The in vitro effects of chemokines on microglia: Implications for multiple sclerosis.

Microglia, the resident macrophages of the central nervous system (CNS), are the primary cell to respond to injury in the brain, both in inflammation e.g. in multiple sclerosis (MS), and in trauma. Microglia can be activated in vitro by proinflammatory cytokines such as IFNgamma and TNFalpha and respond by secretion of cytokines, chemokines, proteases, nitric oxide and superoxide radicals. As MS is a putative autoimmune disease of the CNS, chemokines (chemotactic cytokines), secreted by many cell types including T cells and macrophages in MS lesions, have been postulated as key players in the orchestration of the inflammatory response by recruitment and activation of inflammatory cells.As potential mediators of microglial cell recruitment to sites of injury, the ability of microglia to migrate in response to alpha and beta chemokines was assessed. All chemokines tested demonstrated the ability to induce migration and changes in the distribution of filamentous actin in adult rat microglia and a human microglial cell line, CHME3, in vitro. This study indicates that chemokines secreted by activated T cells in the CNS may attract microglia to areas of central nervous system inflammation where they could exert their well documented effects.Proteases are also found in close proximity to MS plaques in increased amounts as agents of myelin breakdown in MS. Matrix metalloproteinases (MMPs) are zinc-dependant enzymes, capable of degrading extracellular matrix proteins and are known to be produced by microglia. The control of MMP activity by plasmin, as well as the balance between MMP and levels of their natural inhibitors, TIMPs, may determine lytic or anti-lytic activity in MS. The in vitro control of MMPs 2 and 9, TIMPs 1 and 2 and urokinase type plasminogen activator (uPA) by microglia was examined in response to several chemokines using ELISA and zymography techniques. The chemokines tested were all found to significantly increase the secretion of MMPs and TIMPs by CHME3 cells after 24 hours stimulation. The chemokines, MCP1, MIP1beta and Fractalkine were also shown to increase MMP9 secretion by rat microglia with little or no effect on MMP2. Interestingly, MCP1, MIP1alpha/beta and RANTES significantly decreased the secretion of uPA by CHME3 cells which may indicate an increase in uPAR expression. These results suggest that chemokine mediated control of MMP activity in the breakdown of the blood brain barrier could allow further recruitment of immune cells into the CNS and also cause demyelination of axons by lysis of myelin basic protein.Since chemokines, as well as cytokines, are possible candidates for microglia activation, microglia were tested in their response to several chemokines by measurement of nitrite production, superoxide secretion and Fc receptor expression. The majority of the chemokines tested were able to increase superoxide and nitrite production as well as Fc receptor expression with the exception of the beta chemokine, RANTES, whereby only CHME3 cells produced a significant increase in superoxide production and Fc receptor expression above unstimulated levels suggesting a difference in chemokine receptor expression or a difference in the binding affinity and signal transduction in different cell types.The present study has led to a further understanding of the possible chemokine control of microglial migration, proteinase production and effector function in CNS diseases, as well as their well documented role in recruitment of lymphocytes. Targetting these chemokines in MS patients, with specific antibodies or use of synthetic inhibitors to block their receptors may lead to downregulation of the immune response and a decrease in the severity of the disease.