Bone Marrow Recipients Research Articles

Efficacy of Early Post-Transplant Vaccination Using a Prime and Boost Strategy Is Independent of a Prior History of GvHD: Results from BMT CTN 0201

Introduction: BMT CTN 0201 was a randomized trial of bone marrow (BM) vs G-CSF mobilized peripheral blood (G-PB) grafts in 550 patients with leukemia or MDS undergoing allogeneic hematopoietic stem cell transplantation from unrelated donors. Responses to the 23-valent-pneumococcal vaccine are poor in elderly and immune-compromised patients, including allo-transplant recipients. A secondary objective of BMT CTN 0201 compared responses to post-transplant vaccination using protein-based diphtheria-tetanus (dT) vaccine followed by two doses of a diphtheria-pneumococcal polysaccharide conjugate (PCV7) and the 23-valent pneumococcal polysaccharide vaccine (PPV23).Methods: Vaccination started at 6 months post-transplant with dT followed by PCV7 at 7 and 9 months and PPV23 at 11 months post-transplant. Hepatitis B vaccination occurred at 6, 7 and 11 months. Serological responses in BMT CTN 0201 patients surviving to one year post-transplant were compared among those vaccinated per protocol (N=71), those that received partial vaccination (N=58) or no vaccines (N=39). Primary end-points were the fractions of vaccinated individuals with protective antibody titers to the 14 common pneumococcal bacterial serotypes, Hepatitis B, diphtheria and tetanus. Clinical history of acute or chronic GVHD, blood B-cells and T-cells, and serum immunoglobulin levels were studied as covariates of protective immunological responses.Results: Overall survival at 3 years across the cohorts that received full, partial, or no vaccination was similar (81%, 87%, 84%). Lack of compliance with protocol vaccination did not vary according to whether subjects had a history of acute or chronic GVHD. The frequency of patients with prior history of grade 2-4 acute GVHD or chronic GVHD were similar across the three cohorts (82%, 90%, 82%), with comparable rates of severe grades 3-4 acute (10%, 12%, 13%) and extensive chronic GVHD (63%, 67%, 64%, respectively). Serological responses to vaccines did not differ between recipients of BM vs G-PB grafts, or whether patients had a history of grade 2-4 acute GVHD or chronic GVHD. Serological responses were highly associated with protocol-specified vaccination, with median (range) of titers (ug/mL) for fully vaccinated versus non-vaccinated individuals for HepB: 10.81 (0.05-110) versus 0.49 (0.05-63) p=0.01; tetanus, 2.80 (0.16-20) versus 0.40 (0.05-20) p<0.001; diphtheria: 1.62 (0.05-20) versus 0.08 (0.05-1.2) p<0.001; as well as protective antibody titers to each of the component pneumococcal epitopes shared between PCV7 and PPV23 (Table 1, N=71 fully vaccinated and N=48 with no pneumococcal vaccines). Among the fully-vaccinated cohort, a greater proportion had protective antibodies to individual pneumococcal serotypes present in the PCV7 “prime” vaccine (median 66%) versus those restricted to the PPV23 “boost” vaccine (median 41%). Serum levels of IgA, IgM, and IgG at 12 months post-transplant were correlated with the kinetics of B-cell recovery in the blood at 3, 6, and 12 months. Antibody titers to tetanus, diphtheria and 8/14 of the tested pneumococcal serotypes were correlated with total serum IgG levels at 12 months post-transplant. Titers to diphtheria, a T-cell dependent antigen, were correlated with day +100 blood levels of T-cells, myeloid dendritic cells and B-cells (p<0.05), while antibody levels to serotypes in PCV7 were correlated with day +100 levels of B-cells in the blood.Conclusion: The efficacy of vaccination to specific pneumococcal serotypes among recipients of allogeneic transplants is enhanced by priming with dT and PCV7 prior to vaccination with the PPV23 vaccine. Vaccination is effective in patients with a history of acute or chronic GVHD. Strategies that enhance B-cell recovery post-transplant may improve responses to vaccines, especially those that are T-cell independent. [Display omitted] DisclosuresWaller:National Institutes of Health: Research Funding; Helocyte: Consultancy; Chimerix: Equity Ownership; Cerus: Equity Ownership; Coulter Foundation: Research Funding; PRA: Consultancy; Novartis Pharmaceuticals Corporation: Consultancy, Honoraria, Research Funding; Cambium Medical Technologies: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties; Katz Foundation: Research Funding; AMGEN: Consultancy; Celldex: Consultancy.

Impact of Post-Transplant High-Dose Cyclophosphamide on Reconstitution in Bone Marrow Recipients

