Persistent clotting protein pathology in Long COVID/Post-Acute Sequelae of COVID-19 (PASC) is accompanied by increased levels of antiplasmin
Pretorius E, Vlok M, Venter C, Bezuidenhout JA, Laubscher GJ, Steenkamp J, Kell DB
Cardiovascular Diabetology (2021) · n=49
The foundational microclot-in-PASC observation paper — fluorescent staining and proteomics on plasma from 11 PASC patients identified amyloid fibrin deposits resistant to trypsin-mediated fibrinolysis, with α2-antiplasmin elevated approximately eightfold within the resistant pellet versus healthy controls.
This 2021 Cardiovascular Diabetology paper from Pretorius, Kell, and colleagues at Stellenbosch and Liverpool is the foundational observation paper for the fibrinaloid-microclot signal in post-acute SARS-CoV-2 sequelae (PASC). The investigators applied fluorescent thioflavin-T (ThT) microscopy and shotgun proteomics — built on two-step trypsin digestion followed by liquid chromatography-mass spectrometry — to platelet-poor plasma from 13 healthy controls, 10 type 2 diabetes patients, 15 acute COVID-19 patients, and 11 long-COVID/PASC patients, alongside enzyme-linked immunosorbent assay for serum amyloid A and CD62P / PAC-1 platelet-activation markers by flow cytometry. The headline observation is that anomalous amyloid-staining fibrin deposits ("microclots") were detected in both acute COVID-19 and long-COVID/PASC plasma samples, and that the long-COVID deposits were resistant to fibrinolytic dissolution even after trypsin pre-treatment — meaning the structures persist beyond the kinetic window in which normal fibrin polymers are cleared by the body's fibrinolytic machinery.
The proteomic characterisation of the trypsin-resistant pellets quantified the protein cargo trapped within the microclot deposits and is the most-cited structural finding in the paper. α2-antiplasmin (the principal endogenous fibrinolysis inhibitor) was reported approximately 7.98-fold higher in the long-COVID pellet versus healthy-control pellet (p=0.0002); fibrinogen chains were significantly elevated; serum amyloid A4 was 17.5-fold elevated (p=0.01); and von Willebrand factor was approximately 10.2-fold higher in PASC samples than in controls. The authors frame this profile as evidence of a self-reinforcing pathology in which trapped α2-antiplasmin locally inhibits fibrinolysis around the same structures it is being sequestered into, accompanied by markers of platelet hyperactivation. The paper's discussion proposes that "continued anticoagulation therapy" warrants investigation as a means to support the fibrinolytic system's recovery in PASC. The microclot signal documented here has subsequently been cited as a candidate mechanism for the vascular-coagulopathy hypothesis of long COVID in the Davis 2023 Nat Rev Microbiol review and as the empirical basis for the broader theoretical framework articulated in the Kell, Laubscher and Pretorius 2022 Biochemical Journal hypothesis paper.
This is a hypothesis-generating characterisation study, not a causally-established mechanism — the paper convincingly demonstrates that ThT-positive amyloid-fibrin deposits are detectable in PASC plasma, that those deposits resist enzymatic fibrinolysis, and that they sequester a distinctive protein cargo; it does not demonstrate that the microclots are the cause of long-COVID symptomatology. The framework articulated by the broader Kell/Pretorius programme — that fibrinaloid microclots block capillary perfusion and thereby drive PASC symptoms — is a hypothesis built on top of this observation paper, not established by it; see the dedicated Kell, Laubscher and Pretorius 2022 entry for that synthesis. The sample size — 11 PASC patients, 15 acute COVID-19, 10 type 2 diabetes, 13 healthy controls — is small for a discovery study, and the authors themselves acknowledge in the discussion that "statistical power of the current set is limited" and recommend confirmation in larger cohorts with additional methods. The ThT staining methodology used to visualise microclots is reproducible but is not the same as the histopathological characterisation of in-vivo amyloid deposits — what is demonstrated is amyloid-character fluorescence in ex-vivo plasma fibrin structures, not in-situ tissue amyloid. Subsequent independent groups have variously replicated the basic microclot-detection finding while diverging on the clinical-symptom correlation; the Pretorius and Kell 2024 Research and Practice in Thrombosis and Haemostasis reassessment acknowledges that the microclot signal is detectable across multiple post-viral and POTS phenotypes, not only PASC. The clinical implication articulated in the paper — that "continued anticoagulation therapy" may help — is a proposed direction motivated by the observed pathology, not a tested treatment recommendation, and any anticoagulation in PASC patients carries the same haemorrhagic-risk profile as in any other indication and has not been validated in a controlled PASC trial.
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