The Determinative Role of Hematopoietic Stem and Progenitor Cell Graft Composition on the Engraftment Kinetics after HSCT

The importance of hematopoietic stem and progenitor cell (HSPC) dose in the outcome of hematopoietic stem cell transplant (HSCT) has been demonstrated by analyses of the threshold dose of CD34+ cells required to achieve donor engraftment, usually defined as endpoints of donor-derived neutrophil (PMN) or platelet (PLT) absolute numbers, and RBC transfusion independence. We recently described the heterogeneous HSPC composition of prospectively obtained umbilical cord blood samples and lack of correlation between CD34+ cell dose and the frequency of hematopoietic stem cells (HSC)(Mantri S et al, Blood Adv 2020). We describe here a pilot analysis of the relationship between HSPC graft composition and engraftment after HSCT. The study population is comprised of 17 children (3.4-22 years, median 13 years) treated with αβT/CD19B - cell depleted mobilized peripheral blood stem cell (PBSC) HSCT (αβhaplo-HSCT). Eight patients had acute lymphoblastic leukemia, seven had acute myeloid leukemia, and two myelodysplastic syndrome. All patients received serotherapy with Thymoglobulin and Rituximab, and a myeloablative conditioning consisting of either TBI 1200 cGy in combination with Fludarabine/Thiotepa (15 patients), Melphalan/Thiotepa (1 patient), or TBI (400 cGy) with Fludarabine/Thiotepa (1 patient receiving a second HSCT). Only 2/17 patients received 1-2 doses of G-CSF before Day+30. The CD34+ Lin- cells in the PBSC products were analyzed for HSPC composition, including HSC (CD38- CD90+ CD45RA-), Multipotent Progenitors (MPP, CD38- CD90- CD45RA-), Common Myeloid Progenitors (CMP, CD38+ CD123+ CD45RA-), Granulocyte-Monocyte Progenitors (GMP, CD38+ CD123+ CD45RA+), Megakaryocyte-Erythroid Progenitors (MEP, CD38+ CD123- CD45RA-) and Common Lymphoid Progenitors (CLP, CD38+ CD127+). Similar HSPC analyses were performed on marrow obtained at Days +30, +60, and +90 post-HSCT. The CD34+ cell dose in the grafts ranged from 8.5-40 x 10e6/kg recipient body weight (mean 15.6). The median time to Absolute Neutrophil Count (ANC) > 500 or 1000/mm 3 were Days +12 and +14, respectively, and for PLT > 50K or 100K/mm 3 Days +14 and +15, respectively. The time to either PMN or PLT engraftment did not correlate with the HSC dose. In contrast, the GMP dose was predictive of PMN engraftment: 7/8 patients receiving the highest GMP dose achieved ANC > 500 at 0-3 days before the median day of engraftment, while PMN engraftment was delayed by 1-5 days beyond the median in 6 of the 9 receiving the lowest GMP dose (χ 2 = 8.14, p=0.004)(Fig A). Of the patients with the highest MEP dose, 7/8 achieved PLT >50K/mm 3 0-4 days before the median, while 5/9 receiving the lowest dose engrafted 1-6 days beyond the median (χ 2 4.9, p=0.026) (Fig B). In the first 100 days post-HSCT, naïve CD4+ T-cells were all CD31+ CD45RA+ Recent Thymic Emigrants (RTE), indicating that they were newly produced T cells and not adoptively transferred from the αβhaplo-HSCT. The HSC dose did not correlate with the number of naïve CD4+ T cells until Day+180 (Fig C). Like PMN and PLT recovery, early T lymphoid recovery after HSCT was mainly derived from infused CLP, and likely switched to HSC-derived lymphopoiesis by Day +180. Consistent with this concept, HSC dose in the PBSC products was unrelated to the numbers of committed progenitors, e.g., CMP, MEP, CLP, in early (Day +30) post-transplant marrow samples. These data are consistent with clonal analyses of patients receiving lentiviral gene therapy and murine experiments demonstrating prolonged steady-state contribution of committed progenitors to peripheral blood cell maintenance (Biasco L et al, Cell Stem Cell 2016; Sun J et al, Nature 2014). While post-HSCT PMN or PLT numbers are frequently equated with "HSC engraftment" and naïve T-cell numbers with HSC-derived immune reconstitution, these early events reflect blood cell production by committed progenitors, which are variably present in the grafts. Although CD34+ cell dose is currently used to predict post-transplant engraftment and to qualify stem cell products for release, more accurate clinical predictions may be determined by the HSPC grafts' composition. Further, engineering of the progenitor composition of clinical HSCT products, e.g., co-transplantation of additional committed progenitors like GMP or CLP, could be strategically used to control post-transplant lymphohematopoietic recovery. Figure 1 Figure 1. Disclosures Parkman: Jasper Biotech: Consultancy. Bertaina: Cellevolve Bio: Membership on an entity's Board of Directors or advisory committees; Neovii: Membership on an entity's Board of Directors or advisory committees; AdicetBio: Membership on an entity's Board of Directors or advisory committees.

