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Background Acute Graft-versus-Host-Disease (aGvHD) is a major cause of morbidity and mortality after allogeneic hematopoietic (stem) cell transplantation (HCT). Studies evaluating the relation between immune reconstitution (IR) and aGvHD are largely lacking. We previously showed that successful CD4+ IR strongly predicts non-relapse mortality (NRM) and survival after HCT. We hypothesized that adequate CD4+ IR may influence outcomes in patients who develop aGvHD, because (CD4+) T regulatory (Tregs) play a crucial role in tolerance. Therefore, we studied the relation between CD4+ IR after HCT and outcomes (survival, NRM) in patients who developed moderate to severe aGvHD, in 2 separate pediatric / young adult cohorts. Methods Patients receiving their first allogeneic HCT at the UMC Utrecht / Princess Maxima Center (UMC/PMC; 2004 and 2018) and at Memorial Sloan Kettering Cancer Center (MSK; 2008 – 2018), New York were included. No restrictions applied in terms of donor source, indication, conditioning, age. CD4+ IR was measured every 2-4 weeks. The main outcomes of interest were 5-year overall survival (OS) and 5-year NRM, stratified for aGvHD 2-4 and adequate CD4+ IR (twice >50*106 CD4+/uL) prior to aGvHD onset. Fine and Gray competing risk models and cox-proportional hazard models were used. Results 591 patients (276 UMC/PMC; 315 MSK) were included (table 1); median age at UMC/PMC was 7.06 years (range 0.16-22.74) and at MSK 10.4 years (range 0.1-35.6). At UMC/PMC 73 (26.4%) and at MSK 70 (22.2%) patients developed aGvHD 2-4, while 29 (10.5%) and 32 (10.2%) developed aGvHD 3-4. At UMC/PMC 35% (26/73) developed aGvHD in the presence of CD4+ IR and at MSK 64% (45/70). The 5-yr OS for patients without aGvHD at UMC/PMC and MSK was 75% and 72%, resp. The only multivariate predictor for OS and NRM in patients with aGvHD 2-4 was CD4+ IR. In both cohorts, CD4+ IR before onset of aGvHD 2-4 was associated with higher 5-yrs OS compared to those who developed aGvHD 2-4 without CD4+ IR; 77% vs 48%, p Interpretation In 2 separate cohorts adequate ‘CD4+ IR prior to onset aGvHD' was found to be a simple and robust predictor for survival and NRM. These findings provide insight in the importance of adequate IR for HCT patients to survive moderate to severe aGvHD. Finding strategies to better predict CD4+ T cell recovery after HCT may influence survival chances.

Introduction Rabbit anti-thymocyte globulin (ATG) is used in allogeneic hematopoietic cell transplantation (HCT) to prevent graft-versus-host-disease (GvHD) and graft failure (GF). Its main and unpredictable toxicity includes poor immune reconstitution associated with increased relapse, viral reactivations and subsequently higher mortality. Early ( Methods We performed an open label, phase II historically controlled non-randomized prospective clinical trial investigating individualized versus fixed dosing of ATG (Thymoglobulin). Individualized dosing was based on the results from a previous validated population PKPD model1,2 with cumulative doses varying between 2 to 10 mg/kg starting based on weight, recipient lymphocyte counts before first dose of ATG and stem cell source, starting day -9. Primary endpoint was successful CD4+ T-cell reconstitution (CD4+ >50/mm3 at 2 consecutive timepoints within 100 days after HCT1) in patients alive without relapse or graft failure Results We included 58 patients between 2015-2018, and compared to 112 historical controls (table 1). CD4+ reconstitution was significantly better in the individualized dosing group: 83% versus 54% in the fixed dosing group; HR 2.4 (95% CI 1.6-3.6), p Conclusions Individualized ATG dosing significantly increases the chance of rapid immune reconstitution without affecting the incidence of aGvHD and graft failure. Together, this is an important step towards predictable CD4+ T cell reconstitution and improved survival changes.

