Complement activation on neutrophils initiates endothelial adhesion and extravasation

Area of Science:

• Immunology
• Rheumatology
• Cell Biology

Background:

• Neutrophils play a key role in inflammatory diseases.
• The K/BxN mouse model of inflammatory arthritis requires the alternative pathway (AP) of complement and Fc gamma receptors (FcγRs), but not the classical pathway, for disease development.

Purpose of the Study:

• To investigate the differential requirement of complement pathways in K/BxN arthritis.
• To elucidate the mechanism of neutrophil activation and extravasation in this model.

Main Methods:

• K/BxN serum injection in mice.
• Measurement of complement activation on neutrophils via C3 fragment deposition.
• In vivo two-photon microscopy.
• Assessment of neutrophil extravasation and endothelial barrier integrity.
• Pharmacological inhibition of C5a receptor.

Main Results:

• Complement activation (CA) on circulating neutrophils occurs rapidly after K/BxN serum injection, requiring AP factor B and FcγRs.
• CA on neutrophils is essential for their extravasation and C5a generation at inflammatory sites.
• C5a stimulates neutrophil protease release, degrading VE-cadherin and promoting transendothelial migration.
• C5a receptor antagonism inhibits protease release and neutrophil migration.

Conclusions:

• The alternative pathway of complement and FcγRs initiate inflammatory cascades by triggering intravascular neutrophil-endothelial interactions.
• Neutrophil complement activation is critical for neutrophil extravasation and subsequent tissue damage in inflammatory arthritis.
• These findings may apply to neutrophil extravasation in other inflammatory conditions.