Established Mixed Chimeras Research Articles

Distinct roles for major and minor antigen barriers in chimerism-based tolerance under irradiation-free conditions.

Eliminating cytoreductive conditioning from chimerism‐based tolerance protocols would facilitate clinical translation. Here we investigated the impact of major histocompatibility complex (MHC) and minor histocompatibility antigen (MiHA) barriers on mechanisms of tolerance and rejection in this setting. Transient depletion of natural killer (NK) cells at the time of bone marrow (BM) transplantation (BMT) (20 × 106 BALB/c BM cells → C57BL/6 recipients under costimulation blockade [CB] and rapamycin) prevented BM rejection. Despite persistent levels of mixed chimerism, BMT recipients gradually rejected skin grafts from the same donor strain. Extending NK cell depletion did not improve skin graft survival. However, F1 (C57BL/6×BALB/c) donors, which do not elicit NK cell‐mediated rejection, induced durable chimerism and tolerance. In contrast, if F1 donors with BALB/c background only were used (BALB/c×BALB.B), no tolerance was observed. In the absence of MiHA disparities (B10.D2 donors, MHC‐mismatch only), temporal NK cell depletion established stable chimerism and tolerance. Conversely, MHC identical BM (BALB.B donors, MiHA mismatch only) readily engrafted without NK cell depletion but no skin graft tolerance ensued. Therefore, we conclude that under CB and rapamycin, MHC disparities provoke NK cell‐mediated BM rejection in nonirradiated recipients whereas MiHA disparities do not prevent BM engraftment but impede skin graft tolerance in established mixed chimeras.

Vascularized Composite Allograft Tolerance Across MHC Barriers in a Large Animal Model

Vascularized composite allograft (VCA) transplantation can restore form and function following severe craniofacial injuries, extremity amputations or massive tissue loss. The induction of transplant tolerance would eliminate the need for long-term immunosuppression, realigning the risk-benefit ratio for these life-enhancing procedures. Skin, a critical component of VCA, has consistently presented the most stringent challenge to transplant tolerance. Here, we demonstrate, in a clinically relevant miniature swine model, induction of immunologic tolerance of VCAs across MHC barriers by induction of stable hematopoietic mixed chimerism. Recipient conditioning consisted of T cell depletion with CD3-immunotoxin, and 100 cGy total body irradiation prior to hematopoietic cell transplantation (HCT) and a 45-day course of cyclosporine A. VCA transplantation was performed either simultaneously to induction of mixed chimerism or into established mixed chimeras 85-150 days later. Following withdrawal of immunosuppression both VCAs transplanted into stable chimeras (n=4), and those transplanted at the time of HCT (n=2) accepted all components, including skin, without evidence of rejection to the experimental end point 115-504 days posttransplant. These data demonstrate that tolerance across MHC mismatches can be induced in a clinically relevant VCA model, providing proof of concept for long-term immunosuppression-free survival.

Nonalloreactive T Cells Prevent Donor Lymphocyte Infusion–Induced Graft-versus-Host Disease by Controlling Microbial Stimuli

In mice, graft-versus-host reactions, associated with powerful graft-versus-tumor effects, can be achieved without graft-versus-host disease (GVHD) by delayed administration of donor lymphocyte infusions (DLI) to established mixed chimeras. However, GVHD sometimes occurs after DLI in established mixed chimeric patients. In contrast to mice, in which T cell recovery from the thymus occurs prior to DLI administration, human T cell reconstitution following T cell-depleted hematopoietic cell transplantation is slow, resulting in lymphopenia at the time of DLI. We demonstrate in this study that T cell lymphopenia is an independent risk factor for GVHD following DLI in the absence of known inflammatory stimuli. DLI-induced GVHD was prevented in lymphopenic recipients by prior administration of a small number of nonalloreactive polyclonal T cells, insufficient to prevent lymphopenia-associated expansion of subsequently administered T cells, through a regulatory T cell-independent mechanism. GVHD was not inhibited by T cells with irrelevant specificity. Moreover, administration of antibiotics reduced the severity of GVHD in lymphopenic hosts. Accumulation of DLI-derived effector T cells and host hematopoietic cell elimination were markedly diminished by regulatory T cell-depleted, nonalloreactive T cells. Finally, thymectomized mixed chimeras showed increased GVHD following delayed DLI. Collectively, our data demonstrate that in the absence of known conditioning-induced inflammatory stimuli, T cell lymphopenia is a risk factor for GVHD in mixed chimeras receiving delayed DLI. Our data suggest that the predisposition to GVHD can at least in part be explained by the presence of occult inflammatory stimuli due to the absence of T cells to control microbial infections.

