Glomerular Complement Factor H–Related Protein 5 (FHR5) Is Highly Prevalent in C3 Glomerulopathy and Associated With Renal Impairment

Abstract

Introduction: Therapeutic agents that target complement are increasingly available for glomerular diseases. However, the mechanisms linking glomerular complement deposition with inflammation and damage are incompletely understood. Complement factor H-related protein 5 (FHR5) interacts with complement C3 and is considered to promote activation. Circulating and glomerular FHR5 associates with IgA nephropathy and abnormal FHR5 associates with familial C3 glomerulopathy (C3G). We characterized glomerular FHR5 staining in C3G and assessed its relationships with histological features of glomerular injury and clinical outcome. Methods:We developed FHR5 staining protocols for formalin-fixed paraffin-embedded (FFPE) renal tissue and applied them to surplus biopsy sections from a C3G cohort.Results: Glomerular FHR5 was highly prevalent in native and transplant C3G and correlated with glomerular C3 and C5b-9 staining. Glomerular FHR5 staining correlated negatively with estimated glomerular filtration rate (eGFR) (P ¼ 0.04, difference of medians 19.7 ml/min per 1.73 m 2 ; 95% confidence interval [CI] 1.1-43.0) and positively with a membranoproliferative glomerulonephritis pattern at diagnostic biopsy (odds ratio 18; 95% CI 1.6-201; P ¼ 0.049). Glomerular FHR5 staining intensity positively correlated with glomerular complement C3b/iC3b/C3c (Pearson's correlation coefficient [R] ¼ 0.59; P ¼ 0.0008), C3dg (R ¼ 0.47; P ¼ 0.02) and C5b9 (R ¼ 0.44, P ¼ 0.02).Conclusions: Glomerular FHR5 is highly prevalent in C3G, interacts with glomerular C3, and is associated with markers of disease severity. Glomerular FHR5 likely exacerbates complement-mediated glomerular damage in C3G and its interaction with glomerular complement might be exploited to target complement therapeutic agents.

In Search of C3G Tissue Biomarkers

“…Interestingly, the authors found a similar array of complement proteins as described by mass spectrometry, with comparable results in C3GN and DDD. A novel finding was the high prevalence of FHR5 in C3G glomeruli, where the intensity of staining correlated positively with the presence of a membranoproliferative pattern and with the staining intensities of C3 activation products (C3b/iC3b/C3c and C3dg) and C5b-9, and correlated negatively with estimated glomerular filtration rate at biopsy 5 . These results suggest potential mechanistic relevance.…”

“…In this issue, Medjeral-Thomas et al. 5 applied a large battery of antibodies to individual complement factors and their regulatory proteins (including FHR5, FHR1, fH, C3b/iC3b/C3c, C3dg, C5b9, C1q, C4d, and properdin) to formalin-fixed paraffin-embedded (FFPE) biopsy samples from 19 patients with C3G (including 13 with C3GN and 6 with DDD). They catalogued the spectrum of immunohistochemistry results and made correlations with clinical and outcome parameters.…”

“…Prior direct comparisons between immunohistochemistry on FFPE samples and immunofluorescence on frozen sections have suggested that it is harder to quantitate staining intensity by immunohistochemistry, and even the staining results for C3c used to diagnose C3G are not always comparable 6 . Use of random, archived FFPE sections precluded comparison of the intensity and distribution of staining in a given glomerulus across serial sections, as would be needed to control for interglomerular heterogeneity and to compensate for the lack of systematic co-localization studies 5 . Another potential confounder is the nonspecificity of FHR5 for C3G.…”

“…Another potential confounder is the nonspecificity of FHR5 for C3G. FHR5 staining has been identified in glomeruli of diabetic nephropathy, IgA nephropathy, and membranous nephropathy, as well as sclerosing glomeruli irrespective of etiology 5, 7. Thus, larger datasets of complement-mediated diseases and appropriate controls will be needed before conclusions can be drawn about the sensitivity of FHR5 as a marker of C3G.…”

Complement activity is regulated in C3 glomerulopathy by IgG–factor H fusion proteins with and without properdin targeting domains

“…We speculate that glomerular FHR proteins are only likely to clear from glomeruli when C3d has been removed. In this context, it is notable that in human C3G, factor H-related protein 5 associates with glomerular C3d 29 .…”