Objectives Non-conventional aPL have been described in patients presenting clinical manifestations of antiphospholipid syndrome but negative for conventional markers. Among them, detection of autoantibodies against prothrombin has been proposed to improve diagnosis and management of these patients. However autoantibodies against prothrombin are heterogeneous and their use in clinical practice still remains unclear. The aim of this study was to evaluate the interest of IgG and IgM autoantibodies directed against the prothrombin only (aPT). Methods We retrospectively studied IgM and IgG aPT results, conventional antiphospholipid syndrome markers and clinical data of a large cohort of 441 patients referred for antiphospholipid syndrome exploration with aPT detection over a period of 5 years. Results We observed a total prevalence of 17% of aPT-positive patients (75/441). A significant association was found between aPT and thrombosis (P = 0.035), with 70% of patients having unexplained thrombosis, aPT representing the sole aPL detected. aPT positivity was significantly more frequent in venous thrombosis than in arterial thrombosis (P = 0.004). Interestingly, we demonstrated for the first time that aPT IgG levels were higher in recurrent thrombosis than in isolated thrombosis (P = 0.013), leading us to propose a predictive level of recurrence for thrombosis. Conclusion Our results show that aPT are associated with thrombosis and demonstrate the interest of assessing both IgG and IgM aPT, in particular in venous thrombosis when conventional markers are negative. Quantification of aPT could predict recurrence of thrombosis and influence subsequent treatment strategy. Prospective clinical studies are now required to confirm these results.

The detection of anti-phosphatidylethanolamine autoantibodies (aPEs) has been proposed to improve the diagnosis and management of patients presenting clinical manifestations of antiphospholipid syndrome (APS), such as thrombosis, and who are persistently negative for conventional markers. After selecting the most specific ELISA for their detection, we evidenced the interest of aPEs in the exploration of thrombosis when APS conventional markers were negative through a 1-year retrospective study including 1131 consecutive patients routinely tested for aPEs. To validate this result, we assessed aPEs in a newly selected population of 77 patients with unexplained deep vein thrombosis (DVT). With a total prevalence of 19.5%, we confirmed the interest of aPE detection in patients with unexplained DVT who were devoid of other aPLs markers. Since endosomal compartment, a source of ROS production, has been recently identified as the cellular target of aPEs in vitro, we then investigated an association between aPE positivity and reactive oxygen species (ROS) production by measuring the production of thiobarbituric acid-reactive substances. We showed, for the first time, a significant association between aPE positivity and systemic ROS production in patients which led us to hypothesize a new mechanism of action of aPEs in thrombosis through a signaling related to oxidative stress.

| 899 with ARDS (16%) without any specific aPL profile. Thus, it is difficult to draw conclusions based on so few aPL-positive patients. Because of the lack of a standardized assay and aPL heterogeneity, their detection is dependent on enzyme-linked immunosorbent assays (2), which were provided by different manufacturers in the 2 studies. In particular, coating or buffer composition can influence the interaction between cardiolipin and cofactor proteins (3), and can influence whether antibodies are detected or not. Moreover, knowing the difficulties of interpreting aPL positivity, we pay close attention to their routine detection. All positive samples are controlled in duplicate, and, to address the specificity, the absorbance of uncoated wells treated in the same conditions is subtracted in order to avoid nonspecific binding (4). In contrast to Frapard and colleagues who focused on patients in the ICU, we also included patients with severe COVID-19 who were not admitted to the ICU and observed that 29% were positive for IgG aCL (Figure 1A). There was no difference in IgG aCL levels between aCL-positive patients admitted versus those not admitted to the ICU (Figure 1B), suggesting that IgG aCL positivity in severe COVID-19 is not attributable to the pulmonary complications alone.If the link between viral infection and aPL detection is well described in the literature (5), the relationship between aPL positivity and lung injury is not obvious. To support their hypothesis, Frapard et al referenced a study by Wiedermann et al (6), in which an association between aPL positivity and lung injury was not shown except in 1 patient with catastrophic antiphospholipid syndrome (CAPS). In the literature, the association between aPL positivity and ARDS is rarely described and only reported in the context of APS or CAPS (7). Consistent with our results, Zuo et al ( 8) recently reported that half of patients hospitalized for