Comparative study of antibody removal before pig-to-baboon and human ABO-incompatible renal transplantation

Abstract

THE MECHANISM of rejection following pig-to-baboon xenotransplantation (XenoTx) has been compared with that following ABO-incompatible allotransplantation (ABO-Tx), because both involve natural antibody (Ab) against carbohydrate epitopes. Recently, ABO-Tx has shown excellent outcomes following pretreatment with double filtration plasmapheresis (DFPP). We compared changes in anti-donor Ab and histopathology of the renal graft between ABO-Tx and XenoTx. Before renal transplantation (Tx), DFPP was performed on days 26, 24, 22, and 21 in ABO-Tx (n 5 25) and on days 22 and 0 (immediately before reperfusion) in XenoTx (n 5 4). Cyclosporine (or tacrolimus), steroid, and cyclophosphamide were administered to both patients and baboons. Baboons were divided into two groups according to the extent of Ab removal: XenoTx Group I, Ab removal to the same extent as in ABO-Tx (n 5 2); XenoTx Group II, nearly total removal of Ab (n 5 2). Anti-A/B and anti-pig IgM (IgG) Abs were measured by flow cytometry. Twenty-five patients underwent ABO-Tx. All grafts remain functioning, except in two cases where the grafts were lost from recurrence of the original disease after 6 months and from chronic rejection after 5 years, although 3 had reversible humoral rejection. In pig-to-baboon renal transplantation without DFPP pretreatment, hyperacute rejection (HAR) was observed 60 and 90 minutes after transplantation. In XenoTx Group I, no HAR was observed. Grafts were rejected on days 6 and 7, and showed coagulative necrosis caused by vascular rejection; biopsies on days 1, 4, and 5 showed only minimal changes. In XenoTx Group II, the baboons could not tolerate the extra DFPP and died 4 hours and 2 days after transplantation. Immunohistochemical study of the grafts showed IgM, IgG, and C3 binding to endothelial cells within 1 hour post-Tx in XenoTx Group I, and IgM and C3 binding as early as 1 hour post-Tx in XenoTx Group II, while no Ab binding was detected at any time (1 hour to 2 years) after ABO-Tx. The same extent of Ab removal in XenoTx Group I as in ABO-Tx inhibited HAR but could not prevent delayed vascular rejection. Even when anti-pig Ab was depleted by 98% (XenoTx Group II), the small amount of remaining Ab still had the ability of binding to the pig graft within 1 hour. For successful xenografting, we suggest that the concentration of xenoantigen epitopes expressed on pig grafts may need to be reduced by genetic engineering to the level of the ABO antigens on human kidneys.