Phagocytotic removal of apoptotic, inflammatory lymphocytes in the central nervous system by microglia and its functional implications

Guard of delinquency? A role of microglia in inflammatory neurodegenerative diseases of the CNS

Activation of microglia and inflammation-mediated neurotoxicity are believed to play an important role in the pathogenesis of several neurodegenerative disorders, including multiple sclerosis. Studies demonstrate complex functions of activated microglia that can lead to either beneficial or detrimental outcomes, depending on the form and the timing of activation. Combined with genetic and environmental factors, overactivation and dysregulation of microglia cause progressive neurotoxic consequences which involve a vicious cycle of neuron injury and unregulated neuroinflammation. Thus, modulation of microglial activation appears to be a promising new therapeutic target. While current therapies do attempt to block activation of microglia, they indiscriminately inhibit inflammation thus also curbing beneficial effects of inflammation and delaying recovery. Multiple signaling cascades, often cross-talking, are involved in every step of microglial activation. One of the key challenges is to understand the molecular mechanisms controlling cytokine expression and phagocytic activity, as well as cell-specific consequences of dysregulated cytokine expression. Further, a better understanding of how the integration of multiple cytokine signals influences the function or activity of individual microglia remains an important research objective to identify potential therapeutic targets for clinical intervention to promote repair.

The paradox of chronic neuroinflammation, systemic immune suppression, autoimmunity after traumatic chronic spinal cord injury

During the transition from acute to chronic stages of recovery after spinal cord injury (SCI), there is an evolving state of immunologic dysfunction that exacerbates the problems associated with the more clinically obvious neurologic deficits. Since injury directly affects cells embedded within the "immune privileged/specialized" milieu of the spinal cord, maladaptive or inefficient responses are likely to occur. Collectively, these responses qualify as part of the continuum of "SCI disease" and are important therapeutic targets to improve neural repair and neurological outcome. Generic immune suppressive therapies have been largely unsuccessful, mostly because inflammation and immunity exert both beneficial (plasticity enhancing) and detrimental (e.g. glia- and neurodegenerative; secondary damage) effects and these functions change over time. Moreover, "compartimentalized" investigations, limited to only intraspinal inflammation and associated cellular or molecular changes in the spinal cord, neglect the reality that the structure and function of the CNS are influenced by systemic immune challenges and that the immune system is 'hardwired' into the nervous system. Here, we consider this interplay during the progression from acute to chronic SCI. Specifically, we survey impaired/non-resolving intraspinal inflammation and the paradox of systemic inflammatory responses in the context of ongoing chronic immune suppression and autoimmunity. The concepts of systemic inflammatory response syndrome (SIRS), compensatory anti-inflammatory response syndrome (CARS) and "neurogenic" spinal cord injury-induced immune depression syndrome (SCI-IDS) are discussed as determinants of impaired "host-defense" and trauma-induced autoimmunity.

Plasminogen activator inhibitor type 1 regulates microglial motility and phagocytic activity

Background

Plasminogen activator inhibitor type 1 (PAI-1) is the primary inhibitor of urokinase type plasminogen activators (uPA) and tissue type plasminogen activators (tPA), which mediate fibrinolysis. PAI-1 is also involved in the innate immunity by regulating cell migration and phagocytosis. However, little is known about the role of PAI-1 in the central nervous system.

Methods

In this study, we identified PAI-1 in the culture medium of mouse mixed glial cells by liquid chromatography and tandem mass spectrometry. Secretion of PAI-1 from glial cultures was detected by ELISA and western blotting analysis. Cell migration was evaluated by in vitro scratch-wound healing assay or Boyden chamber assay and an in vivo stab wound injury model. Phagocytic activity was measured by uptake of zymosan particles.

Results

The levels of PAI-1 mRNA and protein expression were increased by lipopolysaccharide and interferon-γ stimulation in both microglia and astrocytes. PAI-1 promoted the migration of microglial cells in culture via the low-density lipoprotein receptor-related protein (LRP) 1/Janus kinase (JAK)/signal transducer and activator of transcription (STAT)1 axis. PAI-1 also increased microglial migration in vivo when injected into mouse brain. PAI-1-mediated microglial migration was independent of protease inhibition, because an R346A mutant of PAI-1 with impaired PA inhibitory activity also promoted microglial migration. Moreover, PAI-1 was able to modulate microglial phagocytic activity. PAI-1 inhibited microglial engulfment of zymosan particles in a vitronectin- and Toll-like receptor 2/6-dependent manner.

Conclusion

Our results indicate that glia-derived PAI-1 may regulate microglial migration and phagocytosis in an autocrine or paracrine manner. This may have important implications in the regulation of brain microglial activities in health and disease.

The CSF-1 receptor ligands IL-34 and CSF-1 exhibit distinct developmental brain expression patterns and regulate neural progenitor cell maintenance and maturation

