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The cellular mechanisms required to ensure homeostasis of the hematopoietic niche and the ability of this niche to support hematopoiesis upon stress remain elusive. We here identify Wnt5a in Osterix+ mesenchymal progenitor and stem cells (MSPCs) as a critical factor for niche-dependent hematopoiesis. Mice lacking Wnt5a in MSPCs suffer from stress-related bone marrow (BM) failure and increased mortality. Niche cells devoid of Wnt5a show defective actin stress fiber orientation due to an elevated activity of the small GTPase CDC42. This results in incorrect positioning of autophagosomes and lysosomes, thus reducing autophagy and increasing oxidative stress. In MSPCs from patients from BM failure states which share features of peripheral cytopenia and hypocellular BM, we find similar defects in actin stress fiber orientation, reduced and incorrect colocalization of autophagosomes and lysosomes, and CDC42 activation. Strikingly, a short pharmacological intervention to attenuate elevated CDC42 activation in vivo in mice prevents defective actin-anchored autophagy in MSPCs, salvages hematopoiesis and protects against lethal cytopenia upon stress. In summary, our study identifies Wnt5a as a restriction factor for niche homeostasis by affecting CDC42-regulated actin stress-fiber orientation and autophagy upon stress. Our data further imply a critical role for autophagy in MSPCs for adequate support of hematopoiesis by the niche upon stress and in human diseases characterized by peripheral cytopenias and hypocellular BM.

Most cancers become more dangerous by the outgrowth of malignant subclones with additional DNA mutations that favor proliferation or survival. Using chronic lymphocytic leukemia (CLL), a disease that exemplifies this process and is a model for neoplasms in general, we created transgenic mice overexpressing the enzyme activation-induced deaminase (AID), which has a normal function of inducing DNA mutations in B lymphocytes. AID not only allows normal B lymphocytes to develop more effective immunoglobulin-mediated immunity, but is also able to mutate nonimmunoglobulin genes, predisposing to cancer. In CLL, AID expression correlates with poor prognosis, suggesting a role for this enzyme in disease progression. Nevertheless, direct experimental evidence identifying the specific genes that are mutated by AID and indicating that those genes are associated with disease progression is not available. To address this point, we overexpressed Aicda in a murine model of CLL (Eμ-TCL1). Analyses of TCL1/AID mice demonstrate a role for AID in disease kinetics, CLL cell proliferation, and the development of cancer-related target mutations with canonical AID signatures in nonimmunoglobulin genes. Notably, our mouse models can accumulate mutations in the same genes that are mutated in human cancers. Moreover, some of these mutations occur at homologous positions, leading to identical or chemically similar amino acid substitutions as in human CLL and lymphoma. Together, these findings support a direct link between aberrant AID activity and CLL driver mutations that are then selected for their oncogenic effects, whereby AID promotes aggressiveness in CLL and other B-cell neoplasms.

