Donor Hematopoietic Stem Cell Engraftment Research Articles

Busulfan for Allogeneic Hematopoietic Stem Cell Transplantation in Children with Severe Aplastic Anemia: A Retrospective Study

Introduction: This retrospective study aimed to compare a range of conditioning regimens in children with severe aplastic anemia (SAA) undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT) at the Seventh Medical Center of PLA General Hospital between January 2008 and June 2017. Methods: Patients were categorized into the Bu (Bu + Flu + Cy + ATG-F regimen) and control (Flu + Cy + ATG-F) groups, with a median follow-up time after HSCT of 3.5 (range, 3.1–6.2) and 3.7 (3.2–5.9) years in the Bu and control groups, respectively. Results: No differences were observed between the two groups regarding the median time of peripheral blood neutrophil and platelet engraftment (p = 0.538 and p = 0.491); the 28-day engraftment rates of neutrophils were similar (p = 0.199), although higher for platelets with Bu (p = 0.044). Additionally, graft failure was 0% and 20.0% in the Bu and control groups, respectively (p = 0.004). In both groups, the incidence of grades III–IV (or grades II–IV) acute graft-versus-host disease (GVHD) and chronic GVHD was not significantly different (p > 0.05). Moreover, the 3-year overall survival and failure-free survival did not show significant differences (p = 0.670 and p = 0.908). Discussion: In children with SAA undergoing allo-HSCT, conditioning regimen with Bu + Flu + Cy + ATG-F is capable of enhancing the myeloablation effect, promoting donor hematopoietic stem cell engraftment, and reducing the graft failure rate. Furthermore, it does not increase the incidence of complications, including GVHD.

Mitochondrial Transfer to Host Cells from Ex Vivo Expanded Donor Hematopoietic Stem Cells.

Mitochondrial dysfunction is observed in various conditions, from metabolic syndromes to mitochondrial diseases. Moreover, mitochondrial DNA (mtDNA) transfer is an emerging mechanism that enables the restoration of mitochondrial function in damaged cells. Hence, developing a technology that facilitates the transfer of mtDNA can be a promising strategy for the treatment of these conditions. Here, we utilized an ex vivo culture of mouse hematopoietic stem cells (HSCs) and succeeded in expanding the HSCs efficiently. Upon transplantation, sufficient donor HSC engraftment was attained in-host. To assess the mitochondrial transfer via donor HSCs, we used mitochondrial-nuclear exchange (MNX) mice with nuclei from C57BL/6J and mitochondria from the C3H/HeN strain. Cells from MNX mice have C57BL/6J immunophenotype and C3H/HeN mtDNA, which is known to confer a higher stress resistance to mitochondria. Ex vivo expanded MNX HSCs were transplanted into irradiated C57BL/6J mice and the analyses were performed at six weeks post transplantation. We observed high engraftment of the donor cells in the bone marrow. We also found that HSCs from the MNX mice could transfer mtDNA to the host cells. This work highlights the utility of ex vivo expanded HSC to achieve the mitochondrial transfer from donor to host in the transplant setting.

Antibody based conditioning for allogeneic hematopoietic stem cell transplantation.

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a curative therapeutic option for many patients with hematological malignancies and nonmalignant hematopoietic disorders. To achieve stable engraftment of donor hematopoietic stem cells (HSCs), recipient HSC deletion is needed to create space for incoming donor HSCs and donor HSCs must escape immune rejection by the recipient. Conventional allo-HSCT requires high dose of irradiation and/or chemotherapy to produce sufficient host stem cell and immune system ablation to permit donor HSC engraftment. However, these procedures also result in nonspecific tissue injury that can cause short- and long-term adverse effects as well as incite and amplify graft-versus-host-disease (GVHD). The delivery of targeted radiotherapy to hematopoietic tissues with the use of a radioimmunoconjugate (ROIC) as a part of transplant preparative regimen has shown clinical benefits. ROIC clinical data provide evidence for decreased relapse without increased transplant-related mortality by delivering higher targeted radiation to sites of malignancy than when given in a nontargeted fashion. An alternative approach to allo-HSCT has been developed and tested in preclinical mouse models in which nonmyeloablative preconditioning with low dose of the alkylating agent (busulfan) or lower systemic dose of irradiation combined with co-stimulatory pathway blockade (CTLA4-Ig, anti-CD40L monoclonal antibody) and/or immunosuppressive drugs have been used. Under these conditions, mixed chimerism and transplantation tolerance to fully MHC mismatched donor marrow was observed. Recently, several novel proof-of-concept antibody-mediated preconditioning methods have been developed that can selectively target hematopoietic stem and immune cells with minimal overall toxicity. Antibody-drug-conjugate (ADC) combined with reduced intensity conditioning or high dose ADC as single dose monotherapy have shown promise for allo-HSCT in preclinical models. The purpose of the current review is to discuss the literature exploring antibody-based conditioning that includes native antibody, radiolabeled antibody conjugates, and ADC for allo-HSCT.

Non-genotoxic Restoration of the Hematolymphoid System in Fanconi Anemia

Hematopoietic stem cell transplantation (HSCT) is a curative treatment for patients with many different blood and immune diseases; however, current treatment regimens contain non-specific chemotherapy and/or irradiation conditioning, which carry both short-term and long-term toxicities. The use of such agents may be particularly harmful for patients with Fanconi anemia (FA), who have genetic mutations resulting in deficiencies in DNA repair, leading to increased sensitivity to genotoxic agents. mAb-based conditioning has been proposed as an alternative conditioning strategy for HSCT that minimizes these toxicities by eliminating collateral tissue damage. Given the high need for improved treatments for FA patients, we aimed to evaluate the efficacy of different αCD117 mAb agents and immunosuppression on hematopoietic stem cell (HSC) depletion and explored their ability to safely establish therapeutic donor hematopoiesis post-HSCT in FA disease models. We evaluated the effects of different concentrations of αCD117 mAbs in vitro and in vivo on HSC growth and depletion. To further assess the efficacy of mAb-based conditioning, Fancd2-/- animals were treated with αCD117 mAb and combination agents with αCD47 mAb and antibody-drug-conjugates (ADCs) for syngeneic HSCT. Immunosuppression αCD4 mAb was added to all in vivo experiments due to a slightly mismatched background between the donor grafts and recipients. Immunosuppressant cocktails were also given to Fancd2-/- animals to evaluate the efficacy of mAb-based conditioning in the haploidentical setting. Statistical analyses were done using the unpaired t-test. We found that antagonistic αCD117 mAbs alone do not deplete host HSCs or enhance HSCT effectively in FA mouse models; however, the potency of αCD117 mAbs can be safely augmented through combination with αCD47 mAbs and with ADCs, both of which lead to profound HSC depletion and establishment of long-term donor engraftment post-syngeneic HSCT. This is the first time these approaches have been tested in parallel in any disease setting, with the greatest donor engraftment observed after CD117-ADC conditioning. Interestingly, our data also suggest that HSC-targeted conditioning is not necessary in HSCT for FA, as high donor HSC engraftment was observed with mAb-based immune suppression alone with immunologically matched and mismatched haploidentical grafts. These results demonstrate the safety and efficacy of several different non-genotoxic mAb-based conditioning strategies in the FA setting. In addition, they show that if sufficient immunosuppression is given to obtain initial donor HSC engraftment, turnover of a majority of the hematolymphoid system can result, likely owing to the survival advantage of wild-type HSCs over FA HSCs. Such non-toxic all-mAb-based conditioning strategies could be transformative for FA patients and those with other hematolymphoid diseases.

