The platelet aggregating factor(s) of thrombotic thrombocytopenic purpura.

Thrombotic thrombocytopenic purpura is characterized by disseminated platelet aggregates throughout the microcirculation and a large body of clinical, experimental and pathological evidence implicates in vivo platelet aggregation as being pivotal in the disease. For this reason considerable energy has been spent by many investigators to identify a platelet aggregating factor. The understated reason for these studies is that the characterization of such a factor could lead to the subsequent identification of a medication or a plasma factor that could neutralize the aggregating factor. Although a platelet aggregating factor has been suspected to play a causal role in TTP for almost 50 years, only in recent years have investigators systematically attempted to identify such a factor. One group has identified a 37 kD protein in the plasma of TTP patients which can initiate platelet aggregation through a unique pathway of aggregation and this factor can be inhibited by normal IgG. The other group of workers (ourselves) have provided evidence that the platelet aggregating factor of TTP is calpain. Thus, the very nature of the aggregating factor, and by implication its inhibitors, is very different for the two groups, and it may take other investigators to resolve whether there are multiple platelet aggregating factors of TTP. Nonetheless, it is apparent that recent research has dramatically improved our understanding of this uncommon but important disorder and it is possible that such investigations will lead to more successful treatments of this condition.