Does complement activation cause endothelial injury and elevated D-dimer by promoting thrombosis?

Complement activation is a well-established driver of endothelial dysfunction and systemic hypercoagulability. Evidence confirms that the activation of the complement cascade initiates a series of events—including vascular injury and the induction of a prothrombotic state—that ultimately result in elevated D-dimer levels, a hallmark of active fibrinolysis.

Endothelial injury and activation

Complement activation directly damages the vascular lining through both signaling molecules and physical membrane disruption:

• Anaphylatoxin signaling: C5a binds to receptors (C5aR1) on endothelial cells, triggering the degradation of the glycocalyx—the protective layer of the vessel wall. This leads to RhoA-mediated cell stiffening and reduced nitric oxide bioavailability, which are primary markers of endothelial dysfunction.
• Adhesion and recruitment: C5a stimulation upregulates critical adhesion molecules, such as ICAM-1, VCAM-1, and E-selectin. These molecules facilitate the recruitment of leukocytes, further intensifying local inflammation and vascular permeability.
• Membrane Attack Complex (MAC): The formation of the terminal MAC (C5b-9) on endothelial surfaces causes injury. At sublytic concentrations, MAC induces noncanonical NF-κB signaling, promoting the release of pro-inflammatory cytokines like IL-1β and driving a procoagulant shift in the vessel wall.

Mechanisms of thrombosis

The complement system promotes thrombosis through extensive crosstalk with the coagulation cascade:

• Tissue Factor (TF) upregulation: C5a fragments stimulate monocytes and neutrophils to produce Tissue Factor, the primary initiator of the extrinsic coagulation pathway. This axis is a major contributor to microvascular thrombosis in inflammatory diseases.
• Platelet modulation: The terminal complement complex (sC5b-9) induces platelets to release ADP, which promotes aggregation and fibrin deposition.
• Coagulation feedback: The relationship is bidirectional; coagulation proteases like thrombin and Factor Xa can directly cleave C5 into its active form (C5a), creating a self-perpetuating cycle of "immunothrombosis" where inflammation and clotting reinforce one another.

Clinical implications for D-dimer

D-dimer serves as a clinical biomarker for the fibrinolysis that occurs following complement-mediated clotting:

• Biomarker correlation: In systemic inflammatory conditions such as COVID-19 and Systemic Lupus Erythematosus (SLE), decreased serum C3 and C4 (indicating high complement consumption) are strongly associated with elevated D-dimer levels.
• Thromboinflammation: Because complement activation triggers the coagulation cascade, the subsequent breakdown of resulting fibrin clots leads to the elevation of D-dimer, reflecting the degree of underlying thromboinflammatory activity.

Bottom line

Complement activation is a primary driver of endothelial injury and thrombosis. It promotes a prothrombotic environment via Tissue Factor expression and platelet activation, leading to fibrin formation and a subsequent rise in D-dimer as those clots are degraded.