, all these aspects will be taken into consideration for developing novel therapeutic strategies to combat SARS-CoV-2-induced neuroinflammation.

Differential regulation of microglial states by colony stimulating factors

Recent studies have emphasized the role of microglia in the progression of many neurodegenerative diseases. The colony stimulating factors, CSF-1 (M-CSF), granulocyte-macrophage CSF (GM-CSF) and granulocyte CSF (G-CSF) regulate microglia through different cognate receptors. While the receptors for GM-CSF (GM-CSFR) and G-CSF (G-CSFR) are specific for their ligands, CSF-1 shares its receptor, the CSF-1 receptor-tyrosine kinase (CSF-1R), with interleukin-34 (IL-34). All four cytokines are expressed locally in the CNS. Activation of the CSF-1R in macrophages is anti-inflammatory. In contrast, the actions of GM-CSF and G-CSF elicit different activated states. We here review the roles of each of these cytokines in the CNS and how they contribute to the development of disease in a mouse model of CSF-1R-related leukodystrophy. Understanding their roles in this model may illuminate their contribution to the development or exacerbation of other neurodegenerative diseases.

Tissue-resident immune cells in the pathogenesis of multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease of the central nervous system (CNS) in which genetic and environmental factors contribute to disease progression. Both innate and adaptive immune cells, including T cells, B cells, activated macrophages and microglia, have been identified to be involved in the pathogenesis of MS, leading to the CNS inflammation, neurodegeneration and demyelination. In recent years, there has been considerable progress in understanding the contribution of tissue-resident immune cells in the pathogenesis of MS.

METHODS

We performed a keyword-based search in PubMed database. We combined "multiple sclerosis" with keywords, such as tissue-resident memory T cells, microglia to search for relevant literatures in PubMed.

RESULTS AND CONCLUSION

In this review, we comprehensively describe the characteristics of tissue-resident memory T cells and microglia, summarize their role in the pathogenesis of MS, and discuss their interaction with other immune cells in the CNS.

TH Cells and Cytokines in Encephalitogenic Disorders

The invasion of immune cells into the central nervous system (CNS) is a hallmark of the process we call neuroinflammation. Diseases such as encephalitides or multiple sclerosis (MS) are characterised by the dramatic influx of T lymphocytes and monocytes. The communication between inflammatory infiltrates and CNS resident cells is primarily mediated through cytokines. Over the years, numerous cytokine networks have been assessed to better understand the development of immunopathology in neuroinflammation. In MS for instance, many studies have shown that CD4⁺ T cells infiltrate the CNS and subsequently lead to immunopathology. Inflammatory CD4⁺ T cells, such as T_H1, T_H17, GM-CSF-producing helper T cells are big players in chronic neuroinflammation. Conversely, encephalitogenic or meningeal regulatory T cells (T_REGs) and T_H2 cells have been shown to drive a decrease in inflammatory functions in microglial cells and thus promote a neuroprotective microenvironment. Recent studies report overlapping as well as differential roles of these cells in tissue inflammation. Taken together, this suggests a more complex relationship between effector T cell subsets in neuroinflammation than has hitherto been established. In this overview, we review the interplay between helper T cell subsets infiltrating the CNS and how they actively contribute to neuroinflammation and degeneration. Importantly, in this context, we will especially focus on the current knowledge regarding the contribution of various helper cell subsets to neuroinflammation by referring to their helper T cell profile in the context of their target cell.

Tertiary Lymphoid Structures in the Central Nervous System: Implications for Glioblastoma

Glioblastoma is the most common and aggressive brain tumor, which is uniformly lethal due to its extreme invasiveness and the absence of curative therapies. Immune checkpoint inhibitors have not yet proven efficacious for glioblastoma patients, due in part to the low prevalence of tumor-reactive T cells within the tumor microenvironment. The priming of tumor antigen-directed T cells in the cervical lymph nodes is complicated by the shortage of dendritic cells and lack of appropriate lymphatic vessels within the brain parenchyma. However, recent data suggest that naive T cells may also be primed within brain tumor-associated tertiary lymphoid structures. Here, we review the current understanding of the formation of these structures within the central nervous system, and hypothesize that promotion of tertiary lymphoid structures could enhance priming of tumor antigen-targeted T cells and sensitize glioblastomas to cancer immunotherapy.

Senescent accelerated prone 8 (SAMP8) mice as a model of age dependent neuroinflammation

BACKGROUND

Aging and age-related diseases are strong risk factors for the development of neurodegenerative diseases. Neuroinflammation (NIF), as the brain's immune response, plays an important role in aged associated degeneration of central nervous system (CNS). There is a need for well characterized animal models that will allow the scientific community to understand and modulate this process.

METHODS

We have analyzed aging-phenotypical and inflammatory changes of brain myeloid cells (bMyC) in a senescent accelerated prone aged (SAMP8) mouse model, and compared with their senescence resistant control mice (SAMR1). We have performed morphometric methods to evaluate the architecture of cellular prolongations and determined the appearance of Iba1⁺ clustered cells with aging. To analyze specific constant brain areas, we have performed stereology measurements of Iba1⁺ cells in the hippocampal formation. We have isolated bMyC from brain parenchyma (BP) and choroid plexus plus meningeal membranes (m/Ch), and analyzed their response to systemic lipopolysaccharide (LPS)-driven inflammation.

RESULTS

Aged 10 months old SAMP8 mice present many of the hallmarks of aging-dependent neuroinflammation when compared with their SAMR1 control, i.e., increase of protein aggregates, presence of Iba1⁺ clusters, but not an increase in the number of Iba1⁺ cells. We have further observed an increase of main inflammatory mediator IL-1β, and an augment of border MHCII⁺Iba1⁺ cells. Isolated CD45⁺ bMyC from brain parenchyma (BP) and choroid plexus plus meningeal membranes (m/Ch) have been analyzed, showing that there is not a significant increase of CD45⁺ cells from the periphery. Our data support that aged-driven pro-inflammatory cytokine interleukin 1 beta (IL-1β) transcription is enhanced in CD45⁺BP cells. Furthermore, LPS-driven systemic inflammation produces inflammatory cytokines mainly in border bMyC, sensed to a lesser extent by the BP bMyC, showing that IL-1β expression is further augmented in aged SAMP8 compared to control SAMR1.

