Myelin phagocytosis in Wallerian degeneration. Properties of millipore diffusion chambers and immunohistochemical identification of cell populations

Voluntary wheel running prevents formation of membrane attack complexes and myelin degradation after peripheral nerve injury

Regular aerobic activity is associated with a reduced risk of chronic pain in humans and rodents. Our previous studies in rodents have shown that prior voluntary wheel running can normalize redox signaling at the site of peripheral nerve injury, attenuating subsequent neuropathic pain. However, the full extent of neuroprotection offered by voluntary wheel running after peripheral nerve injury is unknown. Here, we show that six weeks of voluntary wheel running prior to chronic constriction injury (CCI) reduced the terminal complement membrane attack complex (MAC) at the sciatic nerve injury site. This was associated with increased expression of the MAC inhibitor CD59. The levels of upstream complement components (C3) and their inhibitors (CD55, CR1 and CFH) were altered by CCI, but not increased by voluntary wheel running. Since MAC can degrade myelin, which in turn contributes to neuropathic pain, we evaluated myelin integrity at the sciatic nerve injury site. We found that the loss of myelinated fibers and decreased myelin protein which occurs in sedentary rats following CCI was not observed in rats with prior running. Substitution of prior voluntary wheel running with exogenous CD59 also attenuated mechanical allodynia and reduced MAC deposition at the nerve injury site, pointing to CD59 as a critical effector of the neuroprotective and antinociceptive actions of prior voluntary wheel running. This study links attenuation of neuropathic pain by prior voluntary wheel running with inhibition of MAC and preservation of myelin integrity at the sciatic nerve injury site.

Advances in the repair of segmental nerve injuries and trends in reconstruction

Despite advances in surgery, the reconstruction of segmental nerve injuries continues to pose challenges. In this review, current neurobiology regarding regeneration across a nerve defect is discussed in detail. Recent findings include the complex roles of nonneuronal cells in nerve defect regeneration, such as the role of the innate immune system in angiogenesis and how Schwann cells migrate within the defect. Clinically, the repair of nerve defects is still best served by using nerve autografts with the exception of small, noncritical sensory nerve defects, which can be repaired using autograft alternatives, such as processed or acellular nerve allografts. Given current clinical limits for when alternatives can be used, advanced solutions to repair nerve defects demonstrated in animals are highlighted. These highlights include alternatives designed with novel topology and materials, delivery of drugs specifically known to accelerate axon growth, and greater attention to the role of the immune system.

Deletion of SIRPα (signal regulatory protein-α) promotes phagocytic clearance of myelin debris in Wallerian degeneration, axon regeneration, and recovery from nerve injury

BACKGROUND

Recovery of function from traumatic nerve injury depends on the ability of severed axons to grow/regenerate back to their target tissues. This is achieved by successfully crossing the lesion site where physical impact severed axons, determined by the type of trauma, followed by successfully growing throughout the Wallerian degenerating nerve segment located distal to and beyond the lesion site, determined by the nature of Wallerian degeneration. The protracted removal of myelin debris in Wallerian degeneration, which leads residual myelin debris to slow down axon growth, impedes recovery of function. We focused in this study on mechanism(s) that delay the removal of myelin debris in Wallerian degeneration and so impede recovery. Previously, we showed that myelin debris inhibited its own phagocytosis in primary cultured macrophages and microglia as CD47 on myelin ligated SIRPα (signal regulatory protein-α) on phagocytes, and sequentially, SIRPα generated "don't eat me" signaling. We also demonstrated that serum inhibited phagocytosis in a SIRPα-dependent manner. Herein, we aimed to determine whether SIRPα-dependent inhibition of phagocytosis in macrophages impedes the in vivo removal of myelin debris in Wallerian degeneration, further leading to impaired healing.

METHODS

Using SIRPα null (SIRPα-/-) and littermate wild-type (SIRPα+/+) mice, we studied the recovery of sensory and motor functions from nerve injury and, further, axon regeneration, SIRPα expression, myelin debris removal, and the phagocytic capacity and presence of macrophages in Wallerian degeneration.

RESULTS

Myelin debris removal, axon regeneration, and the recovery of functions were all faster in SIRPα-/- mice than in wild-type mice. Between the two cell types that mostly scavenge myelin debris, macrophages but not Schwann cells expressed SIRPα in wild-type mice, and furthermore, SIRPα-/- macrophages phagocytosed significantly more than wild-type macrophages.

CONCLUSIONS

Our findings suggest an intrinsic normally occurring SIRPα-dependent mechanism that impedes the in vivo removal of myelin debris in Wallerian degeneration by inhibiting the phagocytosis of myelin debris in macrophages, hence preventing fast growing axons from fully implementing their regenerative potential. Thus, accelerating the removal of myelin debris by eliminating SIRPα-dependent inhibition of phagocytosis will most likely advance recovery of functions from nerve injury.

Macrophage biology in the peripheral nervous system after injury

Neuroinflammation has positive and negative effects. This review focuses on the roles of macrophage in the PNS. Transection of PNS axons leads to degeneration and clearance of the distal nerve and to changes in the region of the axotomized cell bodies. In both locations, resident and infiltrating macrophages are found. Macrophages enter these areas in response to expression of the chemokine CCL2 acting on the macrophage receptor CCR2. In the distal nerve, macrophages and other phagocytes are involved in clearance of axonal debris, which removes molecules that inhibit nerve regeneration. In the cell body region, macrophage trigger the conditioning lesion response, a process in which neurons increase their regeneration after a prior lesion. In mice in which the genes for CCL2 or CCR2 are deleted, neither macrophage infiltration nor the conditioning lesion response occurs in dorsal root ganglia (DRG). Macrophages exist in different phenotypes depending on their environment. These phenotypes have different effects on axonal clearance and neurite outgrowth. The mechanism by which macrophages affect neuronal cell bodies is still under study. Overexpression of CCL2 in DRG in uninjured animals leads to macrophage accumulation in the ganglia and to an increase in the growth potential of DRG neurons. This increased growth requires activation of neuronal STAT3. In contrast, in acute demyelinating neuropathies, macrophages are involved in stripping myelin from peripheral axons. The molecular mechanisms that trigger macrophage action after trauma and in autoimmune disease are receiving increased attention and should lead to avenues to promote regeneration and protect axonal integrity.

