Complement in Kidney Disease: Core Curriculum 2015 Joshua M. Thurman, MD  American Journal of Kidney Diseases  Volume 65, Issue 1, Pages 156-168 (January 2015) DOI: 10.1053/j.ajkd.2014.06.035 Copyright © 2014 National Kidney Foundation, Inc. Terms and Conditions

Figure 1 Overview of the complement system. The complement system is activated through 3 distinct pathways: the classical pathway, mannose-binding lectin pathway, and alternative pathway. Activation through the classical and mannose-binding lectin pathways causes cleavage of the protein C4 and fixation of C4d (a fragment of C4) to nearby tissues. Activation through all pathways leads to cleavage of C3. Cleavage of C3 generates a soluble fragment (C3a) and tissue-bound fragment (C3b). Further proteolysis of C3b generates iC3b and finally C3d. Full activation of the complement system also generates C5a and C5b-9, important mediators of tissue inflammation and injury. American Journal of Kidney Diseases 2015 65, 156-168DOI: (10.1053/j.ajkd.2014.06.035) Copyright © 2014 National Kidney Foundation, Inc. Terms and Conditions

Figure 2 Mechanisms of complement activation in kidney disease. Immune complex deposition within glomeruli activates the classical pathway of complement in some forms of glomerulonephritis, and antibodies specific to renal antigens also activate the classical pathway. Uncontrolled activation of the alternative pathway of complement is associated with atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy, and the alternative pathway also has been implicated in antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis and poststreptococcal glomerulonephritis. Mannose-binding lectins (MBLs) have been detected in the kidneys of some patients with immunoglobulin A (IgA) nephropathy, membranous nephropathy, and poststreptococcal glomerulonephritis. Abbreviations: GBM, glomerular basement membrane; IgA, immunoglobulin A; MPGN, membranoproliferative glomerulonephritis. American Journal of Kidney Diseases 2015 65, 156-168DOI: (10.1053/j.ajkd.2014.06.035) Copyright © 2014 National Kidney Foundation, Inc. Terms and Conditions

Figure 3 Histology of atypical hemolytic uremic syndrome (aHUS). A kidney biopsy specimen from a patient with aHUS and dysregulated alternative pathway activity demonstrates typical findings of thrombotic microangiopathy. (A) Thrombi are seen within the glomerular capillaries (*). (B) A thrombus also is seen in an arteriole (#). Tissue was stained with Masson trichrome. Reproduced by permission from Macmillan Publishers Ltd: Fakhouri F and Fremeaux-Bacchi V. Does hemolytic uremic syndrome differ from thrombotic thrombocytopenic purpura? Nat Clin Pract Nephrol. 2007;3:679-687. American Journal of Kidney Diseases 2015 65, 156-168DOI: (10.1053/j.ajkd.2014.06.035) Copyright © 2014 National Kidney Foundation, Inc. Terms and Conditions

Figure 4 Biopsy characteristics of C3 glomerulopathy. The diagnosis of C3 glomerulopathy is based on the detection of C3 in the relative absence of immunoglobulin or classical pathway proteins. (A) C3 fragments are seen in the mesangium and capillary loops of a patient with C3 glomerulopathy. (B) C3 and (C) immunoglobulin G from the same patient with C3 glomerulopathy, demonstrating that some immunoglobulin deposition occasionally is seen. (D) Electron microscopy from a patient with dense-deposit disease demonstrates dense intramembranous deposits. (E) Electron-dense deposits are seen in the glomerular basement membrane of patients with C3 glomerulopathy, although they are not as dense or well defined as those in patients with dense-deposit disease. (F) Large humped subepithelial deposits sometimes are seen in patients with C3 glomerulopathy, resembling those seen in patients with postinfectious glomerulonephritis. Reproduced with permission from Macmillan Publishers Ltd: Pickering MC, D’Agati VD, Nester CM et al. C3 glomerulopathy: consensus report. Kidney Int. 2013;84(6):1079-1089 and with permission from BMJ Publishing Group Ltd: Servais A, Frémeaux-Bacchi V, Lequintrec M, et al. Primary glomerulonephritis with isolated C3 deposits: a new entity which shares common genetic risk factors with haemolytic uraemic syndrome. J Med Genet. 2007;44(3):193-199. American Journal of Kidney Diseases 2015 65, 156-168DOI: (10.1053/j.ajkd.2014.06.035) Copyright © 2014 National Kidney Foundation, Inc. Terms and Conditions

Figure 5 Complement regulation in the glomerulus by factor H. Factor H inhibits alternative pathway activation in the fluid phase and on the surface of cells and the basement membrane. It is a string-like protein; the complement regulatory region is at the amino terminus and a region that mediates binding to surfaces is at the carboxy terminus. Factor H defects in patients with atypical hemolytic syndrome are predominantly in the binding region of the protein, suggesting that the disease is caused by impaired complement regulation by factor H on endothelial cells and/or the glomerular basement membrane (GBM). Some patients with C3 glomerulopathy have an absolute deficiency in complement regulation by factor H. Disease in these patients may be caused by uncontrolled fluid phase activation of the alternative pathway or insufficient regulation on the GBM. American Journal of Kidney Diseases 2015 65, 156-168DOI: (10.1053/j.ajkd.2014.06.035) Copyright © 2014 National Kidney Foundation, Inc. Terms and Conditions