An IgM Cleaving Enzyme for Clearance of Anti-Pig Xenoreactive Antibodies in a Nonhuman Primate Model.

Background: The removal of preformed antibodies with cleaving enzymes like IdeS (imlifidase) has demonstrated therapeutic potential in organ transplantation for sensitized recipients. However, preformed xenoreactive antibodies (XAbs) against porcine glycans are predominantly IgM and considered detrimental in pig-to-human xenotransplantation.

Methods: Recombinant IceM, an endopeptidase cleaving IgM, was generated in Escherichia coli (E. coli). Four maximally MHC-mismatched rhesus macaques underwent two serial skin transplantations to model allosensitized patients awaiting xenotransplantation. IceM was administered IV in allosensitized animals at 28 and 56 days after the first skin transplantation to assess in vivo IgM cleavage. Total IgG and IgM were quantified with western blot, and anti-pig (xenoreactive) IgG/IgM were evaluated using flow crossmatch. B cells and their subpopulations were assessed using flow cytometry.

Results: IceM selectively cleaved human IgM, while showing no cleavage activity toward other isotypes, including IgG, IgA, IgD, and IgE. Additionally, IceM cleaves only human and nonhuman primate IgM in vitro, but not in sera from other species. At a dose of 0.5 mg/kg, IceM reduced xenoreactive IgM levels to 13.76% ± 4.98% of baseline (B cell flow crossmatch) at 24 h postadministration, with baseline levels restored approximately 2 weeks after treatment. Additionally, animals showed similar kinetics of xenoreactive IgM degradation with the repeated dose of IceM.

Conclusion: In this study, we report a recombinant bacterial enzyme that selectively cleaves IgM in human and nonhuman primate sera. Repeat administration of IceM in macaques enables selective, robust clearance of circulating xenoreactive IgM. This approach will be useful in treating preformed natural and rebound IgM in xenotransplantation.