Elsevier

Brain Research

Volume 896, Issues 1–2, 30 March 2001, Pages 86-95
Brain Research

Research report
Interleukin-6 promotes post-traumatic healing in the central nervous system

https://doi.org/10.1016/S0006-8993(01)02013-3Get rights and content

Abstract

The central nervous system (CNS) is an immune-privileged site where the role of immune cells and mediators in traumatic brain injury is poorly understood. Previously we have demonstrated that interleukin (IL)-6, a cytokine that acts on a wide range of tissues influencing cell growth and differentiation, is an agonist for vascular endothelial growth factor (VEGF), in in vitro vascularization assays for brain microvessel endothelial cells. In this present work we focus on the role of IL-6 in promoting tissue repair in the CNS in vivo. An aseptic cerebral injury (ACI) was created in the right parietal cortex, using both wild type (C57Bl/6J) and IL-6-deficient (C57Bl/6J–IL-6−/−) mice to study the consequences of the absence of IL-6 on the pathology of brain injuries. We monitored the immediate, early, and late responses to this traumatic injury by characterizing several histologic features in the CNS at days 1, 4, 7 and 14 following injury. Acellular necrosis, cellular infiltration, and re-vascularization were characterized in the injured tissues, and each of these histologic features was individually graded and totaled to assign a healing index. IL-6-deficient mice were found to have a comparatively slower rate of recovery and healing. Furthermore, fluorescein isothiocyanate (FITC)–dextran intravenous injection demonstrated leaky vessels in IL-6-deficient but not in wild type animals following ACI. Additionally, chronic expression of IL-6 in the CNS using transgenic GFAP–IL-6 mice resulted in more rapid healing following ACI. The accelerated tissue repair in GFAP–IL-6 transgenic animals is primarily due to extensive re-vascularization as detected by endothelial cell markers. Combined, this data suggests an important role of IL-6 in tissue repair processes following traumatic injury in the CNS.

Introduction

Interleukin (IL)-6 is a plurifunctional and primarily pro-inflammatory cytokine. It plays a role in hematopoiesis as well as in enhancing T cell activation and B cell immunoglobulin synthesis. In nervous tissue, it is involved in early vasculogenesis; IL-6 mRNA coincides with the expression of vascular endothelial growth factor (VEGF) in the developing brain, and declines following birth to very low constitutive levels [11], [18], [29]. It is important in the activation of astrocytes and microglia, as well as in microglial proliferation, with subsequent gliosis [19]. IL-6 has both neuroprotective and neurotrophic functions [1], [35], [43]. In trauma and certain pathologic conditions, IL-6 is up-regulated [5], [6], [12], [17], [19], [26], [27], [39], [42] together with other pro-inflammatory cytokines, such as IL-1α and TNF-α. While there has been a great deal of data regarding the role of pro-inflammatory cytokines TNF-α and IL-1 in severe tissue trauma, bacterial infection, and initiation in the acute phase response, the information on the role of IL-6 in CNS trauma is more limited. We have published previously that brain endothelial cells upon challenge with interleukin-1 beta (IL-1β), lipopolysaccharide (LPS) or an oxidative challenge (hypoxia) release IL-6 [35]; others have shown similar data of synthesis and release of IL-6 in response to LPS and IL-1 [1], [15], [27], [35]. CNS cells producing IL-6 include astrocytes, macrophages, microglia [19], neurons [38] and brain endothelial cells [15].

Astrocytes are an ideal cell from which cytokine production and neuropathology can be estimated. They respond to an array of instigations and insults, including embryonic development, healing, and disease-mediation, and produce soluble mediators including growth factors, cytokines, and other signaling molecules which eventually impact the function of neurons, microglia, and oligodendrocytes [5], [16], [19], [20], [21], [25]. Astrocytes act to maintain the microenvironment around neurons, regulate the water-ion balance in the CNS via podocytes on capillaries, and via these podocytes and astrocyte-derived secretory factors induce endothelial tight junction formation, creating the blood–brain barrier (BBB) [3], [6], [10], [23], [24], [36]. Normal astrocytic reaction to IL-6 inflammatory stimuli includes increased synthesis of cytoskeletal proteins, namely glial fibrillary acidic protein (GFAP), as well as a change in shape to a more fibrillar, stellar cell type [3], [10], [21], [27]. Microglia can also express receptors for IL-6 and proliferate in response to elevated IL-6 levels [35]. Inflammation and repair are related to the phagocytic abilities of microglia as well as their production of oxidative radicals. Once activated, IL-6 leads to cytokine production and further proliferation – activated microglia (CD11 b/Mac-1+) can also act as antigen presenting cells [3], [10], [31]. Both microglia and other glial cells (astrocytes, oligodendrocytes, ependymal cells) contribute to the gliosis seen following injury.

