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The Primary Immune Deficiency Treatment Consortium (PIDTC) performed a retrospective analysis of 662 patients with severe combined immunodeficiency (SCID) who received a hematopoietic cell transplantation (HCT) as first-line treatment between 1982 and 2012 in 33 North American institutions. Overall survival was higher after HCT from matched-sibling donors (MSDs). Among recipients of non-MSD HCT, multivariate analysis showed that the SCID genotype strongly influenced survival and immune reconstitution. Overall survival was similar for patients with RAG, IL2RG, or JAK3 defects and was significantly better compared with patients with ADA or DCLRE1C mutations. Patients with RAG or DCLRE1C mutations had poorer immune reconstitution than other genotypes. Although survival did not correlate with the type of conditioning regimen, recipients of reduced-intensity or myeloablative conditioning had a lower incidence of treatment failure and better T- and B-cell reconstitution, but a higher risk for graft-versus-host disease, compared with those receiving no conditioning or immunosuppression only. Infection-free status and younger age at HCT were associated with improved survival. Typical SCID, leaky SCID, and Omenn syndrome had similar outcomes. Landmark analysis identified CD4(+) and CD4(+)CD45RA(+) cell counts at 6 and 12 months post-HCT as biomarkers predictive of overall survival and long-term T-cell reconstitution. Our data emphasize the need for patient-tailored treatment strategies depending upon the underlying SCID genotype. The prognostic significance of CD4(+) cell counts as early as 6 months after HCT emphasizes the importance of close follow-up of immune reconstitution to identify patients who may need additional intervention to prevent poor long-term outcome.

Background Outcomes of COVID-19 infection among children with underlying malignancy remain unclear, with limited data available to date. Here we report clinical characteristics and outcomes following COVID-19 infection among children with underlying malignancy or a history of hematopoietic stem cell transplant (HSCT) during the first year of the global COVID-19 pandemic, in Dallas, TX. Methods: We retrospectively reviewed children with a history of malignancy or prior HSCT and positive SARS-CoV-2 polymerase chain reaction (PCR) result between March 1, 2020 - March 31, 2021. Demographic, clinical, and outcome data were reviewed, and analyzed according to underlying condition and disease severity. Results: Forty-six patients with underlying malignancy or HSCT met inclusion criteria. Median age was 9 years, with twenty (43%) female, and twenty-seven (59%) of Hispanic ethnicity. Underlying conditions included Acute Lymphoblastic Leukemia (67%), Acute Myelogenous Leukemia (9%), HSCT (11%), sarcoma (9%), solid tumors (9%), and Hodgkin's lymphoma (2%). Fifteen subjects (33%) were asymptomatic, with twenty-four children (52%) classified as having mild, five (11%) with severe, and two (4%) with critical COVID-19 disease. The most common symptoms at presentation were fever (46%), cough (39%), and gastroenteritis (19%). Thirty-four children (74%) were hospitalized, with seven (15%) requiring intensive care unit (ICU) care. 17% of patients were admitted for treatment of COVID-19 alone, with other indications including neutropenic fever, relapse, and chemotherapy. Six (13%) patients required non-invasive ventilation, and two patients (4%) required invasive mechanical ventilation. Median length of stay was 9 days (IQR 4.5-24.5), with median ICU stay of 3.5 days (IQR 2-26.7). Seven children (15%) received COVID-19 targeted therapy including steroids, remdesivir or convalescent plasma. Among severe and critically ill children, six (86%) had underlying hematologic malignancies (5 HR ALL, 1 AML), and two (28%) a prior history of HSCT. Two children (4%) died. Thirty-six patients (78%) achieved documented clearance of SARS-CoV-2 by PCR, at median of 43.5 days (IQR 28.7-65.2). Conclusion: Among children with underlying malignancy or a history of HSCT, COVID-19 results in a spectrum of illness ranging from asymptomatic disease to death. Rates of hospitalization are high compared to the general pediatric population, and illness may be complicated by additional factors including co-infection, neutropenic fever, relapse of malignancy and need for chemotherapy. Children with hematologic malignancy or a history of HSCT may be at increased risk for severe disease. Additional studies are urgently needed to elucidate risk factors for severe/critical COVID-19 disease among children with underlying malignancy. Disclosures No relevant conflicts of interest to declare.

