P899 Aims: In experimental models, long-term allograft survival has been attributed to the development of operational tolerance; however, the mechanisms underlying long-term acceptance of a kidney allograft in humans under minimal or even no immunosuppression are still poorly understood. The aim of the current study was to determine the TCR repertoire in kidney transplanted patients with long-term renal allograft survival and compare with other transplanted patients, dialyzed patients and healthy controls. Methods: In the present study we analyzed the T cell receptor (TCR) repertoires in circulating T cells of 19 patients with long-term (≥9 years) renal allograft survival (LTS), using TCR Vβ family-specific RT-PCR and spectrotyping analysis of the TCR CDR3 region genes. Results: 3 LTS patients had well functioning grafts even though immunosuppression was stopped for the last 3 years. T cells of patients with long-term allograft survival exhibited strongly altered TCR Vβ usage, including an increased frequency of oligoclonality and a decreased frequency of polyclonality. 15 of the 19 LTS patients demonstrated oligoclonality in at least 3 or more TCR Vβ families, and the frequency of oligoclonality in these patients was significantly higher as compared to patients with well-functioning grafts at 3 years (p<0.01), an uncomplicated course during the first year (p<0.001), acute (p<0.001) or chronic rejection (p<0.01), dialysis patients (p<0.001), or healthy controls (p<0.001). In contrast to LTS patients, all other studied patient groups exhibited a normal polyclonal TCR repertoire with Gaussian distribution. Conclusions: Our data indicate that TCR alteration is a common feature of long-term allograft outcome which might be explained by clonal deletion, exhaustion of alloreactive T cells, or clonal expansion of particular T cell subpopulations, such as regulatory T cells.