Low-affinity LFA1-dependent outside-in signaling mediates avidity modulation via the Rabin8-Rab8 axis.

ABSTRACT

Lymphocyte interactions mediated by leukocyte integrin lymphocyte function-associated antigen 1 (LFA1) and intercellular adhesion molecules (ICAMs) are important for lymphocyte trafficking and antigen recognition. Integrins are regulated by the modulation of ligand-binding affinity and avidity (valency). Although the mechanism underlying high-affinity LFA1 binding has been investigated extensively, the molecular mechanisms by which low-affinity multivalent binding initiates adhesion remain unclear. We previously showed that ICAM1 and monoclonal antibodies that recognize specific LFA1 conformations induce the accumulation of LFA1 at the contact surface. In this study, we found that the small GTPase Rab8 is critical for intracellular transport and accumulation of LFA1 at cell contact areas mediated by low-affinity LFA1-dependent outside-in signaling. Super-resolution microscopy revealed that Rab8 co-localized with LFA1 in small vesicles near the contact membrane. Inactivation of Rab8 decreased ICAM1-dependent adhesion and substantially reduced LFA1 density on the contact membrane. The GTP-bound active form of Rab8 increased cell adhesiveness and promoted LFA1 accumulation at the contact area through co-trafficking with LFA1. Rab8 activation was induced by low-affinity conformation-dependent outside-in signaling via the guanine exchange factor Rabin8, which induced Rab8 activation at the cell contact area independent of Rap1. Single-molecule imaging of ICAM1 on a supported planner lipid bilayer demonstrated that Rab8 increased the frequency of LFA1-ICAM1 interactions without affecting their binding lifetime, indicating that Rab8 is mainly involved in the modulation of LFA1 avidity rather than LFA1 affinity. The present findings underscore the importance of low-affinity conformation-dependent outside-in signaling via the Rabin8-Rab8 axis leading to the initiation of LFA1 transport to the contact area.