The mitochondrial protein TCAIM regulates activation of T cells and thereby promotes tolerance induction of allogeneic transplants.

Primary T cell activation and effector cell differentiation is required for rejection of allogeneic grafts in naïve recipients. It has become evident, that mitochondria play an important role for T cell activation. Expression of several mitochondrial proteins such as TCAIM (T cell activation inhibitor, mitochondrial) is down-regulated upon T cell receptor triggering. Here we report that TCAIM inhibited spontaneous development of memory and effector T cells. CD4(+) T cells from Tcaim knock-in (KI) mice showed reduced activation, cytokine secretion and proliferation in vitro. Tcaim KI T cells tolerated allogeneic skin grafts upon transfer into Rag-1 KO mice. CD4(+) and CD8(+) T cells from these mice did not infiltrate skin grafts and kept a naïve or central memory phenotype, respectively. They were unable to acquire effector phenotype and functions. TCAIM altered T cell activation-induced mitochondrial distribution and reduced mitochondrial reactive oxygen species (mROS) production. Thus, TCAIM controls T cell activation and promotes tolerance induction probably by regulating TCR-mediated mitochondrial distribution and mROS production.

Mechanisms and rescue strategies of calcineurin inhibitor mediated tolerance abrogation induced by anti-CD4 mAb treatment.

To ensure safety tolerance induction protocols are accompanied by conventional immunosuppressive drugs (IS). But IS such as calcineurin inhibitors (CNI), for example, cyclosporin A (CsA), can interfere with tolerance induction. We investigated the effect of an additional transient CsA treatment on anti-CD4mAb-induced tolerance induction upon rat kidney transplantation. Additional CsA treatment induced deteriorated graft function, resulting in chronic rejection characterized by glomerulosclerosis, interstitial fibrosis, tubular atrophy and vascular changes. Microarray analysis revealed enhanced intragraft expression of the B cell attracting chemokine CXCL13 early during CsA treatment. Increase in CXCL13 expression is accompanied by enhanced B cell infiltration with local and systemic IgG production and C3d deposition as early as 5 days upon CsA withdrawal. Adding different CNIs to cultures of primary mesangial cells isolated from glomeruli resulted in a concentration-dependent increase in CXCL13 transcription. CsA in synergy with TNF-α can enhance the B cell attracting and activating potential of mesangial cells. Transient B cell depletion or transfer of splenocytes from tolerant recipients 3 weeks after transplantation could rescue tolerance induction and did inhibit intragraft B cell accumulation, alloantibody production and ameliorate chronic rejection.

Permanent CNI treatment for prevention of renal allograft rejection in sensitized hosts can be replaced by regulatory T cells.

Recent data suggest that donor-specific memory T cells (T(mem)) are an independent risk factor for rejection and poor graft function in patients and a major challenge for immunosuppression minimizing strategies. Many tolerance induction protocols successfully proven in small animal models e.g. costimulatory blockade, T cell depletion failed in patients. Consequently, there is a need for more predictive transplant models to evaluate novel promising strategies, such as adoptive transfer of regulatory T cells (Treg). We established a clinically more relevant, life-supporting rat kidney transplant model using a high responder (DA to LEW) recipients that received donor-specific CD4(+)/ 8(+) GFP(+) T(mem) before transplantation to achieve similar pre-transplant frequencies of donor-specific T(mem) as seen in many patients. T cell depletion alone induced long-term graft survival in naïve recipients but could not prevent acute rejection in T(mem)(+) rats, like in patients. Only if T cell depletion was combined with permanent CNI-treatment, the intragraft inflammation, and acute/chronic allograft rejection could be controlled long-term. Remarkably, combining 10 days CNI treatment and adoptive transfer of Tregs (day 3) but not Treg alone also induced long-term graft survival and an intragraft tolerance profile (e.g. high TOAG-1) in T(mem)(+) rats. Our model allows evaluation of novel therapies under clinically relevant conditions.