BACKGROUND:Regenerating islet-derived 3-alpha (REG3α) is a serum biomarker in patients with graft-versus-host disease (GVHD) linked to 6-month mortality. REG3α is produced by intestinal Paneth cells, which are implicated in Crohn's disease (CD) pathophysiology. OBJECTIVE:To assess associations between serum REG3α and progressive CD DESIGN: Serum REG3α was measured in two cross-sectional (M: Mount Sinai, L: Leuven) and a pre-diagnostic cohort (P: PREDICTS) with serial samples up to 10 years before CD diagnosis. Tissue REG3α expression was assessed via bulk RNA sequencing from paired ileal and colonic biopsies. Serum REG3α and tissue REG3α were associated with CD progression (hospitalization, surgery, steroid course, or new advanced therapy). Single-cell RNA sequencing data explored associations between REG3α expression, Paneth cell phenotypes, and CD. RESULTS:In 394 patients, high serum REG3α associated with CD progression, independent of C-reactive protein and endoscopic activity (M: HR 1.9 (95%CI 1.3-2.8); L: HR 2.9 (95%CI 1.9-4.6), both p<0.001). The association persisted in patients with mild or inactive CD. In the pre-diagnostic cohort, high serum REG3α predicted the development of CD, particularly complicated (B2/3) and surgical presentations, up to 10 years before diagnosis (P). Analysis of REG3α expression and Paneth cell transcriptomes suggested that CD is associated with loss of regenerative Paneth cell populations and enrichment in REG3α-expressing populations, suggesting a mechanism through which changes in serum REG3α associate with complicated CD. CONCLUSION:Serum REG3α holds potential as a non-invasive, prognostic biomarker in CD, independent of disease activity. High serum REG3α, even years before diagnosis, is linked to a complicated disease course.

Background The Blood and Marrow Transplant Clinical Trials Network (BMT CTN) Infection Grading System (IGS), a three-grade scale with increasing infection severity, was developed in 2011 to address limitations in the CTCAE system in capturing infection-related morbidity in hematopoietic cell transplant (HCT) trials. This study aimed to validate the prognostic value of the BMT CTN IGS in predicting 6-month overall survival (OS) following allogeneic HCT Methods A retrospective analysis was conducted using prospectively collected participant-level data from five BMT CTN multicenter trials (1202, 1203, 1301, 1502, and 1703), sourced via BioLINCC after IRB approval. Overall survival (OS) using Cox regression, where infection grade (2, 3) was treated as a time-varying covariate (Cordonnier et al. 2006). Results Among 3014 allogeneic HCT recipients across 5 trials, a total of 1327 grade 2 and 3 infections (defined by the grading system, Shahid et al. 2024) episodes were documented. Basic demographic characteristics by study are summarized in Figure 1; detailed information is available in the respective original publications. The median follow-up across studies was 721 days.The total cumulative incidence of infections (grades 2 and 3) was 20%. Grade 2 infections were most frequent (76%, 1009/1327), followed by grade 3 (24%,318/1327) infections. Of these infections, 49% were bacterial, 45% viral, and 5% fungal in origin. The cumulative incidence of grade 2 and 3 infection across the studies was variable (Figure 2a), with grade 3 infections exceeding a 5% cumulative incidence across all studies combined (Figure 2b).A total of 2984 (99%) of the patients were included in the outcomes analysis. Grade 3 infections were significantly associated with increased mortality across all studies combined (hazard ratio (HR) = 2.5, CI 2.0- 3.1, p = <0.001) while grade 2 infections were not (HR = 1.2, CI 0.97-1.42, p = 0.1) across all studies. Subgroup analyses stratified by study demonstrated an association between grade 2 infections and mortality in protocols 1203 and 1301(Figure 3a). The cumulative hazard for grade 2 infection reached ∼0.85 by day 1500 compared to Grade 3 infections at around ∼0.35 (Figure 3b). Conclusion Infectious complications are frequent post-allogeneic HCT, and grade 3 infections are strongly predictive of increased mortality. This analysis validates the BMT CTN IGS as a reliable and clinically meaningful grading tool to assess infection-related mortality risk in allogeneic HCT recipients and underscores its utility for standardized infection grading in future clinical trials.

