**9.1 Induction of tolerance in transplant patients**

Clinical allograft transplantation research has been conducted to identify methods to induce full or partial tolerance in transplant patients. These strategies are not ready for general clinical use until further evidence-based studies are available.

#### *Full tolerance*

The holy grail of organ transplantation is full immunologic tolerance, a state of indefinite survival of a well-functioning allograft that does not require maintenance immunosuppression. In addition, the host must retain a normal immune response and not suffer from immunosuppression-related infections, neoplasia, or other drug-related adverse effects. Rare cases of operational tolerance after transplantation, with complete cessation of immunosuppressive therapy have been observed and reported; these cases generally are associated with patient noncompliance regarding therapy.

Most studies concerning the intentional induction of immunologic tolerance have involved patients with hematologic malignancies. Full tolerance was achieved with myeloablative therapy prior to organ transplantation in combination with induced donor chimerism by means of bone marrow transplantation and excellent human leukocyte antigen (HLA) matching. Mixed chimerism retains a graft-versus-host T-cell effect that allows for transplant acceptance despite the subsequent disappearance of the donor chimerism.

Myeloablative therapy includes total body irradiation and lymphoablative methods, such as total body irradiation and the use of azathioprine and corticosteroids. However, the complications of full tolerance and the unpredictable timing organ transplantation with regards to the time required for myeloablative therapy prior to transplantation precludes the routine application of these therapies.

Cosimi and Sachs studied mixed chimerism in a small number of patients. They used nonmyeloablative conditioning, such as peritransplantation low-dose total-body irradiation or thymic irradiation plus anti-thymocyte globulin therapy combined with splenectomy. Donor-specific marrow infusion was given at the time of transplantation. Cyclosporine was given for about a month after transplantation and then stopped. Patients had transient chimerism for several weeks, and graft survival was approximately 70% over the long term.

Tregs are responsible for maintaining tolerance by broadening suppression through a mechanism termed linked-suppression, where tolerance to a specific epitope is spread to all epitopes of that protein; this tolerance is also spread to cohorts of naïve T-cells as they develop. Immunologist Herman Waldmann described this phenomenon as a process of infectious tolerance. Tregs from tolerant animals can be transferred to naive animals, in which they subsequently confer antigen-specific tolerance, including tolerance to skin and organ allografts. Although not completely understood, this is referred to as adoptive tolerance, and it has been recognized since the 1990s when Dr. Metcalfe at Cambridge published a landmark paper describing this phenomenon.

immune tolerance, methods to induce states of "partial tolerance" have been discovered; in these cases, lower-than-conventional amounts of ongoing pharmacologic immunosuppression are required to prevent rejection. Nonetheless, immune tolerance remains the holy grail of

Clinical allograft transplantation research has been conducted to identify methods to induce full or partial tolerance in transplant patients. These strategies are not ready for general

The holy grail of organ transplantation is full immunologic tolerance, a state of indefinite survival of a well-functioning allograft that does not require maintenance immunosuppression. In addition, the host must retain a normal immune response and not suffer from immunosuppression-related infections, neoplasia, or other drug-related adverse effects. Rare cases of operational tolerance after transplantation, with complete cessation of immunosuppressive therapy have been observed and reported; these cases generally are

Most studies concerning the intentional induction of immunologic tolerance have involved patients with hematologic malignancies. Full tolerance was achieved with myeloablative therapy prior to organ transplantation in combination with induced donor chimerism by means of bone marrow transplantation and excellent human leukocyte antigen (HLA) matching. Mixed chimerism retains a graft-versus-host T-cell effect that allows for

Myeloablative therapy includes total body irradiation and lymphoablative methods, such as total body irradiation and the use of azathioprine and corticosteroids. However, the complications of full tolerance and the unpredictable timing organ transplantation with regards to the time required for myeloablative therapy prior to transplantation precludes

Cosimi and Sachs studied mixed chimerism in a small number of patients. They used nonmyeloablative conditioning, such as peritransplantation low-dose total-body irradiation or thymic irradiation plus anti-thymocyte globulin therapy combined with splenectomy. Donor-specific marrow infusion was given at the time of transplantation. Cyclosporine was given for about a month after transplantation and then stopped. Patients had transient chimerism for several weeks, and graft survival was approximately 70% over the long term. Tregs are responsible for maintaining tolerance by broadening suppression through a mechanism termed linked-suppression, where tolerance to a specific epitope is spread to all epitopes of that protein; this tolerance is also spread to cohorts of naïve T-cells as they develop. Immunologist Herman Waldmann described this phenomenon as a process of infectious tolerance. Tregs from tolerant animals can be transferred to naive animals, in which they subsequently confer antigen-specific tolerance, including tolerance to skin and organ allografts. Although not completely understood, this is referred to as adoptive tolerance, and it has been recognized since the 1990s when Dr. Metcalfe at Cambridge

transplant acceptance despite the subsequent disappearance of the donor chimerism.

transplantation immunology and clinical transplantation[29].

clinical use until further evidence-based studies are available.

associated with patient noncompliance regarding therapy.

published a landmark paper describing this phenomenon.

the routine application of these therapies.

**9.1 Induction of tolerance in transplant patients** 

*Full tolerance* 

Tolerance induction through the expansion and transfer of donor Tregs to an allograft recipient or by means of the ex vivo development of Treg from recipient T cells are intriguing but yet-untested strategies in humans. Currently, in heart transplantation, analysis of whether FOXP3 gene expression in peripheral blood cells reflects anti-donor immune responses is underway. Overall, these possibilities represent exciting ways to broaden the translational approach of tolerance induction through the use of T regulatory cells.

#### *Partial tolerance*

At present, partial tolerance that requires minimal immunosuppression is possible. This allows for the minimal use of immunosuppressive drugs, which results in reduced risks for infection, neoplasia, and drug-related adverse effects. Partial, or incomplete, donor-specific tolerance has been termed minimal immunosuppression tolerance or prope tolerance from the Latin word for near.

Professor Sir Roy Calne postulated that prope tolerance preserves some of the transplant recipient's immune responses to infection and other antigens, reducing morbidity and mortality caused by immunosuppressive effects.

Although numerous researchers are investigating assays to monitor the degree of immunosuppression, no assays or tests are currently available to monitor tolerance. Dr. Sarwal at Stanford University is currently using exciting microarray technologies to describe genetic identifiers of allograft recipients that are rendered tolerant. This technology may be what is required to overcome the current barriers to allograft tolerance.

Identifying either prope or complete tolerance depends on the elimination, withdrawal, or reduction of maintenance immunosuppression followed by the observation of a favorable response. Allograft biopsies may or may not be helpful in identifying rejection at an early stage if the strategy is unsuccessful. Indeed, a specific directive of granting agencies, such as the National Institutes of Health and the Immune Tolerance Network, is to fund research to develop tolerance assays.

The demonstration of immune tolerance induction in many rodent models stands in stark contrast to the lack of success in humans and primates, with the exception of myeloablative therapy followed by donor-derived stem cell infusion. The specific pathogen-free environment in which rodents are housed for their lifetime limits the number of memory T cells that they develop. In contrast, humans and primates are exposed to many viruses during their long and less pathogen-free lives. In addition, they generate a considerable pool of self-renewing memory T cells; in fact, nearly half of circulating T cells in adult humans are memory T cells. Therefore, they are less immunologically naïve compared to experimental rodents. Many of these memory T cells can cross-react with foreign MHC. Therefore, the translation of tolerance induction strategies from the rodent laboratory models to large animals and then to humans may need to account for differences in previous specific and net immunologic memory.
