Improving engraftment and survival of pig islet xenografts through transgenic expression of human CD47

In selected patients with Type 1 Diabetes whose blood glucose levels are difficult to manage under standard treatment, islet (containing the cells that produce insulin) transplantation provides excellent blood glucose control and prevents occurrence of hypoglycemia events and their associated morbidity and mortality. The insufficient availability of donor pancreata and the substantial number of islets required for each islet recipient, due largely to an early and rapid loss of islets transplanted in the liver, limit the widespread application of this treatment to patients in need. While solutions to improving allogeneic, or same species, transplant efficacy are being sought, transplantation of xenogeneic, or cross-species (e.g. porcine to human), islets remains the most viable option for significant expansion of the donor islet pool. Porcine islets are capable of normalizing blood glucose level of diabetic animals including non-human primates under immune suppression. Unlimited number of islets can potentially be prepared from designated pathogen-free donor pigs. The pig islets are, however, subject to more severe and rapid non-specific “innate” and specific “adaptive” human immune rejection, compared to that of human islets, due to incompatible protein interactions between the two different species. For example, one of the important cellular surface proteins, CD47, functions as a “marker of self” that upon binding with its receptor on macrophages, a non-specific immune cell, sends a “don’t eat me” signal and protects itself from being eaten by the macrophages. The pig CD47 proteins, however, are not recognized by their receptor on the human macrophages as “self”, hence the pig cells are destroyed by the macrophages soon after transplant. In addition, specific immune rejection that involves T cells will be stimulated as a result of activation of macrophages and other innate immune cells. We hypothesize that expression of human CD47 on pigs through genetic modification will allow their cells to be accepted by the human macrophages and will subsequently reduce rejection by both the innate and adaptive immune responses. brbrFor this one-year pilot research, we propose to test our hypothesis utilizing pigs expressing human CD47 generated by our team that have demonstrated significantly improved survival of bone marrow stem cells when transplanted into non-human primates and in mice reconstituted with a human immune system (i.e., humanized mice). Two mouse models are proposed as the pig islet transplant recipients in this application. One is NSG mice whose macrophages, like that of the human counterparts, recognize human, but not the pig CD47 protein. Another is the humanized mouse model generated by our core facility. In the first model, macrophages are the major immune cell population that mediates rejection, while rejection in the second model involves both human macrophages and adaptive immunity (mainly T cells). Like that for human islet transplantation, the pig islets will be transplanted into the liver of these mice (with pre-induced diabetes by chemical treatment) to determine whether carrying the human CD47 protein will make them less foreign and thus engraft better compared to the control pig islets. Islet graft function will be evaluated by measuring blood glucose and porcine islet hormone levels, and by examining immune cell activation and islet-graft rejection. While studies utilizing the NSG mice will specifically assess the protective effect of human CD47 expression against macrophage-mediated rejection, studies in the humanized mice will determine the potential of human CD47 protein to prevent pig islet rejection by human M?s and human T cells. The proof of success with our strategies in mice reconstituted with human immune components will provide a direct conduit for clinical translation.