α2-Macroglobulin Is a Significant In Vivo Inhibitor of Activated Protein C and Low APC:α2M Levels Are Associated with Venous Thromboembolism.

BACKGROUND:  Activated protein C (APC) is a major regulator of thrombin formation. Two major plasma inhibitors form complexes with APC, protein C inhibitor (PCI) and α1 -antitrypsin (α1 AT), and these complexes have been quantified by specific enzyme-linked immunosorbent assays (ELISAs). Also, complexes of APC with α2 -macroglobulin (α2 M) have been observed by immunoblotting. Here, we report an ELISA for APC:α2 M complexes in plasma.

METHODS:  Plasma samples were pre-treated with dithiothreitol and then with iodoacetamide. The detection range of the newly developed APC:α2 M assay was 0.031 to 8.0 ng/mL of complexed APC. Following infusions of APC in humans and baboons, complexes of APC with α2 M, PCI and α1 AT were quantified. These complexes as well as circulating APC were also measured in 121 patients with a history of venous thromboembolism (VTE) and 119 matched controls.

RESULTS:  In all the in vivo experiments, α2 M was a significant APC inhibitor. The VTE case-control study showed that VTE patients had significantly lower APC:α2 M and APC levels than the controls ( p  < 0.001). Individuals in the lowest quartile of APC:α2 M or the lowest quartile of APC had approximately four times more VTE risk than those in the highest quartile of APC:α2 M or of APC. The risk increased for individuals with low levels of both parameters.

CONCLUSION:  The APC:α2 M assay reported here may be useful to help monitor the in vivo fate of APC in plasma. In addition, our results show that a low APC:α2 M level is associated with increased VTE risk.