Introduction: Hematopoetic stem cell transplantation (HSCT) is the only curative therapy for many patients with hematologic malignancies. Occurrence of complications and mortality after allo HSCT is still high and it's strongly associated with immune reconstitution. Despite the wide-spread of Post-Transplant High-Dose Cyclophosphamide (PTCy) immune reconstitution and immunological safety of this method is still poorly understood.Aim: to evaluate immune reconstitution profile in patients who was underwent HSCT with a bone marrow as a stem cell source and with or without PTCy.Patients and methods: In National Research Center for Hematology 41patients after allogeneic HSCT were analyzed in 2 groups; patients with PTCy (n=17) and without PTCy (n=24). The total cohort had 12 males and 29 females, and had a median age of 33 years (range 19-61). All patients were diagnosed with a hematological malignancy. 14 patients were underwent myeloablative conditioning and 27 patients - non-myeloalative prior HSCT.The GVHD prophylaxis consisted of ATG-PTCy-CsA-MMF (n=10), ATG-PTCy (n=5), Mono-PTCy (n=1), ATG-CsA-MMF-MTX (n=20), CsA-MTX (n=2), ATG-CsA-MTX (n=2), CsA-MMF-MTX(n=1). The patients who only had received ATG were included on the analysis.Immune reconstitution profile was tested via serial flow cytometry analysis of peripheral blood on day +14, +30, +60 and +90 after allo-HSCT. Anti-CD3 FITC, anti-CD16PE, anti-CD56 PE, anti-CD45 Per-CP-CYTM5.5, anti-CD4 PE-CyTM7, anti-CD19 APC, anti-CD8 APC-Cy7; anti-CD62L FITC (BD Biosciences, USA); anti-CD14PE, anti-CD16PE, anti-HLA-DR APC (eBiosciences, USA) were used to define white blood cell subsets.Results: Reported detailed data is presented in Table 1. In bone marrow recipients the number of CD4+- cells was significantly lower on day +14 and +30, NK-cells on day +30 and CD8+-cells on day +60 in case of using PTCy versus immunosuppression without PTCy.The number of classical monocytes on day +30 and patrol monocytes on day +30 and +90 was significantly lower in case of using PTCy versus a standard protocol.Conclusion:Lymphocyte and monocyte reconstitution was impaired for the PTCy groups in the immediate post-HSCT period but quickly recovered. The mechanism of tolerance induction with PTCy on day+ 3, + 4 is not limited only to deletion of alloreactive T-cell clones, but also affects other leukocyte subpopulations (B cells, monocytes, granulocytes). The use of PTCy on day+ 3, + 4 is an immunologically safe method for prevention of GVHD.Table 1. Short-term reconstitution in BM recipients with and without PTCy [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Role of Signal Transducing Adaptor Protein (STAP) Family in Chronic Myelogenous Leukemia

Role of Signal Transducing Adaptor Protein (STAP) Family in Chronic Myelogenous Leukemia

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.

MTG16 Variants Provide Context to Notch Signaling to Direct Leukemic Phenotype

Notch signaling guides hematopoietic stem and progenitor cells (HS/PCs) toward distinct cell fates and when dysfunctional, contributes to development of hematopoietic malignancies. Normally, Notch signaling favors T-cell fate and causes attenuated expression of myeloid-specific genes. In contrast, Notch pathway inactivation causes HS/PCs to diverge disproportionately toward a myeloid fate and is accompanied by T-lymphopenia. Mutations that constitutively activate Notch signaling trigger T-cell leukemia, while Notch inactivation is associated with myeloid leukemias. This suggests that Notch-regulated cell fate decisions are context dependent and that determinants downstream of Notch can influence Notch-dependent phenotypes in development and disease.Myeloid Translocation Gene (MTG) 16 binds the intracellular domain of Notch (NICD) and this interaction is necessary to establish CD4+/CD8+ (double positive-DP) T-cell fate. Mtg16 -/- progenitors give rise to GR1+/Mac1+ progeny in response to Notch activation. In ex vivo assays, normal DP T-cell development can be restored to Mtg16 -/- progenitors by enforced expression of MTG16, but not an MTG16 derivative which lacks NICD binding and mimics a naturally occurring Mtg16 splice variant (MTG16c). These findings suggest the status of expressed Mtg16 isoforms can determine lineage allocation in response to Notch signaling and intimates that Notch signaling could trigger alternate phenotypes depending upon the context in which it is activated.To assess impact of the NICD-MTG16 interaction as a determinant of malignant phenotype in the context of constitutive Notch signaling, we created bicistronic retroviral constructs whose NICD and MTG16 expression cassettes were separated by the “self-cleaving” 2A peptide of Foot and Mouth disease virus (FMDV). In each transduced progenitor cell, this insures concurrent stoichiometric expression of NICD and the chosen MTG16 derivative (wild type or NICD binding-deficient). Transduced Mtg16 -/- HS/PCs were then transplanted into irradiated, syngeneic recipients and leukemic phenotype established by flow cytometry. Mice transplanted with Mtg16 -/- cells transduced with vector only fail to reconstitute hematopoiesis in the recipient bone marrow due to a previously described HS/PC defect. Instead, these mice succumb to marrow failure by day 18 post-transplant. However, enforced expression of NICD alone enables survival of Mtg16 -/- progenitors, reconstitution of hematopoiesis and development of T-cell leukemia from transplanted donor elements. This same phenotype was observed when NICD was co-expressed with MTG16, indicating MTG16 is required for normal T-lymphopoiesis but is dispensible for NICD-driven T-cell leukemia. Notably, when Mtg16 -/- progenitors are transduced with NICD and a NICD binding deficient derivative of MTG16, recipients develop T-myeloid bilineal leukemia. This finding suggests that integration of NICD into transcriptional regulatory complexes involving MTG16 can alter the features of leukemic blasts along a continuum from lymphoid to myeloid phenotypes. To gain additional insights, we characterized promoter occupancy and Notch transcription complex (NTC) composition upon expression of MTG16 and its NICD binding deficient derivative. We find both MTG16 forms can exist in the same transcriptional complexes via their NHR2 oligomerization domains and can co-occupy the Notch-responsive Hes1 promoter. Given that NICD-MTG16 binding disrupts interactions between MTG16, histone deacetylases (HDACs) and other co-repressors, these data suggest that leukemic phenotype arising from constitutive Notch signaling may be influenced by combinatorial diversity among MTG16 splice variants and differential integration of the NICD into the NTC. DisclosuresNo relevant conflicts of interest to declare.

