Several epitopes on native human complement C9 are involved in interaction with the C5b-8 complex and other C9...

Area of Science:

• Immunology
• Complement System Biology

Background:

• Human complement protein C9 is the terminal component of the complement cascade.
• C9 polymerizes to form the membrane attack complex (MAC), crucial for cell lysis.
• Previous studies identified monoclonal antibodies (mAbs) inhibiting C9 hemolytic activity.

Purpose of the Study:

• To characterize the epitopes of human C9 recognized by inhibitory monoclonal antibodies.
• To investigate the role of specific C9 epitopes in C9 polymerization and MAC formation.
• To elucidate the relationship between C9 polymerization and its lytic function.

Main Methods:

• Enzyme-linked immunosorbent assay (ELISA) to assess mAb reactivity with monomeric C9 (mC9), polymerized C9 (pC9), and SC5b-9.
• Flow cytometry to analyze mAb binding to MAC on rabbit erythrocytes.
• Investigation of mAb effects on zinc-induced C9 polymerization.

Main Results:

• Monoclonal antibodies were categorized into two groups based on their binding affinities to different C9 forms.
• One mAb targeting the C9b C-terminal region showed no inhibition of lysis or polymerization.
• Nine mAbs against the C9a region exhibited varied inhibitory effects on hemolysis and/or polymerization.
• mAb binding patterns suggest distinct epitopes are involved in C9-C5b-8 and C9-C9 interactions.

Conclusions:

• Specific epitopes on C9 regulate its polymerization and interaction with other complement components.
• C9 polymers are not essential for the hemolytic activity of the complement system.
• Understanding C9 epitope mapping provides insights into complement-mediated effector functions.