Diabetes is a leading cause of both cardiovascular disease and end stage renal disease (ESRD), and incidence is increasing across the country and across the globe (1). Human islet transplantation is an effective treatment for diabetic patients but requires lifelong immunosuppression: prospective islet transplant recipients must weigh the risks of immunosuppression against the short- and long-term complications of diabetes. Patients with diabetic nephropathy represent a unique – and growing – population that would benefit from both islet and kidney transplantation. Indeed, the favorable risk-to-benefit considerations of combined islet and kidney transplantation in this population inspired recent promising clinical studies in islet after kidney transplantation led by the Clinical Islet Transplantation (CIT) Consortium (2). However, at present these procedures are rare, due, in part, to a shortage of deceased donor organs (3). Xenotransplantation using organs derived from pigs may overcome this organ shortage and allow for broader application of combined islet and kidney transplantation.

The past several years have seen enormous progress in the field of xenotransplantation, with advances in gene-editing and immunosuppression leading to long-term survivals of both kidney and heart xenografts in pig-to-nonhuman primate (NHP) studies (4–7), as well as early studies (preclinical and clinical) in humans (8–10). Clinical translation of porcine islet transplantation predated these recent successes in solid organ xenotransplantation, with encouraging pig-to-NHP studies leading to several small clinical studies using porcine islets in humans (11–21). However, results of these early studies in clinical islet xenotransplantation have been mixed. While these differences between outcomes of preclinical and clinical xenogeneic islet transplantation may be partly explained by differences in the immunosuppression regimens used in the clinical trials – notably, CD40/CD40L costimulatory blockade, which has been critical to success in most preclinical studies, was not utilized – further trials have been limited by more recent consensus guidelines outlining an international framework to promote standardized clinical translation of pig-to-human islet transplantation from source pig development and manufacturing to patient monitoring (22).

Moreover, in the many years since these clinical studies in islet xenotransplantation were conducted, the landscape of diabetes management has changed. Patients with diabetes have other options for durable disease management. Innovations in glucose monitoring and the rapid development of hybrid closed-loop insulin delivery systems have improved quality of life for patients living with diabetes (23), and ongoing clinical trials of novel stem-cell derived islet cell therapy have published early and highly promising results (24). However, porcine islet xenotransplantation remains a compelling therapeutic possibility for patients with diabetic nephropathy who need both kidney and islet replacement. In these patients, there are minimal added risks associated with islet transplantation, as these patients are already on immunosuppression for their kidney grafts; in fact, islets may help protect against premature kidney graft loss associated with diabetes (25) as well as improve long-term vascular diabetic outcomes (26). Here, we will briefly highlight immunologic barriers in porcine islet transplantation, chronicle preclinical progress in the field, and summarize our own experience in combined islet and kidney transplantation, using both 1) vascularized islets in an islet-kidney composite graft, and 2) our more recent strategy of sequential kidney followed by islet xenotransplantation.

Porcine xenografts, including pig islets, elicit robust immune responses in humans. These responses involve both innate barriers (27) – including preformed natural antibodies (Nabs) and species incompatibilities in complement and coagulation systems leading to dysregulation – and adaptive immune components (reviewed in (28)). As with transplantation of solid organs, humoral immunity remains a key obstacle to long-term xenograft survival, and T cell-targeted immunosuppression strategies have been critical for prolonging islet survival (20, 29),. Unlike transplantation of other organs, however, transplanted islets also trigger an immediate inflammatory response, known as instant blood-mediated inflammatory reaction (IBMIR) (30, 31), related to expression of tissue factor on islets and leading to activation of innate responses that subsequently consume islets (32, 33). Although IBMIR is seen in auto- and allogeneic islet transplantation, greater immune barriers in xenotransplantation may lead to more pronounced islet losses (34, 35) as high as 70% in some studies (36).

Various strategies have been developed to overcome these short- and long-term immunologic hurdles, including islet encapsulation and source pig genetic modifications – both of which are intended to reduce the immunogenicity of the porcine islets.

Broadly, islet encapsulation technologies include microencapsulation of islets in alginate matrix, and macro-encapsulation of immobilized islets in bi-layered PTFE with a common oxygenation chamber (37). This microencapsulation technique successfully reversed diabetes for up to six months in preclinical studies of rhesus macaques (38), and was subsequently used in two nationally regulated clinical studies of porcine islet xenotransplantation in New Zealand and Argentina. Follow-up studies confirmed modest clinical benefit including reduction in HbA1c, hospitalization, and severe hypoglycemic and/or hyperglycemic events (21, 39 ) The key advantage of these technologies is that encapsulation may protect islets from the recipient immune system and obviate the need for immunosuppression; whereas islet transplantation alone is currently reserved for patients with hypoglycemic unawareness due to the morbidity of immunosuppression, transplantation of encapsulated islets without immunosuppression may tilt the risk-benefit ratio in favor of islet transplantation for cure of diabetes. Still, important hurdles remain in broader clinical application of this technology including variable recipient immune responses to the encapsulation material, which may lead to fibrosis of encapsulated grafts.

