Detection of Neutrophils in the Sciatic Nerve Following Peripheral Nerve Injury

Injury to the sciatic nerve leads to degeneration and debris clearance in the area distal to the injury site, a process known as Wallerian degeneration. Immune cell infiltration into the distal sciatic nerve plays a major role in the degenerative process and subsequent regeneration of the injured motor and sensory axons. While macrophages have been implicated as the major phagocytic immune cell participating in Wallerian degeneration, recent work has found that neutrophils, a class of short-lived, fast responding white blood cells, also significantly contribute to the clearance of axonal and myelin debris. Detection of specific myeloid subtypes can be difficult as many cell-surface markers are often expressed on both neutrophils and monocytes/macrophages. Here we describe two methods for detecting neutrophils in the axotomized sciatic nerve of mice using immunohistochemistry and flow cytometry. For immunohistochemistry on fixed frozen tissue sections, myeloperoxidase and DAPI are used to specifically label neutrophils while a combination of Ly6G and CD11b are used to assess the neutrophil population of unfixed sciatic nerves using flow cytometry.

Acute injury in the peripheral nervous system triggers an alternative macrophage response

BACKGROUND

The activation of the immune system in neurodegeneration has detrimental as well as beneficial effects. Which aspects of this immune response aggravate the neurodegenerative breakdown and which stimulate regeneration remains an open question. To unravel the neuroprotective aspects of the immune system we focused on a model of acute peripheral nerve injury, in which the immune system was shown to be protective.

METHODS

To determine the type of immune response triggered after axotomy of the sciatic nerve, a model for Wallerian degeneration in the peripheral nervous system, we evaluated markers representing the two extremes of a type I and type II immune response (classical vs. alternative) using real-time quantitative polymerase chain reaction (RT-qPCR), western blot, and immunohistochemistry.

RESULTS

Our results showed that acute peripheral nerve injury triggers an anti-inflammatory and immunosuppressive response, rather than a pro-inflammatory response. This was reflected by the complete absence of classical macrophage markers (iNOS, IFN γ, and IL12p40), and the strong up-regulation of tissue repair markers (arginase-1, Ym1, and Trem2). The signal favoring the alternative macrophage environment was induced immediately after nerve damage and appeared to be established within the nerve, well before the infiltration of macrophages. In addition, negative regulators of the innate immune response, as well as the anti-inflammatory cytokine IL-10 were induced. The strict regulation of the immune system dampens the potential tissue damaging effects of an over-activated response.

CONCLUSIONS

We here demonstrate that acute peripheral nerve injury triggers an inherent protective environment by inducing the M2 phenotype of macrophages and the expression of arginase-1. We believe that the M2 phenotype, associated with a sterile inflammatory response and tissue repair, might explain their neuroprotective capacity. As such, shifting the neurodegeneration-induced immune responses towards an M2/Th2 response could be an important therapeutic strategy.

Wallerian degeneration: gaining perspective on inflammatory events after peripheral nerve injury

In this review, we first provide a brief historical perspective, discussing how peripheral nerve injury (PNI) may have caused World War I. We then consider the initiation, progression, and resolution of the cellular inflammatory response after PNI, before comparing the PNI inflammatory response with that induced by spinal cord injury (SCI).In contrast with central nervous system (CNS) axons, those in the periphery have the remarkable ability to regenerate after injury. Nevertheless, peripheral nervous system (PNS) axon regrowth is hampered by nerve gaps created by injury. In addition, the growth-supportive milieu of PNS axons is not sustained over time, precluding long-distance regeneration. Therefore, studying PNI could be instructive for both improving PNS regeneration and recovery after CNS injury. In addition to requiring a robust regenerative response from the injured neuron itself, successful axon regeneration is dependent on the coordinated efforts of non-neuronal cells which release extracellular matrix molecules, cytokines, and growth factors that support axon regrowth. The inflammatory response is initiated by axonal disintegration in the distal nerve stump: this causes blood-nerve barrier permeabilization and activates nearby Schwann cells and resident macrophages via receptors sensitive to tissue damage. Denervated Schwann cells respond to injury by shedding myelin, proliferating, phagocytosing debris, and releasing cytokines that recruit blood-borne monocytes/macrophages. Macrophages take over the bulk of phagocytosis within days of PNI, before exiting the nerve by the circulation once remyelination has occurred. The efficacy of the PNS inflammatory response (although transient) stands in stark contrast with that of the CNS, where the response of nearby cells is associated with inhibitory scar formation, quiescence, and degeneration/apoptosis. Rather than efficiently removing debris before resolving the inflammatory response as in other tissues, macrophages infiltrating the CNS exacerbate cell death and damage by releasing toxic pro-inflammatory mediators over an extended period of time. Future research will help determine how to manipulate PNS and CNS inflammatory responses in order to improve tissue repair and functional recovery.

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.

Chronic restraint stress during early Theiler's virus infection exacerbates the subsequent demyelinating disease in SJL mice: II. CNS disease severity

Theiler's murine encephalomyelitis virus (TMEV) infection is a well-characterized model of multiple sclerosis (MS). Previous research has shown that chronic restraint stress (RS) during early TMEV infection exacerbates behavioral signs of the disease. The present data suggest that RS-induced increases in CNS inflammation, demyelination, and axonal degeneration may underlie this exacerbation. In addition, we report that males exhibit greater CNS inflammation and higher numbers of demyelinating lesions while females show greater susceptibility to RS-induced exacerbation. These findings indicate that RS during early TMEV infection increases CNS lesion formation during the late phase and suggest that the effects of RS are sex-dependent.

Neuroprotection without immunomodulation is not sufficient to reduce first relapse severity in experimental autoimmune encephalomyelitis

OBJECTIVES

Multiple sclerosis can be characterized by a strong neuroinflammatory and progressive neurodegenerative component leading to prolonged disability. The synthetic compound R(+)WIN55,212-2 is reported to be neuroprotective at moderate doses and both neuroprotective and immunomodulatory at high doses, most likely due to differences in receptor affinities. In order to investigate the effects of neuroprotection and immunomodulation in an animal model of multiple sclerosis, we examined the impact of increasing concentrations of R(+)WIN55,212-2 on the inflammatory profile in CNS during first relapse and related this to demyelination, axonal degeneration and relapse severity.

METHODS

Experimental autoimmune encephalomyelitis was induced in Dark Agouti rats and treatment with R(+)WIN55,212-2 was initiated at symptom debut. The animals were scored clinically throughout the experiment, and axonal degeneration, demyelination, T cells, microglia/macrophages, TNF-alpha, IL-12, IFN-gamma, IL-10 and the T(H)17 response were estimated at the peak of the first relapse.

RESULTS

Treatment with high-dose R(+)WIN55,212-2 (10 and 20 mg/kg) significantly improved the clinical performance of the animals during relapse. Interestingly, treatment at any dosage did not affect the brain levels of TNF-alpha, IL-12 and IFN-gamma (T(H)1 response), whereas high-dose cannabinoid treatment reduced the number of T cells and microglia/macrophages in addition to the T(H)17 response. At the same time, we observed a significant reduction in axonal degeneration in all treatment groups whereas only high-dose treatment resulted in reduced demyelination.

CONCLUSION

High-dose R(+)WIN55,212-2 treatment reduces demyelination and axonal degeneration and has immunomodulatory effects which significantly improve clinical performance, whereas a reduction in axonal degeneration on its own, induced by 5 mg/kg R(+)WIN55,212-2, has no impact on first relapse severity.