JSP191 As a Single-Agent Conditioning Regimen Results in Successful Engraftment, Donor Myeloid Chimerism, and Production of Donor Derived Naïve Lymphocytes in Patients with Severe Combined Immunodeficiency (SCID)

JSP191 As a Single-Agent Conditioning Regimen Results in Successful Engraftment, Donor Myeloid Chimerism, and Production of Donor Derived Naïve Lymphocytes in Patients with Severe Combined Immunodeficiency (SCID)

Inducible Sbds deletion impairs bone marrow niche capacity to engraft donor bone marrow after transplantation

AbstractBone marrow (BM) niche-derived signals are critical for facilitating engraftment after hematopoietic stem cell (HSC) transplantation (HSCT). HSCT is required for restoration of hematopoiesis in patients with inherited BM failure syndromes (iBMFSs). Shwachman-Diamond syndrome (SDS) is a rare iBMFS associated with mutations in SBDS. Previous studies have demonstrated that SBDS deficiency in osteolineage niche cells causes BM dysfunction that promotes leukemia development. However, it is unknown whether BM niche defects caused by SBDS deficiency also impair efficient engraftment of healthy donor HSC after HSCT, a hypothesis that could explain morbidity noted after clinical HSCT for patients with SDS. Here, we report a mouse model with inducible Sbds deletion in hematopoietic and osteolineage cells. Primary and secondary BM transplantation (BMT) studies demonstrated that SBDS deficiency within BM niches caused poor donor hematopoietic recovery and specifically poor HSC engraftment after myeloablative BMT. We have also identified multiple molecular and cellular defects within niche populations that are driven by SBDS deficiency and are accentuated by or develop specifically after myeloablative conditioning. These abnormalities include altered frequencies of multiple niche cell subsets, including mesenchymal lineage cells, macrophages, and endothelial cells; disruption of growth factor signaling, chemokine pathway activation, and adhesion molecule expression; and p53 pathway activation and signals involved in cell cycle arrest. Taken together, this study demonstrates that SBDS deficiency profoundly impacts recipient hematopoietic niche function in the setting of HSCT, suggesting that novel therapeutic strategies targeting host niches could improve clinical HSCT outcomes for patients with SDS.

5-Azacytidine depletes HSCs and synergizes with an anti-CD117 antibody to augment donor engraftment in immunocompetent mice

AbstractDepletion of hematopoietic stem cells (HSCs) is used therapeutically in many malignant and nonmalignant blood disorders in the setting of a hematopoietic cell transplantation (HCT) to eradicate diseased HSCs, thus allowing donor HSCs to engraft. Current treatments to eliminate HSCs rely on modalities that cause DNA strand breakage (ie, alkylators, radiation) resulting in multiple short-term and long-term toxicities and sometimes even death. These risks have severely limited the use of HCT to patients with few to no comorbidities and excluded many others with diseases that could be cured with an HCT. 5-Azacytidine (AZA) is a widely used hypomethylating agent that is thought to preferentially target leukemic cells in myeloid malignancies. Here, we reveal a previously unknown effect of AZA on HSCs. We show that AZA induces early HSC proliferation in vivo and exerts a direct cytotoxic effect on proliferating HSCs in vitro. When used to pretreat recipient mice for transplantation, AZA permitted low-level donor HSC engraftment. Moreover, by combining AZA with a monoclonal antibody (mAb) targeting CD117 (c-Kit) (a molecule expressed on HSCs), more robust HSC depletion and substantially higher levels of multilineage donor cell engraftment were achieved in immunocompetent mice. The enhanced effectiveness of this combined regimen correlated with increased apoptotic cell death in hematopoietic stem and progenitor cells. Together, these findings highlight a previously unknown therapeutic mechanism for AZA which may broaden its use in clinical practice. Moreover, the synergy we show between AZA and anti-CD117 mAb is a novel strategy to eradicate abnormal HSCs that can be rapidly tested in the clinical setting.

The Application of ‘Drug-Reversible’ CAR-T Cells Directed Against Recipient Hematopoietic Cells As a Selective Conditioning Strategy for Stem Cell Transplantation

