Cross-species transplantation tolerance: rat bone marrow-derived cells can contribute to the ligand for negative selection of mouse T cell receptor V beta in chimeras tolerant to xenogeneic antigens (mouse + rat----mouse)

Development of an anti-porcine CD34 monoclonal antibody that identifies hematopoietic stem cells

OBJECTIVES

The isolation of porcine hematopoietic stem cells (HSC) would be an important step toward development of porcine-to-human chimerism for induction of tolerance in clinical xenotransplantation. CD34 is a common marker of HSC and has not been developed as a marker in pigs. In this study we have generated and characterized a monoclonal antibody (mAb) that identifies porcine CD34 on a subset of porcine bone marrow (BM) stem/progenitor cells.

METHODS

The porcine CD34 gene was cloned and a recombinant protein produced. An anti-porcine CD34 mAb was produced that could detect both the recombinant protein and a subset of porcine BM cells. The CD34(+) cells were phenotyped by lineage and HSC associated markers. Furthermore, the CD34(+) cells were analyzed by colony-forming unit (CFU) assay.

RESULTS

Two splice variants of the porcine CD34 gene were cloned and a recombinant protein produced for mAb production. The mAb developed can detect both the recombinant protein and the native CD34 protein on a range of pig tissues, including BM. This subset of BM cells was negative for hematopoietic lineage makers, including CD3, CD14, and CD21 and positive for other known porcine HSC markers, including CD90, CD172a, histocompatibility complex (MHC) class I, and MHC class II. Moreover, the CD34(+) BM cells were enriched for multilineage progenitor cells as determined by CFU assay.

CONCLUSIONS

Similar to human and mouse CD34, pig CD34 detects a subset of BM progenitor cells. This mAb will now provide a means for isolating porcine CD34(+) cells to be further analyzed for HSC activity and to assess their potential to develop pig-to-human chimeras to induce xenograft tolerance.

Mixed Xenogeneic Chimerism Induces Donor-Specific Humoral and Cellular Immune Tolerance for Cardiac Xenografts

Xenotransplantation has been suggested as a potential solution to the critical shortage of donor organs. However, success has been limited by the vigorous rejection response elicited against solid organs transplanted across species barriers. Mixed xenogeneic bone marrow chimeras resulting from the transplantation of a mixture of host and donor marrow (B10 mouse + F344 rat --> B10 mouse) results in donor-specific cross-species transplantation tolerance for subsequent nonvascularized skin and islet grafts. Furthermore, compared with fully xenogeneic chimeras (rat --> mouse), mixed xenogeneic chimeras exhibit superior immunocompetence for infectious agents in vivo and in vitro, suggesting that the immune system is intact. The ability to establish long-term humoral and cellular tolerance for primarily vascularized xenografts in vivo, in the setting of both recipient and donor Ig and effector cell production, has not previously been characterized. Mixed xenogeneic chimeras exhibit donor-specific humoral tolerance as evident by the absence of anti-donor Ab and Ab-dependent donor-specific cytotoxicity in vitro and intravascular IgM deposition within donor-strain (F344) cardiac xenografts in vivo. F344 cardiac xenografts are accepted (median > or =180 days) without clinical or histologic evidence of rejection, suggesting cellular tolerance. In contrast, MHC-disparate third-party mouse (B10.BR) and rat (ACI or WF) grafts are rejected (median of 23 and 41 days, respectively) in association with extensive mononuclear cell infiltration and vascular deposits of mouse IgM. These results demonstrate that mixed xenogeneic chimerism establishes donor-specific humoral and cellular tolerance and permits the successful transplantation of even primarily vascularized xenografts in the setting of intact Ab production.