Molecular inflammatory mediators in peripheral nerve degeneration and regeneration

Wallerian degeneration, the self-destructive set of cellular and molecular processes by which degenerating axons and myelin are cleared after injury, is initiated by macrophages and Schwann cells. Molecular inflammatory mediators such as cytokines (IL-1, IL-6, IL-10, and TNF-alpha, among others), transcription factors (NF-kappaB, c-Jun), the complement system and arachidonic acid metabolites have been shown to modulate these processes in various studies. However, the exact role that each of these mediators plays during axonal degeneration and regeneration has not been fully established. Understanding the molecular basis of these interactions between the immune system and peripheral nerve injury would open the possibility of targeting these inflammatory mediators as therapeutic interventions. In this review we attempt to integrate the current evidence generated around this issue, and to explore the therapeutic possibilities that arise.

The role of macrophages in optic nerve regeneration

Following injury to the nervous system, the activation of macrophages, microglia, and T-cells profoundly affects the ability of neurons to survive and to regenerate damaged axons. The primary visual pathway provides a well-defined model system for investigating the interactions between the immune system and the nervous system after neural injury. Following damage to the optic nerve in mice and rats, retinal ganglion cells, the projection neurons of the eye, normally fail to regenerate their axons and soon begin to die. Induction of an inflammatory response in the vitreous strongly enhances the survival of retinal ganglion cells and enables these cells to regenerate lengthy axons beyond the injury site. T cells modulate this response, whereas microglia are thought to contribute to the loss of retinal ganglion cells in this model and in certain ocular diseases. This review discusses the complex and sometimes paradoxical actions of blood-borne macrophages, resident microglia, and T-cells in determining the outcome of injury in the primary visual pathway.

Toll-like receptors in peripheral nerve injury and neuropathic pain

Peripheral nerve injury triggers a series of responses in the injured nerve, such as the dissolution of distal axons, the activation of Schwann cells, the production of various proinflammatory mediators, and the infiltration of circulating immune cells. These orchestrated events regulate the degeneration and subsequent regeneration of the injured nerve. In addition, peripheral nerve injury often accompanies chronic pain. Studies in this field have revealed that spinal cord microglia activation plays a critical role in the development of pain hypersensitivity. Recent studies using genetically modified mice indicate that Toll-like receptors (TLRs) are involved in nerve degeneration (Wallerian degeneration) and chronic pain (neuropathic pain) development after nerve injury. Here, we review studies that have implicated TLRs in mediating nerve degeneration/regeneration and neuropathic pain following nerve injury. In addition, we discuss possible mechanisms underlying the roles of TLRs in these neurological disorders.

CCL5 and CCR5 genotypes modify clinical, radiological and pathological features of multiple sclerosis

Chemokines mediate selective recruitment of leukocyte subsets into the CNS during inflammatory episodes. We hypothesised that functional polymorphisms in CCR5 and CCL5 influence perivascular leukocyte infiltration, inflammation, axonal loss, and remyelination, and disease course. Therefore, we determined genotypes at four possibly functional polymorphisms in CCR5 and CCL5 for 637 patients and 92 brain donors with multiple sclerosis (MS). For a subset of 192 patients, MRI data were available. We found that low-producer allele CCL5-403*G was associated with reduced risk of severe axonal loss, whereas high-producer allele CCL5-403*A was associated with a worse clinical disease course measured by the MS Functional Composite Score and MS Severity Score. Low-producer allele CCR5+303*G was associated with reduced T2 hyperintense and T1 hypointense lesion volumes on MRI, and high-producer allele CCR5+303*A with early age at onset. Furthermore, low-producer allele CCR5Delta32 was associated with reduced T2 lesion volume, lower black hole ratio on MRI, and with a higher percentage of lesions with signs of remyelination, histopathologically. In summary, our multifaceted study supports the notion that polymorphisms in CCL5 and CCR5 modify the course of MS.

Targeting inflammatory demyelinating lesions to sites of Wallerian degeneration

In Theiler's murine encephalomyelitis virus (TMEV) infection, an animal model for multiple sclerosis (MS), axonal injury precedes inflammatory demyelinating lesions, and the distribution of axonal damage present during the early phase of infection corresponds to regions where subsequent demyelination occurs during the chronic phase. We hypothesized that axonal damage recruits inflammatory cells to sites of Wallerian degeneration, leading to demyelination. Three weeks after TMEV infection, axonal degeneration was induced in the posterior funiculus of mice by injecting the toxic lectin Ricinus communis agglutinin (RCA) I into the sciatic nerve. Neuropathology was examined 1 week after lectin injection. Control mice, infected with TMEV but receiving no RCA I, had inflammatory demyelinating lesions in the anterior/lateral funiculi. Other control mice that received RCA I alone did not develop inflammatory lesions. In contrast, RCA I injection into TMEV-infected mice induced lesions in the posterior funiculus in addition to the anterior/lateral funiculi. We found no differences in lymphoproliferative responses or antibody titers against TMEV among the groups. This suggests that axonal degeneration contributes to the recruitment of inflammatory cells into the central nervous system by altering the local microenvironment. In this scenario, lesions develop from the axon (inside) to the myelin (outside) (Inside-Out model).

PD-L1 (B7-H1) regulation in zones of axonal degeneration

Fibre tract injury evokes recruitment of antigen-presenting- and T cells, but does not cause autoimmune demyelination. This implies that immune tolerance to myelin is actively maintained or readily re-established. Using entorhinal cortex lesion (ECL) to induce axonal degeneration in the hippocampus of adult mice, we studied the induction of B7-H1 (PD-L1) in zones of axonal degeneration. This member of the B7-family has been shown to be expressed on parenchymal cells of various organs, where it strongly down-modulates the activity of T cells. Real-time reverse transcriptase (RT)-PCR revealed low mRNA levels in brain compared to lung and spleen under normal conditions. After ECL, a twofold increase could be observed. Immunocytochemistry revealed astrocytes as source of B7-H1, while immune positive microglia were not detected. Thus, axonal degeneration induces astrocytes to express B7-H1, a potent inhibitor of effector T cells.

Antibodies against light neurofilaments in multiple sclerosis patients

OBJECTIVES

Axonal damage in multiple sclerosis (MS) may be reflected by antibodies against axon-specific proteins - the light subunit of neurofilaments (NFL).

MATERIALS AND METHODS

The serum and cerebrospinal fluid obtained from 58 MS patients, 24 normal controls (CN), 49 control patients with miscellaneous diseases (CD) and 31 patients with neurodegenerative disorders (CDEG) were tested for both immunoglobulin G and M antibodies against NFL, using an ELISA.

RESULTS

Intrathecal IgG antibodies to NFL were elevated in MS patients compared with that in CD patients (P = 0.001) and were not related to clinical variables. No differences in IgM anti-NFL levels were found between the MS and CN/CD groups. IgM to NFL was higher in the CDEG group than in either the CD group or even the MS group (P < 0.0005). CONCLUSIONS - Intrathecal IgM or IgG antibodies to NFL are not useful surrogate markers for axonal damage or disease subtypes in MS.

Early cytokine expression in mouse sciatic nerve after chronic constriction nerve injury depends on calpain

Nerve injury initiates Wallerian degeneration with subsequent alterations of cytokine expression contributing to neuropathic pain. To investigate the very early temporal pattern of cytokine regulation we studied 140 mice of C57Bl/6J background after chronic constriction injury (CCI) of the right sciatic nerve and measured the relative mRNA expression of the pro-inflammatory cytokines tumor necrosis factor-alpha (TNF) and interleukin-1beta (IL-1beta) and of the anti-inflammatory cytokines IL-4 and IL-10 with quantitative real-time polymerase chain reaction (qRT-PCR). The measurements were performed in ipsi- and contralateral sciatic nerves and dorsal root ganglia (DRG) 1, 3, 6, 9, 12, 24 h, and 3 and 7 days after CCI. We found an ipsilateral upregulation of TNF, IL-1beta and IL-10 mRNA levels as early as one hour after CCI. To investigate upstream regulatory mechanisms, we used inhibitors to the N-methyl-d-aspartate (NMDA) receptor ((+)-MK-801) and to calpain (MDL-28170). MDL-28170, but not (+)-MK-801 inhibited TNF and IL-1beta upregulation one hour after CCI. This leads us to suggest that calpain is one of the earliest mediators of cytokine upregulation in injured peripheral nerves.

