Persistent Hypercoagulability and Further Characterization of Microclot Complexes in Long COVID

Background: Growing evidence suggests persistent thrombotic endothelial damage (together with elevated (fibrinaloid) microclot complexes (FMCs)) and immune dysfunction in the pathophysiology of Long COVID. Recently we proposed that there are different FMC phenotypes. Here we seek to determine the nature of these FMCs and aggregates in platelet-poor plasma (PPP) by using different markers, as well as thromboelastography (TEG) to assess for hypercoagulability of samples. Material and Methods: Whole-blood and PPP from control (n=19) and Long COVID (n=20) participants were assessed by thromboelastography. FMCs were quantified by imaging flow cytometry of Thioflavin-T (ThT)-stained PPP, 10X diluted PPP, and resuspended PPP pellets. The resuspended pellets were separately stained with a CD62P-PE antibody or Hoechst 33342 to label aggregates and FMCs containing amyloid, platelet, and nuclear material. ThT and CellMask Red were co-stained for confocal microscopy. ThT and myeloperoxidase (MPO), and ThT, Congo Red, and Hoechst were co-stained for fluorescence and polarized microscopy. Whole-blood smears were imaged by scanning electron microscopy (SEM). Results: Long COVID samples showed pronounced hypercoagulability in both whole blood and PPP, with shortened R, K and TMRTG and elevated alpha-angle and MRTG, but unchanged MA and TTG, indicating altered clotting kinetics. Persistence of this phenotype in PPP implicates soluble plasma constituents. ThT-positive FMCs were significantly increased in Long COVID across undiluted, diluted, and resuspended pellet samples; counts were processing-sensitive and a substantial ThT-positive population remained in the supernatant after centrifugation, indicating heterogeneity in density. Across probes, leukocyte material was the most abundant, then platelet material, and ThT-positive FMCs were the least abundant, with the three populations exhibiting unique morphology and occupying distinct size domains. Platelet-derived material was significantly elevated in Long COVID, whereas nuclear material was not. Co-stained samples subject to confocal, fluorescence, and polarized microscopy imaging showed that FMCs are heterogeneous, including events positive for ThT, CellMask, Hoechst, MPO, and Congo Red, and also a distinct subset of membrane-free, ThT-only events. Conclusion: In this Long COVID cohort, plasma is characterised by hypercoagulability and an increased burden of ThT-positive FMCs that are numerically minor relative to, and morphologically distinct from, aggregates and amyloidogenic FMCs marked with platelet- and leukocyte-derived material. The increased burden of platelet debris in PPP is likely indicative of persistent platelet activity. The existence of membrane-free, ThT-only FMCs, in addition to FMCs associated with cellular material, confirms an amyloid-dominated FMC population. Furthermore, positive Congo Red signal further confirms the amyloid nature of FMCs in PPP.