Investigation of chemokine expression and modulation following traumatic brain injury
Rhodes, Jonathan K.J.
Over the last 20 years, advances in our understanding of the pathophysiology of severe traumatic brain injury (TBI) and in particular the contribution of secondary injury to poor outcome, has served to improve clinical management and reduce the mortality in these patients. However despite many promising preclinical studies there has been a failure to introduce a specific therapeutic intervention to further improve outcome. Inflammation, with cytokine release and leucocyte infiltration, is a significant secondary injury processes. However the inflammatory response to brain injury, its control and modulation remain incompletely described. Chemotactic cytokines, known as chemokines, are mediators of leucocyte recruitment and activation. Expression of chemokines and the resultant recruitment of leucocytes into the brain are generally thought to be integral to the enlargement of cerebral contusions which accompany clinical deterioration following severe TBI. Previous studies indicated that the main neutrophil chemokine, macrophage inflammatory protein-2 (MIP-2/CXCL2) and the monocytic chemokine, monocyte chemoattractant protein-1 (MCP-1/CCL2) are derived from glial. However the origin of these chemokines following TBI, has not been established. Furthermore, little is known about the modulation of these chemokines: The relationship of serum levels of pro-inflammatory mediators such as the human neutrophil chemokine, interleukin-8 (IL-8/CXCL8; a functional homologue of MIP-2/CXCL2), MCP-1/CCL2 and soluble interleukin-6 receptor (sIL-6R) to contusion enlargement has not been investigated. In this thesis, I investigated chemokine expression and modulation both in in-vitro, in-vivo models and in a clinical study. Initially, I compared chemokine expression in rodent and human glial cell cultures and investigated the modulation of chemokine expression by interleukin-6, the glucocorticoid dexamethasone and the immunosuppressant agent FK506. To investigate chemokine expression in-vivo I established the rat lateral fluid percussion injury (LFPI) model of TBI and measured MIP-2/CXCL2 and MCP-1/CCL2 expression in the brain following injury. Inhibition of this expression by dexamethasone and FK506 was then investigated. To identify the cellular source of chemokine expression I developed an immunohistochemical protocol for MIP-2/CXCL2 and MCP-1/CCL2. Finally, in a clinical study of serum chemokine and sIL-6R concentrations after severe TBI, I examined the relationship between these inflammatory mediators and clinical deterioration. Rat glia (microglia and astrocytes) produce chemokines with a response profile that was qualitatively similar to that of human derived cells. These chemokines were increased in the ipsilateral hemisphere following TBI. Surprisingly, immunohistochemical studies identified marked chemokine expression localised to cells with the morphology of degenerating neurones in contused tissue, rather than in glia. Furthermore, while dexamethasone significantly inhibited both MIP-2/CXCL2 and MCP-1/CCL2 expression in a rat astrocyte derived cell line, only MCP-1/CCL2 expression was reduced by steroid treatment in-vivo. Clinically, serum IL-8/CXCL8, MCP-1/CCL2 or sIL-6R were not significantly different in patients that deteriorated due to contusion enlargement from those that remained stable. However these inflammatory mediators were significantly increased in those patients that died. These studies indicate that astrocytes may not be the major source of chemokines following TBI and highlight the need for caution when inferring pathophysiological mechanisms from in-vitro studies.