['To define whether immunosuppressive agents that block the interleukin (IL)-2 pathway could prevent activation-induced cell death of activated T cells in the graft, we measured expression of IL-2, IL-2 receptor alpha chain (CD25), IL-15, Fas, and Fas ligand by real time reverse transcription-polymerase chain reaction in cardiac allografts.', 'We characterized the phenotype of the infiltrating cells (CD3, CD68, CD25) by immunohistochemistry. The proportion of apoptotic graft-infiltrating cells was determined by TUNEL (terminal deoxynucleotidyl transferase dUTP nick-end labeling) staining. We analyzed endomyocardial biopsy specimens from cardiac allograft recipients who were treated with anti-CD25 monoclonal antibody (mAb) induction therapy (daclizumab) or with matching placebo in combination with cyclosporine, steroids, and mycophenolate mofetil.', 'Treatment with anti-CD25 mAb affected the number of infiltrating CD3 and CD68 cells and the IL-2-regulated apoptotic pathway. During anti-CD25 mAb treatment, significantly lower intragraft IL-2 and CD25 mRNA transcription levels and decreased numbers of CD25+ T cells were found compared with the levels measured in endomyocardial biopsy specimens from placebo-treated patients (5- to 10-fold, P=0.002 and P<0.0001, respectively). In these samples the intragraft mRNA expression levels of IL-15 were also lower (P=0.02). Inhibition of the IL-2 pathway by anti-CD25 mAb therapy was accompanied by reduced mRNA and protein of Fas ligand and not by reduced Fas expression (P=0.001 and P=0.03). TUNEL staining revealed that the proportion of graft-infiltrating cells was lower in the anti-CD25 mAb patient group than the proportion of apoptotic cells in patients receiving placebo (P=0.06).'] Does anti-CD25 monoclonal antibody therapy affect the death signals of graft-infiltrating cells after clinical heart transplantation?