On the basis of the hypothesis of Starzl and Demetris20
regarding the interrelationship between chimerism and tolerance, there have been previous attempts to induce allograft tolerance in clinical kidney transplantation by infusing donor bone marrow into recipients of solid organ transplants receiving conventional immunosuppressive therapy.21,22
This approach, however, has led to very low levels of chimerism, generally detectable only by means of PCR, and subsequent withdrawal of immunosuppressive therapy has seldom been accomplished. Combined kidney and bone marrow transplantation has been attempted with the use of conditioning with total lymphoid irradiation by Millan and colleagues5
; transient mixed chimerism developed in three of four recipients, but rejection occurred when withdrawal of immunosuppressive therapy was attempted.
In this study of combined HLA-mismatched bone marrow and kidney transplantation in five patients, we were able to discontinue all immunosuppressive therapy in four patients, and they have retained the graft and maintained stable renal function for 2.0 to 5.3 years after the transplantation (i.e., 1.2 to 4.6 years after complete withdrawal of immunosuppressive therapy).
The mechanism responsible for stable graft function without exogenous immunosuppressive therapy in these four recipients remains under investigation. In studies in mice, the immune tolerance that follows the induction of chimerism by bone marrow transplantation has been associated with elimination of cells in the thymus that are reactive to donor antigen (called “central deletion”). The specific loss of in vitro reactivity of the recipient's T cells against the donor's cells in all four of these patients () is consistent with this mechanism, but several other mechanisms are also possible.
By contrast with the permanent chimerism that can be achieved with nonmyeloablative regimens in patients with malignant conditions or other diseases, the chimerism detected in monkeys10,12,13
and in the recipients described here has been transient. Although nonmyeloablative conditioning has achieved long-term chimerism when it has been used for the treatment of hematologic malignant conditions in humans, this kind of conditioning has generally been available only for HLA-matched donor–recipient combinations, since graft-versus-host disease has been a major limitation in the use of HLA-mismatched donors. In the present study, transient chimerism was achieved without the development of graft-versus-host disease, despite HLA mismatches between the donor and the recipient.
Evidence from our studies in monkeys12
suggests that the mechanism of tolerance may switch from central deletion to a peripheral mechanism that may include regulatory T cells. After this switch, the kidney allograft may be essential to the maintenance of the tolerant state.23
High levels of FOXP3+, a marker of regulatory T cells, without a concomitant inflammatory response (as indicated by decreased granzyme B expression) in the renal allografts of these patients suggest such a regulatory mechanism, and they may indicate active involvement of intragraft regulatory T cells for the maintenance of unresponsiveness, as has been shown in mice.24
We did not anticipate the early development of antidonor antibodies, since all of the patients initially received maintenance regimens that suppress T cells, and since anti-HLA antibody production is thought to be a T-cell–dependent response.25
This complication had not been seen in our studies involving nonhuman primates,10
nor in the HLA-identical patients treated with our previous nonmyeloablative regimen for multiple myeloma.16
As a result of this finding and because of the possibility that B cells might have been activated during the capillary leak syndrome, which occurred during the second post-transplantation week in all patients, we modified the protocol to include B-cell depletion with the use of rituximab before the transplantation procedures. Since the capillary leak syndrome is usually reversed by corticosteroids,26
we added a short course of corticosteroids during the post-transplantation period.
The fourth patient had a low titer of antidonor HLA class II antibodies 2 months after complete withdrawal of immunosuppressive medications, and there were C4d deposits in the allograft, with segmental duplication of the glomerular basement membrane in some glomeruli (see Fig. 1 in the Supplementary Appendix). The clinical significance of C4d deposition in the absence of other evidence of rejection remains unclear.27,28
Since the minor histologic changes in this patient have not progressed and there is no other evidence of rejection, we have chosen an approach of watchful waiting. Our observations show that stable graft function after planned, complete withdrawal of immunosuppressive drugs is feasible in recipients of HLA-mismatched grafts.