CONCLUSION

Our data validate the SAMP8 model to study age-associated neuroinflammatory events, but careful controls for age and strain are required. These animals show morphological changes in their bMyC cell repertoires associated to age, corresponding to an increase in the production of pro-inflammatory cytokines such as IL-1β, which predispose the brain to an enhanced inflammatory response after LPS-systemic challenge.

Oligodendroglial connexin 47 regulates neuroinflammation upon autoimmune demyelination in a novel mouse model of multiple sclerosis

In multiple sclerosis plaques, oligodendroglial connexin (Cx) 47 constituting main gap junction channels with astroglial Cx43 is persistently lost. As mice with Cx47 single knockout exhibit no demyelination, the roles of Cx47 remain undefined. We aimed to clarify the effects of oligodendroglia-specific Cx47 inducible conditional knockout (icKO) on experimental autoimmune encephalomyelitis (EAE) induced by myelin oligodendrocyte glycoprotein peptide (MOG35-55) in PLP/CreERT;Cx47fl/fl mice at 14 d after tamoxifen injection. Cx47 icKO mice demonstrated exacerbation of acute and chronic relapsing EAE with more pronounced demyelination than Cx47 flox (fl)/fl littermates. CD3+ T cells more abundantly infiltrated the spinal cord in Cx47 icKO than in Cx47 fl/fl mice throughout the acute to chronic phases. CXCR3-CCR6+CD4+ and IL17+IFNγ-CD4+ helper T (Th) 17 cells isolated from spinal cord and brain tissues were significantly increased in Cx47 icKO mice compared with Cx47 fl/fl mice, while MOG35-55-specific proliferation and proinflammatory cytokine production of splenocytes were unaltered. Microarray analysis of isolated microglia revealed stronger microglial activation toward proinflammatory and injury-response phenotypes with increased expressions of chemokines that can attract Th17 cells, including Ccl2, Ccl3, Ccl4, Ccl7, and Ccl8, in Cx47 icKO mice compared with Cx47 fl/fl mice. In Cx47 icKO mice, NOS2+ and MHC class II+ microglia were more enriched immunohistochemically, and A1-specific astroglial gene expressions and astroglia immunostained for C3, a representative A1 astrocyte marker, were significantly increased at the acute phase, compared with Cx47 fl/fl mice. These findings suggest that oligodendroglia-specific Cx47 ablation induces severe inflammation upon autoimmune demyelination, underscoring a critical role for Cx47 in regulating neuroinflammation.

When encephalitogenic T cells collaborate with microglia in multiple sclerosis

Immune cells mediate critical inflammatory and neurodegenerative processes in the CNS in individuals with multiple sclerosis (MS). In MS, activated microglia, border-associated macrophages and monocyte-derived macrophages in the CNS can encounter T cells that have infiltrated the brain parenchyma from the circulation. Although microglia and T cells both contribute to normal CNS development and homeostasis, evidence suggests that the meeting of activated microglia and macrophages with encephalitogenic T cells exacerbates their capacity to inflict injury. This crosstalk involves many cell-surface molecules, cytokines and neurotoxic factors. In this Review, we summarize the mechanisms and consequences of T cell-microglia interactions as identified with in vitro experiments and animal models, and discuss the challenges that arise when translating this preclinical knowledge to MS in humans. We also consider therapeutic approaches to MS of which the mechanisms involve prevention or modulation of T cell and microglia responses and their interactions.

FGL2 promotes tumor progression in the CNS by suppressing CD103+ dendritic cell differentiation

Few studies implicate immunoregulatory gene expression in tumor cells in arbitrating brain tumor progression. Here we show that fibrinogen-like protein 2 (FGL2) is highly expressed in glioma stem cells and primary glioblastoma (GBM) cells. FGL2 knockout in tumor cells did not affect tumor-cell proliferation in vitro or tumor progression in immunodeficient mice but completely impaired GBM progression in immune-competent mice. This impairment was reversed in mice with a defect in dendritic cells (DCs) or CD103+ DC differentiation in the brain and in tumor-draining lymph nodes. The presence of FGL2 in tumor cells inhibited granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced CD103+ DC differentiation by suppressing NF-κB, STAT1/5, and p38 activation. These findings are relevant to GBM patients because a low level of FGL2 expression with concurrent high GM-CSF expression is associated with higher CD8B expression and longer survival. These data provide a rationale for therapeutic inhibition of FGL2 in brain tumors.

The role of T cells in the pathogenesis of Parkinson's disease

Recent evidence has shown that neuroinflammation plays a key role in the pathogenesis of Parkinson's disease (PD). However, different components of the brain's immune system may exert diverse effects on neuroinflammatory events in PD. The adaptive immune response, especially the T cell response, can trigger type 1 pro-inflammatory activities and suppress type 2 anti-inflammatory activities, eventually resulting in deregulated neuroinflammation and subsequent dopaminergic neurodegeneration. Additionally, studies have increasingly shown that therapies targeting T cells can alleviate neurodegeneration and motor behavior impairment in animal models of PD. Therefore, we conclude that abnormal T cell-mediated immunity is a fundamental pathological process that may be a promising translational therapeutic target for Parkinson's disease.

Asiaticoside counteracts the in vitro activation of microglia and astrocytes: Innuendo for multiple sclerosis

BACKGROUND

During the development of Multiple Sclerosis (MS) there is a marked activation of microglia and astrocyte, leading to progressive inflammation and degeneration of myelin sheath which results in axonal loss and neuron damage.