The Axon-Myelin Unit in Development and Degenerative Disease

Axons are electrically excitable, cable-like neuronal processes that relay information between neurons within the nervous system and between neurons and peripheral target tissues. In the central and peripheral nervous systems, most axons over a critical diameter are enwrapped by myelin, which reduces internodal membrane capacitance and facilitates rapid conduction of electrical impulses. The spirally wrapped myelin sheath, which is an evolutionary specialisation of vertebrates, is produced by oligodendrocytes and Schwann cells; in most mammals myelination occurs during postnatal development and after axons have established connection with their targets. Myelin covers the vast majority of the axonal surface, influencing the axon's physical shape, the localisation of molecules on its membrane and the composition of the extracellular fluid (in the periaxonal space) that immerses it. Moreover, myelinating cells play a fundamental role in axonal support, at least in part by providing metabolic substrates to the underlying axon to fuel its energy requirements. The unique architecture of the myelinated axon, which is crucial to its function as a conduit over long distances, renders it particularly susceptible to injury and confers specific survival and maintenance requirements. In this review we will describe the normal morphology, ultrastructure and function of myelinated axons, and discuss how these change following disease, injury or experimental perturbation, with a particular focus on the role the myelinating cell plays in shaping and supporting the axon.

Nerve Regeneration: Understanding Biology and Its Influence on Return of Function After Nerve Transfers

Poor functional outcomes are frequent after peripheral nerve injuries despite the regenerative support of Schwann cells. Motoneurons and, to a lesser extent, sensory neurons survive the injuries but outgrowth of axons across the injury site is slow. The neuronal regenerative capacity and the support of regenerating axons by the chronically denervated Schwann cells progressively declines with time and distance of the injury from the denervated targets. Strategies, including brief low-frequency electrical stimulation that accelerates target reinnervation and functional recovery, and the insertion of cross-bridges between a donor nerve and a recipient denervated nerve stump, are effective in promoting functional outcomes after complete and incomplete injuries.

Electrical Stimulation to Enhance Axon Regeneration After Peripheral Nerve Injuries in Animal Models and Humans

Injured peripheral nerves regenerate their lost axons but functional recovery in humans is frequently disappointing. This is so particularly when injuries require regeneration over long distances and/or over long time periods. Fat replacement of chronically denervated muscles, a commonly accepted explanation, does not account for poor functional recovery. Rather, the basis for the poor nerve regeneration is the transient expression of growth-associated genes that accounts for declining regenerative capacity of neurons and the regenerative support of Schwann cells over time. Brief low-frequency electrical stimulation accelerates motor and sensory axon outgrowth across injury sites that, even after delayed surgical repair of injured nerves in animal models and patients, enhances nerve regeneration and target reinnervation. The stimulation elevates neuronal cyclic adenosine monophosphate and, in turn, the expression of neurotrophic factors and other growth-associated genes, including cytoskeletal proteins. Electrical stimulation of denervated muscles immediately after nerve transection and surgical repair also accelerates muscle reinnervation but, at this time, how the daily requirement of long-duration electrical pulses can be delivered to muscles remains a practical issue prior to translation to patients. Finally, the technique of inserting autologous nerve grafts that bridge between a donor nerve and an adjacent recipient denervated nerve stump significantly improves nerve regeneration after delayed nerve repair, the donor nerves sustaining the capacity of the denervated Schwann cells to support nerve regeneration. These reviewed methods to promote nerve regeneration and, in turn, to enhance functional recovery after nerve injury and surgical repair are sufficiently promising for early translation to the clinic.

Role of macrophages in Wallerian degeneration and axonal regeneration after peripheral nerve injury

The peripheral nervous system (PNS) has remarkable regenerative abilities after injury. Successful PNS regeneration relies on both injured axons and non-neuronal cells, including Schwann cells and immune cells. Macrophages are the most notable immune cells that play key roles in PNS injury and repair. Upon peripheral nerve injury, a large number of macrophages are accumulated at the injury sites, where they not only contribute to Wallerian degeneration, but also are educated by the local microenvironment and polarized to an anti-inflammatory phenotype (M2), thus contributing to axonal regeneration. Significant progress has been made in understanding how macrophages are educated and polarized in the injured microenvironment as well as how they contribute to axonal regeneration. Following the discussion on the main properties of macrophages and their phenotypes, in this review, we will summarize the current knowledge regarding the mechanisms of macrophage infiltration after PNS injury. Moreover, we will discuss the recent findings elucidating how macrophages are polarized to M2 phenotype in the injured PNS microenvironment, as well as the role and underlying mechanisms of macrophages in peripheral nerve injury, Wallerian degeneration and regeneration. Furthermore, we will highlight the potential application by targeting macrophages in treating peripheral nerve injury and peripheral neuropathies.

Tissue engineered constructs for peripheral nerve surgery

BACKGROUND

Tissue engineering has been defined as "an interdisciplinary field that applies the principles of engineering and life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function or a whole organ". Traumatic peripheral nerve injury resulting in significant tissue loss at the zone of injury necessitates the need for a bridge or scaffold for regenerating axons from the proximal stump to reach the distal stump.

METHODS

A review of the literature was used to provide information on the components necessary for the development of a tissue engineered peripheral nerve substitute. Then, a comprehensive review of the literature is presented composed of the studies devoted to this goal.

RESULTS

Extensive research has been directed toward the development of a tissue engineered peripheral nerve substitute to act as a bridge for regenerating axons from the proximal nerve stump seeking the distal nerve. Ideally this nerve substitute would consist of a scaffold component that mimics the extracellular matrix of the peripheral nerve and a cellular component that serves to stimulate and support regenerating peripheral nerve axons.

CONCLUSIONS

The field of tissue engineering should consider its challenge to not only meet the autograft "gold standard" but also to understand what drives and inhibits nerve regeneration in order to surpass the results of an autograft.

A derivative of the plasma protease inhibitor alpha(2)-macroglobulin regulates the response to peripheral nerve injury

Peripheral nerve injury induces endoneural inflammation, controlled by diverse cytokines and extracellular mediators. Although inflammation is coupled to axonal regeneration, fulminant inflammation may increase nerve damage and neuropathic pain. alpha(2)-Macroglobulin (alpha2M) is a plasma protease inhibitor, cytokine carrier, and ligand for cell-signaling receptors, which exists in two well-characterized conformations and in less well-characterized intermediate states. Previously, we generated an alpha2M derivative (alpha(2)-macroglobulin activated for cytokine binding; MAC) similar in structure to alpha(2)M conformational intermediates, which binds tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta), and inhibits endotoxin toxicity. In this study, we report that the continuum of cytokines that bind to MAC includes IL-6 and IL-18. MAC inhibited TNF-alpha-induced p38 mitogen-activated protein kinase activation and cell death in cultured Schwann cells. When administered by i.p. injection to mice with sciatic nerve crush injury, MAC decreased inflammation and preserved axons. Macrophage infiltration and TNF-alpha expression also are decreased. MAC inhibited TNF-alpha expression in the chronic constriction injury model of nerve injury. When MAC was prepared using a mutated recombinant alpha2M, which does not bind to the alpha2M receptor, low-density lipoprotein receptor-related protein-1, activity in the chronic constriction injury model was blocked. These studies demonstrate that an alpha2M derivative is capable of regulating the response to peripheral nerve injury by a mechanism that requires low-density lipoprotein receptor-related protein-1.