Facilitating cells as a venue to establish mixed chimerism and tolerance

Graft rejection and the toxicity associated with the use of non-specific immunosuppression remain the major limitations in pediatric solid organ transplantation. The induction of tolerance in transplant recipients is an elusive but achievable goal that will decrease the dependence on immunosuppressive agents. BMT is associated with a robust form of donor-specific transplantation tolerance. It achieves a state of chimerism, defined as the presence of donor marrow cells in the recipient. The two major toxicities in conventional bone marrow transplantation that have prevented its clinical application to induce tolerance are the toxicity of ablative conditioning and GVHD. Two forms of chimerism exist: full chimerism and mixed chimerism. In full chimerism, the hematopoietic system of the recipient is replaced by that of the donor following ablative conditioning. Full chimerism is associated with a relatively impaired immunocompetence for primary immune responses and an increased risk of GVHD. In addition, the 7-10% regimen-related mortality associated with ablation could not be accepted in solid organ allograft recipients. In mixed chimerism the donor hematopoietic system co-exists with that of the recipient. Mixed chimerism induces donor-specific tolerance and is associated with superior immunocompetence and a relative resistance to GVHD compared with full chimerism. Moreover, it can be achieved with partial conditioning, thereby reducing the regimen-related morbidity associated with myeloablation. Approaches to establish mixed chimerism using non-myeloablative-conditioning regimens have been aggressively pursued over the past decade. Mixed chimerism can be safely established with minimal conditioning, resulting in a significant reduction in risk compared with ablative conditioning. GVHD is the final hurdle that has prevented the widespread application of chimerism to induce tolerance. Donor T cells are the primary effector cells for GVHD. Although T cell depletion of the donor marrow avoids GVHD, it results in an increase in the rate of graft failure in MHC-disparate recipients. The dichotomy between GVHD and T cell depletion graft failure has recently been dissociated by the discovery of CD8+/TCR- graft FC. Purified HSC engraft readily in syngeneic recipients but not in MHC-disparate allogeneic recipients. The addition of small numbers of facilitating cells permits durable HSC engraftment in allogeneic recipients and avoids GVHD. Using FC to promote HSC engraftment following non-myeloablative conditioning could be a promising approach to establish tolerance in solid organ transplantation. This invited review focuses on recent developments in stem cell chimerism and tolerance that could bring the use of this approach to induce tolerance to solid organ transplantation one step closer to reality.

Xenotransplantation and tolerance

The application of xenotransplantation faces daunting immunological hurdles, some of which might be overcome with the induction of tolerance. Porcine organs transplanted into primates are subject to several types of rejection responses. Hyperacute rejection mediated by naturally occurring xenoreactive antibodies and complement can be overcome without tolerance. Acute vascular rejection and cellular rejection, however, may present important opportunities for immunological tolerance, and humoral rejection might be approached by various mechanisms including (i) clonal deletion, (ii) anergy, (iii) immune deviation, (iv) induction of immunoregulatory or suppressor cells, or (v) veto cells. B-cell tolerance, useful for preventing humoral rejection, might be approached through clonal anergy. It remains to be determined, however, whether tolerance induction is required for xenotransplantation and by which means the various mechanisms of tolerance can be applied in the setting of xenotransplantation. Regardless, the study of tolerance will surely expand understanding of the physiology and pathophysiology of the immune system.

Mixed chimerism

Induction of mixed chimerism has the potential to overcome the current limitations of transplantation, namely chronic rejection, complications of immunosuppressive therapy and the need for xenografts to overcome the current shortage of allogeneic organs. Successful achievement of mixed chimerism had been shown to tolerize T cells, B cells and possibly natural killer cells, the lymphocyte subsets that pose major barriers to allogeneic and xenogeneic transplants. Current understanding of the mechanisms involved in tolerization of each cell type is reviewed. Considerable advances have been made in reducing the potential toxicity of conditioning regimens required for the induction of mixed chimerism in rodent models, and translation of these strategies to large animal models and in a patient are important advances toward more widespread clinical application of the mixed chimerism approach for tolerance induction.