Introduction Emicizumab is a recombinant, humanized bispecific monoclonal antibody that mimics the function of factor VIII (FVIII) which results in a significant reduction in the annualized bleeding rate in patients with hemophilia A (HA), however, the degree with which emicizumab corrects the coagulation defect remains unclear. The objective of this study was to compare the current available laboratory methods in clinical practice; one-stage clotting factor assays (OSCA), bovine and human chromogenic FVIII activity (bovCHR and humCHR, respectively) and FVIII Equivalency of Emicizumab by Thrombin Generation (F8EmT). Aims The aim of this study is to address the differences of FVIII activity with different techniques in patients with severe HA with inhibitors on emicizumab. Materials and Methods Factor VIII levels are determined with an activated partial thromboplastin time (aPTT), OSCA using SynthASil on the ACL TOP 500 (Instrumentation Laboratory, Bedford, MA). Factor VIII activity is also determined photometrically via the Chromogenix Coatest® SP4 FVIII chromogenic assay kit (bovCHR, Diapharma Group, West Chester, OH) and the Biophen FVIII:C chromogenic assay kit (humCHR, Aniara Diagnostica, West Chester, OH). For F8EmT, linear regression was utilized to model the FVIII levels as a function of the endogenous thrombin potential (ETP) and peak thrombin values for patients with mild/moderate hemophilia. Then, we used the ETP and peak thrombin results of the severe HA patients on emicizumab with the calibration curve to calculate their F8EmT. Association between patient weight and their F8EmT were also examined and evaluated by linear regression. Results Data is presented for eight patients with severe HA with inhibitors on emicizumab in the non-bleeding state (Table-1). All patients' FVIII levels measured with OSCA are in or above the normal range (94.0-289.1). Bovine chromogenic FVIII activity is in the severe hemophilia range for five out of eight patients, for the rest it is in the moderate hemophilia range. Human chromogenic FVIII activity ranged between 12.5-49.8%. Factor VIII Equivalency of Emicizumab by Thrombin Generation is either in the mild hemophilia or normal range in all participants of the study. Conclusion One-stage clotting factor assays demonstrated falsely high results as expected since it is activated partial thromboplastin time based. Bovine chromogenic FVIII activity results were consistent with the severe HA range of the patients though a few had results slightly above that level. Previous literature has stated that the humCHR in patients on emicizumab results in FVIII levels of ∼30% when emicizumab is at its therapeutic concentration (∼50 mcg/ml). This study also demonstrated similar results with 5/8 patients having levels 30-50%. F8EMT levels were mostly consistent with the humCHR. In conclusion, understanding the degree to which emicizumab corrects the coagulation defect of is an important goal as it has clinical implications.Certainly, additional studies with higher participant numbers are needed to confirm these findings. Figure 1 Figure 1. Disclosures Young: Apcintex, BioMarin, Genentech/Roche, Grifols, Novo Nordisk, Pfizer, Rani, Sanofi Genzyme, Spark, Takeda, and UniQure: Consultancy; Genentech/Roche, Grifols, and Takeda: Research Funding.

The emergence of CAR-T cell therapy has transformed the treatment of the previously refractory/relapsed multiple myeloma (MM). Yet, autologous CAR-T cells suffer from inconsistent manufacturing, long manufacturing timelines, and high cost, which can limit patient accessibility. To address these issues, we engineered a fully allogeneic anti-BCMA CAR-T cell candidate for MM from healthy donors (P-BCMA-ALLO1). Herein, we demonstrate that P-BCMA-ALLO1 maintains a T stem cell memory phenotype (T SCM) through genetic editing, which correlates with antitumor efficacy. Using Poseida's proprietary non-viral piggyBac® (PB) DNA Delivery System, in combination with the high-fidelity Cas-CLOVER™ (CC) Site-Specific Gene Editing System and a proprietary "booster molecule", we generated P-BCMA-ALLO1 from healthy donor T cells. We used CC to eliminate surface expression of TCR and MHC class I to make fully allogeneic CAR-T cells. In addition to the CAR molecule, PB enables the delivery of a selectable marker allowing the generation of a final cell product that is >95% CAR-positive. The inclusion of the "booster molecule" in the manufacturing process improves the expansion of gene-edited cells without compromising memory phenotype or function. This process can produce up to hundreds of patient doses from a single manufacturing run using one healthy