C and CX3C chemokines: cell sources and physiopathological implications

Within the fascinating world of chemokines, C and CX3C chemokines have long been regarded as two minor components, even though they present unique features and show less redundancy than the other chemokine families. Nevertheless, the body of data on their expression and role in various inflammatory disorders has grown in the past few years. The C chemokine family is represented by two chemokines, XCL1/lymphotactin-alpha and XCL2/lymphotactin-beta, whereas the CX3C chemokine family contains only one member, called CX3CL1/ fractalkine. In this review, we present an overview on the structure, expression and signaling properties of these chemokines and their respective receptors and examine how they contribute to inflammation and the regulation of leukocyte trafficking, as well as their potential role in the pathophysiology of human inflammatory diseases. Taken together, these data expand the biological importance of C and CX3C chemokines from that of simple immune modulators to a much broader biological role, even though their precise commitment within the framework of immune responses has still to be determined.