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Background: Hematopoietic stem cell graft manipulation strategies, such as αβT-cell/CD19 B-cell depleted hematopoietic stem cell transplantation (αβhaplo-HSCT), address the lack of matched donors and reduce the incidence of severe acute graft-versus-host disease (aGvHD). However, grade II-IV aGvHD still occurs in 25-30% of αβhaplo-HSCT recipients . Studies aimed at understanding the pathogenesis underlying aGVHD in αβhaplo-HSCT are lacking. We hypothesized that αβT cells adoptively transferred with the HSCT ( Methods: Six patients with hematologic malignancies receiving fully myeloablative αβhaplo-HSCT at Lucile Packard Children's Hospital, Stanford, between 08/2018 and 05/2020 were enrolled upon signing IRB approved informed consent. Three patients developed grade II-IV aGvHD, while three did not. Aliquot of the graft and of peripheral blood collected at the time of aGvHD onset or at corresponding time points for the patients who did not develop aGvHD, were analyzed. Single sorted CD4 + and CD8 + T cells were profiled by single-cell barcode chip assay from IsoPlexis system (IsoPlexis, Branford, CT) after stimulation with PMA (50 ng/mL) and Ionomycin (1mcg/mL). Following the Human Adaptive Immune Panel, cytokines from CD4 + and CD8 + single T cells were captured by fluorescence ELISA, which measured the numbers of cytokine-producing cells (secretion frequency) and numbers of cytokines produced by individual cells across five functional groups: effector, stimulatory, chemoattractive, regulatory and inflammatory (Table 1). Polyfunctionality was defined as the secretion of 2+ cytokines from each CD4 + and CD8 + T cell. The T cell polyfunctional strength Index (PSI) was defined as the percentage of polyfunctional cells, multiplied by the sum of the mean fluorescence intensity of the proteins secreted by those cells. Additional statistical analysis was performed using the Student's t test. Results: We compared the combinatorial cytokine secretion signature of individual CD4 + and CD8 + T cells isolated from grafts infused into patients, who eventually did or didn't develop aGvHD. We are comparing the signature of post-HSCT CD4 + and CD8 + T cells isolated from patients who did or did not develop aGvHD. Collectively, we considered three variables: cytokine secretion frequency, numbers of cytokines produced by individual cells and characteristics of the cytokines secreted (functional group) upon stimulation. Single-cell functional heterogeneity evaluated by t-Distributed Stochastic Neighbor Embedding (t-SNE), showed higher CD4 + and CD8 + T-cell polyfunctionality (up to 4+ cytokines) with effector and stimulatory dominant functions in the grafts of patients who developed aGvHD, compared to those who did not develop aGvHD (Fig1). The average PSI (driven by Granzyme B, TNF-α, IFN-γ, MIP-1β, IL2, and IL-8) was found to be higher in both CD4 + and CD8 + T cells from the grafts of patients who developed aGvHD (Fig 2). Combinatorial cytokine secretion analysis showed that T cells from grafts of patients who did not develop aGvHD had unique signatures with CD4 + T cells having the predominant cytokine secretion signature of IL2 and TNF-α, and CD8 + T cells having three predominant cytokine secretion signatures: IL2, IL8, TNF-α; MIP-1β, IL8; and MIP-1β, IFN-γ (Fig3). Conclusions: Preliminary data from αβhaplo-HSCT pediatric recipients obtained using IsoPlexis single-cell functional proteomics for CD4 + and CD8 + T cells showed that an increased donor T-cell polyfunctionality with a Th1 dominant functional phenotype may be predictive of an increased risk of aGvHD, while CD4 + and CD8 + T cells infused into patients who didn't develop aGvHD, had combinations with limited cytokine secretion signatures. Ongoing analysis suggest that polyfunctional CD8 + T cells present in the graft of patients who developed aGvHD, are present at the time of aGvHD initiation, while the polyfunctional CD4 + T cell are not present at the onset of aGvHD. Correlation with ongoing studies on circulating cytokines and clonotypic analysis of αβT cells infused with the graft will be crucial to elucidate the cross talking between the donor's immune system and recipient's inflammatory milieu. 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.

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.

