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Background: Chimeric antigen receptor T-cell (CAR-T) therapy has revolutionized the treatment of relapsed/refractory B-cell lymphoid malignancies. Axicabtagene ciloleucel (axi-cel), an anti-CD19 CAR-T not only targets the malignant B-cell, but can potentially also target and eliminate normal B-cells. This can interfere with the normal B-cell repertoire, compromising host humoral immunity such as decreased titers of common vaccines (DTaP, MMR). To assess if this was a clinically significant problem, we evaluated all recipients of axi-cel at our center between 6/2018 through 7/2019. Patients and methods: For patients who received commercial axi-cel, humoral immunity was evaluated both quantitatively [absolute lymphocyte count (ALC)] as well as qualitatively by serology titers [IgG antibodies (Abs)] for diphtheria, tetanus, pertussis, measles, mumps, and rubella, and total Immunoglobulin G (IgG) levels. Data was collected within 30 days prior to CAR-T infusion, then at day +30 and between days +60 and +100 after CAR-T infusion. Results: We identified 10 patients (males = 5, 50%), with a median age of 49.9 years (range 30-65) who received commercial axi-cel during the study period. Patient characteristics and indications for CAR-T therapy are shown in Table 1; the cohort represented a heavily pre-treated aggressive B-cell lymphoma patient population. Baseline information on antibody (Ab) titers was available in 8 patients. At baseline, all patients had positive tetanus IgG Abs (≥0.01 IU/mL), 7 had positive diphtheria IgG Abs (≥0.01 IU/mL), 6 had positive measles IgG Abs (≥ 1.1 AI), 5 had positive rubella IgG Abs (≥ 1.0 AI), 3 had positive mumps IgG Abs (≥ 1.1 AI). None of the patients had a positive pertussis IgG Abs (≥100 IU/mL). At follow-up, all patients with positive Ab at baseline maintained titers in the positive range at day +30 and between days +60 and +100 (Figure 1). None of the patients demonstrated a clinically meaningful decrease in Abs titers, despite a drop in ALC and IgG levels (table 2). Conclusions: Albeit a small sample size, IgG Ab titers for diphtheria, tetanus, measles, mumps, and rubella did not appear to be affected by axi-cel at a short interval follow-up after infusion (up to day +100). We plan to extend this analysis in a larger cohort with a longer-term prospective follow-up to validate our findings, especially in light of dropping absolute lymphocyte counts and IgG levels. Disclosures Ailawadhi: Celgene: Consultancy; Amgen: Consultancy, Research Funding; Cellectar: Research Funding; Janssen: Consultancy, Research Funding; Takeda: Consultancy; Pharmacyclics: Research Funding. Foran:Agios: Honoraria, Research Funding. Tun:Celgene: Research Funding; Curis: Research Funding; TG Therapeutics: Research Funding; Mundi-pharma: Research Funding; DTRM Biopharma: Research Funding; BMS: Research Funding. Chanan-Khan:Pharmacyclics: Research Funding; Merck: Research Funding; Jansen: Research Funding; Mayo Clinic: Employment; Ascentage: Research Funding; Millennium: Research Funding; AbbVie: Research Funding; Xencor: Research Funding. Kharfan-Dabaja:Daiichi Sankyo: Consultancy; Pharmacyclics: Consultancy.

