• Alloantigen-specific;
  • alloresponse;
  • anergy;
  • regulatory T cell (Treg);
  • CD8 suppresser cell;
  • costimulatory molecule blockade;
  • immune tolerance

Allostimulation with concurrent costimulatory blockade induces alloantigen-specific hyporesponsiveness in responder T cells (“alloanergization”). Alloanergized responder cells also acquire alloantigen-specific suppressive activity, suggesting this strategy induces active immune tolerance. While this acquired suppressive activity is mediated primarily by CD4+FOXP3+ cells, other cells, most notably CD8+ suppressor cells, have also been shown to ameliorate human alloresponses. To determine whether alloanergization expands CD8+ cells with allosuppressive phenotype and function, we used mixed lymphocyte cultures in which costimulatory blockade was provided by belatacept, an FDA-approved, second-generation CTLA-4-immunoglobulin fusion protein that blocks CD28-mediated costimulation, as an in vitro model of HLA-mismatched transplantation. This strategy resulted in an eightfold expansion of CD8+CD28 T cells which potently and specifically suppressed alloresponses of both CD4+ and CD8+ T cells without reducing the frequency of a range of functional pathogen-specific T cells. This CD8-mediated allosuppression primarily required cell–cell contact. In addition, we observed expansion of CD8+CD28 T cells in vivo in patients undergoing alloanergized HLA-mismatched bone marrow transplantation. Use of costimulatory blockade-mediated alloanergization to expand allospecific CD8+CD28 suppressor cells merits exploration as an approach to inducing or supporting immune tolerance to alloantigens after allogeneic transplantation.