Magnetic resonance imaging of enhanced nerve repair with mesenchymal stem cells combined with microenvironment immunomodulation in neurotmesis.

INTRODUCTION: The immuno-microenvironment of injured nerves adversely affects mesenchymal stem cell (MSC) therapy for neurotmesis. Magnetic resonance imaging (MRI) can be used noninvasively to monitor nerve degeneration and regeneration. The aim of this study was to investigate nerve repair after MSC transplantation combined with microenvironment immunomodulation in neurotmesis by using multiparametric MRI. METHODS: Rats with sciatic nerve transection and surgical coaptation were treated with MSCs combined with immunomodulation or MSCs alone. Serial multiparametric MRI examinations were performed over an 8-week period after surgery. RESULTS: Nerves treated with MSCs combined with immunomodulation showed better functional recovery, rapid recovery of nerve T2, fractional anisotropy and radial diffusivity values, and more rapid restoration of the fiber tracks than nerves treated with MSCs alone. DISCUSSION: Transplantation of MSCs in combination with immunomodulation can exert a synergistic repair effect on neurotmesis, which can be monitored by multiparametric MRI.

Systemic Immune Response to Traumatic CNS Injuries-Are Extracellular Vesicles the Missing Link?

Inflammation following traumatic injury to the central nervous system (CNS) persists long after the primary insult and is known to exacerbate cell death and worsen functional outcomes. Therapeutic interventions targeting this inflammation have been unsuccessful, which has been attributed to poor bioavailability owing to the presence of blood-CNS barrier. Recent studies have shown that the magnitude of the CNS inflammatory response is dependent on systemic inflammatory events. The acute phase response (APR) to CNS injury presents an alternative strategy to modulating the secondary phase of injury. However, the communication pathways between the CNS and the periphery remain poorly understood. Extracellular vesicles (EVs) are membrane bound nanoparticles that are regulators of intercellular communication. They are shed from cells of the CNS including microglia, astrocytes, neurons and endothelial cells, and are able to cross the blood-CNS barrier, thus providing an attractive candidate for initiating the APR after acute CNS injury. The purpose of this review is to summarize the current evidence that EVs play a critical role in the APR following CNS injuries.

Therapeutics targeting the inflammasome after central nervous system injury.

Innate immunity is part of the early response of the body to deal with tissue damage and infections. Because of the early nature of the innate immune inflammatory response, this inflammatory reaction represents an attractive option as a therapeutic target. The inflammasome is a component of the innate immune response involved in the activation of caspase 1 and the processing of pro-interleukin 1ß. In this article, we discuss the therapeutic potential of the inflammasome after central nervous system (CNS) injury and stroke, as well as the basic knowledge we have gained so far regarding inflammasome activation in the CNS. In addition, we discuss some of the therapies available or under investigation for the treatment of brain injury, spinal cord injury, and stroke.

Immune Surveillance of the CNS following Infection and Injury.

The central nervous system (CNS) contains a sophisticated neural network that must be constantly surveyed in order to detect and mitigate a diverse array of challenges. The innate and adaptive immune systems actively participate in this surveillance, which is critical for the maintenance of CNS homeostasis and can facilitate the resolution of infections, degeneration, and tissue damage. Infections and sterile injuries represent two common challenges imposed on the CNS that require a prompt immune response. While the inducers of these two challenges differ in origin, the resultant responses orchestrated by the CNS share some overlapping features. Here, we review how the CNS immunologically discriminates between pathogens and sterile injuries, mobilizes an immune reaction, and, ultimately, regulates local and peripherally-derived immune cells to provide a supportive milieu for tissue repair.

M. leprae components induce nerve damage by complement activation: identification of lipoarabinomannan as the dominant complement activator.

