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Our group has taken a lead role in defining effective antirejection therapy directed against the T cell: both the efficacy of antithymocyte globulin (ATG) as well as the first human trials of the monoclonal antibody OKT3.
Renal Allograft Rejection
Until recently, most studies on the mechanism of renal allograft rejection have focused predominantly on the central role of the T cell and cell-mediated mechanisms that lead to allograft injury and destruction. And indeed our group has taken a lead role in defining effective antirejection therapy directed against the T cell: both the efficacy of antithymocyte globulin (ATG) as well as the first human trials of the monoclonal antibody OKT3.
The deleterious effects of HLA antibodies in renal transplantation also have long been recognized. Since the early 1960s, it was known that if an allograft was placed in patient with preformed HLA antibodies (i.e. positive cross match) inevitable graft loss would occur (hyperacute rejection). Thus, the present requirement for a negative cross match prior to renal transplantation.
In recent years, however, it has been increasingly recognized that the detection of anti-HLA donor specific antibodies (DSA) de novo after transplantation (i.e. negative pretransplant cross match that subsequently became positive, post transplant), is also associated with important clinical sequelae due to antibody-mediated effector mechanisms of tissue injury. Over the last 6 years, we have made contributions to the understanding of the clinical pathologic syndromes associates with humoral rejection in renal transplant recipients with important therapeutic implications for patient management. The identification of the complement fragment C4d as a specific marker for humoral rejection in pertitubular capillaries of renal allograft biopsies has been critical to define and characterize these syndromes and has been shown to highly correlate with the detection of de novo anti-HLA donor specific antibodies in the serum of patients with acute humoral rejection. Thus, an important diagnostic tool has been devised and now is being implemented and confirmed nationally and internationally. The C4d stain has recently been adopted as a key pathologic feature of antibody mediated rejection at the First Antibody Working Group Conference in 2002. Clinically, patients with acute humoral rejection (AHR) present with early and severe allograft dysfunction with a 50-80% risk of graft failure in the first three months post transplant. Since 1995, we have utilized a new therapeutic approach consisting of plasmapheresis combined with tacrolimus/mycophenolate rescue plus IVIG for recipients suffering from AHR refractory to both steroids and antilymphocyte therapy with a gratifying 80% graft survival and 100% patient survival to date. These observations have recently been extended to the treatment of chronic humoral rejection associated with donor-specific antibodies and C4d deposits. We have exchange, results in sustained decreases in DSA titers with stabilization of renal allograft function. These preliminary findings confirm that suppression of alloantibody production is possible with this drug combination. We are now studying the efficacy of similar strategies to desensitize patients pretransplant with high levels of preformed positive antibodies (highly sensitized patients) in an effort to convert a positive crossmatch to negative.
Simultaneous Bone Marrow and Renal Transplant
Renal transplantation has made great strides over the last 25 years, with dramatic improvement in both patient and allograft survival. However, despite our success, renal transplant remains plagued by chronic rejection, the life-long need for immunosuppresion, and the risks of infectious complications, malignancies, and metabolic complications of the drugs, such as hypoglycemia, hyperlipidemia, and hypertension. In addition, there is a long-term concern about the nephrotoxicity of the calcineurin inhibitors. Thus, the pursuit of tolerance remains the Holy Grail of transplantation.
While multiple strategies have been employed to achieve tolerance and many have been successfully applied in small animal models, extension of these approaches to large primate models or humans has been long in coming. One approach, that of mixed chimerism, has been championed by the Transplantation Biology Unit of Mass General Hospital - Dr. David Sachs, Dr. Megan Sykes, Dr. Tatsuo Kawai, and Dr. A. Benedict Cosimi. New protocols recently developed in large animal models have shown that a non-myeloablative conditioning regimen with limited toxicity can induce mixed chimerism and donor-specific tolerance in solid organ allografts.
Recently, several investigators at our institution (Dr. Megan Sykes, Dr. Thomas Spitzer) demonstrated the efficacy of HLA matched or mismatched bone marrow for the treatment of refractory hematologic malignancies. When a non-myeloablative conditioning regimen was employed, a striking graft versus tumor effect has been observed. In these trials, mixed chimerism was achieved using high-dose cyclophosphamide in combination with in vivo T cell depletion and thymic irradiation before bone marrow transplant. HLA matched bone marrow was administered and cyclosporine was utilized for graft-versus-host disease (GVHD) prophylaxis. The anti-tumor response noted in these patients with refractory malignancies suggests that allogeneic bone marrow transplantation without myeloablative conditioning may have potent immunotherapeutic benefits without excessive GVHD. Based on these findings, our group has extended these observations in a pilot study to patients with myeloma and dialysis-dependent renal failure. Standard renal transplantation was contraindicated because of the underlying myeloma, and likewise these patients were not candidates for full myeloablative bone marrow transplants because of the accompanying renal failure. Dr. Nina Tolkoff-Rubin has been the key transplant nephrologist involved with the multidisciplinary effort between the bone marrow and renal transplantation teams. Two HLA matched patients have now received simultaneous bone marrow and renal transplants, using an adaptation of the non-myeloablative protocol previously described. In each of these patients, an antimyeloma effect is measured by urine kappa light chain excretion. In the first case, complete remission has been achieved now >4 years post transplant. In the second case, there is a marked decline in urine light chain excretion and a normal bone marrow 2 years after transplant. Both of these patients have normal renal function, off all immunosuppresion, without GVHD. Our first patient has been the subject of a Lifetime television documentary.
Based on these observations, a multi-institution immune tolerance network (NIH/JOFR funded) trial has now begun, and four additional patients have undergone therapy with similar results. The question now is - can we expand the envelope? As part of the ITN, we have initiated a tolerance protocol utilizing a nonmyeloablative simultaneous bone marrow and renal transplant in patients with nonmalignant renal failure and one haplotype matches. The first patient is out over a year and off immunosuppresion!
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