Features of circulating factor H immune complexes in complement-mediated diseases

• Published in: Immunobiology (1979) Vol. 230; no. 4; p. 153043
• Main Authors: Cook, Stephanie, Shao, Dingwu, Jellison, Sydney, de Cordoba, Santiago Rodriguez, Nester, Carla, Smith, Richard, Zhang, Yuzhou
• Format: Journal Article
• Language: English
• Published: Elsevier GmbH 01.07.2025
• ISSN: 0171-2985; 1878-3279
• DOI: 10.1016/j.imbio.2025.153043

Complement factor H (FH) regulates the alternative pathway (AP) through two functions: the N-terminal domain mediates decay-accelerating activity (DAA) and cofactor activity (CA), while the C-terminal domain mediates binding to host surfaces. Factor H autoantibodies (FHAAs) can disrupt these functions, with the specific disease consequence of C3 glomerulopathy (C3G) or complement-mediated thrombotic microangiopathy (cm-TMA) reflecting FHAA binding site specificity. Enzyme-linked immunosorbent assays (ELISA) typically detect free FHAAs by binding to immobilized FH (FHAA ELISA) or circulating FH–anti-FH immune complexes (FHICs) using sandwich ELISA. However, free FHAAs can form complexes before or during testing, causing interference and false-negative results. The clinical role of FHICs remains unclear. We screened 120 samples using FHAA ELISA, FHIC sandwich ELISA, and a newly developed lateral flow assay to identify patients with detectable free FHAAs or circulating immune complexes only. Based on these screening results, six samples (four cm-TMA and two C3G patients) underwent further characterization. FHICs were isolated using a protein G pull-down assay, and their specificity, affinity, and stability were analyzed. We observed significant heterogeneity in FH-FHAA affinity and epitope specificity profiles between patients with cm-TMA and those with C3G. Functional assays showed that FHAAs and FHICs impair FH regulatory functions in the fluid phase and on cell surfaces, enhancing AP activation. Our findings demonstrate the complexity and clinical relevance of FHICs in complement-mediated diseases, while highlighting limitations of standard diagnostic tests. Because FHICs may impact disease severity and therapeutic response, further studies should focus on standardizing diagnostic algorithms to characterize FHAAs thereby facilitating appropriate personalized therapeutic interventions. Supported in part by National Institutes of HealthR01 DK110023. Supported in part by National Institutes of Health R01 DK110023.