Gliosis is a double-edged sword: it involves the generation of neurotrophic factors (neuronal growth factor, basic fibroblast growth factor), and protection from toxins, but may also allow for the secretion of neurotoxins such as nitrous oxide, quinolinic acid, and cytokines [1], [4], [6], [7]. Transient elevations in IL-6 mRNA are noted following traumatic brain injury, again felt to be reflective of the re-vascularization occurring during intense tissue reorganization [12], [17]. The pathologic changes seen in transgenic mice with astrocyte-targeted expression of IL-6 are similar to those seen in AIDS dementia complex, Alzheimer’s, Multiple Sclerosis (MS) and acute Experimental Allergic Encephalitis (EAE), Systemic Lupus Erythematosus (SLE), stroke, and both viral and bacterial meningitis [6], [19], [39], [42].

Previously, we have shown IL-6 is involved in angiogenesis, re-vascularization, and healing in vivo and in situ [17]. Vasculogenesis, the first wave of vessel formation, is a phase of endothelial cell differentiation and proliferation, with the formation of primitive vessels; it is especially dependent on VEGF, augmented by IL-6 [14], [17], [36], [45]. Vasculogenesis occurs during normal embryonic brain development, tumoral growth, and in inflammation and healing. Angiogenesis [7], [17], [22], [31], [36], the second wave of vessel formation, involves remodeling of the primitive tubules into a mature vessel network.

Use of the IL-6-deficient model to elucidate the function of endogenous IL-6 in an injured CNS has led to characterization of a specific phenotype, as described by Klein et al. [25]. IL-6-deficient mice develop normally, but display impaired immune responsiveness, especially with antigen presentation [40]. Constitutive IL-6 receptor expression is unchanged on motor neurons and on astrocytes, but the number of activated (GFAP+) astrocytes is markedly diminished, as are the number of activated (CD11 b/Mac-1+) microglia [25]. Neurons, microglia, and astrocytes are all affected by the absence of the pro-inflammatory cytokine, with a hierarchy existing in the targets of IL-6 in the CNS. The reactive astrocyte, demonstrating a high IL-6 receptor level, is the presumed primary target. In IL-6-deficient mice, the normally acute astrocytic activation is much attenuated, and thus contributes less to post-traumatic cellular activation and repair [25]. Also seen in these IL-6-deficient mice were faulty hematopoiesis, chemokine induction, leukocyte recruitment, and neuroglial activation [25].

To elucidate the importance of IL-6 in CNS repair following injury, we applied a cerebral injury model to wild type, IL-6-deficient, and constitutively IL-6-expressing animals. The data presented here demonstrate an important role for this cytokine in post-traumatic healing in the brain.

Section snippets

Materials and methods

Exploiting varying amounts of IL-6 expression by comparing wild type, IL-6-deficient, and IL-6 transgenic chronic expressor mice, the injured brains underwent harvesting, snap freezing, and sectioning as described below. Microscopic evaluation using standard hematoxylin and eosin preparations as well as immunohistochemistry and confocal imaging allowed for assessment of the effect of IL-6 on healing in the injured brain.

IL-6 deficiency results in delayed post-traumatic healing in the brain

Injury results in tissue destruction, followed eventually by an inflammatory response consisting of cellular infiltration and activation of surrounding cells involved in the healing response. GFAP upregulation marking astrocytic activation is demonstrable. Endothelial cells enter the area of injury to form new blood vessels, preceding astrocytic establishment of contact with the vessel.