The use of systemic corticosteroids is recommended only for grades II to IV acute graft-versus-host disease (GVHD), but many patients are often treated at the onset of grade I in real-world practice. We retrospectively analyzed outcomes of 1143 patients with grade I GVHD from 24 Mount Sinai Acute GVHD International Consortium (MAGIC) transplant centers. Approximately half received "up-front" systemic steroids, whereas the remainder received topical steroids alone; GVHD progressed to grade II to IV in only one-third of these patients. Up-front steroids decreased the risk for grade II to IV GVHD but not grade III to IV acute GVHD or the need for a second line of treatment. Systemic steroids were equally effective at treating grade I or II GVHD; however, up-front therapy exposed 3 times more patients to systemic steroids, with a corresponding threefold increase in GVHD that required additional therapy and a threefold rise in infection-related deaths. In multivariable analyses, up-front steroids were significantly associated with higher nonrelapse mortality (NRM; P = .026). MAGIC biomarkers identified most patients with grade I GVHD as low risk and unlikely to progress to severe GVHD regardless of treatment. Nevertheless, low-risk patients who received up-front steroids experienced a threefold increase in infectious deaths compared with those treated topically. This study supports the consensus recommendation of topical therapy for patients with low-risk grade I GVHD, a strategy that can be supported by biomarkers to avoid unnecessary steroid exposure and increased NRM.

Allogeneic hematopoietic cell transplantation (allo-HCT) is a potentially curative treatment for many hematological malignancies, but graft-versus-host disease (GVHD) is a common complication. Low gut microbiome diversity is associated with higher GVHD risk and shorter survival in multiple studies. Recently, the BMT CTN 1703 clinical trial demonstrated superiority of a GVHD-prophylaxis regimen including post-transplant cyclophosphamide (PTCy) compared to the standard prophylaxis (tacrolimus and methotrexate, Tac/MTX) in terms of GVHD-free, relapse-free survival at one year among reduced intensity conditioning allo-HCT recipients. However, the effect of PTCy on the gut microbiome and its association with clinical outcome have not been described. Here, we report on a companion randomized clinical controlled trial (BMT CTN 1801), which collected 2575 longitudinal stool samples from 304 study participants. Samples were obtained up to weekly up to day 84 post allo-HCT and at less frequent intervals thereafter, up to 2 years. Microbiome diversity and absolute microbial load were lower in the PTCy group compared to the Tac/MTX group on days 14-28 post-HCT. However, diversity at the timepoint closest to neutrophil engraftment was not significantly associated with non-relapse mortality after one year or other clinical outcomes, contrary to expectations from previous studies. Microbial domination events, when a single species exceeds 30% relative abundance, were comparable across treatment arms and reflected both pathogen blooms as well as less severe disruptions of the microbial community. Clostridium scindens and secondary bile acid metabolism pathways were less prevalent in the PTCy arm than in the Tac/MTX arm post-HCT, yet presence of secondary bile acid metabolism pathways was associated with a lower risk of chronic GVHD. Given that PTCy was associated with a greater disruption of the microbiome as measured by diversity, absolute microbial abundance, and bile acid metabolism capability, but better clinical outcomes overall, these data suggest that the importance of the microbiome in modulating the host immune systems after allo-HCT is specific to different types of GVHD prophylaxis.

We report the 5-year analysis of tisagenlecleucel in 79 pediatric and young adult patients with relapsed or refractory (r/r) B-cell ALL (B-ALL) from the global phase II ELIANA trial (ClinicalTrials.gov identifier: NCT02435849), with a median follow-up of 79.4 months. Tisagenlecleucel was administered as a single infusion with dosing normalized by weight in patients ≤50 kg. Key long-term end points included relapse-free survival (RFS), overall survival (OS), and safety. Censoring included loss to follow-up, withdrawal of consent, new anticancer therapy (± stem cell transplantation [SCT]), and death. The estimated 5-year RFS among responders (n = 70) with and without inclusion of SCT in censoring for new anticancer therapies was 47.3% and 51.0%, respectively. The median time to B-cell recovery was not reached with censoring for all further anticancer therapies, including SCT. The median OS was not reached. The estimated OS at 5 years was 55.0% and 62.4% with and without inclusion of SCT in censoring for new anticancer therapies, respectively. In total, 17 responders received a postinfusion SCT, 14 while still in complete remission. No new or unexpected adverse events were reported. These findings continue to support the potential of tisagenlecleucel as definitive therapy for many heavily pretreated pediatric and young adult patients with r/r B-ALL.