The transplant of autologous or allogeneic hematopoietic stem cells (HSCs) has the proven ability to treat a wide array of malignant and non-malignant hematological diseases. The preparative regimen, however, routinely entails aggressive and genotoxic treatment with total body irradiation and/or chemotherapy, which brings severe and even life-threatening complications that limit its broader application. Previous experimental studies have established that depletion of recipient HSCs is an essential requirement of these conditioning regimens in allowing successful engraftment of the composite donor HSCs. Animal and clinical studies have also indicated that alloreactive anti-HSC donor T cells additionally facilitate stem cell engraftment, but this is often accompanied by the risks of GvHD. This has prompted the consideration of alternative conditioning methods for the depletion of HSCs with less toxic side-effects, such as anti-c-kit and anti-CD45 antibody-directed treatments. In this way, more precise HSC targeting may also be achieved by the application of short-lived, genetically engineered chimeric antigen receptor (CAR)-T cells for stem cell transplantation conditioning.We developed a novel and controllable CAR-T approach for recipient HSC targeting via genetic modification using the non-viral piggyBac™ (PB) transposon system. As opposed to viral vector delivery systems, the relatively large carrying capacity of PB allows the stable introduction of at least three separate genes encoded within the same tri-cistronic transgene cassette. This includes a second-generation CAR that targets either human c-kit (CD117) or prominin-1 (CD133), markers known to be antigenically expressed on the surface of HSCs. In addition, a drug resistance element serves as a selection gene that, in combination with a non-genotoxic drug, provides an effective method of CAR-T cell purification during manufacture. Importantly, a small molecule drug-inducible safety switch gene is also included to facilitate rapid in vivo clearance of the CAR-T cells after depletion of recipient HSCs and prior to donor HSC transplant. Lastly, as a result of the manufacturing process, the majority of the CAR-T cells express chemokine receptors such as CXCR4 that can allow more selective trafficking to the bone marrow (BM) for eradication of resident HSCs.To select a lead candidate from a panel of anti-HSC CAR constructs, CD3/CD28 stimulated T cells from human peripheral blood were first electroporated with mRNA encoding each of the CAR candidates directed against either c-kit or CD133. CAR surface expression was confirmed in transfected T cells by flow cytometry. In vitro functional assays were performed by co-culturing mRNA-transfected CAR-T cells with mouse or human cell lines (EML-C1, TF-1 and K562), expressing either c-kit or CD133, as well as mouse and human primary BM cells. Lead CAR candidates were identified from their specific activation of the CAR-T cells through degranulation according to CD107a expression and secretion of IFNγ. Furthermore, those CARs were also capable of selectively depleting c-kit or CD133 positive cells. Interestingly, some mRNA-transfected CAR-T cells retained effector activity against target c-kit+ TF-1 cells even in the presence of its soluble ligand, stem cell factor. Next, lead CAR candidates were co-expressed with the selection and drug-inducible safety switch genes in the same tri-cistronic transgene and then stably delivered to T cells using PB. The manufacturing process yielded CAR-T cells that were mainly of the T memory stem cell (Tscm) phenotype, as determined by positive expression of CD62L and CD45RA, and also expressed high levels of the CXCR4 chemokine receptor. Similar to the mRNA-transfected CAR-T cells, these stably-transposed cells were capable of extensive effector capabilities including specific depletion of c-kit or CD133 expressing target cells.Future studies will evaluate PB-produced lead anti-HSC CAR-T cells in immune-deficient NSG mice with pre-established xenogeneic human hematopoietic chimerism, along with standard busulfan or radiation conditioning controls. This approach constitutes a novel targeted biological therapy that is envisaged to lead the way towards minimally toxic transplant regimens for depletion of endogenous HSCs in the BM and to procure their replacement with engrafted allogeneic or gene-corrected stem cells. DisclosuresTimberlake:Poseida Therapeutics, Inc.: Employment. Richter:Poseida Therapeutics, Inc.: Employment. Wang:Poseida Therapeutics, Inc.: Employment. Cam:Poseida Therapeutics, Inc.: Employment. Tan:Poseida Therapeutics, Inc.: Employment. Ostertag:Poseida Therapeutics, Inc.: Employment, Equity Ownership. Shedlock:Poseida Therapeutics, Inc.: Employment. Down:Poseida Therapeutics, Inc.: Employment.

Inducible SBDS Deficiency and Germline FANCC/FANCG Deficiency Differentially Impact the Capacity of Bone Marrow Niches to Engraft Hematopoietic Stem Cells after Transplantation

Strategies to optimize interactions between the recipient bone marrow (BM) microenvironment and donor hematopoietic stem cells (HSC) could improve hematopoietic stem cell transplantation (HSCT) outcomes by reducing graft failure rates, improving blood and immune reconstitution efficiency, and reducing intensive cytotoxic conditioning requirements. Such methods are particularly relevant for patients with inherited bone marrow failure syndromes (iBMFs), rare genetic disorders for which HSCT is required to cure bone marrow failure (BMF), but for which HSCT is also associated with a high incidence of delayed donor engraftment, graft failure and morbidity due to chemotherapy and radiation sensitivity.In previous studies of the MPL-/- model of congenital amegakaryocytic thrombocytopenia, we discovered that decreased numbers of megakaryocytes and/or long-term (LT)-HSC caused by absent thrombopoietin/MPL signaling resulted in diminished capacity of the MPL-/- BM niche microenvironment to efficiently engraft wildtype (WT) donor HSC after HSCT. This observation led us to hypothesize that pre-existing BMF, regardless of cause, may generally alter HSC niche elements critical for engraftment after HSCT. We now present data testing this hypothesis in mouse models of Fanconi Anemia (FA) and Shwachman-Diamond Syndrome (SDS).To model FA, we used double mutant FANCC-/-;FANCG-/- mice, which have previously been demonstrated to develop manifestations of BMF. In our facility, FANCC-/-;FANCG-/- mice did not develop evidence of BMF, as measured by blood counts, BM cellularity, and HSC quantitation out to 1 year of age even with induction of HSC cycling by polyinosinic-polycytidilic acid (pIpC). Using SCT assays in which GFP+ WT donor BM was transplanted into FA recipients (FANCC-/-, FANCG-/-, FANCC-/-;FANCG-/-) or WT controls after 800-1100 cGy total body irradiation (TBI), efficiency and durability of donor engraftment was assessed at 3 weeks and 5 months, respectively, after primary SCT by competitive secondary HSCT. Unexpectedly, all FA recipients demonstrated more rapid BM reconstitution and enhanced early hematopoietic progenitor engraftment of WT donor BM compared with WT controls. However, there was no significant difference in LT-HSC engraftment efficiency or durability in the FA recipient groups versus WT controls.To model SDS, we first attempted to generate a model of osteoblast-specific SBDS deletion by crossing Col1A1Cre mice with mice carrying floxed SBDS alleles (SBDSfl/fl). However, this Col1A1CreSBDSExc model proved to be embryonic lethal at E12-E15 due to profound developmental abnormalities. We thus generated an inducible model by crossing SBDSfl/fl mice with Mx1Cre+ mice, and inducing SBDS deletion in Mx1-inducible BM hematopoietic and osteolineage niche cells by pIpC administration. SBDS deletion induced by pIpC was confirmed with PCR methods.Within 4 weeks of pIpC induction, Mx1CreSBDSExc mice show evidence of BMF, including a 40% reduction in platelet counts, inverted myeloid/lymphoid cell ratio in blood, and up to 80% reduction in immunophenotypic BM LT-HSC. When we attempted HSCT with GFP+ WT donor BM in Mx1CreSBDSExc and littermate control recipients following 1100 cGy TBI, 90% of Mx1CreSBDSExc recipients died by day 9 after HSCT, whereas all littermate controls survived. BM sections from Mx1CreSBDSExc recipients demonstrated persistent aplasia out to 1 week post-HSCT, suggesting that Mx1CreSBDSExc died from primary graft failure. Competitive secondary HSCT performed from BM harvested 1 week after primary HSCT demonstrated severe deficits in GFP+ WT donor HSC engraftment in Mx1CreSBDSExc recipients. Our initial studies as to why donor engraftment fails in Mx1CreSBDSExc recipients have identified that compared to WT, Mx1CreSBDSExc mice show significantly reduced expansion of osteolineage niche cells after TBI, a response that we have previously shown to be critical for efficient donor engraftment after HSCT.Taken together, our data demonstrate that pre-existing BMF and SBDS deficiency in BM osteolineage cells, but not FANCC or FANCG deficiency without co-existent BMF, significantly reduce the capacity of the BM niche to engraft donor HSC. Future studies defining specific niche deficits in our inducible SBDS deficiency model will provide critical insights into cellular and molecular pathways required for efficient engraftment after HSCT. DisclosuresNo relevant conflicts of interest to declare.

