title: Testing the Activity of Complement Convertases in Serum/Plasma for Diagnosis of C4NeF-Mediated C3 Glomerulonephritis.
abstract: Autoantibodies termed C3-nephritic factor (C3NeF), which stabilize convertases of the alternative complement pathway, often stimulate autoinflammatory diseases. However, knowledge about analogous autoantibodies acting on the classical pathway (C4NeF) is limited to a few reports, which indicate association with kidney dysfunction, systemic lupus erythematous, and infections. C4NeF may appear independently from C3NeF, but the lack of a routine diagnostic method predisposes C4NeF for being an underestimated player in autoinflammatory episodes. We tested the activity of classical convertases directly in serum/plasma to screen samples from 13 patients with C3 glomerulopathies and identified one patient showing significantly prolonged half-life of these enzymes. Observed effect was reproduced by immunoglobulins purified from patient's plasma and additionally confirmed on classical convertase built from purified components. Isolated immunoglobulins protected classical convertases from both spontaneous and inhibitor-driven decay but not from C4b proteolysis. The patient had a decreased serum level of C3, elevated sC5b-9, and normal concentrations of factor B and C4. Neither C3NeF nor other autoantibodies directed against alternative pathway proteins (factor H, factor B, factor I, C3, and properdin) were found. Genetic analysis showed no mutations in C3, CFB, CFH, CFI, MCP, THBD, and DGKE genes. Renal biopsy revealed a membranoproliferative pattern with intense C3 deposits. Our results underline the importance of C4NeF as an independent pathogenic factor and a need for the implementation of routine examination of classical convertase activity. Proposed method may enable robust inspection of such atypical cases.
authors.affiliation: Department of Translational Medicine, Lund University, 20502, Malmö, Sweden.
authors.forename: Anna M
authors.initials: AM
authors.lastname: Blom
authors.affiliation: Immunology Unit, University Hospital La Paz, IdiPAZ, Madrid, Spain.
authors.forename: Fernando
authors.initials: F
authors.lastname: Corvillo
authors.affiliation: Department of Translational Medicine, Lund University, 20502, Malmö, Sweden.
authors.forename: Michal
authors.initials: M
authors.lastname: Magda
authors.affiliation: Department of Medical Biotechnology, Intercollegiate Faculty of Biotechnology UG‑MUG, Medical University of Gdańsk, Dębinki 1 street, 80-210, Gdańsk, Poland.
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Stasiłojć
authors.affiliation: Immunology Unit, University Hospital La Paz, IdiPAZ, Madrid, Spain.|Unit 754, Centre for Biomedical Research on Rare Diseases (CIBERER), Madrid, Spain.
authors.forename: Pilar
authors.initials: P
authors.lastname: Nozal
authors.affiliation: Servicio de Nefrología Hospital Universitario Virgen del Rocío, Sevilla, Spain.
authors.forename: Miguel Ángel
authors.initials: MÁ
authors.lastname: Pérez-Valdivia
authors.affiliation: Servicio de Nefrología Hospital Universitario Virgen del Rocío, Sevilla, Spain.
authors.forename: Virginia
authors.initials: V
authors.lastname: Cabello-Chaves
authors.affiliation: Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas (CIB-CSIC), Centro de Investigación Médica en Red (CIBERER U738), Madrid, Spain.
authors.forename: Santiago
authors.initials: S
authors.lastname: Rodríguez de Córdoba
authors.affiliation: Immunology Unit, University Hospital La Paz, IdiPAZ, Madrid, Spain.|Unit 754, Centre for Biomedical Research on Rare Diseases (CIBERER), Madrid, Spain.
authors.forename: Margarita
authors.initials: M
authors.lastname: López-Trascasa
authors.affiliation: Department of Medical Biotechnology, Intercollegiate Faculty of Biotechnology UG‑MUG, Medical University of Gdańsk, Dębinki 1 street, 80-210, Gdańsk, Poland. marcin.okroj@gumed.edu.pl.
authors.forename: Marcin
authors.initials: M
authors.lastname: Okrój
mesh_terms: D000936:Antigen-Antibody Complex / Q000378:metabolism*; D001323:Autoantibodies / Q000378:metabolism*; D001798:Blood Proteins / Q000378:metabolism*; D003167:Complement Activation; D050577:Complement C3-C5 Convertases / Q000378:metabolism*; D015935:Complement C4b / Q000378:metabolism; D015938:Complement Membrane Attack Complex / Q000378:metabolism; D003165:Complement System Proteins / Q000378:metabolism*; D004252:DNA Mutational Analysis; D005260:Female; D005838:Genotype; D015432:Glomerulonephritis, Membranoproliferative / Q000175:diagnosis*; D006801:Humans; D007668:Kidney / Q000378:metabolism* / Q000473:pathology; D008297:Male; D008875:Middle Aged
chemical_list: D000936:Antigen-Antibody Complex; D001323:Autoantibodies; D001798:Blood Proteins; D015938:Complement Membrane Attack Complex; C047610:SC5b-9 protein complex; C058872:complement 4 nephritic factor; D015935:Complement C4b; D003165:Complement System Proteins; D050577:Complement C3-C5 Convertases
keywords: C3 glomerulonephritis; C4NeF; Complement system; complement convertase