Functional clonal deletion versus active suppression in transplantation tolerance induced by total-lymphoid irradiation

Transplant Tolerance, Not Only Clonal Deletion

The quest to understand how allogeneic transplanted tissue is not rejected and how tolerance is induced led to fundamental concepts in immunology. First, we review the research that led to the Clonal Deletion theory in the late 1950s that has since dominated the field of immunology and transplantation. At that time many basic mechanisms of immune response were unknown, including the role of lymphocytes and T cells in rejection. These original observations are reassessed by considering T regulatory cells that are produced by thymus of neonates to prevent autoimmunity. Second, we review "operational tolerance" induced in adult rodents and larger animals such as pigs. This can occur spontaneously especially with liver allografts, but also can develop after short courses of a variety of rejection inhibiting therapies. Over time these animals develop alloantigen specific tolerance to the graft but retain the capacity to reject third-party grafts. These animals have a "split tolerance" as peripheral lymphocytes from these animals respond to donor alloantigen in graft versus host assays and in mixed lymphocyte cultures, indicating there is no clonal deletion. Investigation of this phenomenon excludes many mechanisms, including anti-donor antibody blocking rejection as well as anti-idiotypic responses mediated by antibody or T cells. This split tolerance is transferred to a second immune-depleted host by T cells that retain the capacity to effect rejection of third-party grafts by the same host. Third, we review research on alloantigen specific inhibitory T cells that led to the first identification of the CD4+CD25+T regulatory cell. The key role of T cell derived cytokines, other than IL-2, in promoting survival and expansion of antigen specific T regulatory cells that mediate transplant tolerance is reviewed. The precise methods for inducing and diagnosing operational tolerance remain to be defined, but antigen specific T regulatory cells are key mediators.

Hematopoietic stem cells and solid organ transplantation

Solid organ transplantation provides lifesaving therapy for patients with end stage organ disease. In order for the transplanted organ to survive, the recipient must take a lifelong cocktail of immunosuppressive medications that increase the risk for infections, malignancies and drug toxicities. Data from many animal studies have shown that recipients can be made tolerant of their transplanted organ by infusing stem cells, particularly hematopoietic stem cells, prior to the transplant. The animal data have been translated into humans and now several clinical trials have demonstrated that infusion of hematopoietic stem cells, along with specialized conditioning regimens, can permit solid organ allograft survival without immunosuppressive medications. This important therapeutic advance has been made possible by understanding the immunologic mechanisms by which stem cells modify the host immune system, although it must be cautioned that the conditioning regimens are often severe and associated with significant morbidity. This review discusses the role of hematopoietic stem cells in solid organ transplantation, provides an understanding of how these stem cells modify the host immune system and describes how newer information about adaptive and innate immunity might lead to improvements in the use of hematopoietic stem cells to induce tolerance to transplanted organs.

Induction of graft-versus-leukemia (GVL) effect without graft-versus-host disease (GVHD) by pretransplant donor treatment with immunomodulators

Pretransplant donor treatment with immunomodulators such as complete Freund's adjuvant (CFA) or oligodeoxynucleotide sequences expressing CpG motifs (CpG), was applied in sublethally irradiated host mice inoculated with murine models of mammary carcinoma (4T1) or B cell leukemia (BCL1). Spleen cells or IL-2 activated splenocytes (lymphokine activated killer [LAK]) derived from donor mice treated with CpG emulsified in incomplete Freund's adjuvant (IFA), (CpG + IFA) did not cause graft-versus-host disease (GVHD), but were not efficient enough to induce a significant graft-versus-tumor (GVT) response against 4T1 cells. In contrast, an efficient graft-versus-leukemia (GVL) effect was evident in BCL1-bearing mice inoculated with spleen cells from donors pretreated with CFA or CpG + IFA. Pretransplant donor treatment with CFA prolonged survival to a median of 62 days with 3 of 27 mice remaining GVHD- and leukemia-free for >200 days, compared to GVHD-related death of all mice inoculated with naïve cells (median 17 days), or leukemia-related death of all mice inoculated with leukemia cells (median of 27 days). Pretransplant donor treatment with CpG + IFA exerted a more efficient GVL effect with reduced GVHD resulting in 12 of 26 GVHD- and leukemia-free survivors for >200 days. Our results suggest that it may be possible to prevent GVHD while sparing an efficient GVL effect by using pretransplant donor treatment with immunomodulators prior to allogeneic stem cell transplantation and/or donor lymphocyte infusions in hematologic malignancies.

