Effect of HIT Components on the Development of Breast Cancer Cells

The Binding of the SARS-CoV-2 Spike Protein to Platelet Factor 4: A Proposed Mechanism for the Generation of Pathogenic Antibodies

Pathogenic platelet factor 4 (PF4) antibodies contributed to the abnormal coagulation profiles in COVID-19 and vaccinated patients. However, the mechanism of what triggers the body to produce these antibodies has not yet been clarified. Similar patterns and many comparable features between the COVID-19 virus and heparin-induced thrombocytopenia (HIT) have been reported. Previously, we identified a new mechanism of autoimmunity in HIT in which PF4-antibodies self-clustered PF4 and exposed binding epitopes for other pathogenic PF4/eparin antibodies. Here, we first proved that the SARS-CoV-2 spike protein (SP) also binds to PF4. The binding was evidenced by the increase in mass and optical intensity as observed through quartz crystal microbalance and immunosorbent assay, while the switching of the surface zeta potential caused by protein interactions and binding affinity of PF4-SP were evaluated by dynamic light scattering and isothermal spectral shift analysis. Based on our results, we proposed a mechanism for the generation of PF4 antibodies in COVID-19 patients. We further validated the changes in zeta potential and interaction affinity between PF4 and SP and found that their binding mechanism differs from ACE2-SP binding. Importantly, the PF4/SP complexes facilitate the binding of anti-PF4/Heparin antibodies. Our findings offer a fresh perspective on PF4 engagement with the SARS-CoV-2 SP, illuminating the role of PF4/SP complexes in severe thrombotic events.

Breast cancer cell-based ELISA: a potential material for better detection of heparin-induced thrombocytopenia antibodies

Heparin-induced thrombocytopenia (HIT) is caused by newly formed platelet-activating antibodies against complexes formed between platelet factor 4 (PF4) and heparin (H). HIT can result in life-threatening complications; thus, early detection of HIT antibodies is crucial for the treatment of the disease. The enzyme-linked immune absorbance assay (ELISA) for the identification of HIT antibodies is widely used in many laboratories, but in general, this test provides only ∼50% accuracy while other methods show multiple limitations. Here, we developed a new cell-based ELISA to improve the detection of HIT antibodies. Instead of immobilizing PF4 or PF4/H complexes directly onto a plate as in the standard ELISA, we added the complexes on breast cancer cells, i.e., cell line MDA-MB-231, and applied the same protocol for antibody detection. Using confocal laser scanning microscopy and flow cytometry for the characterization of bound complexes, we identified two types of HIT-mimicked antibodies (KKO and 1E12), which were able to differentiate from the non-HIT antibody (RTO). PF4-treated MDA-MB-231 cells allowed binding of HIT-mimicked antibodies better than PF4/H complexes. With human sera, the cell-based ELISA allowed better differentiation of clinically relevant from non-clinically relevant HIT antibodies as compared with the standard ELISA. Our findings provide a potential approach that contributes to the development of better assays for the detection of HIT antibodies.

When Will Fondaparinux Induce Thrombocytopenia?

The pentasaccharide Fondaparinux, a synthetic selective factor Xa inhibitor, is one of the safest anticoagulants in the heparin family that is recommended as an alternative drug for patients with hypersensitivity to other drugs such as heparin-induced thrombocytopenia (HIT). However, some observations of Fondaparinux-induced thrombocytopenia (FIT) have been reported while others claimed that FIT does not occur in patients with fondaparinux therapy, indicating that the mechanism of FIT remains controversial. Here, we utilized different methodologies including dynamic light scattering, immunosorbent and platelet aggregation assays, confocal laser scanning microscopy, and flow cytometry to gain insights into FIT. We found that at a certain concentration, Fondaparinux formed sufficient large and stable complexes with PF4 that facilitated binding of the HIT-like monoclonal KKO antibody and enhanced platelet aggregation and activation. We proposed a model to describe the role of Fondaparinux concentration in the formation of complexes with platelet factor 4 and how it promotes the binding of KKO. Our results clarify controversial observations of FIT in patients as each contains a dissimilar PF4:Fondaparinux concentration ratio.