Targeting LFA-1 and CD154 suppresses the in vivo activation and development of cytolytic (CD4-idependent) CD8⁺ T cells

Short-term immunotherapy targeting both LFA-1 and CD40/CD154 costimulation produces synergistic effects such that long-term allograft survival is achieved in the majority of recipients. This immunotherapeutic strategy has been reported to induce the development of CD4⁺ regulatory T cells. In the current study, the mechanisms by which this immunotherapeutic strategy prevents CD8⁺ T cell-dependent hepatocyte rejection in CD4 knockout mice were examined. Combined blockade of LFA-1 and CD40/CD154 costimulation did not influence the overall number or composition of inflammatory cells infiltrating the liver where transplanted hepatocytes engraft. Expression of T cell activation markers CD43, CD69, and adhesion molecule CD103 by liver-infiltrating cells was suppressed in treated mice with long-term hepatocellular allograft survival compared to liver-infiltrating cells of untreated rejector mice. Short-term immunotherapy with anti-LFA-1 and anti-CD154 mAb also abrogated the in vivo development of alloreactive CD8⁺ cytotoxic T cell effectors. Treated mice with long-term hepatocyte allograft survival did not reject hepatocellular allografts despite adoptive transfer of naive CD8⁺ T cells. Unexpectedly, treated mice with long-term hepatocellular allograft survival demonstrated prominent donor-reactive delayed-type hypersensitivity responses, which were increased in comparison to untreated hepatocyte rejectors. Collectively, these findings support the conclusion that short-term immunotherapy with anti-LFA-1 and anti-CD154 mAbs induces long-term survival of hepatocellular allografts by interfering with CD8⁺ T cell activation and development of CTL effector function. In addition, these recipients with long-term hepatocellular allograft acceptance show evidence of immunoregulation which is not due to immune deletion or ignorance and is associated with early development of a novel CD8⁺CD25^high cell population in the liver.