Fully MHC-Disparate Mixed Hemopoietic Chimeras Show Specific Defects in the Control of Chronic Viral Infections

The establishment of mixed allogeneic chimerism can induce donor-specific transplantation tolerance across full MHC barriers. However, a theoretical disadvantage of this approach is the possibility that the state of mixed chimerism might negatively affect the recipient's immune competence to control pathogens. Previous studies using murine models have not supported this hypothesis, because they indicate that acute viral infections are cleared by chimeric animals with similar kinetics to that of unmanipulated controls. However, chronic or persistent viral infections often require a more complex and sustained response with cooperation between CD4 Th cells, CTL, and B cells for effective control. The current study indicates that profound defects become manifest in the control of chronic pathogenic infections in MHC-disparate mixed allogeneic chimeric mice. Furthermore, we show that ineffective priming of the donor-restricted CTL response leads to virus persistence, as well as severe T cell exhaustion. Our results further suggest that either T cell adoptive immunotherapy or selected MHC haplotype matching partially restore immune competence. These approaches may facilitate the translation of mixed chimerism therapeutic regimens.

Analysis of the underlying cellular mechanisms of anti-CD154-induced graft tolerance: the interplay of clonal anergy and immune regulation

Although it has been shown that CD4(+)CD25(+) regulatory T cells (T(reg)) contribute to long-term graft acceptance, their impact on the effector compartment and the mechanism by which they exert suppression in vivo remain unresolved. Using a CD4(+) TCR transgenic model for graft tolerance, we have unveiled the independent contributions of anergy and active suppression to the fate of immune and tolerant alloreactive T cells in vivo. First, it is shown that anti-CD154-induced tolerance resulted in the abortive expansion of the alloreactive, effector T cell pool. Second, commensurate with reduced expansion, there was a loss of cytokine production, activation marker expression, and absence of memory T cell markers. All these parameters defined the tolerant alloreactive T cells and correlated with the inability to mediate graft rejection. Third, the tolerant alloreactive T cell phenotype that is induced by CD154 was reversed by the in vivo depletion of T(reg). Reversal of the tolerant phenotype was followed by rapid rejection of the allograft. Fourth, in addition to T(reg) depletion, costimulation of the tolerant alloreactive T cells or activation of the APC compartment also reverted alloreactive T cell tolerance and restored an activated phenotype. Finally, it is shown that the suppression is long-lived, and in the absence of anti-CD154 and donor-specific transfusion, these T(reg) can chronically suppress effector cell responses, allowing long-lived graft acceptance.

Transplantation of hematopoietic stem cells for induction of unresponsiveness to organ allografts

Although it has been recognized since the early days of Owen and Medawar that engraftment of donor stem cells, induced in utero spontaneously or intentionally neonatally, results in life-long unresponsiveness to donor alloantigens. However, successful induction of transplantation tolerance in adult life still represents an unsolved problem. Engraftment of donor stem cells using conventional modalities involves intensive myeloablative or lymphoablative immunosuppression, which is associated with toxicity and mortality and such methods are not suitable for organ allograft recipients. In this chapter, we present an innovative approach for induction of donor-specific unresponsiveness to bone marrow and organ allografts without myeloablative conditioning. Our methods is based on cyclophosphamide-induced, alloantigen-primed lymphocyte depletion. Cyclophosphamide is administered 1 day following infusion of donor hematopoietic cells, thus eliminating predominantly host T lymphocytes reacting against donor cell challenge, and resulting in relative unresponsiveness to donor alloantigens. Subsequently, life-long tolerance to fully mismatched donor skin allografts can be accomplished by a second infusion of stem cells from the same donor, with donor T cells displacing residual alloreactive host cells that may have escaped deletion. Taken together, we believe that induction of true permanent and specific tolerance to organ allografts using donor hematopoietic cells could become a clinical reality in the foreseeable future.

