Intrathecal administration of neutralizing antibody against Fas ligand suppresses the progression of experimental autoimmune encephalomyelitis

Intracisternal delivery of PEG-coated gold nanoparticles results in high brain penetrance and long-lasting stability

Background

The increasing use of gold nanoparticles (AuNPs) in the field of neuroscience instilled hope for their rapid translation to the clinical practice. AuNPs can be engineered to carry therapeutics or diagnostics in the diseased brain, possibly providing greater cell specificity and low toxicity. Although there is a general enthusiasm for these tools, we are in early stages of their development. Overall, their brain penetrance, stability and cell specificity are critical issues that must be addressed to drive AuNPs to the clinic.

Results

We studied the kinetic, distribution and stability of PEG-coated AuNPs in mice receiving a single injection into the cisterna magna of the 4th ventricle. AuNPs were conjugated with the fluorescent tag Cy5.5 (Cy5.5-AuNPs) to track their in vivo distribution. Fluorescence levels from such particles were detected in mice for weeks. In situ analysis of brains by immunofluorescence and electron microscopy revealed that Cy5.5-AuNPs penetrated the brain parenchyma, spreading in the CNS parenchyma beneath the 4th ventricle. Cy5.5-AuNPs were preferentially found in neurons, although a subset of resting microglia also entrapped these particles.

Conclusions

Our results suggest that the ICM route for delivering gold particles allows the targeting of neurons. This approach might be pursued to carry therapeutics or diagnostics inside a diseased brain with a surgical procedure that is largely used in gene therapy approaches. Furthermore, this approach could be used for radiotherapy, enhancing the agent’s efficacy to kill brain cancer cells.

Electronic supplementary material

The online version of this article (10.1186/s12951-019-0481-3) contains supplementary material, which is available to authorized users.

Reverse Signaling of Tumor Necrosis Factor Superfamily Proteins in Macrophages and Microglia: Superfamily Portrait in the Neuroimmune Interface

The tumor necrosis factor (TNF) superfamily (TNFSF) is a protein superfamily of type II transmembrane proteins commonly containing the TNF homology domain. The superfamily contains more than 20 protein members, which can be released from the cell membrane by proteolytic cleavage. Members of the TNFSF function as cytokines and regulate diverse biological processes, including immune responses, proliferation, differentiation, apoptosis, and embryogenesis, by binding to TNFSF receptors. Many TNFSF proteins are also known to be responsible for the regulation of innate immunity and inflammation. Both receptor-mediated forward signaling and ligand-mediated reverse signaling play important roles in these processes. In this review, we discuss the functional expression and roles of various reverse signaling molecules and pathways of TNFSF members in macrophages and microglia in the central nervous system (CNS). A thorough understanding of the roles of TNFSF ligands and receptors in the activation of macrophages and microglia may improve the treatment of inflammatory diseases in the brain and periphery. In particular, TNFSF reverse signaling in microglia can be exploited to gain further insights into the functions of the neuroimmune interface in physiological and pathological processes in the CNS.

Overcoming the Blood-Brain Barrier: The Role of Nanomaterials in Treating Neurological Diseases

Therapies directed toward the central nervous system remain difficult to translate into improved clinical outcomes. This is largely due to the blood-brain barrier (BBB), arguably the most tightly regulated interface in the human body, which routinely excludes most therapeutics. Advances in the engineering of nanomaterials and their application in biomedicine (i.e., nanomedicine) are enabling new strategies that have the potential to help improve our understanding and treatment of neurological diseases. Herein, the various mechanisms by which therapeutics can be delivered to the brain are examined and key challenges facing translation of this research from benchtop to bedside are highlighted. Following a contextual overview of the BBB anatomy and physiology in both healthy and diseased states, relevant therapeutic strategies for bypassing and crossing the BBB are discussed. The focus here is especially on nanomaterial-based drug delivery systems and the potential of these to overcome the biological challenges imposed by the BBB. Finally, disease-targeting strategies and clearance mechanisms are explored. The objective is to provide the diverse range of researchers active in the field (e.g., material scientists, chemists, engineers, neuroscientists, and clinicians) with an easily accessible guide to the key opportunities and challenges currently facing the nanomaterial-mediated treatment of neurological diseases.

