Factor H as a regulator of the classical pathway activation

Area of Science:

• Immunology
• Complement System Biology

Background:

• The classical complement pathway, initiated by C1q, plays a dual role in host defense and tissue homeostasis.
• Dysregulated classical pathway activation can lead to tissue damage, necessitating precise regulatory mechanisms.
• Factor H is a known regulator of the alternative complement pathway.

Purpose of the Study:

• To investigate the potential role of Factor H as a regulator of the classical complement pathway.
• To determine if Factor H can directly interfere with C1q-mediated activation.
• To explore Factor H's impact on C1q interactions with apoptotic cells.

Main Methods:

• In vitro assays measuring C1q-ligand binding.
• Assessment of classical pathway activation in the presence of Factor H.
• Analysis of monocyte-cell interactions involving C1q and Factor H.

Main Results:

• Factor H directly competed with C1q for binding to anionic phospholipids, lipid A, and E. coli.
• Factor H significantly downregulated classical pathway activation induced by these C1q ligands.
• Factor H reduced C1q-enhanced monocyte uptake of apoptotic cells.

Conclusions:

• Factor H acts as a direct down-regulator of the classical complement pathway, independent of its alternative pathway function.
• Factor H's ability to compete for C1q ligands suggests a role in balancing inflammatory responses in autoimmunity and infection.
• Factor H may mitigate inflammation associated with C1q deposition on apoptotic cells.