CD44, a
cell-surface receptor for the extracellular matrix
glycosaminoglycan hyaluronan, can mediate leukocyte rolling on
hyaluronan substrates and has been implicated in leukocyte migration to sites of
inflammation. CD44-mediated binding to
hyaluronan is of low affinity, and effective cell/matrix interaction depends on multiple interactions with the multivalent
ligand. We replaced the Link module of CD44 with the homologous region of TSG-6, a
hyaluronan-binding protein secreted in response to
inflammation whose Link module has a higher affinity for
ligand.
Monoclonal antibodies raised against the CD44/TSG-6 chimera recognized recombinant human TSG-6 and native mouse TSG-6 and blocked
hyaluronan binding to these
proteins. Cells expressing the CD44/TSG-6 molecule bound
hyaluronan with higher avidity than cells expressing CD44. This resulted in changes in the
hyaluronan binding properties characteristic of cells expressing CD44 such as requirements for threshold levels of receptor expression and for
hyaluronan of high molecular mass. In parallel plate flow assays used to model leukocyte rolling, cells expressing CD44/TSG-6 failed to roll on
hyaluronan. Instead, they stuck and remained "tethered" to the substrate under fluid flow. This result argues that the low affinity of CD44 for its
ligand is important for rolling, an early phase of leukocyte extravasation from the blood.