Identification of complement regulatory domains in human factor H

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• Factor H is a key regulator of the complement system, preventing damage to host cells.
• It comprises 20 short consensus repeat (SCR) domains, crucial for its C3b/C4b-binding and regulatory functions.

Purpose of the Study:

• To identify the specific short consensus repeat (SCR) domains within Factor H responsible for its complement regulatory cofactor activity.
• To determine the minimal and optimal SCR units required for Factor H's function in C3b cleavage and cell protection.

Main Methods:

• Constructing and expressing deletion mutants of Factor H's N-terminal SCR domains (1-5) in Chinese hamster ovary (CHO) cells.
• Assessing the cofactor activity of secreted Factor H fragments for factor I-mediated C3b cleavage.
• Evaluating cell protection against complement-mediated lysis using cells expressing functional Factor H constructs.

Main Results:

• Deletion of SCR 2 or 3 abolished cofactor activity, while SCR 1, 4, and 5 deletions retained partial or full activity.
• The SCR 1-3 unit was sufficient for cofactor activity, but SCR 1-4 was necessary for full activity.
• Cells expressing a Factor H SCR 1-5 construct with a GPI anchor were protected against complement-mediated cytotoxicity.

Conclusions:

• The N-terminal SCR domains of Factor H, specifically SCRs 1-3, are critical for its cofactor activity in complement regulation.
• A functional unit of three to four consecutive SCR domains is likely involved in the regulation of C3 convertase by Factor H.
• These findings delineate the functional domains of Factor H and provide insights into complement system regulation.