Bone Marrow Graft Rejection Research Articles

A Review of Cyclophosphamide-Induced Transplantation Tolerance in Mice and Its Relationship With the HLA-Haploidentical Bone Marrow Transplantation/Post-Transplantation Cyclophosphamide Platform.

The bone marrow transplantation (BMT) between haplo-identical combinations (haploBMT) could cause unacceptable bone marrow graft rejection and graft-versus-host disease (GVHD). To cross such barriers, Johns Hopkins platform consisting of haploBMT followed by post-transplantation (PT) cyclophosphamide (Cy) has been used. Although the central mechanism of the Johns Hopkins regimen is Cy-induced tolerance with bone marrow cells (BMC) followed by Cy on days 3 and 4, the mechanisms of Cy-induced tolerance may not be well understood. Here, I review our studies in pursuing skin-tolerance from minor histocompatibility (H) antigen disparity to xenogeneic antigen disparity through fully allogeneic antigen disparity. To overcome fully allogeneic antigen barriers or xenogeneic barriers for skin grafting, pretreatment of the recipients with monoclonal antibodies (mAb) against T cells before cell injection was required. In the cells-followed-by-Cy system providing successful skin tolerance, five mechanisms were identified using the correlation between super-antigens and T-cell receptor (TCR) Vβ segments mainly in the H-2-identical murine combinations. Those consist of: 1) clonal destruction of antigen-stimulated-thus-proliferating mature T cells with Cy; 2) peripheral clonal deletion associated with immediate peripheral chimerism; 3) intrathymic clonal deletion associated with intrathymic chimerism; 4) delayed generation of suppressor T (Ts) cells; and 5) delayed generation of clonal anergy. These five mechanisms are insufficient to induce tolerance when the donor-recipient combinations are disparate in MHC antigens plus minor H antigens as is seen in haploBMT. Clonal destruction is incomplete when the antigenic disparity is too strong to establish intrathymic mixed chimerism. Although this incomplete clonal destruction leaves the less-proliferative, antigen-stimulated T cells behind, these cells may confer graft-versus-leukemia (GVL) effects after haploBMT/PTCy.

Transfusion with Stored Antigen-Negative Blood Impairs T Cell Cross Priming to Red Cell Alloantigen in a Subsequent Transfusion

BACKGROUND: Donor red blood cell (RBC) alloantigens are efficiently cross-presented by recipient antigen presenting cells (APCs), thereby priming recipient naive CD8 T cells; however, this process and its downstream immunological consequences are poorly understood. RBC antigen cross presentation has been linked to cytotoxic T cell responses driving bone marrow graft rejection, but in other systems it causes tolerance. The conventional dendritic cell (cDC) subset, cDC1, is required for cross-presentation of RBC antigen. Macrophages are insufficient for cross-presentation in the absence of cDC1s, but do promote RBC antigen cross-presentation by cDCs in vitro. After prolonged storage, a portion of transfused RBCs are rapidly cleared by the spleen. In murine models, transfused stored RBCs have multiple effects on splenic APCs, including architectural distortion, dendritic cell activation, and macrophage depletion via ferroptosis due to increased erythrophagocytosis. Thus, the antigen presentation machinery controlling alloimmunization to RBC antigens may be influenced by the storage duration of previously transfused RBCs.AIMS: To better understand how recent transfusions may influence the immunogenicity of future transfusions, we used a mouse model to determine whether prior transfusion of stored, antigen-matched RBCs influences CD8 T cell cross priming to antigen mismatched RBCs in a subsequent transfusion.METHODS: In an in vivo cross-priming assay, OVA-expressing RBCs from transgenic HOD mice (with RBC surface expression of hen egg lysozyme, ovalbumin (OVA), and Duffy antigens) were the transfused antigen source and naïve OT-1 CD8 T cells, which express an MHC class I restricted OVA-specific TCR transgene, were the responders. Magnetically-selected CD8+ OT-1 cells, labeled with CellTrace Far Red (CTFR), were adoptively transferred into MHC identical (H-2Kb) UBC-GFP C57BL6/J recipients. After 16 hours, these mice were transfused with fresh or stored (12 days in CPDA-1), leukoreduced UBC-GFP RBCs or PBS. Five hours after the initial transfusion, recipients were transfused again with fresh leukoreduced HOD RBCs. To determine the effect of stored RBC transfusion on T cell priming to a non-RBC associated antigen, groups of mice carrying adoptively transferred OT-1 T cells were transfused with fresh or stored RBCs and then challenged 5 hours later with soluble OVA protein (10 µg, IV). After 4 days, splenic OT-1 T cells were analyzed for proliferation (by CTFR dye dilution) and activation (by CD44 and CD122 expression).RESULTS: Proliferation and activation of OT-1 T cells in response to challenge with soluble OVA protein were significantly increased in mice that had been previously transfused with stored RBCs compared to those transfused with fresh RBCs (Fig. 1A). In contrast, OT-1 T cell activation in response to HOD RBC transfusion was decreased in mice that had previously received a stored RBC transfusion, as compared to those receiving fresh RBCs or PBS (Fig. 1B).CONCLUSION: The increased T cell priming after soluble OVA challenge in the mice previously transfused with stored vs. fresh RBCs may be related to the increased expression of co-stimulatory molecules on DCs after stored RBC transfusion. However, T cell priming to the RBC-associated antigen requires additional factors, which are disrupted by previous transfusion of stored, but not fresh, RBCs. Splenic red pulp macrophages potentiate OT-1 T cell cross priming by dendritic cells in vitro and become severely depleted after stored RBC transfusion in vivo. Thus, macrophage help may be needed for T cell cross priming to an RBC antigen in vivo, and stored RBC transfusions may disrupt macrophage function, thereby affecting the immunogenicity of subsequent RBC alloantigen exposure.Figure 1. Effect of fresh and stored RBC transfusion on priming after antigenic challenge with soluble OVA or HOD RBCs.Representative contour plots show proliferation-induced dye dilution and CD44 expression on OT-1 cells in the spleen 4 days after transfusion of fresh (left) or stored (right) RBCs followed 5 hours later with IV injection of soluble OVA protein (A) or HOD RBCs (B). Dot plots at the right of representative contour plots show compiled data from two independent experiments quantifying the percentage of activated and proliferated cells (CTFR diluted, CD44hi) in the OT-1 subgate (mean ± SEM, n = 4-9). *P < 0.05, **P < 0.01, unpaired t test. DisclosuresNo relevant conflicts of interest to declare.