Xenotransplantation of the liver

Xenotransplantation of the liver, in its broadest conception, might involve the transplantation of an intact organ or xenogeneic hepatocytes, or the use of an intact xenogeneic liver or cells as an ex vivo "device." The indications for xenotransplantation include not only hepatic failure but also, potentially, the treatment of metabolic diseases. The hurdles to xenotransplantation include immune, physiologic, and infectious complications. New information and progress in experimental systems are bringing xenotransplantation closer to clinical application.

Mouse xenoantigens contribute to rat T-cell Vbeta repertoire generation in mixed xenogeneic bone marrow chimeras

We previously demonstrated that rat bone-marrow-derived cells in mixed xenogeneic chimeras (rat + mouse --> mouse) contribute to peripheral selection of mouse T-cell receptor (TCR) variable betas (Vbetas) repertoire. In this study, we analysed rat T cells that developed in the chimeras to assess the contribution of mouse xenoantigens to the development of rat TCR repertoire. The expression of rat Vbetas was analysed using flow cytometry and a reverse transcription-polymerase chain reaction (RT-PCR) method that allows for both semiquantitative analysis of rat Vbeta gene expression and size heterogeneity of the complementarity determining region 3 (CDR3) domain. Three distinct patterns of Vbeta expression were detected. Partial deletion was observed for Vbeta5, 7, 12, 14, 16, 17 and 20 that exhibited reduced levels of peripheral expression by 3.4-, 1.8-, 8.7-, 2.0-, 7.8-, 9.5- and 1.8-fold, respectively, compared with the levels of Vbetas in naYve rats. Higher levels of peripheral expression were detected for three rat Vbeta genes; Vbeta6 (2.2-fold), Vbeta8.2 (3.2-fold), and Vbeta9 (1.7-fold). The relative expression of the other 10 known rat Vbeta families in chimeras was unchanged as compared with that of normal rats. We did not observe detectable changes in the pattern of CDR3 expression in chimeras, suggesting that the mouse xenogeneic environment exerted its influence on the development of rat T cells via the Vbeta-encoded CDR1/2 domains. Our data demonstrate that the rat T-cell repertoire in chimeras is shaped by both contractions as well as expansions of selected Vbetas and suggest that mouse xenoantigens and/or superantigens of endogenous mouse retroviruses may contribute as ligands for these selection processes

Establishment of fully xenogeneic (mouse-->rat) bone marrow chimeras: evidence for normal development and clonal deletion of mouse T cells

BACKGROUND

Xenotransplantation is a potential solution to the critical shortage of transplantable organs. However, conventional immunosuppressive agents do not control the vigorous cellular and humoral rejection across species disparities. The induction of donor specific tolerance via bone marrow chimerism may be a method to avoid xenograft rejection. In xenogeneic chimeras, T cell repertoire selection plays an important role in the induction of tolerance. Until now a model of mouse-->rat multilineage chimerism has not been reported. This study reports the establishment of fully xenogeneic mouse-->rat multilineage chimeras and evaluates the role of T cell development and repertoire selection in tolerance induction in a xenogeneic environment.

METHODS

Recipient rats were irradiated at a dose of total body irradiation ranging between 800-1100 cGy and injected with 120-300x10(6) donor mouse bone marrow cells. Chimeras were typed for engraftment at 4 weeks and then monthly thereafter. T cell repertoire was evaluated in chimeras using two-color flow cytometry and monoclonal antibodies directed against the variable portion of the beta chain of the T cell receptor.

RESULTS

Fully xenogeneic multilineage bone marrow chimerism was produced in a mouse-->rat model by using ablative radiation and a high dose of donor cells. Mouse T cells develop in a phenotypically normal fashion in chimeric rats and the host rat is capable of deleting T cells that are reactive to the donor mouse strain.

CONCLUSION

Long-term multilineage bone marrow chimerism can be produced in a mouse-->rat bone marrow transplant model. Mouse T cells develop in a phenotypically normal fashion and negative selection of specific T cell receptor-Vbeta occurs in a xenogeneic environment in a predictable fashion paralleling that for syngeneic or allogeneic transplantation.

