IgM single antigen bead HLA-assay is affected by imlifidase through the cleavage of IgG but not IgM

Abstract

AimThe aim of this study was to investigate if human IgM is a cleavable substrate for imlifidase and to explain an observed effect in anti-HLA IgM single antigen bead (SAB) assays in sensitized patients. MethodsSerum samples collected pre- and 24 h post-imlifidase administration from sensitized patients enrolled in a phase II trial were investigated for anti-HLA IgG and IgM using SAB assays, with and without in vitro IgG depletion using a CaptureSelect™ affinity matrix. In addition, pre-dose samples and purified human IgM samples were treated with imlifidase in vitro and evaluated by SDS-PAGE, Western blot (PE-conjugated anti-human IgM) and SAB (IgG, IgM) assays. ResultsBy comparing the mean fluorescence intensity (MFI) of HLA-beads, pre- and post-imlifidase administration, three IgM-related patterns were observed; IgM-specific HLA-SABs with an increased MFI post-imlifidase, IgM-specific HLA-SABs with a decreased MFI post-imlifidase, and IgM-specific HLA-SABs with a marginal MFI difference between the pre- and post-imlifidase administration. These IgM signal patterns were observed despite neither purified IgM nor serum IgM could be cleaved by imlifidase. After removing IgG, the effects observed on anti-HLA IgM was largely eliminated with the biggest differences seen in patients with very high anti-HLA IgG in pre-dose samples. ConclusionWe demonstrate that imlifidase does not cleave human IgM, including HLA-specific IgM antibodies from highly sensitized subjects. Observed decreases of SAB-HLA IgM signals after imlifidase treatment may result from the cleavage of IgG-IgM complexes which are bound to SAB-HLA. Serum analysis of patients with high levels of anti-HLA IgG will result in a more accurate SAB-HLA IgM reading after IgG depletion.