Richard M. Ransohoff

Axonal Transection in the Lesions of Multiple Sclerosis

BACKGROUND Multiple sclerosis is an inflammatory demyelinating disease of the central nervous system and is the most common cause of neurologic disability in young adults. Despite antiinflammatory or immunosuppressive therapy, most patients have progressive neurologic deterioration that may reflect axonal loss. We conducted pathological studies of brain tissues to define the changes in axons in patients with multiple sclerosis. METHODS Brain tissue was obtained at autopsy from 11 patients with multiple sclerosis and 4 subjects without brain disease. Fourteen active multiple-sclerosis lesions, 33 chronic active lesions, and samples of normal-appearing white matter were examined for demyelination, inflammation, and axonal pathologic changes by immunohistochemistry and confocal microscopy. Axonal transection, identified by the presence of terminal axonal ovoids, was detected in all 47 lesions and quantified in 18 lesions. RESULTS Transected axons were a consistent feature of the lesions of multiple sclerosis, and their frequency was related to the degree of inflammation within the lesion. The number of transected axons per cubic millimeter of tissue averaged 11,236 in active lesions, 3138 at the hypocellular edges of chronic active lesions, 875 in the hypocellular centers of chronic active lesions, and less than 1 in normal-appearing white matter from the control brains. CONCLUSIONS Transected axons are common in the lesions of multiple sclerosis, and axonal transection may be the pathologic correlate of the irreversible neurologic impairment in this disease.

Microglial Physiology: Unique Stimuli, Specialized Responses

Microglia, the macrophages of the central nervous system parenchyma, have in the normal healthy brain a distinct phenotype induced by molecules expressed on or secreted by adjacent neurons and astrocytes, and this phenotype is maintained in part by virtue of the blood-brain barriers exclusion of serum components. Microglia are continually active, their processes palpating and surveying their local microenvironment. The microglia rapidly change their phenotype in response to any disturbance of nervous system homeostasis and are commonly referred to as activated on the basis of the changes in their morphology or expression of cell surface antigens. A wealth of data now demonstrate that the microglia have very diverse effector functions, in line with macrophage populations in other organs. The term activated microglia needs to be qualified to reflect the distinct and very different states of activation-associated effector functions in different disease states. Manipulating the effector functions of microglia has the potential to modify the outcome of diverse neurological diseases.

Microglia sculpt postnatal neural circuits in an activity and complement-dependent manner

Microglia are the resident CNS immune cells and active surveyors of the extracellular environment. While past work has focused on the role of these cells during disease, recent imaging studies reveal dynamic interactions between microglia and synaptic elements in the healthy brain. Despite these intriguing observations, the precise function of microglia at remodeling synapses and the mechanisms that underlie microglia-synapse interactions remain elusive. In the current study, we demonstrate a role for microglia in activity-dependent synaptic pruning in the postnatal retinogeniculate system. We show that microglia engulf presynaptic inputs during peak retinogeniculate pruning and that engulfment is dependent upon neural activity and the microglia-specific phagocytic signaling pathway, complement receptor 3(CR3)/C3. Furthermore, disrupting microglia-specific CR3/C3 signaling resulted in sustained deficits in synaptic connectivity. These results define a role for microglia during postnatal development and identify underlying mechanisms by which microglia engulf and remodel developing synapses.

Control of microglial neurotoxicity by the fractalkine receptor

Microglia, the resident inflammatory cells of the CNS, are the only CNS cells that express the fractalkine receptor (CX3CR1). Using three different in vivo models, we show that CX3CR1 deficiency dysregulates microglial responses, resulting in neurotoxicity. Following peripheral lipopolysaccharide injections, Cx3cr1−/− mice showed cell-autonomous microglial neurotoxicity. In a toxic model of Parkinson disease and a transgenic model of amyotrophic lateral sclerosis, Cx3cr1−/− mice showed more extensive neuronal cell loss than Cx3cr1+ littermate controls. Augmenting CX3CR1 signaling may protect against microglial neurotoxicity, whereas CNS penetration by pharmaceutical CX3CR1 antagonists could increase neuronal vulnerability.

Expression of specific chemokines and chemokine receptors in the central nervous system of multiple sclerosis patients

Chemokines direct tissue invasion by specific leukocyte populations. Thus, chemokines may play a role in multiple sclerosis (MS), an idiopathic disorder in which the central nervous system (CNS) inflammatory reaction is largely restricted to mononuclear phagocytes and T cells. We asked whether specific chemokines were expressed in the CNS during acute demyelinating events by analyzing cerebrospinal fluid (CSF), whose composition reflects the CNS extracellular space. During MS attacks, we found elevated CSF levels of three chemokines that act toward T cells and mononuclear phagocytes: interferon-gamma-inducible protein of 10 kDa (IP-10); monokine induced by interferon-gamma (Mig); and regulated on activation, normal T-cell expressed and secreted (RANTES). We then investigated whether specific chemokine receptors were expressed by infiltrating cells in demyelinating MS brain lesions and in CSF. CXCR3, an IP-10/Mig receptor, was expressed on lymphocytic cells in virtually every perivascular inflammatory infiltrate in active MS lesions. CCR5, a RANTES receptor, was detected on lymphocytic cells, macrophages, and microglia in actively demyelinating MS brain lesions. Compared with circulating T cells, CSF T cells were significantly enriched for cells expressing CXCR3 or CCR5. Our results imply pathogenic roles for specific chemokine-chemokine receptor interactions in MS and suggest new molecular targets for therapeutic intervention.