Initial upregulation of growth factors and inflammatory mediators during nerve regeneration in the presence of cell adhesive peptide-incorporated collagen tubes

Neurotrophic factors play an important modulatory role in axonal sprouting during nerve regeneration involving the proliferation of hematogenous and Schwann cells in damaged tissue. We have exposed lesioned sciatic nerves to a collagen prosthesis with covalently bonded small cell adhesive peptides Arg-Gly-Asp-Ser (RGDS), Lys-Arg-Asp-Ser (KRDS), and Gly-His-Lys (GHK) to study local production of growth factors and cytokines in the regenerating tissues. Western/enzyme-linked immunosorbent assay (ELISA) studies were performed after 10 days of regeneration, when the tubular prosthesis is filled with fibrous matrix infiltrated by hematogenous cells and proliferating Schwann cells with growth factors produced locally. Regeneration was also analyzed by morphometrical methods after 30 days. The quantification of growth factors and proteins by ELISA revealed that there was an enhanced expression of the neurotrophic factors nerve growth factor (NGF) and neurotrophins (NT-3 and NT-4) in the regenerating tissues. This was further established by Western blot to qualitatively analyze the presence of the antigens in the regenerating tissues. Schwann cells were localized in the regenerating tissues using antibodies against S-100 protein. Other growth factors including growth-associated protein 43 (GAP-43), apolipoprotein E (Apo E), and pro-inflammatory cytokine like interleukin-1alpha (IL-1alpha) expression in the peptide groups were evaluated by ELISA and confirmed by Western blotting. Cell adhesive integrins in the proliferating cells were localized using integrin-alpha V. The combined results suggest that the early phase of regeneration of peripheral nerves in the presence of peptide-incorporated collagen tubes results in the enhanced production of trophic factors by the recruited hematogenous cells and Schwann cells, which in turn help in the secretion of certain vital trophic and tropic factors essential for early regeneration. Furthermore, hematogenous cells recruited within the first 10 days of regeneration help in the production of inflammatory mediators like interleukins that in turn stimulate Schwann cells to produce NGF for axonal growth.

The role of CD36 in peripheral nerve remyelination after crush injury

We previously demonstrated that the deficiency of class A macrophage scavenger receptor type I/II was involved in the delayed phagocytosis of degraded myelin by macrophages in class A macrophage scavenger receptor type I/II knockout mice after crush injury of the sciatic nerve [Naba et al. (2000) Exp. Neurol., 166, 83-89]. In order to elucidate the role of CD36, one of the scavenger receptors, here we inflicted crush injury to the sciatic nerves of CD36 knockout mice and investigated the remyelination after crush injury in comparison with that of class A macrophage scavenger receptor type I/II knockout mice. Although we previously reported a lot of onion-bulbs in class A macrophage scavenger receptor type I/II knockout mice at 3 weeks, the number of onion-bulbs was limited both in CD36 knockout mice and wild-type mice. In the morphometry, the remyelination was seriously delayed, and the infiltrating macrophages into the nerve fascicles were quite frequent in CD36 knockout mice compared with wild-type mice at 3 and 6 weeks postinjury. The immunohistochemistry with the monoclonal antibody reacted with oxidized phosphatidylcholine and oil red O staining were positive in wild-type mice, but were negative in CD36 knockout mice, suggesting that the oxidation of phosphatidylcholine and the generation of neutral lipids in macrophages were disturbed in CD36 knockout mice. We hypothesize that the delayed phagocytosis by macrophages and the defect in reuse of lipids from degraded myelin are related to seriously delayed remyelination and a small number of onion-bulbs in CD36 knockout mice.

Denervated Schwann cells attract macrophages by secretion of leukemia inhibitory factor (LIF) and monocyte chemoattractant protein-1 in a process regulated by interleukin-6 and LIF

Injury to peripheral nerves results in the infiltration of immune cells, which remove axonal- and myelin-derived material. Schwann cells could play a key role in this process by regulating macrophage infiltration. We show here that medium conditioned by primary denervated Schwann cells or the Schwannoma cell line RN22 produces chemotactic activity for macrophages. The presence of blocking antibodies to macrophage chemoattractant protein-1 (MCP-1) or leukemia inhibitory factor (LIF) reduced this activity to approximately 35 and 65% of control levels, respectively, and only 15% remained in the presence of both antibodies. The presence of chemotactic LIF in Schwann cell-conditioned medium was confirmed by using cells from lif-/- mice. Although interleukin-6 (IL-6) is not itself a chemotactic factor, we found that medium from il-6-/- nerves showed only 40% of the activity secreted by wild-type nerves. Furthermore, IL-6 rapidly induced LIF mRNA in primary Schwann cells, and LIF rapidly induced MCP-1 mRNA expression. Treatment of RN22 Schwannoma cells with IL-6 or LIF enhanced the secretion of the chemotactic activity of these cells. These observations show that Schwann cells attract macrophages by secreting MCP-1 and LIF. They also provide evidence for an autocrine-signaling cascade involving IL-6, LIF, and MCP-1, which amplifies the Schwann cell-derived chemotactic signals gradually, in agreement with the delayed entry of macrophages to injured nerves.

Myelin phagocytosis by macrophages and nonmacrophages during Wallerian degeneration

The literature concerning Schwann cells (SCs) and macrophages in myelin phagocytosis during Wallerian degeneration is reviewed. SCs carry out the first step in the removal of myelin by segmenting myelin and then incorporating the degraded myelin. The recruited macrophages then join in the myelin-phagocytosis event, appearing to make full use of their original phagocyte abilities until the end of myelin clearance. The molecular mechanisms of the two cells underlying myelin phagocytosis are thought to be different; myelin phagocytosis by SCs being lectin-mediated, i.e., opsonin-independent, whereas that of macrophages is mainly opsonin-dependent. It is important to note that SCs and macrophages cooperatively accomplish myelin phagocytosis.

