It is well known that transplants from one animal into another animal of the same species, such as human to human, are a routine treatment option for many serious conditions including kidney, heart, lung, liver and other organ disease and skin damage such as severe burn disease. However, it is well known that there are not enough suitable organs available for transplant to meet current or expected clinical demands for organ transplants. More than 18,000 human patients are on the UNOS liver transplant national waiting list, yet less than 7,000 transplants are performed annually in the United States. In patients with kidney failure, dialysis increases the length of time the patient can wait for a transplant. There is no system comparable to dialysis available for patients with liver disease or liver failure.
Xenotransplantation, the transplant of organs, tissues or cells from one animal into another animal of a different species, such as the transplantation of a pig organ into a human recipient has the potential to reduce the shortage of organs available for transplant, potentially helping thousands of people worldwide. However, xenotransplantation using standard, unmodified pig tissue into a human or other primate is accompanied by severe rejection of the transplanted tissue. The rejection may be a hyperacute rejection, an acute rejection, a chronic rejection or may involve survival limiting thrombocytopenia coagulopathy. The human hyperacute rejection response to pig antibodies present on transplanted tissue is so strong that the transplant tissue is typically damaged by the human immune system within minutes or hours of transplant into the human. Attempts have been made to transplant human patients with livers from other organisms such as pigs; however, immediately upon reperfusion of the liver xenotransplant, the patient begins to undergo survival limiting thrombocytopenia coagulopathy. The mechanism by which the thrombocytopenia occurs is unknown; although in 1983 it was reported that hepatocytes bind the platelets causing the thrombocytopenia. Thrombocytopenia or thrombopenia is an abnormal decrease in the platelet number of a patient that also occurs in multiple hemorrhagic conditions.
Many strategies have been employed to address the rejection response including removing porcine genes encoding α(1,3) galactosyltransferase and CMAH to prevent expression of the enzymes. However, progress in this field is critically dependent upon the development of genetically modified pigs. Unfortunately, developing homozygous knockout pigs is a slow process, requiring as long as three years using traditional methods of homologous recombination in fetal fibroblasts followed by somatic cell nuclear transfer (SCNT), and then breeding of heterozygous knockout animals to yield a homozygous knockout pig. The development of new knockout pigs for xenotransplantation has been hampered by the lack of pluripotent stem cells, relying instead on the fetal fibroblast as the cell upon which genetic engineering was carried out. For instance, the production of the first live pigs lacking any functional expression of α(1,3) galactosyltransferase (GTKO) was first reported in 2003. U.S. Pat. No. 7,795,493 to Phelps et al describes a method for the production of a pig that lacks any expression of functional αGal.
Profound thrombocytopenia has been transmitted to recipients in cases where a donor liver was procured from an individual suffering from idiopathic thrombocytopenic purpura (ITP). The thrombocytopenia in the recipients was refractory to treatment and only alleviated once the ITP donor liver was removed and the patient retransplanted with another liver. Patients with preexisting ITP who received liver transplants for other diseases have experienced alleviation of the ITP (Taylor et al (2006) Liver Transplant 12(5):781-791; Diaz et al (2008) Liver Transplant 14(12):1803-1809; and Pereboom et al (2009) Transpl Int, herein incorporated by reference in their entirety.
Platelets survive in the human/baboon circulation for 7-10 days and are removed by the liver and spleen. Cold stored platelets are rapidly cleared from the circulation, forcing exogenous platelets to be used for transfusion to be stored at room temperature. Storage at room temperature reduces platelet shelf live to 5 days. Cold-stored platelets have been thought to be removed from the circulation by a mechanism involving the Kupffer cells and hepatocytes. Mac-1 receptors on Kupffer cells irreversibly recognize clustered β-GlcNac-terminating immature glycans on GPIb receptors on short term (<4 hours) cooled (0° C.) platelets. See Hoffmeister et al (2003), Cell 112:87-97 and Josefsson et al (2005) J. Biol. Chem 280:18025-18032, herein incorporated by reference in their entirety.