Alpha-2-Macroglobulin Is a Major Determinant Of a Lower Thrombin Generation In Infants and Children Compared To Adults
Authors: Kremers Romy , Bloemen Saartje , Al Dieri Raed , Hemker Coen H , Karlaftis Vasiliki , Attard Chantal , de Laat Bas , Monagle Paul , Ignjatovic Vera 
Published: October 2019

Abstract

Introduction
Developmental Haemostasis, the concept describing maturation of the haemostatic system is based on established age specific differences in the quantity and function of haemostatic proteins. Interestingly, infants and children have a lower incidence of thromboembolic events than adults. Understanding the precise mechanism of thromboprotection in infants and children may therefore provide novel prevention and treatment strategies for adults. We hypothesized that a significant component of the thromboprotective mechanism is contributed to by the age-specific differences in thrombin generation.

Aim
This study was designed to investigate differences in thrombin generation in different age groups from infants to adults, to obtain more insight into age related differences in thromboembolic rates.

Materials and Methods
Plasma samples from 86 infants, children and adults were tested individually and the results were grouped according to age: 1 month to 1 year (N=13), 1-5 years (N=20), 6-10 years (N=19), 11-16 years (N=16) and adults (N=18). Thrombin generation (TG) was measured using Calibrated Automated Thrombinography (CAT) triggered with tissue factor (TF) at 5pM and 1pM final concentration. Parameters measured from the TG curve included: lag time, maximal thrombin concentration (peak), time to peak, as well as the area under the curve (endogenous thrombin potential, ETP). The raw data from TG was also used to calculate the overall, as well as alpha-2-macroglobulin (A2M) dependent decay constants. A2M function was measured with a thrombin inhibition assay developed in house.

Results
Both ETP and peak thrombin in infants and children was consistently significantly lower than in adults at both 1pM and 5pM tissue factor concentrations. The difference in ETP ranged from 27% to 35% (p<0.0001), while the difference in peak thrombin generated ranged from 28% to 37% (p<0.005). The thrombin decay constant (total inhibition of thrombin) was 15% (p<0.01) higher in infants and children compared to adults. The A2M-specific decay constant was on average 93% (p<0.001) higher in the younger populations and represented 14% of the total thrombin inhibition, compared to 9% in adults. The functional A2M-levels were 54% (p<0.05) higher in infants and children compared to adults; and correlated significantly with age (r=0.564, p<0.001).

Discussion and Conclusions
This study confirms that TG in children (including adolescents) is lower than in adults and shows for the first time that this is predominantly caused by higher thrombin inhibition, largely due to an increase of A2M activity.
Considering that the levels of A2M are increased by 50%, Antithrombin is decreased by approximately 37%, whilst the levels of Prothrombin, Fibrinogen and Heparin Cofactor II remain the same in infants and children compared to adults; therefore A2M is an important inhibitor of thrombin in early life and during childhood.
Age-specific differences in thrombin generation as demonstrated by our study possibly play an important role in the thromboprotective mechanism functioning in infants and children.

Disclosures:
No relevant conflicts of interest to declare.