Induction of donor-specific tolerance or sensitization as measured by sequential MLC reactivity up to 24 months after renal transplantation

Tracking donor-reactive T cells: Evidence for clonal deletion in tolerant kidney transplant patients

T cell responses to allogeneic major histocompatibility complex antigens present a formidable barrier to organ transplantation, necessitating long-term immunosuppression to minimize rejection. Chronic rejection and drug-induced morbidities are major limitations that could be overcome by allograft tolerance induction. Tolerance was first intentionally induced in humans via combined kidney and bone marrow transplantation (CKBMT), but the mechanisms of tolerance in these patients are incompletely understood. We now establish an assay to identify donor-reactive T cells and test the role of deletion in tolerance after CKBMT. Using high-throughput sequencing of the T cell receptor B chain CDR3 region, we define a fingerprint of the donor-reactive T cell repertoire before transplantation and track those clones after transplant. We observed posttransplant reductions in donor-reactive T cell clones in three tolerant CKBMT patients; such reductions were not observed in a fourth, nontolerant, CKBMT patient or in two conventional kidney transplant recipients on standard immunosuppressive regimens. T cell repertoire turnover due to lymphocyte-depleting conditioning only partially accounted for the observed reductions in tolerant patients; in fact, conventional transplant recipients showed expansion of circulating donor-reactive clones, despite extensive repertoire turnover. Moreover, loss of donor-reactive T cell clones more closely associated with tolerance induction than in vitro functional assays. Our analysis supports clonal deletion as a mechanism of allograft tolerance in CKBMT patients. The results validate the contribution of donor-reactive T cell clones identified before transplant by our method, supporting further exploration as a potential biomarker of transplant outcomes.

Down-regulated donor-specific T-cell reactivity during successful tapering of immunosuppression after kidney transplantation

Stable cadaveric renal transplant patients were routinely converted from cyclosporin A (CsA) to either azathioprine (AZA) or mycophenolate mofetil (MMF) 1 year after transplantation to reduce the side effects of long-term immunosuppressive therapy. Thereafter, the AZA and MMF dose was gradually tapered to 50% at 2 years after transplantation. We questioned whether a reduction of immunosuppressive treatment results in a rise of donor-specific T-cell reactivity. Before transplantation (no immunosuppression), 1 year (high dose immunosuppression) and 2 years (low dose immunosuppression) after transplantation, the T-cell reactivity of peripheral blood mononuclear cells (PBMC) against donor and third-party spleen cells was tested in mixed lymphocyte cultures (MLC) and against tetanus toxoid (TET) to test the general immune response. We also measured the frequency of donor and third-party reactive helper (HTLpf) and cytotoxic (CTLpf) T-lymphocyte precursors in a limiting dilution assay. Donor-specific responses, calculated by relative responses (RR = donor/third-party reactivity), were determined. Comparing responses after transplantation during high dose immunosuppression with responses before transplantation (no immmunosuppression), the donor-specific MLC-RR (P = 0.04), HTLp-RR (P = 0.04) and CTLp-RR (P = 0.09) decreased, while the TET-reactivity did not change. Comparing the responses during low dose with high dose immunosuppression, no donor- specific differences were found in the MLC-RR, HTLp-RR and CTLp-RR, although TET-reactivity increased considerably (P = 0.0005). We observed a reduction in donor-specific T-cell reactivity in stable patients after renal transplantation during in vivo high dose immunosuppression. Tapering of the immunosuppressive load had no rebound effect on the donor-specific reactivity, while it allowed recovery of the response to nominal antigens.

Mutual tolerance after liver and not after heart transplantation? Evaluation of patient-anti-donor and donor-anti-patient responses by mixed lymphocyte culture

The ultimate goal in organ transplantation is the induction of donor-specific transplantation tolerance. The fact that in some patients it is possible to withdraw immunosuppressive therapy completely, suggests that immunological adaptation or donor-specific nonresponsiveness can occur following transplantation. In earlier studies we have shown that after blood transfusion, the mixed lymphocyte reactivity of the donor against patient peripheral blood mononuclear lymphocytes taken after blood transfusion gradually decreased with time. This may reflect the induction of an immunoregulatory mechanism, which protects the recipient against an immune reaction of the donor, enhancing a state of mixed chimerism. A similar phenomenon might also play a role in the immunological mechanism leading to transplantation tolerance. Therefore, we studied responses in patients with a well-functioning liver and heart transplant using a primed lymphocyte test (PLT) and a mixed lymphocyte culture (MLC). Two years after liver transplantation the PLT and MLC responses of patient against donor were decreased significantly compared to the situation before transplantation. The response of donor against patient was also lower two years after transplantation. The decreased responses were donor-specific since responses to third-party cells generally remained unchanged. In heart transplant recipients we could not detect a donor-specific downregulation. The reversed response, of donor against patient, was not different from responses of third-party against patient cells. Therefore, we conclude that donor-specific nonresponsiveness is not induced in patients with well-functioning heart transplants. In contrast, after a successful liver transplantation the response of patient against donor is decreased, as is the reversed response. It may be valuable to test whether in liver transplant patients withdrawing or reducing of maintenance immunosuppression is permitted for patients who appear to have developed two-way donor-specific hyporeactivity.