Targeting of Deciduous Tooth Pulp Stem Cell-Derived Extracellular Vesicles on Telomerase-Mediated Stem Cell Niche and Immune Regulation in Systemic Lupus Erythematosus.

Systemic transplantation of stem cells from human exfoliated deciduous teeth (SHED) is used to treat systemic lupus erythematosus (SLE)-like disorders in MRL/lpr mice. However, the mechanisms underlying the SHED-based therapy remain unclear. In this study, we hypothesized that trophic factors within SHED-releasing extracellular vesicles (SHED-EVs) ameliorate the SLE-like phenotypes in MRL/lpr mice. SHED-EVs were isolated from the culture supernatant of SHED. SHED-EVs were treated with or without RNase and systemically administered to MRL/lpr mice. Subsequently, recipient bone marrow mesenchymal stem cells (BMMSCs) isolated from SHED-EV-administered MRL/lpr mice were examined for the in vitro and in vivo activity of hematopoietic niche formation and immunoregulation. Furthermore, the recipient BMMSCs were secondarily transplanted into MRL/lpr mice. The systemic SHED-EV infusion ameliorated the SLE-like phenotypes in MRL/lpr mice and improved the functions of recipient BMMSCs by rescuing Tert mRNA-associated telomerase activity, hematopoietic niche formation, and immunoregulation. The secondary transplantation of recipient BMMSCs recovered the immune condition and renal functions of MRL/lpr mice. The RNase treatment depleted RNAs, such as microRNAs, within SHED-EVs, and the RNA-depleted SHED-EVs attenuated the benefits of SHED-EVs in MRL/lpr mice. Collectively, our findings suggest that SHED-secreted RNAs, such as microRNAs, play a crucial role in treating SLE by targeting the telomerase activity of recipient BMMSCs.

Megakaryocyte progenitor cell function is enhanced upon aging despite the functional decline of aged hematopoietic stem cells.

SummaryAge-related morbidity is associated with a decline in hematopoietic stem cell (HSC) function, but the mechanisms of HSC aging remain unclear. We performed heterochronic HSC transplants followed by quantitative analysis of cell reconstitution. Although young HSCs outperformed old HSCs in young recipients, young HSCs unexpectedly failed to outcompete the old HSCs of aged recipients. Interestingly, despite substantial enrichment of megakaryocyte progenitors (MkPs) in old mice in situ and reported platelet (Plt) priming with age, transplanted old HSCs were deficient in reconstitution of all lineages, including MkPs and Plts. We therefore performed functional analysis of young and old MkPs. Surprisingly, old MkPs displayed unmistakably greater regenerative capacity compared with young MkPs. Transcriptome analysis revealed putative molecular regulators of old MkP expansion. Collectively, these data demonstrated that aging affects HSCs and megakaryopoiesis in fundamentally different ways: whereas old HSCs functionally decline, MkPs gain expansion capacity upon aging.

Cryopreservation of Unrelated Hematopoietic Stem Cells from a Blood and Marrow Donor Bank During the COVID-19 Pandemic: A Nationwide Survey by the Japan Marrow Donor Program

During the COVID-19 pandemic, donor hematopoietic stem cell grafts are frequently cryopreserved to ensure the availability of graft before starting a conditioning regimen. However, the safety of cryopreservation has been controversial in unrelated hematopoietic stem cell transplantation (HSCT), especially for bone marrow (BM) grafts. In addition, in unrelated HSCT, the effect of the time from harvest to cryopreservation of donor grafts required for the transportation of donor graft has not been fully clarified. In this study, we retrospectively analyzed the first 112 patients with available data who underwent cryopreserved unrelated blood and marrow transplantation through the Japan Marrow Donor Program during the COVID-19 pandemic. There were 112 patients, including 83 who received BM grafts and 29 who received peripheral blood stem cell (PBSC) grafts. The median time from stem cell harvest to cryopreservation was 9.9 hours (range, 2.6 to 44.0 hours), and the median time from cryopreservation to infusion was 231.2 hours. The incidence of neutrophil engraftment at day 28 after HSCT was 91.1%, and among 109 patients (excluding 3 patients with early death), all but 1 patient achieved neutrophil engraftment within 60 days after HSCT. The time to neutrophil engraftment and time to platelet engraftment were shorter in PBSC transplantation compared with BM transplantation (BMT), but the differences were not statistically significant (P = .064 and .18). Multivariate analysis among BM recipients revealed that a higher number of frozen nucleated cells and the absence of HLA mismatch were associated with faster neutrophil engraftment. The time to neutrophil engraftment after unrelated cryopreserved BMT was not different from that after unrelated BMT without cryopreservation. Our findings suggest that unrelated donor BM and PBSC grafts can be safely cryopreserved even after transit from the harvest center to the transplantation center. In the current COVID-19 pandemic, cryopreservation can be considered as an option while balancing the risks and benefits of the procedure.

An aging mouse model of human chronic myeloid leukemia.

Chronic myeloid leukemia (CML) is an age-dependent blood malignancy. Like many other age-dependent human diseases, laboratory animal research of CML uses young mice that do not factor in the influence of aging. To understand how aging may impact animal modeling of human age-dependent diseases, we established the first aging mouse model of human CML in BALB/c mice in the advanced age defined by 75% survival. This model was developed by noncytotoxic depletion of bone marrow lineage-positive cells followed by BCR-ABL retroviral transduction and transplantation. CML developed in aging mice shared many similarities to that in young mice, but had increased incidence of anemia that is often seen in human CML. Importantly, we showed that aging of both donor hematopoietic stem cells and recipient bone marrow niche impacted BCR-ABL mediated leukemogenesis and leukemia spectrum. Optimal CML induction relied on age-matching for donors and recipients, and cross-transplantation between young and old mice produced a mixture of different leukemia. Therefore, our model provides initial evidence of the feasibility and merit of CML modeling in aging mice and offers a new tool for future studies of CML stem cell drug resistance and therapeutic intervention in which aging would be taken into consideration as an influencing factor.