Three or more routes for leukocyte migration into the central nervous system.

Leukocyte migration into and through tissues is fundamental to normal physiology, immunopathology and host defence. Leukocyte entry into the central nervous system (CNS) is restricted, in part, because of the blood–brain barrier (BBB). During the past decade, crucial components that are involved in the process of leukocyte migration have been identified and progress has been made in understanding the mechanisms of neuroinflammatory reactions. In this review, present knowledge of the trafficking determinants that guide the migration of leukocytes is superimposed onto the vascular and compartmental anatomy of the CNS. We discuss three distinct routes for leukocytes to enter the CNS and consider how different populations of leukocytes use trafficking signals to gain entry.

Interferons at age 50: past, current and future impact on biomedicine.

The family of interferon (IFN) proteins has now more than reached the potential envisioned by early discovering virologists: IFNs are not only antivirals with a spectrum of clinical effectiveness against both RNA and DNA viruses, but are also the prototypic biological response modifiers for oncology, and show effectiveness in suppressing manifestations of multiple sclerosis. Studies of IFNs have resulted in fundamental insights into cellular signalling mechanisms, gene transcription and innate and acquired immunity. Further elucidation of the multitude of IFN-induced genes, as well as drug development strategies targeting IFN production via the activation of the Toll-like receptors (TLRs), will almost certainly lead to newer and more efficacious therapeutics. Our goal is to offer a molecular and clinical perspective that will enable IFNs or their TLR agonist inducers to reach their full clinical potential.

Identification of a unique TGF-β–dependent molecular and functional signature in microglia

Microglia are myeloid cells of the CNS that participate both in normal CNS function and in disease. We investigated the molecular signature of microglia and identified 239 genes and 8 microRNAs that were uniquely or highly expressed in microglia versus myeloid and other immune cells. Of the 239 genes, 106 were enriched in microglia as compared with astrocytes, oligodendrocytes and neurons. This microglia signature was not observed in microglial lines or in monocytes recruited to the CNS, and was also observed in human microglia. We found that TGF-β was required for the in vitro development of microglia that express the microglial molecular signature characteristic of adult microglia and that microglia were absent in the CNS of TGF-β1–deficient mice. Our results identify a unique microglial signature that is dependent on TGF-β signaling and provide insights into microglial biology and the possibility of targeting microglia for the treatment of CNS disease.

Inflammatory cortical demyelination in early multiple sclerosis

BACKGROUND Cortical disease has emerged as a critical aspect of the pathogenesis of multiple sclerosis, being associated with disease progression and cognitive impairment. Most studies of cortical lesions have focused on autopsy findings in patients with long-standing, chronic, progressive multiple sclerosis, and the noninflammatory nature of these lesions has been emphasized. Magnetic resonance imaging studies indicate that cortical damage occurs early in the disease. METHODS We evaluated the prevalence and character of demyelinating cortical lesions in patients with multiple sclerosis. Cortical tissues were obtained in passing during biopsy sampling of white-matter lesions. In most cases, biopsy was done with the use of stereotactic procedures to diagnose suspected tumors. Patients with sufficient cortex (138 of 563 patients screened) were evaluated for cortical demyelination. Using immunohistochemistry, we characterized cortical lesions with respect to demyelinating activity, inflammatory infiltrates, the presence of meningeal inflammation, and a topographic association between cortical demyelination and meningeal inflammation. Diagnoses were ascertained in a subgroup of 77 patients (56%) at the last follow-up visit (at a median of 3.5 years). RESULTS Cortical demyelination was present in 53 patients (38%) (104 lesions and 222 tissue blocks) and was absent in 85 patients (121 tissue blocks). Twenty-five patients with cortical demyelination had definite multiple sclerosis (81% of 31 patients who underwent long-term follow-up), as did 33 patients without cortical demyelination (72% of 46 patients who underwent long-term follow-up). In representative tissues, 58 of 71 lesions (82%) showed CD3+ T-cell infiltrates, and 32 of 78 lesions (41%) showed macrophage-associated demyelination. Meningeal inflammation was topographically associated with cortical demyelination in patients who had sufficient meningeal tissue for study. CONCLUSIONS In this cohort of patients with early-stage multiple sclerosis, cortical demyelinating lesions were frequent, inflammatory, and strongly associated with meningeal inflammation. (Funded by the National Multiple Sclerosis Society and the National Institutes of Health.).

The myeloid cells of the central nervous system parenchyma

A microglial cell is both a glial cell of the central nervous system and a mononuclear phagocyte, which belongs to the haematopoietic system and is involved in inflammatory and immune responses. As such, microglia face a challenging task. The neurons of the central nervous system cannot divide and be replenished, and therefore need to be protected against pathogens, which is a key role of the immune system, but without collateral damage. In addition, after physical injury, neural cells need restorative support, which is provided by inflammatory responses. Excessive or chronic inflammatory responses can, however, be harmful. How microglia balance these demands, and how their behaviour can be modified to ameliorate disorders of the central nervous system, is becoming clear.