PURPOSE

In this study, we have explored the action of Asiaticoside A against the activated microglia and astrocytes.

METHODS

Primary microglia and astrocyte cultures were used for the study and the activity were evaluated using cell proliferation assay, nitrate assay and TNFα estimation using ELISA.

RESULTS

Asiaticoside A inhibited the production of nitric oxide and TNFα in LPS activated primary microglia and astrocyte cultures.

CONCLUSION

This study suggests that Asiaticoside A may be effective against the progression of MS.

Microglia and Aging: The Role of the TREM2-DAP12 and CX3CL1-CX3CR1 Axes

Depending on the species, microglial cells represent 5-20% of glial cells in the adult brain. As the innate immune effector of the brain, microglia are involved in several functions: regulation of inflammation, synaptic connectivity, programmed cell death, wiring and circuitry formation, phagocytosis of cell debris, and synaptic pruning and sculpting of postnatal neural circuits. Moreover, microglia contribute to some neurodevelopmental disorders such as Nasu-Hakola disease (NHD), and to aged-associated neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and others. There is evidence that human and rodent microglia may become senescent. This event determines alterations in the microglia activation status, associated with a chronic inflammation phenotype and with the loss of neuroprotective functions that lead to a greater susceptibility to the neurodegenerative diseases of aging. In the central nervous system (CNS), Triggering Receptor Expressed on Myeloid Cells 2-DNAX activation protein 12 (TREM2-DAP12) is a signaling complex expressed exclusively in microglia. As a microglial surface receptor, TREM2 interacts with DAP12 to initiate signal transduction pathways that promote microglial cell activation, phagocytosis, and microglial cell survival. Defective TREM2-DAP12 functions play a central role in the pathogenesis of several diseases. The CX3CL1 (fractalkine)-CX3CR1 signaling represents the most important communication channel between neurons and microglia. The expression of CX3CL1 in neurons and of its receptor CX3CR1 in microglia determines a specific interaction, playing fundamental roles in the regulation of the maturation and function of these cells. Here, we review the role of the TREM2-DAP12 and CX3CL1-CX3CR1 axes in aged microglia and the involvement of these pathways in physiological CNS aging and in age-associated neurodegenerative diseases.

Natural Docosahexaenoic Acid in the Triglyceride Form Attenuates In Vitro Microglial Activation and Ameliorates Autoimmune Encephalomyelitis in Mice

Many neurodegenerative diseases are associated, at least in part, to an inflammatory process in which microglia plays a major role. The effect of the triglyceride form of the omega-3 polyunsaturated fatty acid docosahexaenoic acid (TG-DHA) was assayed in vitro and in vivo to assess the protective and anti-inflammatory activity of this compound. In the in vitro study, BV-2 microglia cells were previously treated with TG-DHA and then activated with Lipopolysaccharide (LPS) and Interferon-gamma (IFN-γ). TG-DHA treatment protected BV-2 microglia cells from oxidative stress toxicity attenuating NO production and suppressing the induction of inflammatory cytokines. When compared with DHA in the ethyl-ester form, a significant difference in the ability to inhibit NO production in favor of TG-DHA was observed. TG-DHA inhibited significantly splenocyte proliferation but isolated CD4+ lymphocyte proliferation was unaffected. In a mice model of autoimmune encephalomyelitis (EAE), 250 mg/kg/day oral TG-DHA treatment was associated with a significant amelioration of the course and severity of the disease as compared to untreated animals. TG-DHA-treated EAE mice showed a better weight profile, which is a symptom related to a better course of encephalomyelitis. TG-DHA may be a promising therapeutic agent in neuroinflammatory processes and merit to be more extensively studied in human neurodegenerative disorders.

Pathologic and Protective Roles for Microglial Subsets and Bone Marrow- and Blood-Derived Myeloid Cells in Central Nervous System Inflammation

Inflammation is a series of processes designed for eventual clearance of pathogens and repair of damaged tissue. In the context of autoimmune recognition, inflammatory processes are usually considered to be pathological. This is also true for inflammatory responses in the central nervous system (CNS). However, as in other tissues, neuroinflammation can have beneficial as well as pathological outcomes. The complex role of encephalitogenic T cells in multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE) may derive from heterogeneity of the myeloid cells with which these T cells interact within the CNS. Myeloid cells, including resident microglia and infiltrating bone marrow-derived cells, such as dendritic cells (DC) and monocytes/macrophages [bone marrow-derived macrophages (BMDM)], are highly heterogeneous populations that may be involved in neurotoxicity and also immunoregulation and regenerative processes. Better understanding and characterization of myeloid cell heterogeneity is essential for future development of treatments controlling inflammation and inducing neuroprotection and neuroregeneration in diseased CNS. Here, we describe and compare three populations of myeloid cells: CD11c⁺ microglia, CD11c⁻ microglia, and CD11c⁺ blood-derived cells in terms of their pathological versus protective functions in the CNS of mice with EAE. Our data show that CNS-resident microglia include functionally distinct subsets that can be distinguished by their expression of CD11c. These subsets differ in their expression of Arg-1, YM1, iNOS, IL-10, and IGF-1. Moreover, in contrast to BMDM/DC, both subsets of microglia express protective interferon-beta (IFNβ), high levels of colony-stimulating factor-1 receptor, and do not express the Th1-associated transcription factor T-bet. Taken together, our data suggest that CD11c⁺ microglia, CD11c⁻ microglia, and infiltrating BMDM/DC represent separate and distinct populations and illustrate the heterogeneity of the CNS inflammatory environment.