Allogeneic versus syngeneic killer splenocytes as effector cells for the induction of graft-versus-tumor effect

The effect of allogeneic versus syngeneic killer cells derived from normal or severe combined immunodeficiency disease (SCID) mice was evaluated for induction of antitumor reaction in a murine model of mammary carcinoma. Tumor cells of H-2d origin were injected intravenously into H-2(d/b) mice 24 hours after total body irradiation (4 Gy). On the following day, lymphokine-activated killer (LAK) splenocytes, derived from either minor (H-2d) or major (H-2b) histocompatibility complex (MHC)-mismatched parental normal mice or MHC (H-2b)-mismatched SCID mice, were given intravenously. LAK cells of H-2d normal or SCID mice, syngeneic to the tumor, were inoculated in parallel. The results show that LAK cells derived from minor histocompatibility complex-mismatched or MHC-mismatched parental normal mice improved the probability of tumor-free survival as compared with LAK cells syngeneic to the tumor cells, but they aggravated the severity of graft-versus-host disease. SCID splenocytes serving as a source of natural killer (NK) cells were expanded and activated in vitro by rIL-2 to obtain a sufficient number of DX5+ CD3- CD8- NK cells (SCID-LAK). H-2b SCID-LAK cells did not cause graft-versus-host disease and significantly delayed tumor growth compared with syngeneic H-2d SCID-LAK cells, as indicated by tumor colony assays in vitro and adoptive transfer experiments. However, the graft-versus-tumor effect was not long lasting, and treated mice finally died of tumor. Our results show an advantage of allogeneic over syngeneic cell therapy for achieving a graft-versus-tumor effect by rIL-2-activated T cells and NK cells. Periodic repetition of NK treatments may be required to achieve more durable antitumor effects.

Non-myeloablative hematopoietic stem cell transplantation (NST) in the treatment of human malignancies: from animal models to clinical practice

NST is becoming a widely accepted method for allogeneic HSCT. Much experience has been gained, and the biology, indications and limitations are becoming clearer. Nonmyeloablative conditioning allows consistent engraftment of allografts from matched related, unrelated, and even partially matched donors. NST has been able to reduce the toxicity of allogeneic HSCT. The better immediate outcome produces better overall DFS. NST was feasible in elderly patients with almost no upper age limit, and in patients with organ dysfunction or other comorbidities precluding standard ablative conditioning. NST has also reduced the regimen-related toxicity of allogeneic HSCT in high-risk setting such as HSCT in heavily pretreated patients or following failure of a prior transplant procedure and in the unrelated setting. NST is rapidly becoming the treatment of choice in these indications where toxicity of standard ablative therapy is unacceptable. In certain malignancies such as in NHL, Hodgkin's disease and multiple myeloma, standard ablative NST has been reported to result in exceptionally high treatment related mortality, and NST is being investigated as a more reasonable alternative. NST may reduce the toxicity of the procedure even in younger patients who are eligible for ablative HSCT as well, however the long-term impact on patient outcome in this group is not yet established, and NST merits further investigation in prospective comparative trials. As described above, the known susceptibility of the underlying malignancy to GVT, the response to prior chemotherapy and bulk of residual disease, and the type of donor are other factors to consider when considering NST, and when selecting a regimen. The optimal preparative regimen needs to be defined. Ultimately less chemotherapy will be used and more specific immune-modulation, rather than intense nonspecific immunosuppression, will be used to achieve HVG tolerance. Preliminary animal models using costimulation blockade for specific induction of tolerance are promising steps towards achievement of this goal. Although much progress has been achieved with consistent achievement of engraftment with NST, GVHD and disease recurrence remain major obstacles to successful treatment. Existing clinical data suggest that NST does limit the incidence and severity of GVHD. Limitation of regimen-related toxicity, and bilateral transplantation tolerance afforded by mixed chimerism, are believed to have a major role in limiting GVHD. However GVHD remains the primary cause of treatment-related mortality. The development of techniques to separate GVHD and GVL are essential for further improvement of NST outcome. Better understanding of the biology and targets of GVHD and GVL may allow the elimination of alloreactive T-cells responsible for GVHD from the graft while retaining T-cells with GVL and infection control potential. Recurrence of the underlying malignancy is a major complication when NST is attempted in patients with chemo-refractory diseases and with high tumor bulk. Reduced toxicity regimens such as the FB/ATG regimen have been somewhat more successful in controlling disease progression until a potent GVT effect is established. However novel approaches are urgently required. NST serves as a platform for cellular immunotherapy. Judicious use of pre-emptive DLI needs to be explored. DLI may be amplified by activation of donor lymphocytes with IL-2 or in vivo administration of IL-2. Identification of tumor antigens will lead the way to ex-vivo generation and expansion of tumor specific cytotoxic T-lymphocytes to be used as potent immunotherapy without the hazards of GVHD. Allogeneic transplantation is rapidly changing from administration of supralethal doses of chemotherapy and radiation, trying to physically eliminate the 'last tumor cell', to the more subtle and tolerated sophisticated immunotherapy. This effort will focus on specific induction of HVG tolerance followed by induction of tumor-specific GVT effect to cure the underlying malignancy.