We have previously shown that IL-6 is a potent inducer of brain microvessel endothelial cell proliferation and

Discussion

The tissue repair process in response to injury is very complex and may be different for various mechanisms of injury. The immune system seems to have a fundamental role in this process regardless of etiology of injury. This paper demonstrates the role of IL-6 in response to an in vivo insult, ACI. Aspects of this model included cerebral cell death and injury due to thermal damage; infiltration into the injured area by blood-borne cells; activation of primary CNS cells adjacent to and remote

Acknowledgements

Special thanks to Dr. Dominic Fee, Dr. Michael N. Hart, Dr. Robert J. Dempsey, Toshi Kinoshita, and the Keck Neural Imaging Laboratory; this work was supported by grants to Z.F. from the National Institute of Health (NS 37570-01A2) and the Multiple Sclerosis Society (MS RG3113A1/1) and to I.L.C. from the National Institute of Health (MH 50426).

References (47)

  • T.M Reyes et al.

    Brain endothelial cell production of a neuroprotective cytokine, interleukin-6, in response to noxious stimuli

    Brain Res.

    (1999)
  • S.C Steffensen et al.

    Site-specific hippocampal pathophysiology due to cerebral overexpression of interleukin-6 in transgenic mice

    Brain Res.

    (1994)
  • S Toulmond et al.

    Local infusion of interleukin-6 attenuates the neurotoxic effects of NMDA on rat striatal cholinergic neurons

    Neurosci. Lett.

    (1992)
  • E.N Benveniste

    Inflammatory cytokines within the central nervous system: sources, function, and mechanism of action

    Am. J. Physiol.

    (1992)
  • U.A.K Betz et al.

    Postnatally induced inactivation of gp130 in mice results in neurological, cardiac, hematopoietic, immunological, hepatic, and pulmonary defects

    J. Exp. Med.

    (1998)
  • I.L Campbell

    Structural and functional impact of the transgenic expression of cytokines in the CNS

    Ann. NY Acad. Sci.

    (1998)
  • I.L Campbell et al.

    Neurologic disease induced in transgenic mice by cerebral overexpression of interleukin 6

    Proc. Natl. Acad. Sci. USA

    (1993)
  • P.A Cancilla et al.

    Freeze injury and repair of cerebral microvessels

    Adv. Exp. Med. Biol.

    (1980)
  • P.A Cancilla et al.

    Regeneration of cerebral microvessels: a morphologic and histochemical study after local freeze-injury

    Lab. Invest.

    (1979)
  • C.S Chiang et al.

    Reactive gliosis as a consequence of interleukin-6 expression in the brain: studies in transgenic mice

    Dev. Neurosci.

    (1994)
  • P.O Couraud

    Infiltration of inflammatory cells through brain endothelium

    Pathol. Biol. (Paris)

    (1998)
  • S Esser et al.

    Vascular endothelial growth factor induces endothelial fenestrations in vitro

    J. Cell Biol.

    (1998)
  • Z Fabry et al.

    Nervous tissue as an immune compartment: the dialect of the immune response in the CNS

    Immunol. Today

    (1994)
  • Cited by (145)

    • Memory decline correlates with increased plasma cytokines in amyloid-beta (1–42) rat model of Alzheimer's disease

      2020, Neurobiology of Learning and Memory
      Citation Excerpt :

      We propose that the initial upsurge in plasma level of IL-1β may be due to its role in initiating immune response to neuro-inflammatory cascade and this initial stimulus for elevation is likely the result of microglia activation to the lesioning effect of Aβ(1–42) (Simard, Soulet, Gowing, Julien, & Rivest, 2006). The plasma level of IL-6 showed a clear upsurge as observed in the post-lesion day 14 group suggestive of its role as a critical cytokine controlling the transition from innate to acquired immunity, which is imperative for dealing properly with injured or infected CNS tissue (Swartz et al., 2001). Studies also suggest that the detrimental influence of IL-6 on memory processes is potentiated over a progressive period of time in animals, this may therefore account for the late elevation of this cytokine as observed in this study.

    View all citing articles on Scopus
    View full text