Exploring the phenotypic assessment of antithrombin deficiency by means of exosomes

Antithrombin (AT) is the most important plasma inhibitor for the activated coagulation factors. Its primary target is thrombin followed by Factor Xa, IXa and VIIa. AT deficiency is associated with an increased risk of deep vein thrombosis and pulmonary embolism, major causes of morbidity and death. When reduced activity levels are identified it is important to measure the AT antigen levels to differentiate type I (quantitative) from type II (qualitative) disorders, as type II defects have varying thrombotic risk. No functional routine diagnostic assay, however, can be assumed to detect all forms of deficiency, due to lack of specificity. In our study we first show by a nanoplasmonic assay that the exosomal molar concentration in Type I, Type II and healthy individuals is the same. Exosomal concentration normalization allowed to reveal for the first time that AT is physiologically present in plasma exosomes and Type II defect-derived vesicles harbour a higher copy number of AT compared to controls. Furthermore Type II deficient exosomal AT shows a unique pattern in 2D electrophoresis, probably related to differential glycosylation. These findings suggest plasma exosomes could complement routine diagnostic tools to unravel qualitative defects in AT deficient patients.