Graft-versus-host disease (GVHD), an often-fatal complication of allogeneic hematopoietic stem cell transplantation (HCT), is driven by inflammatory injury that damages target organs of gastrointestinal (GI) tract, skin and liver. Effective therapies must not only suppress GVHD reactivity of donor lymphocytes but permit regeneration of the damaged epithelium, particularly in the GI tract. Systemic corticosteroids are the standard first-line treatment for GVHD due to their powerful immunosuppressive and anti-inflammatory properties but may retard epithelial repair. Ruxolitinib, a selective JAK1/2 inhibitor, is an approved therapy for steroid-refractory GVHD, although its direct effects on epithelial repair are unknown. Intestinal stem cells (ISCs) that are critical for maintaining gut integrity and barrier function are key cellular targets of GVHD. We employed both ileal and colonic organoid cultures to study the direct effects of methylprednisolone and ruxolitinib under conditions that controlled the strength of the GVH reaction. Ruxolitinib prevented inflammatory apoptosis in both human and murine organoids and preserved ISC function and proliferation, while corticosteroids offered no protection and in fact suppressed proliferation. This study highlights the importance of GVHD therapies that facilitate epithelial repair and regeneration, protect target tissues and suppress alloreactivity of donor T cells.

Acute graft-versus-host disease (GVHD) is a well-established complication of allogeneic hematopoietic cell transplantation. Although the incidence of severe acute GVHD is decreasing with advances in prophylactic approaches, many clinical questions remain. Herein, acute GVHD diagnosis, grading, and treatment are reviewed and critically evaluated. Specific criteria were used for searching the published literature, grading the quality and strength of evidence, and grading the strength of recommendations. A panel of experts developed consensus recommendations for the clinical management of acute GVHD, providing guidance on behalf of the American Society for Transplantation and Cellular Therapy. Key recommendations include the following: (1) The MAGIC grading scheme is supported as the consensus grading scheme for acute GVHD. (2) Clinical tools and noninvasive blood-based biomarkers can aid in risk stratification of patients with newly diagnosed acute GVHD. (3) Corticosteroids remain the recommended first-line systemic therapy for acute GVHD despite innovative approaches to improve front-line treatment. (4) Ruxolitinib is supported as the second-line therapy standard while acknowledging that remestemcel-L is a recognized alternative standard for pediatric patients. No consensus treatment exists for third-line therapy. Ongoing and future studies will seek to improve upon the current treatment paradigm through risk-adapted strategies. Innovative investigative approaches will be needed to address the unmet needs in acute GVHD.

Introduction Several prognostic biomarkers have been studied in acute graft versus host disease (aGVHD), but none are targetable. Thymic stromal lymphopoietin (TSLP) is an IL7-like cytokine produced by epithelial cells in skin and gastrointestinal tract, in response to environmental and proinflammatory stimuli. Two isoforms of TSLP are known. The 7kDa short isoform is constitutively expressed and mediates homeostatic functions while the 18kDa long isoform is upregulated in inflammation. The heterodimeric TSLP receptor contains the IL-7Ra subunit and is expressed on dendritic cells, B cells, activated and memory T cells and absent on regulatory T cells. TSLP-receptor binding leads to downstream activation of JAK-STAT, driving antigen exposed CD4+ T cells towards Th-2 differentiation and production of IL4, IL5, IL13 and TNFa. TSLP is implicated in type-2 and Th-17 type immune responses, with the latter known to play a role in GVHD. Anti-TSLP monoclonal antibody Tezepelumab is approved for steroid refractory asthma. TSLP levels when measured by ELISA do not differentiate the isoforms. We measured TSLP isoforms with nano immunoassay (NIA) in plasma and serum of allogeneic stem cell transplant (HCT) recipients with and without aGVHD. Objective To test the association of TSLP isoform levels with presence and/or severity of aGVHD. Methods TSLP levels were measured in plasma and serum samples obtained from 19 HCT patients without GVHD and 23 HCT patients with aGVHD from Stanford University, and 60 patients with steroid refractory lower gastrointestinal tract (SR-LGI) aGVHD at start of second line therapy obtained from the Mount Sinai Acute GVHD International Consortium (MAGIC) biorepository. TSLP was measured using Peggy Sue NIA as relative units of chemiluminescence. Using the Wilcoxon rank sum test, p values were generated for pairwise comparisons with Holm-Bonferroni multiple comparisons correction. Results Using NIA we discovered novel, high molecular weight isoforms (HMWI) of TSLP ranging from 29kDa to 40kDa size. Median TSLP HMWI levels were 33952 (IQR 29410, 41953) without aGVHD, doubled to 68305 (IQR 46349, 95888) with aGVHD (any organ), and quadrupled to 130152 (IQR 105578, 194797) in SR-LGI-aGVHD (Table 1). Pairwise comparisons between no GVHD vs aGVHD (p=0.0008) and aGVHD vs SR-LGI-aGVHD (p<0.0001) demonstrated significance. (Table2 & Fig1). In SR-LGI-aGVHD, there was no significant difference in TSLP HMWI levels between survivors and 6mNRM group (p=0.8). Conclusions Novel HMW isoforms of TSLP measured with NIA are significantly higher in aGVHD and increase further in SR-LGI-aGVHD, indicating that TSLP may have value as a diagnostic and predictive biomarker in aGVHD. Molecular characterization of the newly discovered TSLP isoforms, the role of TSLP in the pathogenesis of aGVHD and therapeutic targeting of TSLP in aGVHD with monoclonal antibodies warrant investigation.

