Abstract. Data from various rodent models have implicated a role for anergic T cells in the maintenance of self and transplantation tolerance. The relevance of donor-specific T cell anergy to clinical transplantation, however, has not been demonstrated. Previous studies have reported that recipients of solid organ transplant often have reduced frequencies of CD4 ϩ T cells with anti-donor direct pathway allospecificity after transplantation. The underlying mechanism(s) of this donorspecific hyporesponsiveness is unclear but likely to contribute to the diminished immunosuppressive requirement of transplant patients with time after transplantation. This study shows that ex vivo treatment of CD4 ϩ T cells from renal transplant recipients with IL-2 could specifically increase the anti-donor frequency in all the patients with evidence of donor-specific hyporesponsiveness. It also shows that the IL-2-induced recovery of anti-donor frequency is unlikely to result from nonspecific stimulation or selective clonal expansion of activated, allospecific CD4 ϩ T cells. Taken together, the data suggest that T cell anergy plays an important role in the direct pathway hyporesponsiveness that evolves in many human renal transplant recipients.Heightened interest in clinical transplant tolerance stems from the acquisition of insights into the mechanisms and the advent of novel biologic reagents that may promote such a tolerant state. For these reasons, it is necessary to understand the evolution of anti-donor responses using current approaches to immunosuppression. We and others (1-3) have previously described that a substantial proportion of patients with kidney or other solid organ transplants have markedly reduced numbers of T cells with direct allospecificity, particularly in longterm live-related transplant recipients. Although markedly decreased frequencies of direct pathway T cells does not equate to tolerance, it is tempting to speculate that mechanisms that contribute to peripheral T cell tolerance may be operative in diminishing the anti-donor T cell repertoire. It is also important to note that direct pathway hyporesponsiveness can often be observed in patients with chronic transplant rejection. In these patients, it appears that the indirect pathway of anti-donor alloimmunity provides an immunologic drive to this indolent process.Two attractive, yet not mutually exclusive, candidate mechanisms of direct pathway hyporesponsiveness are deletion and anergy. In vitro studies using renal epithelial cells as antigenpresenting cells can result in T cell anergy (4), and peripheral deletion has been shown to be an important mechanism in maintaining tolerance to foreign antigens (5,6). In this study, we investigated whether these mechanisms are operative in the maintenance of donor-specific hyporesponsiveness. Because of the lack of known specific marker of anergic cells, we have relied on the functional properties of anergic T cells to distinguish these possibilities. One of the characteristics of anergic T cells is that their re...