Introduction: Inborn errors of immunity such as hemophagocytic lymphohistiocytosis (HLH) and chronic granulomatous disease (CGD) are characterized by hyperinflammation. Hematopoietic cell transplantation (HCT) in the setting of hyperinflammation leads to high morbidity and mortality. Consequently, there is increasing use of less intense conditioning regimens, which can increase risk of mixed chimerism or graft failure. We sought to study the effect of common regimens on outcomes after HCT using data reported to the Center for International Blood and Marrow Transplant Research. Methods : 365 patients aged Results : Patient demographics were similar across the three treatment groups. Patients with HLH were more likely to receive the Flu/Mel regimen. Although unrelated donor HCTs were predominant across the treatment groups, cord blood graft was more common in the Bu/Cy group. Conditioning regimens changed over the study period with most Flu/Mel/TT and Flu/Bu regimens used after 2010. Consequently, outcomes were censored 2-years post-HCT to account for differences in follow-up. The day-100 incidence of VOD was higher with Bu/Cy (18%) compared to Flu/Mel (4%) and Flu/Mel/TT or Flu/Bu (7%) regimens (p Conclusion : The data does not support the use of a reduced intensity Flu/Mel regimen for hyperinflammatory inborn errors of immunity. Although we did not observe differences in event-free survival between Bu/Cy and Flu/Mel/TT or Flu/Bu regimens, lower incidences of VOD and bacterial infections favor Flu/Mel/TT or Flu/Bu regimens. Disclosures Pulsipher: Bellicum: Honoraria; Jasper: Honoraria; Novartis: Honoraria; Miltenyi: Honoraria, Research Funding; Mesoblast: Honoraria; Adaptive: Research Funding. Stenger:ISCT: Membership on an entity's Board of Directors or advisory committees; Bluebird Bio: Research Funding.

Background Allogeneic hematopoietic stem cell transplantation (HSCT) is a well-established treatment for children with Acute Leukemia (AL). For patients lacking a compatible matched related or unrelated donor, HLA-haploidentical HSCT (haplo-HSCT) from a relative represents a viable alternative. Promising results were reported with a novel method of selective depletion of αβ T and B cells (Locatelli, Blood 2017). This approach is associated with limitations such as suboptimal adaptive immune reconstitution, increased risk of infection and disease relapse. BPX-501 is an allogeneic product consisting of T cells modified to express the inducible caspase-9 (iC9) safety switch and truncated CD19 to allow monitoring and expansion of BPX-501 following transplant. The polyclonal nature of the BPX-501 provides broad virus and tumor-specific immunity, while the safety switch provides the unique ability to promptly and durably resolve graft-versus-host disease (GvHD) symptoms following the administration of rimiducid which induces dimerization and activation of iC9, inducing apoptosis of BPX-501. Aims To evaluate the safety and efficacy of BPX-501 administered after a αβ T and B-cell depleted haplo-HSCT in pediatric patients with AL in morphological complete remission (CR). The objective was to determine whether BPX-501 infusion can increase relapse-free survival (RFS) and overall survival (OS) through an enhanced graft-versus-leukemic (GvL) effect, while maintaining a low risk of GvHD. Methods This multicenter US (NCT03301168) and EU (NCT02065869), prospective trial utilizes αβ-T and B-cell-depleted haplo-HSCT followed by infusion of donor lymphocytes genetically modified with iC9 safety switch (BPX-501) in patients with malignant or non-malignant disorders. A subset of patients had acute high-risk leukemias (AML and ALL). BPX-501 was planned to be infused on day14±4 after the allograft. No post-transplant pharmacological GvHD prophylaxis was employed. Patients who develop GvHD resistant to conventional steroid therapy could receive ≥1 dose of rimiducid to activate iC9. The efficacy-evaluable population (EEP) was defined as any patient with AL who received HSCT, BPX-501 infusion, and at least one