Tolerogenic interactions between CD8+ dendritic cells and NKT cells prevent rejection of bone marrow and organ grafts

AbstractThe combination of total lymphoid irradiation and anti-T-cell antibodies safely induces immune tolerance to combined hematopoietic cell and organ allografts in humans. Our mouse model required host natural killer T (NKT) cells to induce tolerance. Because NKT cells normally depend on signals from CD8+ dendritic cells (DCs) for their activation, we used the mouse model to test the hypothesis that, after lymphoid irradiation, host CD8+ DCs play a requisite role in tolerance induction through interactions with NKT cells. Selective deficiency of either CD8+ DCs or NKT cells abrogated chimerism and organ graft acceptance. After radiation, the CD8+ DCs increased expression of surface molecules required for NKT and apoptotic cell interactions and developed suppressive immune functions, including production of indoleamine 2,3-deoxygenase. Injection of naive mice with apoptotic spleen cells generated by irradiation led to DC changes similar to those induced by lymphoid radiation, suggesting that apoptotic body ingestion by CD8+ DCs initiates tolerance induction. Tolerogenic CD8+ DCs induced the development of tolerogenic NKT cells with a marked T helper 2 cell bias that, in turn, regulated the differentiation of the DCs and suppressed rejection of the transplants. Thus, reciprocal interactions between CD8+ DCs and invariant NKT cells are required for tolerance induction in this system that was translated into a successful clinical protocol.

Dendritic Cell Cross Presentation of RBC Antigens in-Vivo Is Not Affected By RBC Storage Duration and Requires Red Pulp Macrophage "Help" in-Vitro

BACKGROUND: Chronic transfusion is associated with an increased risk of bone marrow graft rejection. In prior studies with mouse models, minor histocompatibility antigens in red blood cell (RBC) and platelet products were presented in the context of recipient MHC class I to prime recipient cytotoxic T cells; these represent cross presentation and cross priming, respectively. Once primed in the recipient, these T cells may attack allogeneic bone marrow grafts. Thus, understanding the mechanism of transfusion-induced T cell cross priming may lead to new methods to reduce bone marrow graft rejection in chronically transfused patients. Cross presentation is performed by a specialized subset of antigen presenting cells (APCs) - CD11b lo, XCR1+ dendritic cells (XCR DCs). Transfusion of RBCs after prolonged refrigerated storage induces erythrophagocytosis and pro-inflammatory gene expression in the spleen. Fluorescent tracking of transfused RBCs showed that splenic red pulp macrophages (RPMs) ingest the majority of damaged RBCs, but RPMs are weak APCs. Splenic dendritic cells, including XCR DCs, also display increased uptake of stored vs. fresh RBCs. These data suggested that refrigerated storage may increase cross presentation of RBC antigens, thereby enhancing T cell cross priming.AIMS: To compare T cell cross priming after transfusion of fresh and refrigerator-stored RBCs, a mouse model was used with transgenic OVA-carrying RBCs as the antigen source and transgenic naive MHC class I-restricted (H-2Kb) OVA-specific T cells (OT-1) as responders. In parallel, an in-vitro system was established to determine the cellular elements required for cross presentation of RBC antigens.METHODS: Purified CD8 T cells from OT-1 mice were loaded with the cell proliferation tracking dye Cell Trace Far Red (CTFR) and adoptively transferred (4 x 10e6 per mouse) into cohorts of 8-12 week old C57BL/6 GFP+ mice (n=3 per group). The next day mice were transfused with 400 uL of fresh (<24 hours old), or stored RBCs (14 days old) from transgenic HOD mice (RBCs express surface HEL, OVA, and Duffy antigens). After 96 hours, flow cytometry was used to assess OT-1 cell proliferation by dye dilution and the expression of activation markers CD44, CD122 and CD62L. Control transfusions with non-HOD GFP+ RBCs confirmed the antigen specificity of the response, and transfusions of HOD blood that had undergone RBC lysis ruled out direct antigen presentation by MHC identical white cells in the transfusate. For the in-vitro cross presentation assay, 1x10e5 CTFR-loaded OT-1 cells were plated in U-bottom 96 well plates with all possible combinations of fresh or stored HOD RBCs (50 x 10e6 per well), bone marrow Flt3L-derived C57BL/6 murine dendritic cells (Flt3L-DCs, 5 X 10e4 per well), and magnetically selected BALB/c (H-2Kd) RPMs (5 x 10e4 per well). MHC mismatch rules out antigen presentation by BALB/c RPMs (H-2Kd) to OT-I T cells (H-2Kb).RESULTS: Transfusion of both fresh and stored HOD RBCs induced vigorous proliferation and activation of OT-1 cells. After 4 days, no differences were seen in the proliferation and activation profiles of OT-1 cells in mice receiving fresh (78±4% CD44hi, CTFR diluted) vs. stored (79±5%) HOD RBCs. The in-vitro cross presentation assay showed weak to absent OVA-specific OT-1 proliferation with co-cultures including fresh or stored HOD RBCs alone, and fresh or stored HOD RBCs plus Flt3L-DCs. In contrast, cultures including RPMs and Flt3L-DCs showed enhanced OVA-specific OT-1 proliferation with fresh (29±10% CTFR diluted) and stored (25±2%) HOD RBCs.CONCLUSION: Cross priming by fresh RBCs was stronger than expected based on prior studies showing minimal RBC uptake by XCR DCs after fresh GFP + RBC transfusion. Cross priming in this system may be more sensitive due to the high frequency of antigen specific T cells and, thus, is saturated by the small antigen load delivered by fresh transfusion. It is also possible that direct uptake of RBCs by DCs harms the cross-presentation machinery and offsets the effect of increased RBC antigen delivery. Optimal cross-presentation of RBC antigen may require cooperation with cells functionally specialized for metabolizing RBC by-products. Indeed, the in-vitro study shows RPMs promote DC-mediated cross presentation of antigens on fresh and stored RBCs. Thus, RPMs may share ingested RBC antigen with adjacent DCs and increase DC activation by secreting inflammatory cytokines.. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