Multiple Sclerosis and T Lymphocytes: An Entangled Story

Multiple sclerosis (MS) is the prototypic inflammatory disease of the central nervous system (CNS) characterized by multifocal areas of demyelination, axonal damage, activation of glial cells, and immune cell infiltration. Despite intensive years of research, the etiology of this neurological disorder remains elusive. Nevertheless, the abundance of immune cells such as T lymphocytes and their products in CNS lesions of MS patients supports the notion that MS is an immune-mediated disorder. An important body of evidence gathered from MS animal models such as experimental autoimmune encephalomyelitis (EAE), points to the central contribution of CD4 T lymphocytes in disease pathogenesis. Both Th1 (producing interferon-γ) and Th17 (producing interleukin 17) CD4 T lymphocytes targeting CNS self-antigens have been implicated in MS and EAE pathobiology. Moreover, several publications suggest that CD8 T lymphocytes also participate in the development of MS lesions. The migration of activated T lymphocytes from the periphery into the CNS has been identified as a crucial step in the formation of MS lesions. Several factors promote such T cell extravasation including: molecules (e.g., cell adhesion molecules) implicated in the T cell-blood brain barrier interaction, and chemokines produced by neural cells. Finally, once in the CNS, T lymphocytes need to be reactivated by local antigen presenting cells prior to enter the parenchyma where they can initiate damage. Further investigations will be necessary to elucidate the impact of environmental factors (e.g., gut microbiota) and CNS intrinsic properties (e.g., microglial activation) on this inflammatory neurological disease.

Immunotherapy for malignant glioma

Malignant gliomas (MG) are the most common type of primary malignant brain tumor. Most patients diagnosed with glioblastoma (GBM), the most common and malignant glial tumor, die within 12-15 months. Moreover, conventional treatment, which includes surgery followed by radiation and chemotherapy, can be highly toxic by causing nonspecific damage to healthy brain and other tissues. The shortcomings of standard-of-care have thus created a stimulus for the development of novel therapies that can target central nervous system (CNS)-based tumors specifically and efficiently, while minimizing off-target collateral damage to normal brain. Immunotherapy represents an investigational avenue with the promise of meeting this need, already having demonstrated its potential against B-cell malignancy and solid tumors in clinical trials. T-cell engineering with tumor-specific chimeric antigen receptors (CARs) is one proven approach that aims to redirect autologous patient T-cells to sites of tumor. This platform has evolved dramatically over the past two decades to include an improved construct design, and these modern CARs have only recently been translated into the clinic for brain tumors. We review here emerging immunotherapeutic platforms for the treatment of MG, focusing on the development and application of a CAR-based strategy against GBM.

Mechanisms of intimate and long-distance cross-talk between glioma and myeloid cells: how to break a vicious cycle

Glioma-associated microglia and macrophages (GAMs) and myeloid-derived suppressor cells (MDSCs) condition the glioma microenvironment to generate an immunosuppressed niche for tumour expansion. This immunosuppressive microenvironment is considered to be shaped through a complex multi-step interactive process between glioma cells, GAMs and MDSCs. Glioma cells recruit GAMs and MDSCs to the tumour site and block their maturation. Glioma cell-derived factors subsequently skew these cells towards an immunosuppressive, tumour-promoting phenotype. Finally, GAMs and MDSCs enhance immune suppression in the glioma microenvironment and promote glioma growth, invasiveness, and neovascularization. The local and distant cross-talk between glioma cells and GAMs and MDSCs is regulated by a plethora of soluble proteins and cell surface-bound factors, and possibly via extracellular vesicles and platelets. Importantly, GAMs and MDSCs have been reported to impair the efficacy of glioma therapy, in particular immunotherapeutic approaches. Therefore, advancing our understanding of the function of GAMs and MDSCs in brain tumours and targeted intervention of their immunosuppressive function may benefit the treatment of glioma.

Can we switch microglia's phenotype to foster neuroprotection? Focus on multiple sclerosis

Microglia cells, the resident innate immune cells in the brain, are highly active, extending and retracting highly motile processes through which they continuously survey their microenvironment for 'danger signals' and interact dynamically with surrounding cells. Upon sensing changes in their central nervous system microenvironment, microglia become activated, undergoing morphological and functional changes. Microglia activation is not an 'all-or-none' process, but rather a continuum depending on encountered stimuli, which is expressed through a spectrum of molecular and functional phenotypes ranging from so-called 'classically activated', with a highly pro-inflammatory profile, to 'alternatively activated' associated with a beneficial, less inflammatory, neuroprotective profile. Microglia activation has been demonstrated in most neurological diseases of diverse aetiology and has been implicated as a contributor to neurodegeneration. The possibility to promote microglia's neuroprotective phenotype has therefore become a therapeutic goal. We have focused our discussion on the role of microglia in multiple sclerosis, a prototype of inflammatory, demyelinating, neurodegenerative disease, and on the effect of currently approved or on-trial anti-inflammatory therapeutic strategies that might mediate neuroprotection at least in part through their effect on microglia by modifying their behaviour via a switch of their functional phenotype from a detrimental to a protective one. In addition to pharmaceutical approaches, such as treatment with glatiramer acetate, interferon-β, fingolimod or dimethyl fumarate, we address the alternative therapeutic approach of treatment with mesenchymal stem cells and their potential role in neuroprotection through their 'calming' effect on microglia.

Phenotypic Identification of Spinal Cord-Infiltrating CD4+ T Lymphocytes in a Murine Model of Neuropathic Pain

BACKGROUND

Neuropathic pain is one of the most devastating kinds of chronic pain. Neuroinflammation has been shown to contribute to the development of neuropathic pain. We have previously demonstrated that lumbar spinal cord-infiltrating CD4⁺ T lymphocytes contribute to the maintenance of mechanical hypersensitivity in spinal nerve L5 transection (L5Tx), a murine model of neuropathic pain. Here, we further examined the phenotype of the CD4⁺ T lymphocytes involved in the maintenance of neuropathic pain-like behavior via intracellular flow cytometric analysis and explored potential interactions between infiltrating CD4⁺ T lymphocytes and spinal cord glial cells.

