Loss of the Rap1 effector RIAM results in leukocyte adhesion deficiency due to impaired ß2 integrin function in mice.

Talin is an integrin adaptor, which controls integrin activity in all hematopoietic cells. How intracellular signals promote talin binding to the integrin tail leading to integrin activation is still poorly understood, especially in leukocytes. In vitro studies identified an integrin activation complex whose formation is initiated by the interaction of active, guanosine triphosphate (GTP)-bound Ras-related protein 1 (Rap1) with the adapter protein Rap1-GTP-interacting adapter molecule (RIAM) followed by the recruitment of talin to the plasma membrane. Unexpectedly, loss-of-function studies in mice have shown that the talin-activating role of RIAM is neither required for development nor for integrin activation in platelets. In this study, we show that leukocyte integrin activation critically depends on RIAM both in vitro and in vivo. RIAM deficiency results in a loss of ß2 integrin activation in multiple leukocyte populations, impaired leukocyte adhesion to inflamed vessels, and accumulation in the circulation. Surprisingly, however, the major leukocyte ß1 integrin family member, α4ß1, was only partially affected by RIAM deficiency in leukocytes. Thus, although talin is an essential, shared regulator of all integrin classes expressed by leukocytes, we report that ß2 and α4 integrins use different RIAM-dependent and -independent pathways to undergo activation by talin.

Alarming and Calming: Opposing Roles of S100A8/S100A9 Dimers and Tetramers on Monocytes.

Mechanisms keeping leukocytes distant of local inflammatory processes in a resting state despite systemic release of inflammatory triggers are a pivotal requirement for avoidance of overwhelming inflammation but are ill defined. Dimers of the alarmin S100A8/S100A9 activate Toll-like receptor-4 (TLR4) but extracellular calcium concentrations induce S100A8/S100A9-tetramers preventing TLR4-binding and limiting their inflammatory activity. So far, only antimicrobial functions of released S100A8/S100A9-tetramers (calprotectin) are described. It is demonstrated that extracellular S100A8/S100A9 tetramers significantly dampen monocyte dynamics as adhesion, migration, and traction force generation in vitro and immigration of monocytes in a cutaneous granuloma model and inflammatory activity in a model of irritant contact dermatitis in vivo. Interestingly, these effects are not mediated by the well-known binding of S100A8/S100A9-dimers to TLR-4 but specifically mediated by S100A8/S100A9-tetramer interaction with CD69. Thus, the quaternary structure of these S100-proteins determines distinct and even antagonistic effects mediated by different receptors. As S100A8/S100A9 are released primarily as dimers and subsequently associate to tetramers in the high extracellular calcium milieu, the same molecules promote inflammation locally (S100-dimer/TLR4) but simultaneously protect the wider environment from overwhelming inflammation (S100-tetramer/CD69).