Donor Bone Marrow-Derived T Cells Inhibit GVHD Induced by Donor Lymphocyte Infusion in Established Mixed Allogeneic Hematopoietic Chimeras

Delayed administration of donor lymphocyte infusion (DLI) to established mixed chimeras has been shown to achieve anti-tumor responses without graft-vs.-host disease (GVHD). Herein we show that de novo donor BM-derived T cells that are tolerant of the recipients are important in preventing GVHD in mixed chimeras receiving delayed DLI. Mixed chimeras lacking donor BM-derived T cells developed significantly more severe GVHD than those with donor BM-derived T cells after DLI, even though both groups had comparable levels of total T cells at the time of DLI. Post-DLI depletion of donor BM-derived T cells in mixed chimeras, as late as 20 days after DLI, also provoked severe GVHD. Although both CD4 and CD8 T cells contributed to the protection, the latter were significantly more effective, suggesting that inhibition of GVHD was not mainly mediated by CD4 regulatory T cells. The lack of donor BM-derived T cells was associated with markedly increased accumulation of DLI-derived alloreactive T cells in parenchymal GVHD target tissues. Thus, donor BM-derived T cells are an important factor in determining the risk of GVHD and therefore, offer a potential therapeutic target for preventing and ameliorating GVHD in the setting of delayed DLI in established mixed chimeras.

Host CD11c+ Dendritic Cells Are Required for Priming the Lympho-Haematopoietic Graft-Versus-Host Response but Not Graft-Versus-Host Disease.

Abstract 2450Poster Board II-427Following allogeneic bone marrow transplantation (BMT), delayed donor leukocyte infusion (DLI) at a point when conditioning-induced inflammation has resolved, is employed to induce graft-versus-leukemia (GVL) responses while limiting the risk of host injury in terms of graft-versus-host disease (GVHD). We have previously shown that host bone marrow (BM)-derived antigen presenting cells (APC) are required to prime GVL responses following delayed donor T cell transfer into MHC-mismatched allogeneic chimeras. The APC population(s) required for induction of graft-versus-host reactivity are not known, although it has been demonstrated by others that certain host-derived dendritic cell (DC) populations (in situ or transferred) are capable of inducing GVHD in models of immediate T cell transfer to freshly conditioned recipients. We hypothesised that host CD11c+ conventional DC would be required for the induction of graft-versus-host reactivity in a model of delayed T cell transfer to established mixed chimeras (MC). To test this hypothesis, we reconstituted lethally irradiated B6 (H2b) mice with a mixture of BALB/c (H2d) and B6.CD11c-DTR T cell depleted bone marrow. In the resulting MC, host CD11chigh cells (predominantly conventional DC) are uniquely sensitive to killing by diphtheria toxin (DT) due to their selective expression of a high affinity DT receptor. 8-10 week old established MC received DLI in form of congenic Thy1.1+ BALB/c splenocytes. MC were injected with DT (or PBS) every 72 hours from day -1 to day 10 following DLI. Depletion of host conventional DC (>90%) abrogated accumulation of donor Thy1.1+ CD4 and CD8 T cells in recipient spleens and reduced in vivo cytotoxicity against host target B cells. These effects associated with a lack of conversion from mixed to full donor chimerism, demonstrating an absolute requirement for host CD11chigh DC in priming the lympho-haematopoietic graft-versus-host response in the steady state.We have shown previously that induction of systemic inflammation by injection of a synthetic TRL7 agonist (R-848) is sufficient to induce systemic GVHD in MC after delayed DLI (Chakraverty et al, JEM, 2006). We therefore considered the requirement for CD11chigh DC in priming graft-versus-host reactivity in the presence of inflammation. We generated BALB/c + B6.CD11c-DTR→ B6 MC as before and then used DT treatment to deplete host conventional DC in the presence of R-848 (given every 72h from day -1 to day 10 following DLI). In MC without depletion of CD11chigh DC, R-848 treatment enhanced accumulation of Thy1.1+ BALB/c CD4 and CD8 donor cells and increased in vivo cytotoxicity against host B cell targets as compared to non-R-848 treated MC. This was associated with histological evidence of GVHD in the skin, gut and liver. In MC with depletion of CD11chigh DC (>90% deletion), R-848 treatment was associated with equivalent donor T cell accumulation and in vivo killing of host targets as observed in non-DC depleted controls. Furthermore, DC-depleted MC treated with R-848 also developed histological GVHD. In vitro experiments using CD11c+ stimulator cells purified from the spleens and lymph nodes of R-848 or PBS-treated mice suggested that priming of the allo-response in the presence of inflammation was not due to a residual population of CD11chigh DC remaining after DT treatment.Taken together, these data demonstrate for the first time that conventional host CD11chigh DC are required to prime lympho-haematopoietic graft-versus-host responses in the steady state following delayed DLI to MC. In contrast, their role is redundant in the context of TLR-agonist induced GVHD. This suggests that other APC populations can prime GVHD in the presence of systemic inflammation. Disclosures:No relevant conflicts of interest to declare.