Role of Tumor Necrosis Factor Superfamily in Neuroinflammation and Autoimmunity

Tumor necrosis factor superfamily (TNFSF) molecules play an important role in the activation, proliferation, differentiation, and migration of immune cells into the central nervous system (CNS). Several TNF superfamily molecules are known to control alloimmunity, autoimmunity, and immunity. Development of transgenic and gene knockout animals, and monoclonal antibodies against TNFSF molecules have increased our understanding of individual receptor-ligand interactions, and their intracellular signaling during homeostasis and neuroinflammation. A strong clinical association has been observed between TNFSF members and CNS autoimmunity such as multiple sclerosis and also in its animal model experimental autoimmune encephalomyelitis. Therefore, they are promising targets for alternative therapeutic options to control autoimmunity. Although, TNFSF ligands are widely distributed and have diverse functions, we have restricted the discussions in this review to TNFSF receptor-ligand interactions and their role in the pathogenesis of neuroinflammation and CNS autoimmunity.

Agile delivery of protein therapeutics to CNS

A variety of therapeutic proteins have shown potential to treat central nervous system (CNS) disorders. Challenge to deliver these protein molecules to the brain is well known. Proteins administered through parenteral routes are often excluded from the brain because of their poor bioavailability and the existence of the blood-brain barrier (BBB). Barriers also exist to proteins administered through non-parenteral routes that bypass the BBB. Several strategies have shown promise in delivering proteins to the brain. This review, first, describes the physiology and pathology of the BBB that underscore the rationale and needs of each strategy to be applied. Second, major classes of protein therapeutics along with some key factors that affect their delivery outcomes are presented. Third, different routes of protein administration (parenteral, central intracerebroventricular and intraparenchymal, intranasal and intrathecal) are discussed along with key barriers to CNS delivery associated with each route. Finally, current delivery strategies involving chemical modification of proteins and use of particle-based carriers are overviewed using examples from literature and our own work. Whereas most of these studies are in the early stage, some provide proof of mechanism of increased protein delivery to the brain in relevant models of CNS diseases, while in few cases proof of concept had been attained in clinical studies. This review will be useful to broad audience of students, academicians and industry professionals who consider critical issues of protein delivery to the brain and aim developing and studying effective brain delivery systems for protein therapeutics.

Physiology of the intrathecal bolus: the leptomeningeal route for macromolecule and particle delivery to CNS

Presently, there are no effective treatments for several diseases involving the CNS, which is protected by the blood-brain, blood-CSF, and blood-arachnoid barriers. Traversing any of these barriers is difficult, especially for macromolecular drugs and particulates. However, there is significant experimental evidence that large molecules can be delivered to the CNS through the cerebrospinal fluid (CSF). The flux of the interstitial fluid in the CNS parenchyma, as well as the macro flux of CSF in the leptomeningeal space, are believed to be generally opposite to the desirable direction of CNS-targeted drug delivery. On the other hand, the available data suggest that the layer of pia mater lining the CNS surface is not continuous, and the continuity of the leptomeningeal space (LMS) with the perivascular spaces penetrating into the parenchyma provides an unexplored avenue for drug transport deep into the brain via CSF. The published data generally do not support the view that macromolecule transport from the LMS to CNS is hindered by the interstitial and CSF fluxes. The data strongly suggest that leptomeningeal transport depends on the location and volume of the administered bolus and consists of four processes: (i) pulsation-assisted convectional transport of the solutes with CSF, (ii) active "pumping" of CSF into the periarterial spaces, (iii) solute transport from the latter to and within the parenchyma, and (iv) neuronal uptake and axonal transport. The final outcome will depend on the drug molecule behavior in each of these processes, which have not been studied systematically. The data available to date suggest that many macromolecules and nanoparticles can be delivered to CNS in biologically significant amounts (>1% of the administered dose); mechanistic investigation of macromolecule and particle behavior in CSF may result in a significantly more efficient leptomeningeal drug delivery than previously thought.

