Preformed and De Novo Xenoreactive IgM Antibodies Contribute to Early Xenograft Loss in Pig-to-Monkey Kidney Xenotransplantation

In gene-edited pig-to-primate kidney xenotransplantation, antibody-mediated rejection (AMR) and thrombotic microangiopathy (TMA) remain major barriers to prolonged xenograft survival. To reduce the incidence of AMR, recipient selection has largely focused on preformed anti-donor pig IgG levels. However, our findings suggest that preexisting anti-donor pig IgM may also be an important determinant of early xenograft outcomes, especially in the development of TMA.
Methods: Pretransplant flow crossmatching with donor pig PBMCs was performed in 60 candidate rhesus monkeys, and six animals with relatively low anti-pig IgM/IgG binding and complement- dependent cytotoxicity (CDC) were selected. Kidneys from GTKO/β4GalNT2KO/hCD55/hTBM (4GE) pigs were then transplanted into these recipients under induction therapy with ATG, C1-INH, and anti-CD20, followed by maintenance immunosuppression with anti-CD154, tacrolimus, and steroids.
Results: Median survival among the NHP recipients was 166 days (range, 45 to >500 days); five of six recipients were euthanized because of TMA, AMR, or both, whereas one survived for more than 500 days with stable graft function. This heterogeneity in outcome was closely associated with pretransplant anti-pig IgM, but not IgG, levels: recipients with lower preformed IgM showed longer xenograft survival. Consistent with this association, pathological analysis demonstrated very early glomerular IgM deposition after transplantation, accompanied by complement deposition, endothelial swelling, podocyte injury, microvascular thrombosis, and TMA-like changes, supporting a role for IgM in early graft injury. Posttransplant de novo IgM elevation was further associated with worsening proteinuria and TMA progression, whereas later rebound of de novo IgG was associated with delayed AMR and transplant glomerulopathy. Notably, proteinuria preceded serum creatinine elevation and was often accompanied by reduced anti-CD154 levels and detectable urinary antibody, suggesting urinary loss of anti-CD154, inadequate maintenance immunosuppression, and subsequent amplification of humoral rejection. Complement inhibitory therapy partially delayed, but did not reverse, ongoing antibody-mediated injury.
Conclusion: In conclusion, low preformed anti-pig IgM levels were closely associated with prolonged xenograft survival. IgM-mediated complement activation, together with glomerular endothelial and podocyte injury, may play a key role in early TMA and graft injury. These findings support the potential value of pretransplant IgM-based risk stratification in improving xenograft outcomes.