Uncoupling key determinants of hematopoietic stem cell engraftment through cell-specific and temporally controlled recipient conditioning.

SummaryHematopoietic stem cells (HSCs) are typically characterized by transplantation into irradiated hosts in a highly perturbed microenvironment. Here, we show that selective and temporally controlled depletion of resident HSCs through genetic deletion of Gata2 constitutes efficient recipient conditioning for transplantation without irradiation. Strikingly, we achieved robust engraftment of donor HSCs even when delaying Gata2 deletion until 4 weeks after transplantation, allowing homing and early localization to occur in a completely non-perturbed environment. When HSCs from the congenic strains Ly5.1 and Ly5.2 were competitively transplanted, we found that the more proliferative state of Ly5.2 HSCs was associated with superior long-term engraftment when using conditioning by standard irradiation, while higher CXCR4 expression and a better homing ability of Ly5.1 HSCs strongly favored the outcome in our inducible HSC depletion model. Thus, the mode and timing of recipient conditioning challenges distinct functional features of transplanted HSCs.

First Report of Non-Genotoxic Conditioning with JSP191 (anti-CD117) and Hematopoietic Stem Cell Transplantation in a Newly Diagnosed Patient with Severe Combined Immune Deficiency

Successful hematopoietic stem cell transplantation (HSCT) requires vacating recipient hematopoietic stem cell (HSC) niches in the bone marrow to permit donor HSC engraftment that can provide life-long hematopoietic and immune function. Currently, HSCT in SCID relies on DNA damaging chemotherapy to eliminate recipient HSC and achieve niche clearance. We have pursued a non-toxic approach to target and deplete HSC using a humanized monoclonal antibody, JSP191, that binds human CD117 (c-Kit). We previously showed the safety and successful HSC engraftment in a Phase 1 trial of the first 6 patients with severe combined immunodeficiency (SCID), who underwent a second transplant because of HSC engraftment failure and poor immunity after their first transplantation. In these re-transplant patients even a low level of stringently measured myeloid chimerism resulted in significant and sustained generation of naive T cells and clinical improvement. Based on these results, the study of JSP191 (NCT#02963064)has opened a cohort of newly diagnosed infants with SCID. Here we report data from the first patient in this cohort, a SCIDX1 patient who received a primary HSCT with haploidentical CD34+ cells after conditioning with JSP 191. The patient had a c.270-15A>G variant in the IL2RG gene, which is predicted to cause a null phenotype. Besides a T- B+ NK- phenotype typical of SCIDX1 including dysfunctional B cells, the patient had anemia and intermittent neutropenia and thrombocytopenia. Despite evidence of maternal T cell engraftment, the patient had no clinical graft-versus-host disease (GVHD). The patient was initially enrolled in a trial of lentiviral gene therapy, but harvested bone marrow cells died in vitro during transduction and culture. The patient also mobilized poorly with G-CSF/Plerixafor. Further investigation revealed heterozygosity for loss-of-function mutations in two genes involved in DNA repair, BRCA1 and RAD51; Diepoxybutane (DEB) breakage study showed greater than normal pathologic chromosomal breaks, but less than that seen in Fanconi anemia. Because of concern for possible hypersensitivity to alkylating agent-based conditioning, the patient was referred for transplant with JSP191 conditioning. The patient received a CD34+ peripheral blood HSCT from his father after conditioning with 0.3 mg/kg of JSP 191 antibody intravenously over an hour on Day -8 and rATG (Thymoglobulin) on Day -5, -4, -3 and -2 (3.5 mg/kg total) to prevent rejection by the maternal T cells. The cryopreserved donor CD34+ cells were administered after sufficient clearance of the JSP191 serum level. The antibody infusion was well tolerated without toxicity, and the post-transplant course was uneventful without acute toxicities or GVHD. As a surrogate marker for HSC engraftment, CD15+ myeloid cells from peripheral blood were stringently sorted by flow cytometry and donor levels were quantified by short-tandem repeat (STR) analysis. Progressive levels of myeloid engraftment were observed beginning at Week 4. The level of donor chimerism at 12 weeks was 8% in the sorted CD15+ blood cells, and a marrow aspirate showed 25% donor CD34+ cells. By 3 months pre-existing abnormal CD19-CD20+ host B lymphocytes were significantly reduced, and CD19+ donor-derived B lymphocytes were emerging. At 2 months, CD4+ recent thymic emigrant and naïve T lymphocytes were observed, and by 3 months, overall T and NK lymphocyte numbers were 390/uL and 117/uL, respectively. Normal blastogenic responses to the T cell mitogen PHA were observed at 3 months. These first-in-class results provide proof of concept of the safety and efficacy of the use of JSP191 antibody to clear host marrow niche space to enable sufficient donor HSC engraftment and immune reconstitution as primary therapy of SCID. Non-genotoxic conditioning with JSP191 may replace conventional conditioning for newly diagnosed infants with SCID, thereby avoiding toxicities of chemotherapy. Disclosures Kohn: Allogene Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees; Orchard Therapeutics: Consultancy, Patents & Royalties, Research Funding. De Oliveira:Orchard Therapeutics: Research Funding; bluebird bio, Inc.: Research Funding. Czechowicz:Rocket Pharmaceuticals, Inc.: Research Funding. Brown:Merck: Membership on an entity's Board of Directors or advisory committees; Ansun: Membership on an entity's Board of Directors or advisory committees; Cidara: Membership on an entity's Board of Directors or advisory committees; Allogene: Membership on an entity's Board of Directors or advisory committees; Cellerant Therapeutics: Membership on an entity's Board of Directors or advisory committees. Shizuru:Jasper Therapeutics, Inc: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees.