Antiviral Cytotoxic Activity Across a Species Barrier in Mixed Xenogeneic Chimeras: Functional Restriction to Host MHC

Reconstitution of lethally irradiated mice with a mixture of mouse and rat bone marrow cells (mouse + rat→mouse) results in mixed xenogeneic chimerism and donor-specific tolerance. The current study demonstrates that mouse and rat T lymphocytes that have developed in xenogeneic chimeras are restricted to Ag presentation by mouse, but not rat, APC. Restriction to host Ags results in functional immunocompetence with generation of antiviral cytotoxic activity in vivo, within and across species barriers. These data demonstrate for the first time that the host thymus is sufficient to support development and positive selection of functional cross-species T lymphocytes. The superior immunocompetence, as compared with fully xenogeneic (rat→mouse) chimeras, may prove to be of significant benefit in the clinical application of xenotransplantation to solid organ transplantation and immune reconstitution for AIDS.

Differential APC requirements of self- and nonself-reactive T cells and T cell hybridomas specific for retinal S-antigen

Autoreactive T cell hybridomas specific for pathogenic peptides of retinal S-Ag require a novel radiosensitive APC activity for IL-2 secretion that is distinct from Ag presentation by MHC class II. Antigen-dependent IL-2 secretion by self-reactive hybridomas was much more efficient with splenic APC than with thymic APC, although both provided similar levels of hybridoma TCR occupancy as measured by activation-induced cell death. Furthermore, thymic APC did stimulate IL-2 secretion by a non-self reactive hybridoma. To test the hypothesis that this activity was provided by a distinct cell population, fractionated splenocytes were tested for their ability to present Ag to these hybridomas. The most potent Ag presentation for IL-2 secretion was found to segregate with low-density, B cell-enriched fractions while adherent cells, or purified T cells were unable to support IL-2 production. Together with previous results, the data show that antigen presentation leading to IL-2 secretion by these autoreactive T cell hybridomas requires activated B cells, whereas TCR occupancy can be provided by several APC subsets.

A minimal conditioning approach to achieve stable multilineage mouse plus rat chimerism

Transplantation of untreated rat bone marrow into lethally irradiated (950 cGy) mouse recipients results in durable xenogeneic (rat-->mouse) chimerism and confers donor-specific transplantation tolerance for subsequent xenografts. The purpose of the present study was to characterize the minimal dose of total body irradiation (TBI) which would allow engraftment of rat bone marrow in mouse recipients. We report here that durable and stable lymphohaematopoietic cross-species chimerism can be achieved using a less than totally ablative radiation-based conditioning approach. The percentage of B10 mouse recipients which engrafted with rat bone marrow cells correlated with the dose of TBI. Engraftment of rat bone marrow stem cells occurred in all animals receiving 750 cGy prior to bone marrow transplantation, while no engraftment was detected at doses less than 650 cGy. Although most of the recipients were repopulated with mixed mouse and rat multilineage chimerism, some exhibited a predominance of rat cells. Although mixed xenogeneic rat/mouse chimeras prepared by lethal TBI produced only mouse derived RBC (red blood cells), chimeras prepared by sublethal conditioning produced both rat and mouse RBC. Only animals with detectable chimerism exhibited specific functional transplantation tolerance to donor xenoantigens, as assessed in vitro by mixed lymphocyte reaction assay. This model may offer an in vivo approach to study the role of species-specific growth factors in stem cell biology as well as the mechanisms for the induction of tolerance across species barriers.

