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We have studied organ allograft survival in rhesus monkeys conditioned with myeloablative total-body irradiation and T cell-depleted autologous bone marrow transplantation then given a heterotopic MHC-mismatched cardiac allograft in the immediate postmyeloablative period. This model has enabled us to investigate the role of T cells in vascularized organ allograft rejection. We previously reported (1) that recipients of marrow depleted of T cells below a critical threshold (0.16% residual marrow T cells, or 0.14 x 10(5) infused T cells/kg) experienced a period of freedom from acute rejection associated with a profound nonspecific immune deficiency (determined by skin grafting). Resolution of the nonspecific immune deficiency was associated with late graft rejection. In the present report, we correlate the results of peripheral immune reconstitution studies and direct immunohistochemical analysis with allograft status in order to study T cell subsets involved in late rejection. We report that, in contrast with CD8+/CD28- T cells, CD16+ NK cells, and CD20+ B cells, late allograft rejection was associated with the return of peripheral CD4+ T cells and CD8+/CD28+ T cells, suggesting a critical role for one or both of these subsets in late allograft rejection in this model.

In the present study, we tested the possibility that a vascularized allograft might induce immunological tolerance in a myeloablated host, similar to the tolerance induced by allogeneic bone marrow grafts. To this end, we developed a rhesus monkey model consisting of myeloablative total-body irradiation and T cell-depleted autologous marrow transplantation followed by MHC-mismatched heterotopic cardiac allograft implantation. Limiting dilution analysis was used to quantify residual marrow T cells following depletion. We found that (1) allograft survival was substantially prolonged in the absence of immunosuppressive drugs (median survival = 160 days) over that seen in controls treated identically but receiving non-T cell-depleted marrow (median survival = 14 days); (2) there was a correlation between allograft survival prolongation and the extent of marrow T cell depletion, with a maximum survival of 329 days associated with a residual marrow T cell content of 0.00014%; (3) nonspecific immune deficiency--and, possibly, specific unresponsiveness of limited duration (determined by cryopreserved donor and third-party skin grafting)--contributed to the rejection-free period seen in recipients of extensively depleted marrow; (4) late allograft rejection occurred in 3 of 3 long-term survivors, thereby demonstrating that permanent tolerance was not induced by the allograft across MHC barriers; and (5) as few as 1.4 x 10(4) infused marrow T cells/kg were sufficient to mediate acute allograft rejection, a threshold approximately 10-fold lower than that reported for the induction of acute graft-versus-host disease following allogeneic bone marrow transplantation.

Studies of postmyeloablative immune reconstitution have been reported for allogeneic bone marrow transplantation and also for non-T cell-depleted autologous/syngeneic BMT. However, there is a paucity of information regarding immune recovery following T cell-depleted autologous/syngeneic BMT. We have developed a primate transplantation tolerance model in which rhesus monkeys were conditioned with total-body irradiation and extensively T cell-depleted autologous BMT and given a major histocompatibility complex-mismatched heterotopic cardiac allograft. This model provided an opportunity to study peripheral immune recovery following T cell-depleted autologous BMT. Limiting dilution analysis was used to quantify marrow T cells following depletion (2.8% to 25.6% marrow T cells predepletion, 0.00014% to 0.036% residual marrow T cells postdepletion). We found that (1) hematopoietic engraftment was prompt despite extensive marrow T cell depletion, (2) reconstitution of CD4+ helper T cells and CD8+ cytotoxic T cells were substantially delayed (6-12 months) compared with the recovery of CD8+ suppressor T cells, CD16+ NK cells, and CD20+ B cells, (3) distinction between CD8+ cytotoxic T cells and CD8+ suppressor T cells by the CD28 marker was critical in revealing the markedly discrepant recoveries of those subsets, and (4) immune reconstitution resembled that observed in recipients of T cell-depleted allogeneic and non-T cell-depleted autologous/syngeneic BMT, suggesting that the pattern of immune recovery following BMT is not substantially influenced by either allogeneic effects or the number of transferred T cells over a range of values.

We have developed a rhesus monkey model consisting of myeloablative total-body irradiation and T lymphocyte-depleted autologous bone marrow transplantation followed by MHC-mismatched heterotopic cardiac allograft implantation that has provided an opportunity to study the role of marrow T cells in cardiac allograft rejection. In order to assess quantitatively the effects of low numbers of residual marrow T cells following depletion, methods to deplete rhesus marrow extensively and to detect residual T cells following depletion at levels below the sensitivity of standard assays have been developed. A rhesus marrow limiting dilution assay has been developed that quantifies less than 1 T cell in 10(5) marrow cells and is superior to traditional detection methods by at least 3 logs. In a direct comparison of four T cell depletion methods, effective depletion has been achieved with complement-mediated cytotoxicity (C'MC), erythrocyte rosetting, and counterflow centrifugal elutriation (CCE), the latter with a simplified single-flow rate protocol. Median marrow T cell depletions of 2.1, 1.1, and 3.1 logs, and total nucleated cell losses of 40%, 61%, and 42% respectively, have been observed. A reported use of ricin A-chain-like toxins for the enhancement of C'MC was of low efficacy with rhesus peripheral blood T cell targets. CCE followed by C'MC has resulted in a median 4.8 logs depletion with residual marrow T cell contents less than 0.001%. Thus, C'MC, E-rosetting, and particularly CCE are effective methods of T cell depletion--and, when used in combination, extensively eliminate marrow T cells. A rhesus marrow limiting dilution assay detects residual T cells at these low levels. These techniques provide a basis for the quantitative study of the role of T cells in organ graft rejection following T lymphocyte-depleted autologous marrow transplantation.