LRBA is Essential for Allogeneic Responses in Bone Marrow Transplantation.

The PH-BEACH-WD40 (PBW) protein family members play a role in coordinating receptor signaling and intracellular vesicle trafficking. LPS-Responsive-Beige-like Anchor (LRBA) is a PBW protein whose immune function remains elusive. Here we show that LRBA-null mice are viable, but exhibit compromised rejection of allogeneic, xenogeneic and missing self bone-marrow grafts. Further, we demonstrate that LRBA-null Natural Killer (NK) cells exhibit impaired signaling by the key NK activating receptors, NKp46 and NKG2D. However, induction of IFN-γ by cytokines remains intact, indicating LRBA selectively facilitates signals by receptors for ligands expressed on the surface of NK targets. Surprisingly, LRBA limits immunoregulatory cell numbers in tissues where GvHD is primed or initiated, and consistent with this LRBA-null mice also demonstrate resistance to lethal GvHD. These findings demonstrate that LRBA is redundant for host longevity while being essential for both host and donor-mediated immune responses and thus represents a unique and novel molecular target in transplant immunology.

LRBA is essential for allogeneic responses in bone marrow transplantation

Abstract Members of the PH-BEACH-WD40 repeat gene family (PBW) play a role in coordinating receptor signaling and intracellular vesicle trafficking. LPS-Responsive Beige-like Anchor (LRBA) is a PBW protein whose immune function has not been defined. Here we show that LRBA−/− mice are viable with normal and healthful life span, but exhibit compromised rejection of allogeneic, xenogeneic and missing self bone marrow grafts. Further we demonstrate that LRBA−/− Natural Killer (NK) cells exhibit impaired signaling by the key NK activating receptors, NKp46 and NKG2D. However, induction of γ-IFN by co-culture with IL12 and IL18 remains intact, indicating LRBA selectively facilitates signals by receptors for ligands expressed on the surface of NK targets. Surprisingly, LRBA limits immunoregulatory cell numbers in tissues where Graft versus Host Disease (GvHD) is primed or initiated, and consistent with this LRBA−/− mice demonstrate resistance to lethal GvHD. These findings demonstrate that LRBA is redundant for host longevity while being essential for both host and donor-mediated immune responses in allogeneic transplantation and thus represent a unique and novel molecular target in transplant immunology.

Strategies to Inhibit Alloantibody Production in Alloprimed Murine Recipients of Hematopoietic Stem Cell Grafts

Alloantibody, not primed T cells, is the major barrier to bone marrow (BM) engraftment in allosensitized mice. We have shown that a single intravenous injection of donor splenocytes, to mimic a blood transfusion, results in high, sustained levels of serum alloantibody sufficient to eliminate donor BM within 3 h, resulting in uniform mortality in lethally irradiated allogeneic recipients. Current studies focused preventing and treating allopriming. Blockade of B cell survival signals with mTACI-Ig pre- and postpriming was ineffective, as was the B cell but not plasma cell depleting anti-CD20 mAb. Germinal center formation inhibition by lymphotoxin-beta receptor-Ig (LβR-Ig) diminished allosensitization, although conditional Prmd1 (Blimp-1) deletion in CD19+ cells was highly effective. By combining anti-CD20 mAb to reduce B cells and LTβR-Ig to diminish the frequency of B cells that could form germinal centers pre- and postpriming, allosensitization was precluded, permitting long-term survival in T- and NK-depleted, irradiated allogeneic recipients, whereas combined therapy postpriming alone was ineffective. As evidence of the critical role of B cells, the proteosomal inhibitor, bortezomib, given unencapsulated or encapsulated, proved ineffective in influencing allosensitization. These data extend our understanding of allopriming and provide a potential therapy for patients at risk for allosensitization and BM graft rejection.

