A Threshold Level of Von Willebrand Factor Is Required for Disease Pathogenesis in a Mouse Model of TTP.

Thrombotic Thrombocytopenic Purpura (TTP) is a life threatening systemic illness associated with inherited or acquired deficiency of the ADAMTS13 metalloprotease. ADAMTS13 deficiency results in the appearance of ultra-large multimers of von Willebrand Factor (UL-VWF) in the circulation, which are thought to play a central role in the formation of the platelet-rich thrombi which are the pathological hallmark of this disease. Despite the critical role postulated for VWF in TTP pathogenesis, direct experimental evidence in support of this hypothesis is lacking. Recently we demonstrated that a TTP-like syndrome can be induced in ADAMTS13-deficient mice of a mixed CASA/Rk genetic background (Adamts13^CASA−/−) by the bacterial agent shigatoxin. As pure CASA/Rk mice exhibit VWF levels 5 to 10-fold higher than most of the common strains of laboratory mice, we hypothesized that increased plasma VWF levels in the Adamts13^CASA−/− mice (which ranged from 200–600% of C57BL/6) were responsible for their increased TTP susceptibility. Surprisingly however, plasma VWF levels did not correlate with the degree of thrombocytopenia or hemolytic anemia induced in Adamts13^CASA−/− mice by shigatoxin challenge, with mice inheriting the lowest VWF levels having TTP pathology equal to those inheriting the highest levels. To further examine the requirement for VWF in TTP pathogenesis, we generated ADAMTS13-deficient mice on a susceptible CASA/Rk genetic background that were also either haploinsufficient (Vwf+/−) or completely deficient (Vwf−/−) in VWF. Absolute deficiency of VWF resulted in complete protection from shigatoxin-induced thrombocytopenia. However, thrombocytopenia was consistently observed in Vwf+/− mice similar to that previously reported in Adamts13^CASA−/− mice with wild-type VWF. Importantly, again no correlation was observed between VWF level and degree of thrombocytopenia among the ADAMTS13-deficient mice that were Vwf haploinsufficient (Vwf+/−), confirming our previous observation. In summary, we now have demonstrated an absolute requirement for VWF in the pathogenesis of TTP in this mouse model system. Together with the observations that increasing VWF levels do not correlate with increasing TTP pathology, these results suggest that a threshold level of VWF is required for induction of TTP, but that further increases of VWF level do not result in worsening disease. Though these findings remain to be confirmed in humans, our data suggest that the wide variation in plasma VWF levels in the human population will not be a significant determinant of TTP susceptibility or disease severity.