Peripheral Alloantigen Drives Early Dysfunction and Eventual Exhaustion of CTL Following Delayed Donor Leukocyte Infusions.

T cell immunotherapies for cancer should ideally generate high levels of anti-tumor activity, with minimal host injury and permit the prolonged survival of functional memory/effector cells to prevent tumor recurrence. Following allogeneic stem cell transplantation, delayed donor leukocyte infusion (DLI) is one strategy employed to induce graft-versus-leukemia (GVL) responses while limiting the risk of host injury in terms of graft-versus-host disease. However, patients remain at significant risk of relapse following DLI and murine models of delayed DLI indicate that this results from the eventual loss of functional, alloreactive cytotoxic T lymphocytes (CTL) [Mapara et al. Transplantation 2003]. We hypothesised that the loss of functional CTL is driven by persistent stimulation of donor CD8 cells by alloantigen expressed by peripheral tissues. In order to follow and characterise an alloreactive CD8 response under conditions in which alloantigen was present or absent in peripheral tissues, we employed a model in which either parental B6 (H2b) or B6 x DBA-2 F1 (BDF1, H2dxb) mice were lethally irradiated and reconstituted with a mixture of B6 and BDF1 T cell depleted bone marrow. 8-10 weeks later congenic CD45.1 B6 splenocytes were transferred into the established mixed chimeras. This allowed us to test the importance of peripheral antigen in the loss of alloreactive CTL responses, since alloantigen was either restricted to the hematopoietic system (B6 +BDF1 → B6) or was ubiquitously expressed (B6 +BDF1 → BDF1). Following transfer of CD45.1 B6 splenocytes, the ensuing alloantigenspecific T cell response in both groups led to the elimination of alloantigen-positive (BDF1-derived) hematopoietic elements. Thereafter, alloreactive CD8 cells resided in an environment in which peripheral alloantigen was present (PA+) or absent (PA-). We observed similar kinetics of initial CD45.1+ CD8 cell proliferation and expansion and similar acquisition of a CD44highCD62Llow phenotype. However, by day 60, there were striking differences in the phenotype and function of transferred CD8 cells. In PA- hosts, CD45.1+ CD8 cells killed allogeneic target cells effectively both in vitro and in vivo, underwent rapid proliferation in a mixed leukocyte reaction and produced the effector cytokine, IFN-γ. In contrast CD45.1+ CD8 cells from PA+ hosts had little or no cytotoxic activity, did not proliferate to alloantigen and were IFN-γlow. Moreover, CD45.1+ CD8 cells from PA+ hosts displayed high levels of the co-inhibitory receptor PD-1, low levels of the IL-7Rα chain and responded poorly to IL-7 and IL-15 in vitro, a phenotype typical of the ‘exhaustion’ signature observed in CTL following chronic antigen exposure. In comparison, CD45.1+ CD8 cells from PA- hosts expressed significantly lower levels of PD-1, higher levels of IL-7Rα and demonstrated better responsiveness to IL-7 and IL-15 in vitro. In vitro PD-1 or PD-L1 blockade restored IFN-γ generation to CD45.1+ CD8 cells from PA+ hosts, suggesting that the PD-1 pathway may play a functional role in driving exhaustion of these cells. Importantly we observed no loss of long-term alloreactive CD4 responses in either PA+ or PA- hosts. This finding is consistent with a model in which peripheral alloantigen drives exhaustion since the majority of cells expressing Class II alloantigens in PA+ and PA- hosts would be restricted to the hematopoietic system and thus, would have been cleared in the initial alloresponse. The full exhausted phenotype of alloreactive CD8 cells described above was not seen until at least 30 days after transfer to PA+ hosts. However, as early as day 14, CTL primed in PA+ hosts produced less IFN-γ in comparison to those primed in PA-hosts, even though they were still equivalent in terms of their cytotoxicity. Furthermore, when CD8 cells primed in PA+ hosts were transferred to secondary antigen-free hosts, they still displayed reduced ‘fitness’ compared to CTL originally primed in PA- hosts. These data show that peripheral alloantigen qualitatively affects donor CTL function during priming and drives their eventual exhaustion. Additionally they suggest that blockade of co-inhibitory signals may have potential in restoring function to such cells as has been demonstrated in models of chronic infection.

