Through half a century of experimental research and ever increasing number of clinical reports, it is now clear that a mature immune system can be retrained to accept allogeneic transplants without maintenance immunosuppression. In animal models, two essential elements of transplantation tolerance are reduction of donor-reactive effector T cells and active regulation of residual anti-graft responses. Mice spontaneously accept allogeneic liver transplant and become tolerant to the donor allotype. This is due to the ability of the liver grafts to withstand alloimmune attacks through regeneration, when the effector T cells become exhausted and regulatory arm of the immune system takes over. For more fragile grafts, such as pancreatic islets, both deletion of donor-reactive T cells and infusion of regulatory T cells (Tregs) are needed for long-term suppression-free graft survival. In this setting, donor-reactive Tregs are five to ten times more potent than polyclonal Tregs.
To enable clinical application of Tregs, we have developed approaches to manufacture polyclonal and donor-alloantigen-reactive Tregs using good-manufacturing-process compliant reagents. The processes rely on fluorescent activated cell sorting to purify Tregs followed by two-week ex vivo expansion. The products thus generated are characterized by high percentage of CD4+FOXP3+ cells with demethylated Treg-specific demethylated region. The cells can be labeled with deuterium during the expansion, permitting tracking of the cells after infusion into patients. Preliminary data demonstrates that polyclonal expanded Tregs persist for up to 1 year after infusion in patients, suggesting that at least some of the cells are long lived. Several scenarios of clinical trial design of Treg therapy in transplantation will be discussed.