The pathology of MS: new insights and potential clinical applications

BACKGROUND

The pathological hallmarks of the multiple sclerosis (MS) lesion consist of focal demyelination, inflammation, scar formation, and variable axonal destruction. Despite years of classical histopathological study and more recent intensive use of magnetic resonance technology, the MS lesion is incompletely understood. How it is initiated, how it changes over time, how it correlates with clinical symptoms and other markers of disease activity, and how it is impacted by therapeutic intervention are all largely unknown. As the site of disease pathology, the MS lesion remains the target of attack for therapy. Therefore it is essential we better understand MS lesion evolution and its clinical as well as paraclinical correlates.

REVIEW SUMMARY

In this review, we focus on what can be learned about MS from detailed pathological analysis. We discuss the literature on both the traditional and more recent changing concepts about MS pathogenesis. We also review the work of the Multiple Sclerosis Lesion Project, an international collaborative effort to study the pathologic, clinical, and radiologic correlates of the MS lesion.

CONCLUSIONS

The introduction of new technologies has contributed to a better appreciation regarding the complexity of the MS lesion. The discovery of heterogeneity in demyelinating lesions has suggested that different mechanisms may be involved in MS pathogenesis. This observation may be important for future studies on the etiology and therapy of the disease. However the potential to apply these findings to the clinic will rely on the development of technologies that allow the stratification of MS subtypes without being dependent on brain biopsies.

Enhanced visualization of axonopathy in EAE using thy1-YFP transgenic mice

It is widely accepted that chronic disabilities in multiple sclerosis (MS) patients are due in part to neuronal damage. The central aim of this study was to characterize axonal disruption in the spinal cord of mice with myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis (MOG-EAE), a model of progressive MS. To accomplish this goal, we induced MOG-EAE in thy1-yellow fluorescent (thy-YFP)-transgenic mice in which all spinal motorneurons express the YFP reporter protein. We demonstrate that a build-up of YFP fluorescence occurs in profiles reminiscent of tortuous fragmented axons and axonal spheroids/globules as seen in various neurodegenerative/neuroinflammatory diseases. Approximately two-thirds of these damaged axons were decorated by the monoclonal antibody SMI 32, which recognizes hypophosphorylated neurofilament-H (hypoP-NF-H), an established marker of CNS axonal pathology. Unexpectedly, one third of damaged axons were hypoP-NF-H negative but could be visualized by their expression of the YFP transgene, whilst the remaining profiles were hypoP-NF-H positive but did not exhibit YFP fluorescence. Thus, using YFP transgenic mice in conjunction with hypoP-NF-H immunoreactivity provides a more comprehensive depiction of axonopathy in the ventral-lateral aspect of lumbosacral spinal cord in MOG-EAE. When YFP fluorescence was used in conjunction with a monoclonal antibody that recognizes CD11b; a marker of subsets of inflammatory cells, we were able to discern evidence of an early inflammatory attack on white matter axons. Finally, we show the accumulation of hyperphosphorylated neurofilament-H (hyperP-NF-H) expression in YFP+, lesioned WM areas and in a subpopulation of neuronal perikarya in the lumbar spinal cords of EAE mice.

Spatiotemporal quantification of tumor necrosis factor-alpha and interleukin-10 after crush injury in rat sciatic nerve utilizing immunohistochemistry

The purpose of this study was to investigate quantitatively the longitudinal temporal, spatial changes of the tumor necrosis factor-alpha (TNF) and interleukin-10 (IL-10) immunopositive cells during Wallerian degeneration and the following regeneration after crush injury in rat sciatic nerve using immunohistochemistry and enzyme linked immunosorbent assay (ELISA). The number of TNF-immunopositive cells reached its peak and increased significantly in all the segments distal to the crush site 3 days after injury. On Day 7, TNF-immunopositive cells decreased in all the segments distal to the crush site, and a significant decrease was observed 14 days after injury. From Day 21 to Day 56, there were no significant differences in the numbers of TNF-immunopositive cells. The average size of TNF immunopositive cells became significantly larger with degeneration. The number of IL-10-immunopositive cells decreases significantly 1 day after crush injury. IL-10-immunopositive cells increased on Day 3, returning to control levels. Seven days after injury, a significant increase in the number of IL-10-immunopositive cells was observed. There was also no significant difference in the number of IL-10-immunopositive cells beyond Day 14 except for a part of distal segments. The number of IL-10-immunopositive cells showed no significant differences in all the segments on Day 56. The protein levels of IL-10 measured by ELISA were similar to the result of immunohistochemistry. These results suggest that the significant change in IL-10 occurred prior to the significant change in TNF and that IL-10 may be the key to the change in TNF.

Tissue sparing and functional recovery following experimental traumatic brain injury is provided by treatment with an anti-myelin-associated glycoprotein antibody

Axonal injury is a hallmark of traumatic brain injury (TBI) and is associated with a poor clinical outcome. Following central nervous system injury, axons regenerate poorly, in part due to the presence of molecules associated with myelin that inhibit axonal outgrowth, including myelin-associated glycoprotein (MAG). The involvement of MAG in neurobehavioral deficits and tissue loss following experimental TBI remains unexplored and was evaluated in the current study using an MAG-specific monoclonal antibody (mAb). Anesthetized rats (n=102) were subjected to either lateral fluid percussion brain injury (n=59) or sham injury (n=43). In surviving animals, beginning at 1 h post-injury, 8.64 microg anti-MAG mAb (n=33 injured, n=21 sham) or control IgG (n=26 injured, n=22 sham) was infused intracerebroventricularly for 72 h. One group of these rats (n=14 sham, n=11 injured) was killed at 72 h post-injury for verification of drug diffusion and MAG immunohistochemistry. All other animals were evaluated up to 8 weeks post-injury using tests for neurologic motor, sensory and cognitive function. Hemispheric tissue loss was also evaluated at 8 weeks post-injury. At 72 h post-injury, increased immunoreactivity for MAG was seen in the ipsilateral cortex, thalamus and hippocampus of brain-injured animals, and anti-MAG mAb was detectable in the hippocampus, fimbria and ventricles. Brain-injured animals receiving anti-MAG mAb showed significantly improved recovery of sensorimotor function at 6 and 8 weeks (P<0.01) post-injury when compared with brain-injured IgG-treated animals. Additionally, at 8 weeks post-injury, the anti-MAG mAb-treated brain-injured animals demonstrated significantly improved cognitive function and reduced hemispheric tissue loss (P<0.05) when compared with their brain-injured controls. These results indicate that MAG may contribute to the pathophysiology of experimental TBI and treatment strategies that target MAG may be suitable for further evaluation.

A distinct subgroup of chronic inflammatory demyelinating polyneuropathy with CNS demyelination and a favorable response to immunotherapy

To explore subclinical central nervous system (CNS) involvement in chronic inflammatory demyelinating polyneuropathy (CIDP), we recorded somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) using transcranial magnetic stimulation, to measure central sensory conduction time (CSCT) and central motor conduction time (CMCT) and examined brain and spinal cord MRI in patients with probable CIDP based on the American Academy of Neurology AIDS Task Force criteria. Eighteen patients with probable CIDP (12 males and 6 females; mean age at examination+/-SD, 45.8+/-17.0 years; range, 17-72) were included in the study. Of the 13 patients who underwent SEPs, one had prolonged CSCT (8%) and of the 13 who underwent MEPs, four had abnormal CMCT (31%). Cranial MRI revealed five of 18 patients had abnormal scans, only one of which showed multiple ovoid periventricular lesions suggestive of demyelination while none showed any intramedullary lesion on spinal cord MRI. Thus, 6 of the 18 patients were considered to have subclinical demyelinative CNS involvement which had lower disability on Global Neurological Disability Score (GNDS) (p=0.0061), a male preponderance (0.0537) and a larger compound muscle action potential (CMAP) amplitude in the median nerve (p=0.005) than those without. The decrease of GNDS with immunologic therapies was nearly significant in the former (p=0.0556) but not in the latter. The results of the present study suggest that subclinical CNS involvement in CIDP is not uncommon in Japanese patients and that CIDP with subclinical CNS involvement is more demyelinative thus responsive to immunotherapies while those without have more axonal damage and less responsive to immunotherapies.

