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Ann. N.Y. Acad. Sci. 961: 346–349 (2002). © 2002 New York Academy of Sciences.

Chemokines in Neurological Trauma Models

RICHARD M. RANSOHOFF

The Lerner Research Institute, Cleveland, Ohio 44195, USA

KEYWORDS: axotomy; chemokines; neurological trauma

Introduction of synthetic materials or engineered cells into the nervous system(either central [CNS] or peripheral [PNS]) will engender a tissue reaction. For thisreason, successful tissue engineering will require us to decipher mechanisms of in-flammatory tissue reactions in the nervous system. Inflammation in the mammaliannervous system follows a diversity of insults, but in many cases the inflammatory re-action is restricted in cellular composition to mononuclear phagocytes. Signals thatgovern selective recruitment of mononuclear phagocytes to the nervous system havebeen enigmatic.1 Classical studies by Perry et al. demonstrated that adhesion mole-cules are readily induced on CNS microvessels by injection of lipopolysaccharide(LPS), so that failure of neutrophil infiltration could not be accounted for by absenceof an activated, adhesive microvascular substrate.2 Studies in which chemokineswere delivered either by recombinant adenoviruses or CNS-directed transgenesshowed that the infiltration of CNS tissues by neutrophils required nothing morethan the presence of a neutrophil-directed chemokine.3,4 Chemokines are chemoat-tractant peptides which signal selectively to subpopulations of leukocytes via G-pro-tein–coupled receptors (GPCRs). In concert with leukocyte and endothelial adhesionmolecules, the chemokines govern the ceaseless process of leukocyte trafficking,both inflammatory and physiological. Chemokine expression profiles have been as-sessed in a variety of models of neural trauma. The results indicate that chemokineexpression in post-traumatic inflammation is generally restricted to the monocytechemoattractant CCL2/MCP-1, and occurs before hematogenous cell entry into neu-ral tissues. Therefore CCL2 is an excellent candidate for a mediator of leukocyte re-cruitment in these settings. Evidence that chemokines selectively recruit targetleukocytes focused attention on the possibility that restricted chemokine expressioncould account in part for the unusual monocyte-rich inflammation that follows ner-vous-system trauma. This line of thinking was supported also by the finding that pa-renchymal neural cells were competent to express chemokines under physiologicalcircumstances.5,6

Address for correspondence: Richard Ransohoff, The Lerner Research Institute, 9500 EuclidAvenue, Room NC30, Cleveland, OH 44195. Voice: 216-444-8939; fax: 216-444-7927.

ransohr@ccf.org

347RANSOHOFF: CHEMOKINES IN NEUROLOGICAL TRAUMA MODELS

SCIATIC NERVE AXOTOMY

In collaboration with Dr. John Griffin (Department of Neurology, Johns HopkinsUniversity School of Medicine), we evaluated chemokine expression duringWallerian degeneration of peripheral nerves in mice and found selective early ex-pression of CCL2; in situ hybridization localized CCL2 message in Schwann cells,preceding recruitment of macrophages. Both macrophage recruitment and clear-ance of debris were delayed in CCL2-null mice after sciatic axotomy (our unpub-lished observations).

AXOTOMY OF SUPERIOR CERVICAL GANGLION

Normal rat SCGs contain only resident ED2+ macrophages; by 48 hours postaxotomy, abundant ED1+ macrophages enter ganglia. In collaboration with Drs. Ri-chard Zigmond and Rebecca Schreiber (Department of Neurosciences, Case West-ern Reserve University School of Medicine), we showed that CCL2 mRNAincreased dramatically by 6 hours post axotomy, with the message localized to neu-rons near the axotomy site.7,8

