Abstract: SA-PO628
Detection and Characterization of Prolonged Classical Pathway Convertase Activity: C4 Nephritic Factor and a Non-Autoantibody Serum Factor
Session Information
- Glomerular Diseases: Immunology, Inflammation - II
November 09, 2019 | Location: Exhibit Hall, Walter E. Washington Convention Center
Abstract Time: 10:00 AM - 12:00 PM
Category: Glomerular Diseases
- 1202 Glomerular Diseases: Immunology and Inflammation
Authors
- Michels, Marloes, Amalia Children's Hospital, Radboud university medical center, Nijmegen, Netherlands
- Van De Kar, Nicole, Amalia Children's Hospital, Radboud university medical center, Nijmegen, Netherlands
- Okroj, Marcin, Medical University of Gdansk, Gdansk, Poland
- Blom, Anna, Lund University, Malmö, Sweden
- van Kraaij, Sanne, Radboud university medical center, Nijmegen, Netherlands
- Van den heuvel, Lambertus P.W.J., Amalia Children's Hospital, Radboud university medical center, Nijmegen, Netherlands
- Volokhina, Elena, Amalia Children's Hospital, Radboud university medical center, Nijmegen, Netherlands
Group or Team Name
- on behalf of the COMBAT consortium
Background
C3 glomerulopathy (C3G) is a renal disease caused by overactivity of the complement system, particularly of the alternative pathway (AP). Autoantibodies such as C3 nephritic factors (C3NeFs) or mutations in complement genes are commonly found as pathogenic causes. Recent findings also reveal the presence of C4 nephritic factors (C4NeFs) in some C3G cases. By stabilizing the convertases of the classical pathway (CP), these autoantibodies contribute to the complement dysregulation in C3G. In this study, we investigated C4NeF activity in a cohort of patients with complement-mediated renal diseases.
Methods
We used a recently described hemolytic method to measure convertase activity directly in serum, using a C5-blocker to separate the CP into 2 steps: a time-variable first step for convertase formation out of test serum and a standardized second step for hemolysis readout.
Results
Serum samples of 17 healthy controls and 47 patients with (suspected) C3G and closely related complement-mediated disorders were analyzed. Convertase activity levels in controls consistently returned to background levels after 10 min. In contrast, convertase activity was significantly prolonged until 20 min in 2/47 (4%) patients (P1 and P2), indicating the presence of CP convertase-stabilizing factors. Addition of purified Igs from P1 to control serum supported prolonged convertase activity, confirming the autoantibody nature of the stabilizing factor. Previously, the Igs of this patient were also shown to have AP convertase-stabilizing activity, i.e. C3NeF activity. Further investigation showed that both the C3NeF and C4NeF activity resided in the Ig fractions of the kappa light chain type. In addition, both the AP and CP convertase-stabilizing activities remained present over the disease course of the patient. In contrast to P1, the Igs of P2 did not support convertase stabilization when added to control serum, indicating a non-autoantibody factor caused the increased convertase half-life. The nature of this factor, possibly genetic, is currently under investigation.
Conclusion
This study offers new opportunities for the detection and characterization of (previously unrecognized) CP convertase-dysregulating factors in patients with complement-mediated renal diseases.