Proinflammatory cytokine synthesis in the injured mouse spinal cord: multiphasic expression pattern and identification of the cell types involved

We have studied the spatial and temporal distribution of six proinflammatory cytokines and identified their cellular source in a clinically relevant model of spinal cord injury (SCI). Our findings show that interleukin-1beta (IL-1beta) and tumor necrosis factor (TNF) are rapidly (<5 and 15 minutes, respectively) and transiently expressed in mice following contusion. At 30-45 minutes post SCI, IL-1beta and TNF-positive cells could already be seen over the entire spinal cord segment analyzed. Multilabeling analyses revealed that microglia and astrocytes were the two major sources of IL-1beta and TNF at these times, suggesting a role for these cytokines in gliosis. Results obtained from SCI mice previously transplanted with green fluorescent protein (GFP)-expressing hematopoietic stem cells confirmed that neural cells were responsible for the production of IL-1beta and TNF for time points preceding 3 hours. From 3 hours up to 24 hours, IL-1beta, TNF, IL-6, and leukemia inhibitory factor (LIF) were strongly upregulated within and immediately around the contused area. Colocalization studies revealed that all populations of central nervous system resident cells, including neurons, synthesized cytokines between 3 and 24 hours post SCI. However, work done with SCI-GFP chimeric mice revealed that at least some infiltrating leukocytes were responsible for cytokine production from 12 hours on. By 2 days post-SCI, mRNA signal for all the above cytokines had nearly disappeared. Notably, we also observed another wave of expression for IL-1beta and TNF at 14 days. Overall, these results indicate that following SCI, all classes of neural cells initially contribute to the organization of inflammation, whereas recruited immune cells mostly contribute to its maintenance at later time points.

Current trends in multiple sclerosis research: an update on pathogenic concepts

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) of presumed autoimmune origin which develops in a genetic susceptible individual triggered by additional environmental factors. In this review we will provide an update of basic pathogenic concepts. In addition, we will discuss newly evolving concepts in MS pathogenesis such as pathogenic heterogeneity, importance of axonal loss and the role of CD8+ T lymphocytes in tissue injury. In the last part of this review we will briefly describe currently approved MS treatments and summarize some promising therapeutic approaches that are currently under evaluation.

Tauopathy-like abnormalities and neurologic deficits in mice immunized with neuronal tau protein

BACKGROUND

A possible role of autoimmunity in Alzheimer disease pathogenesis has recently attracted increasing attention. Vaccination with amyloid-beta peptide was reported to cause marked reduction in amyloid deposition, but it also induced encephalitis. Not much is known regarding neurofibrillary tangle-related autoimmune effects.

OBJECTIVE

To use the main component of tangles-microtubule-associated tau protein-to test the feasibility of active induction of a neuroautoimmune disorder in mice.

DESIGN

Prospective, randomized controlled animal study.

SETTING

University medical center research laboratory. Subjects Female C57BL/6 mice.

INTERVENTIONS

Inoculation with recombinant human tau protein emulsified in complete Freund adjuvant and with pertussis toxin.

MAIN OUTCOME MEASURES

Clinical, immunologic, pathologic, and behavioral evaluations were performed.

RESULTS

Vaccination with tau protein induced histopathologic features of Alzheimer disease and tauopathies, indicated by the presence of neurofibrillary tangle-like structures, axonal damage, and gliosis. Also, mononuclear infiltrates without demyelination in the central nervous system, accompanied by neurologic deficits (such as a limp tail and limb paralysis), were observed. Anti-tau antibodies were detected in the serum of tau-immunized mice.

CONCLUSIONS

These results provide a link between tau autoimmunity and tauopathy-like abnormalities and indicate potential dangers of using tau for immunotherapy. This experimental autoimmune tauopathy-like model is due to a pathogenic immune response against an intraneuronal antigen and is not related to myelin antigens.

Absence of perforin expression confers axonal protection despite demyelination

Current evidence suggests that demyelination may be a necessary but not a sufficient condition for neurologic deficits associated with multiple sclerosis. Axon injury that occurs within the permissive environment of the demyelinated lesion is better correlated with functional deficits, but the mechanisms and cellular effectors of this injury are largely unknown. In an effort to identify potential axon injury mediators, we examined demyelination, motor function, and the number of spinal axons in perforin-deficient mice. Perforin is a critical molecular mediator of cytotoxic immunological injury and we hypothesized that genetic deletion of perforin expression would protect demyelinated axons. Indeed, we found that while perforin-deficient mice had considerable spinal cord demyelination 180 days after infection with Theiler's murine encephalomyelitis virus, such mice exhibited functional and axonal preservation comparable to non-demyelinated perforin-competent controls. We conclude that perforin-dependent effector cells such as cytotoxic T cells, gammadelta T cells, and natural killer cells may play a role in axon damage that is dependent upon but separable from demyelination.

Bone marrow stromal cells reduce axonal loss in experimental autoimmune encephalomyelitis mice

We investigated the ability of human bone marrow stromal cell (hBMSC) treatment to reduce axonal loss in experimental autoimmune encephalomyelitis (EAE) mice. EAE was induced in SJL/J mice by injection with proteolipid protein (PLP). Mice were injected intravenously with hBMSCs or PBS on the day of clinical onset, and neurological function was measured daily (score 0-5) until 45 weeks after onset. Mice were sacrificed at week 1, 10, 20, 34, and 45 after clinical onset. Bielshowsky silver was used to identify axons. Immunohistochemistry was performed to measure the expression of nerve growth factor (NGF) and MAB1281, a marker of hBMSCs. hBMSC treatment significantly reduced the mortality, the disease severity, and the number of relapses in EAE mice compared with PBS treatment. Axonal density and NGF(+) cells in the EAE brain were significantly increased in the hBMSC group compared with the PBS group at 1, 10, 20, 34, and 45 weeks. Disease severity was significantly correlated with decreased axonal density and decreased NGF, and increased axonal density was significantly correlated with reduced loss of NGF expression after hBMSC treatment. Most of the NGF(+) cells are brain parenchymal cells. Under 5% of MAB1281(+) cells colocalized with NG2(+), a marker of oligodendrocyte progenitor cells. Nearly 10% of MAB1281(+) cells colocalized with GFAP, a marker of astrocytes, and MAP-2, a marker of neurons. Our findings indicate that hBMSCs improve functional recovery and may provide a potential therapy aimed at axonal protection in EAE mice, in which NGF may play a vital role.

