Factor H-related protein 3 (FHR-3) deposition in kidney allografts – localization and correlation with complement activation

Abstract

Introduction: Factor H-related proteins (FHRs) have emerged as novel players in complement-mediated diseases, as they exhibit structural resemblances to factor H but lack the regulatory domains, enabling them to antagonize factor H and increase complement activation through several activities. Despite the widely importance of the complement system in kidney transplantation, FHRs have not been studied in this context. Utilizing a novel monoclonal antibody, we investigated the presence of FHR-3 in kidney allografts. Methods: The RTEC-2 monoclonal antibody was validated using immunohistochemistry, Western Blot analysis, and immunoprecipitation combined with mass spectrometry. FHR-3 deposition, localization, and the relationships to complement activation were analyzed in human kidney biopsies obtained pre-transplantation from living and deceased donors, and post-transplantation in cases with acute tubular necrosis, acute cellular and vascular rejection, or chronic rejection. Results: Glomerular FHR-3 deposition was detected in kidneys from deceased, but not living, donors before transplantation. Additionally, we observed FHR-3 deposition in post-transplant settings, both in cases of rejection and non-rejection. While tubular and vascular deposition of FHR-3 was observed in some cases, FHR-3 was predominantly seen in the glomeruli, where it was primarily localized to podocytes. Moreover, co-localization of FHR-3 and C3d was rarely detected, with most cases exhibiting separate and non-overlapping staining patterns for both antigens, However, there was a moderate correlation between the staining intensity of the FHR-3 and C3d in the kidney biopsies (r=0.38, P=0.01). Conclusion: We detected FHR-3 deposition in kidney allografts under inflammatory conditions, primarily colocalizing with podocytes in both the presence and absence of complement activation.