An inflammatory checkpoint regulates recruitment of graft-versus-host reactive T cells to peripheral tissues

Transfer of T cells to freshly irradiated allogeneic recipients leads to their rapid recruitment to nonlymphoid tissues, where they induce graft-versus-host disease (GVHD). In contrast, when donor T cells are transferred to established mixed chimeras (MCs), GVHD is not induced despite a robust graft-versus-host (GVH) reaction that eliminates normal and malignant host hematopoietic cells. We demonstrate here that donor GVH-reactive T cells transferred to MCs or freshly irradiated mice undergo similar expansion and activation, with similar up-regulation of homing molecules required for entry to nonlymphoid tissues. Using dynamic two-photon in vivo microscopy, we show that these activated T cells do not enter GVHD target tissues in established MCs, contrary to the dogma that activated T cells inevitably traffic to nonlymphoid tissues. Instead, we show that the presence of inflammation within a nonlymphoid tissue is a prerequisite for the trafficking of activated T cells to that site. Our studies help to explain the paradox whereby GVH-reactive T cells can mediate graft-versus-leukemia responses without inducing GVHD in established MCs.

Requirement for CD4 Help for Optimal GVL Effects of Alloreactive T Cells Given to Established Mixed Chimeras but Not Freshly Irradiated Mice.

Donor lymphocyte infusion (DLI) may lead to lymphohematopoietic graft-versus-host (GVH) reactions without graft-versus-host-disease (GVHD). The graft-versus-leukemia (GVL) effect of DLI is greater in mixed chimeras than in full chimeras following fully MHC-mismatched bone marrow transplantation (BMT). Some studies showed, following MHC-mismatched BMT, that only CD8+ T cells are involved in GVL effects of DLI against MHC class I+ class II− tumors, but others showed that the GVL effect of DLI is dependent on both CD8+ and CD4+ T cells. Previously, we have observed that GVL against EL4 is mediated by CD8+ T cells but not CD4+ T cells following DLI in freshly irradiated mice treated with cytokines to reduce GVHD mortality in fully MHC-mismatched BMT. The exact contribution of CD4+ and CD8+ T cells to the GVL effects of DLI in established mixed chimeras is not known. We hypothesized that lethal conditioning leads to the production of many proinflammatory cytokines which may help to generate a CD4 cell-independent, CD8 cell-mediated anti-tumor effect. However, in delayed DLI recipients, CD4+ helper T cells might be needed to induce CD8+ anti-tumor effector cells. We evaluated the role of T-cell subsets in GVL effects of DLI following fully major histocompatibility complex (MHC)-mismatched BMT in freshly irradiated mice and mixed chimeric delayed DLI recipients. While GVL effects of delayed DLI given to established mixed chimeras were dependent on both CD4+ and CD8+ T cells, CD4+ T cells did not contribute to GVL effects in freshly irradiated mice. Consistent with this, MHC class II expression on host antigen-presenting cells (APCs) was needed to optimize GVL effects of DLI in established mixed chimeras. To address whether cross-presentation of tumor antigens (Ag) via class I would permit GVL responses in full chimeras, we generated full chimeras in which donor and recipient shared a class I locus. However, there were no GVL effects of delayed DLI in full chimeras, providing no evidence that cross-presentation of tumor Ag through class I molecules shared by donor and the tumor led to anti-tumor effects in this model. We conclude that following delayed DLI to established mixed chimeras, the GVL response requires extensive MHC disparity, MHC class II expression on host APCs and the presence of CD4+ T cells. In contrast, the requirement for CD4+ T cells can be bypassed in freshly irradiated recipients, but this occurs at the expense of a greater risk of GVHD. Administration of delayed DLI to mixed chimeras permits achievement of optimal GVL effects because large numbers of GVH-reactive T cells can be administered without inducing GVHD.