Approximately 30% of patients with acute graft-versus-host disease (GVHD) develop steroid-refractory disease and have very poor outcomes. Ruxolitinib has become the standard of care for steroid-refractory acute GVHD, but it is unclear which patients derive benefit. The MAGIC Composite Score (MCS), an algorithm that combines clinical symptoms and biomarkers, has been validated to predict outcomes at the start of primary GVHD treatment. Here, we evaluated its performance at the initiation of second-line treatment in 278 patients. MCS stratified patients into three risk groups (MCS1-3), with the majority (88%) classified as intermediate or high risk. Increasing MCS score was associated with progressively higher 1-year non-relapse mortality (NRM) rates (16%, 41%, and 73%; p<0.001), lower 1-year survival (77%, 56%, and 24%; p<0.001), and lower complete response (CR) rates at day 28 (47%, 38%, and 20%, respectively; p<0.01). The area under the receiver operating characteristic curve (AUROC) for 1-year NRM was significantly higher with MCS compared to clinical symptoms alone (0.70 vs. 0.63; p=0.023). Among patients treated with ruxolitinib, higher MCS similarly predicted higher NRM and lower survival and CR rates. Patients classified as MCS2/3 had poor outcomes despite ruxolitinib, underscoring the need for novel therapies in this patient population. In conclusion the MCS is an accurate predictor of outcomes for patients who require second-line treatment and may be of use as an eligibility criterion for future clinical trials in this high-risk population.

10000 Background: Persistent MRD portends a high risk of relapse in B-ALL. In the Children’s Oncology Group (COG) AALL0232 frontline trial for HR B-ALL, children and young adults with MRD ≥0.01% at the EOC had poor 5 y disease-free survival (DFS) of 39% (Borowitz, Blood , 2015). We report the primary analysis of a phase 2 trial, in collaboration with COG, of tisa-cel, a CD19-directed chimeric antigen receptor (CAR) T cell therapy, in this very high-risk (VHR) population. Methods: COG AALL1721/CASSIOPEIA (NCT03876769) is a global (34 sites in the US/Canada, 10 sites in EU/UK) phase 2 study of tisa-cel in patients (pts) aged 1-25 y with National Cancer Institute HR B-ALL and persistent MRD ≥0.01% by flow cytometry at EOC. A single dose of tisa-cel (0.2-5×10 6 CAR+ T cells/kg for pts ≤50 kg or 0.1-2.5×10 8 CAR+ T cells for pts >50 kg) was given after standard lymphodepleting chemotherapy. Reinfusion was available for pts with B-cell recovery within 6 mo and/or MRD ≥0.01% at any time. Primary endpoints were 4 y overall survival (OS) and 5 y DFS, defined as time from first infusion to morphologic relapse, secondary malignancy, or death, without censoring for new anticancer therapy, including stem cell transplant (SCT). Secondary endpoints included DFS with censoring, MRD negative rate (MRD <0.01%) at 3 mo, and safety. Results: The primary analysis included 121 infused pts; 1 pt was not infused due to adverse event. Median age was 14 y (range: 1-24 y), 67% were male, 6% had trisomy 21. As of August 20, 2025, median time from infusion to last follow-up was 38 mo. OS at 4 y was 85% (95% CI: 76-90%). DFS (95% CI) at 3 y and 5 y without censoring for new therapy was 71% (61-79%) and 64% (53-73%). DFS (95% CI) at 3 y and 5 y with censoring was 69% (55-79%) and 62% (46-74%). MRD negative rate at day 29 and 3 mo post infusion was 95% and 86%. Relapse occurred in 31 pts, of which 7 occurred after new therapy/SCT. 41 (34%) pts received a second infusion. 58 (48%) pts received new anticancer therapy/SCT without prior relapse. Median duration of B-cell aplasia was 5.6 mo. Probability (95% CI) of ongoing remission and B-cell aplasia at 3, 6, and 12 mo was 76% (67-83%), 47% (38-56%), and 31% (23-41%). Following first infusion, cytokine release syndrome (CRS) occurred in 37% (grade ≥3, 2%), immune effector cell–associated neurotoxicity syndrome (ICANS) in 3% (grade ≥3, 1%). There were 18 deaths, all post new therapy/SCT and/or relapse. Conclusions: Durable remissions, with very low rates of severe CRS/ICANS, were achieved with tisa-cel in a VHR population with persistent MRD in frontline therapy, who historically have low 5 y DFS. Nearly 50% of pts received new anticancer therapy, including SCT, without prior relapse. Longer follow-up is needed to confirm 5 y DFS. Clinical trial information: NCT03876769 .

