Acute inflammatory responses to mechanical lesions in the CNS: differences between brain and spinal cord

Interferon-gamma-mediated site-specific clearance of alphavirus from CNS neurons

Recovery from viral encephalomyelitis requires immune-mediated noncytolytic clearance from neurons by mechanisms assumed to be the same for all neurons. In alphavirus encephalomyelitis, antibody clears infectious virus from neurons in all regions of the central nervous system (CNS), but CD8 T cells contribute to elimination of viral RNA. To understand the role of T cells in clearance, we infected antibody knockout mice with Sindbis virus. Virus was cleared from spinal cord and brain stem neurons, but not from cortical neurons, and required both CD4 and CD8 T cells. Infection with cytokine-expressing recombinant viruses suggested that T cells used interferon-gamma, but not tumor necrosis factor alpha, in clearing virus and that populations of neurons differ in responsiveness to this effector pathway.

Abeta-induced inflammatory processes in microglia cells of APP23 transgenic mice

A microglial response is part of the inflammatory processes in Alzheimer's disease (AD). We have used APP23 transgenic mice overexpressing human amyloid precursor protein with the Swedish mutation to characterize this microglia response to amyloid deposits in aged mice. Analyses with MAC-1 and F4/80 antibodies as well as in vivo labeling with bromodeoxyuridine demonstrate that microglia in the plaque vicinity are in an activated state and that proliferation contributes to their accumulation at the plaque periphery. The amyloid-induced microglia activation may be mediated by scavenger receptor A, which is generally elevated, whereas the increased immunostaining of the receptor for advanced glycation end products is more restricted. Although components of the phagocytic machinery such as macrosialin and Fc receptors are increased in activated microglia, efficient clearance of amyloid is missing seemingly because of the lack of amyloid-bound autoantibodies. Similarly, although up-regulation of major histocompatibility complex class II (IA) points toward an intact antigen-presenting function of microglia, lack of T and B lymphocytes does not indicate a cell-mediated immune response in the brains of APP23 mice. The similar characteristics of microglia in the APP23 mice and in AD render the mouse model suitable to study the role of inflammatory processes during AD pathogenesis.

Up-regulation of CD81 (target of the antiproliferative antibody; TAPA) by reactive microglia and astrocytes after spinal cord injury in the rat

We examined the expression of CD81 (also known as TAPA, or target of the antiproliferative antibody) after traumatic spinal cord injury in the rat. CD81, a member of the tetraspanin family of proteins, is thought to be involved in reactive gliosis. This is based on the antiproliferative and antiadhesive effects of antibodies against CD81 on cultured astrocytes, as well as its up-regulation after penetrating brain injury. CD81 expression following dorsal hemisection of the spinal cord was determined immunohistochemically at time points ranging from 1 day to 2 months postlesion (p.l.). In the unlesioned cord a low background level of CD81 was observed, with the exception of the ependyma of the central canal and the pia mater, which were strongly CD81-positive. One day p.l., CD81 was diffusely up-regulated in the spinal cord parenchyma surrounding the lesion site. From 3 days onward, intensely CD81-positive round cells entered the lesion site, completely filling it by 7 days p.l. Staining with the microglial markers OX-42 and Iba1 revealed that these cells were reactive microglia/macrophages. At this time, no significant CD81 expression by GFAP-positive reactive astrocytes was noted. From the second week onward, CD81 was gradually down-regulated; i.e., its spatial distribution became more restricted. The CD81-positive microglia/macrophages disappeared from the lesion site, leaving behind large cavities. After 2 months, astrocytes that formed the wall of these cavities were strongly CD81-positive. In addition, CD81 was present on reactive astrocytes in the dorsal funiculus distal from the lesion in degenerated white matter tracts. In conclusion, the spatiotemporal expression pattern of CD81 by reactive microglia and astrocytes indicates that CD81 is involved in the glial response to spinal cord injury.

MHC antigen expression in human first trimester spinal cord with implications for clinical transplantation procedures

We report human leukocyte antigen (HLA) class I expression in 5-17% and class II in 0-9% of first trimester human spinal cord cells. After 8 days in culture with gamma-interferon, >87% of the spinal cord cells expressed HLA class II. However, mixed cultures of adult human peripheral lymphocytes and immature human spinal cord cells, showed no induction of lymphocyte proliferation prior to or after gamma-interferon exposure in culture. In conclusion, we report non-immunogenic expression of HLA antigens in the human first trimester spinal cord.

