USE OF LOW-DOSE IL-2 TO EXPAND ENDOGENOUS REGULATORY T CELLS AND ACHIEVE TRANSPLANTATION TOLERANCE

A population of immune cells, called regulatory T cells or Tregs, have been shown in experimental animal models to regulate the immune system and to play a central role in preventing organ rejection and in controlling autoimmune and inflammatory diseases. In animal models of transplantation, Tregs are key to induce immunological tolerance, a situation in which the transplanted organ is indefinitely accepted in the absence of anti-rejection medication. The use of IL-2 at low doses has been shown to selectively and safely increase the number of Tregs in humans, but whether this can be used to induce tolerance in transplanted patients has never been investigated. Liver transplantation is an optimal clinical setting to do so, given that there are evidences indicating that transplantation tolerance can spontaneously occur in these patients (albeit only many years after transplantation), and that liver transplantation is the only solid organ transplantation setting in which it is safe to investigate the development of tolerance by intentionally discontinuing all anti-rejection medications.

Our hypothesis is that administration of a short course of low-dose IL-2 will transiently expand the number of Tregs and allow for the permanent discontinuation of anti-rejection medication. In the current project we propose to conduct a clinical study in which liver transplant recipients who have a very low probability of achieving tolerance spontaneously will receive daily injections of IL-2 during the period of time when their anti-rejection medication is gradually withdrawn. Patients will be carefully studied to investigate the following:

1) Whether IL-2 is efficacious in expanding the number of Tregs in liver transplant patients.
2) Whether the expansion of Tregs in the circulation and/or the liver facilitates the successful discontinuation of anti-rejection medication.
3) What are the effects of an increased number of Tregs in the immune system of a transplanted patient.

Ultimately, the project will serve to clarify whether donor-specific Tregs are essential to establish transplantation tolerance, and the extent to which low-dose IL-2 is an effective strategy to promote this phenomenon. This will have implications for a number of other inflammatory diseases, and will open the door to future clinical trials in which low-dose IL-2 could used in combination with other tolerance-promoting medications.

Transplantation remains the most successful treatment for end-stage organ failure, but the need to administer life-long immunosuppression (IS) to prevent rejection limits patient survival. Liver transplantation is the only transplantation setting in which a sizeable proportion of patients spontaneously develop "operational tolerance", a phenomenon defined by the maintenance of stable graft function in the absence of destructive immune responses without the need of IS. Unfortunately this phenomenon preferentially develops in elderly recipients and several years after transplantation. To maximize the benefit derived from IS discontinuation there is a need to find strategies to intentionally induce tolerance in young recipients in whom accumulated IS toxicity has not yet occurred. Our studies have revealed that successful IS discontinuation is associated with a transient intra-graft immune regulatory response with preferential accumulation of regulatory T cells (Tregs). This suggests that short-term enhancement of Treg numbers and/or function at the time of IS withdrawal may facilitate the acquisition of tolerance in patients who are not predisposed to spontaneously develop it. IL-2 is a cytokine that is essential for the optimal development, survival and function of Tregs. Several clinical studies have shown that low-dose IL-2 preferentially expands Tregs and is safe and efficacious in patients with autoimmunity or GVHD. In these studies, Treg frequency increased up to 2 to 8-fold without significant changes in the number of effector T cells. Our objective is to investigate if administration of a short-course of low-dose IL-2 to liver transplant recipients facilitates the discontinuation of IS. We propose to conduct a prospective, phase II, efficacy, single-arm clinical trial in which liver recipients <50 years old and 2-6 years after transplantation will receive IL-2 and gradually discontinue their IS medication.

The current project will provide fundamental information on how regulatory T cells (Tregs) exert their suppressive function in the setting of liver transplantation, and will answer the question of whether Tregs play a central role in the establishment of immunological tolerance in humans, and the extent to which low-dose IL-2 is efficacious in harnessing Tregs towards this goal. This information will have direct implications for the future development of Treg immunotherapy, which has the potential to be of great benefit to patients with organ transplantation or with chronic inflammatory disorders requiring long-term immunosuppressive therapy, as it may allow for a reduction or cessation in the amount of immunosuppression required with the consequent improvement in immunosuppression-related morbidity and mortality.

The scope of the project goes beyond the field of transplantation, however, as Tregs act as mediators or modulators in virtually all inflammatory responses. A deeper understanding on how human Tregs function in vivo will provide clues as to the optimal conditions required to promote Treg function, or, alternatively, point out to novel strategies to reduce their effects.

Patients and the wide academic community will therefore be the major beneficiaries. In addition, given the commercial potential of Treg therapies, the cell therapy biotech industry may be an additional beneficiary, which is an area we are actively exploring through our collaboration with the UK Cell Therapy Catapult.