The broad long-term objective of this application is to achieve sustained life-supporting cardiac xenograft survival in the pig-to-primate model using a protocol consisting of a genetically-engineered donor heart, a combination of currently available immunosuppressive drugs and techniques, and modalities to detect and treat rejection episodes that will allow clinical application of cardiac xenotransplantation. Pig-to-primate xenotransplantation (XTx) has been limited by antibody mediated rejection (AMR) initially to the Galactose a -1,3 Galactose (a-Gal) antigen. Anti-Gal AMR has been prevented in our laboratory using a-Gal polymers and specific immunosuppressive (IS) regimens. After 7 years of XTx research a median survival of 96 days in heterotopic transgenic pig heart to baboon transplants has been achieved. In the orthotopic, life-supporting position, survival of over 5 and 8 weeks has been reached, the best outcomes to date. As an alternative to a-Gal polymers, we have bred homozygous pigs deficient in the synthesis of the a-Gal antigen (GGTA1-KO). Recent heterotopic transplants with GGTA1-KO donors exhibit similar prolonged survival. Despite eliminating anti-Gal antibody, the xenograft (Xg) is rejected by a non-Gal anti-pig antibody response. The overall hypotheses of this proposal are 1) that the optimal pig donor to resist AMR is a GGTA1-KO;hCRP transgenic;2) the use of clinically applied immunosuppression can result in prolonged cardiac Xg function in the absence of a-Gal antigen;and 3) that Xg rejection can be diagnosed and treated by current methods for antibody control in patients with preformed antibodies. The proposal has three specific aims.
Specific Aim 1, to determine the preferred transgenic donor for pig-to-primate cardiac XTx by comparing survival, histology, immunohistology and immune responses using GGTA1-KO and GGTA1-KO;hCRP donor organs. The outcome addresses a central issue in pig-to-primate XTx, namely the role of the a-Gal antigen and the impact of hCRP's, on both graft outcomes and mechanisms of rejection. Furthermore, this work will define the base genetics that should be used for future genetic modifications.
Specific Aim 2 is to identify the minimal level of immunosuppression compatible with prolonged organ survival. Using donor organs defined in SA 1 these experiments will determine whether an immunosuppressive regimen based on current clinical IS for presensitized allograft recipients can control Xg rejection resulting in prolonged Xg survival. These experiments will define the relative contribution of basal IS and therapies that seek to block indirect antigen presentation and T-cell help.
Specific Aim 3 is to use orthotopic cardiac XTx to define the physiological function of the Xg and identify parameters (functional, biochemical, hematological and biophysical) that allow the detection of rejection and test methodologies to reverse these events. While Xg failure is associated with the presence of anti-pig antibody there is as yet no definitive proof that rejection is antibody mediated. These life-supporting transplants will allow the development of methods to accurately detect rejection based on cardiac function. In addition, by attempting to reverse rejection episodes with plasmapheresis we will identify the initiating mechanism of graft rejection. This proposal, if successful, lays the groundwork for the clinical application of cardiac XTx.
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