Long-term survival of cardiac xenografts in fully xenogeneic (mouse --> rat) bone marrow chimeras

BACKGROUND: The shortage of human hearts remains a major barrier to the efficacy of heart transplantation for the treatment of end-stage heart disease. One potential solution to the supply problem would be the use of hearts from nonhuman donors (xenografts). We have established a model of mouse to rat xenogeneic bone marrow chimerism, and in this study we have hypothesized that such chimeric rats will accept both donor and recipient specific heart grafts while rejecting third-party mouse and rat grafts. We also investigated humoral responses in naive and chimeric rats with and without donor murine cardiac grafts. METHODS: Recipient Lewis rats (n = 22) were given 1100 cGy lethal total body irradiation and the same day received 300 x 10(6) donor B10.BR mouse bone marrow cells intravenously. Peripheral blood of surviving rats (n = 18) was typed at 4 weeks and then monthly thereafter. Donor and recipient specific and third-party heterotopic heart transplantations were performed at 6 to 8 weeks after reconstitution with bone marrow. RESULTS: Multilineage bone marrow chimerism was produced in all experimental animals with complete replacement of recipient marrow by donor cells. Murine donor and rat recipient strain hearts transplanted in chimeric rats survived indefinitely. Third-party rat and mouse hearts were rejected, though at a slower rate than bone marrow matched naive controls. High levels of antimouse antibodies were detected in rats with rejected hearts. These antibodies were absent in chimeric animals with long-term surviving heart grafts. CONCLUSIONS: Long-term multilineage bone marrow chimerism can be produced in a mouse --> rat bone marrow transplant model. Long-term survival of donor specific and recipient specific vascularized cardiac grafts can be produced in these chimeric animals. These animals are clinically normal but show signs of subclinical immunosuppression regimen as they reject third-party hearts later than naive animals. Our results suggest that antibodies also play a significant role in concordant xenograft rejection, and induction of bone marrow chimerism can overcome this barrier.     

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.     

Kinetics of early T-cell repopulation in fully xenogeneic chimeras (F344 rat----B10 mouse): evidence for rat T-cell maturation in a xenogeneic mouse thymus.

We recently reported the model of fully xenogeneic chimerism achieved by transplantation of rat bone marrow into mouse recipients (F344 rat----B10 mouse), resulting in stable long-term rat lymphoid chimerism. We have now extended this model to examine whether developing precursor rat T cells from rat bone marrow stem cells can undergo normal differentiation in mature lymphocytes under the influence of a xenogeneic mouse thymus. We examined thymic and splenic lymphoid cells from fully xenogeneic chimeras starting 1 week after bone marrow transplantation to characterize early T-cell repopulation and phenotype. Our data suggest that developing rat precursor T cells are able to undergo normal differentiation in the mouse thymus. The first precursor T cells appeared 2 weeks after reconstitution and by week 10 accounted for more than 90% of thymocytes present in the chimeras. In chimeras, developing rat T lymphocytes in the mouse thymus exhibited an immature pattern (Thy 1.1+, alpha beta-TCRdull, CD4+ plus CD8+) when analyzed by flow cytometry. This pattern was similar to a normal rat. In contrast, splenic T-lymphoid cells showed a mature rat phenotype (Thy 1.1-, alpha beta-TCRhi, CD4+ or CD8+), again similar to a normal rat. This development began 2 weeks after bone marrow transplantation, and both thymus and spleen from chimeras exhibited "normal" rat T-cell staining profiles by 10 weeks after reconstitution. Overall, these data indicate that developing rat T cells are capable of undergoing normal maturation in a xenogeneic mouse thymus of tolerant animals.     

Clonal deletion and clonal anergy in allogeneic bone marrow chimeras prepared with TBI or TLI

Abstract The evolution of Vβ6 - expressing C₃H (H^k₂, Thy 1.2, Mls a—) lymphocytes was investigated in C3H recipients mice pretreated with total body irradiation (TBI) or total lymphoid irradiation (TLI) and infusion of AKR (H^k₂, Thy 1.1, MIS a +) cells. After TBI (9.5 Gy) all Vβ6+ Thy 1.2 (C₃H) cells, which are capable of reacting against the MIS a antigen that like expressed by AKR mice, were deleted in the thymus and the periphery in stable bone marrow (BM) chimeras obtained by infusion of 5 times 10⁶ T-cell-depleted (TCD) AKR BM cells. When, in the opposite combination, 30 times 10⁶ C₃H spleen cells were infused into TBI-treated AKR cells, all animals developed graft-versus-host disease (GVHD) with no clonal deletion and in contrast, showed an increase in Vβ6+ C₃H cells. After injection of 30 times 10⁶ AKR BM cells into TLI-treated C₃H mice no C₃H cells were detected in the thymus and only a small percentage in the periphery. Within these C₃H cells Vβ6+ cells were only partially deleted and anergized as they did not respond in vitro after stimulation with Mls a + AKR cells or anti-Vβ6 mAb. Cells suppressing anti-Mls a-reacting C₃H cells were not found. After injection of 15 times 10⁶ AKR cells more C₃H cells were found in the thymus, but only a minority of Vβ6+ cells persisted in the periphery of these animals. In conclusion in TBI-prepared chimeras only clonal deletion occurred, whereas in TLI-prepared chimeras both clonal deletion and anergy occurred in maintaining tolerance.     