Rapid response of identified resident endoneurial macrophages to nerve injury

Macrophages play a central role in the pathogenesis of peripheral neuropathy but the role of resident endoneurial macrophages is undefined because no discriminating markers exist to distinguish them from infiltrating hematogenous macrophages. We identified and characterized resident endoneurial macrophages during Wallerian degeneration in radiation bone marrow chimeric rats created by transplanting wild-type Lewis rat bone marrow into irradiated TK-tsa transgenic Lewis rats. In such animals, resident cells carry the transgene, whereas hematogenous cells do not. As early as 2 days after sciatic nerve crush and before the influx of hematogenous macrophages, resident transgene-positive endoneurial macrophages underwent morphological and immunophenotypic signs of activation. At the same time, resident macrophages phagocytosing myelin were found, and proliferation was detected by bromodeoxyuridine incorporation. Continuous bromodeoxyuridine feeding revealed that resident endoneurial macrophages sequentially retracted their processes, proliferated, and expressed the ED1 antigen, rendering them morphologically indistinguishable from hematogenous macrophages. Resident endoneurial macrophages thus play an early and active role in the cellular events after nerve lesion before hematogenous macrophages enter the nerve. They may thus be critically involved in the pathogenesis of peripheral neuropathy particularly at early stages of the disease and may act as sensors of pathology much like their central nervous system counterparts, the microglial cells.

Onion-bulb formation after a single compression injury in the macrophage scavenger receptor knockout mice

Onion-bulb (OB) formation is often encountered in acquired neuropathies such as chronic inflammatory demyelinating polyradiculoneuropathy and diabetic neuropathy and is believed to require repeated injuries to peripheral nerves. Although this suggests that remaining damaged cell membranes, including myelin debris, might trigger OB formation, the molecular mechanism remains unclear. In this study, we were successful in producing many small OBs after a single compression injury to peripheral nerves of the knockout mice deficient of macrophage scavenger receptor class A (MSR-A). Although morphometry showed no difference in the average densities of the remaining myelinating fibers between wild-type and MSR-A knockout mice after the compression injury, there were more macrophages and myelin debris positive for oxidized-phosphatidylcholine in the nerves from the MSR-A knockout mice. We believe that OB formation was induced after a single compression injury as the result of delayed phagocytosis of myelin debris possessing oxidized lipids by MSR-A deficient macrophages. The present work shed light on the molecular mechanism of OB formation seen in chronic neuropathies and provided a model for further investigation.

Teased-fiber technique for peripheral myelinated nerves: methodology and interpretation

Teased-fiber technique is the best approach for studying peripheral myelinated nerve fibers in their continuity. It enables the assessment of size of myelin segments formed by Schwann cells and characterization of pathologic changes affecting the internodia, the paranodal regions, and the invested axons. Fiber teasing is performed on prestained proximodistally oriented portions of peripheral nerves. Specimens about 10 mm long are stained for 24-48 hours in Sudan black and then transferred to glycerin, where, using a pair of fine forceps and a stereomicroscope, they are separated into smaller fiber bundles from which single fibers are isolated. The work is performed on a glass slide with an adhesive surface (albuminized or "superfrost"), on which the fibers are placed in strict proximodistal orientation. Following drying in an oven, the slides are mounted with glycerin-gelatine (same as used for frozen sections). The changes, when present, can usually be recognized during the preparation, but fibers are reexamined and changes confirmed in mounted slides. Photographic reconstruction of the fibers facilitates their assessment and enables the documentation of findings. The teased-fiber technique is auxiliary to histopathology, and to limit the workload and save costs, it can be performed on only a few specimens selected for better characterization of changes recognized or suspected in tissue sections. In particular, segmental demyelination and early stages of Wallerian or secondary axonal degeneration can be recognized in teased fibers. Segmental demyelination is characterized by loss of fully myelinated segments and their replacement by newly formed short and thin segments, remyelinating the preserved axon. The early stage of secondary axonal degeneration is recognized by formation of ovoidal fiber fragments in the midinternodal region.

Signals for proinflammatory cytokine secretion by human Schwann cells

Wallerian degeneration is a post-traumatic process of the peripheral nervous system whereby damaged axons and their surrounding myelin sheaths are phagocytosed by infiltrating leukocytes. Our studies indicate that Schwann cells could initiate the process of Wallerian degeneration by releasing proinflammatory cytokines involved in leukocyte recruitment and differentiation including IL-1beta, MCP-1, IL-8 and IL-6. A comparison of the secretory pattern between nerve explants and cultured Schwann cells showed that each cytokine was differentially regulated by growth factor deprivation or axonal membrane fragments. Since Wallerian-like degeneration occurs in a wide variety of peripheral neuropathies, Schwann cell-mediated cytokine production may play an important role in many disease processes.

Complement depletion reduces macrophage infiltration and activation during Wallerian degeneration and axonal regeneration

After peripheral nerve injury, macrophages infiltrate the degenerating nerve and participate in the removal of myelin and axonal debris, in Schwann cell proliferation, and in axonal regeneration. In vitro studies have demonstrated the role serum complement plays in both macrophage invasion and activation during Wallerian degeneration of peripheral nerve. To determine its role in vivo, we depleted serum complement for 1 week in adult Lewis rats, using intravenously administered cobra venom factor. At 1 d after complement depletion the right sciatic nerve was crushed, and the animals were sacrificed 4 and 7 d later. Macrophage identification with ED-1 and CD11a monoclonal antibodies revealed a significant reduction in their recruitment into distal degenerating nerve in complement-depleted animals. Complement depletion also decreased macrophage activation, as indicated by their failure to become large and multivacuolated and their reduced capacity to clear myelin, which was evident at both light and electron microscopic levels. Axonal regeneration was delayed in complement-depleted animals. These findings support a role for serum complement in both the recruitment and activation of macrophages during peripheral nerve degeneration as well as a role for macrophages in promoting axonal regeneration.

Concentration-dependent effects of pentoxifylline on migration and myelin phagocytosis by macrophages

The effects of pentoxifylline (POX) on macrophage migration and myelin uptake were studied in an in vitro model of myelin phagocytosis. The POX is a phosphodiesterase inhibitor which inhibits TNF-alpha (tumor necrosis factor alpha) production and reduces ICAM-1 (intercellular adhesion molecule-1) expression by macrophages. Both of these molecules have earlier been shown to be involved in the process of myelin recognition and degradation. In the present series of experiments, cocultured peripheral nerves and macrophages were treated with different concentrations of POX. Untreated controls were massively invaded by macrophages which ingested the degenerating myelin sheaths. High concentrations of POX (100 microg ml(-1)) inhibited macrophage invasion of the nerves. Lower POX concentrations (50 microg ml(-1)), in contrast, lead to an increased myelin uptake by phagocytic cells without affecting macrophage migration. These data indicate that POX may regulate different effector functions of macrophages such as migration and myelin phagocytosis during Wallerian degeneration. This is important for inflammatory demyelinating conditions in the central or peripheral nervous system (PNS) in which macrophages are also important effector cells. Since POX is used as an immunomodulatory drug in demyelinating diseases, its effects on the described macrophage functions may be of high relevance. An increased myelin uptake during Wallerian degeneration may also support a more efficient axonal regeneration by removing axonal outgrowth inhibitors.