New strategies for bone marrow transplantation

Adoptive immunotherapy of hematologic malignancies and metastatic solid tumors by donor lymphocyte infusion following induction of host-versus-graft transplantation tolerance against can best be achieved following nonmyeloablative stem-cell transplantation (NST). Induction of mixed chimerism may represent the best approach for induction of transplantation tolerance to donor alloantigens. Thus NST may become the optimal approach for the treatment of nonmalignant diseases, where replacement of host with donor hematopoietic cells is indicated: for correction of genetic or stem cell deficiency diseases; as a platform for immunotherapy of autoimmune or infectious diseases; or for induction of tolerance to organ allografts.

Nonmyeloablative conditioning to induce bilateral tolerance after allogeneic bone marrow transplantation in mice

We recently described a new nonmyeloablative method to induce stable and specific transplantation tolerance to allogeneic tissues in adult mice. It included total lymphoid irradiation (TLI) of recipients with six fractions of 200 cGy each, inoculation with donor bone marrow (BM) cells and cyclophosphamide (Cy) for selective elimination or inactivation of residual donor-reactive cells of the host, and infusion with T-cell depleted donor BM cells after Cy. Here, we investigated the possibility to induce stable bilateral graft-vs-host and host-vs-graft transplantation tolerance using non-T-cell depleted allogeneic BM. Our results show that the dose of BM required for the induction of transplantation tolerance was inversely correlated with the intensity of the conditioning. Transfer of a low dose (3 x 10(6)) of total donor BM cells to recipients preconditioned with a less intensive regimen (two or three TLI fractions instead of six) diminished graft-vs-host disease (GVHD)-related mortality of recipients to 40% and converted 89% of the survivors into GVHD-free mixed hematopoietic chimeras that maintained donor skin allografts >180 days. A tenfold increase in the number of donor BM cells (3 x 10(7) instead of 3 x 10(6)) reduced the rate of GVHD-related mortality of recipients to 20% and resulted in bilateral transplantation tolerance in 100% of nonirradiated survivors.

Thymic dependence of loss of tolerance in mixed allogeneic bone marrow chimeras after depletion of donor antigen. Peripheral mechanisms do not contribute to maintenance of tolerance

A nonmyeloablative conditioning regimen has recently been developed that allows allogeneic marrow engraftment with induction of permanent mixed chimerism and donor-specific tolerance across fully MHC-mismatched allogeneic barriers. We recently demonstrated that tolerance can be broken in these chimeras by administration of an anti-donor class I-specific monoclonal antibody that eliminates donor hematopoietic cells. We have now investigated the role of the thymus in the loss of tolerance observed when chimerism is eliminated in this manner. Mixed chimeras were prepared in B10 (H2b) recipients by treatment with depleting anti-CD4 and anti-CD8 mAbs, 3-Gy whole body irradiation, and 7-Gy thymic irradiation, followed by B10.A (H2a) bone marrow transplantation. Chimeras were thymectomized 7 weeks later, and were either untreated or were depleted of donor cells with anti-donor class I (Dd-specific) mAb 34-2-12. Control chimeras that were not thymectomized also received anti-donor monoclonal antibodies or no further treatment. Of the four groups, only euthymic animals that were depleted of donor antigen showed a loss of tolerance, as evidenced by rejection of B10.A skin grafts. In contrast to untreated control and thymectomized, anti-Dd-treated chimeras, these euthymic anti-Dd-treated chimeras showed significant recovery of Vbeta11+ T cells, which can recognize Mtv antigens presented by donor I-E molecules. The requirement for a thymus for loss of tolerance in the absence of donor antigen was verified in an adoptive transfer model, in which chimera (B10.A-->B10) spleen cells were depleted of donor-type cells ex vivo, adoptively transferred into B6 nu/nu mice, and then further depleted of donor-type antigen with monoclonal antibody treatment in vivo. These B6 nu/nu mice maintained donor-specific tolerance to B10.A skin grafts. The absence of active suppression as a potent mechanism of tolerance in long-term mixed chimeras was confirmed by the loss of mixed chimerism and of tolerance that was readily induced by injection of naive host-type spleen cells. Together, our results suggest that in mixed allogeneic chimeras, intrathymic clonal deletion, and not peripheral suppression or anergy, is the major mechanism maintaining donor-specific tolerance.