Peripheral nerve damage is the hallmark of leprosy pathology but its etiology is unclear. We previously identified the membrane attack complex (MAC) of the complement system as a key determinant of post-traumatic nerve damage and demonstrated that its inhibition is neuroprotective. Here, we determined the contribution of the MAC to nerve damage caused by Mycobacterium leprae and its components in mouse. Furthermore, we studied the association between MAC and the key M. leprae component lipoarabinomannan (LAM) in nerve biopsies of leprosy patients. Intraneural injections of M. leprae sonicate induced MAC deposition and pathological changes in the mouse nerve, whereas MAC inhibition preserved myelin and axons. Complement activation occurred mainly via the lectin pathway and the principal activator was LAM. In leprosy nerves, the extent of LAM and MAC immunoreactivity was robust and significantly higher in multibacillary compared to paucibacillary donors (p = 0.01 and p = 0.001, respectively), with a highly significant association between LAM and MAC in the diseased samples (r = 0.9601, p = 0.0001). Further, MAC co-localized with LAM on axons, pointing to a role for this M. leprae antigen in complement activation and nerve damage in leprosy. Our findings demonstrate that MAC contributes to nerve damage in a model of M. leprae-induced nerve injury and its inhibition is neuroprotective. In addition, our data identified LAM as the key pathogen associated molecule that activates complement and causes nerve damage. Taken together our data imply an important role of complement in nerve damage in leprosy and may inform the development of novel therapeutics for patients.

Neurobiology of microglial action in CNS injuries: receptor-mediated signaling mechanisms and functional roles.

Microglia are the first line of immune defense against central nervous system (CNS) injuries and disorders. These highly plastic cells play dualistic roles in neuronal injury and recovery and are known for their ability to assume diverse phenotypes. A broad range of surface receptors are expressed on microglia and mediate microglial 'On' or 'Off' responses to signals from other host cells as well as invading microorganisms. The integrated actions of these receptors result in tightly regulated biological functions, including cell mobility, phagocytosis, the induction of acquired immunity, and trophic factor/inflammatory mediator release. Over the last few years, significant advances have been made toward deciphering the signaling mechanisms related to these receptors and their specific cellular functions. In this review, we describe the current state of knowledge of the surface receptors involved in microglial activation, with an emphasis on their engagement of distinct functional programs and their roles in CNS injuries. It will become evident from this review that microglial homeostasis is carefully maintained by multiple counterbalanced strategies, including, but not limited to, 'On' and 'Off' receptor signaling. Specific regulation of theses microglial receptors may be a promising therapeutic strategy against CNS injuries.

Wallerian degeneration: the innate-immune response to traumatic nerve injury.

Traumatic injury to peripheral nerves results in the loss of neural functions. Recovery by regeneration depends on the cellular and molecular events of Wallerian degeneration that injury induces distal to the lesion site, the domain through which severed axons regenerate back to their target tissues. Innate-immunity is central to Wallerian degeneration since innate-immune cells, functions and molecules that are produced by immune and non-immune cells are involved. The innate-immune response helps to turn the peripheral nerve tissue into an environment that supports regeneration by removing inhibitory myelin and by upregulating neurotrophic properties. The characteristics of an efficient innate-immune response are rapid onset and conclusion, and the orchestrated interplay between Schwann cells, fibroblasts, macrophages, endothelial cells, and molecules they produce. Wallerian degeneration serves as a prelude for successful repair when these requirements are met. In contrast, functional recovery is poor when injury fails to produce the efficient innate-immune response of Wallerian degeneration.

Roles of autoantibodies in central nervous system injury.

Stroke, traumatic brain injury (TBI), and spinal cord injury (SCI) cause irreversible damage to the nervous system. Although these are neurological disorders, pathology and loss of function also occur outside the nervous system and are often not easily explained by paralysis or impaired neural function. Emerging data indicate that much of the pathological sequelae that accompanies CNS trauma has characteristics of a self-directed immunological disease. Here, we outline those data, describing basic mechanisms of B cell activation and autoantibody synthesis after CNS injury. A summary of the anti-CNS autoantibodies that have been identified in humans and animals is provided along with a discussion of how autoantibodies may affect survival of neuronal and non-neuronal tissues and whether autoimmune reactions are feasible therapeutic targets after CNS trauma.

Dynamic regulation of spinal pro-inflammatory cytokine release in the rat in vivo following peripheral nerve injury.

