Clinical Kidney Xenotransplantation—Why Do We Not Transplant Both Pig Kidneys Into the Recipient?

At present, because of the scarcity of deceased human donor kidneys, only one kidney is transplanted into each recipient. When pig kidney xenotransplantation is fully established as a clinical therapy, the chronic shortage of organs suitable for transplantation will no longer be a limiting factor. However, initial clinical observations suggest that the pig kidney is rather smaller in size compared to a human kidney in a donor of similar body weight. If a smaller pig kidney is transplanted into a larger patient, this may result in a serum creatinine that is persistently higher than normal. It is likely, therefore, that the insertion of both donor kidneys would result in (i) improved immediate function and (ii) longer survival if chronic rejection is slowly developing. Whether the transplantation of both pig kidneys would affect the incidence of proteinuria and/or hydronephrosis seen in pig-to-nonhuman primate models needs to be investigated. A major expense for the companies that will breed gene-edited pigs for the purpose of transplantation will be the cost of housing them under designated pathogen-free conditions. If both kidneys were excised for transplantation in a single patient, the period of pig housing could be reduced, thus reducing the cost, but this would be offset by the need to breed many more pigs. However, if the transplantation of a pair of kidneys increased patient survival, this would warrant the additional expense on humanitarian grounds and possibly also on economic grounds. It could be argued that the transplantation of two kidneys might complicate a subsequent surgical procedure if the patient were eventually allocated a deceased human donor kidney. However, as many of the patients who will be assessed for pig kidney transplantation will be selected because they face a period on the waiting list of 5 or more years, it seems illogical to take this factor into consideration at the time of pig kidney transplantation. Good function of two kidneys may negate the need for the patient to ever require an allograft. The immediate care of the patient should take precedence over possible future care. This work was supported in part by the NIH NIAID U19 grant AI090959 and in part by a Kidney X Prize from the US DHHS and the American Society of Nephrology. D.K.C.C. is a consultant to eGenesis Bio, Cambridge, MA, USA, but the views expressed in this letter are those of the author and do not necessarily reflect those of the company.