follow-up assessment. Results At clinical cut-off (June 30, 2018), 100 patients (EU: 75, US: 25) with AL met the EEP definition. Median follow-up was 14.7 mos (1 - 40.6 mos). Key baseline characteristics are shown in Table 1. The median time for neutrophil and platelet engraftment was 16 (15 - 17) and 12 (11 - 12) days, respectively. Four patients (4.1% [95% CI: 0 - 8%]) experienced primary graft failure. Of 96 evaluable patients, 21 patients developed Grade I-IV aGvHD (21.7% [95% CI: 13.5 - 29.8%]). Five patients developed Grade III-IV aGvHD (3.1% [95% CI: 0 - 6.5%]). Of 82 evaluable patients, 12 patients developed cGvHD (18.1% [95% CI: 8.2 - 22%]), with only three cases being moderate-severe. Rimiducid was administered to 10 patients with steroid-resistant acute GvHD. Best overall clinical response of CR or PR post-rimiducid administration was seen in 8 patients (80%). Among responding patients, 7 patients (87.5%) had a CR. Six patients died after transplantation (6.6% [95% CI: 1.4 - 11.7%]). Relapse Free Survival (RFS) was 82.2% (95% CI: 74.5 - 89.7%). Overall Survival (OS) was 90.1% (95% CI: 83.9 - 96.3%). Efficacy outcomes (TRM, RFS and OS) in AL subsets (AML and ALL) are shown in Table 2. CD3+ and CD3+CD4+ T cells above 500 cells/ml were achieved by 180 and 270 days, respectively. IgA and IgM levels achieved normal values by 180 days. The percentage of circulating and median absolute BPX-501 cells at Day 100 were 9.96% ± 11.6% (0 - 54.9%) and 85.58 ± 165.57 cells/ul (0 - 1001 cells/ml), respectively. Conclusion The adoptive transfer of BPX-501 following αβ-T and B-cell depleted haplo-HSCT represents a novel and highly effective transplantation strategy for pediatric patients with AL. Compared to data from children receiving only αβ T and B-cell depleted haplo-HSCT or matched unrelated donor HSCT (https://bloodcell.transplant.hrsa.gov/research/transplant_data/us_tx_data/survival_data/survival.aspx), this novel approach resulted in a comparable risk of transplant-related mortality and a lower risk of recurrence. Rimiducid was also an effective treatment for patients who developed steroid-resistant GvHD. Disclosures Locatelli: bluebird bio: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Qasim:Orchard: Equity Ownership; Autolus: Equity Ownership; Servier: Research Funding; Bellicum: Research Funding. Nemecek:Novartis Pharmaceuticals Corporation: Other: advisory boards.

Background Allogeneic hematopoietic stem cell transplantation (HSCT) has been shown to rescue trilineage cytopenias and to prevent occurrence of myeloid malignancies in patients with Fanconi Anemia (FA). HLA-identical family donor HSCT is a recommended treatment strategy with an ≥80% cure rate, and outcomes of HSCT from unrelated donors have progressively improved over time. However, HLA-partially matched (haplo) HSCTs have been historically associated with inferior results. Innovative approaches of graft manipulation, such as selective depletion of αβ-T and B cells, have recently been reported to improve the outcomes of haplo-HSCT in children with non-malignant disorders, however obstacles remain, such as delayed recovery of adaptive immunity with a risk of life-threatening infections. Novel strategies aimed at accelerating immune reconstitution are warranted to optimize the outcomes of patients transplanted from a HLA-haploidentical family donor. BPX-501 is an allogeneic product consisting of T cells modified to express the inducible caspase-9 (iC9) safety switch and truncated CD19 to allow monitoring and expansion of BPX-501 following transplant. The polyclonal nature of the BPX-501 provides broad virus and tumor-specific immunity, while the safety switch provides the unique ability to promptly and durably resolve graft-versus-host disease (GvHD) symptoms following the administration of rimiducid which induces dimerization and activation of iC9, inducing apoptosis of BPX-501. Aims To evaluate the safety and efficacy (acceleration of immune recovery and prevention of transplant-related mortality while maintaining