RESULTS

We consistently observed significantly higher numbers of T-Bet⁺, IFN-γ⁺, TNF-α⁺, and GM-CSF⁺, but not GATA3⁺ or IL-4⁺, lumbar spinal cord-infiltrating CD4⁺ T lymphocytes in the L5Tx group compared to the sham group at day 7 post-L5Tx. This suggests that the infiltrating CD4⁺ T lymphocytes expressed a pro-inflammatory type 1 phenotype (Th1). Despite the observation of CD4⁺ CD40 ligand (CD154)⁺ T lymphocytes in the lumbar spinal cord post-L5Tx, CD154 knockout (KO) mice did not display significant changes in L5Tx-induced mechanical hypersensitivity, indicating that T lymphocyte-microglial interaction through the CD154-CD40 pathway is not necessary for L5Tx-induced hypersensitivity. In addition, spinal cord astrocytic activation, represented by glial fibillary acidic protein (GFAP) expression, was significantly lower in CD4 KO mice compared to wild type (WT) mice at day 14 post-L5Tx, suggesting the involvement of astrocytes in the pronociceptive effects mediated by infiltrating CD4⁺ T lymphocytes.

CONCLUSIONS

In all, these data indicate that the maintenance of L5Tx-induced neuropathic pain is mostly mediated by Th1 cells in a CD154-independent manner via a mechanism that could involve multiple Th1 cytokines and astrocytic activation.

The kinetics of myelin antigen uptake by myeloid cells in the central nervous system during experimental autoimmune encephalomyelitis

Induction of experimental autoimmune encephalomyelitis (EAE) in susceptible animals requires reactivation of encephalitogenic CD4(+) T cells by APCs in the CNS. However, it has remained unresolved from where APCs in the CNS acquire myelin Ag for T cell activation and under which conditions, that is, whether only during EAE or also in the naive CNS. In this study, we investigated the kinetics of myelin Ag uptake by CNS APCs during EAE and in the naive CNS. Our results show that during EAE CX3CR1(+)CD11b(+) microglia were the first APCs in the CNS to contain myelin Ag upon induction of disease, albeit in very small numbers. Dendritic cells (DCs) arrived in the CNS in sizable numbers significantly later (day 5 postimmunization), without detectable myelin Ag, but acquired it by day 7 postimmunization. Furthermore, a sharp increase in neuroantigen-containing DCs coincided with the onset of EAE symptoms. Importantly, in naive mice a low but consistent number of microglia contained myelin Ag, suggesting release by oligodendrocytes under steady state conditions. Although microglia isolated from naive brain and spinal cord did not elicit a strong CD4(+) T cell response in vitro, myelin Ag-containing microglia may still play a local role in modulating encephalitogenic CD4(+) T cell responses in early EAE prior to the arrival of other professional APCs, such as DCs. Finally, newly arriving DCs in the CNS not yet loaded with myelin Ag before the onset of EAE may be a potential therapeutic target.

A new type of microglia gene targeting shows TAK1 to be pivotal in CNS autoimmune inflammation

Microglia are brain macrophages and, as such, key immune-competent cells that can respond to environmental changes. Understanding the mechanisms of microglia-specific responses during pathologies is hence vital for reducing disease burden. The definition of microglial functions has so far been hampered by the lack of genetic in vivo approaches that allow discrimination of microglia from closely related peripheral macrophage populations in the body. Here we introduce a mouse experimental system that specifically targets microglia to examine the role of a mitogen-associated protein kinase kinase kinase (MAP3K), transforming growth factor (TGF)-β-activated kinase 1 (TAK1), during autoimmune inflammation. Conditional depletion of TAK1 in microglia only, not in neuroectodermal cells, suppressed disease, significantly reduced CNS inflammation and diminished axonal and myelin damage by cell-autonomous inhibition of the NF-κB, JNK and ERK1/2 pathways. Thus, we found TAK1 to be pivotal in CNS autoimmunity, and we present a tool for future investigations of microglial function in the CNS.

Microglial phenotype and adaptation

Microglia are the prime innate immune cells of the central nervous system. They can transit from a (so-called) resting state under homeostatic conditions towards a pro-inflammatory activation state upon homeostatic disturbances. Under neurodegenerative conditions, microglia have been largely perceived as neurotoxic cells. It is now becoming clear that resting microglia are not inactive but that they serve house-keeping functions. Moreover, microglia activity is not limited to proinflammatory responses, but covers a spectrum of reactive profiles. Depending on the actual situation, activated microglia display specific effector functions supporting inflammation, tissue remodeling, synaptic plasticity and neurogenesis. Many of these functions not only relate to the current state of the local neural environment but also depend on previous experience. In this review, we address microglia functions with respect to determining factors, phenotypic presentations, adaptation to environmental signals and aging. Finally, we point out primary mechanisms of microglia activation, which may comprise therapeutic targets to control neuro-inflammatory and neurodegenerative activity.

Impact of IVIg on the interaction between activated T cells and microglia

When human microglia are co-cultured with activated human T lymphocytes, several cytokines become up-regulated in significant quantities. This condition can also occur at sites of inflammation in autoimmune inflammatory diseases of the central nervous system (CNS), including multiple sclerosis (MS), where T cells infiltrate the brain tissue and come in proximity to microglia. Therefore, T cell-microglia interaction is a potential avenue of drug therapy to decrease neuroinflammation. An immunomodulator used in autoimmune disorders is intravenous immunoglobulins (IVIg). The mechanisms of IVIg activity in diseases such as MS remain unclear. Here, we report that the application of IVIg to activated T cells leads to their decreased ability to engage microglia. As a result of IVIg treatment of T cells, there were reduced levels of tumor necrosis factor-alpha a and interleukin-10 in T cell-microglia co-culture. Our results add to the understanding of how IVIg may affect inflammation of the CNS.

