Immunoglobulin-like Domain of Hs FcμR as a Capture Molecule for Detection of Crimean-Congo Hemorrhagic Fever Virus- and Zika Virus-Specific IgM Antibodies

The cellular surface molecule TOSO/FAIM3/ FcμR has been identified as an IgM-specific Fc receptor expressed on lymphocytes. Here, we show that its extracellular immunoglobulin-like domain ( FcμR-Igl) specifically binds to IgM/antigen immune complexes (ICs) and exploit this property for the development of novel detection systems for IgM antibodies directed against Crimean-Congo hemorrhagic fever virus (CCHFV) and Zika virus (ZIKV). His-tagged FcμR-Igl was expressed in and purified by affinity chromatography, oxidative refolding, and size-exclusion chromatography. Specific binding of FcμR-Igl to IgM/antigen ICs was confirmed, and 2 prototypic ELISAs for the detection of anti-CCHFV and anti-ZIKV IgM antibodies were developed. Thereby, patient sera and virus-specific recombinant antigens directly labeled with horseradish peroxidase (HRP) were coincubated on FcμR-Igl-coated ELISA plates. Bound ICs were quantified by measuring turnover of a chromogenic HRP substrate. Assay validation was performed using paired serum samples from 15 Kosovar patients with a PCR-confirmed CCHFV infection and 28 Brazilian patients with a PCR-confirmed ZIKV infection, along with a panel of a priori CCHFV/ZIKV-IgM-negative serum samples. Both ELISAs were highly reproducible. Sensitivity and specificity were comparable with or even exceeded in-house gold standard testing and commercial kits. Furthermore, latex beads coated with FcμR-Igl aggregated upon coincubation with an IgM-positive serum and HRP-labeled antigen but not with either component alone, revealing a potential for use of FcμR-Igl as a capture molecule in aggregation-based rapid tests. Recombinant FcμR-Igl is a versatile capture molecule for IgM/antigen ICs of human and animal origin and can be applied for the development of both plate- and bead-based serological tests.