Unique Role of the Chemokine Domain of Fractalkine in Cell Capture

The chemokine fractalkine (FK) has two structural features that make it unique in the chemokine family: a CX 3C motif and an extended carboxyl terminus that anchors it to the cell surface. This mucin-like stalk or an equivalent spacer is required for FK to mediate the adhesion of cells expressing its receptor, CX 3CR1. To determine whether the ability of FK to act as a cell adhesion molecule is due to the unique presentation of a chemokine domain on a stalk or to properties of the chemokine domain itself, we created a series of chimeras in which other soluble chemokines (RANTES (regulated on activation normal T cell expressed), monocyte chemoattractant protein 1, macrophage inflammatory protein 1β, secondary lymphoid tissue chemokine, and interleukin 8) were fused to the mucin stalk. When tested in a static-cell adhesion assay, many of these chemokine chimeras demonstrated activity equivalent to that of FK. In flow assays, however, none of the chimeras captured cells as efficiently as FK. Interestingly, FK captured cells expressing either CX 3CR1 or the viral receptor US28. Cells bound to FK without rolling or detaching, whereas the interleukin 8 and monocyte chemoattractant protein 1 chimeras induced primarily cell rolling and detaching, respectively. In binding studies, FK has a significantly slower off-rate from its receptors than any of the other chemokine chimeras had for their cognate receptors. We conclude that presentation of a chemokine atop a mucin-like stalk is not, in and of itself, sufficient to capture cells. The unique ability of FK to mediate adhesion under flow may be a function of its slow receptor off-rate.