Astrocytes inhibit microglial surface expression of dendritic cell-related co-stimulatory molecules through a contact-mediated process

Murine microglia cultured in isolation were treated sequentially with granulocyte/monocyte colony-stimulating factor (GM-CSF) (5 days) and lipopolysaccharide (LPS) (2 days) to elicit a mature dendritic cell-like (DC-like) phenotype. Examined by flow cytometry microglia thus isolated show high surface expression of CD11c together with the co-stimulatory molecules CD40, CD80, and CD86 that are necessary for T-cell activation. In contrast, microglia co-cultured with astrocytes fail to achieve a mature DC-like phenotype. Contact with the astrocytic environment is necessary for the inhibition. Failure was not because of a more rapid degradation of protein. Bone marrow-derived cells, like microglia, were prevented by astrocytes from attaining a mature DC phenotype. Although GM-CSF pre-treatment substantially increases mRNA of co-stimulatory molecules and major histocompatibility complex (MHC) Class II in isolated microglia, co-cultured microglia await treatment with LPS to up-regulate them. In contrast, western blot and immunocytochemical analysis revealed that it is not a failure of transcription or translation, nor is it a more rapid degradation of mRNA that is responsible for the low surface expression; rather microglia co-cultured with astrocytes produce mRNA and protein but do not traffic the protein onto the cell surface.

Differential lumbar spinal cord responses among wild type, CD4 knockout, and CD40 knockout mice in spinal nerve L5 transection-induced neuropathic pain

BACKGROUND

Our previous studies have indicated that both lumbar spinal cord-infiltrating CD4+ T cells and microglial CD40 contribute to the maintenance of mechanical hypersensitivity in a murine model of neuropathic pain spinal nerve L5 transection (L5Tx). To further delineate the CD4 and CD40-mediated mechanisms involved in the development of L5Tx-induced neuropathic pain behaviors, we examined the lumbar spinal cord mononuclear cells of wild type (WT) BALB/c, BALB/c-CD4 knockout (KO), and BALB/c-CD40 KO mice via flow cytometry.

RESULTS

In WT mice, L5Tx induced significant but transient (at day 3 and/or day 7) increases of the total numbers of mononuclear cells, microglial cells (CD45loCD11b+), and infiltrating leukocytes (CD45hi) in the ipsilateral side of the spinal cord. In CD4 KO mice, significant elevation of microglia was detected only on day 7 post-L5Tx, while no significant increase in infiltrating leukocytes post-L5Tx was observed. CD40 KO mice did not exhibit any of the changes observed in WT mice. Furthermore, neutralizing CD40 antibody treatment indicated an early involvement of CD40 signaling in the development of L5Tx-induced mechanical hypersensitivity.

CONCLUSIONS

Altogether, data indicate that both CD4 and CD40 play a role in L5Tx-induced leukocyte infiltration into the lumbar spinal cord but have differential contributions to spinal cord microglial activation following peripheral nerve injury.

Involvement of calcitonin gene-related peptide and CCL2 production in CD40-mediated behavioral hypersensitivity in a model of neuropathic pain

The neuropeptide calcitonin gene-related peptide (CGRP) is known to play a pro-nociceptive role after peripheral nerve injury upon its release from primary afferent neurons in preclinical models of neuropathic pain. We previously demonstrated a critical role for spinal cord microglial CD40 in the development of spinal nerve L5 transection (L5Tx)-induced mechanical hypersensitivity. Herein, we investigated whether CGRP is involved in the CD40-mediated behavioral hypersensitivity. First, L5Tx was found to significantly induce CGRP expression in wild-type (WT) mice up to 14 days post-L5Tx. This increase in CGRP expression was reduced in CD40 knockout (KO) mice at day 14 post-L5Tx. Intrathecal injection of the CGRP antagonist CGRP8-37 significantly blocked L5Tx-induced mechanical hypersensitivity. In vitro, CGRP induced glial IL-6 and CCL2 production, and CD40 stimulation added to the effects of CGRP in neonatal glia. Further, there was decreased CCL2 production in CD40 KO mice compared to WT mice 21 days post-L5Tx. However, CGRP8-37 did not significantly affect spinal cord CCL2 production following L5Tx in WT mice. Altogether, these data suggest that CD40 contributes to the maintenance of behavioral hypersensitivity following peripheral nerve injury in part through two distinct pathways, the enhancement of CGRP expression and spinal cord CCL2 production.

miRNA-mediated immune regulation and immunotherapeutic potential in glioblastoma

Glioblastoma (GB), the most common primary neoplasm of the CNS, remains universally fatal with standard therapies and has a mean overall survival time of only 14.6 months. Even in the most favorable situations most patients do not survive longer than 2 years. Another hallmark of GBs, apart from the poor control of proliferation, is an immune suppressed tumor microenvironment. miRNAs usually bind the 3' untranslated region of target mRNAs and direct their post-transcriptional repression. Certain miRNAs are known to have altered expression levels in GB tumors, and in many immune cell subtypes. miRNAs have been found to serve important roles in gene regulation and are implicated in many processes in oncogenesis and immune deregulation. In this article we focus on the miRNAs involved in gliomagenesis and in the regulation of the immune response. We also present current challenges and miRNA immunotherapeutic strategies that should be investigated further.

