Covid-19 Spike Proteins Form Antigenic Complexes with Platelet Factor 4 that Trigger Production of Pathogenic Antibodies: A New Mechanism of Autoimmunity Thrombocytopenia

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Until March 2021, the WHO reported around 115.887 million infected cases and 2.574 million deaths globally since the start of the pandemic. A 71.4% of the deaths in Covid-19 patients had abnormal coagulation profiles. Recently, different types of pathogenic antibodies Abs that activate platelets via FcγRIIa and caused the worst clotting, have been detected in Covid-19 patients. However, the mechanism of what triggering the body to produce these antibodies has not yet been clarified.Recently, we utilized nanobiophysical methods to discover a new mechanism of autoimmunity mediated by anti-platelet factor 4 (PF4) antibodies (aPF4 Abs) (Nguyen et al, NCOMMS, 2017). These aPF4 Abs have a high binding affinity to PF4 as they cluster PF4 molecules and form PF4/antibody complexes that expose binding epitope for heparin (H)-induced thrombocytopenia (HIT) antibodies. In this project, we aim to transfer our technologies to realize the role of Covid-19 spike proteins (SP) in the production of pathogenic antibodies in Covid-19 patients. We hypothesize that SP have a capacity in clustering PF4 molecules similar to the aPF4 Abs, forming PF4/SP complexes that promote immune cells to produce different types of pathogenic antibodies, which activate platelets and other blood cells via FcγRIIa.To prove this mechanism, we first identify if SP have a high binding affinity to PF4 and also to PF4/Heparin complexes via evaluation of binding forces and kinetic properties of the interactions. High binding affinity indicates that SP can cluster PF4 and PF4/H complexes.We next test if the resulting PF4/SP and PF4/H/SP complexes allow binding different types of platelet-activating antibodies. As a model, we will use well-characterized monoclonal HIT Abs (the KKO), the anti-PF4 Abs, and the kidney Abs. If these bindings occur, we suspect that immune cells may produce a high concentration of these antibodies in Covid-19 patients. We will isolate and determine the concentration of these antibodies including aPF4/SP Abs, aPF4/H/S Abs, HIT Abs, aPF4 Abs, and kidney Abs in Covid-19 patients, and test if they contain FcγRIIa. If concentrations of these Abs are high in sera and they activate platelets, our proposed mechanism is proven. Finally, we aim to identify the heparins that can reduce the binding affinity of SP to the ACE2, PF4, and PF4/H complexes. As we found previously that the synthetic heparins of different lengths exhibited higher activity than the animal-derived heparins, we will apply nanobiophysical methods to identify potential heparins that can reduce the reactivity of SP. We hypothesize that the patches of the positive charge of the virus can be blocked by the negatively charged heparin of equal size. Taken together, this proposal will provide a new mechanism of Covid-19 SP in clustering PF4 that leads to the production of aggressive pathogenic antibodies in Covid-19 patients. Furthermore, we may be able to identify heparins that reduce the reactivity of SP.