donor, thereby significantly reducing manufacturing cost per dose. We characterized the phenotype and functionality of P-BCMA-ALLO1 using flow cytometry and Nanostring to assess their memory phenotype at both the protein and mRNA levels. Also analyzed was antitumor toxicity and proliferative capacity through multiple rounds of activation using in vitro co-culture assays and serial restimulation, respectively. The relationship of all characterizations with in vivo efficacy was then determined, as defined by control of tumor growth in an immunodeficient RPMI-8226 subcutaneous murine tumor model. We found that P-BCMA-ALLO1 is comprised of a high frequency of T SCM after editing (Fig. 1), and the maintenance of that memory phenotype correlates with antitumor efficacy. In vivo, these CAR-T cells are potent in controlling tumor growth, comparable to or better than autologous anti-BCMA CAR-T cells. Our analysis revealed that the expression of memory markers at the surface protein level (CD27, CD62L, CD127, CCR7) and mRNA level significantly correlate with in vivo tumor control. Conversely, suboptimal research products with worse in vivo outcomes express a more exhausted gene expression profile. We reveal from our analysis that the most effective P-BCMA-ALLO1 in vivo share similar characteristics: (1) these products were a result of efficient manufacturing, with >90% CAR+ and >99% TCR-; (2) they carry a memory phenotype, with 50-70% T scm and high proliferative capacity after multiple rounds of restimulation; (3) they are >90% viable; and (4) they show strong antitumor efficacy both in vitro and in vivo. We demonstrate that Tscm percentage in the final product correlates with antitumor activity. P-BCMA-ALLO1 is advancing rapidly towards the clinic (NCT04960579) to positively impact the outcomes of CAR-T therapy for MM patients. Figure 1: Memory composition of P-BCMA-ALLO1 research products. P-BCMA-ALLO1 consists mostly of stem cell memory (T scm) and central memory (T cm) T cells that are CD62L + as opposed to effector memory (T em) and effector (T eff) T cells. Figure 1 Figure 1. Disclosures Tseng: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Zhang: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Cranert: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Richter: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Marquez: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Qiu: Poseida Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Cho: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tan: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tong: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Domingo: Poseida Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Weiss: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Argus: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Sparks: Poseida Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Ostertag: Poseida: Current Employment, Current equity holder in publicly-traded company. Coronella: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Shedlock: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company.

Background: Chimeric Antigen Receptor-modified T cell (CAR-T) therapies have revolutionized cancer immunotherapy, especially in hematological malignancies. Although great results have been achieved during the last years, long-term efficacy is still compromised in some cases and factors behind CAR-T cell disfunction are not fully understood. Recent studies have shown that the control of CAR expression influences CAR-T fitness and antitumoral efficacy 1. Therefore, we hypothesized that CAR density on the membrane of CAR-T cells could directly affect CAR-T cell function. In this study we perform a functional and genomic analysis of FACS-isolated subpopulations of CAR-T cells with different CAR densities (CAR High and CAR Low). Methodology: Second generation CAR-T cells with 4-1BB costimulatory domain targeting BCMA were generated by lentiviral transduction of αCD3/αCD28 activated T cells that were expanded for 12-14 days in the presence of IL-7/IL-15. Phenotypic analyses were performed by flow cytometry before and after coculture with MM cells. Cytotoxic activity and cytokine production were measured by standard procedures. In vivo antitumoral efficacy was evaluated in xenogeneic tumor models in NSG mice. Transcriptomic (RNA-seq) and epigenetic (ATAC-seq) analysis were performed following stablished protocols 2. Single cell analysis was performed using the Chromium Single Cell Immune Profiling solution from 10x Genomic that allows simultaneous analysis of gene expression and paired T-cell receptors from a single cell. Gene