Enteroglial cells act as antigen-presenting cells in chagasic megacolon

Chagas disease is one of the most serious parasitic diseases of Latin America, with a social and economic impact far outweighing the combined effects of other parasitic diseases such as malaria, leishmaniasis, and schistosomiasis. In the chronic phase of this disease, the destruction of enteric nervous system components leads to megacolon development. Besides neurons, the enteric nervous system is constituted by enteric glial cells, representing an extensive but relatively poorly described population within the gastrointestinal tract. Several lines of evidence suggest that enteric glial cells represent an equivalent of central nervous system astrocytes. Previous data suggest that enteric glia and neurons are active in the enteric nervous system during intestinal inflammatory and immune responses. To evaluate whether these cells act as antigen-presenting cells, we investigated the expression of molecules responsible for activation of T cells, such as HLA-DR complex class II and costimulatory molecules (CD80 and CD86), by neurons and enteric glial cells. Our results indicate that only enteric glial cells of chagasic patients with megacolon express HLA-DR complex class II and costimulatory molecules, and hence they present the attributes necessary to act as antigen-presenting cells.

Immunotherapy coming of age: what will it take to make it standard of care for glioblastoma?

With the recent approval by the FDA of an immunotherapy for prostate cancer and another positive immunotherapy trial in melanoma, immunotherapy may finally be coming of age. So what will it take for it to become part of the standard treatment for glioblastoma? To put this question into perspective, we summarize critical background information in neuro-immunology, address immunotherapy clinical trial design, and discuss a number of extrinsic factors that will impact the development of immunotherapy in neuro-oncology.

Centrally administered pertussis toxin inhibits microglia migration to the spinal cord and prevents dissemination of disease in an EAE mouse model

Background

Experimental autoimmune encephalomyelitis (EAE) models are important vehicles for studying the effect of infectious elements such as Pertussis toxin (PTx) on disease processes related to acute demyelinating encephalomyelitis (ADEM) or multiple sclerosis (MS). PTx has pleotropic effects on the immune system. This study was designed to investigate the effects of PTx administered intracerebroventricularly (icv) in preventing downstream immune cell infiltration and demyelination of the spinal cord.

Methods and Findings

EAE was induced in C57BL/6 mice with MOG_35–55. PTx icv at seven days post MOG immunization resulted in mitigation of clinical motor symptoms, minimal T cell infiltration, and the marked absence of axonal loss and demyelination of the spinal cord. Integrity of the blood brain barrier was compromised in the brain whereas spinal cord BBB integrity remained intact. PTx icv markedly increased microglia numbers in the brain preventing their migration to the spinal cord. An in vitro transwell study demonstrated that PTx inhibited migration of microglia.

Conclusion

Centrally administered PTx abrogated migration of microglia in EAE mice, limiting the inflammatory cytokine milieu to the brain and prevented dissemination of demyelination. The effects of PTx icv warrants further investigation and provides an attractive template for further study regarding the pleotropic effects of infectious elements such as PTx in the pathogenesis of autoimmune disorders.

Critical role of microglial CD40 in the maintenance of mechanical hypersensitivity in a murine model of neuropathic pain

We recently demonstrated a contributing role of spinal cord infiltrating CD4+ T lymphocytes in the maintenance of mechanical hypersensitivity in a rodent model of neuropathic pain, spinal nerve L5 transection (L5Tx). It has been demonstrated that microglia play a role in the etiology of pain states. We hypothesized that infiltrating CD4+ T lymphocytes communicate with microglia via a CD40-CD154 interaction. Here, we investigated the role of CD40 in the development of mechanical hypersensitivity post-L5Tx. CD40 KO mice displayed significantly decreased mechanical sensitivity compared with WT mice starting from day 5 post-L5Tx. Using bone marrow chimeric mice, we further identified a pro-nociceptive role of CNS microglial CD40 rather than the peripheral leukocytic CD40. Flow cytometric analysis determined a significant increase of CD40+ microglia in the ipsilateral side of lumbar spinal cord post-L5Tx. Further, spinal cord proinflammatory cytokine (IL-1beta, IL-6, IL-12, and TNF-alpha) profiling demonstrated an induction of IL-6 in both WT and CD40 KO mice post-L5Tx prior to the increase of microglial CD40 expression, indicating a CD40-independent induction of IL-6 following L5Tx. These data establish a novel role of microglial CD40 in the maintenance of nerve injury-induced behavioral hypersensitivity, a behavioral sign of neuropathic pain.

Peripheral formalin injection induces unique spinal cord microglial phenotypic changes

Microglia are resident immune cells of brain and activated by peripheral tissue injury. In the present study, we investigated the possible induction of several microglial surface immunomolecules in the spinal cord, including leukocyte common antigen (LCA/CD45), MHC class I antigen, MHC class II antigen, Fc receptor, and CD11c following formalin injection into the rat's hind paw. CD45 and MHC class I were upregulated in the activated microglia, which was evident on day 3 with the peak expression on day 7 following peripheral formalin injection. There was a very low basal expression of MHC class II, CD11c, and the Fc receptor, which did not change after the formalin injection. These results, for the first time, indicate that peripheral formalin injection can induce phenotypic changes of microglia with distinct upregulation of CD45 and MHC class I antigen. The data suggest that phenotypic changes of the activated microglia may be a unique pattern of central changes following peripheral tissue injury.

Increased T-cell reactivity and elevated levels of CD8+ memory T-cells in Alzheimer's disease-patients and T-cell hyporeactivity in an Alzheimer's disease-mouse model: implications for immunotherapy

Neuroinflammation is observed in neurodegenerative diseases like Alzheimer's disease (AD). However, a little is known about the mechanisms of neural-immune interactions. The involvement of peripheral T-cell function in AD is still far from clear, though it plays an important role in immunotherapy. The aim of this study was to determine peripheral T-cell reactivity in AD patients and in an AD mouse model. Mitogenic activation via ligation of the T-cell receptor (TCR) with PHA-L was measured in T lymphocytes from AD patients and Thy1(APP 751SL) x HMG(PS1 M146L)-transgenic mice (APP x PS1). In order to uncover failures in TCR signaling, the TCR was also bypassed by PMA and ionomycin treatment. All patients were sporadic late onset cases and the transgenic mice expressed no mutant APP in lymphocytes, so that direct interactions of mutant APP on T-cell function can be excluded. CD4+ and CD8+ T-cell showed increased reactivity (tyrosine phosphorylation, CD69 expression, and proliferation) in AD, while APP x PS1 transgenic mice displayed hyporeactive CD8+ T-cells after TCR ligation. Increased levels of CD8+ T memory cells and down regulation of CD8 receptor were found in AD and the animal model. Anergic TCR uncoupling was associated with loss of MAPK signaling (p38, ERK1 and ERK2) in APP x PS1. Our data implicate the generation of reactive memory T-cell in AD and of anergic memory T-cells in transgenic mice and should be taken into concern when designing immunotherapy.

