|Communication from the SSC Hemostasis and Malignancy Subcommittee|
New Commercial Assay to Measure Microparticle Tissue Factor Activity in Plasma
The Hemostasis & Malignancy SSC Subcommittee just published a paper in the Journal of Thrombosis and Haemostasis (JTH) (Tatsumi et al) comparing a new commercial assay to measure microparticle (MP) tissue factor (TF) activity in plasma. MPs are small membrane vesicles that are released from cells. The majority of TF present in plasma resides on MPs.
The Osanto and Mackman groups independently developed similar in-house MP TF activity assays that isolated MPs by centrifugation. TF present in the MPs is quantitated by measuring FXa generation in a reaction containing FVIIa, FX and a chromogenic substrate. An inhibitory anti-TF antibody is used to distinguish TF-dependent from TF-independent FXa generation. The strength of the new commercial assay (Zymuphen MP-TF, Aniara) is that it is a plate-based assay and captures TF-positive MPs present in plasma using a well characterized, non-inhibitory anti-TF antibody called 10H10. However, no anti-TF antibody is used because it was assumed that all the FXa activity of the captured MPs was TF-dependent.
We found that this was not true. One weakness of Zymuphen MP-TF is that it is less sensitive than our in-house assay. This suggests that not all TF-positive MPs are captured. In addition, the commercial assay is less specific for TF and has a higher background than the in-house assay due to the use of a higher FVIIa concentration (12 nM versus 2.5 nM). In conclusion, Zymuphen MP-TF is the first commercial assay to measure TF activity in captured MPs. TF+ MPs may be a useful biomarker to identify patients at risk for thrombosis.
Many thanks to Kohie Tatsumi and Nigel Mackman for this article submission.