Early events in cell-mediated recognition of vascularized xenografts: cooperative interactions between selected lymphocyte subsets and natural antibodies

Cell-mediated early immune recognition of xenogeneic vascularized discordant grafts is poorly characterized. It has been the purpose of our studies to elucidate the role of lymphocytes in the recognition and rejection phenomena. To this end, we have utilized both ex vivo and in vitro model systems. We demonstrate that selected human lymphocyte subpopulations (mainly NK cells) rapidly and specifically adhere to xenogeneic endothelia. Efficient lysis of endothelial cells is subsequently mediated by such a population. Adhesion and cytotoxicity occur via at least two pathways, one dependent on and the other independent of the presence of human natural antibodies of the G class. Both IgG-dependent and IgG-independent adhesion and cytotoxicity can be partly inhibited by the use of anti-leukocyte integrin monoclonal antibodies. IgG-dependent adhesion and cytotoxicity can be also partly inhibited by carbohydrate structures that contain the alpha-galactosyl epitope. A possible role of these events in the eventual outcome of discordant vascularized xenogeneic transplants can be postulated.

Future directions in transplantation

Transplantation has become an established treatment for many diseases where organ replacement is the best or perhaps the only option, but it has not reached its full potential. The use of fetal and infant donors raises ethical problems, as does the use of other species for xenografts. Legal problems include the drafting of appropriate legislation to maximise the donor pool. Economic limitations are imposed by the competition for funding with other areas of medicine. Transplantation of cells, tissues, and organs is rapidly being expanded. Exciting progress in experimental models of new methods of immunosuppression may lead to immunological tolerance of grafts so that chronic non-specific immunosuppression becomes a thing of the past. Xenografts will probably have to become a major source of organs as transplantation is extended to more patients and a broader range of diseases. Genetic engineering may alleviate the problem of supply in some instances; free cell transplants of genetically modified cells are being studied.

Graft-versus-host disease after brown Norway-to-Lewis and Lewis-to-Brown Norway rat intestinal transplantation under FK506

In LEW rats treated daily with variable doses of FK506 for 14 days and weekly thereafter, successful intestinal transplantation from fully allogeneic BN donors never was complicated by fatal GVHD. In contrast, with LEW-to-BN transplantation, rejection was difficult to control and GVHD developed after the end of the daily treatment. However, FK506 in high daily doses continued after the initial 14-day course could prevent this GVHD or even reverse it after allowing its onset. Further experiments did not clarify why the BN rat was an "easy" donor and "difficult" recipient. In unaltered animals the lymphocyte population of normal LEW rats had a higher proportion of T cells, fewer B cells, and a lower CD4:CD8 ratio than normal BN rats. However, one-way MLR reactions of the BN and LEW combinations were generally similar in either direction and not affected differently by the addition of FK506 to the medium. The two-way lymphocyte traffic from graft to host lymphoid organs and vice versa also was similar with BN-to-LEW and LEW-to-BN models. The BN rat may be a useful tool to investigate inadequately explained mechanisms of GVHD.

Islet xenografts in fully xenogeneic (rat----mouse) chimeras: evidence for normal regulation of function in a xenogeneic mouse environment

BACKGROUND

Transplantation of untreated rat bone marrow into mouse recipients conditioned by total-body irradiation results in fully xenogeneic chimerism (rat----mouse). The chimerism is stable for up to 10 months, survival is excellent, and there is no evidence for graft-versus-host disease. We recently reported the long-term survival (greater than 180 days) of donor-specific pancreatic islet xenografts in these fully xenogeneic chimeras.

METHODS

Chimeras were prepared and typed for chimerism at 6 weeks, and diabetes was induced by streptozocin injection. Donor-specific pancreatic islets were placed under the renal capsule and recipient blood glucose levels were followed biweekly. The aim of this study was to examine whether the transplanted pancreatic islets exhibited normal function in a xenogeneic environment and assess whether the islet xenografts were not only sufficient to support euglycemia but also regulated in function in response to a glucose challenge.

RESULTS

We report for the first time that donor-specific rat islet xenografts were capable of producing normal basal and peak levels of insulin and responding to a glucose challenge in a manner similar to that of normal mouse islets.

CONCLUSIONS

These data indicate that donor-specific rat islet xenografts are functional and regulated normally in fully xenogeneic (rat----mouse) chimeras.