Blockade of TREM-2 exacerbates experimental autoimmune encephalomyelitis

Triggering receptor expressed on myeloid cells (TREM-2) is a membrane receptor associated with DAP12 that is expressed primarily in myeloid cells, including dendritic cells and microglia, and promotes fusion of osteoclast precursors into multinucleated cells. A rare autosomal recessive condition, Nasu-Hakola disease (NHD) is associated with loss-of-function mutations in DAP12 and TREM-2. The brain pathology observed in NHD patients suggests that disruption of the TREM-2/DAP12 pathway leads to neurodegeneration with demyelination and axonal loss. In this study, we have characterized TREM-2 protein expression on microglia using a newly produced monoclonal antibody directed against the mouse TREM-2 receptor. We report that TREM-2 expression is up-regulated in the spinal cord during both the early inflammatory and chronic phases of myelin oligodendrocyte glycoprotein (MOG)(35-55)peptide-induced experimental autoimmune encaphalomyelitis (EAE). We also demonstrate that TREM-2 is highly expressed on microglial cells in the central nervous system (CNS) during EAE and that blockade of TREM-2 during the effector phase of EAE results in disease exacerbation with more diffuse CNS inflammatory infiltrates and demyelination in the brain parenchyma. These results demonstrate a critical role for TREM-2 during inflammatory responses in the CNS.

Protection from direct cerebral cryptococcus infection by interferon-gamma-dependent activation of microglial cells

The brain represents a significant barrier for protective immune responses in both infectious disease and cancer. We have recently demonstrated that immunotherapy with anti-CD40 and IL-2 can protect mice against disseminated Cryptococcus infection. We now applied this immunotherapy using a direct cerebral cryptococcosis model to study direct effects in the brain. Administration of anti-CD40 and IL-2 significantly prolonged the survival time of mice infected intracerebrally with Cryptococcus neoformans. The protection was correlated with activation of microglial cells indicated by the up-regulation of MHC II expression on brain CD45(low)CD11b(+) cells. CD4(+) T cells were not required for either the microglial cell activation or anticryptococcal efficacy induced by this immunotherapy. Experiments with IFN-gamma knockout mice and IFN-gammaR knockout mice demonstrated that IFN-gamma was critical for both microglial cell activation and the anticryptococcal efficacy induced by anti-CD40/IL-2. Interestingly, while peripheral IFN-gamma production and microglial cell activation were observed early after treatment, negligible IFN-gamma was detected locally in the brain. These studies indicate that immunotherapy using anti-CD40 and IL-2 can augment host immunity directly in the brain against C. neoformans infection and that IFN-gamma is essential for this effect.

GM-CSF production by autoreactive T cells is required for the activation of microglial cells and the onset of experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a CNS autoimmune disease believed to be triggered by T cells secreting Th1-specific proinflammatory cytokines, such as GM-CSF. In the animal model of MS, experimental autoimmune encephalomyelitis (EAE), Th1 but not Th2 cells have been shown to induce disease; however, to date, no single encephalitogenic T cell-derived cytokine has been shown to be required for EAE onset. Because GM-CSF-deficient mice have been shown to be resistant to EAE following immunization with myelin self-Ag, we investigated the cellular source of the required GM-CSF and found that GM-CSF production by encephalitogenic T cells, but not CNS resident or other peripheral cells, was required for EAE induction. Furthermore, we showed that microglial cell activation, but not peripheral macrophage activation, was a GM-CSF-dependent process. Activation of microglial cells by the injection of LPS abrogated the GM-CSF requirement for EAE induction, suggesting that microglial cell activation is required for EAE onset. These data also demonstrate that GM-CSF is a critical Th1 cell-derived cytokine required for the initiation of CNS inflammation associated with EAE, and likely MS.

Immune response and immunotherapy to Cryptococcus infections

Cryptococcus neoformans is a ubiquitous fungus that can cause lifethreatening infections during immunosuppressive states such as acquired immunodeficiency syndrome (AIDS) and after bone marrow transplantation (BMT). Infected individuals normally succumb to meningitis and meningoencephalitis caused by dissemination of C. neoformans to the brain. In this review, we analyze the current understanding of the interaction between host immune response and C. neoformans as well as the current state of immunotherapeutic strategies for treating cryptococcosis.

Multiple sclerosis: a battle between destruction and repair

Multiple sclerosis (MS) is a chronic neurodegenerative disease of the CNS in which an unrelenting attack from the innate and adaptive arms of the immune system results in extensive demyelination, loss of oligodendrocytes and axonal degeneration. This review summarizes advances in the understanding of the cellular and molecular pathways involved in neurodegeneration following autoimmune-mediated inflammation in the CNS. The mechanisms underlying myelin and axonal destruction and the equally important interaction between degenerative and repair mechanisms are discussed. Recent studies have revealed that the failure of CNS regeneration may be in part a result of the presence of myelin-associated growth inhibitory molecules in MS lesions. Successful therapeutic intervention in MS is likely to require suppression of the inflammatory response, in concert with blockade of growth inhibitory molecules and possibly the mobilization or transplantation of stem cells for regeneration.