Impact of Day 14 Peripheral Blood Chimerism after Allogeneic Hematopoietic Stem Cell Bone Transplantation on the Treatment Outcome of Non-Malignant Disease

Relevance of Recent Thymic Emigrants Following Allogeneic Hematopoietic Cell Transplantation for Pediatric Patients with Inborn Errors of Immunity

BACKGROUND

Allogeneic hematopoietic cell transplantation (HCT) can be curative for many inborn errors of immunity (IEI). Timely neothymopoiesis is paramount to favorable clinical outcomes after HCT. Neothymopoiesis can be quantified by flow cytometric measurement of circulating recent thymic emigrants (RTE; CD31+CD4+CD45RA+ T cells).

OBJECTIVES

We hypothesized that decreased RTE would be associated with baseline HCT characteristics of older age at time of HCT and exposure to greater HCT conditioning intensity, as well as with HCT outcomes including mixed (<95%) lymphoid donor chimerism and presence of acute graft-versus-host disease (GvHD).

STUDY DESIGN

In this retrospective analysis two cohorts of pediatric IEI HCT recipients were identified at two centers that collected RTE data following allogeneic HCT. For both cohorts, patient and HCT information was recorded including but not limited to patient age, lymphoid donor chimerism, and occurrence of acute GvHD. Mixed effects models were fitted for the repeated measures of RTE with these covariates and time.

RESULTS

Between 2012 and 2021, a total of 162 pediatric IEI HCT recipients transplanted across both cohorts were eligible for inclusion. Cohort A (n = 34) included 23 males (68%). Median age at HCT was 2.2 years (interquartile range (IQR) 0.8 to 10.8). Eight (23.5%) underwent reduced intensity (RIC), 23 (67.7%) reduced toxicity myeloablative (RTC), and 3 (8.8%) myeloablative (MAC) conditioning. All received alemtuzumab serotherapy. Cohort B (n = 128) included 87 males (68%). Median age at HCT was 1.4 years (IQR 0.7 to 5.3). Seventy-six (59.4%) underwent RIC, 38 (29.7%) RTC, and 14 (10.9%) MAC. RIC and RTC patients received alemtuzumab serotherapy, MAC antithymocyte globulin. In the linear mixed effect model for RTE at 1 year after HCT for Cohort A, significant negative associations included increasing age (P < .0001) and RTC compared to RIC (P < .01). In the linear mixed effects model for RTE at 1 year after HCT for Cohort B, significant negative associations included increasing age (P < .0001), grade 2 to 4 acute GvHD (compared to grade 0 to 1; P < .01), MAC compared to RIC (P < .0001), MAC compared to RTC (P < .01), and RTC compared to RIC (P = .03).

CONCLUSIONS

Serial measurement of RTE is a useful assessment of thymic function after HCT. In pediatric patients with IEI, older age at transplantation, greater intensity of conditioning, and occurrence of grade 2 to 4 acute GvHD were strongly associated with slower thymic-derived immune reconstitution. Mixed lymphoid donor chimerism was not associated with RTE in the linear mixed effects model. In addition to augmenting current anticipatory guidance on HCT outcomes, these findings may guide personalization of regimens to optimize clinical outcomes in IEI HCT.

Second Allogeneic Hematopoietic Cell Transplantation Following Graft Failure in Children

BACKGROUND

Graft failure (GF) is a major complication of allogeneic hematopoietic cell transplantation (allo-HCT). Secondary transplantation has been recognized as a potential curative intervention.

METHODS

This study aimed to investigate the characteristics and outcomes of salvage transplantation by analyzing the patients who underwent a second HCT for GF following the initial allo-HCT between 1998 and 2020.

RESULTS

Overall, 23 recipients were identified, including 14 and 9 individuals with primary and secondary GF, respectively. Nine recipients underwent a second transplant from the same donor. Familial mismatched donors predominated in the second HCT (86.9%), with reduced-intensity conditioning as the prevailing approach (60.9%). Neutrophil engraftment occurred in 17 patients (73.9%) following the second HCT at a median of 17 days (range: 9-58 days) post-transplantation. However, secondary GF subsequently occurred in 5 patients, and successful engraftment following salvage transplantation was achieved in 12 (52.2%) patients. In the entire study population, the estimated 5-year probability of overall survival (OS) and treatment-related mortality (TRM) were 30.4% and 58.5%, respectively. Among patients who achieved successful engraftment following a second transplantation, the OS and TRM rates were 41.7% and 33.3%, respectively, indicating a trend toward better OS and significantly lower TRM compared to those with GF. Notably, 17 patients died, with infection being the most common cause (n = 12), irrespective of the engraftment status.

CONCLUSION

A successful engraftment following a second allo-HCT reduced the TRM; however, the OS remained suboptimal. The effective control of infectious diseases remains crucial for patients with GF, regardless of the engraftment status following salvage transplantation.

The predictive value of T-cell chimerism for disease relapse after allogeneic hematopoietic stem cell transplantation

Introduction

Chimerism is closely correlated with disease relapse after allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, chimerism rate is dynamic changes, and the sensitivity of different chimerism requires further research.

Methods

To investigate the predictive value of distinct chimerism for relapse, we measured bone marrow (BM), peripheral blood (PB), and T-cell (isolated from BM) chimerism in 178 patients after allo-HSCT.