Cross-species transplantation: NK cell number and function are normal in fully xenogeneic chimeras (rat----mouse).

When untreated F344 rat bone marrow is transplanted into B10 mouse recipients conditioned with total body irradiation, stable fully xenogeneic chimerism (rat----mouse) results. Chimeras are specifically tolerant to the donor strain of rat, survival is excellent (greater than 80% at 8 months), and all stem-cell-derived lineages are produced by the rat stem cell. We have previously demonstrated normal function of T-lymphocytes in these chimeras, but have not examined the immune function of natural killer (NK) cells present. Because NK cells play a critical role in immune surveillance, absence of function could result in a serious immunodeficiency state. We present data here to suggest that rat NK cells that have developed in a mouse stromal environment are normal in function as well as number. In all fully xenogeneic chimeras tested from 8 weeks to 8 months following bone marrow transplantation, NK cells were present at a normal level (10% to 16%). NK cells function in these chimeras, as tested by spontaneous lysis of YAC tumor cell targets, was normal or superior to normal F344 rat and B10 mouse NK cells.     

Clonal deletion: a mechanism of tolerance in mixed bone marrow chimeras

The mechanism of antigen-specific immunologic unresponsiveness which results from lethal irradiation and mixed (syngeneic-allogeneic) bone marrow cell (BMC) reconstitution is unknown. To determine whether clonal deletion is the mechanism of tolerance in this model, monoclonal antibody (Mab) RR-4-4, specific for a T-cell receptor (V beta 6) reactive against the minor alloantigen MLsa, was employed. Six-week-old B10 mice (H-2b, Mlsb, Thyl.2) were tolerized to AKR antigens (H-2k, Mlsa, Thyl.1) by whole body irradiation (950 R) and iv infusion of T-cell-depleted (TCD) B10 BMC + non-TCD AKR BMC. Chimerism and antigen-specific tolerance were documented by flow microfluorometry (FMF), skin grafting, mixed lymphocyte reaction, and cell-mediated lympholysis. When tolerant B10 mice (n = 15) had accepted AKR skin grafts for greater than 100 days, these animals were studied for the presence of host V beta 6+ T cells using Mab RR-4-4. FMF revealed that 0-5% of host (B10) lymph node and spleen cells from chimeras were V beta 6+ while 15-20% of lymph node and spleen cells from control B10 mice expressed V beta 6. These data demonstrate that clonal deletion occurs in the lethal irradiation-mixed reconstitution model as evidenced by the near total elimination of Mlsa-reactive V beta 6+ T cells and suggest that it maybe a mechanism responsible for tolerance in adult mice.     

Long-term survival of donor-specific pancreatic islet xenografts in fully xenogeneic chimeras (WF rat----B10 mouse).

We recently reported that reconstitution of lethally irradiated B10 mouse recipients with 40 x 10(6) untreated WF rat bone marrow cells resulted in stable fully xenogeneic chimerism (WF rat----B10 mouse). In these animals, the tolerance induced for skin xenografts was highly MHC specific in that donor-specific WF rat skin grafts were significantly prolonged while MHC-disparate third-party xenografts were rapidly rejected (median survival time [MST] = 9 days). We have now examined whether islet cell xenografts placed under the renal capsule of chimeras rendered diabetic with streptozotocin would be accepted and remain functional to maintain euglycemia. Animals were prepared, typed for chimerism at 6 weeks, and diabetes induced with streptozotocin. Donor-specific WF (Rt1Au) islet cell xenografts were significantly prolonged (MST greater than 180 days) in WF----B10 chimeras, while MHC-disparate third-party F344 rat (Rt1A1) grafts were rejected with a time course similar to unmanipulated B10 mice (MST = 8 days). The transplanted donor-specific islet cells were functional to maintain euglycemia, since removal of the grafts at from 100 to 180 days in selected individual chimeras uniformly resulted in return of the diabetic state. These data suggest that donor-specific islet cell xenografts are accepted and remain functional in mice rendered tolerant to rat xenoantigens following bone marrow transplantation.     

Characterization of xenogeneic mouse-to-rat bone marrow chimeras. I. Examination of hematologic and immunologic function

Eighteen xenogeneic chimeric rats (survival: greater than 100 days) were established by transplanting bone marrow cells from femurs of 10 gnotobiotic CFW mice into each germfree Sprague-Dawley or Wistar rat. The erythrocytes circulating in the rats were of mouse origin as determined by hemagglutination. Hemoglobin electrophoresis, radial immunodiffusion for IgG, and assay of granulocytic neutrophils for leukocyte alkaline phosphatase verified that true chimerism was achieved. The extent of hematological and immunological reconstitution varied. In general, hematocrit levels were low to normal, white blood cell counts and differentials were within normal limits, and serum protein levels were normal. Levels of circulating IgG of each species were comparable to those of germfree rat and mouse controls. Natural killer (NK) activity was depressed, a phenomenon that may be attributable to the radiation treatment of recipients, or to failure to transfer NK cells or precursors. Mitogenic stimulation reactions were varied, but most chimeric rats demonstrated moderately depressed responses. Reactions as a whole suggested that gnotobiotic rats with xenogeneic bone marrow are incompletely reconstituted, both hematologically and immunologically. No acute graft-versus-host reaction was seen.     