Source pig genetic modification is another strategy to overcome innate immune barriers and can be divided into two major categories: elimination of carbohydrate antigens that are targets of preformed antibodies, and correction of species incompatibilities. In solid organ xenotransplantation, preformed antibody binding leads to hyperacute rejection of graft; in free islet xenotransplantation, preformed antibody binding leads to an amplified IBMIR with islet loss (40). While elimination of targets of preformed Nabs (particularly elimination of α-gal with creation of α-1,3 Galactosyl transferase gene knockout or GalTKO source pigs) has been essential for successful pig-to-NHP heart and kidney xenotransplantation (4, 41, 42), the impact of using GalTKO source pigs on xenograft survival in islet transplantation is less conclusive, which may be a function of changes in α-gal expression with islet maturation (43–45). Similarly, correcting for species incompatibilities between porcine and primate complement regulatory systems through individual insertion of human complement regulatory proteins may not significantly reduce the incidence of IBMIR (44). However, combining carbohydrate antigen gene knockouts with complement regulatory transgenes proves additive: xenogeneic islets from GalTKO.hCD55.hCD59 and GalTKO.hCD39.hCD46 source pigs demonstrated reduced islet loss and attenuated IBMIR (15, 16). More recently, islets derived from neonatal GalTKO.hCD55.hCD59 source pigs demonstrated cure of diabetes with >1 year of insulin independence in the stringent pig-to-baboon model (46).

Marked improvements in diabetes management and emerging therapies have changed the risk-benefit calculus associated with islet transplantation more broadly, and porcine islet xenotransplantation in particular. As described in the preceding section, encapsulation technologies – which may allow for durable glucose control without immunosuppression – remain one relevant application for porcine islet xenotransplantation. Another relevant strategy is combining porcine islet xenotransplantation with solid organ xenotransplantation. This strategy has already been employed with success by the CIT consortium treating diabetic nephropathy with islet transplantation after kidney transplantation, but broader application is limited by the shortage of deceased donor organs. The following sections detail our preclinical experience with combined islet and kidney transplantation, including both composite islet-kidney transplantation as well as kidney-first sequential islet and kidney xenotransplantation.

Definitive evaluation of composite I-K transplantation in a pig-to-NHP model also requires a control: independent kidney and free islet transplantation. Negative controls were present in previous studies of composite IK technologies – in pig-to-pig, baboon-to-baboon, and macaque-to-macaque models – and demonstrated preservation of islets with composite I-K transplantation across allogeneic barriers. In the past year, due to lack of inbred or cloned source pigs, we tested an alternative strategy for combined islet and kidney transplantation across a xenogeneic barrier that would also serve as a control of the composite I-K strategy. This alternative approach involves delayed islet transplantation after kidney and vascularized thymus transplantation (role of vascularized thymus transplantation in the induction of tolerance across xenogeneic barriers reviewed in (28)), using a recipient size-matched kidney and thymus source pig, as well as a large source pig for islets. Notably this approach (without thymus co-transplantation) is also similar to recent work in human islet-after-kidney transplantation conducted by the CIT consortium. Although additional cases are required, we have achieved reversal of diabetes and life-supporting renal function for 180 days with this kidney-first sequential islet and kidney xenotransplantation (52). To our knowledge, this is the first demonstration of maintenance of durable normoglycemia and stable creatinine with porcine kidney and islets in a diabetic and life-supporting pig-to-baboon combined kidney, vascularized thymus and islet xenotransplantation model. These preliminary results were recently presented at the International Xenotransplantation Association Congress (San Diego, 2023), and are described in detail in the following sections:

Porcine islet xenotransplantation is one promising strategy for cure of diabetes among an evolving landscape of emerging therapies in diabetes management. While islet-alone xenotransplantation strategies continue to show improvement with source pig genetic modifications and refinements to immunosuppression regimens, approaches like encapsulation which allow for reversal of diabetes without immunosuppression may be more clinically relevant. Porcine islet xenotransplantation, in conjunction with kidney xenotransplantation, remains a particularly compelling therapeutic possibility for patients with diabetic nephropathy who require both kidney and islet replacement, and who have already committed to immunosuppression for their kidney grafts. Composite islet-kidney transplantation has proven challenging in xenogeneic preclinical models; however, preliminary studies in islet-after-kidney xenotransplantation are promising and may point to a path forward with combined islet and kidney transplantation for diabetic nephropathy.

DE: Conceptualization, Formal Analysis, Investigation, Methodology, Writing – original draft. HI: Data curation, Formal Analysis, Investigation, Writing – review & editing. WLC: Formal Analysis, Methodology, Writing – review & editing. YH: Formal Analysis, Investigation, Writing – review & editing. WXC: Methodology, Writing – review & editing. MS: Formal Analysis, Investigation, Writing – review & editing. AS: Investigation, Writing – review & editing. DG: Methodology, Writing – review & editing. AM: Investigation, Writing – review & editing. KK: Methodology, Project administration, Writing – review & editing. ZS: Methodology, Supervision, Writing – review & editing. DW: Investigation, Methodology, Project administration, Writing – review & editing. KY: Conceptualization, Investigation, Methodology, Project administration, Supervision, Writing – review & editing.