Neural architecture in transected rabbit sciatic nerve after prolonged nonreinnervation

Observations have been made on the rabbit sciatic nerve distal to a transection, with survival periods of up to 26 mo and prevention of reinnervation. It was confirmed that the nerve becomes compartmented by fibroblast processes and that a zone of fine collagen fibrils develops around the Schwann cell columns that constitute the Büngner bands. The Schwann cells become progressively more atrophic but after 6 mo of denervation still expressed low affinity p75 nerve growth factor receptor (NGFR), the latest stage at which this was examined. NGFR was also expressed by the processes of the fibroblasts producing the endoneurial compartmentation. By 26 mo after transection the site of previous nerve fibres was indicated by sharply demarcated domains of approximately circular outline in transverse section consisting of densely packed longitudinally oriented collagen fibrils. Some of these domains still possessed centrally situated Schwann cells or residual basal lamina but many were acellular. The central collagen fibrils in these domains were of smaller diameter than those situated peripherally but were of larger size than those that form around the Büngner bands during wallerian degeneration. The peripherally located fibrils in the domains were of the same calibre as for normal endoneurial collagen. The collagen domains were encircled by fibroblast processes or at times enclosed in a perineurial cell ensheathment. Long-standing axonal loss therefore leads to a striking reorganisation of the internal architecture of peripheral nerve trunks. The findings may be relevant for the interpretation of the appearances in chronic peripheral neuropathies in man.

The role of the mouse macrophage scavenger receptor in myelin phagocytosis

Myelin phagocytosis during Wallerian degeneration and immune-mediated demyelination depends on the action of mononuclear cells of the monocyte/macrophage system. The present study investigated the role of the macrophage scavenger receptor, a trimeric membrane glycoprotein, in myelin uptake by macrophages. Two in vitro models of myelin phagocytosis were used: an organ culture model of mouse peripheral nerves exposed to cocultured macrophages and a quantitative flow cytometric assay. Different concentrations of the monoclonal rat anti-mouse scavenger receptor antibody (2F8) were applied to these systems to selectively block the macrophage scavenger receptor. Concentration-dependent effects on macrophage migration and myelin uptake were seen when the macrophage scavenger receptor was blocked by the antibody 2F8. Low concentrations reduced myelin phagocytosis by the invading macrophages; higher concentrations completely abolished macrophage invasion of the nerves. Using a quantitative flow cytometric assay it was also shown that the 2F8 antibody inhibits phagocytosis of myelin in a dose-dependent manner. These data indicate that the macrophage scavenger receptor is involved in myelin phagocytosis by macrophages.

The role of macrophages in Wallerian degeneration

The present review focuses on macrophage properties in Wallerian degeneration. The identification of hematogenous phagocytes, the involvement of cell surface receptors and soluble factors, the state of activation during myelin removal and the signals and factors leading to macrophage recruitment into degenerating peripheral nerves after nerve transection are reviewed. The main effector cells in Wallerian degeneration are hematogenous phagocytes. Resident macrophages and Schwann cells play a minor role in myelin removal. The macrophage complement receptor type 3 is the main surface receptor involved in myelin recognition and uptake. The signals leading to macrophage recruitment are heterogenous and not yet defined in detail. Degenerating myelin and axons are suggested to participate. The relevance of these findings for immune-mediated demyelination are discussed since the definition of the role of macrophages might lead to a better understanding of the pathogenesis of demyelination.

Liposome-mediated monocyte depletion during wallerian degeneration defines the role of hematogenous phagocytes in myelin removal

Newly recruited hematogenous mononuclear cells of the monocyte/macrophage system are suggested to be important effector cells in myelin removal during Wallerian degeneration. Their role has extensively been studied in various in vitro and in vivo models. However, there has been much controversy concerning the role of hematogenous vs. resident cells of the peripheral nervous system in Wallerian degeneration. The present study used a recently established technique to deplete the hematogenous monocyte population by application of dichloromethylene diphosphonate-containing liposomes. Intravenously injected liposomes containing dichloromethylene diphosphonate (Cl2MDP) are ingested by macrophages and monocytes and cause temporary and selective depletion of these cells. The number of LFA-1- and Mac-1- positive macrophages within the nerves was significantly reduced when liposomes were injected shortly after nerve transsection. In these nerves, myelin degradation was significantly less, indicating an essential role of newly recruited phagocytes in this process. Macrophage invasion of degenerating nerves occurred within the first 2 days after transsection. Resident cells of the peripheral nerve participate in myelin removal since macrophage depletion did not completely abolish myelin degradation. These results confirm the important role of hematogenous phagocytes in myelin removal during Wallerian degeneration.

Peripheral nerve regeneration

Peripheral nerve regeneration comprises the formation of axonal sprouts, their outgrowth as regenerating axons and the reinnervation of original targets. This review focuses on the morphological features of axonal sprouts at the node of Ranvier and their subsequent outgrowth guided by Schwann cells or by Schwann cell basal laminae. Adhesion molecules such as N-CAM, L1 and N-cadherin are involved in the axon-to-axon and axon-to-Schwann cell attachment, and it is suggested that integrins such as alpha 1 beta 1 and alpha 6 beta 1 mediate the attachment between axons and Schwann cell basal laminae. The presence of synaptic vesicle-associated proteins such as synaptophysin, synaptotagmin and synapsin I in the growth cones of regenerating axons indicates the possibility that exocytotic fusion of vesicles with the surface axolemma supplies the membranous components for the extension of regenerating axons. Almost all the subtypes of protein kinase C have been localized in growth cones both in vivo and in vitro. Protein kinase C and GAP-43 are implicated to be involved in at least some part of the adhesion of growth cones to the substrate and their growth activity. The significance of tyrosine kinase in growth cones is emphasized. Tyrosine kinase plays an important role in intracellular signal transduction of the growth of regenerating axons mediated by both nerve trophic factors and adhesion molecules. Growth factors such as NGF, BDNF, CNTF and bFGF are also discussed mainly in terms of the influence of Schwann cells on regenerating axons.

Axonal regeneration

Axons damaged in a peripheral nerve are often able to regenerate from the site of injury along the degenerate distal segment of the nerve to reform functional synapses. Schwann cells play a central role in this process. However, in the adult mammalian central nervous system, from which Schwann cells are absent, axonal regeneration does not progress to allow functional recovery. This is due to inhibitors of axonal growth produced by both oligodendrocytes and astrocytes and also to the decreased ability of adult neurons to extend axons during regeneration compared to embryonic neurons during development. However once provided with a substrate conducive to axonal growth, such as a peripheral nerve graft, many central neurons are able to regenerate axons over long distances. Over the past year this response has been utilised in experimental models to produce a degree of behavioural recovery.

