Coagulation, although primarily regulated by platelets, endothelial cells, and clotting factors, can also be influenced by molecules that are not traditionally seen as related to coagulation, including cytokines, hormones, metabolites, reactive oxygen species, acute phase reactants, and more. Here, we derive pseudoserum or clotting factor-depleted fractions from control, type II diabetes mellitus, and Long COVID platelet-poor plasma (PPP) samples, and expose them to purified, exogenous fibrinogen obtained from healthy donors. Thrombin-induced fibrin networks were then formed and visualized using light and scanning electron microscopy. The results demonstrate that pseudoserum can greatly influence the organization, density, and ultrastructure of fibrin networks formed from purified fibrinogen, emphasizing the role of non-clotting factors in fibrin formation. Fibrin networks formed from purified fibrinogen exposed to control pseudoserum appear homogeneous, exhibiting organised architecture with few regions of unusual density or aggregates, whereas the networks formed using patient pseudoserum show disorganisation, regions of density, fibre-like strands, and anomalous aggregates. These abnormalities are also observed in patient PPP samples, suggesting that fibrin network characteristics in PPP samples are also significantly influenced by non-clotting factors and are somewhat independent of endogenous fibrinogen. Furthermore, fibrinolysis was significantly reduced in the patient groups, demonstrating the ability of pseudoserum to influence the susceptibility of fibrin networks to plasmin-induced degradation. The ability of pseudoserum to drive these changes, despite the essential absence of endogenous fibrinogen and other classical clotting factors, suggests that soluble molecules retained in pseudoserum can directly modify fibrinogen's structural conformation and functionality, influence thrombin-mediated fibrin formation and polymerization, and/or impact Factor XIII's crosslinking capabilities. This study provides a systems-level perspective on the influence of pseudoserum on fibrin networks and highlights the potential of serum and other clotting factor-depleted fractions to yield deeper mechanistic and diagnostic insights into coagulation.
