The 2021 IXA Keith Reemtsma Lecture: Moving xenotransplantation to the clinic

Abstract

Keith Reemtsma was a pioneer in xenotransplantation, the Honorary Founding President of the International Xenotransplantation Association (in 1998), and a wonderful personality. It is a privilege to be invited to give this lecture in his memory. If he were alive today, he would be delighted to see the progress that has been made in pig organ transplantation into nonhuman primate recipients. This progress has largely resulted from two major advances: (i) the increasing availability of pigs with multiple genetic manipulations aimed at protecting the cells of the organ from the primate immune response and (ii) the introduction of novel immunosuppressive agents that block the CD40/CD154 costimulation pathway. There is strong evidence from numerous in vitro studies that the transplantation of a triple-knockout pig organ, particularly if expressing several human protective proteins, into a patient is likely to be significantly more successful than if that same organ is transplanted into a nonhuman primate recipient. With this fact in mind, and in view of the advances currently being made, the time has surely come when we need to consider moving from the laboratory to the clinic. However, there are still questions we need to definitively resolve: (i) What exact genetic modifications do we need in the organ-source pig? (ii) What exact immunosuppressive regimen will we choose? (iii) How will we monitor the immune response and diagnose and treat rejection? and (iv) How do we plan to prevent or treat potential infectious complications? Furthermore, when these matters have been resolved, which patients will be offered a pig organ in the first trial? We have suggested that patients who are very unlikely to survive until a suitable deceased human donor kidney becomes available are those who should be considered for the initial trials. Assessing public attitudes to xenotransplantation is also important before embarking on a clinical trial. I suggest that progress is much more likely to be made from a small clinical trial than if we persist in carrying out experiments in an animal model that no longer mimics the clinical situation.

Keywords: International Xenotransplantation Association; Keith Reemtsma; nonhuman primate; pig; xenotransplantation.

Figure 1:

Keith Reemtsma (1925–2000)

Figure 2:

Macroscopic appearance of the two chimpanzee kidneys (top) and the two native kidneys (bottom) at necropsy 9 months after transplantation. The chimpanzee kidneys were macroscopically normal, and microscopically showed no features of rejection.

Figure 3:

Maximal survival each year of NHPs with pig kidney grafts between 1989 and 2019. Since then, maximal survival has increased to more than 3 years (see Adams AB, reference 8).

Figure 4:

(A) Human serum (n=14) IgM (left) and IgG (right) antibody binding to wild-type (WT), GTKO, double-knockout (i.e., deletion of expression of Gal and Sda), and triple-knockout (TKO, i.e., with additional deletion of expression of Neu5Gc) pig red blood cells (RBCs). Human serum antibody binding to pRBCs was measured by flow cytometry using the relative geometric mean (rGM), which was calculated by dividing the GM value for each sample by the negative control. Negative controls were obtained by incubating the cells with secondary anti-human antibodies only (with no serum). Human IgM and IgG binding to GTKO/β4GalKO/CMAHKO (TKO) pig RBCs was almost at the level of binding to human RBCs, and there was no detectable IgM or IgG binding to TKO RBCs. Binding to TKO pig RBCs was not significantly different from human IgM and IgG binding to human RBCs of blood type O. (*p<0.05, **p<0.01; ns = not significant). (B) Baboon (an Old World NHP, n=14) IgM and IgG antibody binding to WT, GTKO, DKO, and TKO pig RBCs. (Note that deletion of Neu5Gc [CMAH-KO] in pig cells appears to expose a fourth xenoantigen against which baboons have natural antibodies. Note also that the data support the observation that the deletion of expression of Gal has less effect in reducing antigenicity of human serum (70% reduction) (Figure1A), when compared with baboon serum (90% reduction) (*p<0.05, **p<0.01; ns = not significant). (Reproduced in part with permission from Cooper DKC et al. Xenotransplantation 2019; Apr 15:e12516. doi: 10.1111/xen.12516, reference 18).

Figure 5:

IgM (left) and IgG (middle) binding and complement-dependent cytotoxicity (right) of baboon sera to GTKO, GTKO/β4GalNT2KO (DKO), and TKO pig peripheral blood mononuclear cells (PBMCs). IgM and IgG binding and serum cytotoxicity to TKO cells were higher or comparable to binding to GTKO cells. Although mean IgM and IgG binding and mean serum cytotoxicity to DKO cells were less than to TKO cells, many baboons had a high level of cytotoxicity to DKO cells. (**p<0.01). (Reproduced with permission from Yamamoto T et al, Xenotransplantation. 2020; Jun 25:e12596, reference 21).

Figure 6:

Rejection-free survival of GTKO pig kidneys in baboons (Group 1, in black) was significantly longer that that of TKO pig kidneys (Group 2, in red). (Reproduced with permission from Iwase H, et al, Xenotransplantation. 2021. 25 May. e12700, reference 24)

Figure 7:

GTKO pig kidney survival in baboons receiving US FDA-approved immunosuppressive agents (Group A, in red) was much shorter than in those receiving an anti-CD40mAb-based regimen (Group B, in black). (Reproduced with permission from Yamamoto T, et al, Transplantation. 2019;103:2090–2104, reference 29)

Figure 8:

Survival of patients with a living donor kidney (top), a deceased donor kidney (second from top), on the waitlist while on dialysis (second from bottom), and while on dialysis but not waitlisted for a kidney transplant (bottom). Approximately 45% of patients receiving chronic dialysis while on the waitlist for a deceased human donor kidney either die or are removed from the waitlist (as considered no longer acceptable candidates for the procedure) within 5 years. (Reproduced with permission from Jagdale A, et al, Transplantation. 2021;105:1904–1908, reference 44)