The impact of graft cell source on bloodstream infection in the first 100 days after allogeneic hematopoietic cell transplantation.

Bloodstream infection (BSI) is a major infectious complication after allogeneic hematopoietic cell transplantation (HCT). To clarify the impact of graft cell source on the incidence of BSI after transplantation, we retrospectively examined 782 adult patients receiving their first allogeneic HCT: 122 recipients of related peripheral blood stem cells or bone marrow, 215 recipients of unrelated bone marrow, and 445 recipients of unrelated umbilical cord blood (U-CB). The cumulative incidence of BSI was 42.5% at 100 days after transplantation (95% confidence interval, 39.0-46.0). Gram-positive cocci were present in 64.2% of detected isolates. Among the pre-transplant factors including age, performance status, primary disease, disease status, graft cell source, sex and ABO blood type matching, and the intensity of conditioning regimen, U-CB use was identified as the most significant risk factor for BSI by multivariate analysis (hazard ratio, 1.76; 95% confidence interval, 1.40-2.22; p < 0.00001). Among the U-CB recipients, those who are not in remission at the time of transplantation were at the greatest risk of BSI (hazard ratio, 1.69; 95% confidence interval, 1.14-2.50; p < 0.01). The study makes it clear that graft cell source has an impact on BSI development after allogeneic HCT.

Low 5-year health care burden after umbilical cord blood transplantation

AbstractRecipients of allogeneic hematopoietic cell transplantation (HCT) experience a substantial health care burden, with potentially differing patterns of long-term health care requirements using peripheral blood stem cells, bone marrow, and umbilical cord blood (UCB) grafts. We analyzed data from 1077 consecutive adult allogeneic HCT recipients who underwent transplant at the University of Minnesota between 2000 and 2016. To estimate health care burden over time, we compared the number of visits, laboratory studies, medications, and relative value units billed. Health care elements were analyzed both individually and together (ie, total health care elements used per patient days into a density composite score). UCB had the lowest density health care burden composite score from the time of transplant through year 5 (median score 64.0 vs 70.5 for peripheral blood stem cells and 88.0 for bone marrow; P < .01). In multivariate analysis of health care burden between years 1 and 5, recipients of either bone marrow (odds ratio [OR] 0.49 [95% confidence interval (CI) 0.29-0.84]) or peripheral blood stem cells (OR 0.49 [95% CI 0.36-0.67]) were half as likely to experience low health care burden compared with UCB. Adult recipients of UCB have a lower long-term health care burden compared with other graft sources, possibly reflecting a better quality of life.

CD34+ Stem Cell Selection and CD3+ T Cell Add-Back from Matched Unrelated Adult Donors in Children with Primary Immunodeficiencies and Hematological Diseases

Less than 25% of children who require hematopoietic stem cell transplantation (HSCT) for primary immunodeficiencies (PIDs) or genetic hematological diseases have an HLA-identical sibling. For them, a matched unrelated donor (MUD), although baring a greater risk of graft failure, delayed engraftment and immune reconstitution, and severe graft-versus-host disease (GvHD), represents a valid alternative. The stem cell source is also important, as unprocessed peripheral blood stem cells (PBSCs) contain 5 to 10 times more T cells than bone marrow (BM)-derived grafts, a major risk especially for small children with PID. A CD34+ positive selection can mitigate HLA compatibility issues, but the resulting CD3+ T cell depletion hampers engraftment and facilitates infections. To mitigate those problems, we decided to add back a certain number of T cells (30×106 cells/kg body weight [BW]) to the positive CD34+ selection derived from MUD BM or PBSCs and report the results in terms of time to engraftment and immune reconstitution, GvHD incidence, infections, and survival. Our aim was to show not only the feasibility and clinical efficacy of this addback but also that PBSC-derived CD34+ selected grafts with calibrated T cell addback would be equivalent to BM-derived grafts. We analyzed retrospectively our single-center cohort of 76 children (median age, 1.9 years) affected by PID (61) and hematological diseases (15) who received a total of 79 MUD HSCTs with CD34+ selection and addback of 30×106 CD3+ cells/kg BW between 2001 and 2019. We used descriptive and analytic statistics (chi-square, Student's t-test, Mann-Whitney U test, as appropriate) and constructed Kaplan-Meier curves using the log-rank test to compare patients grafted with BM or PBSC-derived inocula. The two groups showed no statistically significant differences in terms of age, sex, HLA-mismatch, or amount of CD3+ cells/kg BW added back to the CD34+ selection. However, the latter being higher in the PBSC group (P=.0001). Overall engraftment rate was 96% (73/76) and occurred faster in the PBSC group than in BM recipients: polymorphonuclear cells, 16 versus 21 days (P=.006); platelets, 15 versus 22 days (P=.001). GvHD incidence was low. No acute GvHD was diagnosed in 24 children, whereas grades I, II, III, and IV occurred in 19, 28, five, and three children, respectively (P not significant). Chronic GvHD was seen in only two children. The CD4+ count at six months after HSCT was higher in PBSC recipients as compared to those receiving BM (184 versus 88 CD4+ cells; P=.003). Overall survival for the whole cohort was 80% at 10 years, with no significant difference between the two stem cell sources (P not significant). Viral infections occurred among five of the PBSC grafted children and 14 in the BM group (P not significant), and no patient suffered from post-transplant lymphoproliferative disorder (PTLD). The results we present show that an addback of 30×106 donor CD3+ cells/kg recipient BW to a MUD BM or PBSC-derived CD34+ selection gives promising results in infants and young children undergoing HSCT for PID or hematological diseases. Furthermore, with this manipulation the inherent limits of PBSC-derived grafts can be overcome, allowing both swift engraftment and immune reconstitution without an increase in GvHD, infections, or PTLD.