Necrotic neuronal cells induce inflammatory Schwann cell activation via TLR2 and TLR3: implication in Wallerian degeneration

Schwann cells play an important role in peripheral nerve regeneration. Upon nerve injury, Schwann cells are activated and produce various proinflammatory cytokines and chemokines, resulting in the recruitment of macrophages and the phagocytosis of myelin debris. However, it is unclear how nerve injury induces Schwann cell activation. Recently, it was reported that necrotic cells induce immune cell activation via toll-like receptors (TLRs). This suggests that the TLRs expressed on Schwann cells may recognize nerve injury by binding to the endogenous ligands secreted by the damaged nerve, thereby inducing Schwann cell activation. To explore such a possibility, we stimulated rat Schwann cells with necrotic neuronal cells (NNC). The stimulation of Schwann cells with NNC induced the expression of various inflammatory mediators, including TNF-alpha and iNOS. Studies on the NNC-mediated intracellular signaling pathways revealed that p38 and JNK are involved in the NNC-mediated Schwann cell activation. In addition, NNC-induced proinflammatory gene expression was reduced in mouse Schwann cells derived from TLR2 or TLR3 knockout mice. In summary, these results suggest that necrotic neuronal cells induce inflammatory Schwann cell activation via TLR2 and TLR3, which might be involved in Wallerian degeneration upon peripheral nerve injury.

Nature of signals that initiate the immune response during Wallerian degeneration of peripheral nerves

Monocyte chemoattractant protein-1 is produced by Schwann cells during Wallerian degeneration of a peripheral nerve and contributes to a selective accumulation of macrophages in the degenerating segment. An in vitro preparation has been developed to analyze the molecules from axons and non-neuronal cells in nerves that stimulate an increased production of monocyte chemoattractant protein-1 mRNA by Schwann cells. For this purpose, Schwann cells obtained from neonatal rats were maintained in culture, exposed to putative molecular stimuli and analyzed for their content of monocyte chemoattractant protein-1 mRNA. Under basal conditions, the concentration of monocyte chemoattractant protein-1 in Schwann cells was low. Freeze-killed fragments or homogenates of nerve (or brain) but not viable nerve or freeze-killed muscle were effective in inducing monocyte chemoattractant protein-1 mRNA. The inductive activity was abolished by heating. Results of dialysis of supernatants of nerve homogenates indicate that a protein or proteins of 1-10 kDa were capable of stimulating synthesis of monocyte chemoattractant protein-1 by Schwann cells. Also, the activity in nerve homogenates was partially inhibited by antibodies to Toll-like receptor-4. The observations suggest that a non-secreted protein is released from disintegrating axons to initiate the innate immune response that characterizes Wallerian degeneration.

A role for CXCL12 (SDF-1alpha) in the pathogenesis of multiple sclerosis: regulation of CXCL12 expression in astrocytes by soluble myelin basic protein

The pathogenic mechanisms that contribute to multiple sclerosis (MS) include leukocyte chemotaxis into the central nervous system (CNS) and the production of inflammatory mediators, resulting in oligodendrocyte damage, demyelination, and neuronal injury. Thus, factors that regulate leukocyte entry may contribute to early events in MS, as well as to later stages of lesion pathogenesis. CXCL12 (SDF-1alpha), a chemokine essential in CNS development and a chemoattractant for resting and activated T cells, as well as monocytes, is constitutively expressed at low levels in the CNS and has been implicated in T cell and monocyte baseline trafficking. To determine whether CXCL12 is increased in MS, immunohistochemical analyses of lesions of chronic active and chronic silent MS were performed. CXCL12 protein was detected on endothelial cells (EC) in blood vessels within normal human brain sections and on a small number of astrocytes within the brain parenchyma. In active MS lesions, CXCL12 levels were high on astrocytes throughout lesion areas and on some monocytes/macrophages within vessels and perivascular cuffs, with lesser staining on EC. In silent MS lesions, CXCL12 staining was less than that observed in active MS lesions, and also was detected on EC and astrocytes, particularly hypertrophic astrocytes near the lesion edge. Experiments in vitro demonstrated that IL-1beta and myelin basic protein (MBP) induced CXCL12 in astrocytes by signaling pathways involving ERK and PI3-K. Human umbilical vein EC did not produce CXCL12 after treatment with MBP or IL-1beta. However, these EC cultures expressed CXCR4, the receptor for CXCL12, suggesting that this chemokine may activate EC to produce other mediators involved in MS. In agreement, EC treatment with CXCL12 was found to upregulate CCL2 (MCP-1) and CXCL8 (IL-8) by PI3-K and p38-dependent mechanisms. Our findings suggest that increased CXCL12 may initiate and augment the inflammatory response during MS.

Endogenous leukemia inhibitory factor production limits autoimmune demyelination and oligodendrocyte loss

Autoimmune injury to oligodendrocytes evokes an endogenous response in the central nervous system, which initially limits the acute injury to oligodendrocytes and myelin, and subsequently promotes remyelination. The key molecular and cellular events responsible for this beneficial outcome are incompletely understood. In this article, we utilize murine autoimmune encephalomyelitis (EAE) to focus on the effect of endogenously produced leukemia inhibitory factor (LIF) upon mature oligodendrocyte survival after demyelinating injury. We show that the mRNA for LIF is markedly upregulated in the spinal cord in the context of acute inflammatory demyelination. After clinical disease onset, administration of neutralizing anti-LIF antibodies over a four day period significantly worsens disease severity in two different murine EAE models. We also show that administration of neutralizing antibodies results in reduced activation of the cognate LIF receptor components in the spinal cord. Histologically, anti-LIF antibody administration increases the extent of acute demyelination (P < 0.01) and doubles the oligodendrocyte loss already induced by EAE (P < 0.05), without altering the extent of inflammatory infiltration into the spinal cord. Although acute EAE induces a rapid, three-fold increase in the proliferation of NG2 positive oligodendrocyte progenitors (P < 0.001), this response is not diminished by antagonism of endogenous LIF. We conclude that endogenous LIF is induced in response to autoimmune demyelination in the spinal cord and protects mature oligodendrocytes from demyelinating injury and cell death, thereby resulting in attenuation of clinical disease severity.

Differential recruitment of CD8+ macrophages during Wallerian degeneration in the peripheral and central nervous system

The strong macrophage response occurring during Wallerian degeneration in the peripheral but not central nervous system has been implicated in tissue remodeling and growth factor production as key requirements for successful axonal regeneration. We have previously identified a population of CD8+ phagocytes in ischemic brain lesions that differed in its recruitment pattern from CD4+ macrophages/microglia found in other lesion paradigms. In the present study we show that crush injury to the sciatic nerve induced strong infiltration by CD8+ macrophages both at the crush site and into the degenerating distal nerve stump. At the crush site, CD8+ macrophages appeared within 24 hours whereas infiltration of the distal nerve parenchyma was delayed to the second week. CD8+ macrophages were ED1+ and CD11b+ but always MHC class II-. Most CD8+ macrophages coexpressed CD4 while a significant number of CD4+/CD8-macrophages was also present. Expression of the resident tissue macrophage marker ED2 was largely restricted to the CD4+/CD8- population. Following intraorbital crush injury to the optic nerve, infiltration of CD8+ macrophages was strictly confined to the crush site. Taken together, our study demonstrates considerable spatiotemporal diversity of CD8+ macrophage responses to axotomy in the peripheral and central nervous system that may have implications for the different extent of axonal regeneration observed in both systems.

TNFalpha-induced MMP-9 promotes macrophage recruitment into injured peripheral nerve

Matrix metalloproteinase-9 (MMP-9) is an extracellular protease that is induced hours after injury to peripheral nerve. This study shows that MMP-9 gene deletion and neutralization with MMP-9 antibody reduce macrophage content in injured wild-type nerves. In mice with delayed Wallerian degeneration (WldS), MMP-9 and tumor necrosis factor alpha (TNFalpha) decline in association with the reduced macrophage recruitment to injured nerve that characterizes this strain of mice. We further determined that TNFalpha acts as an MMP-9 inducer by establishing increased MMP-9 levels after TNFalpha injection in rat sciatic nerve in vivo and primary Schwann cells in vitro. We found reduced MMP-9 expression in crushed TNFalpha knockout nerves that was rescued with exogenous TNFalpha. Finally, local application of MMP-9 on TNFalpha-/- nerves increased macrophage recruitment to the lesion. These data suggest that TNFalpha lies upstream of MMP-9 in the pathway of macrophage recruitment to injured peripheral nerve.

