The immunoalteration of pancreatic allografts.

Since George Snell, in 1957, first postulated that transplantable tissues and organ grafts contain cells or "passenger leucocytes" which, alone, could perhaps be responsible for graft rejection and eventual graft loss, many researchers have endeavoured not only to identify these leucocytes, but have also attempted to eradicate them from the graft itself prior to transplantation. Professor Frank P.Stuart, at the University of Chicago, demonstrated that following irradiation, and subsequent elimination of these cells, animal kidneys enjoyed a prolonged survival when transplanted into non-immunosuppressed recipients. Unfortunately, many of the subsequent pretreatment regimens which were developed, including irradiation, cyclophosphamide or azothiaprine pretreatment, were only effective if performed four or five days prior to transplantation. This is clearly not applicable to the clinical situation. The removal of these passenger leucocytes or dendritic cells from transplantable tissue is fundamentally important in reducing the immunogenicity of that tissue. There is a need, therefore, to develop an effective method for removing these dendritic cells from a whole organ, prior to transplantation, which will not only be effective in reducing graft immunogenicity but also be clinically applicable. With the increasing knowledge of what these immunogenic cells are, and with the concomitant development of monoclonal antibodies, I have been developing an animal model to study the effects of pretreating whole, intact organs with monoclonal antibodies targeted against these dendritic cells, and studying the effect upon allograft survival following transplantation after this pretreatment. An ex vivo perfusion circuit was developed which enabled an isolated intact rat pancreas (or kidney) to be perfused with monoclonal antibodies for up to six hours, without undue damage to the organ, and subsequently be transplanted. The effect on the survival, after transplantation, of these pancreases which had been pretreated in this way is the main issue addressed in this thesis. The thesis is divided into three sections. Section I is the general introduction and gives an historical account of pancreas transplantation, diabetes, and previous studies dealing with tissue immunoalteration. Section II constitutes most of the experimental work surrounding the attempts at 'immune modulation'. It describes the development of the ex vivo perfusion circuit and reports the effects on allograft survival of transplanting pancreases perfused with anti-Class II monoclonal antibodies for different periods of time. In addition to the ex vivo perfusion data, this section contains the results of in vitro studies performed on isolated pancreatic islets. Pancreatic islets were cultured with allogeneic T lymphocytes in mixed islet lymphocyte cultures. The effect of pretreating these islets with anti-Class II MAb's on the stimulation of these T cells was studied. Section III provides evidence that a new programme involving clinical pancreatic transplantation as a safe and reliable alternative for the treatment of diabetes can be set up successfully. Finally, Section III comprises the general discussion of the thesis. It concludes that the pretreatment of vascularised organ allografts can effectively reduce their immunogenicity, but not sufficiently to abrogate allograft rejection altogether. The elimination of the 'passenger' dendritic cells may be an important adjunct in the battle to reduce the severity and frequency of allograft rejection episodes, and perhaps allow reduction in non-specific immunosuppression.