Intravenous immune globulin prevents venular vaso-occlusion in sickle cell mice by inhibiting leukocyte adhesion and the interactions between sickle erythrocytes and adherent leukocytes.

Sickle cell vaso-occlusion is a complex multistep process likely involving heterotypic interactions among sickle erythrocytes (red blood cells [RBCs]), leukocytes (white blood cells [WBCs]), and endothelial cells. Recent data using intravital microscopy in a sickle cell mouse model suggest that adherent leukocytes in postcapillary venules play a critical role in vaso-occlusion by capturing circulating sickle RBCs. In the course of studies to investigate the adhesion receptors mediating sickle RBC-WBC interactions, we found that control nonspecific immunoglobulin G (IgG) preparations displayed significant inhibitory activity. As a result, we studied the effects of commercial intravenous human immune globulin (i.v.IG) preparations and found that i.v.IG inhibits RBC-WBC interactions in cremasteric venules in a dose-dependent manner. i.v.IG of at least 200 mg/kg dramatically reduced these interactions, even after tumor necrosis factor-alpha (TNF-alpha) stimulation, and not only increased microcirculatory blood flow but also improved survival of sickle cell mice. These data raise the possibility that i.v.IG may have a beneficial effect on sickle cell-associated vaso-occlusion.

Cloning, expression, and functional characterization of the von Willebrand factor-cleaving protease (ADAMTS13).

Deficient von Willebrand factor (VWF) degradation has been associated with thrombotic thrombocytopenic purpura (TTP). In hereditary TTP, the specific VWF-cleaving protease (VWF-cp) is absent or functionally defective, whereas in the nonfamilial, acquired form of TTP, an autoantibody inhibiting VWF-cp activity is found transiently in most patients. The gene encoding for VWF-cp has recently been identified as a member of the metalloprotease family and designated ADAMTS13, but the functional activity of the ADAMTS13 gene product has not been verified. To establish the functional activity of recombinant VWF-cp, we cloned the complete cDNA sequence in a eukaryotic expression vector and transiently expressed the encoded recombinant ADAMTS13 in HEK 293 cells. The expressed protein degraded VWF multimers and proteolytically cleaved VWF to the same fragments as those generated by plasma VWF-cp. Furthermore, recombinant ADAMTS13-mediated degradation of VWF multimers was entirely inhibited in the presence of plasma from a patient with acquired TTP. These data show that ADAMTS13 is responsible for the physiologic proteolytic degradation of VWF multimers.