Localization and changes of intraneural inflammatory cytokines and inducible-nitric oxide induced by mechanical compression

STUDY DESIGN

Investigation of intraneural inflammation induced by mechanical compression.

OBJECTIVES

In order to investigate the mechanism of neuropathy, this study used a median nerve compression model in dogs. Immunohistochemistry was used to examine the localization and changes of inflammatory cytokines and nitric oxide (NO).

SUMMARY OF BACKGROUND DATA

The manifestation of pain at sites of inflammation has a close relationship with the release of mediators from macrophages such as interleulin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha), as well as with NO. However, the mediators involved in inflammation of nerve due to mechanical compression remain almost unknown.

METHODS

In this study, the median nerve of dogs was compressed with a clip for three weeks to observe the changes caused by compression. Immunohistochemistry was done by the avidin-biotin-peroxidase complex method to observe the changes of T cells (CD45) and macrophages (Mac-1) after compression. Antibodies against IL-1beta, TNF-alpha, and inducible nitric oxide synthesis (i-NOS) were used to examine the localization and changes of these mediators caused by nerve compression.

RESULTS

In control animals, resident T cells were detected, but there were no macrophages. IL-1beta was positive in the Schwann cells and vascular endothelial cells. However, no cells showed TNF-alpha or i-NOS positively. After nerve compression, numerous T cells and macrophages appeared among the demyelinized nerve fibers. The macrophages were positive for IL-1beta, TNF-alpha and i-NOS.

CONCLUSION

Inflammatory cytokines and NO may be involved in intraneural inflammatory changes arising from mechanical compression. Such mediators may be of importance in the manifestation of neuropathy.

Neuroprotective effect of the immune system in a mouse model of severe dysmyelinating hereditary neuropathy: enhanced axonal degeneration following disruption of the RAG-1 gene

In mouse models of later onset forms of human hereditary demyelinating neuropathies, the immune system plays a crucial pathogenic role. Here, we investigated the influence of immune cells on early onset dysmyelination in mice homozygously deficient of the myelin component P0. In peripheral nerves of P0(-/-) mice, CD8+ T-lymphocytes increased with age. Macrophages peaked at 3 months followed by a substantial decline. They were mainly of hematogenous origin. To evaluate the functional role of immune cells, we cross-bred P0(-/-) mutants with RAG-1-deficient mice. At 3 months, the number of endoneurial macrophages did not differ from the macrophage number of immunocompetent myelin mutants, but the later decline of macrophages was not observed. Quantitative electron microscopy revealed that in plantar nerves of 6-month-old double mutants, significantly more axons had degenerated than in immunocompetent littermates. These data suggest a neuroprotective net effect of T-lymphocytes on axon survival in inherited, early onset dysmyelination.

Upregulation of matrix metalloproteinases following nerve injury is not mediated by mast cell activation

OBJECTIVE

Matrix metalloproteinases (MMPs) contribute to inflammatory and degenerative processes in injured nerves. Since mast cells release mediators which upregulate and activate MMPs, we tested the hypothesis that activation of mast cells is responsible for changes in the expression and activity of MMP-2 and MMP-9 in the injured peripheral nerve.

METHODS

The sciatic nerve was partially ligated in Wistar rats in which mast cells were stabilized with sodium cromoglycate. Expression and activity of MMP-2 and MMP-9 were measured in the injured and contralateral nerve using gelatin zymography, and compared between mast cell-stabilized and control groups.

RESULTS

Expression and activity of MMP-9 were increased in both the injured and contralateral nerve, but activity of MMP2 was slightly reduced by nerve injury. However, stabilization of mast cells did not alter the changes in expression or activity of MMP-2 and MMP-9 following nerve injury.

CONCLUSION

These findings suggest that the contribution of MMP-9 upregulation to the inflammatory and degenerative changes that follow nerve injury is independent of mast cell activation.

Systemic injections of lipopolysaccharide accelerates myelin phagocytosis during Wallerian degeneration in the injured mouse spinal cord

The phagocytic cell response within the injured spinal cord is inefficient, allowing myelin debris to remain for prolonged periods of time within white matter tracts distal to the injury. Several proteins associated with this degenerating myelin are inhibitory to axon growth and therefore prevent severed axons from regenerating. Inflammatory agents such as lipopolysaccharide (LPS) can stimulate both the migration and phagocytic activity of macrophages. Using in situ hybridization, we found that the expression of the LPS membrane receptor, CD14, was enhanced in the mouse dorsal column following a dorsal hemisection. Double labeling studies showed that microglia and macrophages are the two major cell types expressing CD14 mRNA following spinal cord injury (SCI). We therefore tested whether systemic injections of LPS would increase the number and phagocytic activity of macrophages/microglia in the ascending sensory tract (AST) of the mouse dorsal column following a dorsal hemisection. Mice were treated daily via intraperitoneal injections of either LPS or phosphate-buffered saline (PBS). At 7 days post-SCI, greater numbers of activated mononuclear phagocytes were present in the AST undergoing Wallerian degeneration (WD) in LPS-treated animals compared with controls. Animals treated with LPS also exhibited greater Oil Red O staining, which is specific for degenerating myelin and macrophages phagocytosing myelin debris. Myelin clearance was confirmed at 7 days using Luxol Fast Blue staining and on toluidine blue-stained semi-thin sections. These results indicate that it is possible to manipulate the innate immune response to accelerate myelin clearance during WD in the injured mouse spinal cord.

Hematopoietic stem cell transplantation in multiple sclerosis: experimental evidence to rethink the procedures

Acute immunosuppression with lymphocytic agents given at maximally tolerated doses, followed by hematopoietic stem cell rescue achieved by autologous bone marrow or peripheral blood stem cell transplantation (BMT), has proved effective in various experimental models of autoimmunity. The rationale for such an approach in autoimmune diseases is based on the concept of lymphoablation of self-reactive lymphocytes followed by de novo immune system reconstitution, which, in the presence of the autoantigens in the thymus, may reinduce self-tolerance. Our previous work shows that in experimental autoimmune encephalomyelitis (EAE), autologous/syngeneic BMT not only prevents the appearance of paralytic signs, but can also partially reverse chronic disease and induce long-term, antigen-specific tolerance. However, there are serious reservations to be considered when interpreting these data and before applying similar protocols in patients with multiple sclerosis. (1) The model of EAE is not a completely reliable model of multiple sclerosis. (2) In animals with chronic EAE, although further relapses were prevented, the established paralysis was usually not reversible. According to recent data, in chronic multiple sclerosis (MS) lesions, damage caused by axonal loss/transection and cortical/spinal cord atrophy is irreversible and probably amenable to immunotherapy. (3) Long-term, antigen-specific tolerance may be induced with BMT, but not in all cases; in passively induced CR-EAE, many of the mice relapsed upon challenge with myelin antigens, which may indicate that the presence of the immunizing, myelin antigens (on the site of immunization) during the process of immune reconstitution is critical for induction of tolerance. Finally, one should weigh the procedure-related risks (including mortality of up to 5%) of bone marrow or peripheral stem cell transplantation (SCT). A more radical solution for autoimmunity may involve the use of non-myeloablative allogeneic transplantation.

Wallerian degeneration after crush or chronic constriction injury of rodent sciatic nerve is associated with a depletion of endoneurial interleukin-10 protein

We used enzyme-linked immunoassay (ELISA), immunohistochemistry (IHC), and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) to determine whether interleukin (IL)-10 protein is changed after unilateral crush or chronic constriction injury (CCI) of mouse or rat sciatic nerve and whether IL-10 protein and mRNA are differentially regulated. In the mouse sciatic nerve, IL-10 protein declined rapidly to 10-20% of baseline early after crush or CCI, while the IL10 mRNA was up-regulated with a maximum on Days 1 and 3. In the rat sciatic nerve, IL-10 protein was significantly reduced on Day 3 after CCI, and IL-10 mRNA was up-regulated in both models. These results suggest that changes of the local cytokine network during wallerian degeneration include an early deficiency of the antiinflammatory cytokine IL-10 despite up-regulation at the mRNA level.

