| Summary: | The fractalkine receptor (CX3CRl) has been implicated in a range of diseases involving an inflammatory component. While CX3CRl appears to play an anti-inflammatory role in certain conditions it has also been reported to promote inflammation. The mechanism behind this pleiotropic effect has not been studied. The hypothesis of this thesis predicts that functional responses following CX3CRl activation depend on the local cytokine environment and the molecular form wherein fractalkine is presented. The pleiotropic role of CX3CRl furthermore suggests that compounds that inhibit or activate CX3CRl may be therapeutically useful. Aiming at developing such compounds, a series of fractalkine mimicking recombinant proteins were designed, some of which specifically inhibited fractalkine induced calcium signalling in CX3CRl expressing primary cells and cell lines. Surprisingly these same constructs were able to induce chemotaxis in a CX3CRl dependent manner, indicating that these constructs inhibit some signalling pathways while activating others. The hitherto known pathway engaged by fractalkine is pertussis toxin sensitive. Multivalent fractalkine was found to induce migration in a valency dependent but pertussis toxin insensitive manner. Furthermore only surface bound fractalkine, but not soluble monovalent fractalkine, was found to stimulate human cell mediated cytotoxicity in a pertussis toxin insensitive manner. These results are consistent with the presence of an alternative CX3CRl activation pathway which may constitute the molecular basis that allows CX3CRl + cells to differentiate between membrane tethered and soluble fractalkine. The significance of the local inflammatory environment was also investigated. To this end IL-6 was found to transiently alter the migrational potential of a microglial cell line in response to soluble fractalkine, suggesting that at least for these cells CX3CRl responses can be modified by other cytokines. In summary this thesis work introduces useful molecular tools and novel concepts towards the understanding of CX3CRl activation and its pleiotropic outcome.
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