5-Azacytidine Depletes Hematopoietic Stem Cells and Synergizes with an Anti-CD117 Antibody to Augment Engraftment of Donor Stem Cells in Immunocompetent Mice

Monoclonal antibody (mAb)-targeting of CD117 (c-Kit) present on hematopoietic stem cells (HSC) is an emerging conditioning strategy for hematopoietic cell transplantation (HCT) to replace standard-of-care alkylator and radiation therapy used to deplete recipient HSC and permit donor HSC engraftment. In preclinical and clinical studies anti-CD117 mAbs, which inhibit stem cell factor (SCF) from binding to CD117, have shown single agent efficacy in permitting donor HSC engraftment in immunocompromised recipients. However, preclinical data suggest that anti-CD117 mAb, ACK2, used as a single agent is insufficient to enable HSC engraftment in immunocompetent mice. Hence, there is a desire to potentiate the effect of naked anti-CD117 mAbs to permit targeted elimination of host HSC for a range of immune competent bone marrow (BM) states, from monogenic disorders, e.g., hemoglobinopathies, to BM clonal malignant diseases. Here, we show that ACK2 synergizes with the widely used hypomethylating agent, 5-Azacytidine (AZA), to deplete HSC and enable robust long-term HSC engraftment in immunocompetent mice. The cell depleting effect of AZA or ACK2+AZA was first tested on mature and primitive hematopoietic cells of C57BL/6 (B6) mice. Mice received AZA at 5mg/kg i.p. for 5 days either alone or combined with ACK2 at 500 µg i.v. 5 days prior to AZA. BM and spleen were harvested days 6, 10 and 20 after the first AZA dose and analyzed by extended immunophenotype. For HCT studies both congenic [B6 (H-2b, Thy1.1, CD45.1) into B6 recipients (H-2b, Thy1.1, CD45.1/CD45.2)] and allogeneic [B6 (H-2b, Thy1.1, CD45.1) into BALB.B recipients (H-2b, Thy1.2, CD45.2)] transplantation mouse models were tested. Single agent AZA induced rapid depletion of CD117+ cells in the BM, including LT-HSC (Lin-Sca+Kit+[LSK]CD150+CD48-) and ST-HSC (LSKCD150-CD48-). In vivo HSC depletion by AZA has not been previously reported, and was surprising given that most HSC are quiescent, and non-proliferating cells are known to be insensitive to the toxicities of AZA. Examination of HSC populations in the spleen did not suggest that mobilization of LT- or ST-HSC to secondary organs caused the decrease in BM. Rather, AZA-induced myelosuppression was followed by rapid expansion of myeloid-biased MPP and increased HSC proliferation as assessed by Ki67 intracellular protein expression. We hypothesized, that the in vivo HSC-depletive effects of AZA might result from increased sensitivity of proliferating HSC to AZA. Hence, the in vitro effects of different AZA concentrations were tested on proliferating HSC. In both mouse and human HSC cultures, supplemented with SCF and TPO, we observed that AZA reduced the number of cells in a dose dependent manner and decreased cell viability at concentrations of ≥ 0.5 µg/ml. In vivo, combined ACK2+AZA led to more robust HSC depletion and prolonged time to HSC recovery compared to AZA alone. We then evaluated if blockade of CD117 with its ligand SCF by ACK2 was the cause of this prolonged HSC-depletion. 2B8 mAb, which also binds mouse CD117, but only partially inhibits CD117/SCF signaling, did not result in augmented in vivo HSC depletion (Figure 1A). We further tested if immunocompetent mice treated with ACK2+AZA could be "rescued" with exogenous recombinant SCF. SCF-treatment resulted in faster recovery of LT-HSC. Together these studies suggest that the synergy of ACK2+AZA on the depth and duration of HSC-depletion depends of the disruption of CD117/SCF binding. Finally, we tested ACK2+AZA conditioning on donor stem cell engraftment in immunocompetent mice. Transplantation of 15-20x106 WBM cells following ACK2+AZA conditioning led to significantly increased stable mean multilineage donor chimerism of >60% in the congenic (Figure 1B) and >20% in the allogeneic setting. Single agent AZA enabled sustained donor engraftment with levels of myeloid donor chimerism of 10-15%, serving as proof of principle that AZA treatment can indeed clear HSC niches and permit engraftment of true HSC. Similar to WBM grafts, 5x104 purified HSC (LSK) engrafted successfully in the ACK2+AZA group, reaching long-term (26 weeks) mean donor myeloid chimerism >25% in both congenic and allogeneic settings. Our studies reveal a previously unknown effect of AZA on BM HSC and provide a platform for the clinical use of a novel less toxic non-myeloablative conditioning strategy for HCT, which is aimed at the rapid translation into clinical practice. Disclosures Pang: Jasper Therapeutics, Inc: Current Employment, Current equity holder in private company. Shizuru:Jasper Therapeutics, Inc: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees.

Antibody-Drug Conjugates Targeting CD45 Plus Janus Kinase (JAK) Inhibitors As Conditioning for Allogeneic Hematopoietic Stem Cell Transplantation

<h3>Introduction</h3> Allogeneic hematopoietic stem cell transplantation (alloHSCT) is a potentially curative therapy for the treatment of hematologic diseases. AlloHSCT is associated with significant treatment-related morbidity and mortality from conditioning-related toxicities and graft-versus-host disease (GvHD), which limits its use to the most life-threatening indications, such as acute myeloid leukemia (AML). However, for conditions like AML that are primarily diagnosed in elderly patients, alloHSCT may be contraindicated due to poor patient functional status or medical comorbidities. Thus, an unmet clinical need exists for novel, highly-effective alloHSCT conditioning regimens with greater tolerability and minimal toxicity. <h3>Objectives</h3> Previously, antibody-drug conjugates (ADCs) composed of a CD45.2 antibody bound to the ribosome-inactivating protein saporin (CD45-SAP) were shown to allow high-level engraftment of syngeneic murine hematopoietic stem cells (HSCs). Here, we describe ADC-based conditioning regimens for murine alloHSCT which overcome immune barriers to engraftment imposed by minor histocompatibility antigen (miHA) and major histocompatibility complex (MHC) mismatches. <h3>Methods</h3> Mice were conditioned for HSCT with 3 mg/kg CD45-SAP 7 days prior to infusion of 10 × 10⁶ bone marrow cells. Haploidentical F1-to-parent (CB6F1→B6) and miHA-mismatched (BALB/c→DBA/2) HSCT models were used. For GvHD studies, a parent-to-F1 (B6→CB6F1) model was used in which recipients were infused with 25 × 10⁶ donor splenocytes after CD45-SAP treatment or sublethal irradiation. <h3>Results</h3> CD45-SAP combined with T cell depletion was sufficient for high-level engraftment in our alloHSCT models (Fig 1). The selective Janus kinase (JAK) inhibitor baricitinib, previously shown by our lab to prevent GvHD, also permitted HSC engraftment when mice received the drug during the peri-transplant period (Fig 2A-B). Graft stability in baricitinib-treated mice was optimized by either combining daily baricitinib treatment with pre-transplant T cell depletion, or by delivering baricitinib via continuous infusion (Fig 2C-D). Notably, CD45-SAP conditioned mice showed minimal morbidity or mortality from GvHD when infused with allogeneic splenocytes, whereas mice conditioned with sublethal irradiation developed acute GvHD. (Fig 3). <h3>Conclusion</h3> CD45-SAP combined with T cell depletion and/or pharmacologic immunomodulation with baricitinib effectively conditioned mice for engraftment of mismatched HSCs. Strikingly, mice conditioned with CD45-SAP did not develop GvHD even when infused with high doses of allogeneic splenocytes. These studies provide proof-of-principle evidence that targeted immunotherapeutics (ADCs plus JAK inhibitors) can prepare HSCT recipients for engraftment of allogeneic donor HSCs with minimal toxicities and without development of GvHD.

