David H Sachs, MD

Dr. Sachs’ major areas of research are the following:

A. Induction of transplantation tolerance:

A major thrust of transplantation research in this laboratory has been directed toward induction of tolerance as a means of permitting transplantation of organs to be achieved without a requirement for immunosuppressive drugs. Two major models for tolerance induction are studied:

1) Regulatory tolerance: Using miniature swine with defined MHC haplotypes (see below), Dr. Sachs and colleagues demonstrated that a short course of immunosuppression with a high-dose of calcineurin inhibitors is capable of inducing long-term tolerance in juvenile animals. This phenomenon has become and remains an important basic model for the study of tolerance induction across MHC barriers in a large animal model. Among active studies being carried out in this model are: a) the role of the thymus in permitting regulatory tolerance to develop; b) the dependence of tolerance on T regulatory cells; c) the adoptive transfer of tolerance; and d) the mechanism of tolerance induction and maintenance at cellular and molecular levels.

2) Deletional tolerance: It has been known for many years that when a bone marrow transplant is successful, it carries with it tolerance to any other tissue or organ from the bone marrow donor. Dr. Sachs’ research demonstrated that this kind of tolerance does not require a complete bone marrow transplant, but rather only requires that a small number of bone marrow-derived cells persist in the recipient at the time of the organ transplant. This state, involving a mixture of bone marrow from recipient and donor, has been called “mixed chimerism” and is the method which investigators in this laboratory have developed to induce tolerance to transplanted organs without the requirement for a full bone marrow transplant or for lifelong immunosuppressive drugs. Since their original description of this phenomenon in mice, Dr. Sachs and colleagues have pioneered a series of studies, extending the work first to large animals and most recently to clinical applications. Because of these studies, there are now patients who have normal, functioning kidneys without the need to remain on chronic immunosuppressive drugs.

B. Development of miniature swine as a large animal transplantation model:

More than 40 years ago, Dr. Sachs and colleagues recognized the need for a large animal model for studies of transplantation, and chose miniature swine because of their appropriate size, physiologic and immunologic similarity to man, and their breeding characteristics. When full grown, these animals reach sizes approximately the same as human beings, with maximal weights of approximately 200-300 pounds -- in contrast to domestic swine, which can attain weights over 1,000 pounds, and would therefore be a difficult laboratory animal model. Like domestic swine, miniature swine have outstanding reproductive parameters, making it possible to establish lines of miniature swine homozygous for different alleles at the MHC of pigs, known as SLA. These MHC defined animals have become an invaluable resource for preclinical research in the field of organ transplantation, providing numerous insights for understanding tolerance and histocompatibility (see above). In addition, the capacity for genetic engineering of miniature swine has enabled production of both transgenic and knockout animals, making one subline of these inbred miniature swine likely to be the ideal organ xenograft donor (see below).

C. Xenogeneic transplantation

Even if the induction of tolerance is able to eliminate treatment-related complications and chronic rejection as limitations to allogeneic transplantation, it will not solve the third problem in this field, the shortage of available donor organs. It is for this reason that xenotransplantation, the transplantation of organs from animals to humans, has also been the subject of major investigations in Dr. Sachs’ laboratory. For a variety of reasons, most investigators in this field have settled on the pig as the most appropriate potential donor for xenotransplantation of organs to human recipients. Pigs are readily available, have very favorable reproductive characteristics, can be bred in controlled, clean environments to avoid pathogens and are less likely to raise ethical concerns about their use for this purpose than non-human primates, since they are accepted as a food source in most modern societies. Dr. Sachs and colleagues have developed a special strain of miniature pigs through selective breeding over more than 30 years. Unlike domestic swine, which attain adult weights of greater than 1000 pounds, the maximum size of these “miniature swine” is 200 to 300 pounds. They can therefore be used as organ donors at sizes appropriate for any potential human recipient, from a baby to a large adult. In addition, through selective breeding, it has been possible to control the genetics of these animals and achieve a very high coefficient of inbreeding, so that all animals of one strain are essentially identical. This property could be very important for the induction of tolerance, since one could use the cells from one animal to induce tolerance for an organ from another animal of the same strain.

Over the past twenty years, extensive research in this laboratory has been performed using pig to non-human primate transplants as a model for potential clinical xenotransplants. The major stumbling block to these studies originally was the presence of a large amount of natural (i.e. present without intentional induction) antibody in non-human primates and humans directed toward the cells of pigs. The reason for the presence of these antibodies is that during evolution, at the level of Old World primates, a gene was lost for an enzyme that puts the Gal sugar epitope onto cell surface glycoproteins in all species except for Old World primates and humans. Since the Gal antigen is found on bacteria and other environmental antigens, humans and Old World primates make a large amount of antibody against Gal. It is these anti-Gal antibodies that then reacted with pig tissues after xenotransplantation, causing vigorous rejection.

Dr. Sachs’ group was among the first to eliminate this enzyme from their special inbred strains of miniature swine through an intentional “knock-out” mutation. These new pigs, like humans and Old World primates, do not put Gal on to the surface of their cells. As a result, xenotransplants can now be performed without the powerful rejection previously caused by natural anti-Gal antibodies. The results have been remarkable, with increased survivals of both heart and kidney transplants from pigs-to-baboons. Using immunosuppressive drugs, organ survivals were prolonged using these new Gal knock-out (GalT-KO) pigs, but new antibodies soon appeared, causing rejection. However, using a regimen directed toward induction of tolerance, organ survivals were prolonged markedly, and no rejection was seen. Studies of tolerance induction in the pig-to-primate models are a major area of ongoing research in Dr. Sachs’ laboratory.