BACKGROUND:T cell proliferation and repertoire reconstitution is a hallmark of successful hematopoietic cell transplantation (HCT). This process may be modeled as a dynamical system and in such a system, precise telomere length (TL) measurement may reflect the proliferative capacity of donor T cells. TL for different chromosomes span a few orders of magnitude, and different blood cell populations including T cell clones display variable TL; these differences across the cell populations are not represented when examining average leukocyte TL. This study aims to develop a method that integrates the entire spectrum of TL observed within a sample to better understand the influence on clinical outcomes following HCT. METHODS:To better reflect the entire span of TL, we used data generated using the Telomere Shortest Length Assay (TeLSA) that provides discrete measurements of individual telomeres for each blood DNA sample. TeSLA leukocyte TL (LTL) measurements were performed on 72 paired samples collected from the donor pretransplant (D-LTL) and the recipient 90 days following HCT (post-HCT LTL). Area under the curve (AUC) calculations were used to incorporate the full distribution of measured LTL from each sample. The magnitude of LTL shortening after HCT was calculated as the difference between the AUCs for D-LTL and corresponding post-HCT LTL, and referred to as AUC delta-TL. RESULTS:Telomere band lengths ranged from 350 base pairs to 16.7 kilobases with a logarithmically declining distribution in all samples when arrayed in descending order. The AUC delta-TL predicted patient overall survival (OS; P-log rank <.0001); HCT recipients with an intermediate degree of TL shortening (25th to 75th percentile/Q2&3) post-HCT experienced the best outcomes (2 years OS = 92%), whilst donors with minimal (<25th percentile/Q1; 2 years OS = 33%; adjusted HR versus intermediate shortening = 9.3, P = .001) or maximal (>75th percentile/Q4; 2 years OS = 59%; adjusted HR = 6.0, P = .01) TL shortening had worse outcomes. CONCLUSION:The findings described herein suggest that the degree of donor telomere attrition may correlates with clinical outcomes following transplant, possibly reflecting alloreactive T cell expansion. Accounting for the entire span of telomere lengths, may better identify post-transplant risk groups.

This abstract has been removed: please see Elsevier policy on article withdrawal (https://www.elsevier.com/about/policies-and-standards/article-withdrawal). This article has been removed at the request of the author. This abstract has been removed because it was not presented at the 2025 BMT Tandem Meeting.