The CSF-1 receptor (CSF-1R) regulates CNS microglial development. However, the localization and developmental roles of this receptor and its ligands, IL-34 and CSF-1, in the brain are poorly understood. Here we show that compared to wild type mice, CSF-1R-deficient (Csf1r-/-) mice have smaller brains of greater mass. They further exhibit an expansion of lateral ventricle size, an atrophy of the olfactory bulb and a failure of midline crossing of callosal axons. In brain, IL-34 exhibited a broader regional expression than CSF-1, mostly without overlap. Expression of IL-34, CSF-1 and the CSF-1R were maximal during early postnatal development. However, in contrast to the expression of its ligands, CSF-1R expression was very low in adult brain. Postnatal neocortical expression showed that CSF-1 was expressed in layer VI, whereas IL-34 was expressed in the meninges and layers II-V. The broader expression of IL-34 is consistent with its previously implicated role in microglial development. The differential expression of CSF-1R ligands, with respect to CSF-1R expression, could reflect their CSF-1R-independent signaling. Csf1r-/- mice displayed increased proliferation and apoptosis of neocortical progenitors and reduced differentiation of specific excitatory neuronal subtypes. Indeed, addition of CSF-1 or IL-34 to microglia-free, CSF-1R-expressing dorsal forebrain clonal cultures, suppressed progenitor self-renewal and enhanced neuronal differentiation. Consistent with a neural developmental role for the CSF-1R, ablation of the Csf1r gene in Nestin-positive neural progenitors led to a smaller brain size, an expanded neural progenitor pool and elevated cellular apoptosis in cortical forebrain. Thus our results also indicate novel roles for the CSF-1R in the regulation of corticogenesis.

FGF2 gene transfer restores hippocampal functions in mouse models of Alzheimer's disease and has therapeutic implications for neurocognitive disorders

The adult hippocampus plays a central role in memory formation, synaptic plasticity, and neurogenesis. The subgranular zone of the dentate gyrus contains neural progenitor cells with self-renewal and multilineage potency. Transgene expression of familial Alzheimer's disease-linked mutants of β-amyloid precursor protein (APP) and presenilin-1 leads to a significant inhibition of neurogenesis, which is potentially linked to age-dependent memory loss. To investigate the effect of neurogenesis on cognitive function in a relevant disease model, FGF2 gene is delivered bilaterally to the hippocampi of APP+presenilin-1 bigenic mice via an adenoassociated virus serotype 2/1 hybrid (AAV2/1-FGF2). Animals injected with AAV2/1-FGF2 at a pre- or postsymptomatic stage show significantly improved spatial learning in the radial arm water maze test. A neuropathological investigation demonstrates that AAV2/1-FGF2 injection enhances the number of doublecortin, BrdU/NeuN, and c-fos-positive cells in the dentate gyrus, and the clearance of fibrillar amyloid-β peptide (Aβ) in the hippocampus. AAV2/1-FGF2 injection also enhances long-term potentiation in another APP mouse model (J20) compared with control AAV2/1-GFP-injected littermates. An in vitro study confirmed the enhanced neurogenesis of mouse neural stem cells by direct AAV2/1-FGF2 infection in an Aβ oligomer-sensitive manner. Further, FGF2 enhances Aβ phagocytosis in primary cultured microglia, and reduces Aβ production from primary cultured neurons after AAV2/1-FGF2 infection. Thus, our data indicate that virus-mediated FGF2 gene delivery has potential as an alternative therapy of Alzheimer's disease and possibly other neurocognitive disorders.

Key role for spinal dorsal horn microglial kinin B1 receptor in early diabetic pain neuropathy

Background

The pro-nociceptive kinin B₁ receptor (B₁R) is upregulated on sensory C-fibres, astrocytes and microglia in the spinal cord of streptozotocin (STZ)-diabetic rat. This study aims at defining the role of microglial kinin B₁R in diabetic pain neuropathy.

Methods

Sprague-Dawley rats were made diabetic with STZ (65 mg/kg, i.p.), and 4 days later, two specific inhibitors of microglial cells (fluorocitrate, 1 nmol, i.t.; minocycline, 10 mg/kg, i.p.) were administered to assess the impact on thermal hyperalgesia, allodynia and mRNA expression (qRT-PCR) of B₁R and pro-inflammatory markers. Spinal B₁R binding sites ((¹²⁵I)-HPP-desArg¹⁰-Hoe 140) were also measured by quantitative autoradiography. Inhibition of microglia was confirmed by confocal microscopy with the specific marker Iba-1. Effects of intrathecal and/or systemic administration of B₁R agonist (des-Arg⁹-BK) and antagonists (SSR240612 and R-715) were measured on neuropathic pain manifestations.

Results

STZ-diabetic rats displayed significant tactile and cold allodynia compared with control rats. Intrathecal or peripheral blockade of B₁R or inhibition of microglia reversed time-dependently tactile and cold allodynia in diabetic rats without affecting basal values in control rats. Microglia inhibition also abolished thermal hyperalgesia and the enhanced allodynia induced by intrathecal des-Arg⁹-BK without affecting hyperglycemia in STZ rats. The enhanced mRNA expression (B₁R, IL-1β, TNF-α, TRPV1) and Iba-1 immunoreactivity in the STZ spinal cord were normalized by fluorocitrate or minocycline, yet B₁R binding sites were reduced by 38%.

Conclusion

The upregulation of kinin B₁R in spinal dorsal horn microglia by pro-inflammatory cytokines is proposed as a crucial mechanism in early pain neuropathy in STZ-diabetic rats.

Role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination of the CNS

As the resident innate immune cells of the central nervous system (CNS), microglia fulfil a critical role in maintaining tissue homeostasis and in directing and eliciting molecular responses to CNS damage. The human disease Multiple Sclerosis and animal models of inflammatory demyelination are characterized by a complex interplay between degenerative and regenerative processes, many of which are regulated and mediated by microglia. Cellular communication between microglia and other neural and immune cells is controlled to a large extent by the activity of cytokines. Here we review the role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination, highlighting their importance in potentiating cell damage, promoting neuroprotection and enhancing cellular repair in a context-dependent manner.