Regulatory Network (GRN) analysis was performed using SimiC, a novel computational method that infers regulatory dissimilarities 3. Results: RNA-seq and ATAC-seq analysis revealed completely different profiles between CAR High- and CAR Low-T cells in both CD4 +and CD8 + cell subsets, with >3500 differentially expressed genes (2086 for CD4 + and 1553 for CD8 +) that were related with increased tonic signaling, T cell activation and proliferation in CAR High-T cells. Functional studies at resting state (before antigen encounter) corroborated that CAR High-T cells presented increased tonic signaling, that lead to a higher basal activation and a more differentiated phenotype with skewed presence of CCR7 +/CD45RA +/CXCR3 + T SCM cells. After antigen-driven activation, increased cytotoxicity and cytokine production was observed in CAR High-T cells, that also presented higher percentage of terminally differentiated effector cells (CCR7 -/CD45RA +), along with increased exhaustion (PD1 +/LAG3 +/TIGIT +). This effect was also observed in the infusion products of CARTBCMA-HCB-01 clinical trial for patients with R/R MM (NCT04309981), where products enriched in CAR High-T cells presented increased cytotoxic activity. Although no significant differences were observed in the antitumoral efficacy in vivo, CAR Low-T cells presented increased persistence, suggesting that higher CAR levels could reduce long-term efficacy. Further characterization of CAR-T cells at single cell level (scRNA-seq) showed enrichment of CAR High-T cells in activated CD4 + and exhausted CD8 + cell clusters. The analysis of regulatory dissimilarities driven by different CAR densities with SimiC revealed an increased activity of the regulon associated to NR4A1 transcription factor (a well-known TF driving T cell exhaustion 4) in CAR High-T cells, providing mechanistic insights of the regulatory networks behind differential functionality of CAR High-T cells. Finally, to evaluate the impact of CAR density in the clinical outcome of CAR-T therapies, we developed a gene signature associated to increased CAR density, that was applied to transcriptomic data available from public studies 5. We score the infusion products of several clinical trials testing CTL019 (NCT01029366, NCT01747486 and NCT02640209) and we observed an enrichment on CAR High signature in the products from non-responder patients. Conclusions: Our data demonstrate that CAR density on the membrane of engineered T cells plays important roles in CAR-T activity with a significant impact on clinical outcome. Moreover, the comprehension of regulatory mechanisms driven by CAR densities at the single cell level offer an important tool for the identification of key regulatory factors that could be modulated for the development of improved therapies. Figure 1 Figure 1. Disclosures Rodríguez-Otero: Oncopeptides: Honoraria, Membership on an entity's Board of Directors or advisory committees; Kite: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Regeneron: Membership on an entity's Board of Directors or advisory committees; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees; GlaxoSmithKline: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS/Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel and other expenses. Paiva: Bristol-Myers Squibb-Celgene, Janssen, and Sanofi: Consultancy; Adaptive, Amgen, Bristol-Myers Squibb-Celgene, Janssen, Kite Pharma, Sanofi and Takeda: Honoraria; Celgene, EngMab, Roche, Sanofi, Takeda: Research Funding. San-Miguel: AbbVie, Amgen, Bristol-Myers Squibb, Celgene, GlaxoSmithKline, Janssen, Karyopharm, Merck Sharpe & Dohme, Novartis, Regeneron, Roche, Sanofi, SecuraBio, Takeda: Consultancy, Other: Advisory board. Prósper: Oryzon: Honoraria; Janssen: Honoraria; BMS-Celgene: Honoraria, Research Funding.

Introduction: Haploidentical peripheral blood stem cell transplantation ( Haplo-PBSCT) is a curative option in patient with hematology malignancy. In Mexico is very difficult to find an identical HLA donor , for this reason Haplo PBSCT it is an option in our public Health institution. Materials and methods: we aimed a retrospective study since (2017 -2021) in our Hospital Unidad Médica de alta Especialidad 25, Instituto Mexicano del Seguro Social (IMSS UMAE 25) Monterrey, Mexico. We include patients over 15 years of age who were undergo Haplo PBSCT with hematolgy malignancies . We performed Analysis of Overall and Relapse-free survival with kaplan meier curves and incidence of acute and chronic graft versus host disease (GVHD) and percentage of Transplant-Related Mortality (TRM). Results: we analized a 38 patients, the median age was 35 years (range 15-64). Of the total , 25 were male (65%) and 13 famale (34%). The rest of baseline data is described in table 1. The median time of neutrophil engrafment was days 17 ( 11-26 ) and platelets was 19.5 days ( 12-38 ). The Incidence of Primary Graft failure was 18 % ( seven patients). Patients with graft had 99-100% chimerism at day 100. The Incidence of aGVHD was 26 % and the cGVHD was 16%. DLI was used in four patients to treat passenger lymphocyte syndrome (2 patients) and ( 2 patients) with severe graft rejection. The principal cause of death was sepsis in eleven patients (29%). Two patients with acute leukemia (AML and ALL) died after development of post transplant lymphoproliferative disease. The TRM was 28.2%. There were no relapse-related deaths during follow-up after Haplo-PBSCT. The median of survival was 4.5 months ( range 1-44) with Overall Survival (OS) of 42 % at 3 years ( figure 1). Conclusion: Our analysis shows results that were comparable with those published in first world international transplant centers . Relapse remains the major cause of transplant failure, in our population the antigenic disparity between donor and recipient can strengthen the immune response versus disease. Haplo-PBSCT is a feasible transplantation method in patients with hematological malignancies in developing countries with limited resources. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Background Congenital thrombotic thrombocytopenic purpura (cTTP) is an ultra-rare and potentially life-threatening autosomal recessive inherited thrombotic microangiopathy with an estimated prevalence of Study Design and Methods This is a phase 3b, prospective, open-label, multicenter, single-arm continuation study (NCT04683003) to evaluate the long-term safety and efficacy of rADAMTS13 for prophylactic and on-demand treatment in patients with severe cTTP. The study will enroll approximately 77 patients and include approximately 57 patients who completed the phase 3 study (48 patients from the prophylaxis cohort; 9 patients from the on-demand cohort) and at least 20 patients who are naïve to rADAMTS13. Patients from expanded access programs or who had an allergic reaction to prophylactic standard of care therapy in the phase 3 study are also eligible for enrollment. Patients who were 0 to 70 years of age at screening for either the phase 3 study or the continuation study and were diagnosed with severe congenital ADAMTS13 deficiency (ADAMTS13 activity Conclusion Treatment options for cTTP are limited. This study will evaluate the long-term safety and efficacy of rADAMTS13 for the treatment of patients with severe cTTP. Figure 1 Figure 1. Disclosures Jain: Takeda: Current equity holder in publicly-traded company; Takeda Development Center Americas, Inc.,: Current Employment. Marquez: Takeda Development Center Americas, Inc.,: Current Employment; Takeda: Current equity holder in publicly-traded company. Martell: Takeda: Current equity holder in publicly-traded company; Takeda Development Center Americas, Inc.,: Current Employment.

Background: Patients with COVID-19 have an increased risk of thromboembolic disease, this has been partly attributed to an excessive inflammatory response that is associated with hypercoagulability; patients develop thrombotic complications with rates of 6.4% in non-critically ill and 15 to 31 % in critically ill patients. With this data some clinicians have incorporated thromboprophylaxis with higher dose heparin into the management of this patients, to date there´s no information of the effect of this intervention. Methods: We conducted a prospective cohort, including consecutive critical and non-critical adults admitted to a referral center in Mexico City, between March 18 and May 19, all with a positive RT-PCR for SARS-CoV 2. Conventional coagulation test results were collected on admission and during hospitalization; use of anticoagulation, and patient outcomes were recorded, all patients had been discharged at the time of the final analysis. Thromboprophylaxis was administered according to institutional recommendations and individual medical criteria, we defined anticoagulant dose according to each medication. We compared the basal characteristics and outcomes in critical and non-critical patients. We evaluated the factors associated with thrombosis, bleeding, and mortality using the Cox Regression Model. Results: We evaluated 447 consecutive hospitalized patients with COVID-19, median age was 50 years (range,18-91), 62.6 % were male, 111 (24.8%) were critical. At admission 156 patients (34.9%) had D-dimer values above 3000 ng/mL, median fibrinogen was 651 mg/dL (range 130-1095), APTT