Abstract Background Umbilical cord blood (UCB) is an important alternative source of stem cells for patients who lack matched adult donors, particularly in ethnic minorities. The infused total nucleated cell (TNC) and CD34+ cell dose are important prognostic factors on transplant outcomes. UCB units collected from African Americans (AA) have lower TNC and CD34+ cell counts and more likely to be disqualified for banking compared to other racial and ethnic groups. Furthermore, AA, including pregnant women, have increased prevalence of vitamin D deficiency. Vitamin D has an established role in hematopoiesis; therefore, we investigated the racial differences in UCB vitamin D content and its correlation with UCB cell composition and hematopoietic potential. Methods 119 UCB units that did not meet the TNC count banking criteria were analyzed. 51 UCB units were collected from AA mothers and 68 from Caucasian mothers. We analyzed UCB variables including volume, hematocrit (HCT), TNCs, mononucleated cells (MNC), CD34+ cells, plasma 25-Hydroxy vitamin D [25(OH)D] and in vitro progenitor cell hematopoietic potential measured by Colony-Forming Cell (CFC) assay. Results The median values of 25(OH)D in UCB units were significantly lower in AA compare to Caucasians (p= <.0001). Severe vitamin D deficiency < 10 ng/mL was exclusively detected in UCB units from AA (19/51; 37%) and plasma 25(OH)D concentration < 20 ng/mL was more prevalent in AA (43/51 units; 84%), compare to Caucasians (15/68 units; 22%). UCB units from AA had lower absolute numbers of TNC (p= .0016), MNC (p= .0261), and CD34+ cells (p= .0257) compare to Caucasians. No difference in median CFC/10,000 MNC was detected between AA and Caucasians (p= 0.661) (Table 1). Univariate analysis of 25(OH)D effect on UCB variables revealed a correlation between 25(OH)D level and TNC (r= 0.193, 95% CI 0.013-0.36; p= .0353) and UCB HCT (r= 0.196, 95% CI 0.016-0.363; p= .0327). A significantly higher MNC count was noted when 25(OH)D concentration was ≥ 20ng/mL (4.5x 108, range (1.7-9.5) vs. 3.9x 108, range (0.8-9); p= .0329). This correlation between 25(OH)D level and TNC and MNC was not detected after adjustment for race (Figure 1). However, 25(OH)D concentration ≥ 20 ng/mL significantly correlated with CBU HCT in AA race (median 31.9% (24-42) vs. 27.8%, range (21.4 - 40); p= .0124). Conclusion 25(OH)D level, TNC and HCT are all significantly lower in UCB units from AA compare to Caucasians. Independent of race, 25(OH)D directly correlated with TNC and MNC. These data suggest vitamin D and other yet to be determined factors play an important role in the mechanism of low UCB cell and progenitor counts. Disclosures: No relevant conflicts of interest to declare.

Abstract Abstract 349 Failure to collect enough CD34+ cells following poor hematopoietic progenitor cell (HPC) mobilization is generally thought to be due to marrow damage following previous therapies. However, marrow damage alone cannot account for all poor mobilizations since there are donors who have good marrow cellularity but fail to adequately mobilize. In addition, the relationship between CD34+ cell dose and engraftment rate is not linear, suggesting other factors play a role during engraftment. Previous animal studies suggest CD8+ T cells are important for CD34+ cell homing to the marrow. We hypothesized that CD8+ T cells have dual roles by inhibiting CD34+ cell mobilization and facilitating engraftment. To test our hypothesis, we followed 192 autologous HPC donors to assess the role of CD8+ T cells in HPC mobilization and engraftment. The median donor age was 58.5 years (range 18 – 76), and 110 of the 192 patients were males. Donors were diagnosed with plasma cell disease, primarily multiple myeloma, or non-Hodgkin's lymphoma. CD34+ and CD8+ T cell content in the patients' apheresis products were enumerated using flow cytometry, quantitative RT-PCR (qPCR) and immunohistochemistry. The median CD34+ cell/kg was 4.7 × 106. Patients were categorized as low, moderate or high mobilizers according to their total CD34+ cell collection quartile distribution: <3.7, 3.8 – 5.5 and ≥5.6 × 106/Kg respectively. We found that HPC products from low mobilizers contained significantly more CD8+ T cells (median 1.25 × 108/Kg) than HPC products from moderate and high mobilizers (Table 1 p = 0.04). Therefore, the ratio of CD8+ T cells to CD34+ cells was 4 – 8 folds higher in low mobilizers than in moderate and high mobilizers. Moreover, CD8 antigen expression measured using qPCR, normalized with 18s ribosomal RNA, was significantly higher in low mobilizers than in moderate or high mobilizers. Similar analysis with CD4+ T cells did not demonstrate significant correlation with mobilization. To assess the role of CD8+ T cells in HPC mobilization, we purified CD8+ T and CD34+ cells from both low and moderate or high mobilizers. Using chemotaxis assay, we assessed the effect of CD8+ T cells on CD34+ cell migration away from high stromal cell derived factor (SDF1-a) concentrations. Purified CD34+ cells were plated in bone marrow epithelial cell coated trans-wells and co-cultured with purified CD8+ T cells at 1:2 and 1:5 ratios in the presence of SDF-1a at 0, 200 and 400ng/ml concentrations. Cells that migrated through the micro-pores of the trans-well into the lower wells were enumerated and analyzed. Our studies showed CD8+ T cells inhibit migration of CD34+ cells in-vitro independent of SDF1-a concentration. (Figure 1) The inhibitory properties of CD8+ T cells were more pronounced on CD34+ cells from low mobilizers than on CD34+ cells from moderate or high mobilizers. We also assessed if CD8+ cell content was associated with days to neutrophil engraftment. Our analysis showed the higher the CD8+ T cell content per unit CD34+ cell, the faster the rate of neutrophil engraftment. In summary, HPC products from low mobilizers contained significantly higher numbers of CD8+ T cells. CD8+ T cells, independent of SDF1-a concentration, significantly inhibited migration of purified CD34+ cells. In addition, CD8+ T cell content per unit CD34+ cell was inversely proportional to the time to neutrophil engraftment. These findings suggest CD8+ T cells may play dual roles by inhibiting CD34+ cell mobilization and facilitating its homing and engraftment. Table 1 Association of CD8+ T cell number with CD34+ cell mobilization capacity Mobilization Quartiles Low Moderate High p-value Median CD34+ Cell × 106/Kg 3.03 4.55 6.73 <0.001 Median CD8+ T cell × 108/Kg 1.25 0.50 0.33 0.04 CD8+ T: CD34+ cell ratio 41 11 5 Figure 1 Figure 1. Disclosures: No relevant conflicts of interest to declare.