Abatacept is effective as GVHD prophylaxis in unrelated donor stem cell transplantation for children with severe sickle cell disease

AbstractWe report results of a phase 1 multicenter stem cell transplantation (SCT) trial from HLA-matched (n = 7) or one-antigen–mismatched (n = 7) unrelated donors (URD) using bone marrow or cord blood as stem cell source, following reduced-intensity conditioning (RIC) in severe sickle cell disease (SCD). Conditioning included distal alemtuzumab, fludarabine, and melphalan (matched donors), with thiotepa (mismatched donors). Abatacept, a selective inhibitor of T cell costimulation, was added to tacrolimus and methotrexate as graft-versus-host disease (GVHD) prophylaxis to offset GVHD risks, and was administered for longer duration in bone marrow recipients than in cord blood recipients because of increased incidence of chronic GVHD with bone marrow. Median age at transplant was 13 years (range, 7-21 years). The incidence of grades II to IV and grades III to IV acute GVHD at day +100 was 28.6% and 7%, respectively. One-year incidence of chronic GVHD was 57% and mild/limited in all but 1 patient who received abatacept for a longer duration. Only 1 patient developed reversible posterior encephalopathy syndrome and recovered. With a median follow-up of 1.6 years (range, 1-5.5 years), the 2-year overall and disease-free survival was 100% and 92.9%, respectively. The encouraging results from the phase 1 portion of this RIC SCT trial, despite risk factors such as older age, URD, and HLA-mismatch, support further evaluation of URD SCT in clinical trial settings. The phase 2 portion of the trial is in progress. This trial was registered atwww.clinicaltrials.govas NCT03128996.

Experimental Competitive Bone Marrow Transplant Assays.

Hematopoietic stem cells have the ability to produce all blood cells. When hematological malignancies occur, transplant of compatible blood or bone marrow cells from a healthy donor to the patient is an efficient solution to restore normal hematopoiesis. Bone marrow transplant in a mouse model is often used to study HSC function and capacity to repopulate an irradiated recipient. This protocol details the different steps of a competitive bone marrow transplant experiment, beginning with total body irradiation of the recipient mice; preparation and administration of the donor and competitor bone marrow samples; peripheral blood analysis to follow reconstitution posttransplant; and finally, the analysis of recipient bone marrow and secondary transplants to evaluate long-term HSC function. Different formulas used to establish transplant efficiency are explained. All the steps are discussed in detail, including tips, variations, and alternative procedures with their advantages and disadvantages.

Single Agent CD45-Targeted Antibody Drug Conjugate Enables Full Mismatch Allogeneic Hematopoietic Stem Cell Transplantation in a Murine HSCT Model