Spinal release of cytokines may play a critical role in the maladapted nociceptive signaling underlying chronic pain states. In order to investigate this biology, we have developed a novel 'high flux' intrathecal microdialysis approach in combination with multiplex bead-based immunoassay technology to concurrently monitor the spinal release of interleukin (IL)-1beta, IL-6 and tumour necrosis factor (TNF)alpha in rats with unilateral sciatic nerve chronic constriction injury (CCI). Intrathecal microdialysis was performed under isoflurane/N(2)O anaesthesia in rats with confirmed mechanical hypersensitivity. In a first study, C-fiber strength electrical stimulation of the operated nerve in neuropathic rats was found to evoke a dramatic increase in IL-1beta efflux ( approximately 15-fold) that was significantly greater than that observed in the sham-operated group. Spinal IL-6 efflux was also responsive to primary afferent stimulation, whereas TNFalpha was not. In a second study, treatment with the glial inhibitor propentofylline for 7days normalized CCI-induced mechanical hypersensitivity. In the same animals, this treatment also significantly reduced intrathecal IL-1beta, IL-6 and TNFalpha and prevented afferent stimulation-evoked cytokine release of both IL-1beta and IL-6. These results provide support for glia as the source of the majority of intrathecal IL-1beta, IL-6 and TNFalpha that accompanies mechanical hypersensitivity in the CCI rat. Moreover, our studies demonstrate the ability of a neurone-glia signaling mechanism to dynamically modulate this release and support a role of spinal IL-1beta in the phasic transmission of abnormal pain signals.

Cellular repair of CNS disorders: an immunological perspective.

Cellular repair is a promising strategy for treating central nervous system (CNS) disorders. Several strategies have been contemplated including replacement of neurons or glia that have been lost due to injury or disease, use of cellular grafts to modify or augment the functions of remaining neurons and/or use of cellular grafts to protect neural tissue by local delivery of growth or trophic factors. Depending on the specific disease target, there may be one or many cell types that could be considered for therapy. In each case, an additional variable must be considered--the role of the immune system in both the injury process itself and in the response to incoming cells. Cellular transplants can be roughly categorized into autografts, allografts and xenografts. Despite the immunological privilege of the CNS, allografts and xenografts can elicit activation of the innate and adaptive immune system. In this article, we evaluate the various effects that immune cells and signals may have on the survival, proliferation, differentiation and migration/integration of transplanted cells in therapeutic approaches to CNS injury and disease.

Central nervous system and non-central nervous system antigen vaccines exacerbate neuropathology caused by nerve injury.

Previously, we showed that autoimmune (central nervous system myelin-reactive) T cells exacerbate tissue damage and impair neurological recovery after spinal cord injury. Conversely, independent studies have shown T cell-mediated neuroprotection after spinal cord injury or facial nerve axotomy (FNAx). The antigen specificity of the neuroprotective T cells has not been investigated after FNAx. Here, we compared the neuroprotective capacity of autoimmune and non-autoimmune lymphocytes after FNAx. Prior to axotomy, C57BL/6 mice were immunized with myelin basic protein, myelin oligodendrocyte glycoprotein (MOG) or ovalbumin (a non-self antigen) emulsified in complete Freund's adjuvant (CFA). FNAx mice receiving injections of phosphate-buffered saline (PBS) only (unimmunized) or PBS/CFA emulsions served as controls. At 4 weeks after axotomy, bilateral facial motor neuron counts were obtained throughout the facial motor nucleus using unbiased stereology (optical fractionator). The data show that neuroantigen immunizations and 'generic' lymphocyte activation (e.g. PBS/CFA or ovalbumin/CFA immunizations) exacerbated neuron loss above that caused by FNAx alone. We also found that nerve injury potentiated the effector potential of autoimmune lymphocytes. Indeed, prominent forelimb and hindlimb motor deficits were accompanied by disseminated neuroinflammation and demyelination in FNAx mice receiving subencephalitogenic immunization with MOG. FNAx or neuroantigen (MOG or myelin basic protein) immunization alone did not cause these pathological changes. Thus, irrespective of the antigens used to trigger an immune response, neuropathology was enhanced when the immune system was primed in parallel with nerve injury. These data have important implications for therapeutic vaccination in clinical neurotrauma and neurodegeneration.