was prolonged (> 3 seconds) in 179 patients (40%), and INR >1.2 in 26 patients (5.8%), median platelet value 215 X103/uL (range 33-666). Thromboprophylaxis' dosages were prophylactic in 267 (59.7%), intermediate in 75 (16.8%) and therapeutic in 91 (20.4%), 14 patients (3.1%) did not receive any medical thromboprophylaxis and 26 patients (5.8%) received aspirin during hospitalization. According to the International Society on Thrombosis and Hemostasis' criteria (ISTH criteria) 40 patients (8.9%) had overt-DIC, sepsis induced coagulopathy (SIC) was present in 28 patients (6.3%), and high risk for bleeding by IMPROVE score ≥7 points was found in 5 patients (1.1%). Overall thrombotic event (TE) was confirmed in five patients (1.1%), arterial thrombosis events in 0.4%, one stroke and one acute myocardial infarction; radiographically confirmed venous thrombosis in 0.67%, two with pulmonary embolism (PE) and one with deep venous thrombosis (DVT). The TE were more common in critical than in non-critical patients (3.6% vs 0.3%). The number of CT pulmonary angiogram or duplex ultrasounds performed when PE/DVT was suspected was eighteen (4%), eight (47%) non-critically ill and ten (53%) in the ICU; the rate of radiographically positive results was 22.2%. The overall major bleeding rate was 2.5%, of these 91% were in the ICU. Mortality was 23.5% in the cohort. Table 1. No factors were found to be associated with thrombosis. The factors associated with bleeding were an INR >1.2 (HR 9.0, 95% CI 1.2-67.3, p 0.03), IMPROVE score ≥7 (HR 81.3, 95% CI 11.9-555.6, p < 0.01), and mechanical ventilation (HR 33.1, 95% CI 4.1-262.2, p 0.01). Factors associated with mortality were: age >70 (HR 2.5, 95% CI 1.5-4.2, p 3000 ng/mL (HR 2.0,95% CI 1.2-3.4, p Discussion: The late outbreak of COVID-19 in Latin America had led to an empiric use of aggressive thromboprophylaxis. Our data shows a low TE rate as compared with other groups, nevertheless we cannot prove a direct impact of the aggressive thromboprophylaxis, firstly because of the low rate or events, and secondly, due to the limitations of an observational study. On the other hand, the incidence of PE/DVT is conditioned by the number of studies performed, yet radiological confirmation has proven difficult due to concerns about virus exposure. Regarding the security of the intervention, major bleeding rates were slightly higher to what has been otherwise reported, but with no bleeding related deaths. The benefit of higher anticoagulant doses most be shown in clinical trials before we can recommend their generalized use in COVID-19 patients. Disclosures No relevant conflicts of interest to declare.

Introduction Cytokine release syndrome (CRS) is a common and potentially severe toxicity associated with haploidentical peripheral blood hematopoietic stem cell transplantation (Haplo-HSCT). CRS is characterized by immune activation and a high level of circulating inflammatory cytokines, particularly interleukin-6 (IL-6). While tocilizumab, an IL-6 receptor-targeted monoclonal antibody has been effectively used to treat CRS in the context of immunotherapy, it has not been evaluated during the acute Haplo-HSCT period. We hypothesized that prophylactic tocilizumab can prevent CRS after Haplo-HSCT. We report the results of a pilot trial to evaluate its use. Methods Patients ≥18 years undergoing Haplo-HSCT of peripheral blood in a single center with any underlying diagnosis were included. The conditioning was myeloablative and was administered as follows: cyclophosphamide 350 mg / m2 and fludarabine 25 mg / m2 (days -5-3) or rabbit antithymocyte globulin (2.5 mg / kg days -4, -3), plus melphalan 200 mg / m2 PO (days -2-1). Prophylaxis of graft versus host disease (GVHD) consisted of cyclophosphamide (50 mg / kg / day, days +3 and +4), cyclosporine and mycophenolate mofetil on day +5. The cells were infused on day 0. Tocilizumab was administered on day -1 in a single dose of 4 mg / kg IV. CRS was rated according to Lee et al. The primary outcome included the incidence and severity of CRS, the adverse effects associated with tocilizumab, hospitalization and graft rates and the incidence of GVHD. Results Ten patients have enrolled, with an average age of 26 years (range 19-32). The underlying diagnoses were acute lymphoblastic leukemia (n = 7), acute myeloid leukemia (n = 1) and non-Hodgkin lymphoma (n = 2). There were no infusion reactions associated with tocilizumab. Six patients (60%) developed CRS, all grades 1-2, with a median day of onset on day +2 (range 1-3) and a median fever duration of 2 days (range 2-3), often accompanied by nausea, diarrhea, dehydration and transaminitis. All patients were hospitalized; hospitalization was not related to CRS in three patients. 