Immune Reconstitution in Recipients of Living Donor Kidney/Hematopoietic Stem + Facilitating Cell Transplants

Solid organ transplant recipients generally require life-long immunosuppressive agents to maintain graft acceptance. This is associated with a number of toxicities, including renal dysfunction, metabolic abnormalities, opportunistic infection and malignancies. Calcineurin inhibitors, the most frequently used agents for immunosuppression, induce nephrotoxicity, with gradual renal decline in many transplant recipients. As a result, approaches to induce tolerance remain a major focus of research. We report here the long-term results from a reduced-intensity nonmyeloablative conditioning approach to establish high levels of donor chimerism without graft-versus-host disease (GVHD) or engraftment syndrome in mismatched related and unrelated recipients of combined living donor kidney/hematopoietic stem cell transplant (HSCT). Recipients were conditioned with 200 cGy total body irradiation plus fludarabine (30 mg/kg day -5, -4, -3) and cyclophosphamide (50 mg/kg days -3 and +3) and administered donor granulocyte colony-stimulating factor mobilized stem cells, enriched for hematopoietic stem cells (HSC) and graft facilitating cells (FCs) and depleted of GVHD-producing cells (FCRx). Subjects ranged in age from 18 to 65 years. All subjects were HLA-disparate from their living kidney/HSCT donors, ranging from 5 of 6 related to 0 of 6 unrelated. Eight were unrelated and 11 related to their donors. Subjects were maintained on tacrolimus and mycophenolate mofetil (MMF) for 6 months. At 6 months, if protocol biopsy is normal and chimerism present, the MMF is discontinued. Similarly, the tacrolimus is discontinued at 12 months if stable renal function, normal protocol biopsy and chimerism are present. 30 subjects have been enrolled and 25 transplanted. Nineteen subjects have > 1 year follow up. Twelve have achieved stable donor chimerism and have been successfully taken off immunosuppression. Time off IS ranges from 8 months to 48 months. None have developed GVHD. We have herein prospectively evaluated immune reconstitution and immunocompetence. Return of CD4⁺ and CD8⁺ T central and effector memory cell populations was rapid. Examination for recent thymic emigrants (T cell staining for CD31⁺ CDRA⁺, TCR excision circle analysis) indicated that the majority (>97%) of these chimeric cells are being produced de novo. The TCR repertoires post-transplant in chimeric subjects were as diverse as pre-transplant donors and recipients but were distinct from donor or recipient pre-transplant. An increase in the CD4⁺ regulatory T cell/CD4⁺ effector T cell ratio was seen early in durably and transiently chimeric patients, suggesting active immunoregulation. Patients have been successfully vaccinated per American Society of Bone and Marrow Transplantation guidelines without loss of chimerism or development of allograft rejection. Of four subjects immunized for hepatitis B and whose donors were not, three retained immunologic memory after transplantation. Most chimeric subjects retained memory for measles, mumps, rubella and varicella. Chimeric subjects generated immune responses to pneumococcal vaccine. Opportunistic viral infections such as BK viremia and cytomegalovirus activation were absent after cessation of immunosuppression. These findings suggest that immunologic recovery is robust in chimeric subjects who receive kidney/FCRx and that durable chimerism is associated with donor-specific tolerance to renal allografts.