Abstract Abstract 1184 Lenalidomide, a second generation immunomodulatory derivative of thalidomide, has demonstrated significant clinical benefit in myelodysplastic syndrome, multiple myeloma and B-cell malignancies. Its side effects include myelosuppression, particularly neutropenia, and impaired filgrastim induced stem cell mobilization in patients with myeloma. The precise mechanism of action of lenalidomide on hematopoietic and bone marrow microenvironment remains largely unknown. In this study, we investigated the effect of lenalidomide on survival, lineage commitment and differentiation of normal purified CD34+ hematopoietic stem cells. Mobilized peripheral blood progenitor cells (PBPCs) were obtained from a normal allogeneic stem cell donor with an informed consent, and were subjected to positive selection with the EasySep magnetic cell selection system to purify CD34+ cells (purity of 95%). Unsorted and CD34- accessory cells were used as controls. To assess the direct cytotoxic and proliferative effect of lenalidomide on purified CD34+ cells, lenalidomide (Celgene, NJ) was dissolved in DMSO and was diluted in culture medium to final concentration of 0.1, 1, 10, or 100 μM. Purified CD34+, CD34- and unsorted cells were plated in triplicates at a density of 2×104/mL in serum-free StemSpan® medium supplemented with growth factors and cytokines. 24 hours after plating the cells, lenalidomide or 0.1% DMSO as control was added. 72 hours following treatment, cells were counted with Beckman coulter counter and phenotypically analyzed using Accuri C6 flow-cytometer. The percentage of viable cells was determined using 7AAD staining. While no difference in viability was noted in the treated vs. untreated purified CD34+ cells, the viability of CD34- cells and unsorted cells was reduced to 30% and 40% respectively by lenalidomide. Contrary to our assumption, lenalidomide, in a dose dependant manner, induced proliferation of purified CD34+ cells. Maximal increase in proliferation was observed between 1mM and 10mM concentrations. The rate of growth plateaued beyond the 10mM concentration and 100mM concentration appeared to have no additional effect. At the peak of growth (10mM), the cell count nearly tripled. (Fig. 1) Lenalidomide appeared to be cytotoxic to CD34- and unsorted cells in a dose dependant manner. At 100mM concentration, the CD34- cell proliferation dropped by 4 fold and the unsorted cells by 7 fold. To examine the effect of lenalidomide on the clonogenic potential of purified CD34+ cells, 2,000 cells were seeded onto MethoCult® media (Stem cell technologies) supplemented with growth factors and cytokines. Cells were cultured in the presence of lenalidomide (0.1, 1, 10 and 100 μM) or 0.1 % DMSO as control. BFU-E, CFU-GM and CFU-mix were scored on day 14 with the use of an inverted microscope and standard morphological criteria. Significance of difference between groups was determined by student's t test for paired samples. Lenalidomide induced a significant increase in clonogenic activity of CD34+ cells (148 ± 27 colonies per 103 CD34+ cells, p = 0.01), most evident at the dose of 10 μM with enhanced formation of BFU-E (102 ± 23/ per 103 CD34+ cells, p = 0.01) but demonstrated no significant impact on CFU-GM (36 ± 14 per 103 CD34+ cells, p = 0.83) compared to control. (Fig. 2) This study demonstrates that lenalidomide has pronounced effects on both CD34+ and CD34- components of mobilized PBPCs. It enhances the proliferative and clonogenic potentials of CD34+ cells with lineage commitment towards erythroid differentiation. It also has cytotoxic activity towards CD34- cells. It is likely that neutropenia and impaired mobilization of stem cells in patients treated with lenalidomide is mediated via its impact on CD34- accessory cells in the bone marrow microenvironment. Further studies exploring the impact of lenalidomide on normal bone marrow stromal cells, T and NK cell mediated signaling are needed. Disclosures: No relevant conflicts of interest to declare.

