1 Molecular Cardiology, Department of Internal Medicine III, University of Frankfurt, Frankfurt, Germany, 2 Division of Clinical Immunology and Allergy, University of Florence, Florence, Italy Submitted 1 November 2006 ; accepted in final form 9 January 2007 Regulatory T cells (T reg ) migrate into allografts and induce tolerance of the graft. Immunosuppressive T reg are found among CD4 + CD25 ++ T cells and specifically express the forkhead/winged transcription factor FOXP3. We hypothesized that activated T cells and T reg might modulate the ongoing inflammation of the cardiac allograft (CA) and that the chronic inflammatory environment might influence the balance between these distinct cell types. We therefore quantified levels of activated T cells and CD4 + CD25 ++ T reg in the cardiac and systemic circulation in heart transplant recipients. To determine the influence of the allograft passage on these cells, transcardiac gradients were evaluated in CA recipients ( n = 22) compared with controls ( n = 18). Systemic levels of circulating T reg were significantly lower in CA recipients (8.9 ± 1.3 µl) compared with controls (15.8 ± 1.6 µl; P = 0.002). Similarly, the proportion of T reg related to the total leukocyte number was significantly lower in CA recipients ( P < 0.01). In contrast, systemic levels of circulating activated CD4 + T cells and of circulating plasmacytoid dendritic cells were similar in both groups. In transplant patients, numbers of T reg significantly decreased during transcardiac passage (3.0 ± 0.3 to 2.4 ± 0.3% of CD4 + T cells, P < 0.01), and FOXP3 + T cells invaded into the allograft. In contrast, numbers of activated CD4 + T cells increased during passage through the allograft, even in the presence of effective immunosuppression. In conclusion, numbers of circulating immunosuppressive T reg are reduced in transplant recipients. Recruitment of T reg into the cardiac allograft during transcoronary passage may induce graft tolerance during subclinical inflammation potentially influencing allograft vasculopathy. coronary circulation; chronic inflammation Address for reprint requests and other correspondence: S. Dimmeler or A. M. Zeiher, Dept. of Internal Medicine III, Division of Cardiology, J. W. Goethe Univ., Theodor-Stern-Kai 7, 60590 Frankfurt, Germany (e-mail: dimmeler{at}em.uni-frankfurt.de )