BACKGROUND:Response to first-line treatment in acute graft-versus-host disease (GVHD) is typically assessed at day 28 (D28) in clinical trials, but this convention was established without accounting for onset severity and thus was optimized for mild-moderate GVHD that comprises the majority of cases. Furthermore, the initiation of second-line therapy, which is considered primary treatment failure, is not based on standardized criteria and thus remains subjective and inconsistent for patients regardless of clinical trial participation. OBJECTIVE:In this study we hypothesized that an early assessment of treatment response at D7 would accurately predict long-term outcomes for patients with severe GVHD and support the initiation of second line therapy in non-responders. STUDY DESIGN:We analyzed six-month outcomes by D7 and D28 response for 1561 patients receiving systemic therapy for acute GVHD in two large trial cohorts - one observational (n = 1008) and one interventional (n = 553) after stratification for onset severity using Minnesota risk criteria. RESULTS:Patients with Minnesota standard risk GVHD comprised approximately 80% of each cohort. D7 responses predicted much smaller differences in 6-month NRM (observational: responders: 9% versus non-responders: 23%; interventional: 12% versus 24%) than D28 responses (observational: 7% versus 35%; interventional: 9% versus 36%) and second line therapy was deferred in ∼85% of patients who had not responded by D7. More than half of this "wait and see" group proved to be slow responders with low 6-month NRM of <10% and as a result the D28 response more accurately predicted 6-month NRM than D7 response (AUC: observational; 0.73 versus 0.63, P < .001; interventional: 0.70 versus 0.60, P = .002). Subset analyses confirmed the superiority of D28 over D7 in children with Minnesota standard risk GVHD and in patients with little or no lower gastrointestinal (GI) GVHD (stage 0 or 1) but not patients with stage 2 GI GVHD. In contrast, among Minnesota high risk patients, D7 (observational: 26% versus 54%; interventional: 20% versus 56%) and D28 (observational: 20% versus 57%; interventional: 22% versus 62%) responses both predicted large differences in 6-month NRM with similar AUCs (observational; 0.65 versus 0.69, P = .171; interventional: 0.68 versus 0.71, P = .581). Subset analyses demonstrated similar AUCs for both D7 and D28 in children with Minnesota high risk GVHD and in patients with severe GI GVHD (stage 2-4). Notably, second line therapy was deferred for 70% of high-risk patients without a response at D7. The "wait and see" approach was common even after the approval of ruxolitinib for steroid-resistant GVHD, and their 6-month NRM was ∼50%. CONCLUSION:These findings support the use of D7 response as an actionable assessment timepoint in high risk acute GVHD and highlight the need for severity-adapted response criteria in both clinical practice and trial design.

ABSTRACT:Overall response (OR) that combines complete (CR) and partial responses (PR) is the conventional end point for acute graft-versus-host disease (GVHD) trials. Because PR includes heterogeneous clinical presentations, reclassifying PR could produce a better end point. Patients in the primary treatment cohort from the Japanese Society for Transplantation and Cellular Therapy (JSTCT) were randomly divided into training and validation sets. In the training set, a classification and regression tree algorithm generated day 28 refined response (RR) criteria based on symptoms at treatment and day 28. We then evaluated RR for primary and second-line treatments, using the area under the receiver operating characteristic curve (AUC) and negative predictive value (NPV) for 6-month nonrelapse mortality as performance measures. RR considered patients with grade 0/1 at day 28 without additional treatment as responders. RR for primary treatment produced higher AUCs than OR with small improvement of NPVs in both validation sets: JSTCT (AUC, 0.73 vs 0.69 [P < .001]; NPV, 92.0% vs 89.6% [P < .001]) and the Mount Sinai Acute GVHD International Consortium (MAGIC; AUC, 0.71 vs 0.68 [P = .032]; NPV, 90.9% vs 89.8% [P = .009]). RR for second-line treatment produced similar AUCs but much higher NPVs than OR in both validation sets of JSTCT (AUC, 0.64 vs 0.63 [P = .775]; NPV, 74.5% vs 66.0% [P < .001]) and MAGIC (AUC, 0.67 vs 0.64 [P = .105]; NPV, 86.8% vs 76.1% [P = .004]). Classifying persistent but mild skin symptoms as responses and residual lower gastrointestinal GVHD as nonresponses were major drivers in improving the prognostic performance of RR. Our externally validated day 28 RR would serve as a better end point than conventional criteria in future first- and second-line treatment trials.

