Induction of tolerance to human renal allografts with bone marrow and antilymphocyte globulin

Alloantigen-induced specific immunological unresponsiveness

When the immune system encounters alloantigen it can respond in any one of a number of different ways. The choice that is made will take into account factors such as where, when and how the contact with the alloantigen takes place, as well as the environmental conditions that prevail at the time the alloantigen is encountered. Alloantigen administration before transplantation either alone or in combination with therapeutic agents that modulate the functional activity of the responding leucocytes can be a powerful way of inducing specific unresponsiveness to alloantigens in vivo. The molecular mechanisms that influence the way the outcome of the immune response to alloantigen develops, either activation or unresponsiveness to the triggering antigen, hold the key to our ability to manipulate the immune system effectively by exposing it to donor antigen for therapeutic purposes. This review will focus on alloantigen-induced immunological unresponsiveness and how insights into the mechanisms of unresponsiveness have driven the development of novel tolerance-induction strategies that show promise for translation into the clinic in the future.

Peripheral tolerance to alloantigen: strategies for the future

The mature, adult immune system is specifically designed to eliminate any foreign material that may enter the body, but not to respond to the body's own tissues and molecules. Indeed, during development, the potential of the immune system to respond to self antigens is removed, by eliminating or effectively silencing any autoreactive cells. These features are well adapted under normal circumstances, as they result in the efficient elimination of potentially harmful agents thereby protecting the body from infection and malignancy. However, in the context of transplantation, this 'normal' response is diametrically opposed to the desired clinical outcome, which is clearly the long term function and survival of the transplanted tissue. To prevent graft rejection the immune system of the transplant recipient has to be manipulated to ensure that it is incapacitated. Immunosuppressive drugs can be used for this purpose and are undoubtedly effective; indeed they have had a dramatic impact on success rates in clinical organ transplantation. However, as the mechanism of action of these chemical immunosuppressants is immunologically non-specific, any immune response the recipient may need to make after transplantation, as well as the rejection response, is suppressed. In addition, to maintain graft survival the drugs have to be taken indefinitely after transplantation and therefore their use is not only associated with immunological complications such as increased risks of infection and malignancy, but also numerous non-immunological side-effects. One way to overcome these problems would be to develop strategies for specific immunosuppression, such that only leukocytes capable of responding to the foreign histocompatibility or alloantigens expressed by the transplanted tissue would be affected. The ability to manipulate or reprogramme the adult immune system in such a way as to induce specific immunological unresponsiveness or tolerance to the alloantigens of the organ donor would offer many advantages over conventional immunosuppressive therapy. Only leukocytes reactive with donor alloantigens would be affected, thus allowing transplant recipients to respond effectively to other immunological stimuli, such as virus infections.

Alternative approaches for the induction of transplantation tolerance

The immunosuppressive drugs currently in use in clinical transplantation are undoubtedly very effective at controlling graft rejection. However, their use is associated with a large number of side-effects, both immunological and non-immunological, particularly in the longer term. From an immunological point of view, the major disadvantage of these agents is that their mode of action is immunologically non-specific, resulting in blanket or pan-immunosuppression of the recipient's immune system. Thus, not only is the recipient's immune response against the organ graft suppressed, but responses to all other antigenic stimuli such as viral infections are also prevented. The transplant recipient can therefore become severely immunocompromised as a result of the drug therapy and is susceptible to opportunistic infections and an increased incidence of cancer. One of the aims for the design of new immunosuppressive therapy is to develop protocols that are both effective and immunologically specific, such that only the immune response to the transplanted organ is suppressed. The optimal approach would be to induce tolerance to the donor histocompatibility antigens before transplantation, permanently paralysing the ability of the recipient's immune system to mount a rejection response against the graft. Some of the approaches for the induction of immunological tolerance currently being explored in clinical transplantation will be discussed. These include total lymphoid irradiation (TLI) and donor bone marrow transfusion combined with anti-lymphocyte globulin (ALG) post-transplantation. In addition some new approaches for the induction of tolerance before transplantation currently being investigated in experimental systems will be presented.