Proteinuria Remains a Significant Hurdle to Successful Pig Kidney Xenotransplantation Despite an Effective Immunosuppressive Regimen

BACKGROUND: This study evaluated the efficacy of an optimized immunosuppressive regimen, including a higher dose of anti-CD154 monoclonal antibody (TNX-1500), in prolonging graft survival in pig-to-baboon kidney xenotransplantation. Despite advances in genetic engineering of organ-source pigs and immunosuppressive regimens, we report that the development of nephrotic-range proteinuria remains a significant problem. METHODS: We assessed the TNX-1500-based immunosuppressive regimen in 9 baboon recipients of gene-edited pig kidney transplants. The regimen included induction with antithymocyte globulin, anti-CD20 monoclonal antibody (rituximab), and C1 esterase inhibitor, and maintenance with TNX-1500, rapamycin, methylprednisolone, and anti-interleukin-6 receptor blockade (tocilizumab). RESULTS: Although an increased dose of TNX-1500 of 30 mg/kg (higher dose, n = 6) versus 20 mg/kg (lower dose, n = 3) improved overall survival (median 214 versus 86 d; P < 0.05), 4 of 9 baboons (including 3 with higher dose) developed persistent nephrotic-range proteinuria. The histopathology of these 4 grafts revealed focal glomerular thrombotic microangiopathy (4/4), transplant glomerulopathy (3/4), focal segmental glomerulosclerosis (2/4), and/or membranous nephropathy (1/4) but no definitive features of rejection. Proteinuria was associated with decreased serum albumin and, very importantly, with loss of TNX-1500 in the urine. CONCLUSIONS: Despite the efficacy of the immunosuppressive regimen, we suggest that proteinuria may lead to inadequate immunosuppressive therapy through loss of the therapeutic antibody, thus increasing the risk of rejection. Further research is needed to develop strategies to prevent this complication.