Objective Because currently available assays that measure circulating immune complexes (ICx) are suboptimal, a novel assay was recently developed measuring C4d, a stable product of activation of the classical complement pathway. The present study aimed to establish the value of measuring plasma C4d levels in a longitudinal cohort of patients with severe refractory SLE who were treated with a combination therapy of rituximab with belimumab (RTX+BLM).
Methods Fifteen patients with SLE who were treated with RTX+BLM in a phase 2A, open label study were included to sequentially measure plasma C4d levels and correlated to well-established markers of ICx-formation, that is, autoantibodies against double-stranded (ds) DNA, autoantibodies against C1q and proteinuria. The performance of plasma C4d measurements, C4 measurements and the ratio of C4d over C4 (C4d:C4) was evaluated.
Results After establishing that on RTX+BLM treatment kinetics of C4d levels was distinct from traditional C3 and C4 levels, we found strong correlation of C4d:C4 with anti-dsDNA (R=0.76, p<0.001) and anti-C1q (R=0.65, p<0.001) autoantibody levels, which outperformed both stand-alone C4 and C4d levels. Additionally, changes in C4d:C4 over time correlated strongly with changes in proteinuria (R=0.59, p<0.001) as well as anti-dsDNA (R=0.46, p=0.003) and anti-C1q (R=0.47, p=0.002).
Conclusion In patients with severe SLE, plasma C4d levels in relation to C4 levels is useful for longitudinal monitoring after RTX+BLM treatment to reflect amelioration of classical complement activation by ICx as well as proteinuria.
- systemic lupus erythematosus
- lupus nephritis
- immunoglobulin-mediated membranoproliferative glomerulonephritis
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Contributors TK, SN, CVK, AMB and YKOT contributed to the ideas for the article. TK, CVK, AMB and YKOT analysed the data, participated in the interpretation of the data and wrote and revised the manuscript. AMB and MO were involved in developing the assay for C4d measurement. All authors approved of the final version.
Funding The study was funded by the Swedish Research Council (2016-01142), King Gustav V’s 80th Anniversary Foundation, Swedish Rheumatism Association, and grants for clinical research (ALF and from the Skåne University Hospital). Dr Kraaij and Dr Teng’s work is supported by the Dutch Kidney Foundation (KJPB12.028 and 17OKG04), Clinical Fellowship from the Netherlands Organization for Scientific Research (90713460). Marcin Okrój’s work is supported by the National Science Centre Poland, grant no. 2014/14/E/NZ6/00182. GlaxoSmithKline (GSK) provided belimumab and an unrestricted grant for the clinical study described in this manuscript.
Competing interests AMB and MO are named as inventors in a patent application including claims to use of C4d as biomarker.
Patient consent for publication Not required.
Ethics approval The study was approved by the LUMC medical ethics committee (Commissie Medische Ethiek LUMC, P14.065).
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement No additional data are available.
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