Humoral Immunity Induced By Viral Infection Provides a Major Barrier To Hematopoietic Cell Transplantation

BackgroundSensitization to MHC antigens because of transfusion, pregnancy, and previous grafts is among the most critical challenges to clinical transplantation. The presence of preformed donor-specific antibody increases the risk for bone marrow and solid organ graft rejection. It is well documented that some microbial molecules and microorganisms can directly induce the proliferation and differentiation of antibody-secreting cells from different B cells, regardless of their antigen specificity. The presence of a strong polyclonal B cell response resulting in hypergammaglobulinemia secretion has been described in several animal models of infection with viruses, parasites, and bacteria. Recent experiments in our lab showed that pathogen infection in naïve animals induces activation of naïve allospecific B cell and formation of de novo alloantibody. This prompted us to analyze the effect of pathogen infection induced alloantibody on hematopoietic cell transplantation. Herein, these studies show that preformed donor specific antibody induced by pathogen infection in the absence of donor-antigen is a major barrier to bone marrow engraftment. Methods/results: LCMV infection is a model system of viral infection and also an established system to examine virus nonspecific, polyclonal hypergammaglobulinemia. Naïve C57BL/6 (H2b) mice were infected with acute and chronic strain of LCMV (Armstrong and clone 13, n=10 each). Virally infected mice were bled weekly to analyze total IgG and formation of de novo alloantibody in the serum. Viral infection induced both hypergammaglobulinemia and the formation of de novo alloantibody (anti-H2d). Peak levels of donor-specific antibody in acute and chronically infected mice were increased to approximate 3 fold when compared to uninfected controls as assessed by flow cytometric crossmatch. The results were consistent with formation of anti-H2d antibody when measured by anti-H2d ELISA with peak level of approximately 336.3 ± 51.84 units/ml when compared to 2.179 ± 0.4597 units/ml in uninfected mice (p<0.0001, Figure 1). Interestingly, the response generated by chronic infection was relatively sustained (up to 75 days) when compared to the transient response induced by acute infection.Next we analyzed if virally induced preformed de novo alloantibody would represent a barrier to bone marrow engraftment. Naïve C57Bl/6 mice were irradiated with 950 rads and received 15x106BALB/c bone marrow. BALB/c bone marrow cells were either incubated ex vivo with 1:6 diluted sera from naïve B6 mice or with the sera from clone 13 infected mice (d28 post-infection). After incubation, bone marrow was washed and complement inactivated prior to infusion. All of the 10 mice receiving bone marrow incubated with naïve mice serum engrafted and survived, while only 1/10 mice survived receiving bone marrow incubated with serum from infected mice (Mean survival = >100 vs 24.5 days respectively, p < 0.0067, figure 2).We also examined the effects of virally induced preformed de novo alloantibody in a costimulation blockade based regimen to induce mixed allogeneic chimerism and transplant tolerance. Naïve C57BL/6 mice received 30x106 BALB/c bone marrow infusion along with costimulation blockade treatment (250ug each of CTLA4-Ig and anti-CD40L (MR1) at 0,2,4,6 days post transplant) after nonirradiation-based conditioning with busulfan (-3,-2 days. 20mg/kg IP injection). BALB/c bone marrow cells were either incubated ex vivo with 1:6 diluted sera from naïve B6 mice or with sera from clone 13 infected mice (d28 post-infection) as described above. Consistent with the experiments using the irradiation based conditioning regimen, mice receiving bone marrow incubated with naïve mice serum engrafted and established stable donor chimerism while none of mice receiving bone marrow incubated with serum from infected mice showed any donor cell engraftment Discussion:Allogeneic hematopoetic cell is used as therapy for hematologic malignancies and for non-malignant hematologic diseases. Our results indicate that preformed donor specific antibody induced by viral infection in the absence of previous exposure to donor-antigen is a major barrier to bone marrow engraftment. These results suggest that an individual's prior immune history to pathogens may significantly impact subsequent humoral responses upon bone marrow transplantation. Disclosures:No relevant conflicts of interest to declare.

The Rac activator DOCK2 regulates natural killer cell–mediated cytotoxicity in mice through the lytic synapse formation

AbstractNatural killer (NK) cells play an important role in protective immunity against viral infection and tumor progression, but they also contribute to rejection of bone marrow grafts via contact-dependent cytotoxicity. Ligation of activating NK receptors with their ligands expressed on target cells induces receptor clustering and actin reorganization at the interface and triggers polarized movement of lytic granules to the contact site. Although activation of the small GTPase Rac has been implicated in NK cell–mediated cytotoxicity, its precise role and the upstream regulator remain elusive. Here, we show that DOCK2, an atypical guanine nucleotide exchange factor for Rac, plays a key role in NK cell–mediated cytotoxicity. We found that although DOCK2 deficiency in NK cells did not affect conjugate formation with target cells, DOCK2-deficienct NK cells failed to effectively kill leukemia cells in vitro and major histocompatibility complex class I–deficient bone marrow cells in vivo, regardless of the sorts of activating receptors. In DOCK2-deficient NK cells, NKG2D-mediated Rac activation was almost completely lost, resulting in a severe defect in the lytic synapse formation. Similar results were obtained when the Rac guanine nucleotide exchange factor activity of DOCK2 was selectively abrogated. These results indicate that DOCK2-Rac axis controls NK cell–mediated cytotoxicity through the lytic synapse formation.