Mixed chimerism to induce tolerance for solid organ transplantation

Chimerism, or the coexistence of tissue elements from more than one genetically different strain or species in an organism, is the only experimental state that results in the induction of donor-specific transplantation tolerance. Transplantation of a mixture of T-cell-depleted syngeneic (host-type) plus T-cell-depleted allogeneic (donor) bone marrow into a normal adult recipient mouse (A + B----A) results in mixed allogeneic chimerism. Recipient mice exhibit donor-specific transplantation tolerance, yet have full immunocompetence to recognize and respond to third-party transplantation antigens. After complete hematolymphopoietic repopulation at 28 days, animals accept a donor-specific skin graft but reject major histocompatibility complex (MHC) locus-disparate third-party grafts. We now report that permanent graft acceptance can also be achieved when the graft is placed at the time of bone marrow transplantation. Histologically, grafts were viable and had only minimal inflammatory changes. This model may have potential future clinical application for the induction of donor-specific transplantation tolerance.

The use of parabiosis for investigating the mechanism of transplantation tolerance in bone marrow chimeras induced by total lymphoid irradiation

The mechanism of transplantation tolerance in total lymphoid irradiation (TLI)-induced semiallogeneic bone marrow chimeras without clinical evidence of graft-versus-host disease (GVHD) was investigated using the technique of surgical parabiosis. When held in parabiosis with normal BALB/c mice, BALB/c---- (BALB/c x C57BL/6)F1 (BALB----F1) chimeras survived 7-9 days, significantly (P less than 0.001) shorter than the 12-19 day survival of normal F1 hybrids kept in parabiosis with normal BALB, and in contrast to indefinite (greater than 200 days) survival of syngeneic BALB parabiotic partners. When C57 skin grafts were placed on BALB mice held in parabiosis with BALB----F1 chimeras, C57 skin grafts survived 50-60 days, in contrast to 10-14 days in normal BALB recipients (P less than 0.001). Lethal GVHD, induced in sublethally irradiated F1 recipients by 10(7) BALB spleen cells, could not be delayed or prevented by cotransfer of 10(7) to 30 x 10(7) tolerant BALB spleen cells obtained from stable BALB----F1 chimeras. GVHD reactivity of BALB spleen cells isolated from BALB----F1 chimeras tolerant of C57 could not be recovered by depletion of Lyt2 cytotoxic suppressor cells. Taken together, in the absence of suppressive capacity by suppressor cells, these data support functional clonal deletion as the primary mechanism responsible for the maintenance of unresponsiveness to host alloantigens in TLI-induced semiallogeneic chimeras, since no protection against induction of GVHD could be documented in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of effector cells of graft vs leukemia following allogeneic bone marrow transplantation in mice inoculated with murine B-cell leukemia

It is now widely accepted that immunocompetent lymphocytes in allogeneic bone marrow grafts exert an antileukemic effect that contributes to the cure of leukemia. Graft vs leukemia (GVL) effects independent of graft vs host disease were investigated in allogeneic bone marrow chimeras tolerant of host and donor alloantigens. The role of Thy1.2, L3T4 and Lyt2 T lymphocytes as effector cells of GVL were investigated in (BALB/c x C57BL/6)F1 mice inoculated with murine B-cell leukemia and subsequently conditioned with total lymphoid irradiation and cyclophosphamide (200 mg/kg). Mice were reconstituted with C57BL/6 bone marrow cells depleted of well-defined T-cell subsets or enriched for stem cells by the soybean agglutination method. Detection of residual tumor cells, an indicator for efficacy of GVL, was carried out by adoptive transfer of peripheral blood or spleen cells obtained from treated chimeras into secondary naive BALB/c recipients at different time intervals following bone marrow transplantation. Treatment of the primary marrow inoculum with monoclonal anti-Thy1.2 or anti-Lyt2 abolished the GVL effects and all secondary BALB/c recipients developed leukemia within 60 days. On the other hand, the treatment with monoclonal anti-L3T4 did not influence the effect of GVL and all treated recipients remained without leukemia. The data suggest that T cells may mediate GVL effects in the absence of graft vs host disease and in circumstances where tolerance to conventional alloantigens is elicited. Effector cells of GVL across the major histocompatibility complex (MHC) in the murine B-cell leukemia tumor model system appear to be Thy1.2+ Lyt2+ L3T4-. Induction of GVL effects by allogeneic cells tolerant of host MHC suggests that these effects may be independent of graft vs host disease.

