Payam Rezaie

Periventricular leukomalacia, inflammation and white matter lesions within the developing nervous system.

Periventricular leukomalacia (PVL) occurring in premature infants, represents a major precursor for neurological and intellectual impairment, and cerebral palsy in later life. The disorder is characterized by multifocal areas of necrosis found deep in the cortical white matter, which are often symmetrical and occur adjacent to the lateral ventricles. There is no known cure for PVL. Factors predisposing to PVL include birth trauma, asphyxia and respiratory failure, cardiopulmonary defects, premature birth/low birthweight, associated immature cerebrovascular development and lack of appropriate autoregulation of cerebral blood flow in response to hypoxic‐ischemic insults. The intrinsic vulnerability of oligodendrocyte precursors is considered as central to the pathogenesis of PVL. These cells are susceptible to a variety of injurious stimuli including free radicals and excitotoxicity induced by hypoxic‐ischemic injury (resulting from cerebral hypoperfusion), lack of trophic stimuli, as well as secondary associated events involving microglial and astrocytic activation and the release of pro‐inflammatory cytokines TNF‐α and IL‐6. It is yet unclear whether activated astrocytes and microglia act as principal participants in the development of PVL lesions, or whether they are representatives of an incidental pathological response directed towards repair of tissue injury in PVL. Nevertheless, the accumulated evidence points to a pathological contribution of microglia towards damage. The topography of lesions in PVL most likely reflects a combination of the relatively immature cerebrovasculature together with a failure in perfusion and/or hypoxia during the greatest period of vulnerability occurring around mid‐to‐late gestation. Mechanisms underlying the pathogenesis of PVL have so far been related to prenatal ischemic injury to the brain initiated within the third trimester, which result in global cognitive and developmental delay and motor disturbances. Over the past few years, several epidemiological and experimental studies have implicated intrauterine infection and chorioamnionitis as causative in the pathogenesis of PVL. In particular, recent investigations have shown that inflammatory responses in the fetus and neonate can contribute towards neonatal brain injury and development‐related disabilities including cerebral palsy. This review presents current concepts on the pathogenesis of PVL and emphasizes the increasing evidence for an inflammatory pathogenic component to this disorder, either resulting from hypoxic‐ischemic injury or from infection. These findings provide the basis for clinical approaches targeted at protecting the premature brain from inflammatory damage, which may prove beneficial for treating PVL, if identified early in pathogenesis.

Expression of the β-chemokine receptors CCR2, CCR3 and CCR5 in multiple sclerosis central nervous system tissue

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) characterised by perivascular inflammatory cell infiltrates and plaques of demyelination. Chemokines have been shown to play an important role in the activation and directional migration of cells to sites of CNS inflammation. The action of chemokines requires the expression of their complementary chemokine receptors by their target cells. We have examined the expression of the beta-chemokine receptors CCR2, CCR3 and CCR5 in post-mortem MS CNS tissue using single- and double-labelling immunocytochemistry techniques. Low levels of CCR2, CCR3 and CCR5 were expressed by microglial cells throughout control CNS tissue. In chronic active MS lesions CCR2, CCR3 and CCR5 were associated with foamy macrophages and activated microglia. CCR2 and CCR5 were also present on large numbers of infiltrating lymphocytes. A smaller number of CCR3-positive lymphocytes were present, but we also noted CCR3 and CCR5 on astrocytes in five of the 14 cases of MS investigated, particularly associated with processes around vessels and at the glia limitans. Ligands for CCR2 and CCR3 include MCP-1 and MCP-3 which were co-localised around vessels with the infiltrating leukocytes, but were also present in unaffected areas of cortex. The elevated expression of CCR2, CCR3 and CCR5 in the CNS in MS suggests these beta-chemokine receptors and their ligands play a role in the pathogenesis of MS.

Colonisation of the developing human brain and spinal cord by microglia: a review

Microglia are the immune effector cells of the nervous system. The prevailing view is that microglia are derived from circulating precursors in the blood, which originate from the bone‐marrow. Colonisation of the central nervous system (CNS) by microglia is an orchestrated response during human fetal development related to the maturation of the nervous system. It coincides with vascularisation, formation of radial glia, neuronal migration and myelination primarily in the 4th–5th months and beyond. Microglial influx generally conforms to a route following white matter tracts to gray areas. We have observed that colonisation of the spinal cord begins around 9 weeks, with the major influx and distribution of microglia commencing around 16 weeks. In the cerebrum, colonisation is in progress during the second trimester, and ramified microglial forms are widely distributed within the intermediate zone by the first half of intra‐uterine life (20–22 weeks). A distinct pattern of migration occurs along radial glia, white matter tracts and vasculature. The distribution of these cells is likely to be co‐ordinated by spatially and temporally regulated, anatomical expression of chemokines including RANTES and MCP‐1 in the cortex; by ICAM‐2 and PECAM on radiating cerebral vessels and on capillaries within the germinal layer, and apoptotic cell death overlying this region. The phenotype and functional characteristics of fetal microglia are also outlined in this review. The need for specific cellular interactions and targeting is greater within the central nervous system than in other tissues. In this respect, microglia may additionally contribute towards CNS histogenesis. Microsc. Res. Tech. 45:359–382, 1999.