O trauma raquimedular

A medula espinhal dos mamíferos adultos não permite a regeneração de axônios. Por razões ainda desconhecidas, as fibras neurais falham em cruzar o sítio da lesão, como se não houvesse crescimento, desde a primeira tentativa. Quais mecanismos poderiam explicar a perda da capacidade de regeneração? As cicatrizes formadas pelas células da glia seriam uma consequência da falha na regeneração ou a causa? Diversas linhas de evidência sugerem que a regeneração da medula espinhal seria impedida no sistema nervoso central pela ação de fatores locais no sítio da lesão, e que o sistema nervoso central não-lesado é um meio permissivo para o crescimento axonal, na direção de alvos específicos. Uma vez que os axônios são induzidos adequadamente a cruzar a lesão com o auxílio de implantes, fármacos ou células indiferenciadas, as fibras em regeneração podem encontrar a via específica e estabelecer conexões corretas. O que ainda não se sabe é que combinação de moléculas induz/inibe o potencial de regeneração do tecido e que mecanismos permitem aos neurônios formarem conexões específicas com os alvos com os quais são programados a fazer.

C1q enhances microglial clearance of apoptotic neurons and neuronal blebs, and modulates subsequent inflammatory cytokine production

The expression of C1q, a recognition molecule of the complement system, is up-regulated following neuronal injury and is detected early in neurodegenerative disorders such as Alzheimer's disease. This multimeric protein triggers an enhancement of phagocytosis of suboptimally opsonized targets by microglia, the phagocytic cells of the CNS, similar to other phagocytes, enhances the uptake of apoptotic cells in peripheral phagocytes, and suppresses inflammatory cytokine production in human monocytes, macrophages and dendritic cells in the absence of activation of the entire complement cascade. The goal of this study was to determine if C1q could influence the inflammatory response to injury in the CNS, using primary rat microglia and neurons. The data show that microglia preferentially ingest apoptotic cells in comparison to live cells, like other professional phagocytes, that microglial ingestion of apoptotic neurons and neuronal blebs is enhanced by the presence of normal serum and that these enhanced levels of uptake are diminished in serum depleted of C1q. In addition, purified C1q bound to apoptotic neurons and neuronal blebs in a dose dependent manner, and alone triggered a significant enhancement of uptake by microglia. Microglia added to C1q coated wells or fed apoptotic neurons or neuronal blebs coated with C1q suppressed the lipopolysaccharide-induced production of proinflammatory cytokines interleukin (IL)-1alpha, IL-1beta, IL-6 and TNF-alpha, while the presence of C1q enhanced levels of the chemokine MCP-1/CCL2. The data are consistent with a protective role for C1q in the CNS during early stages of cell death by enhancing microglial clearance of apoptotic cells and suppressing proinflammatory cytokines.

Intravenous immunoglobulins induce potentially synergistic immunomodulations in autoimmune disorders

The increase in platelets in patients with immune thrombocytopenia (ITP) by intravenous administration of human immunoglobulin concentrates (IVIG) reflects a therapeutic immunomodulatory intervention targeted at the disturbed immune response in many inflammatory and autoimmune disorders. These immunoglobulin concentrates contain large numbers of antibodies as well as trace levels of various other immunologically active molecules. Clinical and laboratory studies have documented various mechanisms of action of IVIG. The complex network of immunological reactions resulting from the infusion of IVIG includes changes in several cytokines, interactions with dendritic cells, T- and B- lymphocyte effects, macrophage effects, mediated by distinct Fc-gamma receptors. In addition, effects on complement components and apoptosis have also been observed. Synergism between the different elements of the immune response characterizes the beneficial effects of IVIG in inflammatory and autoimmune disorders. They have immunopathogeneses and clinical manifestations which are difficult to define and therefore IVIG treatment indications remain heterogeneous. Dose finding studies are missing for most of the indications of the drug. In future research, defining the appropriate subgroups of patients should be undertaken. This may be accomplished by prospective registries collecting data on large numbers of patients with long-term follow-up. Controlled clinical and laboratory studies may follow based on new, validated patient selection criteria and focused on mechanisms of action, leading to more evidence-based indications.

Neuroinflammation and microglia: considerations and approaches for neurotoxicity assessment

BACKGROUND

The impact of an inflammatory response, as well as interactions between the immune and nervous systems, are rapidly assuming major roles in neurodegenerative disease and injury. However, it is now appreciated that the exact nature of such responses can differ with each type of insult and interaction. More recently, neuroinflammation and the associated cellular response of microglia are being considered for their contribution to neurotoxicity of environmental agents; yet, so far, the inclusion of inflammatory end points into neurotoxicity assessment have relied primarily on relatively limited measures or driven by in vitro models of neurotoxicity.

OBJECTIVE

To present background information on relevant biological considerations of neuroinflammation and the microglia response demonstrating the complex integrative nature of these biological processes and raising concern with regards to translation of effects demonstrated in vitro to the in vivo situation. Specific points are addressed that would influence the design and interpretation of neuroinflammation with regards to neurotoxicology assessment.

CONCLUSION

There is a complex and dynamic response in the brain to regulate inflammatory processes and maintain a normal homeostatic level. The classification of such responses as beneficial or detrimental is an oversimplification. Neuroinflammation should be considered as a balanced network of processes in which subtle modifications can shift the cells toward disparate outcomes. The tendency to overinterpret data obtained in an isolated culture system should be discouraged. Rather, the use of cross-disciplinary approaches to evaluate several end points should be incorporated into the assessment of inflammatory contributions to the neurotoxicity of environmental exposures.