FTY720 Markedly Increases Alloengraftment but Does Not Eliminate Host Anti-Donor T Cells that Cause Graft Rejection on Its Withdrawal

The immunomodulator FTY720 (FTY) is beneficial in models of graft-versus-host disease, solid organ transplantation, and autoimmunity and has been approved for use in patients with multiple sclerosis. FTY modifies the homing and migration of many cell types. We report that FTY has profound positive and negative effects on allogeneic bone marrow (BM) engraftment in sublethally irradiated recipients. FTY increased donor hematopoietic progenitors in the BM, resulting in high donor engraftment in the B cell, myeloid cell, and natural killer cell, but not T cell, lineages. Donor T cell progenitors within the thymus of FTY-treated recipients were dramatically reduced, resulting in a lack of donor T cell reconstitution. In addition to preventing the ingress of donor (and host) T cell progenitors, FTY prevented the egress of fully functional host CD4+CD8- and CD4-CD8+ thymocytes that on cessation of FTY administration were able to exit from the thymus and contribute to a rapid and complete rejection of a well-established donor BM graft. When used in combination with anti-CD40L mAbs to block the CD40L:CD40 costimulatory pathway, FTY markedly enhanced anti-CD40L mAb-mediated alloengraftment promotion. In contrast to FTY alone, the combination of anti-CD40L mAb and FTY resulted in a surprisingly stable, multilineage, long-term donor chimerism. These data illustrate FTY's profound migration modulating effects and suggest a use in combinatorial therapy in achieving stable alloengraftment under nonmyeloablative conditions.

Inhibitor and activator: dual functions for SHIP in immunity and cancer

SHIP1 is at the nexus of intracellular signaling pathways in immune cells that mediate bone marrow (BM) graft rejection, production of inflammatory and immunosuppressive cytokines, immunoregulatory cell formation, the BM niche that supports development of the immune system, and immune cancers. This review summarizes how SHIP participates in normal immune physiology or the pathologies that result when SHIP is mutated. This review also proposes that SHIP can have either inhibitory or activating roles in cell signaling that are determined by whether signaling pathways distal to PI3K are promoted by SHIP's substrate (PI(3,4,5)P(3) ) or its product (PI(3,4)P(2) ). This review also proposes the "two PIP hypothesis" that postulates that both SHIP's product and its substrate are necessary for a cancer cell to achieve and sustain a malignant state. Finally, due to the recent discovery of small molecule antagonists and agonists for SHIP, this review discusses potential therapeutic settings where chemical modulation of SHIP might be of benefit.

The requirement for NKG2D in NK cell–mediated rejection of parental bone marrow grafts is determined by MHC class I expressed by the graft recipient

AbstractNatural killer (NK) cells provide a unique barrier to semiallogeneic bone marrow (BM) transplantation. In the setting where the parents donate to the F1 offspring, rejection of parental bone marrow occurs. This “hybrid resistance” is completely NK cell dependent, as T cells in the F1 recipient tolerate parental grafts. Previously, we demonstrated that rejection of BALB/c parental BM by (BALB/c × C57BL/6) F1-recipient NK cells is dependent on the NKG2D-activating receptor, whereas rejection of parental C57BL/6 BM does not require NKG2D. BALB/c and B6 mice possess different NKG2D ligand genes and express these ligands differently on reconstituting BM cells. Herein, we show that the requirement for NKG2D in rejection depends on the major histocompatibility complex haplotype of donor cells and not the differences in the expression of NKG2D ligands. NKG2D stimulation of NK cell–mediated rejection was required to overcome inhibition induced by H-2Dd when it engaged an inhibitory Ly49 receptor, whereas rejection of parental BM expressing the ligand, H-2Kb, did not require NKG2D. Thus, interactions between the inhibitory receptors on F1 NK cells and parental major histocompatibility complex class I ligands determine whether activation via NKG2D is required to achieve the threshold for rejection of parental BM grafts.

Canine Bone Marrow-Derived Mesenchymal Stromal Cells Suppress Alloreactive Lymphocyte Proliferation in Vitro but Fail to Enhance Engraftment in Canine Bone Marrow Transplantation

Stable mixed hematopoietic chimerism has been consistently established in dogs who were mildly immunosuppressed by 200 cGy of total body irradiation (TBI) before undergoing dog leukocyte antigen (DLA)-identical bone marrow (BM) transplantation and who received a brief course of immunosuppression with mycophenolate mofetil (28 days) and cyclosporine (35 days) after transplantation. However, when TBI was reduced from 200 to 100 cGy, grafts were nearly uniformly rejected within 3-12 weeks. Here, we asked whether stable engraftment could be accomplished after a suboptimal dose of 100 cGy TBI with host immunosuppression enhanced by donor-derived mesenchymal stromal cells (MSCs) given after transplantation. MSCs were cultured from BM cells and evaluated in vitro for antigen expression. They showed profound immunosuppressive properties in mixed lymphocyte reactions (MLRs) in a cell dose-dependent manner not restricted by DLA. MSC and lymphocyte contact was not required, indicating that immunosuppression was mediated by soluble factors. Prostaglandin E2 was increased in culture supernatant when MSCs were cocultured in MLRs. The addition of indomethacin restored lymphocyte proliferation in cultures containing MSCs. MSCs expressed CD10, CD13, CD29, CD44, CD73/SH-3, CD90/Thy-1, and CD106/VCAM-1. For in vivo studies, MSCs were injected on the day of BM grafting and on day 35, the day of discontinuation of posttransplantation cyclosporine. MSCs derived from the respective BM donors failed to avert BM graft rejection in 4dogs who received DLA-identical grafts after nonmyeloablative conditioning with 100 cGy TBI in a time course not significantly different from that of control dogs not given MSCs. Although the MSCs displayed in vitro characteristics similar to those reported for MSCs from other species, their immunosuppressive qualities failed to sustain stable BM engraftment in vivo in this canine model.