Transplantation of neural stem cells in a rat model of stroke: assessment of short-term graft survival and acute host immunological response.

The use of progenitors and stem cells for neural grafting is promising, as these not only have the potential to be maintained in vitro until use, but may also prove less likely to evoke an immunogenic response in the host, when compared to primary (fetal) grafts. We investigated whether the short-term survival of a grafted conditionally immortalised murine neuroepithelial stem cell line (MHP36) (2 weeks post-implantation, 4 weeks post-ischaemia) is influenced by: (i) immunosuppression (cyclosporin A (CSA) vs. no CSA), (ii) the local (intact vs. lesioned hemisphere), or (iii) global (lesioned vs. sham) brain environment. MHP36 cells were transplanted ipsi- and contralateral to the lesion in rats with middle cerebral artery occlusion (MCAo) or sham controls. Animals were either administered CSA or received no immunosuppressive treatment. A proliferation assay of lymphocytes dissociated from cervical lymph nodes, grading of the survival of the grafted cells, and histological evaluation of the immune response revealed no significant difference between animals treated with or without CSA. There was no difference in survival or immunological response to cells grafted ipsi- or contralateral to the lesion. Although a local upregulation of immunological markers (MHC class I, MHC class II, CD45, CD11b) was detected around the injection site and the ischaemic lesion, these were not specifically upregulated in response to transplanted cells. These results provide evidence for the low immunogenic properties of MHP36 cells during the initial period following implantation, known to be associated with an acute host immune response and ensuing graft rejection.

Late onset neurodegeneration in the Cln3−/− mouse model of juvenile neuronal ceroid lipofuscinosis is preceded by low level glial activation

Mouse models of neuronal ceroid lipofuscinosis (NCL) exhibit many features of the human disorder, with widespread regional atrophy and significant loss of GABAergic interneurons in the hippocampus and neocortex. Reactive gliosis is a characteristic of all forms of NCL, but it is unclear whether glial activation precedes or is triggered by neuronal loss. To explore this issue we undertook detailed morphological characterization of the Cln3 null mutant (Cln3(-/-)) mouse model of juvenile NCL (JNCL) that revealed a delayed onset neurodegenerative phenotype with no significant regional atrophy, but with widespread loss of hippocampal interneurons that was first evident at 14 months of age. Quantitative image analysis demonstrated upregulation of markers of astrocytic and microglial activation in presymptomatic Cln3(-/-) mice at 5 months of age, many months before significant neuronal loss occurs. These data provide evidence for subtle glial responses early in JNCL pathogenesis.

Expression of β-chemokines and chemokine receptors in human fetal astrocyte and microglial co-cultures: Potential role of chemokines in the developing CNS

Chemokines play specific roles in directing the recruitment of leukocyte subsets into inflammatory foci within the central nervous system (CNS). The involvement of these cytokines as mediators of inflammation is widely accepted. Recently, it has become evident that cells of the CNS (astrocytes, microglia, and neurons) not only synthesize, but also respond functionally or chemotactically to chemokines. We previously reported developmental events associated with colonization of the human fetal CNS by mononuclear phagocytes (microglial precursors), which essentially takes place within the first two trimesters of life. As part of the array of signals driving colonization, we noted specific anatomical distribution of chemokines and chemokine receptors expressed during this period. In order to further characterize expression of these molecules, we have isolated and cultured material from human fetal CNS. We demonstrate that unstimulated subconfluent human fetal glial cultures express high levels of CCR2 and CXCR4 receptors in cytoplasmic vesicles. Type I astrocytes, and associated ameboid microglia in particular, express high levels of surface and cytoplasmic CXCR4. Of the chemokines tested (MIP‐1α, MIP‐1β, MCP‐1, MCP‐3, RANTES, SDF‐1, IL‐8, IP‐10), only MIP‐1α, detected specifically on microglia, was expressed both constitutively and consistently. Low variable levels of MCP‐1, MIP‐1α, and RANTES were also noted in unstimulated glial cultures. Recombinant human chemokines rhMCP‐1 and rhMIP‐1α also displayed proliferative effects on glial cultures at [10 ng/ml], but displayed variable effects on CCR2 levels on these cells. rhMCP‐1 specifically upregulated CCR2 expression on cultured glia at [50 ng/ml]. It is gradually becoming evident that chemokines are important in embryonic development. The observation that human fetal glial cells and their progenitors express specific receptors for chemokines and can be stimulated to produce MCP‐1, as well as proliferate in response to chemokines, supports a role for these cytokines as regulatory factors during development. GLIA 37:64–75, 2002.

Microglia and the pathogenesis of spongiform encephalopathies.

