Pig islet transplantation with costimulation blockade

Rita Bottino (1), Martin Wijkstrom (2), Hidetaka Hara (2), Carol Phelps (3), David Ayares (3), David K.C. Cooper (2), Massimo Trucco (1)

(1)Division of Immunogenetics, Children’s Hospital of Pittsburgh, and (2) Thomas E. Starzl Transplantation Institute, University of Pittsburgh, Pittsburgh, PA, and (3) Revivicor, Blacksburg, VA, USA.

            Considerable progress has been made in pig-to-nonhuman primate (NHP) islet transplantation (Tx), with 6 groups reporting graft function for periods >6 months. Two problems remain to be fully resolved – (i) the loss of islets from the immediate blood-mediated inflammatory reaction (IBMIR), and (ii) the establishment of a clinically-applicable immunosuppressive (IS) regimen that prevents a T-cell response to the graft.

            We shall here concentrate on a clinically-applicable IS regimen. Work progresses on two fronts – (i) the identification of a regimen that prevents a T-cell response, but which is associated with minimal complications, e.g., infection, malignancies, or thromboembolism. This approach might be facilitated if (ii) the islet-source pig could be genetically-engineered to either reduce the intensity of the T-cell response or to provide local protection against this response.

            Our own interest in costimulation blockade as a basis for the IS regimen arose from work by others in alloTx models, e.g., by Kenyon et al, and in the initial report by Buhler et al in 2000 that an anti-CD154 mAb-based regimen prevented a T-cell elicited antibody response in NHPs to transplanted pig hematopoietic cells whereas a cyclosporine-based regimen did not.

            Since 2006, 23 cynomolgous monkeys have undergone pig islet Tx at our center. ATG induction with anti-CD154mAb and MMF as maintenance therapy successfully prevented rejection of islets from genetically engineered pigs in 5 recipients that maintained normoglycemia for periods of >3 months to >1 year. There was neither a T-cell proliferative response on MLR nor an elicited anti-pig antibody response in any recipient. Although there were no clinical features or necropsy findings consistent with thromboembolism (prevention of which was attempted with 40 mg/day aspirin p.o.), the likely unavailability of anti-CD154 mAb for clinical Tx has led us and others to search for an alternative.

            From studies in the pig-to-baboon artery patch Tx model, we have demonstrated that, in the absence of anti-CD154 mAb, blockade of both the CD40:CD154 pathway (with an anti-CD40 mAb) and the CD28:B7 pathway (with belatacept) is required to prevent a T-cell response. We plan to test this regimen in our pig-to-monkey islet Tx program.

            Genetic engineering of the islet-source pig might reduce the intensity of the IS therapy required. The production of pigs transgenic for a mutant human MHC class II transactivator (CIITA-DN) results in downregulation of expression of swine leukocyte antigen (SLA) class II even when the cells are activated. We are uncertain whether SLA class II is expressed on pig islets and so its beneficial effect might be limited to vascular endothelial and immune cells transplanted with the islets. An alternative approach is through the transgenic expression of a costimulation-blockade agent, e.g., CTLA4-Ig, in the beta-cells of the islets using the insulin promoter, but it is too preliminary to know whether this is effective.

Source of funding: US Department of Defense; Revivicor.