End-stage dying glioma cells are engulfed by mouse microglia with a strain-dependent efficacy

Microglia phagocytic activity for apoptotic glioma cells is hardly analysed inspite of its relevance to tissue damage prevention. We provide evidence for a phosphatidylserine-independent clearance of mouse glioma cells at an advanced stage of death, suggesting microglia recognition of late apoptotic markers. Dying cells were immediately cleared or stayed for hours in that stage before engulfment occurred. This phagocytic activity was restricted to a microglia subset representing 30 to 70% of the population according to the used strain. Expression of receptors involved in late apoptotic markers recognition therefore seems confined to a subpopulation of microglia and to be strain-dependent.

Treating autoimmune demyelination by augmenting lymphocyte apoptosis in the central nervous system

The elimination of autoreactive T cells from the central nervous system (CNS) by apoptosis plays an important role in switching off autoimmune attack. B-cell apoptosis in the CNS probably also has a key role in downregulating autoimmunity. Augmenting lymphocyte apoptosis in the CNS is a potential strategy for treating autoimmune CNS diseases such as multiple sclerosis. These strategies involve modulation of the physiological pro-apoptotic and anti-apoptotic pathways that control lymphocyte fate in the CNS. In the case of T cells, apoptosis can be augmented by enhancing activation-induced T-cell apoptosis through the CD95 (Fas) pathway and by inhibiting costimulation-induced anti-apoptotic pathways mediated through BCL-2 and BCL-X L.

Mediators of apoptosis Fas and FasL predict disability progression in multiple sclerosis over a period of 10 years

TNF-alpha, IL-12p35, IL-12p40, IL-4, IL-10, TGF-beta1, CCR3, CXCR3, CCR5, Fas and FasL mRNA levels in PBMC of 25 multiple sclerosis (MS) patients were quantified at baseline by real-time PCR according to a post-hoc study design. The baseline values of the different markers were analysed with respect to their correlation with the increase in disability over a period of 10 years. High levels of Fas mRNA were associated with a favourable disease course in relapsing-remitting (RR) MS (R2 = 0.74, P = 0.0001, n = 13), as measured by the Expanded Disability Status Scale (EDSS); high levels of FasL mRNA were associated with relatively mild disease progression (R2 = 0.86, P = 0.0001, n =12) in secondary progressive (SP) MS. These findings suggest that Fas-mediated apoptosis plays a major role in the mechanism underlying long-term disease progression in MS.:

A novel approach based on nanotechnology for investigating the chronic actions of short-lived peptides in specific sites of the brain

This review presents a novel experimental approach for investigating the chronic actions of short-lived peptides in specific sites of the brain. This method combines the advantages of three different techniques: liposome encapsulation, site-specific microinjection and telemetry. First, liposomes can be designed to remain located at the injection site for a long period of time, where they protect encapsulated peptide from rapid degradation and act as a sustained-release system. Secondly, microinjection allows the administration of peptides in specific sites of the brain with minimal side effects. Finally, using telemetry, it is possible to register physiological parameters and their circadian variations in undisturbed free-moving animals for several days. Angiotensin-(1-7) and angiotensin II were used as peptide models, in order to validate the proposed method. Following the unilateral microinjection of the liposome-encapsulated peptides into the rostral ventrolateral medulla (RVLM) of Wistar rats, long-lasting cardiovascular actions were elicited, for several days. Importantly, new physiological actions of angiotensin-(1-7) at the RVLM were unmasked: modulation of the circadian rhythms of blood pressure and heart rate. It is felt that this method can be applied to a wide variety of short-lived bioactive peptides and should encounter numerous applications in the field of neurosciences.

T-cell apoptosis in human glioblastoma multiforme: Implications for immunotherapy

We used immunohistochemistry and flow cytometry to assess apoptosis in human glioblastoma multiforme (GBM). Our immunohistochemical study revealed apoptosis of glioma cells expressing glial fibrillary acidic protein and of CD3(+) T cells infiltrating GBM. To quantify and phenotype the apoptotic T cells, we performed flow cytometry on lymphocytes separated from GBM. The cells were stained with annexin-V-FLUOS/propidium iodide to identify apoptosis. We found that high proportions of both the CD4(+) and CD8(+) T cells were apoptotic. In particular, we found that T cells expressing Fas ligand (Fas-L, CD95L) were eight times more vulnerable to apoptosis than those not expressing Fas-L, which suggests that the T-cell apoptosis is induced by overactivation of the T-cell receptor, possibly in the absence of appropriate costimulation. Our results have implications for the design of immunotherapies for GBM.

Microglia recognize double-stranded RNA via TLR3

Microglia are CNS resident innate immune cells of myeloid origin that become activated and produce innate proinflammatory molecules upon encountering bacteria or viruses. TLRs are a phylogenetically conserved diverse family of sensors for pathogen-associated molecular patterns that drive innate immune responses. We have recently shown that mice deficient in TLR3 (TLR3(-/-) mice) are resistant to lethal encephalitis and have reduced microglial activation after infection with West Nile virus, a retrovirus that produces dsRNA. We wished to determine whether microglia recognize dsRNA through the TLR3 pathway. In vitro, murine wild-type primary cultured microglia responded to synthetic dsRNA polyinosinic-polycytidylic acid (poly(I:C)) by increasing TLR3 and IFN-beta mRNA and by morphologic activation. Furthermore, wild-type microglia dose dependently secreted TNF-alpha and IL-6 after poly(I:C) challenge, whereas TLR3(-/-) microglia produced diminished cytokines. Activation of MAPK occurred in a time-dependent fashion following poly(I:C) treatment of wild-type microglia, but happened with delayed kinetics in TLR3(-/-) microglia. As an in vivo model of encephalitis, wild-type or TLR3(-/-) mice were injected intracerebroventricularly with poly(I:C) or LPS, and microglial activation was assessed by cell surface marker or phospho-MAPK immunofluorescence. After intracerebroventricular injection of poly(I:C), microgliosis was clearly evident in wild-type mice but was nearly absent in TLR3(-/-) animals. When taken together, our results demonstrate that microglia recognize dsRNA through TLR3 and associated signaling molecules and suggest that these cells are key sensors of dsRNA-producing viruses that may invade the CNS.