Introduction Allogeneic hematopoietic stem cell transplant (Allo-HSCT) is a potentially curative treatment for malignant and non-malignant blood disorders. However, current conditioning regimens limit the use of this curative procedure in many eligible patients due to regimen-related mortality and morbidities, including organ toxicity, infertility, and secondary malignancies. We are developing novel antibody drug conjugates (ADC) as conditioning agents that can achieve full myeloablation as a single agent that may reduce toxicity associated with current conditioning regimens. We have generated an anti-murine ADC targeting CD45 and assessed its effectiveness as single agent conditioning regimen in a fully allogeneic murine HSCT model. Methods Our tool CD45 ADC is engineered for rapid clearance (t1/2=1.7hr) to enable HSCT after conditioning. A single dose of 3 mg/kg is fully myeloablative in C57BL/6 mice. To determine if the tool CD45-ADC could successfully condition recipients for fully mismatched allo-HSCT, we evaluated the ability of a single dose of 5 mg/kg of the tool CD45-ADC to condition C57BL/6 hosts (H-2b, CD45.2+) for transplant with cells from CByJ.SJL(B6) donors (H-2d, CD45.1+). A matched dose of an isotype ADC (Iso-ADC) was used as a negative control, while 9 Gy TBI was used as a conventional conditioning positive control. Conditioned mice were transplanted with 4x107 whole BM cells, and peripheral blood chimerism was assessed over 22 weeks. At 22 weeks, donor hematopoietic cell chimerism was evaluated in the spleen, bone marrow, and thymus of recipients. Results In the fully mismatched Balb/c → C57Bl/6 allo-HSCT model, conditioning with a single dose of 5 mg/kg of CD45-ADC as a single agent was well tolerated and enabled full allogeneic donor chimerism (n=2 separate experiments). Peripheral blood chimerism was observed in mice conditioned with CD45-ADC at week 4 and maintained through week 22 (Figure 1). Multilineage reconstitution was observed in the T-, B-, and myeloid cell compartments with &amp;gt;90% donor chimerism seen in each compartment, indicative of HSC engraftment. These results were comparable to chimerism seen in the 9 Gy TBI positive control. Treatment with a non-targeting isotype ADC at a matched dose was not effective (Figure 1). For all groups, stem cell chimerism in the bone marrow matched that in the periphery. Splenic and thymic donor immune cell reconstitution was similar between CD45-ADC and TBI conditioning at week 22 (Figure 1), demonstrating that CD45-ADC efficiently depletes host lymphocytes in secondary lymphoid organs while preserving the capacity of the host thymus to support de novo generation of donor-derived T cells after transplantation. Conclusion Conditioning with CD45-ADC was well-tolerated, fully myeloablative, and enabled complete chimerism in a full mismatch allo-HSCT model as a single agent. This targeted, readily translatable approach for safer conditioning could improve the risk-benefit profile for allogenic and haploidentical HSCT and may extend the curative potential of HSCT to more patients suffering from blood cancers and other diseases that may benefit from HSCT. Disclosures Hyzy: Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Proctor:Magenta Therapeutics: Current Employment. Gillard:Magenta Therapeutics: Current Employment. Hammond:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Sarma:Magenta Therapeutics: Ended employment in the past 24 months. Clark:Magenta Therapeutics: Current Employment. Bhat:Magenta Therapeutics: Current Employment. Lamothe:Magenta Therapeutics: Current Employment. Palchaudhuri:Magenta Therapeutics: Current Employment. Pearse:Magenta Therapeutics: Ended employment in the past 24 months. McDonagh:Magenta Therapeutics: Ended employment in the past 24 months. Kiem:Magenta Therapeutics: Consultancy; CSL: Consultancy; Homology Medicines: Membership on an entity's Board of Directors or advisory committees; Rocket Pharma: Membership on an entity's Board of Directors or advisory committees; Umoja: Membership on an entity's Board of Directors or advisory committees; Enochian: Membership on an entity's Board of Directors or advisory committees; Vor Biopharma: Membership on an entity's Board of Directors or advisory committees. Wagner:Rocket Pharmaceuticals, Inc.: Consultancy, Current equity holder in publicly-traded company; Novartis: Research Funding; Magenta Therapeutics: Consultancy, Research Funding; BlueRock: Research Funding; Gadeta: Membership on an entity's Board of Directors or advisory committees. Blazar:Magenta Therapeutics: Consultancy; Fate Therapeutics Inc.: Research Funding; BlueRock Therapeutics: Research Funding; Childrens' Cancer Research Fund: Research Funding; KidsFirst Fund: Research Funding; Tmunity: Other: Co-founder; BlueRock Therapeuetic: Consultancy. Boitano:Magenta Therapeutics: Ended employment in the past 24 months, Patents &amp; Royalties. Cooke:Magenta Therapeutics: Ended employment in the past 24 months, Patents &amp; Royalties. Davis:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company.

Impact of Cryopreservation of Donor Grafts on Outcomes of Allogeneic Hematopoietic Cell Transplant (HCT)

Introduction: Cryopreservation of peripheral blood stem cell (PBSC) or bone marrow (BM) grafts is rarely performed, thus information about the effect of cryopreservation on graft characteristics and HCT outcomes is limited. Given the global pandemic leading to changing practices for donor graft collection and increasing utilization of cryopreservation, we evaluated the outcomes of HCT recipients who received either fresh or cryopreserved allogeneic BM or PBSC grafts. The primary endpoint was engraftment. Secondary endpoints were incidence of acute graft-vs.-host disease (aGVHD), relapse, transplant related mortality (TRM), disease free survival (DFS), and overall survival (OS). Methods: We included 7397 patients transplanted between 2013 and 2018. 1883 cryopreserved graft recipients were divided into three cohorts based on graft source and donor type: matched related (MRD) PBSC (n=1,051), matched unrelated (MUD) PBSC (n=678), and matched related or unrelated bone marrow donors (n=154). All patients received conventional calcineurin-based GVHD prophylaxis strategies. Patients who received cryopreserved grafts were matched with 5514 patients who received fresh adjusting for graft type (BM vs. PB), donor source (MRD vs. 8/8 MUD), Disease Risk Index (DRI, low risk vs. intermediate risk vs. high risk), recipient age (within 5-years) and propensity score (within 1 standard deviation from pooled sample). The propensity score was based on age, Karnofsky score, diagnosis, disease risk index, HCT-comorbidity index, and conditioning intensity. The level of statistical significance for the main analyses was p&amp;lt;0.01 due to multiple comparisons. The reason for cryopreservation was available on a subset of URD product recipients. Results: Baseline characteristics for all cohorts are shown in Table 1. CD34+ doses reflect enumeration at infusion. In univariate analyses, rates of graft failure and neutrophil recovery at day 28 were similar for cryopreserved and fresh grafts for BM or related PBSC recipients but differed in cryopreserved vs. fresh MUD PBSC (5% vs 2%, p&amp;lt;0.001 and 87 vs 94%, p&amp;lt;0.001, respectively). Rates of platelet recovery within 28 days were similar in marrow cohorts, but lower with cryopreserved related (92% vs 96%, p&amp;lt;0.001) and MUD PBSC (87 vs 94%, p&amp;lt;0.001) grafts. In matched pair multivariable analyses, there were no significant differences in any outcomes with cryopreserved vs fresh BM grafts, irrespective of donor type. With related donor PBSC, cryopreservation was associated with decreased platelet recovery (HR=0.73, CI=0.68-0.78, p&amp;lt;0.001) and an increased risk of both grade II-IV (HR=1.27, CI=1.09-1.48, p=0.002) and grade III-IV (HR=1.48, CI=1.19-1.84, p&amp;lt;0.001) aGVHD, but had no impact on other outcomes. In contrast, with MUD PBSC grafts, cryopreservation was associated with delayed engraftment of neutrophils (HR=0.77, CI=0.71-0.84, p&amp;lt;0.001) and platelets (HR=0.61, CI=0.56-0.66, p&amp;lt;0.001), an increased risk of TRM (HR=1.4, CI=1.18-1.66, p&amp;lt;0.001) and relapse (HR=1.32, CI=1.11-1.58, p=0.002), and decreased DFS (HR=1.36, CI=1.20-1.55, p&amp;lt;0.001) and OS (HR=1.38, CI=1.22-1.58, p&amp;lt;0.001) but did not affect the incidence of aGVHD. In a subset analysis of 299 URD recipients, the most common reason for cryopreservation was delays due to patient-related events, eg., additional chemotherapy, infection. These recipients had decreased OS compared to products cryopreserved for non-patient reasons (HR=0.65, CI=0.44-0.96, p=0.029). Conclusions: The retrospective nature of this analysis, and the fact that cryopreservation is likely to have been performed in recipients with a different (higher) risk profile compared to those receiving fresh products limits our ability to draw firm confusions. Despite these limitations, we conclude that cryopreservation in some patient populations may have a negative impact on engraftment and transplant outcomes. The decision to cryopreserve donor grafts, particularly PBSC grafts, should be carefully considered and highlights the need for further investigation of the effect of cryopreservation of allogeneic grafts where patient factors can be controlled for. Disclosures Farhadfar: CSL Behring: Research Funding; Incyte pharmaceutical: Other: Member of GVHD advisory forum. Frey:Kite Pharma: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Syntax: Consultancy, Honoraria. Devine:Magenta Therapeutics: Consultancy. Shaw:Orca Bio: Consultancy. Wingard:Shire: Other: Personal Fees - serve on data monitoring committee for clinical trial; Ansun: Other: Personal Fees - serve on data monitoring committee for clinical trial; Merck: Other: Personal Fees - serve on data monitoring committee for clinical trial; Cidara: Other: Personal Fees - serve on data monitoring committee for clinical trial; ReViral: Other: Personal Fees - serve on data monitoring committee for clinical trial.