The role of FasL and Fas in health and disease

The FS7-associated cell surface antigen (Fas, also named CD95, APO-1 or TNFRSF6) attracted considerable interest in the field of apoptosis research since its discovery in 1989. The groups of Shin Yonehara and Peter Krammer were the first reporting extensive apoptotic cell death induction upon treating cells with Fas-specific monoclonal antibodies.1,2 Cloning of Fas3 and its ligand,4,5 FasL (also known as CD178, CD95L or TNFSF6), laid the cornerstone in establishing this receptor-ligand system as a central regulator of apoptosis in mammals. Therapeutic exploitation of FasL-Fas-mediated cytotoxicity was soon an ambitous goal and during the last decade numerous strategies have been developed for its realization. In this chapter, we will briefly introduce essential general aspects of the FasL-Fas system before reviewing its physiological and pathophysiological relevance. Finally, FasL-Fas-related therapeutic tools and concepts will be addressed.

Fas/FasL interaction: a novel immune therapy approach with immobilized biologicals

Systemically applied agents to modulate the Fas/FasL system, e.g., by stimulation of Fas on activated leukocytes or tumor cells failed as strategies in immune therapy due to severe toxic effects in the host. Recently, a novel strategy has been developed by using immobilized immune active biologicals in a medical device that may allow immune management without expensive systemic therapy. This review reports on the potential role of Fas/FasL in immune therapy and summarizes current experimental and clinical data with the leukocyte inhibition module (LIM), an immobilized anti-Fas antibody containing device yet used in extracorporeal blood circulation. This proof of principal may stimulate the development of other devices based on the regulation of Fas/FasL or other targets relevant for immune disorders.

New approaches and therapeutics targeting apoptosis in disease

Apoptosis, the major form of cellular suicide, is central to various physiological processes and the maintenance of homeostasis in multicellular organisms. Presumably, even more important is a causative or contributing role of apoptosis to various human diseases. These include situations with unwanted cell accumulation (cancer) and failure to eradicate aberrant cells (autoimmune diseases) or disorders with an inappropriate loss of cells (heart failure, stroke, AIDS, neurodegenerative diseases, and liver injury). The past decade has witnessed a tremendous progress in the knowledge of the molecular mechanisms that regulate apoptosis and the mediators that either prevent or trigger cell death. Consequently, apoptosis regulators have emerged as key targets for the design of therapeutic strategies aimed at modulating cellular life-and-death decisions. Numerous novel approaches are currently being followed employing gene therapy and antisense strategies, recombinant biologics, or classical organic and combinatorial chemistry to target specific apoptotic regulators. Convincing proof-of-principle evidence obtained in several animal models confirms the validity of strategies targeting apoptosis and revealed an enormous potential for therapeutic intervention in a variety of illnesses. Although numerous apoptotic drugs are currently being developed, several therapeutics have progressed to clinical testing or are already approved and marketed. Here we review the recent progress of apoptosis-based therapies and survey some highlights in a very promising field of drug development.

Fas-disabling small exocyclic peptide mimetics limit apoptosis by an unexpected mechanism

Fas ligand- (FasL) mediated apoptosis is an important element of tissue-specific organ damage. We have developed biologically active small exocyclic peptide mimetics that disable apoptotic functions of Fas. The most effective mimetic binds to both its receptor and FasL with comparable affinity. In vitro, the most effective antagonist blocked FasL-induced cytotoxicity completely and specifically. In vivo, the antagonistic mimetic also prevented Concanavilin A (Con A) induced hepatitis, a CD4(+) T cell-mediated animal model of liver injury. Although current approaches prevent Fas receptor signaling by excluding FasL binding to Fas, the small molecule mimetics reported here disable Fas by promoting a defective Fas-FasL receptor complex. This event desensitizes FasL-mediated apoptosis by inhibiting extracellular signal regulated kinase activity and up-regulating NF-kappaB.