Apoptosis inhibition in T cells triggers the expression of proinflammatory cytokines--implications for the CNS

Stimulation of death receptors such as CD95 or TNF-R1 results in rapid onset of apoptosis. Here we show that inhibition of death receptor-induced apoptosis by the broad range caspase inhibitor ZVAD causes a switch from apoptotic to proinflammatory signaling. In previous studies we have reported that caspase inhibitors induce expression of various proinflammatory cytokines in CD95-stimulated primary T cells, such as TNF-alpha, IFN-gamma and GM-CSF. In this study we provide further evidence for the proinflammatory activity of CD95. Stimulation of CD95 by agonistic antibodies (7C11) resulted in expression of IL-2 in primary T cells, which was further enhanced when caspase activity was blocked by ZVAD. Moreover, CD95 triggered expression of IL-4 and IL-8 when caspase activity was inhibited, but not in the absence of ZVAD. Our findings are of significant importance for the CNS as changes in the cytokine pattern in the periphery affects the entry of various immune cells into the brain. Moreover, invading activated T cells can also directly influence the cytokine profile within the brain, triggering signaling cascades that eventually lead to neuronal cell death. The use of caspase inhibitors to prevent apoptotic cell death should be carefully evaluated in the management of systemic and CNS diseases.

Elevated levels of DNA repair enzymes and antioxidative enzymes by (+)-catechin in murine microglia cells after oxidative stress

(+)-Catechin possesses a broad range of pharmacological properties, including antioxidative effect. However, little is reported on the mechanism by which (+)-catechin protects microglia cells from DNA damage by oxidative stress. In this study, TUNEL assay and DNA electrophorysis indicated that (+)-catechin markedly blocked DNA fragmentation and apoptosis of microglia cells by tBHP exposure. A potent antioxidative effect of (+)-catechin was confirmed by comparison with a putative antioxidant agent, N-acetylcysteine at the lower doses. Furthermore, the increased intracellular ROS by tBHP exposure were scavenged by elevated activities of catalase (CAT) and superoxide dismutase (SOD) after (+)-catechin treatment. (+)-Catechin partially inhibited the activation of caspase-3, thereby both cleavage of poly (ADP-ribose) polymerase (PARP) and degradation of inhibitor of caspase-activated DNase (ICAD) were effectively abolished. In addition, the expression of PARP for repair of impaired DNA was significantly increased by (+)-catechin treatment. Taken together, these data suggest that protective effects of (+)-catechin against oxidative DNA damage of microglia cells is exerted by the increased expression of DNA repair enzyme PARP and antioxidant enzyme activities.

Permanent effector phenotype of neuroantigen-specific T cells acquired in the central nervous system during experimental allergic encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is regarded as an animal model of the human autoimmune disease multiple sclerosis (MS). Autoreactive T cells are present in the peripheral T cell repertoire of healthy mice and mediate clinical autoimmune disease only after activation by immunization or pathogens and migrate into the central nervous system (CNS). Because it is not known whether autoreactive T cells are regulated differentially once entering the CNS we investigated cytokine regulation in T cells from peripheral lymphatic organs and from the inflamed CNS ex vivo obtained from SJL mice after inducing relapsing-remitting EAE with PLP peptide 139-151. We show here that during acute EAE, an interleukin-2 (IL-2) biased T cell response exists in the spleen, while an interferon-gamma (IFN-gamma) biased T cell response prevails in the CNS of mice with acute EAE. The IFN-gamma biased phenotype was stable with optimized costimulation and even after in vitro stimulation with IL-2. After adoptive transfer into naïve syngeneic mice these T cells were only partially reversed to an IL-2 biased phenotype. These findings of our work suggest that a permanent effector phenotype of neuroantigen-specific T cells is finally acquired in the CNS in EAE.

Functional expression of CCL6 by rat microglia: a possible role of CCL6 in cell-cell communication

There is growing evidence that chemokines play important roles in the immune surveillance of central nervous system (CNS). In the CNS, microglia are primary immune effector cells and secrete various chemokines in response to their microenvironment. Using the RT-PCR procedure and indirect immunofluorescence analysis, we found that CCL6 (known as C10/MRP-1 in mouse) was expressed in rat primary microglia without any stimulation, but not in primary astrocytes, although both cell types expressed CCR1 mRNA, which is a receptor for CCL6. Furthermore, immunohistochemical analysis demonstrated that microglia produced CCL6 protein in a normal brain, suggesting that microglia may be the primary source of CCL6 in a normal brain. Recombinant rat CCL6 mediated the migration of microglia and astrocytes in vitro. The CCL6-mediated cell migration was blocked by treating the cells with LY294002, a PI3-kinase inhibitor and Western blot analysis showed that the phosphorylation of Akt could be induced by treating microglia with a recombinant CCL6, suggesting that CCL6 functions by activating the PI3-kinase/Akt pathway. A proinflammatory cytokine, interferon-gamma enhanced the expression of both CCL6 mRNA and protein in microglia, while other proinflammatory cytokines, interleukin-6 and tumor necrosis factor-alpha and an anti-inflammatory cytokine, transforming growth factor-beta exerted no effect on CCL6 expression in microglia. These findings suggest that CCL6 may be a mediator released by microglia for cell-cell communication under physiological as well as pathological conditions of CNS.