Background Graft-versus-Host-Disease (GvHD) is a major cause of morbidity and mortality after allogeneic hematopoietic (stem) cell transplantation (HCT). Studies evaluating the relation between immune reconstitution (IR) and GvHD are largely lacking. We previously showed that successful CD4+ IR strongly predicts event-free survival chances (JACI 2018, Blood 2018). Therefore, we were interested in relating IR to acute and chronic GvHD after HCT. Methods Pediatric patients receiving their first allogeneic HCT between 2004 and 2018 were included. Blood samples and outcome data were collected and registered prospectively. Our main outcomes of interest were 5-year overall survival (OS) and non-relapse mortality (NRM) probabilities, stratified for aGvHD (grade II-IV) and for the presence or absence of adequate CD4+ IR (twice >50*106 CD4+CD3+ cells/L) prior to aGvHD onset. Other outcomes of interest were IR of T-cells, B-cells, NK-cells, monocytes, neutrophils, reticulocytes, and basophils, in patients with or without grade II-IV aGvHD or cGvHD. We applied a time-to-event multivariate Log-rank test to evaluate aGvHD-related survival. IR data were analyzed as continuous variables using multivariate linear-mixed-effects-regression modelling. HCT-source; cord blood transplantation, and bone marrow or peripheral blood transplantation, was analyzed as covariate. Results 391 pediatric patients were included; 97 (24.8%) had moderate-severe aGvHD (grade II-IV) and 42 (10.7%) had cGvHD (limited-extensive). Adequate early CD4+ IR before diagnosis of aGvHD grade II-IV was associated with reduced mortality compared to no CD4 +IR; OS was 77% versus 48% (p Interpretation Adequate CD4+ IR prior to aGvHD onset protects against aGvHD-related mortality after HCT. Either regulatory T-cell development or a lower risk of infection may be possible explanations. A specific delay in B-cell recovery after HCT provides an interesting clinical indicator for cGvHD, which demands further research. Our findings provide insight in the importance of monitoring the immune system after HCT to better understand the biology and determine strategies to prevent and treat GvHD.

Development of autoimmune cytopenia (AIC) after allogeneic hematopoietic cell transplantation (HCT) is a serious complication requiring urgent intensification of immunosuppressive therapy. The pathophysiology and predictors of AIC are not completely understood. In this retrospective cohort analysis of 380 pediatric patients, we evaluated the incidence, outcomes, and related various variables, including immune reconstitution markers to AIC. Three hundred eighty patients (median age, 7.4 years; range, .1 to 22.7) were included, of which 30 patients (7.8%) developed AIC in 1 (n = 6), 2 (n = 6), or 3 (n = 16) cell lineages at a median of 133 days (range, 46 to 445) after HCT. Using multivariate analysis we found that chemo-naivety before HCT, acute graft-versus-host disease (aGVHD) grades II to IV, and serotherapy were associated with the development of AIC. Development of AIC was preceded by increased levels of IgM, IgA, and IgG. Immune profiles of total absolute lymphocytes were very similar between AIC patients and control subjects. However, CD3-CD16+CD56+ natural killer cells, CD3+ T cells, CD3+CD4+ T cell subset, and CD3+CD8+ T cell subset were lower in AIC patients. Overall survival was good, at 83% (similar between AIC patients and control subjects). In conclusion, we identified chemo-naivety before HCT, preceding aGVHD grades II to IV, and serotherapy as predictors for development of AIC. Increasing levels of IgM, IgA, and IgG preceded AIC development. These data provide clues to further study the biology of AIC.