Myelinated nerve fibre regeneration in diabetic sensory polyneuropathy: correlation with type of diabetes

Observations were made on myelinated fibre regeneration in diabetic sensory polyneuropathy assessed in sural nerve biopsy specimens. These confirmed that regenerative clusters initially develop within abnormally persistent Schwann cell basal laminal tubes. The number of regenerating fibres, identified by light microscopy, was found to decline in proportion to the reduction in total myelinated fibre density. The relative number of regenerating fibres was significantly greater in patients with insulin-dependent as compared with those with non-insulin-dependent diabetes after correction for age. There was a slight negative correlation between the relative proportion of regenerating fibres and age, but this was not statistically significant. The progressive reduction in the number of regenerating fibres with declining total fibre density indicates that axonal regeneration fails with advancing neuropathy. The production of nerve growth factor (NGF) and NGF receptors by denervated Schwann cells is likely to be important for axonal regeneration. To investigate whether the failure of axonal regeneration could be related to a lack of NGF receptor production by Schwann cells, we examined the expression of p75 NGF receptors by Büngner bands immunocytochemically. In comparison with other types of peripheral neuropathy, p75 NGF receptor expression appeared to take place normally. It is concluded that failure of axonal regeneration constitutes an important component in diabetic neuropathy. Its explanation requires further investigation.

Mechanisms of macrophage recruitment in Wallerian degeneration

Monocytes/macrophages are important effector cells in myelin removal during Wallerian degeneration. Experiments with the mouse mutant C57BL/Ola revealed prolonged axonal survival and reduced phagocytic cell recruitment after nerve transsection. In the present study, we compared the course of Wallerian degeneration in peripheral nerves of C57BL and C57BL/Ola mice in vivo and in vitro. In vivo experiments confirmed earlier investigations describing a delayed degeneration in the C57BL/Ola mutant compared with C57BL mice which were used as control animals without abnormal degeneration. Quite different results were seen in experiments in vitro: degenerating nerve segments of C57BL/Ola mice revealed pronounced axonal breakdown even in the absence of non-resident phagocytic cells. There was no difference in vitro compared with degenerating nerves from C57BL mice. The differences observed between the in vivo and in vitro situations suggest that axonal breakdown plays an important role in the initiation of macrophage recruitment to degenerating peripheral nerves.

Lectin binding pattern of Schwann cells and macrophages in 2,5-hexanedione-induced axonal degeneration in rats

The lectin binding pattern of both Schwann cells and macrophages has been studied during axonal degeneration induced in the rat sciatic nerve by chronic administration of 2,5-hexanedione (0.8 ml/kg per day i.p. for 20 days). In particular, the present study aimed to establish a possible relationship between macrophage activation and expression of lectin binding sites. To identify and distinguish between Schwann cells and macrophages, electron microscopy was combined with the lectin staining method. On 2,5-hexanedione injury, a drastic disorganization of both axon and myelin sheath occurred and nerve fibers were replaced by a chain of ovoids. Besides the well-established concept that Schwann cells and macrophages cooperate in the removal of the myelin debris during axonal degeneration, evidence is presented that expression of binding sites to lectins is closely related to macrophage activation. Monocytes occasionally present in control nerves were labelled only by Con A and sialidase-peanut sequence; in 2,5-hexanedione degeneration monocytes, prephagocytes (macrophages with minute bubbles) and phagocytes (macrophages with large bubbles) were labelled also by peanut, wheat germ and BSA I-B4; moreover, phagocytes were labelled by soybean as well, thus showing a clearly differentiation-dependent binding pattern. Since changes in lectin binding pattern may reflect changes in complex carbohydrate structures, the results show that the expression of certain glycoproteins may be closely related to activation of macrophages in response to toxic injuries.

Observations on the progress of Wallerian degeneration in transected peripheral nerves of C57BL/Wld mice in the presence of recruited macrophages

The first part of this study is a description of the effect of the intraneural injection of lysophosphatidyl choline into the sciatic nerves of C57BL/Wld mice. This mouse is unusual because its peripheral nerve fibres degenerate very slowly after transection, and few myelomonocytic cells are recruited into the endoneurium after traumatic injury. However, intraneural injection of lysophosphatidyl choline produced a typical demyelinating lesion in which recruited macrophages were active in removal of myelin. In the second part of the study, nerves were transected either before, at the same time as, or some days after, the intraneural injection of lysophosphatidyl choline into the distal stump; the changes within the endoneurium were compared with those seen in distal stumps which had not been injected with lysophosphatidyl choline. Immunohistochemical and ultrastructural examination during the period 1-4 weeks after transection showed that degeneration occurred in the portion of each nerve which had been injected with LPC (and which therefore contained exogenous macrophages) but failed to occur in the portion of nerve which was not penetrated by the injected bolus of lysophosphatidyl choline. It is suggested that the unusual property of sealing off of the tips of the transected axons within the distal stumps may be a significant factor in maintaining 'normal' Schwann cell-axon relationships along transected axons, even though the axons are separated from their cell bodies. Lysophosphatidyl choline destabilises the Schwann cell-axon relationship by initiating myelin breakdown within the Schwann cell. It is suggested that while the Schwann cells remain closely associated with the axons in the distal stumps, they do not behave as denervated cells and consequently may be incapable of signalling their damaged status.

Production and characterization of monoclonal antibodies to the extracellular domain of P0

Seven monoclonal antibodies were raised against the immunoglobulin-like extracellular domain of P0 (P0-ED), the major protein of peripheral nervous system myelin. Mice were immunized with purified recombinant rat P0-ED. After fusion, 7 clones (P01-P07) recognizing either recombinant, rat, mouse, or human P0-ED were selected by ELISA and were characterized by Western blot, immunohistochemistry, and a competition assay. Antibodies belonged to the IgG or IgM class, and P04-P07, reacted with P0 in fresh-frozen and paraffin-embedded sections of human or rat peripheral nerve, but not with myelin proteins of the central nervous system of either species. Epitope specificity of the antibodies was determined by a competition enzyme-linked immunosorbent assay (ELISA) and a direct ELISA using short synthetic peptides spanning the entire extracellular domain of P0. These assays showed that P01 and P02 exhibiting the same reaction pattern in Western blot and immunohistochemistry reacted with different distant epitopes of P0. Furthermore, the monoclonal antibodies P05 and P06 recognized 2 different epitopes in close proximity within the neuritogenic extracellular sequence of P0. This panel of monoclonal antibodies, each binding to a different epitope of the extracellular domain of P0, will be useful for in vitro and in vivo studies designed to explore the role of P0 during myelination and in demyelinating diseases of the peripheral nervous system.