Pathology of lumbar nerve root compression. Part 1: Intraradicular inflammatory changes induced by mechanical compression

STUDY DESIGN

This study is to investigate the intraradicular inflammation induced by mechanical compression using in vivo model.

OBJECTIVES

The relationship between the intraradicular edema and nerve fiber degeneration induced by mechanical compression was determined in the nerve root.

SUMMARY OF BACKGROUND DATA

Recently some studies reported that mechanical compression increased microvascular permeability of the endoneurial capillaries and resulted in an intraradicular inflammation. These changes may be an important factor of the pathogenesis of radiculopathy. However, the natural courses of the intraradicular inflammation after mechanical compression are still poorly understood.

METHODS

In dogs, laminectomy was performed at L7 and the seventh nerve root was exposed to compression at 7.5 gram force (gf) clipping power. The animals were evaluated at 1 and 3 weeks after clipping. After the appropriate period of nerve root compression, Evans blue albumin (EBA) was injected intravenously. The nerve root sections were divided into two groups. The sections were used to investigate the status of the blood-nerve barrier function under the fluorescence microscope. The other sections were used for light and transmission electron microscopic study.

RESULTS

After 1 and 3 weeks, intraradicular edema was observed not only at the site of compression but also in the peripheral zone of a compressed anterior root and in the central zone of a compressed posterior root. The evidence of active Wallerian degeneration was also seen in the area of intraradicular edema. In addition, the nerve roots showing Wallerian degeneration were infiltrated by inflammatory cells, such as macrophages and mast cells.

CONCLUSIONS

Inflammatory reaction, such as Wallerian degeneration, breakdown of blood-nerve barrier and appearance of macrophage, may be deeply involved in radiculitis arising from mechanical compression, and these factors seem to be important in the manifestation of radiculopathy.

Caught in the act: in vivo mapping of macrophage infiltration in nerve injury by magnetic resonance imaging

In vivo tracking of hematogenous macrophages has been a major challenge because these cells are key players in nerve injury and repair. We visualized the spatiotemporal course of macrophage infiltration after acute peripheral nerve injury in living rats by using superparamagnetic iron oxide (SPIO) particles and magnetic resonance imaging (MRI). A signal loss on MR images indicating iron accumulation was present in degenerating sciatic nerves between days 1 and 8 after a crush lesion, ceased thereafter, and corresponded to the transient presence of iron-labeled ED1-positive macrophages in tissue sections. In contrast, no SPIO accumulation was seen after optic nerve crush, which revealed microglial activation but lacked macrophage infiltration. SPIO-enhanced MRI provides a new tool to selectively visualize active periods of macrophage transmigration into the nervous system, thus enabling dynamic views on a fundamental process in a multitude of nerve disorders.

Acute motor axonal neuropathy rabbit model: immune attack on nerve root axons

Macrophages in the periaxonal space and surrounding intact myelin sheath are the most prominent pathological feature of acute motor axonal neuropathy (AMAN). We describe this characteristic in nerve roots from paralyzed rabbits immunized with bovine brain ganglioside or GM1. IgG was deposited on nerve root axons. Distal nerve conduction was preserved, and late F wave components were absent during the acute phase. Initial lesions were located mainly on nerve root axons, as in human AMAN. This study thus provides supportive evidence that the rabbits constitute a model of AMAN.

Neurophysiological changes in demyelinating and axonal forms of acute experimental autoimmune neuritis in the Lewis rat

Myelin-sensitized T- and B-cells (lymph node cells) induced experimental autoimmune neuritis (EAN) in Lewis rats after passive transfer to naive recipients. After 6 days, all recipient rats developed tail paresis that progressed to limb paresis within 12-72 h. Progressive nerve conduction changes consistent with demyelination in the sciatic nerve (conduction block and prolongation of the distal motor latencies) and lumbar nerve roots (initial low F-wave frequencies followed by later prolongation in F-wave latencies) were observed during the disease. For comparison, adoptive transfer experimental autoimmune neuritis (AT-EAN) of differing disease severity was induced by titrating the dose of P2-specific T-cells. In contrast to EAN induced by myelin-sensitized T- and B-cells, AT-EAN was predominantly associated with rapid nerve conduction changes consistent with axonal dysfunction and degeneration. These findings demonstrate that distinct forms of EAN with different pathophysiological mechanisms are induced by the passive transfer of P2-specific T-cell lines or myelin-specific T-cells and B-cells. The electrophysiological changes in EAN induced by myelin-specific T- and B-cells are very similar to those seen clinically during acute inflammatory demyelinating polyneuropathy, whereas AT-EAN has less resemblance to axonal forms of Guillain-Barré syndrome.

Efficacy of leukemia inhibitory factor in experimental autoimmune neuritis

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine that exerts neurotrophic and myotrophic actions. We have investigated the effect of LIF in experimental autoimmune neuritis (EAN), an animal model of Guillain-Barré syndrome (GBS). Treatment with LIF at the onset of the disease showed a slight, but not significant, improvement in the clinical course but no effect on nerve histology.

Anti-inflammatory strategies to prevent axonal injury in multiple sclerosis

Axonal injury in multiple sclerosis has attracted considerable interest during the past few years. It has been demonstrated in association with inflammation within active lesions, but it is also present in normal-appearing white matter. Because axonal loss appears to be responsible for persistent neurological deficits in patients with multiple sclerosis, treatment strategies to prevent damage to neurites and restore function are of paramount importance in controlling the disease process. Some of the currently available immunomodulatory therapies may also reduce axonal damage, as demonstrated using improved imaging technologies, but the precise mechanisms that could protect axons during the inflammatory attack are yet to be identified. Factors that are involved in functional impairment of axonal conduction and those elements that are responsible for direct structural damage to the axon are both potential targets for therapeutic interventions.

gamma delta T cells infiltrating sensory nerve biopsies from patients with inflammatory neuropathy

Sensory nerve biopsy specimens from patients with Guillain Barré syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), and controls consisting of other neuropathies, were examined in order to characterise the nature and intensity of any inflammatory infiltrate. In order to establish whether gamma delta T cells were present in these infiltrates we examined the expression of alpha beta and gamma delta T cell receptors in the biopsy specimens from patients with inflammatory neuropathy. A section of each biopsy specimen was simultaneously cultured in order to attempt to establish T cell lines. T cell lines were established in 4 out of 7 patients with GBS of which 2 were gamma delta in phenotype. There was a significant correlation between the number of mononuclear cells detected by immunostaining within the biopsy specimens and the chance of successfully establishing a T cell line. Histological studies detected gamma delta T cell receptor in 2 out of the 7 patients with GBS, 14 out of the 20 with CIDP and in 5 out of the 13 controls (vasculitis 3, paraneoplastic 1, axonal neuropathy of uncertain cause 1). The presence of T cells of a gamma delta T cell receptor phenotype in nerve biopsy specimens from patients with inflammatory neuropathy is consistent with a possible pathogenetic role of a cellular immune response against non-protein antigens such as gangliosides.