Fas ligand/Fas system in the brain: regulator of immune and apoptotic responses

Apoptosis, also known as programmed cell death, is the major type of cell death involved in normal development, regeneration, proliferation and pathologic degeneration in the central nervous system (CNS). The apoptotic process can be divided further into two pathways depending on the involvement of mitochondria and related biochemical cascades. The internal pathway of apoptosis is initiated by a variety of cytotoxic stimuli and mediated by the release of cytochrome c and subsequent activation of downstream caspases. The external pathway is mainly triggered by ligation of death receptors such as Fas, tumor necrosis factor (TNF)-related apoptosis inducing ligand-R1 (TRAIL-R1), TRAIL-R2 and TNFRp55, and mediated by direct activation of upstream caspases. The Fas-FasL system has been known as a prototypic inducer of extrinsic cell death responsible for cell-mediated cytotoxicity, peripheral immune regulation, immune privilege and "counterattack" of malignant tumor cells against the host immune system. Fas and FasL are expressed in the normal CNS, and expression increases in inflamed and degenerated brains. Like other specialized tissues such as the eye and testis, the Fas-FasL system is thought to be involved in immune suppressed status in the CNS. Expression of Fas and FasL is significantly elevated in a variety of the neurologic disorders, suggesting the possibility that this system may play roles in degenerative and inflammatory responses in the CNS. Therefore, the FasL-Fas system should be considered as a double-edged sword in the CNS: maintaining the immune suppressed status in normal brain and inducing neuronal cell death and inflammation in a variety of neurologic disorders.

Caspase-mediated oligodendrocyte cell death in the pathogenesis of autoimmune demyelination

Multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), are inflammatory diseases of the central nervous system (CNS) characterized by localized areas of demyelination. MS is believed to be an autoimmune disorder mediated by activated immune cells such as T- and B-lymphocytes and macrophages/microglia. Lymphocytes are primed in the peripheral tissues by antigens, and clonally expanded cells infiltrate the CNS. They produce large amounts of inflammatory and cytokines that lead to demyelination and axonal degeneration. Although several studies have shown that oligodendrocytes (OLGs), the myelin-forming glial cells in the CNS, are sensitive to cell death stimuli, such as cytotoxic cytokines, anti-myelin antibodies, nitric oxide, and oxidative stress, in vitro, the mechanisms underlying injury to the OLGs in MS/EAE remain unclear. Transgenic mice that express the anti-apoptotic protein specifically in OLGs and caspase-11-deficient mice are significantly resistant to EAE induction. Histopathological analyses show that the number of caspase-activated OLGs and dead OLGs are reduced in the CNS of these mice. The numbers of infiltrating immune cells and the amounts of cytokines are also markedly reduced in EAE lesions. Therefore, caspase-mediated OLG death leads to the exacerbation of demyelination and the deterioration of neurological manifestations by inducing local inflammatory events.

Cellular FLICE-inhibitory protein: an attractive therapeutic target?

Cellular FLIP (c-FLIP), also known as FLICE-inhibitory protein, has been identified as an inhibitor of apoptosis triggered by engagement of death receptors (DRs) such as Fas or TRAIL (TNF-related apoptosis-inducing ligand). cFLIP is recruited to DR signalling complexes, where it prevents caspase activation. Animal models have indicated that c-FLIP plays an important role in T cell proliferation and heart development. Abnormal c-FLIP expression has been identified in various diseases such as multiple sclerosis (MS), Alzheimer's disease (AD), diabetes mellitus, rheumatoid arthritis (RA) and various cancers. This review focuses on recent insights into c-FLIP dysregulation associated with human diseases and addresses the possibilities of using c-FLIP as a therapeutic target.

