Approaching the clinical application of xenotransplantation

Ig knock-in mice producing anti-carbohydrate antibodies: breakthrough of B cells producing low affinity anti-self antibodies

Natural Abs specific for the carbohydrate Ag Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) play an important role in providing protective host immunity to various pathogens; yet little is known about how production of these or other anti-carbohydrate natural Abs is regulated. In this study, we describe the generation of Ig knock-in mice carrying functionally rearranged H chain and L chain variable region genes isolated from a B cell hybridoma producing alphaGal-specific IgM Ab that make it possible to examine the development of B cells producing anti-carbohydrate natural Abs in the presence or absence of alphaGal as a self-Ag. Knock-in mice on a alphaGal-deficient background spontaneously developed alphaGal-specific IgM Abs of a sufficiently high titer to mediate rejection of alphaGal expressing cardiac transplants. In the spleen of these mice, B cells expressing alphaGal-specific IgM are located in the marginal zone. In knock-in mice that express alphaGal, B cells expressing the knocked in BCR undergo negative selection via receptor editing. Interestingly, production of low affinity alphaGal-specific Ab was observed in mice that express alphaGal that carry two copies of the knocked in H chain. We suggest that in these mice, receptor editing functioned to lower the affinity for self-Ag below a threshold that would result in overt pathology, while allowing development of low affinity anti-self Abs.

Liver transplantation: current status and novel approaches to liver replacement

The major challenge currently facing liver transplantation is the performance of a greater number of liver transplants, which has been fueled by the large and growing disparity between the increasing number of qualified patients listed for transplantation and the relatively static number of available cadaver donor organs. In the past 2 years, approximately 4500 liver transplants have been performed annually, with 1-year survival rates in the 85%-90% range, while the waiting list has expanded as of November 2000 to more than 16,000 patients, resulting in an increasing death rate among listed patients. In the short term, there will continue to be a major focus on more effective use of available cadaver donor organs to balance the competing principles of justice (patients with most urgent need for transplant and lower probability of posttransplant survival) and medical utility (patients with less urgent need for transplant and higher odds of postoperative survival). Over the long term, there will be an increasing application of novel approaches to liver replacement including cadaver split liver transplantation and adult living donor liver transplantation and possibly, in the more distant future, xenotransplantation and hepatocyte transplantation. The treatment, and ideally the prevention, of recurrent disease after liver transplantation, particularly chronic hepatitis C-the most common indication for transplantation-is a major priority to optimize the use of liver grafts. Finally, improved immunosuppressive strategies, including movement toward minimal immunosuppression and steroid withdrawal and the development of safer and more effective drugs, is another important factor that has the potential to increase the success of liver transplantation.

Defining appropriate health status and management programs for specific-pathogen-free swine for xenotransplantation

Swine are expected to be utilized as xenograft donors for both whole-organ and cellular transplantation. In order to meet the criteria for regulatory guidelines, donor animals are going to have to be free of potential zoonoses and other complicating diseases. Screening of animals will have to include tests for viruses, bacteria, parasites, congenital defects, and other inapparent diseases such as neoplasia or metabolic dysfunctions. The term Specific-Pathogen-Free (SPF) swine is a proprietary designation in the U.S. that does not include screening for all appropriate organisms for xenotransplantation. A program for breeding animals as xenograft donors will have to be conducted in a biomedical research facility rather than a conventional farm setting. The research programs at such a facility should include serology, microbiology, necropsy, histology and parasitology. The use of sentinel animal monitoring in a research facility is one method to ensure compliance. It will be impossible to provide complete individual animal screening in a timely fashion prior to performing a xenograft transplant. Quality control measures need to ensure that there is a reasonable confidence that the donor tissue is appropriate for the procedure to be performed. It is suggested that a term such as xenograft-defined flora be used to designate the appropriate health status of donor animals rather than SPF in order to avoid confusion with existing standards.