Non-Genotoxic Anti-CD117 Antibody Conditioning Results in Successful Hematopoietic Stem Cell Engraftment in Patients with Severe Combined Immunodeficiency

Non-Genotoxic Anti-CD117 Antibody Conditioning Results in Successful Hematopoietic Stem Cell Engraftment in Patients with Severe Combined Immunodeficiency

Inducible SBDS Deficiency Impairs Bone Marrow Niche Function to Engraft Donor Hematopoietic Stem Cell after Transplantation

Hematopoietic stem cell (HSC) transplantation (HSCT) is required for curative therapy for patients with high-risk hematologic malignancies, and a number of non-malignant disorders including inherited bone marrow failure syndromes (iBMFS). Strategies to enhance bone marrow (BM) niche capacity to engraft donor HSC have the potential to improve HSCT outcome by decreasing graft failure rates and enabling reduction in conditioning intensity and regimen-associated complications. Several studies in animal models of iBMFS have demonstrated that BM niche dysfunction contributes to both the pathogenesis of iBMFS, as well as impaired graft function after HSCT. We hypothesize that such iBMFS mouse models are useful tools for discovering targetable niche elements critical for donor engraftment after HSCT. Here, we report the development of a novel mouse model of Shwachman-Diamond Syndrome (SDS) driven by conditional Sbds deletion, which demonstrates profound impairment of healthy donor hematopoietic engraftment after HSCT due to pathway-specific dysfunctional signaling within SBDS-deficient recipient niches. We first attempted to delete Sbds specifically in mature osteoblasts by crossing Sbdsfl/flmice with Col1a1Cre+mice. However, the Col1a1CreSbdsExc progenies are embryonic lethal at E12-E15 stage due to developmental musculoskeletal abnormalities. Alternatively, we generated an inducible SDS mouse model by crossing Sbdsfl/flmice with Mx1Cre+ mice, and inducing Sbds deletion in Mx1-inducible BM hematopoietic and osteolineage niche cells by polyinosinic-polycytidilic acid (pIpC) administration. Compared with Sbdsfl/flcontrols, Mx1CreSbdsExc mice develop significantly decreased platelet counts, an inverted peripheral blood myeloid/lymphoid cell ratio, and reduced long-term HSC within BM, consistent with stress hematopoiesis seen in BMF and myelodysplastic syndromes. To assess whether inducible SBDS deficiency impacts niche function to engraft donor HSC, we transplanted GFP+ wildtype donor BM into pIpC-treated Mx1CreSbdsExc mice and Sbdsfl/flcontrols after 1100 cGy of total body irradiation (TBI). Following transplantation, Mx1CreSbdsExc recipient mice exhibit significantly higher mortality than controls (Figure 1). The decreased survival was related to primary graft failure, as Mx1CreSbdsExc mice exhibit persistent BM aplasia after HSCT and decreased GFP+ reconstitution in competitive secondary transplantation assays. We next sought to identify the molecular and cellular defects within BM niche cells that contribute to the engraftment deficits in SBDS-deficient mice. We performed RNA-seq analysis on the BM stromal cells from irradiated Mx1CreSbdsExc mice versus controls, and the results revealed that SBDS deficiency in BM niche cells caused disrupted gene expression within osteoclast differentiation, FcγR-mediated phagocytosis, and VEGF signaling pathways. Multiplex ELISA assays showed that the BM niche of irradiated Mx1CreSbdsExc mice expresses lower levels of CXCL12, P-selectin and IGF-1, along with higher levels of G-CSF, CCL3, osteopontin and CCL9 than controls. Together, these results suggest that poor donor HSC engraftment in SBDS-deficient mice is likely caused by alterations in niche-mediated donor HSC homing/retention, bone metabolism, host monocyte survival, signaling within IGF-1 and VEGF pathways, and an increased inflammatory state within BM niches. Moreover, flow cytometry analysis showed that compared to controls, the BM niche of irradiated Mx1CreSbdsExc mice contained far fewer megakaryocytes, a hematopoietic cell component of BM niches that we previously demonstrated to be critical in promoting osteoblastic niche expansion and donor HSC engraftment. Taken together, our data demonstrated that SBDS deficiency in BM niches results in reduced capacity to engraft donor HSC. We have identified multiple molecular and cellular defects in the SBDS-deficient niche contributing to this phenotype. Such niche signaling pathway-specific deficits implicate these pathways as critical for donor engraftment during HSCT, and suggest their potential role as targets of therapeutic approaches to enhance donor engraftment and improve HSCT outcome in any condition for which HSCT is required for cure. Disclosures Olson: Merck: Membership on an entity's Board of Directors or advisory committees; Bluebird Bio: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria.