Background Treatment strategies for Crohn’s disease (CD) suppress diverse inflammatory pathways but many patients remain refractory to treatment. Autologous haematopoietic stem cell transplantation (SCT) is an emerging therapy for medically refractory CD though the mechanisms through which it circumvents refractory pathophysiology are unknown. Objective The objective of this study is to understand how the immune system reconstitutes post-SCT and whether SCT may function as a cellular therapy restoring appropriately responsive immune cell populations from haematopoietic stem cells (HSCs). Design Adults with CD with active clinical and endoscopic disease who failed available medical therapies were enrolled in a phase II study of SCT for refractory CD (n=19). Blood and intestinal samples were collected longitudinally and analysed using CyTOF and scRNA-seq. Stem cell autografts were functionally assayed in mouse xenograft models. Results scRNA-seq and CyTOF analyses reveal that SCT predominantly affected the intestinal myeloid lineage with loss of inflammatory populations and return of macrophages capable of supporting mucosal healing. Xenograft models using patient HSCs suggested that HSCs support the early reconstitution of the myeloid lineage and reveal an impairment of short and long-term HSC engraftment that may determine SCT outcomes. Conclusions This study suggests SCT functions as a myeloid-directed cellular therapy reinforcing the critical role of macrophages in refractory CD pathophysiology and as a target for cellular therapies. Furthermore, we report an unrecognised functional heterogeneity among HSC subpopulations in CD that may be relevant to our understanding of CD treatment and pathophysiology.

ABSTRACT:Allogeneic hematopoietic cell transplantation (allo-HCT) is a curative option for patients with high-risk malignancies and nonmalignant disorders. Long-term survival depends on robust immune reconstitution (IR), which governs overall immune homeostasis and risks of infection, graft-versus-host disease, and relapse. However, despite its centrality to posttransplant outcomes, IR is not consistently monitored across transplant centers, limiting ability to generate meaningful, comparable, and translatable data. This review synthesizes current knowledge on numerical and functional IR milestones after allo-HCT, with a primary focus on flow cytometry-based monitoring of key immune cell subsets. Importantly, early CD4+ T-cell recovery (achieving >50 cells per μL by day 100 after transplant), is supported by strong clinical evidence and correlates with improved outcomes. Although emerging data suggest that additional subsets (CD8+ T cells, natural killer cells, B cells, naïve and recent thymic emigrant T cells, and γδ T cells) may also influence clinical trajectories, further harmonized, multicenter studies are needed to validate prognostic relevance across transplant settings. We propose practical, evidence-based guidelines for IR monitoring, including recommended time points, preferred assays, and flow cytometry panel components. Additionally, we highlight modifiable factors (eg, immunosuppressive drug exposures, graft manipulation) offering interventional opportunities for influencing IR. Harmonized monitoring strategies will support robust correlation between IR and clinical outcomes, guide real-time risk stratification, and facilitate the development of targeted, individualized transplant approaches. Standardization efforts led by consortia and registries are essential for advancing knowledge and optimizing care. We provide a roadmap for implementing uniform IR monitoring to improve outcomes and quality of life for allo-HCT recipients.

Purpose To determine whether fluorine 18 (18F) fluorothymidine (FLT) PET imaging alone or combined with Mount Sinai Acute GVHD International Consortium (MAGIC) biomarkers could help identify subclinical gastrointestinal graft versus host disease (GI-GVHD) by day 100 following hematopoietic stem cell transplantation (HSCT). Materials and Methods 18F-FLT PET imaging was analyzed in a prospective pilot study (ClinicalTrials.gov identifier no. NCT01338987) with a primary end point of engraftment for a planned secondary end point identifying GI-GVHD. Regions of interest (ROIs) in the colon (1 cm3), jejunum (1 cm3), and ileum (1 cm3) were drawn in the area of greatest signal intensity within each segment of the GI tract by using software. Standardized uptake values (SUVs) were captured on day 28 following transplantation, along with MAGIC serum biomarkers and MAGIC algorithm probability (MAP) scores using MAGIC serum biomarkers collected at days 28-35. Results Among 20 participants (median age, 33.85 years [IQR: 28.65-39.25 years]; 11 female, nine male), seven presented with clinically diagnosed GI-GVHD by 100 days. Increased SUV was observed throughout the GI tract, most predominantly in the jejunum. Maximum and mean SUV by day 100 were significantly elevated in those with GI-GVHD (maximum SUV, 4.81; mean SUV, 3.73; n = 7) compared with those without (maximum SUV, 3.99; mean SUV, 2.56). MAP score (P = .02) was associated with acute GVHD on day 28 but not on day 100. Spearman correlation between maximum SUV in the jejunum and MAP score was r = 0.65 (P = .002). Conclusion These data suggest that 18F-FLT PET may help identify acute GI-GVHD after HSCT and could inform location in areas difficult to biopsy. Keywords: Transplantation, PET/CT, Bone Marrow, Abdomen/GI ClinicalTrials.gov identifier: NCT01338987 © RSNA, 2024.