T-cell cytokines in injury-induced neural damage and repair.

T-cell cytokines are involved in beneficial immune responses, pathological autoimmunity, and tissue inflammation. In this review, we focus on the role of interferon-gamma, interleukin (IL)-2, IL-4, IL-6, and IL-10 in autoimmune diseases such as multiple sclerosis, and primarily "nonimmune" injury of the central nervous system (CNS), in particular focal ischemia and trauma. Resident CNS cells such as microglia and astroglia are additional, and on some occasions major, cellular sources of T-cell cytokines in CNS disease. Collectively, they mediate harmful as well as beneficial functions that depend on the dynamics, cellular source, and compartmental site of their release, the pathophysiological context, and the presence of coexpressed factors. Furthermore, direct neurotoxic and neuroprotective effects of cytokines are evident that are independent from their immunoregulating properties. Whereas these complex interactions are only beginning to be understood, T-cell cytokines nevertheless hold promise as therapeutic targets in a variety of neurological disease conditions.

Controlled autoimmunity in CNS maintenance and repair: naturally occurring CD4+CD25+ regulatory T-Cells at the crossroads of health and disease.

T-cells directed to self-antigens ("autoimmune" T-cells) have traditionally been perceived as tending to attack the body's own tissues, and likely to exert their destructive effects unless they undergo deletion in the thymus during ontogeny. Naturally occurring CD4+CD25+ regulatory T-cells were viewed as thymus-derived cells that constitutively suppress any autoimmune T-cells that escaped thymic deletion. Studies in recent years suggest, however, that some autoimmune T-cells are necessary, at least in the central nervous system for neural maintenance and repair, possibly in part by rendering the resident microglia capable of fighting off adverse conditions, as well as for neural maintenance and repair. In line with this notion, the regulatory T-cells are thought to allow autoimmunity to exist in healthy individuals without causing an autoimmune disease. This proposed immune scenario and its implications for therapy are discussed.

Nervous system injury: focus on the inflammatory cytokine 'granulocyte-macrophage colony stimulating factor'.

Any lesion in the nervous system, be it infectious, immunopathological, ischemic or traumatic, is followed by an inflammatory process that induces rapid activation of glial cells and additional recruitment of granulocytes, T-cells and monocytes/macrophages from the blood stream. Neuroinflammation is a double-sided sword. It can cause neuronal damage and participate in neuropathic pain, but it also has neuroprotective and neurotrophic effects at some stages. Cytokines are the main molecular actors of this 'network of inflammation'. Among them, granulocyte-macrophage colony-stimulating factor (GM-CSF) is a pro-inflammatory hematopoietic cytokine widely used in haematological disorders to stimulate proliferation and differentiation of neutrophilic, eosinophilic and monocytic lineages. GM-CSF and its receptor are expressed in the brain and the cytokine can cross the blood-brain barrier. It is thus likely to affect various nervous system functions. This review will focus on the role of GM-CSF in nervous system disorders and their experimental models with particular emphasis on its possible beneficial effect on axonal regeneration after PNS and CNS injury.

Protective autoimmunity as a T-cell response to central nervous system trauma: prospects for therapeutic vaccines.

Immune activity in general, and autoimmunity in particular, have long been considered as harmful in the context of central nervous system (CNS) trauma. Increasing evidence suggests, however, that the injured CNS can benefit from autoimmune manipulations. Active or passive immunization with CNS-associated self antigens was shown to promote recovery from a CNS insult. It is now also evident that this beneficial 'autoimmunity' is not solely an outcome of immune manipulation but is also a physiological response, evoked by a non-pathogenic insult and apparently designed to counteract the insult-related toxicity which is induced in part by essential physiological compounds present in excess of their normal levels. It appears that when the buffering capacity of constitutive local mechanisms (transporters, enzymes, etc.) that normally regulate these compounds is exceeded, assistance is recruited from the immune system. Like the overactive physiological compounds themselves, the immune system needs to be rigorously regulated in order to produce adequate phagocytic activity and the required quantity of cytokines and growth factors at the right time and place. Boosting of this autoimmune response is potentially a powerful strategy for neuroprotective therapy.