Macrophage responses and myelin clearance during Wallerian degeneration: relevance to immune-mediated demyelination

Macrophages are important effector cells in immune-mediated demyelination. Current concepts regarding their entry and activation focus on the effects of T-cell-derived cytokines. This presentation describes the responses of macrophages and microglia to a non-inflammatory, non-immune injury, Wallerian degeneration. During Wallerian degeneration in the peripheral nervous system (PNS), macrophages are promptly and abundantly recruited from the circulation, and myelin clearance is prompt. In the central nervous system (CNS), the appearance of macrophages is markedly slower, and entry from the circulation is modest or absent. Myelin clearance is similarly delayed. The nature of the factors promoting macrophage entry and activation in Wallerian degeneration, and the bases for the differences between PNS and CNS, are relevant to current issues in immune-mediated demyelination.

TNF-alpha suppresses CR3-mediated myelin removal by macrophages

Mononuclear cells of the monocyte/macrophage system play an important role in myelin ingestion during Wallerian degeneration. The present in vitro study clarifies the role in this process of two macrophage-secreted cytokines, TNF-alpha and interleukin-1. Treatment with TNF-alpha massively reduced the amount of myelin ingested by macrophages via their complement receptor type 3 (CR3). Anti-TNF-alpha antibodies reversed the effect. Immunofluorescence of macrophages indicated that TNF-alpha caused a reduced expression of the CR3 by phagocytic cells. Further experiments revealed an interaction of TNF-alpha with its receptor on the macrophage cell membrane. Interleukin-1 had no effect on myelin ingestion in the in vitro system used in these experiments.

Delayed wallerian degeneration in the central nervous system of Ola mice: an ultrastructural study

The ultrastructural features of wallerian degeneration in the optic nerves of the mutant mouse, C57BL/Ola, was compared with that occurring in age matched control mice, to determine whether the previously described defect in the peripheral nervous system was present in the central nervous system as well. On ultrastructural examination, marked delay in the rate of degeneration was seen in the Ola mice nerves seen most clearly at all stages up to 4 weeks post-enucleation, following which differences progressively became undetectable. Once degeneration began, however, the pattern and mechanisms were similar to those seen in control animals, with macrophages, oligodendrocytes, and astrocytes apparently behaving similarly. In both the experimental animals and the controls, the rate of degeneration was slower than that seen in the peripheral nervous system. This study confirms a previous electrophysiological study that the defect in this mutant affects axons in both the peripheral and the central nervous systems.

P2-peptide induced experimental allergic neuritis: a model to study axonal degeneration

In experimental allergic neuritis (EAN) severity of clinical disease and pathology correlate with the dose of antigen (Hahn et al., Lab Invest 59:115-125, 1988). To avoid axonal membrane contamination of the antigen, EAN was induced with a synthetic peptide, corresponding to residues 53-78 of bovine P2 myelin protein. Severity of EAN correlated with the dose of peptide in the inoculate. The relationship between demyelination, inflammation and axonal degeneration was studied. Low doses resulted in pure demyelination. Axonal degeneration occurred only with high doses of antigen and in association with very active mononuclear inflammation. The role of macrophages in producing axonal damage is discussed.

The role of complement in myelin phagocytosis during PNS wallerian degeneration

Myelin removal in nerves undergoing wallerian degeneration mainly depends on invading, non-resident macrophages. The present study clarifies the role of serum complement components in this process in vitro and in vivo. Macrophages cocultured with degenerating nerves in vitro were unable to invade these nerves in the presence of C3-deficient serum. Application of C3-deficient serum subsequent to cellular invasion abolished the myelin phagocytic capacity of the invaded macrophages. This indicates that opsonization of myelin by complement components is necessary in myelin ingestion via macrophage receptors. In vivo, a monoclonal antibody to the macrophage complement receptor type 3 (CR3) significantly reduced myelin phagocytosis. Immunohistochemistry with anti-C3 antibodies showed a marked reaction in degenerating nerves. Immunoelectron microscopy localized C3 particles at the degenerating myelin sheaths. Haematogenous cells, invading the degenerating nerves, also showed a strong reaction for C3 in their cytoplasm. These results indicate that complement components play a critical role both in macrophage invasion of degenerating nerves and in the ingestion of myelin by these cells.

Anti-macrophage CR3 antibody blocks myelin phagocytosis by macrophages in vitro

Myelin phagocytosis in Wallerian degeneration of peripheral nerves depends on invasion of nerves by non-resident macrophages. The present study was done to clarify the role of the macrophage complement receptor type 3 (CR3) in myelin removal. Myelin phagocytic capacity of invading macrophages was abolished by treatment of cultured nerves and macrophages with anti-CR3 antibody or by serum complement depletion with cobra venom factor. This indicates that myelin phagocytosis is mediated by the macrophage CR3.

Phagocytosis in the rat optic nerve following Wallerian degeneration

Unilateral enucleation of the eye in adult male rats was performed in an attempt to resolve the longstanding controversy as to the nature of the phagocytic cells during Wallerian degeneration in the central nervous system. Previously both resident microglia and circulating monocytes, as well as oligodendrocytes, have all been considered to be the phagocytic cells. In these present experiments macrophages and microglia were studied using lectin histochemistry for Griffonia simplicifolia agglutinin and the monoclonal antibody ED1 at light microscopic level. Oligodendrocytes were demonstrated ultrastructurally using immunohistochemistry with monoclonal antibodies against myelin oligodendrocyte glycoprotein (MOG). Ultrastructural examination of the degeneration optic nerves confirmed longstanding reports of the slow nature of breakdown in the adult central nervous system. During the early periods of breakdown, starting at 1 week and continuing to 1 or 2 months, it was difficult to type, on ultrastructural examination alone, the nature of all the cells undergoing phagocytosis, but many of them resembled microglia/macrophages. Myelin debris cleared very slowly and could still be recognised prominently in the nerve up to 22 months post-enucleation. Lectin and immunochemical examination showed that the early major phagocytic component of phagocytosis was carried out by macrophages, probably both circulating and resident. In addition, however, myelin and axonal debris was taken up or retracted into oligodendrocyte processes, which were stained with antibodies to MOG. This oligodendrocyte component appeared to be small in relationship to the overall degree of debris.(ABSTRACT TRUNCATED AT 250 WORDS)