T-cell immune responses in the brain and their relevance for cerebral malignancies

In order that cellular immune responses afford protection without risk to sensitive normal tissue, they must be adapted to individual tissues of the body. Nowhere is this more critical than for the brain, where various passive and active mechanisms maintain a state of immune privilege that can limit high magnitude immune responses. Nevertheless, it is now clear that immune responses are induced to antigens in the brain, including those expressed by cerebral malignancies. We discuss hypotheses of how this can occur, although details such as which antigen presenting cells are involved remain to be clarified. Antitumor responses induced spontaneously are insufficient to eradicate malignant astrocytomas; many studies suggest that this can be explained by a combination of low level immune response induction and tumor mediated immunosuppression. A clinical objective currently pursued is to use immunotherapy to ameliorate antitumour immunity. This will necessitate a high level immune response to ensure sufficient effector cells reach the tumor bed, focused cytotoxicity to eradicate malignant cells with little collateral damage to critical normal cells, and minimal inflammation. To achieve these aims, priority should be given to identifying more target antigens in astrocytoma and defining those cells present in the brain parenchyma that are essential to maintain antitumour effector function without exacerbating inflammation. If we are armed with better understanding of immune interactions with brain tumor cells, we can realistically envisage that immunotherapy will one day offer hope to patients with currently untreatable neoplastic diseases of the CNS.

Fas and Fas ligand are associated with neuritic degeneration in the AD brain and participate in beta-amyloid-induced neuronal death

It has recently been suggested that neuronal cell death in response to many brain insults may be mediated by the upregulation of tumor necrosis factor receptor (TNFR) family members and their ligands. In the present study, we investigated whether the expression of the TNFR family death domain receptor, Fas, and its ligand, FasL, is altered in association with neuropathology and activated caspase markers in Alzheimer disease (AD) brain, and Abeta-induced neuronal cell death in vitro. To evaluate this hypothesis, we examined Fas and FasL expression in AD and control brain, and Abeta-treated primary neurons, using immunocytochemistry and Western blots. Neurons in both AD brain and Abeta-treated cultures exhibited FasL upregulation and changes in immunoreactivity for Fas receptor. Further, FasL expression was remarkably elevated in senile plaques and neurofilament-positive dystrophic neurites, and in association with caspase activation and neuritic apoptosis in AD brain. Based on these and previous data regarding protection of primary neuronal cultures from Abeta(1-42)-induced apoptosis by blockade of Fas-associated death domain signaling, we also tested the hypothesis that dynamic regulation of Fas and FasL may contribute to Abeta-mediated neuronal cell death. Accordingly, neuronal cultures derived from mice carrying inactivating mutations in Fas (Faslpr) or FasL (Fasgld) exhibited protection from Abeta(1-42)-induced cell death. These findings suggest that Fas-FasL interactions may contribute to mechanisms of neuronal loss and neuritic degeneration in AD.

Protein-based therapeutic approaches targeting death receptors

Death receptors (DRs) are a growing family of transmembrane proteins that can detect the presence of specific extracellular death signals and rapidly trigger cellular destruction by apoptosis. Eight human DRs (Fas, TNF-R1, TRAMP, TRAIL-R1, TRAIL-R2, DR-6, EDA-R and NGF-R) have been identified. The best studied to date is Fas (CD95). Expression and signaling by Fas and its ligand (FasL, CD95L) is a tightly regulated process essential for key physiological functions in a variety of organs, including the maintenance of immune homeostasis. Recently, strong evidence has shown that dysregulation of Fas expression and/or signaling contributes to the pathogenesis of tissue destructive diseases such as graft-versus-host disease, toxic epidermal necrolysis, multiple sclerosis and stroke. With these new developments, strategies for modulating the function of Fas signaling have emerged and provided novel protein-based therapeutic possibilities that will be discussed herein. Selective triggering of DR-mediated apoptosis in cancer cells is an emerging approach that is being intensely investigated as a mode of cancer therapy. Local administration of Fas agonists, and more promisingly, systemic use of soluble recombinant forms of TRAIL have shown efficacy in preclinical models of the disease. Developments in this field that may have important clinical implications for the treatment of cancer are reviewed.