PENETRATING MECHANICAL TRAUMA TO THE CEREBRAL CORTEX

Within the first twenty-four hours after mechanical trauma to the central nervoussystem (CNS), reactive astrogliosis develops and injury sites are infiltrated by acti-vated mononuclear phagocytes derived from blood-borne monocytes and endoge-nous microglia. In collaboration with Drs. V. Balasingam and V.W. Yong (MontrealNeurological Institute and McGill University), we analyzed the time course and cel-lular source of MCP-1 in mouse brain after penetrating mechanical injury, with par-ticular focus on early time points before histological detection of infiltratingmononuclear phagocytes. Steady-state levels of CCL2 mRNA and protein increasedwithin three hours after nitrocellulose membrane stab or implant injury to the adultmouse brain (but not after stab to the neonatal mouse brain). In situ hybridizationcombined with immunohistochemistry for the glial fibrillary acidic protein (GFAP)astrocyte marker showed that astrocytes were the cellular source of CCL2 at theseearly time points after mechanical brain injury.9

CRYOPROBE LESION TO THE CEREBRAL CORTEX

Cryolesion to the cortex is considered a highly reproducible protocol for gener-ating neuronal injury without opening the skull or dura. In collaboration with Dr.Sean Murphy (then at the Department of Pharmacology, University of Iowa Schoolof Medicine), we evaluated chemokine expression in the rodent CNS after cortical

348 ANNALS NEW YORK ACADEMY OF SCIENCES

cryolesion. In the ipsilateral cortex, CCL2 gene expression was increased signifi-cantly at 6 hours, 20-fold at the 12- and 24-hour time points, and declined to controllevels by 48 hours. Unexpectedly, there was significant CCL2 expression contralat-erally, maximally about 40% of that seen in lesioned cortex with a superimposabletime course.10 This result was compatible with several possible explanations: thatdamaging of fibers within lesioned cortex could initiate chemokine expression con-tralateral to the lesion, as a result of signaling from neurons from which the fibersoriginated. Alternatively, injury to fibers in one region of cortex could signal to neu-rons in the contralateral projection field, resulting in chemokine expression.

Further experiments in collaboration with Dr. Roy Weller (Department of Neuro-pathology, University of Southampton School of Medicine, UK), showed that corti-cal cryolesions induced abundant CCL2 in the cerebrum one day after lesionplacement. At this time point (day 9), there was no detectable CCL2 expression inspinal cord, demonstrating that universal chemokine production throughout the CNSwas not provoked by a cortical cryolesion.11

SPINAL CORD PERCUSSION INJURY

Contusion injury of the rat spinal cord produces a reproducible clinical and his-tological evolution. Inflammation in this model is prominent and its role in recoveryremains uncertain. We evaluated chemokine expression in the spinal cords of rats af-ter percussion injury, in collaboration with Dr. Bradford Stokes (Ohio State Univer-sity). After a single-level laminectomy at T8, rats were immobilized in a spinalframe and the dorsal surface of the spinal cord was rapidly and precisely displacedwith a sterile impactor. Histological changes included immediate and transient neu-trophil infiltration; microglial reaction and macrophage accumulation, beginning atthree days and pronounced from seven to 28 days post injury. CCL2 gene expressionwas more than 20-fold increased by 6 hours, and >50-fold elevated by 12 hours postinjury.12,13

Our results to date indicate that chemokine expression in the context of CNS andPNS inflammation is highly regulated and closely associated with the recruitment ofspecific populations of leukocytes into the target tissue. Thus, CCL2 expression cor-relates well with accumulation of macrophages in such diverse settings as sciaticnerve axotomy or spinal cord percussion injury. Our challenge now is to define thefunctions of the inflammatory reaction in these model systems, and in pathologicalchallenges to the nervous system.

REFERENCES

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3. BELL, M.D. & V.H. PERRY. 1995. Adhesion molecule expression on murine cerebralendothelium following the injection of a proinflammagen or during acute neuronaldegeneration. J. Neurocytol. 24: 695–710.

349RANSOHOFF: CHEMOKINES IN NEUROLOGICAL TRAUMA MODELS

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9. GLABINSKI, A.R., M. TANI, V. BALASINGAM, et al. 1996. Chemokine monocytechemoattractant protein-1 (MCP-1) is expressed by astrocytes after mechanicalinjury to the brain. J. Immunol. 156: 4363–4368.

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