Heterogeneity of Hematopoietic Stem and Progenitor Cell (HSPC) Composition in Αβ T-Cell/CD19 B-Cell Depleted Peripheral Blood Cell Stem Cell (PBSC) Transplant Grafts and Correlation with Immune and Hematopoietic Recovery

INTRODUCTION. While total CD34 counts in PBSC graft products have correlated with overall likelihood of hematopoietic recovery after allogeneic hematopoietic stem cell transplantation (HSCT), analyses of the HSPC composition and its relationship to relevant post-transplant clinical outcomes are lacking. In fact, the biological basis for different dynamics of hematopoietic/immune recovery, the risk of infection, and graft-versus-host disease (GvHD) is not fully understood. We have performed the first analysis of HSPC graft composition in 6 αβ T-cell/CD19 B-cell depleted haploidentical (αβhaplo) HSCT. Additionally, we correlated the HSPC graft composition with the distribution of the same HSPC subsets in serial post-HSCT bone marrow aspirates performed at Days 30, 60, and 90, with the peripheral blood counts [neutrophils, monocytes, platelet (PLT)] and with the immune recovery (CD3+, CD3+CD4+, CD3+CD8+, αβT, γδT, NK cells) at the same time points. The patients were divided in two groups: 3 patients had a robust and sustained hematopoietic recovery (Group 1) while 3 patients experienced mild cytopenia after Day 60 (Group 2). All patients were transplanted for acute leukemia and received a myeloablative TBI-based conditioning regimen. See Table 1 for details about patients and graft composition. METHODS. All patients were enrolled in the Stanford IRB approved BMT Protocols 179/351/361 and had peripheral blood (PB) and bone marrow (BM) evaluated at Day 30, 60 and 90 post HSCT for the primitive CD34+ Lin- HSPC subsets: HSC (CD38-CD45RA-CD90+), MPP (CD38+CD45RA+), CMP (CD38+CD45RA-CD123+), GMP (CD38+CD45RA+CD123+), MEP (CD38+CD45RA-CD123+) and CLP (CD38+CD127+). Aliquots of αβhaplo-HSCT grafts were cryopreserved for later analyses. Mononuclear cells were isolated from PBSC, PB and BM by Ficoll-Hypaque (Sigma-Aldrich) density gradient centrifugation. FACS analyses were performed on either fresh or frozen cells on Becton Dickinson (BD) Aria II flow cytometer. At least 5x105 events were acquired and analyzed using FlowJo software (BD). RESULTS. Despite consistent levels of αβT-cell depletion and CD34 enrichment, the frequency of the HSPC subsets varied between the grafts. Notably, while CMP and GMP were very consistent across the 6 grafts, the frequency of HSC, CLP, MEP and MPP showed a 2-fold range of variation (Fig1A). No significant correlation was observed between the HSC frequency in the graft and the hematopoietic/immune recovery. However, the frequency of HSC in the BM at Day 30 is statistically correlated (P=0.027) with the PLT at Day 90 (Fig1B). In these preliminary results, the different distribution of CMP, GMP, MEP and MPP did not impact on the hematopoietic/immune recovery. However, there was a significant correlation (P=0.02) between CLP and γδ T cells reconstitution at Day 90 in Group 1 patients (Fig1C). Additionally, the neutrophils, monocytes and phagocytic cells recovery at Day 90 is statistically correlated with the GMP frequency in the BM at Day 30 (P=0.017; P=0.018; P=0.0132, respectively) (Fig1D). Interestingly, the same strong correlation is observed between the CMP in the BM at Day 60 and the recovery of neutrophils and phagocytic cells (P=0.016, P=0.019) at Day 90 (Fig1E), but the CMP at day 30 are already predictive of a robust engraftment in the Group 1 patients (data not shown). CONCLUSIONS. In αβhaplo-HSCT, previously identified factors influencing hematopoietic recovery have been mainly limited to the enumeration of bulk CD34 counts and of mature effector cells, such as αβ/γδ T and NK cells. On the other hand, the presence of GvHD and thymic injury have been correlated to the kinetics of immune reconstitution. We hypothesized that the HSPC composition of the graft would impact lymphohematopoietic recovery in αβhaplo-HSCT recipients. Although preliminary, our data indicate that even with a consistent method of graft manipulation, the HSPC graft composition is heterogeneous. Variations in HSPC subsets frequency and number can contribute to significant differences in lymphohematopoietic recovery and, therefore, clinical outcome. The evaluation of a larger number of patients with longer follow up after HSCT are required. Comparative studies with unmanipulated T-cell replete and cord blood grafts are ongoing. Such analyses will be instrumental not only for prediction of clinical outcome, but also for optimization of novel graft engineering strategies. Disclosures No relevant conflicts of interest to declare.