low rates of acute and chronic GvHD) of BPX-501 administered after a αβ T and B-cell depleted haplo-HSCT in pediatric patients with FA. Methods This is a multicenter US (NCT03301168) and EU (NCT02065869), prospective trial utilizing αβ-T- and B-cell-depleted haplo-HSCT followed by infusion of donor lymphocytes genetically modified to express iC9 (BPX-501 T cells). BPX-501 cells were planned to be infused on day14±4 after the allograft per protocol. No post-transplant pharmacological GvHD prophylaxis was employed. Patients who develop GvHD resistant to conventional steroid therapy were eligible to receive ≥1 dose of dimerizing rimiducid activating iC9. The efficacy-evaluable population (EEP) was defined as any patient with FA who received HSCT, BPX-501 infusion and had at least one follow-up assessment. Results At the time of clinical cut-off (June 30th, 2018), 14 patients met the EEP definition. Nine and 5 patients were transplanted in the EU and US, respectively. The median follow-up was 23.5 mos (0.75 - 42 mos). Key baseline and transplant characteristics are detailed in Table 1. The median time for neutrophil and platelet engraftment was 14 (11-15) and 10 (8 - 11) days, respectively. One patient experienced primary graft failure (7.7% [95% CI: 0-22.2%]). Only one patient developed Grade I aGvHD (7.7% [95% CI: 0-22.2%]). Two patients developed cGvHD (19.2% [95% CI: 4-43.2%]), both cases being moderate. Rimiducid was not administered to any patient. One patient died after transplantation due to brain hemorrhage 25 days post HSCT. The event was unrelated to BPX-501 (TRM 7.1% [95% CI: 0-20.6%]). No patients died of infectious complications. The probability of both overall survival (OS) and disease-free survival (DFS) was 92.9% (95% CI: 79.4-100%). The median time from last red blood cell (RBC) transfusion was 23.6 mos (0.2 - 41.9 mos). CD3+ and CD3+CD4+ T cells above 500 cells/ml were achieved by 180 and 360 days, respectively. IgA and IgM levels achieved normal values by 30 and 180 days, respectively. The percentage of circulating and median absolute BPX-501 cells at Day 100 were 3.35% ± 2.94% (0 - 8.4%) and 29.09 ± 39.56 cells/ml (0 - 135 cells/ml), respectively. Conclusion These data indicate that the adoptive transfer of BPX-501 following αβ-T and B-cell depleted haplo-HSCT represents a novel and highly effective transplantation strategy for pediatric patients with FA. Further, despite the addition of BPX-501, low rates and severity of both acute and chronic GvHD was observed, a finding that could translate into significant long-term benefit, in terms of reduction of squamous cell carcinoma a well-known complication occurring in FA patients after HSCT. Disclosures Aldinger: Bellicum Pharmaceuticals, Inc.: Employment. Locatelli:Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio: Consultancy; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Familial hemophagocytic lymphohistiocytosis (FHL) is an inherited, fatal disorder of infancy. We report here a 17-day-old female infant who presented with high fever, hepatosplenomegaly, hypertriglyceridemia, hypofibrinogenemia, thrombocytopenia, and liver failure. Leukocytosis was detected with circulating “atypical” lymphoid cells. Flow cytometric studies revealed expanded subpopulations of CD8+ T cells with unusual immunophenotypic features, including a subset that lacked CD5 expression. A liver biopsy showed hemophagocytic lymphohistiocytosis with exuberant infiltrates of CD8+ T cells that lacked perforin. Mutational studies revealed a 666C→A (H222Q) missense mutation in the perforin gene. T-cell receptor studies on flow-sorted T-cell subpopulations revealed no evidence of monoclonality. Analysis of T-cell receptor excision circle levels indicated long proliferative history in the aberrant CD8+ T-cell subsets. This case provides an instructive example of uncontrolled reactive proliferation of CD8+ T cells in FHL, resulting in atypical morphology and unusual immunophenotypic features that might suggest malignancy in other clinical settings.