Prevention of Rat Cardiac Allograft Rejection by Host Th2 Cell Adoptive Transfer

We have recently shown that ex vivo manufactured, rapamycin-resistant donor Th2 cells (Th2.R cells) prevent the rejection of allogeneic bone marrow grafts through a process that involves IL-4-mediated polarization of host T cells towards a Th2 phenotype. Because donor T cell therapy lacks feasibility in the setting of cardiac allograft transplantation using cadaveric donors, we developed an approach to prevent rejection that incorporates host Th2 cell therapy prior to organ transplantation. The aims of the study were to: (1) develop a method for the ex vivo manufacture of rat Th2.R cells and inject such syngeneic Th2.R cells just prior to class I and class II disparate cardiac allografting; (2) determine whether such adoptive host Th2.R cell transfer polarizes post-transplant immunity towards a Th2 phenotype; and (3) evaluate in a preliminary manner whether Th2.R cell transfer reduces graft rejection in a model that uses sub-optimal cyclosporine therapy. Recipient-type (Black Norway, BN) CD4+ T cells were co-stimulated with anti-rat anti-CD3 and –CD28 antibodies in the presence of rapamycin, rrIL4, rhuIL-2, and rhIL7 (3-day culture interval). The Th2.R cell cytokine phenotype was determined by intra-cellular flow cytometry and multi-analyte testing of supernatants obtained after repeat co-stimulation. Th2.R cells were injected (i.v.; 1 × 107 cells) 2–3 hours prior to cardiac allografting (donor strain, Dark Agouti; DA). Cardiac function was evaluated clinically by cardiac palpation through day 28 post-grafting. At day 28, histology studies and immune function studies were performed, including assessment of host-anti-donor alloreactivity. Engraftment controls (cohort #1) and rejection controls (cohort #2) received cyclosporine (CSA) through the full 28 days or only through day 18, respectively; the experimental cohort (#3) received host Th2.R cells followed by the short-course of CSA. Supernatants from the Th2.R cell product contained 1500 pg/ml IL-4 and only 50 pg/ml of IFN-γ; the frequency of IL-4+ cells and IFN-γ+ cells was 10% and 1%, respectively. Clinical rejection was observed in cohort #2 (3/3 subjects) whereas all subjects in cohorts #1 and #3 had full cardiac function through day 28 (cohort #1- 2/3 subjects; cohort #3- 3/3 subjects). Relative to rejection controls, Th2.R cell recipients had a reduced frequency of intra-cardiac CD8+ T cells (%CD8+ T cells, 9.2 ± 6.2 vs. 1.1 ± 0.3; p<0.001) and spleen CD8+ T cells (1.9 ± 0.2 vs. 0.5 ± 0.2; p=0.014). Furthermore, after ex vivo re-stimulation with donor-type dendritic cells, intra-cardiac CD8+ T cells from Th2.R cell recipients tended to produce less IFN-γ relative to rejection controls (pg/ml in supernatant; 15.7 ± 1.4 vs. 44.2 ± 1.4; p=0.07) and tended to produce increased IL-4 relative to rejection controls (1974 ± 140 vs. 260 ± 291; p=0.07). This study demonstrates a feasible and successful methodology for inducing recipient based immunotolerance by generating rapamycin-resistant Th2 cells in the rat species.. Adoptive transfer of host-type Th2.R cells just prior to fully genetically-disparate cardiac allograft transplantation results in a shift towards Th2 cytokines post-transplant, a reduction in the frequency of intra-cardiac T cell infiltration, and shows promise as a novel method to prevent cardiac allograft rejection.

Donor double‐negative Treg promote allogeneic mixed chimerism and tolerance

Bone marrow (BM) transplantation is an efficient approach to develop donor-specific tolerance and prevent chronic rejection. Allogeneic BM transplantation is limited by donor T cell-mediated graft-versus-host disease, requirement of cytoreduction and high numbers of BM cells. In addition of these drawbacks, recent studies demonstrate that not only T cells, but also NK cells can mediate BM rejection, and long-term mixed chimerism depends on NK cell tolerance. Thus, NK cell is another potential barrier against engraftment of BM and an important target in efforts to induce transplant tolerance. We have previously identified a novel type of Treg with the phenotype TCRalphabeta+CD3+CD4-CD8- (double-negative, DN). We and others have demonstrated that DN-Treg can effectively suppress anti-donor T cell responses. In this study, we found that donor-derived DN-Treg can suppress NK cell-mediated allogeneic BM graft rejection in both parent-to-F1 and fully MHC-mismatched BM transplantation models. Perforin and FasL in DN-Treg play important roles in the suppression of NK cells. Furthermore, adoptive transfer of DN-Treg can promote a stable mixed chimerism and donor-specific tolerance without inducing graft-versus-host disease. These results demonstrate a potential approach to control innate immune responses and promote allogeneic BM engraftment.

FTY720 Inhibits Graft-Versus-Host Disease (GVHD) and Allogeneic Bone Marrow (BM) Graft Rejection Independent of Effects on T Cell Egress from Lymphoid Tissue.

