The Role of the Common Cytokine Receptor γ-Chain in Regulating IL-2-Dependent, Activation-Induced CD8+ T Cell Death

IL-2-dependent, activation-induced T cell death (AICD) plays an important role in peripheral tolerance. Using CD8+ TCR-transgenic lymphocytes (2C), we investigated the mechanisms by which IL-2 prepares CD8+ T cells for AICD. We found that both Fas and TNFR death pathways mediate the AICD of 2C cells. Neutralizing IL-2, IL-2Rα, or IL-2Rβ inhibited AICD. In contrast, blocking the common cytokine receptor γ-chain (γc) prevented Bcl-2 induction and augmented AICD. IL-2 up-regulated Fas ligand (FasL) and down-regulated γc expression on activated 2C cells in vitro and in vivo. Adult IL-2 gene-knockout mice displayed exaggerated γc expression on their CD8+, but not on their CD4+, T cells. IL-4, IL-7, and IL-15, which do not promote AICD, did not influence FasL or γc expression. These data provide evidence that IL-2 prepares CD8+ T lymphocytes for AICD by at least two mechanisms: 1) by up-regulating a pro-apoptotic molecule, FasL, and 2) by down-regulating a survival molecule, γc.

Induction of Allograft Tolerance in the Absence of Fas-Mediated Apoptosis

Using certain immunosuppressive regimens, IL-2 knockout (KO) mice, in contrast to wild-type (wt) controls, are resistant to the induction of allograft tolerance. The mechanism by which IL-2 regulates allograft tolerance is uncertain. As IL-2 KO mice have a profound defect in Fas-mediated apoptosis, we hypothesized that Fas-mediated apoptosis of alloreactive T cells may be critical in the acquisition of allograft tolerance. To definitively study the role of Fas in the induction of transplantation tolerance, we used Fas mutant B6.MRL-lpr mice as allograft recipients of islet and vascularized cardiac transplants. Alloantigen-stimulated proliferation and apoptosis of Fas-deficient cells were also studied in vivo. Fas mutant B6.MRL-lpr (H-2b) mice rapidly rejected fully MHC-mismatched DBA/2 (H-2d) islet allografts and vascularized cardiac allografts with a tempo that is comparable to wt control mice. Both wt and B6.MRL-lpr mice transplanted with fully MHC-mismatched islet allografts or cardiac allografts can be readily tolerized by either rapamycin or combined costimulation blockade (CTLA-4Ig plus anti-CD40L mAb). Despite the profound defect of Fas-mediated apoptosis, Fas-deficient T cells can still undergo apoptotic cell death in vivo in response to alloantigen stimulation. Our study suggests that: 1) Fas is not necessarily essential for allograft tolerance, and 2) Fas-mediated apoptosis is not central to the IL-2-dependent mechanism governing the acquisition of allograft tolerance.

Evidence for Immune Regulation in the Induction of Transplantation Tolerance: A Conditional but Limited Role for IL-4

Most experimental models of allograft tolerance depend on manipulation of immune responses at the time of transplant. In such systems, the graft itself probably plays an important role in the induction of unresponsiveness but as a consequence may suffer immune mediated damage. Ideally, recipients would be made specifically unresponsive before transplant such that the graft is protected from the outset. In this report, we demonstrate that CBA mice pretreated with donor-specific transfusion plus anti-CD4 Ab 28 days before transplant accept cardiac allografts indefinitely without further intervention. Adoptive transfer of spleen cells from mice with long term surviving grafts results in donor-specific graft acceptance in naive secondary recipients, indicating that tolerance in this system involves immuneregulation. Regulation develops as a result of the pretreatment protocol alone, since transfer of cells from pretreated but untransplanted mice to naive recipients also leads to prolonged allograft survival without additional therapy. Neutralizing IL-4 at the time of tolerance induction had no effect on graft outcome in primary recipients. However, removal of IL-4 from the adoptive transfer donors at the time of tolerance induction prevented long term engraftment in the majority of secondary recipients. Our data demonstrate that pretreatment of transplant recipients can establish immune regulation powerful enough to override the responses of an intact immune repertoire and that under stringent conditions at least, development of this regulatory population may in part be dependent on IL-4.