Persistent accumulation of cyclooxygenase-1 (COX-1) expressing microglia/macrophages and upregulation by endothelium following spinal cord injury

Acute inflammation following spinal cord injury results in secondary injury and pathological reorganisation of the central nervous system (CNS) architecture. Cyclooxygenases (Prostaglandin Endoperoxide H Synthases, PGH) are key enzymes in the conversion of arachidonic acid into prostanoids which mediate immunomodulation, mitogenesis, apoptosis, blood flow, secondary injury (lipid peroxygenation) and inflammation. Here, we report cyclooxygenase-1 (COX-1) expression following spinal cord injury. In control spinal cords, COX-1 expression was localized by immunohistochemistry to ependymal cells, some neurons, inclusive dorsal and ventral root ganglion cells, few endothelial cells but rarely to brain microglia/macrophages. In injured spinal cords, COX-1(+) microglia/macrophages accumulated highly significantly (P<0.0001) at peri-lesional areas and in the developing necrotic core early after injury. Here numbers of COX-1(+) cells remained persistently elevated up to 4 weeks following injury. Further, COX-1(+) cells were located in perivascular Virchow-Robin spaces, between spared axons and in areas of Wallerian degeneration. Double labeling experiments confirmed co-expression of COX-1 by ED-1(+) and OX-42(+) microglia/macrophages. Transiently after infarction most COX-1(+) microglia/macrophages coexpress the activation antigen OX-6 (MHC class II). However, the prolonged accumulation of COX-1(+) microglia/macrophages at the lesion site enduring the acute post injury inflammatory response points to a role of COX-1 in tissue remodeling or secondary injury. We have identified and localized persistent accumulation of COX-1 expressing cells which might be a potential pharmacological target following spinal cord injury. Therefore, we suggest that approaches based on: (i) short-term; and (ii) selective COX-2 blocking alone might not be a sufficient tool to suppress the local synthesis of prostanoids.

Induction of VEGF and VEGF receptors in the spinal cord after mechanical spinal injury and prostaglandin administration

Vascular endothelial growth factor (VEGF) is an angiogenetic factor that promotes endothelial cell proliferation during development and after injury to various types of tissue, including the central nervous system (CNS). Using immunohistochemical and in situ hybridization methods we have here demonstrated that VEGF and its receptors Flk-1, Flt-1 and Neuropilin-1 mRNAs and proteins are induced after incisions in the rat spinal cord. The inducible enzyme for prostaglandin synthesis cyclooxygenase-2 (COX-2) is known to be upregulated after spinal injury, cerebral ischemia and to stimulate angiogenesis. To test the hypothesis that prostaglandins may be involved in the VEGF response after lesion we investigated whether intraspinal microinjections of prostaglandin F2alpha (PGF2alpha) alters VEGF expression in the spinal cord. Such treatment was followed by a strong upregulation of VEGF mRNA and protein in the injection area. Finally, by use of an in vitro model with cell cultures of meningeal fibroblast and astrocyte origin, resembling the lesion area cellular content after spinal cord injury but devoid of inflammatory cells, we showed that VEGF is expressed in this in vitro model cell system after treatment with PGF2alpha and prostaglandin E2 (PGE2). These data suggest that cells within a lesion area in the spinal cord are capable of expressing VEGF and its receptors in response to mechanical injury and that prostaglandins may induce VEGF expression in such cells, even in the absence of inflammatory cells.

Reactions of oligodendrocytes to spinal cord injury: cell survival and myelin repair

The aim of this study was to elucidate whether oligodendrocytes die in fiber tracts that are spared by a spinal cord injury but are in close vicinity of inflammatory cells. Adult rat spinal cords were studied histologically 1 day to 2 weeks after a contusion lesion that left the ventral white matter largely intact. Massive oligodendrocyte death occurred in the lesion center, along with the death of neurons, microglia, and astrocytes. Oligodendrocytes, specifically positive for proteolipid protein (PLP) mRNA, were counted in the ventral white matter where axons at the rostral and caudal edges of the lesion were histologically intact. Although these regions contained many macrophages and neutrophils hypothesized to contribute to secondary tissue loss, there was no significant loss of oligodendrocytes. In the ventral funiculus, 3 and 6 mm rostral and caudal to the lesion, oligodendrocyte numbers were also unchanged, in spite of the presence of many activated microglial cells. From day 7 on, oligodendrocytes in close vicinity to the lesion increased their expression of PLP mRNA. We conclude that, at least within the first 2 weeks after a spinal cord contusion lesion, there is no major devastating influence of inflammatory cells or their mediators on oligodendrocytes. When death occurs, it may be due to mechanical trauma, ischemia, or excitotoxicity within the lesion or it may occur as a result of axonal degeneration.