Prevention of graft-versus-host disease by treatment of bone marrow with gliotoxin in fully allogeneic chimeras and their cytotoxic T cell repertoire

Gliotoxin, a secondary fungal metabolite, at nanomolar concentrations, irreversibly inhibits murine T cell proliferation to mitogen. Treatment of allogeneic spleen cells with gliotoxin allows their transfer into sublethally irradiated recipients without inducing a GVH reaction. Gliotoxin treatment of bone marrow allows the establishment of fully allogenic bone marrow chimeras free of GVH disease. The cytotoxic T cell repertoire against influenza virus in these animals is restricted to both host- and donor-type MHC. However, their immune competence is severely compromised by their lack of host MHC-type stimulator cells.     

Construction of ectopic xenogeneic bone marrow structure associated with persistent multi-lineage mixed chimerism by engraftment of rat bone marrow plugs into mouse kidney capsules

Abstract: Poor bone marrow (BM) engraftment in a xenogeneic combination results at least in part from the limited engraftment capacity of BM-derived stromal cells, which support hematopoietic repopulation in a species-specific fashion. We attempted to construct a BM stromal microenvironment by engraftment of BM plug fragments into kidney capsules in a rat-to-mouse combination. BM plugs from F344/N Jcl- rnu/rnu (F344 nu ) rats were transplanted into the kidney capsules of C.B-17 scid/scid (C.B-17 scid ) mice treated with rabbit anti-asialo-GM1 serum to deplete natural killer (NK) cells and then with 3 Gy of whole body irradiation. As a conventional control, an equivalent amount of F344 nu bone marrow cells (BMCs) was intravenously injected into C.B-17 scid mice treated with a similar conditioning regimen. In both mouse recipients of rat BM plug engraftment in the kidney capsules and recipients of intravenous injection of rat BMC suspension, comparable extents of donor rat class I⁺ cells were persistently detected in the peripheral blood. However, the differentiation of rat-derived B cells in the mouse recipients of rat BM plugs was more rapid than that in the recipients of rat BMC suspension. In the late phase (10 weeks after BM transplantation), the percentage of rat-derived T cells (CD4⁺ cells) in the mouse recipients of rat BM plugs was significantly higher than that in the recipients of rat BMC suspension. At this time point, ectopic BM structure consisting of bone, mesenchymal cells, and hematopoietic progenitors was constructed in the kidney capsules of mice that received rat BM plugs. Most of the cells in the ectopic BM were derived from the donor rat. Thus, engraftment of BM plugs into the kidney capsules results in the construction of a donor-derived BM microenvironment, facilitating multilineage mixed xenogeneic chimerism.     

Mixed xenogeneic chimeras (rat + mouse to mouse). Evidence of rat stem cell engraftment, strain-specific transplantation tolerance, and skin-specific antigens.

We report the induction of stable and reliably detectable mixed xenogeneic chimerism through the coadministration of a mixture of untreated rat bone marrow plus T cell-depleted mouse bone marrow into B10 recipients conditioned with total body irradiation (TCD B10 mouse + untreated F344 rat----B10 mouse). Recipients repopulated as true mixed lymphopoietic chimeras, with from 1-21.6% rat-derived lymphoid cells in peripheral blood and splenic lymphoid tissue. Production of rat platelets was also demonstrated. Rat platelet and lymphoid chimerism was reliably detectable in chimeras from 1 to 7 months following reconstitution, suggesting engraftment of the rat bone marrow stem cell. Production of each stem cell-derived lineage appeared to be under independent regulation since a significantly greater proportion of platelets were rat-derived (24-81%) than were lymphocytes (1-21.6% rat), while erythrocytes were preferentially syngeneic (less than 2% rat). The tolerance induced by this model was highly donor strain-specific: donor-specific rat and mouse skin grafts were accepted while MHC-disparate third-party mouse (C3H; H-2k) and rat (Wistar Furth; Rt1Au) skin grafts were promptly rejected. Although specifically prolonged xenogeneic donor rat skin grafts underwent a slow chronic rejection, and some totally disappeared. In spite of this, chimeras retained their lymphoid chimerism, suggesting the presence of skin-specific antigens. This model for mixed xenogeneic chimerism with reliably detectable rat lymphoid cells may provide a model to study the existence of tissue-specific antigens across a species barrier, as well as mechanisms responsible for the induction and maintenance of this strain-specific transplantation tolerance.