C4 Nephritic Factors in C3 Glomerulopathy: A Case Series Yuzhou Zhang, PhD, Nicole C. Meyer, BS, Fernando C. Fervenza, MD, Winnie Lau, MD, Adam Keenan, BS, Gabriel Cara-Fuentes, MD, Dingwu Shao, BS, Aalia Akber, MD, Veronique Fremeaux-Bacchi, MD, PhD, Sanjeev Sethi, MD, Carla M. Nester, MD, Richard J.H. Smith, MD  American Journal of Kidney Diseases  Volume 70, Issue 6, Pages 834-843 (December 2017) DOI: 10.1053/j.ajkd.2017.07.004 Copyright © 2017 The Authors Terms and Conditions

Figure 1 The complement cascade. A simplified schematic of the alternative, lectin, and classical pathways of complement. Each of these initiating pathways leads to the generation of C3 convertases (C3bBb or C4b2a) and C5 convertases (C3bBbC3b or C4b2aC3b). C5 convertases cleave C5 and trigger the generation of membrane attack complex (MAC) or C5b-9. The presence of C4 nephritic factor (C4Nef) and/or C3Nef stabilizes C3 convertases, resulting in hyperactivity of the alternative and terminal pathways. Abbreviations: FBAA, factor B autoantibody; FHAA, factor H autoantibody; MASP, mannose-associated serine protease; P, properdin. American Journal of Kidney Diseases 2017 70, 834-843DOI: (10.1053/j.ajkd.2017.07.004) Copyright © 2017 The Authors Terms and Conditions

Figure 2 C4 nephritic factor (C4NeF) assay. Antibody (Ab)-sensitized sheep erythrocytes (EA) are used as index cells to generate EAC1C4b by incubating pooled normal human serum in the presence of TTHA (a strong magnesium chelator) and calcium. After washes, EAC1C4b2a (C3 convertase of the classical pathway) is made by adding purified C2. To test for the presence of C4b2a autoantibodies (C4Nefs), patient-purified immunoglobulin G (IgG) is added and cells are allowed to decay for different periods (7.5, 15, 22.5, and 30 minutes). After each time point, 50 μL of the cell mixture is removed. An equal volume of rat-EDTA serum (1:19 dilution) is added as a source of complement proteins C3 through C9. To stop the reaction after 1 hour, 150 μL of cooled EDTA-GVB buffer is added. Nonlysed cells are removed by centrifugation and hemoglobin in supernatant (200 μL) is measured at optical density at 415nm (OD415). American Journal of Kidney Diseases 2017 70, 834-843DOI: (10.1053/j.ajkd.2017.07.004) Copyright © 2017 The Authors Terms and Conditions

Figure 3 Kidney biopsy from patient 2. (A) Periodic acid–Schiff staining shows a membranoproliferative pattern with mesangial and capillary loop hypercellularity. (B) Glomerular basement membrane (GBM) duplication with deposition of hyaline material is seen on Masson trichrome stain (original magnification, ×40). (C-E) Immunofluorescence findings are positive for (C) C3, but negative for (D) C4d and all other immunoreactants (immunoglobulin G is shown). (F) Electron microscopy shows diffuse effacement of podocyte foot processes and thickening of the GBM by a combination of subendothelial deposits, basement membrane duplication, and mesangial cell interposition. American Journal of Kidney Diseases 2017 70, 834-843DOI: (10.1053/j.ajkd.2017.07.004) Copyright © 2017 The Authors Terms and Conditions

Figure 4 C4 nephritic factors (C4NeFs). (A) In each of 5 patients, purified immunoglobulin Gs (IgGs; 500μg) had varying ability to prolong the half-life of C4b2a and cause hemolysis. (B) The effect was dose dependent (50-500μg of IgG), confirming the ability of these autoantibodies to stabilize C4b2a (green line, pooled normal IgG; patient-purified IgGs: dark blue, patient 1; red, patient 2; purple, patient 3; light blue, patient 4; orange, patient 5). (C, D) In addition, the C4NeFs protected C4b2a from decay by (C) soluble CR1 (sCR1) and (D) C4 binding protein (C4bp; red, patient 1; blue, control). American Journal of Kidney Diseases 2017 70, 834-843DOI: (10.1053/j.ajkd.2017.07.004) Copyright © 2017 The Authors Terms and Conditions