Dynamic complexity of the microglial activation response in transgenic models of amyloid deposition: implications for Alzheimer therapeutics

The presence of activated microglia in postmortem Alzheimer disease specimens is used to support the argument that inflammation contributes to Alzheimer pathogenesis. Transgenic mice overexpressing the amyloid precursor protein (APP) gene form amyloid plaques that are accompanied by local activation of microglia/macrophages in a manner similar to the human disease. Many markers of microglial activation and inflammation increase in an age-dependent manner in these mice. However, manipulation of these inflammatory reactions can lead to unexpected outcomes with several instances of reduced pathology when microglia/macrophages are activated further. In particular, anti-Abeta immunotherapy in amyloid-depositing transgenic mice causes a complex series of changes in microglial markers, negating the implicit belief that such activation is monotonic and represented equally well by any of several "activation" markers. A survey of the peripheral macrophage literature identifies at least 2 distinct activation states of macrophages with different consequences for the surrounding tissue. These different activation states can often be distinguished by the markers that are expressed. Several markers are identified from studies outside the brain that neuroscientists might consider evaluating when attempting to more definitively describe the activation state of the monocyte-derived cells in the brain.

Choroid plexus selectively accumulates T-lymphocytes in normal controls and after peripheral immune activation

We determined T-lymphocyte migration into brain and choroid plexus (CPx) after enterotoxin-induced systemic immune activation. CPx T-lymphocytes/mm2 in control mice were > 3 logs more numerous than brain and increased by as much as 150-fold by post-enterotoxin Day 3 (p < 0.01). Flow cytometry of pooled CPx confirmed post-enterotoxin increases. Brain T-lymphocytes increased up to 17-fold after SEB and accumulated in subependymal and periventricular brain. T cell apoptosis was absent. These results show preferential T-lymphocyte migration to CPx over brain and suggest that brain T cells may be derived from the CPx by direct migration or by cerebrospinal fluid dissemination.

CD40 signaling regulates innate and adaptive activation of microglia in response to amyloid beta-peptide

Although deposition of amyloid beta-peptide (Abeta) as Abeta plaques involves activation of microglia-mediated inflammatory responses, activated microglia ultimately fail to clear Abeta plaques in the brains of either Alzheimer's disease (AD) patients or AD mouse models. Mounting evidence suggests that chronic microglia-mediated immune response during Abeta deposition etiologically contributes to AD pathogenesis by promoting Abeta plaque formation. However, the mechanisms that govern microglia response in the context of cerebral Abeta/beta-amyloid pathology are not well understood. We show that ligation of CD40 by CD40L modulates Abeta-induced innate immune responses in microglia, including decreased microglia phagocytosis of exogenous Abeta(1-42) and increased production of pro-inflammatory cytokines. CD40 ligation in the presence of Abeta(1-42) leads to adaptive activation of microglia, as evidenced by increased co-localization of MHC class II with Abeta. To assess their antigen-presenting cell (APC) function, cultured microglia were pulsed with Abeta(1-42) in the presence of CD40L and co-cultured with CD4(+) T cells. Under these conditions, microglia stimulate T cell-derived IFN-gamma and IL-2 production, suggesting that CD40 signaling promotes the APC phenotype. These data provide a mechanistic explanation for our previous work showing decreased microgliosis associated with diminished cerebral Abeta/beta-amyloid pathology when blocking CD40 signaling in transgenic Alzheimer's mice.

Doxycycline reduces cleaved caspase-3 and microglial activation in an animal model of neonatal hypoxia-ischemia

Neonatal hypoxia-ischemia (HI) is a major contributor to many perinatal neurologic disorders and, thus, the search for therapies and effective treatments for the associated brain damage has become increasingly important. The tetracycline derivative, doxycycline (DOXY), has been reported to be neuroprotective in adult animal models of cerebral ischemia. To investigate the putative neuroprotective effects of DOXY in an animal model of neonatal HI, a time-course study was run such that pups received either DOXY (10 mg/kg) or VEH immediately before hypoxia, 1, 2, or 3 hours after HI (n=6). At 7 days after injury, the pups were euthanized, and the brains were removed and processed for immunohistochemical and Western blot analyses using antibodies against specific markers for neurons, apoptotic markers, microglia, oligodendrocytes, and astrocytes. Results showed that in vulnerable brain regions including the hippocampal formation, thalamus, striatum, cerebral cortex and white matter tracts, DOXY significantly decreased caspase-3 immunoreactivity (a marker of apoptosis), promoted neuronal survival, inhibited microglial activation and reduced reactive astrocytosis compared with VEH-treated HI pups. These effects were found to occur in a time-dependent manner. Taken together, these results strongly suggest that doxycycline has potential as a pharmacological treatment for mild HI in neonates.

Apoptotic Cell Death in Experimental Autoimmune Encephalomyelitis

Particularly in the vulnerable CNS with a low capacity for regeneration specialized mechanisms must be active for the fast and gentle elimination of dysregulated autoaggressive immune cells. In EAE, local apoptosis of autoimmune T-cells has been identified as a safe means for the removal of these unwanted cells. T-cell apoptosis in situ followed by phagocytic clearance of apoptotic remnants by glia assures a minimum of detrimental bystander damage to the local parenchyma and down-regulates the local inflammatory reaction. The pharmacological augmentation of local apoptosis of inflammatory effector cells might gain therapeutic importance also in human neuroimmunological diseases such as multiple sclerosis.

