INTRODUCTION

Anti-glomerular basement membrane (anti-GBM) disease is characterized by pathogenic autoantibodies targeting the alpha-3 chain of type IV collagen located in the glomerular and alveolar basement membranes. Clinically, it causes rapidly progressive glomerulonephritis and pulmonary hemorrhage. Observational data and mechanistic rationale have largely informed current treatment strategies - including plasmapheresis, glucocorticoids, cyclophosphamide, and rituximab. However, overall and renal prognosis remain guarded, and advancements in therapy are needed.

Emerging data have implicated the complement system in the pathogenesis of anti-GBM disease (1Anti–Glomerular Basement Membrane Glomerulonephritis: A Morphologic Study of 80 Cases., 2Groggel G.C. Salant D.J. Darby C. Rennke H.G. Couser W.G. Role of terminal complement pathway in the heterologous phase of antiglomerular basement membrane nephritis., 3Sheerin N. Springall T. Carroll M. Hartley B. Sacks S. Protection against anti‐glomerular basement membrane (GBM)‐mediated nephritis in C3‐and C4‐deficient mice., 4Ma R. Cui Z. Liao Y-h Zhao M-h Complement activation contributes to the injury and outcome of kidney in human anti-glomerular basement membrane disease., 5Ma R. Cui Z. Hu S.-Y. Jia X.-Y. Yang R. Zheng X. et al.The alternative pathway of complement activation may be involved in the renal damage of human anti-glomerular basement membrane disease.). Eculizumab is an anti-C5 monoclonal antibody that blocks the cleavage of C5, which prevents the formation of C5a, the potent leukocyte chemoattractant, and C5b, the initial reagent in the formation of the membrane attack complex (MAC; also known as C5b-9). This provides an immediate inhibition of the downstream proinflammatory and cytotoxic sequelae of the complement system.

Here, we report the use of eculizumab as rescue therapy in two patients with progressive anti-GBM disease on standard therapies, which to our knowledge has not been reported to date. We also present kidney biopsy results with extensive complement staining that add to a limited but growing body of literature on the possible role of complement in the renal damage from anti-GBM disease

DISCUSSIONHistorically, deposition of C3 along the GBM has been frequently observed in biopsy specimens of patients with anti-GBM disease, which theoretically suggested a pathogenic role of complement (1Anti–Glomerular Basement Membrane Glomerulonephritis: A Morphologic Study of 80 Cases.). To date, a limited number of experiments evaluating this theory have been published. In 1984, Groggel et al. demonstrated that rabbits deficient in C6 had less proteinuria and a smaller rise in serum creatinine in an anti-GBM disease model, which suggested the membrane attack complex (MAC; also known as C5b-9) contributes to renal injury (2Groggel G.C. Salant D.J. Darby C. Rennke H.G. Couser W.G. Role of terminal complement pathway in the heterologous phase of antiglomerular basement membrane nephritis.). In 1997, Sheerin et al. demonstrated mice deficient in C3 or C4 had less neutrophilic infiltration, less glomerular capillary thrombosis, and less proteinuria compared to wild-type mice in an anti-GBM disease model (3Sheerin N. Springall T. Carroll M. Hartley B. Sacks S. Protection against anti‐glomerular basement membrane (GBM)‐mediated nephritis in C3‐and C4‐deficient mice.). In 2013, Ma et al. found elevated serum and urinary levels of C5b-9 and C5a in humans with anti-GBM disease (4Ma R. Cui Z. Liao Y-h Zhao M-h Complement activation contributes to the injury and outcome of kidney in human anti-glomerular basement membrane disease.). They also observed an association between the serum C5b-9 level and development of renal failure. Finally, in 2014, Ma et al. found that C1q, factor B, properidin, C3d, C4d, and C5b-9 were detected along the GBM in all the glomeruli in 10 patients with anti-GBM disease (5Ma R. Cui Z. Hu S.-Y. Jia X.-Y. Yang R. Zheng X. et al.The alternative pathway of complement activation may be involved in the renal damage of human anti-glomerular basement membrane disease.). These findings suggest complement activation might play a significant role in renal damage in anti-GBM disease.Our patient’s kidney biopsy from CASE 2 revealed extensive C5b-9 deposition in the glomerular capillary walls along with detection of C3, C3d, and C4d. High circulating levels of and positive biopsy staining for C5b-9 suggest activation of the terminal complement cascade and offer a mechanistic rationale for therapeutic complement inhibition. In the study by Ma et al. C5b-9 was detected clearly in a linear pattern along the glomerular capillary wall in 29 (100%) glomeruli of all 10 consecutive patients with anti-GBM disease (5Ma R. Cui Z. Hu S.-Y. Jia X.-Y. Yang R. Zheng X. et al.The alternative pathway of complement activation may be involved in the renal damage of human anti-glomerular basement membrane disease.). In contrast, no deposits of C5b-9 were found in renal biopsy specimens from patients with minimal change disease (n = 5), a disease where complement activation is not thought to be involved, nor was it detected in normal control renal tissue (obtained from the normal part of a nephrectomized kidney due to renal carcinoma). They also observed stronger intensity C5b-9 staining in glomeruli with crescent formation compared with the glomeruli without crescent formation. While further study is required to validate the role and interpretation of C5b-9 staining in anti-GBM disease, C5b-9 staining has been studied in other diseases causing glomerulonephritis. For example, C5b-9 staining on kidney biopsy specimens can be found in nearly all patients with proliferative lupus nephritis (6Wilson H.R. Medjeral-Thomas N.R. Gilmore A.C. Trivedi P. Seyb K. Farzaneh-Far R. et al.Glomerular membrane attack complex is not a reliable marker of ongoing C5 activation in lupus nephritis.). Although the intensity of staining did not significantly differ between active and chronic forms, post-treatment biopsies did show a reduction in C5b-9 staining intensity. A similar reduction in C5b-9 staining intensity after treatment has been reported in dense deposit disease (7Vivarelli M. Pasini A. Emma F. Eculizumab for the treatment of dense-deposit disease.). In our patient, given the clinical and serologic improvement, a repeat biopsy post-treatment was deferred.