Efficacy of Remestemcel-L Treatment in Steroid Refractory Acute Graft-Versus-Host Disease in Children: An Aggregate Analysis

Introduction: Steroid-refractory acute graft-versus-host dose disease (SR-aGVHD) is a frequent and potentially fatal complication of hematopoietic stem cell transplantation (HSCT). There are no approved therapies specifically indicated for use in children with SR-aGVHD &amp;lt; 12 years old. Treatment with culture-expanded, adult human allogeneic bone marrow (BM)-derived mesenchymal stromal cells (remestemcel-L) has shown excellent safety and efficacy in clinical trials conducted over the past 10 years. Here we describe aggregate results of remestemcel-L treatment of SR-aGVHD in the pediatric population. Methods: 309 pediatric subjects with SR-aGVHD were treated with remestemcel-L across 3 protocols. Data were derived from a phase 3, single arm study as first line after steroid failure with no other second line aGVHD therapies added through day 28 (NCT02336230, n=54), an expanded access program in subjects with aGVHD that had not responded to multiple other treatments (NCT00759018, n=241) and the pediatric subset of a randomized controlled trial (NCT00366145, n=14) in which remestemcel or placebo were given with a concomitant second line aGVHD therapy. Subjects in each protocol received 8 intravenous infusions of remestemcel-L (2 x 106 cells/kg twice weekly for 4 weeks). Those achieving a partial response could receive an additional 4 doses (once weekly for the subsequent 4 weeks). This analysis included 309 subjects who received at least one remestemcel-L dose. Response was assessed 28 days after the first dose. Survival was evaluated through day 100. Results: At baseline, mean age was 8.9 years (range 0.3 to 18.2 years) and aGVHD severity by IBMTR grade was C or D in n=252 (82%). Sources of grafts were bone marrow (BM: 46%), mobilized blood (MB: 20%) and cord blood (CB: 31%). Donor grafts were HLA matched related (9%) or unrelated (44%) or HLA-mismatched (46%). Subjects received a mean (standard deviation) of 10.3 (4.2) remestemcel-L infusions for a mean cumulative dose of 660.8 (484.1) x 106 cells. The primary endpoint, overall response (complete + partial response) at day 28 (OR) was observed in 66% of subjects (204/309) and survival at day 100 (OS) was 68% (211/309). Day 28 OR strongly predicted day 100 survival. Among subjects who were alive at day 28 (N=284), 170 of the 204 (83%) responders survived through day 100. In contrast, 41/80 (51%) of non-responders survived through day 100. Day 28 OR frequencies and day 100 OS rates were consistent across baseline aGVHD grades: Grade B OR 70%, OS 77%; Grade C or D OR 65%, OS 66%. Comparable day 28 response frequencies and day 100 survival rates were observed across type of HSCT: OR was achieved in 65%, 64% and 71% of BM, MB and CB, respectively and 67%, 70% and 70% of BM, MB and CB recipients survived through day 100. Comparable OR and OS were observed in HLA compatibility and donor types with OR in 71%, 66% and 66% and day 100 OS in 64%, 66% and 71% of matched related or unrelated and mismatched, respectively. Remestemcel-L was well tolerated with no infusion-related toxicity or other safety concerns. Most treatment-emergent serious adverse events were attributable to aGVHD or the underlying pre-transplant disease. Conclusions: In this large, diverse sample of pediatric subjects with SR- aGVHD, treatment with remestemcel-L was associated with a high and consistent overall response rate and survival rate through 100 days. Day 28 OR was highly predictive of day 100 survival. Remestemcel-L was well tolerated and effective across transplant types, donor sources, moderate to severe grades of GVHD and prior or concurrent treatment paradigms as defined by the individual protocols in this aggregate analysis. Remestemcel-L is a safe and effective treatment for children with SR- aGVHD addressing an unmet medical need for this highly vulnerable population. Disclosures Kurtzberg: Mesoblast, Inc,: Consultancy, Honoraria. Martin:Abgenomics: Research Funding; Enlivex: Consultancy; Procter and Gamble: Current equity holder in publicly-traded company; Pharmacyclics LLC: Membership on an entity's Board of Directors or advisory committees; Neovii: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pfzier: Membership on an entity's Board of Directors or advisory committees; Xenikos: Research Funding; Talaris: Consultancy; Rigel: Consultancy; Mesoblast, Inc.: Consultancy; Genentech: Consultancy, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria. Prockop:Atara: Research Funding; Jasper: Research Funding; Mesoblast, Inc,: Consultancy, Honoraria, Research Funding. Burke:Mesoblast, Inc,: Current Employment. Segal:Mesoblast, Inc,: Current Employment.