Background: Approximately 3540 children are diagnosed with leukemia in the United States yearly (Bhatia and Robison, Oncology of Infancy and Childhood, 2008). Cooperative group trials have increased survival, particularly for acute lymphoblastic leukemia (ALL), but successful treatment of recurrent leukemia remains an unmet medical need. Resistance pathways and epigenetic alterations suggest a role for histone deacetylase (HDAC) inhibitors in children with leukemia (Burke and Bhatla, Frontiers in Pediatrics, 2014). Panobinostat is an orally administered pan-deacetylase inhibitor with activity against HDACs at concentrations in the nanomolar range (Atadja, Cancer Letters, 2009), and for which there is pre-clinical evidence of activity in pediatric leukemia (Stubbs, et al., ASH 2010). Panobinostat shows promise in a variety of adult hematologic malignancies (Khot et al., Expert Opinion on Investigational Drugs, 2013). We undertook a phase I trial of panobinostat in children with recurrent hematologic malignancies, and herein report the safety and pharmacokinetics (PK) from enrolled children with leukemia. Methods: T2009-012 is a first-in-child study coordinated by Therapeutic Advances in Childhood Leukemia and Lymphoma (TACL). Children with relapsed or refractory leukemia between the ages of 1 and 21 years were enrolled to a multi-center, single agent trial of panobinostat dosed once per day three days per week for four successive weeks. Dose escalation was a standard 3+3 design with three dose levels planned. Subjects underwent lumbar puncture with prophylactic chemotherapy at treatment start and after a 28 day course. Blood was sampled pre-dose, at 0.5, 1, 6, 24 and 28-48 hours following the first dose. PK was obtained from blood on patients concurrently with optional specimens obtained from cerebrospinal fluid (CSF) on Day 29. Subjects who received fewer than 11 of the 12 planned doses and did not experience a dose limiting toxicity (DLT) were considered not evaluable for DLT, but were included in the summary of toxicities. Serial ECGs were monitored. Results: Seventeen subjects were enrolled with a diagnosis of acute leukemia, 10 with ALL and 7 with acute myelogenous leukemia (AML). Five were enrolled at dose level 1, 24 mg/m2/dose, 6 at dose level 2, 30 mg/m2/dose, and 6 at dose level 3, 34 mg/m2/dose. There have been no DLTs. Nine subjects are evaluable for DLT and 4 subjects were taken off study early due to increasing blast count. No subjects required removal from protocol therapy for QTc prolongation. One subject with infant ALL was removed early for progressive Aspergillus infection, 1 subject only received 10 doses owing to electrolyte abnormalities, and 2 subjects had nausea and vomiting after administration of 4 doses and did not continue. Grade 3/4 adverse events occurring in more than 20% of subjects included anemia in 82%, diarrhea in 24%, febrile neutropenia in 65%, hypokalemia in 41%, and hypophosphatemia in 24%. Concentration-time profiles were obtained from 9 subjects ages 16 months to 14 years in the 3 dose levels. Mean ± SE of PK for all subjects were Cmax 28.8 ± 6.1 ng/mL, Tmax 2.0 ± 0.8 hours, and T1/2 12.8 ± 3.0 hours. Two toddlers had the highest dose-normalized AUC0-inf and lowest oral clearance. Apparent oral clearance proportionally increased with increase in BSA. To date, 4 CSF specimens have been evaluated and found to have panobinostat below the lower limit of the quantification of 0.1 ng/mL, despite appreciable levels in the plasma. Two subjects on dose level two began a second cycle of therapy; one completed a second cycle for MLL rearranged leukemia and one discontinued study participation in the second cycle to undergo hematopoietic stem cell transplant for secondary AML after achieving a CRp in the first cycle. Conclusions: Panobinostat was tolerated these heavily pre-treated patients without unanticipated toxicities. PK in larger children and adults appears similar but PK in smaller children needs to be further explored. Penetration of panobinostat into the CSF was negligible. Two of 17 patients were able to receive a second cycle of therapy, but 4 had to be withdrawn early because of rapid increase in blast counts. Future trials will explore combination therapy in children with refractory hematologic malignancies, particularly those known to be driven by epigenetic mechanisms, in order to better control risk of rapid progression and improve efficacy through synergy. Disclosures Off Label Use: panobinostat for leukemia. Manley:Seattle Genetics, Inc.: Employment, Equity Ownership. Thomson:Epizyme, Inc: Employment.