α2Macroglobulin Fails To Compensate For Decreased Thrombin Decay In Severe Antithrombin Deficiency In Liver Cirrhosis Patients
Authors: Kremers Romy , Kleinegris Marie-Claire , De Laat Bas , Wagenvoord Rob , Hemker Coenraad 
Published: October 2019

Abstract

Introduction
In liver cirrhosis patients antithrombin (AT) decreases proportionally to the severity of the disease and in a part of the cases approaches the dangerous levels of heterozygous AT deficiency (50%) associated with thrombosis risk. Moreover, α2Macroglobulin (α2M) has been reported to increase up to fourfold. We investigated in how far the decrease of AT was compensated by the simultaneous increase of α2M.

Methods
Thrombin decay was characterized in plasma samples from healthy controls (n=32) and liver cirrhosis patients (n=29) by measuring AT, α2M and fibrinogen levels as well as the overall thrombin inhibitory capacity of the plasma. Functional AT, α2M and fibrinogen levels were measured with in-house chromogenic assays and the von Clauss method respectively. The overall inhibitory capacity of a plasma sample (total thrombin decay rate) was obtained by triggering thrombin generation (TG) with 50 pM tissue factor and analysis of thrombin decay after prothrombin conversion was over (t = 3 min). From the same TG curves, the rate of thrombin decay by α2M was calculated, and thrombin decay by antithrombin was calculated as the difference between total and α2M-dependent thrombin decay.
In addition, we used a validated computer model to predict thrombin decay in time based on measured plasma levels of AT, α2M and fibrinogen. This allows us to predict thrombin decay under conditions that do not exist in real patient populations, such as liver cirrhosis patients with decreased AT levels, but normal α2M levels. In this way we can quantify the potential protective effect of α2M increase during liver cirrhosis.

Results
Mean AT and α2M concentrations in the healthy control group were 2.01 μM (± 0.36 μM) and 3.34 μM (± 0.89 µM), respectively. In liver cirrhosis patients, the average AT level was significantly decreased (1.62 µM; p = 0.002) and the average α2M level was significantly increased (5.05 µM; p<0.001). In accordance, thrombin decay by α2M is significantly increased in liver cirrhosis patients (180% of control; p<0.001), whereas AT-dependent thrombin decay is significantly decreased (88% of control; p=0.009). The overall thrombin decay was not significantly altered in cirrhosis patients (95% of control; p=0.345), showing that α2M increase indeed compensates for AT decrease. However, in the subset of patients with AT<75% of normal, the rate of thrombin decay is significantly lower than in the control group (76%, p<0.001). Computer simulation of thrombin decay in this subset showed a similarity with the experimental curves to within the limit of experimental error. Replacing the values for the a2M concentration in individual liver cirrhosis patient samples by the average normal value reveals that the elevation in α2M levels increases the thrombin decay rate significantly (10%, p=0.016), but not enough to completely restore physiological thrombin decay rates (76%, p<0.001).

Conclusion
We show that thrombin decay in liver cirrhosis is substantially reduced by the decrease of plasma antithrombin levels, and that a simultaneous increase in α2M levels counteracts this effect. In liver cirrhosis patients with moderate AT loss (<25%) α2M increase fully compensates for AT deficiency, but in patients with severe AT deficiency (<75% of normal levels) this compensation is only partial. In conclusion, the increase of the plasma α2M concentration in liver cirrhosis can be considered to be a (partial) protective mechanism.

Disclosures:
No relevant conflicts of interest to declare.