Altered Macrophage Polarization Induces Experimental Pulmonary Hypertension and Is Observed in Patients With Pulmonary Arterial Hypertension.

To determine whether global reduction of CD68 (cluster of differentiation) macrophages impacts the development of experimental pulmonary arterial hypertension (PAH) and whether this reduction affects the balance of pro- and anti-inflammatory macrophages within the lung. Additionally, to determine whether there is evidence of an altered macrophage polarization in patients with PAH. Approach and Results: Macrophage reduction was induced in mice via doxycycline-induced CD68-driven cytotoxic diphtheria toxin A chain expression (macrophage low [MacLow] mice). Chimeric mice were generated using bone marrow transplant. Mice were phenotyped for PAH by echocardiography and closed chest cardiac catheterization. Murine macrophage phenotyping was performed on lungs, bone marrow-derived macrophages, and alveolar macrophages using immunohistochemical and flow cytometry. Monocyte-derived macrophages were isolated from PAH patients and healthy volunteers and polarization capacity assessed morphologically and by flow cytometry. After 6 weeks of macrophage depletion, male but not female MacLow mice developed PAH. Chimeric mice demonstrated a requirement for both MacLow bone marrow and MacLow recipient mice to cause PAH. Immunohistochemical analysis of lung sections demonstrated imbalance in M1/M2 ratio in male MacLow mice only, suggesting that this imbalance may drive the PAH phenotype. M1/M2 imbalance was also seen in male MacLow bone marrow-derived macrophages and PAH patient monocyte-derived macrophages following stimulation with doxycycline and IL (interleukin)-4, respectively. Furthermore, MacLow-derived alveolar macrophages showed characteristic differences in terms of their polarization and expression of diphtheria toxin A chain following stimulation with doxycycline. These data further highlight a sex imbalance in PAH and further implicate immune cells into this paradigm. Targeting imbalance of macrophage population may offer a future therapeutic option.

Securing the future of kidney transplantation by addressing the challenges of transplant nephrology.

Kidney transplant is a life-changing procedure, and transplant nephrologists, as part of a larger transplant team, play an important role in the field by managing the complex medical needs of transplant patients. The subspecialty of transplant nephrology, however, faces structural challenges related to its workforce, reporting structures, compensation, research and innovation, and health care information technology. The position of transplant nephrology at the academic and operational intersection of medicine and surgery may limit its access to critical resources, hinder academic promotion, and contribute to physician burnout. The authors provide an overview of the subspecialty transplant nephrology and propose solutions. Collaborative efforts that fortify the subspecialty of transplant nephrology will ultimately improve the lives of patients suffering from kidney disease.

3116 – GPRASP FAMILY MEMBERS AS NOVEL REGULATORS OF B-CELL DEVELOPMENT DURING NORMAL AND MALIGNANT HEMATOPOIESIS.

In our search for novel regulators of hematopoietic stem cell transplantation (HSCT), we recently reported Gprasp1 and Gprasp2 as negative regulators of HSCT. These genes encode proteins that are necessary for the stability, cellular trafficking and the proper degradation of CXCR4, a master regulator of hematopoiesis. Consequently, the absence of either Gprasp1 or Gprasp2 causes increased sensitivity to SDF-1 chemoattractant effect, enhancing HSCs repopulating activity after transplant. Strikingly, although Gprasp gene knockdown enhanced the repopulating activity of HSC at early time-points post-transplant, we observed that a significant number of mice transplanted with Gprasp-deficient hematopoietic progenitors developed an immature B-cell hematologic malignancy, typically at >20 weeks post-transplant. Since CXCR4 plays a crucial role in B-cell differentiation and, more specifically, in the movement of immature B-cells between the light and dark zones of the germinal center, we hypothesize that GPRASP family members are key regulators of CXCR4 during B-cell development during normal and malignant hematopoiesis. Thus, we followed mice transplanted with hematopoietic progenitors treated with control, Gprasp1 or Gprasp2-shRNAs for up to one year post-transplant. The onset of B-cell malignancy coincided with an accumulation of bone marrow common lymphoid progenitors, a complete disruption of the immunophenotype of B-cell progenitors and a loss of mature B-cells in the peripheral blood. Transformed cells invaded bone marrow, spleen, and lymph nodes in primary recipients. 100% of secondary recipients of leukemic cells developed a phenotypically identical malignancy <6 weeks post-transplant. As the dark zone of the geminal center is a region where somatic hypermutation is naturally exacerbated, we hypothesize that Gprasp-deficient B-cell progenitors are arrested in an immature state in the germinal center, increasing the likelihood of malignant transformation.