Proliferation, Expansion, Effector Differentiation and Survival of GVH-Reactive T Cells Following Delayed DLI to Mixed Chimeras.

Donor leucocyte infusions (DLI) given to established mixed chimeras (MC) can eliminate normal and malignant hematopoietic cells without causing graft-versus-host disease (GVHD). DLI given immediately following lethal irradiation lead to severe GVHD. We examined the proliferation, expansion, differentiation and survival of GVH-reactive T cells following delayed DLI and compared the outcomes to those observed when identical DLI were administered early following lethal irradiation (TBI). MC recipients were prepared by TBI of BALB/c mice and reconstitution with mixed BALB/c and CD45.2 C57BL/6 (B6) T-cell depleted bone marrow (TCD BM). 10 weeks later, we transferred 1 x 107 CD45.1 B6 and 5 x 106 2C transgenic (tg) splenocytes. CD8+ T cells from 2C tg mice bear TCR specific for recipient class I MHC Ld. Polyclonal DLI-derived CD4+/CD8+ T cell responses were monitored by gating on CD45.1+ events and clonal 2C CD8+ T cell responses tracked using a clonotypic marker. For comparison, identical DLI together with TCD BM was administered to freshly irradiated BALB/c or B6 CD45.2 syngeneic recipients. MC recipients of delayed DLI developed a GVH reaction, as indicated by increases in donor chimerism, but no GVHD. In contrast, allogeneic recipients receiving DLI immediately following TBI developed lethal GVHD (median survival 32d vs. >100d post-delayed DLI, p<0.0001). By day 6, donor CD4+/CD8+ T cells had undergone almost equivalent proliferation as monitored by CFSE-dilution in MC and TBI allogeneic recipients. However, the kinetics and distribution of donor T cell expansion were distinct. Following delayed DLI, marked expansion of donor CD4+ cells (peak day 10) preceded expansion of CD8+ cells (peak day 13) in the spleen, with less accumulation in the lymph nodes, BM, liver and lung, and no accumulation in the gut. Histology revealed transient, mild lymphocytic infiltrates within the lung/liver but no evidence of colitis. In contrast, the kinetics of donor CD4+/CD8+ T cell expansion were more rapid in freshly irradiated recipients with CD4+/CD8+ responses peaking on day 4–7. The distribution was also different with major increases in donor CD4+/CD8+ numbers in the gut but less accumulation in the spleen. Histology confirmed severe colitis. The kinetics of proliferation, expansion and distribution of tg 2C CD8+ T cells showed a similar pattern to the polyclonal donor CD8+ T cell population. Following delayed DLI, 2C CD8+ T cells acquired a memory/activation phenotype (CD44hi, CD45RBlo, CD62Llo, CD49d+, CD27+) with similar kinetics to those observed in TBI mice developing GVHD. Despite the marked differences in trafficking to the gut, 2C CD8+ T cells in both groups of allogeneic recipients expressed equivalent levels of the gut homing receptor, α4β7. However, we also observed important differences: 1) IL-2Rαexpression was absent on 2C CD8+ T cells following delayed DLI, but was expressed at high levels (>50% by day +3) in freshly irradiated DLI recipients; 2) 2C CD8+ T cells showed greater reductions in expression of IL-7Rαfollowing delayed DLI; and 3) high rates of 2C CD8+ T cell apoptosis, as indicated by annexin V staining, were observed following delayed DLI with absolute numbers up to 6-fold greater than in TBI recipients. Thus, in contrast to responses in freshly irradiated mice that develop GVHD, GVH reactions induced by delayed DLI are characterized by delayed kinetics, a distinct distribution, marked apoptosis and reduced expression of cytokine receptors important for CD8+ T-cell survival.