4 patients are alive without relapse (40%), 2 patients alive with relapse (20%), 3 patients died due to transplant-related mortality (30%), one patient dies from relapse (10%). No graft failure occurred and mixed chimerism was observed in a patient. Acute GVHD with steroid response has been diagnosed in 2 cases (20%, grade I / II). The median follow-up is 8.3 months (range 0-20). Conclusion What we observe about the prophylactic use of tocilizumab for Haplo-HSCT of peripheral blood is that its administration is feasible, Grades 1-2 CRS was observed in 60% of patients, grades 3-4 were not observed, however the use of tocilizumab did not prevent hospitalizations. More feasibility data is required before starting a definitive randomized trial to evaluate the use of tocilizumab for Haplo-HSCT peripheral blood. Disclosures Gomez-Almaguer: Janssen: Consultancy, Speakers Bureau; Teva: Consultancy, Speakers Bureau; Takeda: Consultancy, Speakers Bureau; Celgene: Consultancy, Speakers Bureau; Amgen: Consultancy, Speakers Bureau. OffLabel Disclosure: Tocilizumab for Prophylaxis of Cytokine Release Syndrome

Introduction: Patients with severe factor VIII (FVIII) deficiency are defined as having a FVIII level of Methods Patients with mild/moderate hemophilia (group 1) and severe FVIII deficient patients with inhibitors and on steady-state emicizumab (group 2) participated in the study. Blood was drawn from both groups in the non-bleeding state and with no recent factor infusion and assayed for FVIII activity (group 1) and thrombin generation (both groups). Factor VIII activity was measured with a one-stage clotting assay while TGA was performed using the calibrated automated thrombogram on platelet poor plasma using the PPP low reagent. First, linear regression was utilized to model the FVIII levels as a function of the endogenous thrombin potential (ETP) and peak thrombin values for patients in group 1. Then, we used the ETP and peak thrombin results of the emicizumab patients (group 2) with the calibration curve to calculate their predicted FVIII level. Association between patient weight and their predicted FVIII levels were examined and evaluated by linear regression. Results Twenty three patients with moderate and mild FVIII deficiency (levels 1-40%) and 11 patients with severe FVIII deficiency and inhibitors on emicizumab were enrolled. We had insufficient data for one patient in each group and therefore data is presented for 22 and 10 patients for group 1 and 2, respectively. Figure 1 (below on the left) demonstrates the raw data and the regression line for the FVIII levels versus ETP for group 1 and the squares represent the predicted FVIII levels for patients on emicizumab based on their actual ETP. Peak thrombin results are not shown in the figure but are similar to the ETP data. Figure 2 (below on the right) shows the relationship between the inhibitor patient's weight and their predicted FVIII level demonstrating that the heavier patients had lower predicted FVIII levels. Discussion Determining the degree of correction of the clotting defect for patients on emicizumab is important in order to make good clinical decisions regarding activities, response to therapy, and perhaps even surgery. As of yet, no studies have been performed to address this issue. In this study, we made a calibration curve of native FVIII levels and ETP and peak thrombin in mild/moderate hemophilia A patients, and then used the ETP and peak thrombin results of inhibitor patients on emicizumab to predict their FVIII level. All the patients on emicizumab had predicted FVIII levels above 10% with most having levels above 20%. The wide variability in the predicted FVIII level was tightly correlated to weight with the heaviest patients having the lowest FVIII predicted levels. It is unclear at this time why this may be the case, and further data will be collected to assess this relationship. Few bleeding events were reported in our inhibitor patients such that in this small group we cannot correlate the results to bleeding. Moving forward, understanding the correction of the clotting defect of non-replacement therapies is an important goal. Figure Disclosures Young: Uniqure: Consultancy, Honoraria; Freeline: Consultancy, Honoraria; Grifols: Consultancy, Honoraria; Kedrion: Consultancy, Honoraria; Novo Nordisk: Consultancy, Honoraria; Spark: Consultancy, Honoraria; Shire/Takeda: Consultancy, Honoraria; Genentech/Roche: Consultancy, Honoraria, Research Funding; Bioverativ/Sanofi: Consultancy, Honoraria; CSL Behring: Consultancy, Honoraria.