Introduction Nicotinamide-expanded cord blood (NiCord) is a potential alternative source for allogeneic hematopoietic cell transplantation (HCT) when an HLA-id donor is lacking. A phase 1/2 trial with standalone NiCord HCT showed rapid neutrophil- (11 days) and platelet engraftment (34 days). We previously reported that successful CD4+ immune reconstitution (IR) is crucial for infectious and relapse control associated with favorable survival (JACI 2017) and is a better predictor for event-free survival than neutrophil reconstitution. We performed unique in-depth immune monitoring to evaluate and compare IR after NiCord and conventional HCT. Methods In this phase1/2 international multicenter trial, we compared IR after NiCord HCT to cohorts of adolescent and young adult (AYA) patients receiving either unmanipulated cord blood transplantation (unCBT) or T-repleted-unrelated bone marrow transplantation (BMT). All patients received HCT for a hematologic malignancy with myeloablative conditioning without serotherapy. Immune monitoring was performed (harmonized sampling, handling and analyses) in a central lab. The primary endpoint was probability of achieving CD4+ IR (>50*106/L within 100 days). Secondary endpoints were IR of B-cells, CD4+ and CD8+ T-cells, natural killer (NK)-cells, monocytes, and dendritic cells (DC) 7-365 days after HCT. In addition, TREC analyses were performed on CD3+ MACs-sorted cells. Linear-mixed effects modelling in LOESS-regression curves and two-sided log-rank test for univariate comparisons in cumulative incidence plots were used. Results 27 NiCord recipients (median 41.5; 13.4-61.7yrs) were included. NiCord cell dose consisted of median 6.4*106 CD34+/kg, and 2.3*106 CD3+T-cells/kg of the co-infused negative fraction (following CD133+ selection). Of these patients, 91% achieved successful CD4+ IR, which was comparable (p=0.76, Figure 1) to the 27 unCBT (median 15.4; 12.2-22.1 yrs) and 20 BMT (median 14.3; 12.1-19.7 yrs) recipients included in this study. We observed similar reconstitution of T-cells (p=0.15), monocytes (p=0.94), conventional DCs (p=0.41), and plasmacytoid DCs (p=0.52). Interestingly, reconstitution of NK-cells (p Conclusions In-depth immune monitoring reveals fast and full IR after NiCord HCT in adult patients, which is equal or even faster to IR after unCBT or BMT, despite the younger age of the AYA cohorts (expected to reconstitute faster). This may be explained by the higher stem cell dose and higher proliferative capacity of the NiCord-expanded product. Optimal comparison of IR in NiCord vs. unCBT in a randomized phase 3 trial is underway.

Allogeneic (allo) hematopoietic cell transplantation (HCT) has evolved into a potent curative treatment option for a variety of malignant and nonmalignant diseases. The occurrence of complications and mortality after allo-HCT is, however, still high and is strongly associated with immune reconstitution (IR). Therefore, detailed information on IR through immunomonitoring is crucial to improve survival chances after HCT. To date, information about the reconstituting immune system after allo-HCT in pediatric patients is mostly derived from routine standard-of-care measurements. More profound knowledge on IR may provide tools to better predict and modulate adverse reactions and, subsequently, improve survival chances. Here, we provide an overview of IR (eg, immune cell subsets and circulating chemokines/cytokines) after allo-HCT in children, taking into account different cell sources and serotherapy, and discuss strategies to enhance immunomonitoring. We conclude that available IR data after allo-HCT contain limited information on immune cell families (mostly only generic T, B, and NK cells), which would improve with more detailed information on reconstituting cell subsets or effector cell functionality at earlier time points (1 month). In addition, secretome data (eg, multiplex cytokine/chemokine profiles) could add to the understanding of IR mechanisms and cell functionality and may even provide (early) biomarkers for individual disease outcome, such as viral reactivity, graft-versus-host disease, or graft-versus-leukemia. The present data and suggestions for more detailed, standardized, and harmonized immunomonitoring in future (pediatric) allo-HCT studies will pave the path to "precision transplantation:" an individualized HCT approach (including conditioning), based on detailed information on IR and biomarkers, aiming to reduce transplantation related mortality and relapse, and subsequently improve survival chances.

Meeting abstracts Development of novel (immune) therapies is of utmost importance to improve survival in relapsed pediatric-AML (acute myeloid leukemia). We aim to develop a powerful and safe therapy consisting of 2 synergistic components: Cord Blood (CB) HSCT and vaccination with CB-derived Wilms