Microglia phagocytose alloreactive CTL-damaged 9L gliosarcoma cells

Intracranial adoptive transfers of alloreactive cytotoxic T lymphocytes (aCTL) for brain tumor treatment were safe and showed promise in preclinical and early clinical trials. To better understand the endogenous immune responses that may ensue following cellular therapy with aCTL, we examined the ability of microglia to phagocytose aCTL-damaged and undamaged rat 9L gliosarcoma cells in vitro and in vivo. In vitro, 5.5+/-0.9% of microglial cells isolated from adult tumor-bearing rat brains phagocytosed aCTL-damaged 9L cells, whereas microglia did not bind to or ingest undamaged 9L cells. Addition of supernates from either 9L cell cultures or from aCTL+9L co-incubate cell cultures to microglia did not significantly alter their ability to bind to or phagocytose damaged glioma cells even though the latter contained T helper 1 and 2 cytokines. At 3 days following intracranial 9L cell infusion, 17.5+/-0.1% of the microglia phagocytosed CFSE-labeled aCTL-damaged 9L tumor cells within the adult rat brain, confirming the in vitro data. The results suggest that microglia within the tumor microenvironment of the adult rat glioma model selectively remove damaged, but not undamaged, glioma cells.

Cholinergic modulation of baker's yeast cell phagocytosis by rat astrocytes

Cholinergic regulation of baker's yeast cell phagocytosis in rat cultured astrocytes was studied. Phagocytic activity was reduced by 1 x 10(-5) M of atropine or pirenzepine, but not by AF-DX116 or 4-DAMP. In addition, carbachol stimulated phagocytosis in a dose-dependent manner. Furthermore, only 1 x 10(-5)M of atropine, pirenzepine and 4-DAMP significantly reduced enhanced activity induced by 1 x 10(-7)M carbachol. It was also observed that L-NMMA, staurosporine, or U-73122, reduced phagocytosis activity while TFP failed to do so. Nitrite levels in astrocyte supernatants increased after baker's yeast cells were incorporated to astrocyte cultures, correlating with enhanced phagocytosis induced by carbachol stimulation, and were reduced by 1 x 10(-5) M of atropine, pirenzepine or aminopiridine, but not by AF-DX116 or 4-DAMP. Enhanced NO production triggered by astrocyte phagocytosis may have pathological consequences.

Relapsing and remitting multiple sclerosis: pathology of the newly forming lesion

The study describes the clinical and pathological findings in 12 patients with relapsing and remitting multiple sclerosis, who died during or shortly after the onset of a relapse. Pathological changes not previously associated with the formation of new symptomatic lesions were observed in seven cases, namely, extensive oligodendrocyte apoptosis and microglial activation in myelinated tissue containing few or no lymphocytes or myelin phagocytes. No current laboratory model of multiple sclerosis, in particular, experimental allergic encephalomyelitis, is known with these features, which raises the possibility of some novel process underlying new lesion formation in multiple sclerosis.

Site-specific microinjection of liposomes into the brain for local infusion of a short-lived peptide

The short-lived peptide, angiotensin-(1-7) (Ang-(1-7)), was encapsulated in different liposome preparations, in order to evaluate the influence of membrane fluidity, membrane surface, liposome size and dose of peptide on the cardiovascular effects of the encapsulated peptide at a specific site of the brain. These preparations were microinjected unilaterally into the rostral ventrolateral medulla (RVLM) of Wistar rats, and mean arterial blood pressure (MAP) and heart rate (HR) were registered by telemetry. Pegylated, rigid and calibrated (200 nm) liposomes, containing 50 ng of Ang-(1-7), elicited a significant increase of MAP for at least 7 days, in contrast to empty liposomes or non-pegylated liposomes. When a two-fold higher peptide dose was employed or when pegylated liposomes were used in the fluid state or uncalibrated, less pronounced pressor effects were observed. These data show that the cardiovascular responses to the microinjection of Ang-(1-7)-containing liposomes into the RVLM can be modulated through the manipulation of liposome characteristics. These results can be explained by the influence of liposome characteristics on the flux of peptide release. It is expected that this new method will encounter numerous applications in the study of the chronic actions of short-lived bioactive peptides in specific sites of the brain.

Neuronal apoptotic bodies: phagocytosis and degradation by primary microglial cells

Neuronal loss via apoptosis is a key element in numerous neurodegenerative diseases. To avoid accumulation of apoptotic material, the remains of apoptotic cells should be degraded. It was suggested that microglial cells are phagocytosing and degrading apoptotic material. There is only limited information available concerning the fate of the remains of apoptotic neurons. In this study, we investigated the ability of microglial cells to take up and degrade neuronal apoptotic material. We isolated primary microglial cells and used apoptotic bodies of apoptotic neuron-like PC12 cells as a substrate. The apoptotic material was taken up and degraded within the microglial cells. The uptake is clearly activation dependent. We were able to demonstrate that the CD36 scavenger receptor is involved in the uptake of the apoptotic material via competition studies, antibody blockage, and use of a CD36 mutant rat strain. Blockage of other uptake mechanisms was also able to inhibit the uptake to some extent. Furthermore, we were able to demonstrate the role of the microglial lysosomal and proteasomal pathways in the degradation of proteins originating from apoptotic bodies.