FTY720 (FTY), a sphingosine-1-phosphate receptor agonist, inhibits lymphocyte egress from lymphoid tissues although the complete mechanism of its immunomodulatory effects is not well understood. We studied FTY in murine GVHD and engraftment models as a single agent and in combination with known beneficial strategies. FTY (3 mg/kg orally d0-28) inhibited GVHD, increasing the median survival time (MST) of lethally irradiated C57BL/6 (B6) mice given BALB/c BM and a moderate T cell dose from 30d to 71d (p<.001) or a higher T cell dose from 7d to 32d (p<.006). FTY was additive with ex vivo activated and expanded CD4+CD25+ regulatory T cells (Tregs) for GVHD inhibition. In contrast to control mice which all died of GVHD by d70 (MST = 37d), mice receiving FTY or Tregs or both had 100 day survival rates of 50%, 71% and 100%, respectively. The additive effects of FTY and Tregs were reproduced in a second strain combination. Although we hypothesized that FTY trapped Tregs in the lymph nodes allowing for increased contact time for Tregs to inhibit GVHD effector T cells, imaging studies of green fluorescent protein (GFP)+ Tregs did not support this hypothesis. Moreover, imaging studies of GFP+ effectors did not indicate that FTY inhibited donor T cell egress from lymphoid tissues. To study alloantigen-specific responses, B6 TCR Tg CD8+ and CD4+ T cells that are reactive against BALB/c alloantigen were adoptively transferred into B6 rag mice prior to sublethal irradiation and infusion with BALB/c BM. FTY reduced the expansion of Tg CD8+ and CD4+ T cells in the spleen 10d after BMT by 80% and 78%, respectively. In addition to GVHD inhibition, FTY also promoted donor BALB/c BM engraftment in sublethally (5.0 Gy) irradiated B6 mice although donor chimerism was not stable in all mice. In contrast to 4 of 26 water-treated controls, all 29 FTY-treated mice (3 mg/kg d0-13) were engrafted at 1 month as assessed by PBL phenotyping (ave of 9% vs 68% donor. p<.001). However, by 3 months, the engraftment rate in FTY-treated mice had decreased to 12 of 28 mice. All 9 mice receiving FTY for an additional 2 weeks (d0-28) had high donor chimerism levels (>96%) at 1 month but chimerism was lost in 3 mice by 6 months after BMT. In a different strain combination, even a 2 wk FTY course increased donor chimerism that remained stable for 6 months. Imaging studies revealed that FTY did not prevent the egress of adoptively transferred host-type GFP T cells in allogeneic BM recipients. FTY was also tested in combination with anti-CD40L mAb, a potent engraftment-promoting agent in mice. In contrast to anti-CD40L mAb, FTY did not promote engraftment in mice conditioned with very low levels of irradiation (0.5–2.0 Gy) although the combination of anti-CD40L mAb and FTY resulted in superior stable engraftment rates and levels compared to anti-CD40L mAb as a single agent. Interestingly, FTY increased donor chimerism in syngeneic, as well as, allogeneic recipients. These data indicate that FTY is efficacious as a single or adjunct agent for the prevention of GVHD and graft rejection and further indicate that inhibition of lymphocyte egress was not essential for the beneficial effects of FTY in BMT recipients.

Keratinocyte Growth Factor Increases the CD4+Foxp3+ Regulatory T Cell-Pool by an Early Thymus-Independent and a Late Thymus-Dependent Mechanism.

Administration of keratinocyte growth factor (KGF) before and shortly after experimental allogeneic bone marrow transplantation (allo-BMT) has been shown to reduce the incidence and severity of graft versus host disease (GVHD) and to inhibit graft rejection. As naturally occurring CD4+Foxp3+ regulatory T cells (Treg) play an important role in the prevention of GVHD and graft rejection, we evaluated the effect of KGF on peripheral and thymic CD4+Foxp3+ regulatory T cell numbers in normal mice and BMT recipients. KGF (5mg/kg/day) was administered subcutaneously to adult B6 mice for 3 consecutive days. KGF enhanced the frequency of CD4+Foxp3+ Treg in peripheral blood and spleen twofold, to more than 15 % of CD4+ T cells, resulting in an increase in absolute numbers of CD4+Foxp3+ Treg within one week. From 2 weeks onwards, the frequency of CD4+Foxp3+ Treg gradually normalized, but the absolute numbers of Treg remained elevated (> 10 weeks) due to an increase of both CD4 and CD8 T cell numbers. In order to assess the relative contribution of thymic output versus peripherally expanded Treg, we next determined the frequency of T-cell receptor excision circles (TREC) in peripheral blood T cells. Analysis of sorted CD4+Foxp3+ Treg and CD4+Foxp3− conventional T cells showed a decline in TREC frequency in both subsets 1 week after KGF administration, suggesting peripheral expansion. In contrast, the TREC frequency in peripheral T cells was significantly higher at 5 and 10 weeks after KGF, indicating a late thymic dependent effect. Normal B6 mice that were adoptively transferred with congenic T cells showed an enhanced, but transient, increase of the frequency of CD4+Foxp3+ Treg within one week, also suggesting that the early effect of KGF on peripheral Treg is independent of thymic output. In addition, we assessed the effects of KGF on Foxp3+ thymocytes. Thymic weight and thymic cellularity of all subsets, including CD4+Foxp3+ thymocytes, were 2 to 3-fold increased one week after KGF administration as compared to control mice. In addition, we found that KGF transiently affected the thymic architecture. The medullary epithelial compartment was virtually absent one week after KGF administration. Normal thymic architecture gradually reappeared after 2–3 weeks. To determine whether KGF-enhanced Treg numbers in BMT recipients would inhibit graft rejection, KGF was administered from day -3 until day -1 to Rag-1−/− mice. Subsequently, the mice were 3Gy irradiated, supplied with 105 B6 CD45.1 congenic T cells and transplanted with a MHC-matched minor-Ag mismatched T cell depleted 129Sv bone marrow graft. As in normal B6 mice, KGF-treatment of BMT-recipients enhanced peripheral CD45.1+CD4+Foxp3+ Treg numbers. This was associated with a reduced rate of bone marrow graft rejection (2 out of 8 KGF-treated mice as compared to 5 out of 7 PBS-treated mice). Taken together, our data show that KGF enhances the peripheral Treg pool by an early, transient and selective thymus-independent mechanism and thereafter by a non-selective thymus-dependent mechanism. The KGF-mediated expansion of the pool of peripheral Treg may contribute to the inhibitory effects of KGF on graft rejection.