Endoneurial remodeling by TNFalph- and TNFalpha-releasing proteases. A spatial and temporal co-localization study in painful neuropathy

Peripheral nerve injury causing Wallerian degeneration results in endoneurial remodeling initiated by an increase in tumor necrosis factor-alpha (TNF), which is activated from its precursor by extracellular proteases of the matrix metalloproteinase (MMP) family. We used immunohistochemistry to analyze the distribution of TNF, TNF-releasing MMPs, including gelatinases (MMP-2 and MMP-9), and TNF-alpha converting enzyme (TACE) in painful neuropathy caused by chronic constriction injury of rat sciatic nerve. Tissue was analyzed at the injury site and in the corresponding L4 and L5 dorsal root ganglia (DRG) throughout the time-course of the neuropathy. Using confocal laser scanning microscopy, we co-localized TNF with each MMP, and observed spatial and temporal distinction in their distribution. TNF co-localized in vessel endothelium with MMP-2 and in macrophages with MMP-9 and TACE at the period of active immune cell migration. TNF co-localized with myelin degrading MMP-9 within Schwann cells during demyelination, and intraaxonally during remyelination. These studies were performed to explore the role of basal-lamina degrading gelatinases and other TNF-releasing MMPs in TNF-mediated Wallerian degeneration. The data provided in this study may be useful in designing selective therapy for painful neuropathy using synthetic hydroxamate MMP inhibitors.

Degeneration and regeneration of the peripheral nervous system: from Augustus Waller's observations to neuroinflammation

This review article on the degeneration and regeneration of peripheral nerve fibers was presented as a Plenary Lecture at the 2001 meeting of the Peripheral Nerve Society. It is accompanied by a reprint of Augustus Waller's 1850 article, which gave rise to the pathologic process termed Wallerian degeneration. This review is focused on the role of neuroinflammation in Wallerian degeneration and how immune mediators contribute to both axonal degeneration and regeneration. Similarities and differences between the PNS and CNS in terms of inflammation and microglial activation after nerve injury are discussed, and point towards progress in understanding the failure of nerve fiber regeneration in the CNS.

Differential regulation of myelin phagocytosis by macrophages/microglia, involvement of target myelin, Fc receptors and activation by intravenous immunoglobulins

Macrophages/microglia are the key effector cells in myelin removal. Differences exist in the amount and time course of myelin uptake in the central (CNS) and peripheral nervous system (PNS), the basis of this difference, however, is not yet clarified. In the present experiments we studied the phagocytosis rate of CNS or PNS myelin by macrophages and microglia in vitro. Additionally, the effects of intravenous immunoglobulins (IVIg) on this process were investigated. In the PNS experiments, sciatic nerves were cocultured with peritoneal macrophages. Optic nerve fragments were used to characterize the myelin-removing properties of microglia. Cocultures with peritoneal macrophages aimed at investigating the differences in phagocytosis between resident microglia and added macrophages. The myelin phagocytosis in sciatic nerve fragments was higher than in optic nerves, indicating differences in the myelin uptake rate between peripheral macrophages and microglia. IVIg increased the phagocytosis of PNS myelin by macrophages, but not by microglia in optic nerves. The addition of peritoneal macrophages to optic nerve fragments did not lead to an increase in the phagocytosis of CNS myelin either. The IVIg induced phagocytosis of PNS myelin by peripheral macrophages was associated with an increased expression of macrophage Fc receptors measured by FACS. Blocking of Fc receptors by anti-Fc receptor antibody reduced the IVIg induced PNS myelin phagocytosis to basic levels, indicating that the induced but not the basic myelin uptake by macrophages is Fc receptor dependent. In contrast to peripheral macrophages, IVIg did not increase Fc receptor density on microglia. These data indicate that phagocytosis of PNS and CNS myelin by macrophages or microglia is differentially regulated. Local factors within the CNS or PNS may affect this process by modulating the surface receptor profile and activation state of the phagocytic cell or the structure of the myelin sheath.

Sciatic inflammatory neuritis (SIN): behavioral allodynia is paralleled by peri-sciatic proinflammatory cytokine and superoxide production

We have recently developed a model of sciatic inflammatory neuritis (SIN) to assess how immune activation near peripheral nerves influences somatosensory processing. Administration of zymosan (yeast cell walls) around a single sciatic nerve produces dose-dependent low-threshold mechanical allodynia without thermal hyperalgesia. Low (4 microg) doses produce both territorial and extraterritorial allodynia restricted to the injected hindleg. In contrast, higher (40 microg) doses produce territorial and extraterritorial allodynias of both hindlegs, an effect not accounted for by systemic spread of the zymosan. The aim of these experiments was to determine whether these behavioral allodynias were correlated with immunological and/or anatomical changes in or around the sciatic nerve. These experiments reveal that zymosan-induced bilateral allodynia was associated with the following: (a) increased release of both interleukin-1beta and tumor necrosis factor-alpha from peri-sciatic immune cells; (b) increased release of reactive oxygen species from perisciatic immune cells; (c) no change in circulating levels of proinflammatory cytokine; (d) no apparent zymosan-induced influx of immune cells into the sciatic nerve from the endoneurial blood vessels; (e) mild edema of the sciatic, which was predominantly restricted to superficial regions closest to the peri-sciatic immune cells; and (f) no anatomic evidence of changes in either the ipsilateral saphenous nerve or contralateral sciatic nerve that could account for the appearance of extraterritorial or contralateral ("mirror") allodynia, respectively. No reliable differences were found when the low-dose zymosan was compared with vehicle controls. Taken together, these data suggest that substances released by peri-sciatic immune cells may induce changes in the sciatic nerve, leading to the appearance of bilateral allodynia.

Induction of the proinflammatory cytokine interleukin-18 by axonal injury

Interleukin-18 (IL-18) is an important cytokine in innate immunity and in the induction phase of autoimmunity. We report the expression of IL-18 mRNA and protein after nerve crush during Wallerian degeneration (WD) of the rat nervous system. In normal optic nerves (ON) constitutive IL-18 mRNA levels as revealed by semiquantitative reverse transcriptase polymerase chain reaction were higher than in sciatic nerves (SN). After nerve crush, steady-state levels moderately increased in the distal nerve part of the SN but not the ON. By immunocytochemistry no SN or faint ON IL-18 protein expression was detectable in normal nerves. In contrast, IL-18 expression dramatically increased after SN and ON crush. On the cellular level, ED1(+) macrophages infiltrating the crush site strongly expressed IL-18 at days 2 and 4 after SN crush. By days 4 and 8, in addition, the entire distal nerve part was covered by IL-18(+) macrophages. At day 16, IL-18 immunoreactivity had disappeared despite the persistence of large numbers of ED1(+) macrophages. A similar infiltration of IL-18(+) macrophages was seen at the crush site in the ON. Moreover, microglia in the distal ON stump lacking macrophage infiltration and undergoing delayed myelin degradation up-regulated IL-18. In conclusion this study shows that IL-18 is involved in the cytokine network associated with the robust inflammatory response during WD of the SN. Despite up-regulation of the proinflammatory cytokine IL-18, major histocompatibility complex class II, and CD4 molecules similar to macrophages in the PNS, microglial activation after ON injury appears to be insufficient to mount an effective phagocytic response as a prerequisite for successful regeneration in the CNS.

Animal model of axonal Guillain-Barré syndrome induced by sensitization with GM1 ganglioside

Some humans develop the axonal form of Guillain-Barré syndrome after receiving bovine brain ganglioside. On sensitization with the ganglioside mixture, all of a group of rabbits injected developed high anti-GM1 IgG antibody titers, flaccid limb weakness of acute onset, and a monophasic illness course. Pathological findings for the peripheral nerves showed predominant Wallerian-like degeneration, with neither lymphocytic infiltration nor demyelination. IgG was deposited on the axons of the anterior roots, and GM1 was proved to be present on the axons of peripheral nerves. Sensitization with purified GM1 also induced axonal neuropathy, indicating that GM1 was the immunogen in the mixture. A model of human axonal Guillain-Barré syndrome has been established that uses inoculation with a bovine brain ganglioside mixture or isolated GM1. This model may help to clarify the molecular pathogenesis of the syndrome and to develop new treatments for it.