Results

Receiver operating characteristic (ROC) curve showed that T-cell chimerism was more suitable to predict relapse after allo-HSCT compared with PB and BM chimerism. The cutoff value of T-cell chimerism for predicting relapse was 99.45%. Leukemia and myelodysplastic syndrome (MDS) relapse patients' T-cell chimerism was a gradual decline from 2 months to 9 months after allo-HSCT. Higher risk of relapse and death within 1 year after allo-HSCT. The T-cell chimerism rates in remission and relapse patients were 99.43% and 94.28% at 3 months after allo-HSCT (P = 0.009), 99.31% and 95.27% at 6 months after allo-HSCT (P = 0.013), and 99.26% and 91.32% at 9 months after allo-HSCT (P = 0.024), respectively. There was a significant difference (P = 0.036) for T-cell chimerism between early relapse (relapse within 9 months after allo-HSCT) and late relapse (relapse after 9 months after allo-HSCT) at 2 months after allo-HSCT. Every 1% increase in T-cell chimerism, the hazard ratio for disease relapse was 0.967 (95% CI: 0.948-0.987, P<0.001).

Discussion

We recommend constant monitoring T-cell chimerism at 2, 3, 6, and 9 months after allo-HSCT to predict relapse.

The significance of mixed chimaerism and cell lineage chimaerism monitoring in paediatric patients post haematopoietic stem cell transplant

Haematopoietic stem cell transplants (HSCTs) are carried out across the world to treat haematological and immunological diseases which would otherwise prove fatal. Certain diseases are predominantly encountered in paediatric patients, such severe primary immunodeficiencies (PID) and diseases of inborn errors of metabolism (IEM). Chimaerism testing for these disorders has different considerations compared to adult diseases. This review focuses on the importance of cell-lineage-specific chimaerism testing and examines the appropriate cell populations to be assessed in individual paediatric patient groups. By analysing disease-associated subpopulations, abnormalities are identified significantly earlier than in whole samples and targeted clinical decisions can be made. Chimaerism methods have evolved over time and lead to an ever-increasing level of sensitivity and biomarker arrays to distinguish between recipient and donor cells. Short tandem repeat (STR) is still the gold standard for routine chimaerism assessment, and hypersensitive methods such as quantitative and digital polymerase chain reaction (PCR) are leading the forefront of microchimaerism testing. The rise of molecular methods operating with minute DNA amounts has been hugely beneficial to chimaerism testing of paediatric samples. As HSCTs are becoming increasingly personalised and risk-adjusted towards a child's individual needs, chimaerism testing needs to adapt alongside these medical advances ensuring the best possible care.

Hematopoietic stem cell transplantation with mesenchymal stromal cells in children with metachromatic leukodystrophy

Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder primarily affecting the white matter of the nervous system that results from a deficiency of the arylsulfatase A (ARSA). Mesenchymal stem cells (MSCs) are able to secrete ARSA and have shown beneficial effects in MLD patients. In this retrospective analysis, 10 pediatric MLD patients (MSCG) underwent allogeneic hematopoietic stem cell transplantation (HSCT) and received two applications of 2 x 106 MSCs/kg bodyweight at day +30 and +60 after HSCT between 2007 and 2018. MSC safety, occurrence of graft-versus-host disease (GvHD), blood ARSA levels, chimerism, cell regeneration and engraftment, MRI changes, and the gross motor function were assessed within the first year of HSCT. The long-term data included clinical outcomes and safety aspects of MSCs. Data were compared to a control cohort of seven pediatric MLD patients (CG) who underwent HSCT only. The application of MSC in pediatric MLD patients after allogeneic HSCT was safe and well tolerated and long-term potentially MSC-related adverse effects up to 13.5 years after HSCT were not observed. Patients achieved significantly higher ARSA levels (CG: median 1.03 nmol∙10-6, range 0.41-1.73 | MSCG: median 1.58 nmol∙10-6, range 0.44-2.6; p<0.05), as well as significantly higher leukocyte (p<0.05) and thrombocyte (p<0.001) levels within 365 days of MSC application compared to CG patients. Statistically significant effects on acute GvHD, regeneration of immune cells, engraftment, MRI changes, gross motor function, and clinical outcomes were not detected. In conclusion, the application of MSCs in pediatric MLD patients after allogeneic HSCT was safe and well tolerated. The two applications of 2 x 106/kg allogeneic MSCs were followed by improved engraftment and hematopoiesis within the first year after HSCT. Larger, prospective trials are necessary to evaluate the impact of MSC application on engraftment and hematopoietic recovery.

Chimerism in the Realm of Hematopoietic Stem Cell Transplantation for Non-malignant Disorders-A Perspective

Hematopoietic stem cell transplantation (HCT) is a curative intervention in non-malignant disorders (NMD) that benefit from donor-derived hematopoiesis, immunity, and establishment of vital cells or enzyme systems. Stability or reversal of disease symptoms depends on adequacy and long-term stability of donor cell engraftment in the compartment of interest. Unlike hematologic malignancies where complete replacement with donor derived hematopoiesis is desirable for a cure, NMD manifestations can often be controlled in the presence of mixed chimerism. This allows for exploration of reduced intensity conditioning regimens that can limit organ toxicity, late effects, and increase tolerability especially in young recipients or those with a large burden of disease related morbidity. However, the levels of donor chimerism conducive to disease control vary between NMD, need to focus on the hematopoietic lineage necessary to correct individual disorders, and need to be assessed for stability over time, i.e., a whole lifespan. An enhanced ability to reject grafts due to recipient immune competence, alloimmunization, and autoimmunity add to the complexity of this balance making NMD a highly diverse group of unrelated disorders. The addition of donor factors such as stem cell source and Human-Leukocyte-Antigen match extend the complexity such that 'one size does not fit all'. In this perspective, we will discuss current knowledge of the role of chimerism and goals, approach to HCT, and emerging methods of boosting engraftment and graft function, and monitoring recommendations. We draw attention to knowledge gaps and areas of necessity for further research and research support.