Critical Roles of Memory T Cells and Antidonor Immunoglobulin in Rejection of Allogeneic Bone Marrow Cells in Sensitized Recipient Mice

Allosensitization is a major risk factor for graft failure in clinical bone marrow transplantation, even with an human leukocyte antigen (HLA)-matched combination under radiation-based conditioning regimens. The critical components of immunological memory in donor bone marrow graft rejection in allosensitized hosts remain unclear at present. C57BL/6-recipient mice, which had been intraperitoneally injected with splenocytes from donor C3H mice on day -35 (sensitized recipients), had been lethally irradiated with 10-Gy whole-body irradiation and were intravenously injected with T-cell-depleted bone marrow cells (TCD-BMC) from C3H mice or third-party SJL mice. Lethally irradiated recipient mice, which had been sensitized by donor splenocytes 5 weeks before the transplantation of TCD-BMC, completely rejected the donor-BMC in a donor-specific manner, whereas none of the nonsensitized recipient mice, all of which showed full allogeneic chimerism, rejected the donor TCD-BMC. Antibody-mediated T cell and/or Natural Killer (NK) cell depletion did not improve the ability of the sensitized recipients to overcome the rejection even when a megadose of TCD-BMC was administered to the sensitized recipients. Furthermore, BMC rejection occurred in sensitized B cell-deficient mice. In adoptive transfer experiments, naive mice, which received a transfer of purified T cells from sensitized mice, rejected the donor BMC, but not those from nonsensitized mice. Moreover, naive mice, which received a transfer of serum containing antidonor immunoglobulin, rejected the donor BMC. These findings suggest that alloreactive memory T cells and antidonor immunoglobulin independently function to reject donor BMC in sensitized recipients.

Overcoming T cell–mediated rejection of bone marrow allografts by T-regulatory cells: Synergism with veto cells and rapamycin

Recently, we have shown that anti-third-party cytotoxic T lymphocytes (CTLs) depleted of alloreactivity against the host are endowed with marked veto activity and can facilitate bone marrow (BM) allografting without graft-versus-host disease. We also demonstrated synergism between rapamycin (RAPA) and the veto cells. CD4(+)CD25(+) T-regulatory (Treg) cells are suppressor cells that can enhance alloengraftment. We investigated whether donor Tregs would be synergistic with veto CTLs and RAPA in augmenting alloengraftment or, conversely, would suppress veto CTL effects. Lethally irradiated C3H mice were transplanted at day 2 after irradiation with Balb-nude BM. Graft rejection was induced by purified host-type T cells infused 1 day prior to BMT. The addition of Tregs led to moderate enhancement of engraftment. RAPA at different doses was synergistic with Tregs. The addition of veto CTLs to Tregs enabled reducing the effective RAPA dose fourfold. Combining all three agents was necessary to overcome rejection at low-dose RAPA. Chimerism analysis at 5 to 9 months revealed a significant presence of host-type cells coexisting with the predominant donor T cells, suggesting that tolerance had been attained. The synergistic effects between Tregs, veto CTLs, and RAPA offer an attractive approach for facilitating alloengraftment.

Suppression of natural killer cell-mediated bone marrow cell rejection by CD4+CD25+ regulatory T cells.

Naturally occurring CD4(+)CD25(+) T regulatory (Treg) cells have been shown to inhibit adaptive responses by T cells. Natural killer (NK) cells represent an important component of innate immunity in both cancer and infectious disease states. We investigated whether CD4(+)CD25(+) Treg cells could affect NK cell function in vivo by using allogeneic (full H2-disparate) bone marrow (BM) transplantation and the model of hybrid resistance, in which parental marrow grafts are rejected solely by the NK cells of irradiated (BALB/c x C57BL/6) F(1) recipients. We demonstrate that the prior removal of host Treg cells, but not CD8(+) T cells, significantly enhanced NK cell-mediated BM rejection in both models. The inhibitory role of Treg cells on NK cells was confirmed in vivo with adoptive transfer studies in which transferred CD4(+)CD25(+) cells could abrogate NK cell-mediated hybrid resistance. Anti-TGF-beta mAb treatment also increased NK cell-mediated BM graft rejection, suggesting that the NK cell suppression is exerted through TGF-beta. Thus, CD4(+)CD25(+) Treg cells can potently inhibit NK cell function in vivo, and their depletion may have therapeutic ramifications for NK cell function in BM transplantation and cancer therapy.