The suppression of T cell apoptosis influences the severity of disease during the chronic phase but not the recovery from the acute phase of experimental autoimmune encephalomyelitis in mice

The elimination of T cells by apoptosis is considered to be one of the regulatory factors in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. To address further the role of apoptotic T cell death in EAE, we investigated myelin oligodendrocyte glycoprotein (MOG)-induced EAE in transgenic mice overexpressing the anti-apoptotic gene, bcl-2, in T cells. During the acute phase of EAE, no significant difference was observed in the clinical course, pathology and T cell response to MOG between bcl-2 transgenic mice and wild-type littermates. While the recovery from the first attack of EAE was not impaired in the bcl-2 transgenic mice, a more severe disease was observed during the chronic phase of the disease even though T and B cell responses to MOG were comparable to those of wild-type littermates. A flow cytometric analysis by Annexin V showed a significant decrease of apoptotic T cells in the central nervous system (CNS) of the bcl-2 transgenic mice with EAE compared with controls during the chronic as well as the acute phase of disease. These results suggest that while T cell apoptosis in the CNS may play a regulatory role in EAE, the spontaneous recovery from acute EAE cannot solely be explained by T cell apoptosis.

Intrathecal Fas ligand infusion strengthens immunoprivilege of central nervous system and suppresses experimental autoimmune encephalomyelitis

Fas ligand (FasL) is an essential molecule strongly expressed in some immunoprivileged sites, but is expressed at very low levels in normal CNS. In this study, acute experimental autoimmune encephalomyelitis (EAE) was induced in Lewis rats with guinea pig myelin basic protein. Intrathecal infusion of recombinant FasL before EAE onset dose dependently suppressed acute EAE and alleviated pathological inflammation in lumbosacral spinal cord. This treatment greatly increased apoptosis in CNS inflammatory cells, but did not inhibit systemic immune response to myelin basic protein. Systemic administration of a similar dose of rFasL was ineffective. In vitro, encephalitogenic T cells were highly sensitive to rFasL-induced cell death, and activated macrophages were also susceptible. In addition, in vitro rFasL treatment potentiated the immunosuppressive property of rat cerebrospinal fluid. We conclude that intrathecal infusion of rFasL eliminated the initial wave of infiltrating T cells and macrophages, and therefore blocked the later recruitment of inflammatory cells into CNS. Although Fas receptor expression was observed on spinal cord neurons, astrocytes, and oligodendrocytes, no damage to these cells or to the myelin structure was detected after rFasL infusion.

Apoptotic death following Fas activation in human oligodendrocyte hybrid cultures

The purpose of this study was to determine how oligodendrocytes die following Fas receptor activation. An immortalized human oligodendrocyte hybrid line (MO3.13) was challenged with Fas ligand (FasL), and cell death was assessed by flow cytometry and DNA gel electrophoresis. Caspase activation was determined by either Western immunoblotting on cell extracts or by whole-cell flow cytometry. FasL challenge clearly induced substantial apoptotic cell death in the oligodendrocyte hybrid cell line, as judged by flow cytometry and by the presence of prominent low molecular weight DNA banding patterns after gel electrophoresis. Western immunoblots showed marked increases in cleaved caspase-1, 8, and 3, indicating that the extrinsic Fas death receptor-induced pathway was activated. The intrinsic mitochondrial pathway was also activated, but only at a minimal level. These findings demonstrate that there are several independent molecular sites within the extrinsic caspase cascade in oligodendrocytes where inhibitory compounds may be capable of blocking cell death in vivo.