ESPN 51th Annual Meeting

ESPN 2018


 
Two interesting cases of prolonged classical pathway convertase activity: C4NeF and a non-autoantibody serum factor
MARLOES AHM MICHELS 1 NICOLE CAJ VAN DE KAR 1 MARCIN OKRÓJ 2 ANNA M BLOM 3 SANNE AW VAN KRAAIJ 1 LAMBERTUS PWJ VAN DEN HEUVEL 1 ELENA B VOLOKHINA 1

1- RADBOUD UNIVERSITY MEDICAL CENTER
2- MEDICAL UNIVERSITY OF GDAńSK
3- LUND UNIVERSITY
 
Introduction:

C3 glomerulopathy (C3G) is characterized by overactivation of the complement system, particularly of the alternative pathway. This can be caused by autoantibodies such as C3 nephritic factor (C3NeF) or by mutations in complement genes. Recent findings reveal presence of so-called C4 nephritic factors (C4NeFs) in some C3G cases. These may contribute to the complement dysregulation by stabilizing the C3 convertase of the classical pathway (CP). However, studies and routine diagnostic procedures are scarce. Using a recently described method for measuring convertase activity in whole serum, we detected C4NeF activity in a cohort of patients with complement-mediated renal diseases. 

Material and methods:

Convertase stability was measured in a hemolytic assay using the C5-blocker eculizumab to separate the CP into two steps: formation of C3/C5 convertases by test sera in a time-variable step 1 and formation of lytic membrane attack complexes in a standardized step 2 for readout. Samples of 17 healthy controls and 47 patients with (suspected) C3G and closely related complement-mediated disorders were analyzed.

Results:

Healthy control convertase activity profiles consistently showed maximal convertase activity after 0.5-2.5 min; activity returned to background levels at t=10. In contrast, 2/47 (4%) patients showed significant convertase activity at t=10. This prolonged activity indicates presence of CP convertase-stabilizing factors. We confirmed presence of a stabilizing autoantibody in the first patient, which was also positive for C3NeF. The Ig-fraction from the second patient’s serum did not support convertase stabilization when added to control serum, indicating another non-autoantibody factor is present causing increased convertase half-life. The nature of this factor is under investigation.

Conclusions:

Prolonged CP convertase activity was found in two patients: one case was Ig-mediated, a C4NeF, and one was not. This study offers new opportunities for detection of (previously unrecognized) convertase-dysregulating factors of the CP in patients with various complement-mediated renal diseases.