Sensitive and specific assays for C3 nephritic factors clarify mechanisms underlying complement dysregulation

Area of Science:

• Immunology
• Complement System Biology
• Renal Pathology

Background:

• C3 nephritic factors (C3NFs) are autoantibodies implicated in renal disease by prolonging C3 convertase activity.
• Their precise role in disease pathogenesis remains debated.
• Current methods for detection and characterization are limited.

Purpose of the Study:

• To optimize and compare a panel of assays for identifying and interrogating C3 nephritic factor activities.
• To characterize the functional impact of C3NFs on complement regulation.
• To provide a rational approach for C3NF detection in patients.

Main Methods:

• Development and comparison of an assay panel for C3NF detection.
• Assessment of autoantibody binding and convertase stabilization in patient samples.
• Evaluation of C3NF effects on complement regulatory proteins (Factor H, DAF, CR1).
• Biacore analysis of purified IgG samples to confirm convertase stabilization and activity.

Main Results:

• 48 out of 101 patients with renal disease tested positive for C3NFs.
• Properdin-dependent C3NF binding and convertase stabilization were detected in 39 and 36 samples, respectively.
• Most C3NFs inhibited convertase decay by Factor H, DAF, and CR1.
• Properdin-independent C3NFs did not affect terminal pathway activity, while properdin-dependent C3NFs enhanced it.
• Purified C3NFs significantly increased C3 convertase half-life and activity, with varying effects based on properdin dependence.

Conclusions:

• The developed assay panel provides a rational and effective approach to detect and characterize C3 nephritic factors.
• C3NFs exhibit distinct functional properties, including properdin dependence/independence, influencing complement activation.
• Characterization of C3NFs is crucial for understanding their role in renal disease pathogenesis.