Alterations in the phenotype and function of microglia, the resident mononuclear phagocytes of the central nervous system, are among the earliest indications of pathology within the brain and spinal cord. The prion diseases, also known as spongiform encephalopathies, are fatal neurodegenerative disorders with sporadic, genetic or acquired infectious manifestations. A hallmark of all prion diseases is the aberrant metabolism and resulting accumulation of the prion protein. Conversion of the normal cellular protein [PrP(c)] into the abnormal pathogenic (or disease-causing) isoform [PrP(Sc)] involves a conformational alteration whereby the alpha-helical content is transformed into beta-sheet. The histological characteristics of these disorders are spongiform change, astrocytosis, neuronal loss and progressive accumulation of the protease-resistant prion isoform. An additional upregulation in microglial response has been reported in Kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), scrapie, in transgenic murine models and in culture, where microglial activation often accompanies prion protein deposition and neuronal loss. This article will review the roles of microglia in spongiform encephalopathies.

Mesoglia & microglia: a historical review of the concept of mononuclear phagocytes within the central nervous system

More than a century and a half has elapsed since the first accounts of mesodermal phagocytic elements were proposed within the central nervous system. Over the intervening decades, body and substance were added to this concept through the advancement of histological techniques at the disposal of the researcher and the acute and keen-minded skills of the pathologist. Notable among these pioneering efforts were the contributions of W. Ford Robertson, Santiago Ramon y Cajal, Pio del Rio-Hortega and Wilder Penfield amongst an entire cavalcade of other noteworthy figures. The term ‘mesoglia’ and ‘third element of the nervous system’ was bestowed upon these cells towards the beginning of the twentieth century to account for their separate origins from neurons and macroglia. It was later amended by del Rio-Hortega in 1919, to ‘microglia’ in order to further discriminate between true mesodermal elements and oligodendrocytes, previously regarded as a component of ‘mesoglia’. This particular contention sparked much controversy among del Rio-Hortega’s peers and resulted in an escalation of fruitful research throughout Europe that eventually declined up to the outbreak of the Second World War. The post-war years were a period of the ‘dark ages’ that cast doubt on the very existence and nature of microglia, until the ‘renaissance’ of research was once again rejuvenated in the 1960s, by a new cohort of intrigued minds: Cammermeyer, Blinzinger, Kreutzberg and others who saw in the ‘third element’ the potential that is now commonly ascribed to microglia: the intrinsic immune effector cells of the CNS. It is now universally accepted that microglia are involved as the first line of rapid defence in any pathology of the nervous system, and as such, present a diagnostic tool for the neuropathologist. Although our knowledge of microglia stems from an extensive body of work conducted over the last two decades, much of the earlier work (pre-1960s) has remained somewhat obscure. This is partly accountable due to the limited availability of translated works, and additionally to the lack of a compendium of these articles. This paper will present a comprehensive overview of the pioneering research on mononuclear phagocytes within the central nervous system, which has direct bearing on our present-day understanding of the concept of microglia.

The neuropathology of autism: where do we stand?

The neurobiology and neuropathology of the autism spectrum disorders (ASD) remain poorly defined. Brain imaging studies suggest that the deficits in social cognition, language, communication and stereotypical patterns of behaviour that are manifest in individuals with ASD, are related to functional disturbance and ‘disconnectivity’, affecting multiple brain regions. These impairments are considered to arise as a consequence of abnormal pre‐ and postnatal development of a distributed neural network. Examination of the brain post mortem continues to provide fundamental information concerning the cellular and subcellular alterations that take place in the brain of autistic individuals. Neuropathological observations that have emerged over the past decade also point towards early pre‐ and postnatal developmental abnormalities that involve multiple regions of the brain, including the cerebral cortex, cortical white matter, amygdala, brainstem and cerebellum. However, the neuropathology of autism is yet to be clearly defined, and there are several areas that remain open to further investigation. In this respect, more concerted efforts are required to examine the various aspects of cellular pathology affecting the brain in autism. This paper briefly highlights four key areas that warrant further evaluation.

Expression of chemokines and their receptors by human brain endothelium: implications for multiple sclerosis.

Leukocyte migration into the central nervous system (CNS) is mediated by chemokines expressed on CNS endothelial cell surfaces. This study investigated the production of chemokines and expression of chemokine receptors by human brain endothelial cells (HBECs) in vitro and in situ. Four chemokines (CCL2, CCL5, CXCL8, and CXCL10) were demonstrated by immunohistochemistry in endothelial cells in brain samples from patients with multiple sclerosis. CXCL8 and CCL2 were constitutively released and increased by primary HBECs and the brain endothelial cell line hCEMC/D3 in response to tumor necrosis factor and/or interferon &ggr;. CXCL10 and CCL5 were undetectable in resting endothelial cells but were secreted in response to these proinflammatory cytokines. Tumor necrosis factor strongly increased the production of CCL2, CCL5, and CXCL8; interferon &ggr; upregulated CXCL10 exclusively. CCL3 was not secreted by HBECs and seemed to be confined to astrocytes in situ. The chemokine receptors CXCR1 and CXCR3 were expressed by HBECs both in vitro and in situ; CXCR3 was upregulated in response to cytokine stimulation in vitro. In contrast, CXCR3 expression was reduced in noninflammatory (silent) multiple sclerosis lesions. The particularly high levels of CXCL10 and CXCL8 expressed by brain endothelium may contribute to the predominant TH1-type inflammatory response observed in chronic inflammatory conditions such as multiple sclerosis.