Balancing thymocyte adhesion and motility: a functional linkage between beta1 integrins and the motility receptor RHAMM.

Thymocyte differentiation involves several processes that occur in different anatomic sites within the thymus. Therefore, thymocytes must have the ability to respond to signals received from stromal cells and adopt either adhesive or motile behavior. We will discuss our data indicating human thymocytes use alpha4beta1 integrin, alpha5beta1 integrin and RHAMM to mediate these activities. Immature multinegative (MN; CD3-4-8-19-) thymocytes use alpha4beta1 and alpha5beta1 integrins to mediate weak and strong adhesion. This subset also uses alpha4beta1 integrin to mediate motility. As thymocytes differentiate, they begin to express and use RHAMM to mediate motility in conjunction with alpha4beta1 and alpha5beta1 integrins. Motile thymocytes use beta1 integrins to maintain weakly adhesive contacts with substrate to provide traction for locomoting cells, thus weak adhesion is a requirement of motile behavior. Hyaluronan (HA) is also required by thymocytes to mediate motility. HA binding to cell surface RHAMM redistributes intracellular RHAMM to the cell surface where it functions to mediate motility. We propose that the decision to maintain adhesive or motile behavior is based on the balance between low and high avidity binding conformations of beta1 integrins on thymocytes and that RHAMM:HA interactions decrease high avidity binding conformations of integrins pushing the balance toward motile behavior.