Blocking both signal 1 and signal 2 of T-cell activation prevents apoptosis of alloreactive T cells and induction of peripheral allograft tolerance

Abstract

The alloimmune response against fully MHC-mismatched allografts, compared with immune responses to nominal antigens, entails an unusually large clonal size of alloreactive T cells. Thus, induction of peripheral allograft tolerance established in the absence of immune system ablation and reconstitution is a challenging task in transplantation. Here, we determined whether a reduction in the mass of alloreactive T cells due to apoptosis is an essential initial step for induction of stable allograft tolerance with non-lymphoablative therapy. Blocking both CD28-B7 and CD40-CD40 ligand interactions (co-stimulation blockade) inhibited proliferation of alloreactive T cells in vivo while allowing cell cycle-dependent T-cell apoptosis of proliferating T cells, with permanent engraftment of cardiac allografts but not skin allografts. Treatment with rapamycin plus co-stimulation blockade resulted in massive apoptosis of alloreactive T cells and produced stable skin allograft tolerance, a very stringent test of allograft tolerance. In contrast, treatment with cyclosporine A and co-stimulation blockade abolished T-cell proliferation and apoptosis, as well as the induction of stable allograft tolerance. Our data indicate that induction of T-cell apoptosis and peripheral allograft tolerance is prevented by blocking both signal 1 and signal 2 of T-cell activation.

Induction of apoptosis and modulation of activation and effector function in T cells by immunosuppressive drugs

“…The importance of apoptosis induction for peripheral transplantation tolerance has been demonstrated in recent reports [26,49]. Wells et al showed that mice transgenic for bcl-x L expression in T cells or IL-2 deficient mice are resistant to the induction of transplantation tolerance due to defective passive or active T cell apoptosis pathways.…”

Lack of Role for CsA‐Sensitive or Fas Pathways in the Tolerization of CD4 T Cells Via BMT and Anti‐CD40L

“…To address this in our protocol for inducing mixed chimerism and tolerance, we treated B6 mice with our standard regimen (3 Gy TBI, anti-CD8 mAb, 20 Â 10 6 B10.A BMCs, and MR1) and compared the incidence of chimerism and the deletion of donor-reactive CD4 + T cells with that in a group receiving the same protocol plus daily CsA treatment (20 mg/kg) beginning on the day of BMT and continuing for 14 days. This CsA dosing regimen has been shown by other groups to impair allograft survival in mice receiving costimulatory blockade (2,21). In recipients of the standard regimen, 80% of the mice demonstrated long-term multilineage chimerism for at least 27 weeks post-BMT ( Figure 1A).…”

“…All chimeric WT animals accepted donor skin grafts >200 days (top), while rejecting third party grafts (bottom), whereas all Bcl-x L recipient mice rejected both donor and third party grafts. (2,39,40), has been reported to be detrimental to the prolongation of graft survival in various protocols using costimulatory blockade (2,21,22). One critical function of calcineurin is the dephosphorylation and activation of NFAT, a key transcription factor involved in the production of IL-2 (41), which promotes FAS-mediated AICD by down-regulating the amount of FLIP, the down-regulator of apoptosis, associated with the Fas molecule (36).…”

“…However, with few exceptions (2), most of these approaches do not lead to primary skin graft acceptance across full MHC barriers in euthymic animals, the most stringent measure of tolerance. Recently, we and others have combined the use of costimulatory blockade with bone marrow transplantation (BMT) to induce high levels of donor hematopoietic chimerism across full MHC barriers (3)(4)(5)(6), with or without the addition of multiple minor histoincompatibilities.…”

“…Fas) signals to the T cell [reviewed in (18)]. Data involving costimulatory blockade protocols have also suggested a role for both pathways of T-cell apoptosis in the establishment of peripheral tolerance (2,15,19,20) and impairment of graft prolongation has been reported in recipients in which AICD is blocked by calcineurin inhibitors (2,21,22). Therefore, it is important to understand the role of each type of apoptosis in the induction of lasting chimerism and tolerance using costimulatory blockade and BMT.…”