Tracking of TCR-Transgenic T Cells Reveals That Multiple Mechanisms Maintain Cardiac Transplant Tolerance in Mice

Abstract

Solid organ transplantation tolerance can be achieved following select transient immunosuppressive regimens that result in long-lasting restraint of alloimmunity without affecting responses to other antigens. Transplantation tolerance has been observed in animal models following costimulation or coreceptor blockade therapies, and in a subset of patients through induction protocols that include donor bone marrow transplantation, or following withdrawal of immunosuppression. Previous data from our lab and others have shown that proinflammatory interventions that successfully prevent the induction of transplantation tolerance in mice often fail to break tolerance once it has been stably established. This suggests that established tolerance acquires resilience to proinflammatory insults, and prompted us to investigate the mechanisms that maintain a stable state of robust tolerance. Our results demonstrate that only a triple intervention of depleting CD25+ Tregs, blocking PD-L1 signals, and transferring low numbers of alloreactive T cells was sufficient to break established tolerance. We infer from these observations that Tregs and PD-1/PD-L1 signals cooperate to preserve a low alloreactive T cell frequency to maintain tolerance. Thus, therapeutic protocols designed to induce multiple parallel mechanisms of peripheral tolerance may be necessary to achieve robust transplantation tolerance capable of maintaining one allograft for life in the clinic.

Distinct Graft-Specific TCR Avidity Profiles during Acute Rejection and Tolerance

“…In addition, it has been shown that peripheral tolerance to self-antigens and the induction of oral tolerance can result in abortive proliferation of antigen-specific T cells (Sun et al, 2003; Townsend and Goodnow, 1998). Studies using carboxyfluorescein succinimidyl ester (CFSE)labeled TCR-Tg T cells transferred into tolerant recipients have shown that tolerance induction does not inhibit proliferation of T cells (Chai et al, 2015; Miller et al, 2016a) but does prevent their accumulation. Although studies using TCR-Tg T cells suggest there was no defect in proliferation in a tolerant environment, those studies only compared proliferation of T cells of a single specificity and affinity and thus did not fully recapitulate the endogenous response to alloantigens where interclonal competition occurs.…”

“…Short-term therapeutic blockade of costimulation induces transplantation tolerance, which is thought to depend on peripheral mechanisms of T cell tolerance. Many studies have examined these mechanisms using monoclonal allospecific TCR-transgenic T cells with fixed affinities (Iwakoshi et al, 2000; Miller et al, 2016a; Pinelli et al, 2013; Quezada et al, 2005). However, a polyclonal endogenous response to a given donor antigen comprises multiple T cell clones of various affinities and/or avidities competing for antigen (Honjo et al, 2000; Tsang et al, 2011), and the fates of T cells of differing affinities and/or avidities in rejection versus tolerance has not been analyzed.…”

Mechanisms of Mixed Chimerism-Based Transplant Tolerance

“…Preclinical studies suggest several promising approaches for enhancing peripheral tolerance via deletion, exhaustion and suppression, that may allow further reductions in the need for lymphoablation and cytoreduction in future protocols. Murine studies demonstrating the potency of multiple parallel tolerance mechanisms in promoting robust transplantation tolerance [113] and particularly the combination of central deletion and regulation that is achieved with incomplete lymphodepletion and Treg-promoting mixed chimerism regimens [30, 114] are useful in guiding these approaches. Together, major new insights into transplant tolerance mechanisms, along with the emergence of new immunotherapies and tools for immune monitoring, open up new avenues for developing clinical tolerance protocols with reduced risks and greater efficacy (see Outstanding Questions).…”

Harnessing Apoptotic Cells for Transplantation Tolerance: Current Status and Future Perspectives

“…While induction of tolerance by apoptotic cells has been shown to involve a multitude of mechanisms including regulation, deletion and anergy, it appears that maintenance of tolerance relies on anergy more than any other mechanism [59, 60]. Our own studies using donor ECDI-SP support this notion and reveal that once tolerance is established, thorough depletion of CD25 + cells does not result in breaking of tolerance or precipitation of graft rejection.…”