Mice Lacking the Signaling Molecule, CalDAG-GEFI, Represent a Mouse Model for Leukocyte Adhesion Deficiency Type III.

Defective inside-out activation of β 1, β 2, and β 3 integrins in platelets and leukocytes is a main characteristic of patients with leukocyte adhesion deficiency (LAD)-III syndrome. We have recently shown that CalDAG-GEFI, a member of the CalDAG-GEF/RasGRP family of intracellular signaling molecules that catalyzes the exchange of GTP for GDP bound to Rap1, plays a key role for the activation of α IIbβ 3 in murine platelets. Here we studied the role of CalDAG-GEFI for neutrophil function as well as the activation of β 1 integrins in platelets. Neutrophils from CalDAG-GEFI−/ − mice showed normal surface expression of key adhesion receptors such as L-selectin, PSGL-1, or β 1/β 2 integrins. Calcium flux, degranulation, and oxygen radical formation were similar in wild-type (WT) and mutant cells. In contrast, β 2 integrin-mediated adhesion to fibrinogen was significantly reduced in cells lacking CalDAG-GEFI when compared to controls. In vivo, CalDAG-GEFI-deficient neutrophils showed normal rolling along stimulated venules, while firm adhesion was almost completely inhibited. A similar defect in firm adhesion was observed in WT mice pre-treated with blocking antibodies against β 2 integrins. To determine the role of CalDAG-GEFI in neutrophil emigration, inflammation was induced in the peritoneum or the skin. In both models, neutrophil infiltration was significantly reduced in CalDAG-GEFI−/ − mice when compared to controls. We further demonstrate that CalDAG-GEFI regulates the activation of β 1 integrins in platelets and that CalDAG-GEFI-deficiency leads to a complete inhibition of arterial thrombus formation in mice. Due to its central role in the activation of β 1, β 2, and β 3 integrins, we propose CalDAG-GEFI as a candidate gene defective in LAD-III patients.