In vivo analysis reveals different apoptotic pathways in pre- and postmigratory cerebellar granule cells of rabbit

Naturally occurring neuronal death (NOND) has been described in the postnatal cerebellum of several species, mainly affecting the cerebellar granule cells (CGCs) by an apoptotic mechanism. However, little is known about the cellular pathway(s) of CGC apoptosis in vivo. By immunocytochemistry, in situ detection of fragmented DNA, electron microscopy, and Western blotting, we demonstrate here the existence of two different molecular mechanisms of apoptosis in the rabbit postnatal cerebellum. These two mechanisms affect CGCs at different stages of their maturation and migration. In the external granular layer, premigratory CGCs undergo apoptosis upon phosphorylation of checkpoint kinase 1 (Chk1), and hyperphosphorylation of retinoblastoma protein. In postmigratory CGCs within the internal granular layer, caspase 3 and to a lesser extent 7 and 9 are activated, eventually leading to poly-ADP-ribose polymerase-1 (PARP-1) cleavage and programmed cell death. We conclude that NOND of premigratory CGCs is linked to activation of DNA checkpoint and alteration of normal cell cycle, whereas in postmigratory CGCs apoptosis is, more classically, dependent upon caspase 3 activation.

CD44, alpha(4) integrin, and fucoidin receptor-mediated phagocytosis of apoptotic leukocytes

Various types of phagocytes mediate the clearance of apoptotic cells. We previously reported that human and murine high endothelial venule (HEV) cells ingest apoptotic cells. In this report, we examined endothelial cell fucoidin receptor-mediated phagocytosis using a murine endothelial cell model mHEV. mHEV cell recognition of apoptotic leukocytes was blocked by fucoidin but not by other phagocytic receptor inhibitors such as mannose, fucose, N-acetylglucosamine, phosphatidylserine (PS), or blocking anti-PS receptor antibodies. Thus, the mHEV cells used fucoidin receptors for recognition and phagocytosis of apoptotic leukocytes. The fucoidin receptor-mediated endothelial cell phagocytosis was specific for apoptotic leukocytes, as necrotic cells were not ingested. This is in contrast to macrophages, which ingest apoptotic and necrotic cells. Endothelial cell phagocytosis of apoptotic cells did not alter viable lymphocyte migration across these endothelial cells. Antibody blocking of CD44 and alpha4 integrin on the apoptotic leukocyte inhibited this endothelial cell phagocytosis, suggesting a novel function for these adhesion molecules in the removal of apoptotic targets. The removal of apoptotic leukocytes by endothelial cells may protect the microvasculature, thus ensuring that viable lymphocytes can successfully migrate into tissues.

Phagocytosis of apoptotic inflammatory cells by microglia and its therapeutic implications: termination of CNS autoimmune inflammation and modulation by interferon-beta

Apoptosis of autoaggressive T-cells in the CNS is an effective, noninflammatory mechanism for the resolution of T-cell infiltrates, contributing to clinical recovery in T-cell-mediated neuroinflammatory diseases. The clearance of apoptotic leukocytes by tissue-specific phagocytes is critical in the resolution of the inflammatory infiltrate and leads to a profound downregulation of phagocyte immune functions. Adult human microglia from surgically removed normal brain tissue was used in a standardized, light-microscopic in vitro phagocytosis assay of apoptotic autologous peripheral blood-derived mononuclear cells (MNCs). Microglia from five different patients had a high capacity for the uptake of apoptotic MNCs in contrast to nonapoptotic target cells with the phagocytosis rate for nonapoptotic MNCs amounting to only 61.6% of the apoptotic MNCs. A newly described phosphatidylserine receptor, critical in the phagocytosis of apoptotic cells by macrophages, is also expressed at similar levels on human microglia. The effects of the therapeutically used immunomodulatory agent interferon-beta (IFNbeta) were investigated using Lewis rat microglia and apoptotic, encephalitogenic, myelin basic protein-specific autologous T-cells. Also, rat microglia had a high capacity to phagocytose apoptotic T-cells specifically. IFNbeta increased the phagocytosis of apoptotic T-cells to 36.8% above the untreated controls. The enhanced phagocytic activity was selective for apoptotic T-cells and was not mediated by increased IL-10 secretion. Apoptotic inflammatory cells may be efficiently and rapidly removed by microglial cells in the autoimmune-inflamed human CNS. The in vitro increase of phagocytosis by IFNbeta merits further investigations whether this mechanism could also be therapeutically exploited.

Effects of polyclonal immunoglobulins and other immunomodulatory agents on microglial phagocytosis of apoptotic inflammatory T-cells

T-cell apoptosis in the CNS is an effective mechanism for the noninflammatory resolution of autoimmune T-cell infiltrates. Ingestion of apoptotic leukocytes by microglia results in an efficient clearance of the inflammatory infiltrate, followed by a profound downregulation of proinflammatory phagocyte immune functions. The effects of different immunomodulatory agents on Lewis rat microglial phagocytosis of apoptotic autologous thymocytes or myelin-basic protein (MBP)-specific, encephalitogenic T-cells were investigated using a standardized, light microscopical in vitro phagocytosis assay. Pretreatment of microglia with polyclonal 7S immunoglobulins (IVIg) decreased the phagocytosis of apoptotic thymocytes by 38.2% (p<0.0001). Also, immunoglobulin F(ab')(2) fragments decreased microglial phagocytosis, suggesting an Fc receptor-independent mechanism. Similar results were obtained using MBP-specific T-cells. Pretreatment of microglia with IFN-gamma increased the phagocytosis of apoptotic cells by 65.4%, which was to a large extent counteracted by IVIg. Glatiramer acetate (GLAT) did not exert an effect on microglial phagocytosis, while methylprednisolone (MP) induced microglial apoptosis in vitro. These results indicate that IVIg has a high potential to inhibit microglial phagocytosis of apoptotic inflammatory T-cells even under proinflammatory conditions and extend our view of the complex immunomodulatory effects of IVIg.