L-fucosidase treatment blocks myelin phagocytosis by macrophages in vitro

Myelin phagocytosis in nerves undergoing Wallerian degeneration has been shown to depend on their invasion by non-resident, hematogenous macrophages. This process can be studied in vitro using organ cultures of peripheral nerves exposed to cultured peritoneal macrophages. The present report concerns the role of cell surface carbohydrates in the invasion of degenerating nerves and in the recognition and ingestion of myelin by the phagocytic cells. Additional experiments explored the effect of pH, calcium and cytochalasin D on myelin phagocytosis. Organ cultures with peritoneal macrophages were treated with 14 simple or complex sugars or with eight sugar-splitting enzymes. Macrophage invasion was diminished by many simple or complex sugars, but exposure to sugars had no effect on the recognition or ingestion of myelin by the invading macrophages. Macrophage invasion was abolished upon treatment with beta-mannosidase. Exposure to L-fucosidase abolished the myelin phagocytic capacity of invading macrophages completely without affecting their capacity to ingest carbon or latex particles. The results indicate that the phagocytosis of myelin by macrophages is an L-fucosidase-sensitive process, probably by interaction with their complement receptor type C3.

Production and characterization of monoclonal antibodies to the major peripheral myelin glycoprotein P0

Several monoclonal antibodies were generated against the major glycoprotein P0 of human peripheral nervous system myelin. Antibodies were selected for their reactivity with P0 in Western blots. The antibodies were of the immunoglobulin G subclass and reacted with the glycopeptidase F-treated P0, indicating that the reactive epitope resides in the protein backbone. In fresh frozen and paraffin-embedded sections of central and peripheral nervous system of rat and human, P0 antibody 592 reacted with myelin sheaths of peripheral, but not central, nervous system.

Macrophage responses in inflammatory demyelinating neuropathies

Macrophages are prominent participants in inflammatory demyelinating neuropathies. To provide a different means of evaluating macrophage behavior, we used immunostaining of teased nerve fibers and endoneurial blood vessels. We assessed the frequency with which macrophages were seen in inflammatory demyelinating neuropathies, their relationship to normal and demyelinating fibers, and their expression of major histocompatibility Class II markers (Ia antigen). In 6 patients with chronic inflammatory neuropathy and 1 with Guillain-Barré syndrome, we found regularly that macrophages were adherent to teased blood vessels. Cells presumed to be entering the nerve were elongated, often with a polarized appearance suggesting motility, and were Ia-positive. After entry into nerve, the Ia-positive macrophages were adherent to both normal and demyelinating fibers. They often retained their Ia positivity after penetrating into the nerve fiber and removing myelin. Foamy macrophages, judged to be postphagocytic, were Ia-negative. The foamy macrophages found adhering to blood vessels were presumed to be leaving the nerve. This pattern of entry as Ia-positive prephagocytic cells and evolution to Ia-negative foamy macrophages was compared with other experimental and human neuropathies.

Activation of macrophages by recombinant interferon-gamma has no effect on myelin phagocytosis but hinders invasion of nerves in organ culture

Myelin phagocytosis in nerves undergoing Wallerian degeneration was shown to depend on their invasion by non-resident, hematogenous macrophages. This process can be studied in vitro using organ cultures of peripheral nerves exposed to cultured peritoneal macrophages. The present report concerns the effect of recombinant interferon-gamma (rIFN-gamma) on luminol-dependent chemiluminescence, macrophage migration and myelin phagocytosis in organ cultures. Chemiluminescence was activated by rIFN-gamma compared to untreated cells. The macrophage population was capable of activation at any phase of exposure to organ cultures. The engagement of macrophages in myelin phagocytosis, however, varied with the timing of the application of rIFN-gamma. Application from the start of the experiment led to activation of chemiluminescence and also to a complete inhibition of macrophage invasion of the organ culture, thus preventing myelin removal. Application of rIFN-gamma at a later phase of the experiment had no effect on cell invasion and also no detectable effect on the efficiency of myelin phagocytosis. There was no indication that myelin phagocytosis by itself activated chemiluminescence in untreated cultures. Phagocytosis of myelin appears to be a function of macrophages independent of activation causing production of oxygen radicals.

Wallerian degeneration in the peripheral nervous system: participation of both Schwann cells and macrophages in myelin degradation

This study examined the role of Schwann cells and hematogenous macrophages in myelin degradation and Ia antigen expression during Wallerian degeneration of rodent sciatic nerve. To identify and distinguish between macrophages and Schwann cells we used, in addition to electron microscopy, immunocytochemical staining of teased nerve fibres and 1 microns thick cryosections. Before the appearance of adherent macrophages the myelin sheath fragmented into ovoids, small whorls of myelin debris appeared within Schwann cell cytoplasm and the Schwann cell displayed numerous lipid droplets. However, at least in large fibres most myelin degradation and removal was accomplished or assisted by macrophages, identified by their expression of the ED1 marker. These cells began entering the nerve from blood vessels by day 2, migrated to degenerating nerve fibres and adhered to nerve fibres in the regions of the ovoids. There they penetrated the Schwann cell basal lamina to occupy an intratubal position and phagocytose myelin. During Wallerian degeneration a subpopulation of ED1-positive monocytes/macrophages expressed Ia antigen; Schwann cells were Ia-negative. Ia expression by monocytes/macrophages appeared to be a transient event and was not seen in post-phagocytic macrophages, as indicated by the fact that ED1-positive phagocytes with large vacuoles were Ia-negative. Our data show that both Schwann cells and macrophages play important roles in degrading and removing myelin during Wallerian degeneration. The expression of Ia antigen during Wallerian degeneration indicates that Ia expression need not necessarily reflect specific immune events but in some instances can represent a nonspecific response to PNS damage.

The rate of Wallerian degeneration in the absence of immunoglobulins. A study in chick and mouse peripheral nerve

Sciatic nerves undergoing Wallerian degeneration are subject to massive invasion by monocytes bearing Fc receptors. The present experiments were done to explore the potential role of the Fc receptors. The effect of suppressed immunoglobulin levels on the rate of Wallerian degeneration was studied in the n. medioulnaris of the chick and in murine sciatic nerve. In the avian model, total immunoglobulin production was eliminated by bursectomy. The mice were injected after birth with antibodies against IgM, causing a selective reduction of immunoglobulin M levels. Myelin degeneration was measured in both groups by computer-assisted line sampling and corrected for intrafascicular edema. In both models, there were only minor differences with the controls. Immunoglobulins, accordingly, do not seem to play a significant role in the efficiency of myelin phagocytosis by monocytes during Wallerian degeneration. The study also includes data on the degree and the distribution of edema in nerves undergoing Wallerian degeneration.