Mesenchymal stem cells (MSCs) are becoming an increasingly popular therapeutic option among patients with a broad range of ailments to modulate immunity and induce regeneration. The majority of patients receiving these MSC therapies are on concurrent medication or have ongoing infection. In the present study, we examined the effect of immunosuppressive drugs and lipopolysaccharides (LPS)/endotoxins on the secretory profile, migration towards site of injury, and suppression of lymphocyte proliferation of bone marrow-derived MSCs (BMSCs). Generally, LPS coculture augmented the secretory capacity of BMSCs while exposure to immunosuppressive drugs resulted primarily in no change or attenuated secretion, with some cases of increased secretion, dependent on the cytokine assayed. Among the immunosuppressants evaluated, Hydrocortisone had the most widespread inhibitory effect, while LPS from E. coli O111:B4 had the most potent stimulatory effect. In addition, we also showed that Hydrocortisone or LPS from E. coli O111:B4 affected the migratory and immunosuppressive capacity of BMSCs. Following simulation with Hydrocortisone, BMSC migration was attenuated, and immunosuppressive capacity against T cell proliferation was enhanced, however, the opposite effects were seen with LPS from E. coli O111:B4. Our data suggests that the clinical outcomes of MSC-based therapy are affected by the use of immunosuppressive medication or the presence of endotoxemia in patients.

AbstractBackgroundMobilization regimen choice is a significant contributing factor for successful hematopoietic progenitor cell (HPC) collection by leukocytapheresis and reaching the target CD34+ cell dose. How mobilization regimen affects collection efficiency and the quality of products collected using the Spectra Optia apheresis instrument is not fully known.MethodsWe evaluated the impact of granulocyte‐colony stimulating factor (GCSF) and GCSF/plerixafor mobilization regimens on CE and product composition. We studied 373 leukocytapheresis HPC collections for 147 autologous transplants from January 1, 2010 to December 31, 2014. Patients were categorized in two groups; good mobilizers, mobilized with GCSF only (GM) and poor mobilizers, mobilized with GCSF and Plerixafor (PM).ResultsOverall, compared with PM group, total nucleated cell (TNC) yield was significantly lower in GM group (P = <.001). In contrast, median percent mononuclear cell (MNC) collected from GM (86.5%) was significantly higher than products collected from PM group (79.5%; P < .001). Compared with GM group, CD34+ cell CE was about 10% lower in PM group (P < .008). In addition, daily CD34+ cell/Kg yield was significantly higher in GM (2.08 × 10/Kg) compared with PM group (1.64 x 10/Kg, P = .019). Overall, the median number of collections per patient was two for GM and three for PM (P = .004).ConclusionProducts collected from PM group contained higher TNC content relative to GM group but had lower MNC enrichment, CD34+ cell CE and daily CD34+ cell yield per Kg.

BACKGROUND Expanding quantities of mesenchymal stem cells (MSCs) sufficient to treat large numbers of patients in cellular therapy and regenerative medicine clinical trials is an ongoing challenge for cell manufacturing facilities. STUDY DESIGN AND METHODS We evaluated options for scaling up large quantities of bone marrow–derived MSCs (BM‐MSCs) using methods that can be performed in compliance with Good Manufacturing Practices (GMP). We expanded BM‐MSCs from fresh marrow aspirate in αMEM supplemented with 5% human platelet lysate using both an automated cell expansion system (Quantum, Terumo BCT) and a manual flask‐based method using multilayer flasks. We compared MSCs expanded using both methods and assessed their differentiation to adipogenic and osteogenic tissue, capacity to suppress T‐cell proliferation, cytokines, and growth factor secretion profile and cost‐effectiveness of manufacturing enough BM‐MSCs to administer a single dose of 100 × 10 6 cells per subject in a clinical trial of 100 subjects. RESULTS We have established that large quantities of clinical‐grade BM‐MSCs manufactured with an automated hollow‐fiber bioreactor were phenotypically (CD73, CD90, CD105) and functionally (adipogenic and osteogenic differentiation and cytokine and growth factor secretion) similar to manually expanded BM‐MSCs. In addition, MSC manufacturing costs significantly less and required less time and effort when using the Quantum automated cell expansion system over the manual multilayer flasks method. CONCLUSION MSCs manufactured by an automated bioreactor are physically and functionally equivalent to the MSCs manufactured by the manual flask method and have met the standards required for clinical application.