In addition to complement deposition in the glomerular capillary walls, we detected staining for C5b-9, C3, C3d, and C4d in the tubular basement membrane and vessels outside the glomerulus (Figure S3-4). Given that the non-collagenous domain of the alpha 3 chain of type IV collagen, also known as the Goodpasture antigen, is largely localized to glomerular and alveolar basement membranes, possible mechanisms to explain complement activation outside of the glomerulus include the presence and recognition of the target autoantigen, cross-reactivity with another antigen, or non-specific staining from nearby complement activation. Future biopsy studies with controls would be informative.

There is growing evidence that complement blockade in ANCA-associated vasculitis (AAV) may be effective (9, S1), and it is possible that the potential therapeutic response the patient in CASE 1 had to eculizumab was to the component of his presentation due to AAV. Moreover, the patient’s initial symptomatology and time course can be typical for AAV as the predominant driver of disease. However, the patient had an unequivocally positive anti-GBM titer and developed relentlessly worsening kidney function despite high-dose glucocorticoids, plasmapheresis, rituximab, and cyclophosphamide, which can be seen with the aggressive nature of many cases of anti-GBM disease. Despite patient 1 initially possessing good prognostic features, the impetus for utilizing eculizumab as rescue therapy related to the rate of rise in serum creatinine and rate of decline in urine output without an alternative explanation while already receiving full conventional therapy.

The overall and renal prognosis of anti-GBM disease depends largely on how quickly immunosuppressive therapy is initiated. In a retrospective observational study, in patients presenting with a SCr ≥ 5.7 mg/dL but not requiring dialysis within 3 days of admission (n = 13), the patient and renal survival were 83% and 82% at 1 year, respectively (8Levy J.B. Turner A.N. Rees A.J. Pusey C.D. Long-term outcome of anti–glomerular basement membrane antibody disease treated with plasma exchange and immunosuppression.). In patients who required dialysis within 3 days of admission (n = 39), patient and renal survival were 65% and 8% at 1 year, respectively. Lastly, all patients who required dialysis within 3 days and had 100% crescents on renal biopsy remained dialysis dependent. In our series, patient 2’s presenting SCr was 3.4 mg/dL, however immunosuppressive therapy was not initiated until the SCr was 6.9 mg/dL. She required dialysis 6 days after admission, and her biopsy revealed 70% crescents. Furthermore, she received one dose of eculizumab 16 days after a diagnosis was established (22 days after admission). It is possible that the limited and delayed initiation of eculizumab blunted any potential efficacy due to onset of irreversible kidney damage. Given the patient’s on-going dialysis-dependence, eculizumab here was utilized here as an immediate acting anti-inflammatory agent at the level of the end organ to further bridge the patient until other therapies addressing more upstream pathogenic autoreactive B cells, plasma cells, and circulating autoantibodies took greater effect. It is difficult to say if our patient’s clinical outcome of dialysis independence would have been expected without eculizumab. Future studies investigating anti-complement therapy should include earlier initiation and potentially more prolonged therapy courses. Furthermore, no adverse events directly attributable to complement inhibition were observed in our patients, both of whom received vaccinations for meningococcus serogroups ACWY and B, as well as chemoprophylaxis with penicillin. However, a control group is necessary to determine the efficacy and safety of eculizumab in anti-GBM disease. Our observations serve as grounds for further scientific investigation for targeting complement in a disease where treatment advancements are needed.