Abstract WP326: Microclots, Erythrocytes, and the Coagulation Cascade in Long COVID

Abstract

Introduction: The coagulation cascade in mammalian blood consists of a series of enzyme activation events in which proteins are activated in the next step of the cascade via limited proteolysis, resulting in polymerization of fibrin and the activation of platelets. The extrinsic, intrinsic, and common pathways are well-described. While typically protective, these pathways may also promote pathologic thrombosis, which is a leading cause of death and disability. An emerging concept is that COVID-induced endothelial injury initiates systemic activation of the extrinsic pathway, marked by direct protein-protein interactions with soluble fibrinogen molecules. In controlled in vitro settings, microclots can be induced using an array of substances including SARS-CoV-2 S1 spike protein. In this study, scanning electron microscopy (SEM) was used to visually demonstrate the detailed features of microclot interaction with erythrocytes at an ultrastructural level. Methods: SEM was performed on citrated whole blood of healthy individuals and patients with Long COVID. Whole blood smears from healthy controls and patients with Long COVID were placed on a glass coverslip, fixed in 4% formaldehyde, dehydrated in serial alcohol steps, coated with carbon and viewed with a scanning electron microscope (Carl Zeiss, Germany). Results: Proteolytic resistant fibrin microclots, spanning a size range of submicron to 200 μm, have a profound effect on the structure and function of erythrocytes (Figure 1). Conclusion: Long COVID microclots are characterized by an anomalous form of fibrin that is resistant to proteolysis (fibrinolysis). In addition to the possibility of microvascular ischemia, persistent microclots alter erythrocyte structure and may also entrap other proteins and stimulate production of various autoantibodies. Further study on the mechanism for microclot formation in Long COVID may lead to desperately needed novel treatment strategies.