Mixed chimerism and permanent specific transplantation tolerance induced by a nonlethal preparative regimen

The use of allogeneic bone marrow transplantation as a means of inducing donor-specific tolerance across MHC barriers could provide an immunologically specific conditioning regimen for organ transplantation. However, a major limitation to this approach is the toxicity of whole body irradiation as currently used to abrogate host resistance and permit marrow engraftment. The present study describes methodology for abrogating host resistance and permitting marrow engraftment without lethal irradiation. Our preparative protocol involves administration of anti-CD4 and anti-CD8 mAbs in vivo, 300-rad WBI, 700-rad thymic irradiation, and unmanipulated fully MHC-disparate bone marrow. B10 mice prepared by this regimen developed stable mixed lymphohematopoetic chimerism without any clinical evidence of graft-vs.-host disease. Engraftment was accompanied by induction of specific tolerance to donor skin grafts (B10.D2), while third-party skin grafts (B10.BR) were promptly rejected. Mice treated with the complete regimen without bone marrow transplantation appeared healthy and enjoyed long-term survival. This study therefore demonstrates that stable mixed chimerism with donor-specific tolerance can be induced across an MHC barrier after a nonlethal preparative regimen, without clinical GVHD and without the risk of aplasia.

Induction of transplantation tolerance by intraportal injection of allogeneic bone marrow cells. Possible implications for intrauterine bone marrow transplantation across major histocompatibility barriers

Intraportal inoculation of C57BL/6 marrow cells into sublethally (400 rad) irradiated BALB/c recipients resulted in durable chimerism and the permanent acceptance of C57BL/6 skin allografts. Sublethally irradiated recipients of a similar number of marrow cells inoculated systemically did not develop chimerism or any significant prolongation of the survival of C57BL/6 skin allografts. Consequently, lethal graft-versus-host disease developed only in recipients of intraportal marrow allografts (80%). The intraportal injection of allogeneic C57BL/6 marrow cells into nonirradiated recipients resulted in significant, although not permanent, prolongation of skin allograft survival without durable chimerism, suggesting that the introduction of alloantigens intraportally may favor the induction of nonresponsiveness to alloantigens even across strong major histocompatibility barriers. The relevance of these findings is discussed regarding the intraportal inoculation of allogeneic bone marrow cells for the treatment of genetic disorders in utero through the induction of neonatal tolerance.

Alloantigen persistence in induction and maintenance of transplantation tolerance

Infusion of parental bone marrow cells into F1 hybrids conditioned by total lymphoid irradiation (TLI) results in chimeras with a high percentage of donor-type cells, and without clinical signs of graft-vs.-host reaction. In these chimeras, a state of tolerance has been shown to be associated with paucity of cytotoxic T lymphocyte percursors (pCTL) reactive with host-type alloantigens. To determine whether the presence of tolerizing alloantigens is essential for maintenance of unresponsiveness, lymphohematopoietic cells obtained from such tolerant chimeras were transferred into supralethally irradiated recipients of two different genotypes: in one case the adoptive recipients were syngeneic with host-type cells, and in the other they were syngeneic with donor-type cells of the original chimeras, thus providing the chimeric cells with a tolerogen-free environment. After "parking" for 4 d in syngeneic donor-type mice, the transferred cells displayed a marked increase in the frequency of pCTL directed against tolerizing alloantigens, whereas a low pCTL frequency directed against the same H-2 target cells was maintained in allogeneic tolerizing-type adoptive recipients. Multiple injections of adoptive donor-type mice with tolerizing-type cells of the original chimera reestablished a low level of cytotoxic precursors. Cytotoxic activity against unrelated alloantigens was independent of the presence of tolerogen-presenting cells in the adoptively transferred mice. Our experimental model suggests that persistence of cells bearing tolerizing alloantigens is an essential requirement for maintenance of previously established tolerance.