Regulatory T Cell Adoptive Immunotherapy Promotes B Cell Immunity after Haploidentical Transplantation

Background: Adoptive transfer of CD4+CD25+FOXP3+ regulatory T cells (Tregs) effectively prevents conventional T cell (Tcons) mediated Graft versus Host Disease (GvHD) while it does not impair graft-versus-leukemia effect in haploidentical hematopoietic cell transplantation (haplo-HCT). Moreover Treg immunotherapy promotes fast donor T cell recovery after transplant. Recent studies showed that mouse bone marrow (BM) Tregs localize in the hematopoietic stem cell (HSC) niche, where they contribute to HSCs maintenance and promote donor engraftment and B cell lymphopoiesis. We are investigating if human Tregs promote B cell reconstitution and immunity in preclinical models and in haplo-HCT patients. Methods: Human sample analysis: B cell reconstitution was analysed monthly by FACS in BM and peripheral blood (PB) samples from 66 patients who underwent either Treg/Tcon haplo-HCT (45 patients), or T-cell depleted haplo-HCT (8 patients) or haplo-HCT with post-transplant cyclophosphamide (PTCy, 13 patients). Diagnosis was acute leukemia in 52 patients, lymphoma in 11 and multiple myeloma in 3. PB total immunoglobulin (Ig), anti-Cytomegalovirus (CMV) IgM and CMV viremia were monitored. Humanized mouse model: donor derived human Tregs and purified CD34+ HSCs were co-infused in sublethally irradiated (2 Gy) immune-deficient NSG mice. Donor engraftment and B cell reconstitution were analysed by FACS and histology. Results: B cell reconstitution was faster after Treg/Tcon haplo-HCT when compared to other haplo-HCT protocols. B cell counts were higher in PB of patients that received Treg/Tcon haplo-HCT (p&lt;.05) and were comparable to those of healthy subjects by 4 months after transplant (117±148 cells/mm, Fig.1A). We could detect early frequencies of CD34+CD38+CD10+CD127+ common lymphoid progenitors, CD45+CD10+CD38+CD19- Pre/Pro-B, CD45+CD10+CD38+CD19+ Pre-B, and Pro-B cells in the BM of these patients, that resulted in an increased production of CD38+CD19+CD5-IgM+ immature B cells, CD38+CD19+CD5+IgM+ transitional B cells and CD19+CD20+ mature B cells. We used a mouse model of xenotransplantation to understand whether donor B cell reconstitution in Treg/Tcon haplo-HCT is boosted by a Treg-mediated effect on donor human HSCs. We found that infusion of human Tregs facilitated donor HSC engraftment. BM and PB human chimerism was increased (p&lt;.05) in mice that received Treg infusion. BM histology revealed the presence of in vivo expanded human CXCR4+ Tregs in the femurs of Treg receiving mice 1 month after transplant. In the same samples we observed a lower number of CD34+ HSCs, suggesting BM Tregs facilitate donor HSC differentiation. Moreover, Treg treatment allowed human HSCs to localize preferentially in the epiphyseal areas of the femurs, where donor cells started to engraft (p&lt;.04). When looking at donor B cell reconstitution, we found HSC-derived mature B cells rapidly abundant and easily detectable starting 30 days after HSC infusion in PB of Treg-treated animals (p&lt;.05). To further assess whether HSC-derived B cells were functional in patients that received Treg/Tcon haplo-HCT, we analysed total Ig production and specific responses to CMV reactivation. Post-transplant hypogammaglobulinemia was rapidly corrected in Treg/Tcon haplo-HCT patients. Total IgM were higher compared to other haplo-HCT protocols and reached normal levels by 3 months after transplant (96±155 mg/dL, Fig.1B). CMV reactivation rate was lower (47% vs 79%) and it occurred later after Treg/Tcon immunotherapy (51±19 vs 40±48 days). New production of anti-CMV specific IgM was documented in 43% of CMV seropositive patients 98±48 days after Treg/Tcon haplo-HCT, while anti-CMV specific IgM were almost undetectable after other haplo-HCT protocols within the first 6 months after transplant (Fig.1C). Such potent B cell responses coupled with T cell reconstitution resulted in a reduction of CMV second reactivations (Fig.1D p&lt;.05) with no patient that died with CMV disease. Conclusions: Adoptive transfer of human Tregs boosts donor HSC engraftment and facilitates the reconstitution of functional HSC-derived donor B cells. Such results suggest that Treg/Tcon haplo-HCT patients could be early vaccinated after transplant. Treg/Tcon immunotherapy promotes control of infections and B cell immunity in patients undergoing haplo-HCT. Disclosures No relevant conflicts of interest to declare.

Translating HSC Niche Biology for Clinical Applications

Over the final 3 decades of the twentieth century, the advent and evolution of hematopoietic stem cell transplantation (HSCT) to cure refractory malignancies and severe non-malignant diseases created a need to better understand basic scientific mechanisms underlying the maintenance and migration of hematopoietic stem cells (HSC). Over the past 20 years, researchers have discovered complex overlapping bone marrow (BM) HSC niches that utilize a number of signaling pathways to tightly regulate HSC physiology. Here, we review recent efforts to define critical mediators of HSC niche function in health and disease, and how these discoveries are now enabling the development of the next generation of cellular therapies for cancer and severe non-malignant diseases. A number of cellular interactions and molecular pathways critical for HSC mobilization, donor HSC engraftment after HSCT, and HSC/progenitor cell recovery following chemotherapy have recently been identified. Additional studies have defined mechanisms by which cancer and bone marrow failure states disrupt the normal function of these pathways. Translational investigators are now leveraging these discoveries to develop preclinical and clinical strategies to target the niche for regenerative and cancer therapy. Ongoing research to define fundamental aspects of HSC niche biology will lead to further refinements and precision approaches to improve the safety and efficacy of clinical cell therapy.

Toxicity-Free Hematopoietic Stem Cell Engraftment Achieved with Anti-CD117 Monoclonal Antibody Conditioning

Successful hematopoietic cell transplantation (HCT) requires vacating recipient hematopoietic stem cell (HSC) niches to permit donor HSC engraftment to provide life-long hematopoietic and immune function. Currently HCT relies on DNA damaging radiation or chemotherapy to achieve HSC niche clearance. We have pursued a non-toxic approach to target and deplete HSC using humanized monoclonal antibody, AMG 191, that binds human CD117 (c-Kit). We opened a Phase 1 dose escalation trial using AMG 191 as the sole conditioning agent to achieve donor HSC engraftment in patients undergoing HCT for severe combined immunodeficiency (SCID). The primary endpoint is to assess the safety of administering AMG 191. Secondary endpoints include AMG 191 pharmacokinetics (PK), host HSC depletion, and determination of the dose of AMG 191 that achieves adequate donor HSC engraftment, defined as >5% donor granulocyte chimerism at 24 weeks. We have completed the first dose cohort of patients receiving 0.1 mg/kg AMG 191 and treated the first two patients in the second cohort (0.3 mg/kg). All five patients tolerated the AMG 191 infusion and the subsequent infusion of their CD34-selectd donor cells without clinical problems. Here we report efficacy in the first two patients, who have reached the 24-week timepoint. Both patients had T-B-NK+ SCID with mutations in the DCLRE1C (Artemis) gene. Both previously received unconditioned HCT as infants, failed to develop donor B cells and remained dependent on exogenous immunoglobulin. CD34-selected mobilized peripheral blood cells from the original donors were infused when the AMG 191 serum level was Conclusion: These data are proof of concept that a humanized monoclonal antibody targeting CD117 can safely clear human HSC niches and facilitate donor HSC engraftment. This study is ongoing and open for enrollment.

