T-Cell costimulatory pathways relevant to transplant rejection and tolerance

Cyclin-dependent kinases: molecular switches controlling anergy and potential therapeutic targets for tolerance

A large body of research has established the importance of costimulatory signals and proliferation for the generation of productive T cell immune responses. While costimulation and cell cycle progression are each individually necessary for CD4+ effector T cell differentiation, it has become clear that neither of these processes alone is sufficient to avoid anergy. This review outlines the links between T cell differentiation, tolerance, and the cell cycle, and highlights recent work that has implicated cyclin-dependent kinases as important regulators and potential targets for modulation of T cell immunity and tolerance.

Gene transfer of programmed death ligand-1.Ig prolongs cardiac allograft survival

BACKGROUND

The CD28 homologue programmed death-1 (PD-1) and its ligands, PD-L1 and PD-L2 (which are homologous to B7), constitute an inhibitory pathway of T cell costimulation. The PD-1 pathway is of interest for immune-mediated diseases given that PD-1-deficient mice develop autoimmune diseases. We have evaluated the effect of local overexpression of a PD-L1.Ig fusion protein on cardiac allograft survival.

METHODS

Adenovirus-mediated PD-L1.Ig gene transfer was performed in F344 rat donor hearts placed in the abdominal position in Lewis recipients. Inflammatory cell infiltrates in the grafts were assessed by immunohistochemistry.

RESULTS

Allografts transduced with the PD-L1.Ig gene survived for longer periods of time compared with those receiving noncoding adenovirus or virus dilution buffer alone: median survival time (MST), 17 (range: 16-20) days vs. 11 (8-14) and 9 (8-13) days, respectively (P < 0.001). PD-L1.Ig gene transfer combined with a subtherapeutic regimen of cyclosporin A (CsA) was superior to CsA alone: MST, 25 (15-42) vs. 15 (13-19) days (P < 0.05). PD-L1.Ig gene transfer was associated with decreased numbers of CD4 cells and monocytes/macrophages infiltrating the graft (P < 0.05).

CONCLUSIONS

Localized PD-L1.Ig expression in donor hearts attenuates acute allograft rejection in a rat model. The effect is additive to that of a subtherapeutic regimen of CsA. These results suggest that targeting of PD-1 by gene therapy may inhibit acute cardiac allograft rejection in vivo.

Tolerance in Heart Transplantation: The Holy Grail, or an Attainable Goal?

The father of cardiac transplantation, Norman Shumway, famously predicted that tolerance was the future of the field, and always would be. Although his prediction remains true to date, significant progress has been made toward this goal, the "Holy Grail" for transplant clinicians. Current efforts are fueled by disappointing long-term outcomes associated with chronic immunosuppression, and the promise that partial or complete tolerance will impact long-term results favorably. This article provides a clinical definition of tolerance primarily based on lessons learned from animal heart allograft models. It reviews several promising strategies for inducing tolerance and detecting its presence through the use of biomarkers in peripheral blood or the graft, and outlines a possible path toward making this vision a clinical reality.

Differential Effects of B and T Lymphocyte Attenuator and Programmed Death-1 on Acceptance of Partially versus Fully MHC-Mismatched Cardiac Allografts

Although fully MHC-mismatched murine cardiac allografts are rapidly rejected, allografts mismatched at only MHC class I or class II alleles survive long term; the immunologic basis for the long-term survival of MHC class I- or II-mismatched allografts is unknown. We examined the roles of two recently described inhibitory receptors, B and T lymphocyte attenuator (BTLA) and programmed death-1 (PD-1), in the survival of partially or fully MHC-mismatched allografts using gene-deficient recipients as well as through use of blocking mAbs in wild-type hosts. Partially MHC-mismatched allografts showed strong induction of BTLA, but not PD-1 mRNA and survived long term in wild-type recipients, whereas targeting of BTLA or its ligand, herpesvirus entry mediator, but not PD-1, prompted their rapid rejection. By contrast, fully MHC-mismatched cardiac allografts were acutely rejected in wild-type recipients despite the induction of both BTLA and PD-1. Targeting of PD-1 in several fully MHC-mismatched models accelerated rejection, whereas targeting of BTLA unexpectedly enhanced PD-1 induction by alloreactive CD4 and CD8 T cells and prolonged allograft survival. In vitro studies using allogeneic dendritic cells and T cells showed that at low levels of T cell activation, BTLA expression was primarily induced, but that with increasing degrees of T cell activation, the expression of PD-1 was strongly up-regulated. These data suggest that BTLA and PD-1 exert distinct inhibitory actions in vivo, with the BTLA/herpesvirus entry mediator pathway appearing to dominate in regulating responses against a restricted degree of allogeneic mismatch.