A major complication of mismatched unmanipulated hematopoietic stem cell transplant (HSCT) is Graft-versus-Host Disease (GvHD), which results in significant morbidity and increased non-relapse mortality. Novel strategies to reduce GvHD and improve long-term tolerance between mismatched donor-host pairs include regulatory T cell therapy. We are investigating a new cell product named T-allo10 which contains suppressive anergic cells and is enriched in IL-10 producing regulatory type 1 T cells (Tr1). The main advantage of adoptive immunotherapy with Tr1 compared to other regulatory T cells is the host alloantigen specificity that is established in the donor Tr1 during in vitro culture in the presence of IL-10 and host tolerogenic dendritic cells. Here we report the preliminary results of our phase I trial (IND 17292) on the use of escalating doses of T-allo10 cells for patients aged 3-45 years affected by hematological malignancies. At present, we completed the first cohort of the phase I portion of the study (Table 1). The donors were class I mismatched for patient 1 and class II HLA-mismatched for patients 2 and 3, respectively. The GvHD prophylaxis consisted of Sirolimus and Mycophenolate and was serotherapy and Calcineurin inhibitory-free. Patients received 1 × 106 T-allo10 cells/Kg on Day-1. No adverse events were observed after the T-allo10 cell infusion. All 3 patients met the safety criteria and are alive and disease-free at 2 years, 15 months and 2 months post-HSCT, respectively. Tr1, phenotypically defined as CD4+CD45RA−CD49b+LAG3+ were detectable in the peripheral blood shortly after the infusion at 21% and 26% in patient 2 and 3, respectively. At day 28 post-HSCT, Tr1 represented 12% of circulating memory CD4+ T cells in patient 1, 3% in patient 2 and 2% in patient 3. TCR sequencing of T-allo10 cells prior and after infusion showed a restricted clonotype diversity with persistence of certain clonotypes in vivo, demonstrating Tr1 cell recirculation and survival. These data show that the T-allo10 cell infusion is safe and well tolerated and demonstrate that T-allo10 cells are detectable in the recipients after the infusion and traceable by TCR clonotype analysis.

INTRODUCTION: Allogeneic hematopoietic stem cell transplantation (HSCT) remains the treatment of choice for pediatric patients with high risk or refractory leukemia. In the absence of related or unrelated HLA-matched donors, alternative approaches such as HLA-haploidentical HSCT have been implemented. Our group has developed one such approach, αβ T-cell/CD19 B-cell depletion (αβhaplo-HSCT), and demonstrated its clinical efficacy (Bertaina A, Blood 2018). In αβhaplo-HSCT, anti-thymocyte globulin (ATG) is used for preventing graft rejection and graft-versus-host disease (GvHD). However, the optimal dose still needs to be elucidated. Here, we present the first analysis comparing 2 different Thymoglobulin® ATG doses: one fixed at 3.75 mg/Kg, as established in previous European studies with the closely related ATG Grafalon®, and one based on a newly developed algorithm that also integrates absolute lymphocyte count (ALC). METHODS: Between March 2017 and April 2020, 27 pediatric patients (median age 12 years) with hematological malignancies were transplanted at Stanford University's Lucile Packard Children's Hospital. Importantly, 60% of these patients were in CR2 or greater and 2/27 had active disease at the time of HSCT. All children received a fully myeloablative conditioning regimen. On days -9/-8/-7 before αβhaplo-HSCT, patients were given a regimen of Thymoglobulin® ATG. No patient received post-HSCT pharmacological GvHD prophylaxis. The fixed dosage was given to 14 patients in cohort 1 (ATG: 3.75 mg/Kg) and the novel ALC-based dosage to 13 in cohort 2 (ATG ranging between 3-6 mg/Kg). All patients enrolled in this study (BMT Protocol 179 and 351 approved from our IRB) had a minimum follow-up of 100 days, evaluated at Day 30 and 90. Following Admiraal R (Lancet Hematology 2015), we defined CD4 immune reconstitution (IR) as more than 50 CD4 T-cells/ul twice within the first 100 days after HSCT. PBMC were enriched by Ficoll-Hypaque (Sigma Aldrich) density gradient centrifugation. Flow analyses were performed on fresh cells resuspended in PBS 2% FBS on Cytek DxP 10 flow-cytometer. At least 5x104 events of total cells were acquired and analyzed using FloJo software. RESULTS: With a median follow-up of 555 and 124 days for cohort 1 and 2 respectively, 12 patients (85.7%) in cohort 1 and 7 patients (54%) in cohort 2 achieved CD4 immune reconstitution. In cohort 2, CD3 αβ T cells were significantly lower at both Day 30 and Day 90 (P=0.0003, Figure 1A). At Day 90, both the CD4 and CD8 subpopulations were significantly depressed (P=0.01 and P=0.056, respectively, Figure 1C). In both subpopulations, the memory compartment was the most reduced (Figure 1D). The absolute numbers of CD3 γδ T cells did not differ between the cohorts at either Day 30 or 90 (Figure 1B). Viral reactivations were higher in cohort 1 (10/14, 71%) than in cohort 2 (6/13, 46%, P=NS). Half of the reactivations were CMV reactivations, but no patient developed organ disease. There was no statistically significant difference in overall survival and the incidence of relapse in the two cohorts. Three patients developed grade III-IV aGvHD: 2 in cohort 1 (14%) and 1 in cohort 2 (8%, P=NS). Remarkably, the only patient, who developed grade IV aGvHD in cohort 2, did not experience symptoms until Adenovirus reactivation 138 days after HSCT. CONCLUSION: Our analysis confirms that the optimal dose of ATG Thymoglobulin® before αβhaplo-HSCT remains elusive. There were no significant clinical differences between the 2 ATG regimens. However, the ALC-based regimen resulted in the more pronounced reduction of donor-derived memory T cells. Our analysis suggests two intriguing explanations for the observed pattern of results. First, the selective depletion of the memory compartment in both CD4 and CD8 T cells may well be due to a priming effect of ATG Thymoglobulin® on the few αβ T cells left over in the graft. Second, the equivalent reconstitution of naive T cells in the 2 cohorts is likely because the ATG has no impact on the thymus-dependent IR. Remarkably, in our overall cohort, 70% of the patients achieved CD4 IR by 90 days after αβhaplo-HSCT. This result is superior to the best results from other ex vivo T-cell depleted approaches 54%, recently reported by Van Roessel (Cytotherapy 2020). In vivo studies of the pharmacokinetics of ATG in αβhaplo-HSCT recipients and a comparison with the use of Grafalon® are required to shed more light on this crucial topic. Disclosures No relevant conflicts of interest to declare.

Purpose ‘Caring Letters’ is a suicide prevention intervention that involves sending brief messages of care and concern to high-risk patients following psychiatric hospitalisation. This is the only intervention that has been previously shown to reduce suicide mortality rates in a civilian population. A multi-site randomised controlled trial is now underway at US military and VA hospitals. This presentation provides an in-progress report on how the caring contacts trial is providing an active crisis intervention benefit. Methods and approach This in-progress multi-site study enrolls active duty military personnel and veterans who receive inpatient treatment at six psychiatric facilities. Comprehensive interviews are conducted to assess predictors of treatment effect. Patients are randomised into either ‘usual care’ or ‘Caring Letters’ intervention group. The latter receives 13 scheduled caring emails for two years. Both groups are contacted at end of the study to evaluate the intervention’s effectiveness. Primary outcomes include suicide mortality rates and self-directed violence. Results VA Palo Alto has enrolled 311 participants and 147 have been randomised to the Caring Emails group. The site has received 44 reply emails from patients. The majority expressed appreciation while 12 patients indicated acute distress due to overwhelming stressors, worsening symptoms, substance abuse relapse, homicidal and suicidal ideation. The research team followed established safety protocol to resolve potential crisis and reconnect patients to care. Conclusions Caring Letters provide a means to reconnect patients in crisis to care. By following established safety protocols, patients can be reconnected to care who may have otherwise may not have received additional intervention. Significance These in-process findings provide an essential step to validating a low-cost, high-impact intervention. A standardised Caring Letters program at US military and VA hospitals could provide an effective crisis management process as part of posthospitalisation safety planning.