Impact of Daratumumab on Stem Cell Collection, Graft Composition and Engraftment Among Multiple Myeloma Patients Undergoing Autologous Stem Cell Transplant

Background High dose chemotherapy followed by autologous stem cell transplantation (ASCT) plays an important role in the treatment of transplant-eligible multiple myeloma (MM) patients and yields deep responses, prolongs progression-free survival (PFS) and overall survival (OS). Daratumumab (Dara), a humanized monoclonal antibody that binds to CD38+ on the surface of myeloma cells and is increasingly used upfront to treat MM. Up to 75% of mobilized CD34+ hematopoietic progenitors also express CD38 and, hence exposure to Dara may potentially impact stem cell mobilization, and, given the extended half-life of Dara, also impair engraftment. The Cassiopeia trial (Moreau, P. 2019) showed the utility of Dara in combination with bortezomib and thalidomide as upfront treatment for MM without a negative impact on stem cell collection yield. Similarly, the Griffin trial (Voorhees, PM, 2020) demonstrated the safety of upfront Dara when combined with bortezomib and lenalidomide also with no impact on stem cell collection or engraftment. However, others(Al Saleh, A. 2019) reported delayed neutrophil engraftment in patients treated with Dara. Given the conflicting findings of these studies and scarcity of data on the direct impact of Dara on erythroid and myeloid progenitors, we investigated the effect of Dara on stem cell collection yield, graft composition and time to engraftment among patients who underwent ASCT. Methods We evaluated all patients with MM who underwent ASCT at our institute between 2017-2020 and excluded patients undergoing 2nd transplant or tandem ASCT. Disease risk stratification was assessed as per International Staging System (ISS) for MM. Treatment response prior to transplant was assessed as per response criteria definitions by International Myeloma Working Group (IMWG). Stem cells were collected according to institutional protocol and prior to cryopreservation, 1x105 peripheral blood cells are plated in duplicate in semi-solid media- MethoCult™ H4434 (Stem Cell Technologies, Vancouver, BC) and cultured in 37°C, 5% CO2 following which CFU-GM (colony forming unit- granulocyte macrophage) and BFU-E (burst forming units- erythroid) colonies are scored on day 14 based on morphological recognition, and reported as 104 colonies per kg of recipient weight. CFU-C (colony forming unit combined) is calculated by combining BFU-E and CFU-GM. Results Patients (N=108) that underwent ASCT between 2017-2020 were identified and demographic data are summarized (Table 1). Sixteen patients received Dara as part of upfront treatment prior to stem cell collection. Median age was 62 years old for the entire group with no significant age difference between patients who received Dara vs. those who did not receive. Similarly, there was no difference in race, ISS stage, pre-transplant response, plerixafor or lenalidomide use between the groups. Pts who received Dara were more likely to have received more than one chemotherapy regimen prior to transplant (62.5% vs. 30%, p-value=0.014). Although, total and day 1 collection of CD34+ stem cells was lower in the Dara treated group (7.18 x 106/kg) compared to Dara untreated (8.78 x 106/kg), the difference was not statistically significant. (p-value=0.151). Stem cell product composition based on measurement of BFU-E, CFU-GM and CFU-C colonies were similar independent of Dara use. Prior exposure to Dara was also not predictive of day 1 stem cell collection failure (defined as < 5x106 CD34+ cells/kg) as determined by multivariate analysis. Median time to absolute neutrophil count (ANC) recovery (defined as ANC >1500) was 12 days in both groups (p-value=0.09). Median time to platelet recovery (defined as platelet count >20k) was 12 days in the Darauntreated group vs. 13 days Dara treated group (p-value=0.06). Discussion In this single institute experience, there was a trend towards lower CD34+ collection as well as lower progenitor scoring with Dara use but was not statistically significant and also there was no difference in time to ANC or platelet recovery. The majority of patients in this study received plerixafor for mobilization, which might abrogate the effect of Dara and lenalidomide on the graft and stem cell collection. Further studies to investigate the impact of Dara on CD38+ mobilized hematopoietic cells is warranted. Disclosures Manjappa: Pfizer: Research Funding. Caimi:Amgen: Other: Advisory Board; Bayer: Other: Advisory Board; Verastem: Other: Advisory Board; Celgene: Speakers Bureau; Kite pharmaceuticals: Other: Advisory Board; ADC therapeutics: Other: Advisory Board, Research Funding. de Lima:BMS: Other: Personal Fees, advisory board; Incyte: Other: Personal Fees, advisory board; Kadmon: Other: Personal Fees, Advisory board; Celgene: Research Funding; Pfizer: Other: Personal fees, advisory board, Research Funding. Malek:Clegene: Other: Advisory board , Speakers Bureau; Janssen: Other: Advisory board, Speakers Bureau; Sanofi: Other: Advisory board; Amgen: Honoraria; Medpacto: Research Funding; Cumberland: Research Funding; Bluespark: Research Funding; Takeda: Other: Advisory board , Speakers Bureau.