Reduction in Bone Marrow Insulin-like Growth Factor-1 Levels Enhances Long-Term Hematopoietic Stem Cell Engraftment Following Bone Marrow Transplantation

Efficient donor hematopoietic stem cell (HSC) engraftment is critical for successful bone marrow transplantation (BMT) and requires interactions between donor HSC and host-derived HSC niche elements. Due to disruption of perivascular stem cell niches induced by myeloablative BMT conditioning, initial engraftment of HSC following BMT occurs in niches located near the endosteum. We have previously shown that myeloablative BMT conditioning with total body irradiation (TBI) induces expansion of endosteal niche osteolineage cells in a time course correlating with increases in bone marrow (BM) levels of insulin-like growth factor (IGF)-1. We also have shown that inhibition of IGF-1 Receptor (IGF1R) signaling blocks osteolineage cell expansion after TBI, resulting in poor engraftment of long-term (LT)-HSC after BMT and suggesting a critical role for IGF-1/IGF1R interactions in regulating engraftment after BMT. We now present data in which we have defined the cellular sources of BM IGF-1 following TBI and have surprisingly discovered that reduction in BM IGF-1 levels promotes LT-HSC retention at baseline and donor LT-HSC engraftment following BMT.Using mice with floxed IGF-1 alleles (IGF1L/L) and with tissue-specific expression of Cre-recombinase, we generated models with deletion of IGF-1 derived from mature osteolineage cells (Col1A1CreIGF1Exc), hematopoietic cells (CD45CreIGF1Exc), or liver (AlbCreIGF1Exc). We additionally generated a conditional IGF-1 deletion model (MX1CreIGF1Exc) in which polyI:polyC (pI:pC) treatment disrupts IGF-1 production from all 3 of the above sources.By qPCR analysis, CD45CreIGF1Exc BM showed marked reduction (>95%) in BM IGF-1 mRNA at baseline and following TBI, suggesting that hematopoietic cells produce most of the locally derived IGF-1 in BM. Unexpectedly however, neither deletion of osteoblast-derived (Col1A1CreIGF1Exc) nor hematopoietic cell-derived (CD45CreIGF1Exc) IGF-1 significantly reduced BM extracellular or intracellular IGF-1 protein levels at baseline, or extracellular BM IGF-1 levels following TBI. In contrast, elimination of liver-derived IGF-1 in AlbCreIGF1Exc mice not only reduced plasma IGF-1 levels by 80%, but surprisingly reduced BM extracellular IGF-1 protein levels by ≥80% both at baseline and 48 hours post-TBI, and reduced BM intracellular IGF-1 protein levels by >80% and >60% at baseline and post-TBI, respectively. These data suggest that most IGF-1 protein in BM during BMT is derived from liver. Conditional IGF-1 deletion in MX1CreIGF1Exc mice resulted in severe reduction of both BM IGF-1 mRNA and protein levels, combining effects seen in CD45CreIGF1Exc and AlbCreIGF1Exc mice. None of these models, however, showed deficits in TBI-induced endosteal cell expansion, suggesting that other ligands or perhaps only low levels of IGF-1 are required to induce IGFR1 signaling-dependent endosteal cell expansion.Unexpectedly, reduction of BM IGF-1 in ALBCreIGF1Exc and MX1CreIGF1Exc mice caused a 3-4 fold increase (% of total BM) in BM Lin-cKit+Sca1+CD48-CD150+ or Lin-cKit+Sca1+CD34-CD135- HSC. ALBCreIGF1Exc BM displayed enhanced multi-lineage long-term repopulating capacity in competitive transplant assays, proving that these cells are functional LT-HSC. Given this baseline LT-HSC enhancement, we asked if reduction in BM IGF-1 levels impacted capacity of ALBCreIGF1Exc BM niches to engraft donor HSC after BMT. Using competitive secondary transplantation assays to assess numbers of engrafted WT GFP+ LT-HSC 3 weeks after primary BMT into ALBCreIGF1Exc or control mice, we found that reduced IGF-1 in the ALBCreIGF1Exc BM microenvironment resulted in >2.5 fold enhancement of donor LT-HSC engraftment. In an initial search of downstream mediators of this effect, we discovered that reduction of IGF-1 levels in MX1CreIGF1Exc BM results in a 2.5-fold increase in post-TBI CXCL12 mRNA expression, suggesting that reducing BM IGF-1 may enhance donor LT-HSC engraftment by promoting LT-HSC homing and maintenance within HSC niches.Taken together our data define a critical role for IGF-1/IGF1R interactions in regulating the efficiency of donor HSC engraftment following BMT. Therapeutic strategies to reduce BM IGF-1 may prove to be a valuable approach to improving engraftment following clinical BMT. DisclosuresNo relevant conflicts of interest to declare.

Blood Stem Cell Activity Is Arrested by Th1-Mediated Injury Preventing Engraftment following Nonmyeloablative Conditioning.

T cells are widely used to promote engraftment of hematopoietic stem cells (HSCs) during an allogeneic hematopoietic cell transplantation. Their role in overcoming barriers to HSC engraftment is thought to be particularly critical when patients receive reduced doses of preparative chemotherapy and/or radiation compared with standard transplantations. In this study, we sought to delineate the effects CD4+ cells on engraftment and blood formation in a model that simulates clinical hematopoietic cell transplantation by transplanting MHC-matched, minor histocompatibility-mismatched grafts composed of purified HSCs, HSCs plus bulk T cells, or HSCs plus T cell subsets into mice conditioned with low-dose irradiation. Grafts containing conventional CD4+ T cells caused marrow inflammation and inhibited HSC engraftment and blood formation. Posttransplantation, the marrows of HSCs plus CD4+ cell recipients contained IL-12-secreting CD11c+ cells and IFN-γ-expressing donor Th1 cells. In this setting, host HSCs arrested at the short-term stem cell stage and remained in the marrow in a quiescent cell cycling state (G0). As a consequence, donor HSCs failed to engraft and hematopoiesis was suppressed. Our data show that Th1 cells included in a hematopoietic allograft can negatively impact HSC activity, blood reconstitution, and engraftment of donor HSCs. This potential negative effect of donor T cells is not considered in clinical transplantation in which bulk T cells are transplanted. Our findings shed new light on the effects of CD4+ T cells on HSC biology and are applicable to other pathogenic states in which immune activation in the bone marrow occurs such as aplastic anemia and certain infectious conditions.