Host Environment Dictates the Outcome Following Transfer of Graft-Versus-Host Reactive Effector/Memory T Cells.

Donor leucocyte infusions (DLI) given to established mixed chimeras (MC) can eliminate normal and malignant hematopoietic cells without causing graft-versus-host disease (GVHD). Identical DLI transferred immediately following lethal irradiation lead to severe GVHD. In contrast to freshly irradiated mice developing GVHD, established MC receiving DLI show delayed kinetics of effector/memory T cell (TE/M) expansion and a distinct distribution with no accumulation of cells in the gut. Lethal irradiation is associated with tissue injury, altered chemokine expression and cytokine dysregulation, and resolution of these changes is important in limiting GVHD following delayed DLI. In an attempt to define the respective roles of ‘intrinsic' T cell versus ‘extrinsic' environmental factors in shaping the character of the GVH response elicited, TE/M developing following GVHD induction were transferred to established MC. Conversely, TE/M developing following delayed DLI to MC were transferred to lethally-irradiated 2° recipients. In the first series of experiments, we transferred 1 x 107 C57BL/6 (B6) splenocytes and 5 x 106 B6 T-cell depleted bone marrow (TCD BM) to lethally-irradiated BALB/c recipients. On day 4 following BMT, when GVH-reactive T cell activation and expansion was maximal, 3 x 106 nylon wool-passaged TE/M derived from recipient spleens (equivalent to 1 recipient spleen to 1 secondary recipient) were transferred to established B6 → BALB/c MC or to lethally irradiated BALB/c 2° recipients. The latter mice received B6 TCD BM to prevent death from aplasia. Secondary established MC recipients were resistant to GVHD whereas the lethally-irradiated 2° recipients developed severe, lethal GVHD (median survival time-MST- mixed chimeras >100d versus irradiated mice 52d, p<0.0001). Histological studies confirmed that 4/5 lethally-irradiated 2° recipients developed colitis versus 0/4 MC. TE/M derived from GVHD mice did not increase donor chimerism in 2° MC recipients. In reciprocal experiments, we transferred 2.5 x 107 B6 CD45.1 splenocytes to established B6 CD45.2 → B6D2F1 MC. This number of splenocytes represents the dose required to convert 100% of MC to full donor chimerism without induction of lethal GVHD. On day 12 post-DLI, when GVH-reactive T-cell expansion was maximal, a cohort of recipient mice was sacrificed and splenocytes harvested. On the basis of 1 recipient spleen to 1 secondary recipient, 2.7 x 107 nylon wool-passaged lymphocytes were transferred to 2° freshly-irradiated B6D2F1 recipients together with B6 TCD BM. In parallel, we depleted host-derived cells by sorting for DLI-derived CD45.1+ lymphocytes and transferred these cells on the basis of 1 recipient to 1 2° recipient (7 x 106 lymphocytes) to a second cohort of lethally-irradiated recipients. After a lag-period of about 20 days, 2° recipients of non-fractionated cells lost >10% weight and developed clinical signs of mild GVHD, but then showed almost complete recovery by day 60 post-BMT. In sharp contrast, after a similar lag period, 2° recipients of CD45.1+-selected lymphocytes developed severe lethal GVHD (MST CD45.1+ cells 55d vs >100d non-fractionated cells, p<0.0005). In conclusion, these findings suggest that the host environment strongly influences the capacity of TE/M to induce GVHD. Furthermore, host-derived cells pre-existing in or developing following delayed DLI, reduce the potential of TE/M to induce GVHD.