How and when should we monitor chimerism after allogeneic stem cell transplantation?

Outcomes and prognostic factors associated with relapse after haploidentical stem cell transplantation for paediatric T-cell acute lymphoblastic leukaemia

The outcomes are poor for paediatric patients with T-cell acute lymphoblastic leukaemia (T-ALL) who relapse after haematopoietic stem cell transplantation (HSCT). However, studies focusing on paediatric patients with T-ALL following haploidentical HSCT (haplo-HSCT) are limited. We retrospectively identified a consecutive cohort comprising of 128 paediatric T-ALL after haplo-HSCT from 2642 consecutive ALL patients between January 2010 and June 2022. The 2-year overall survival and leukaemia-free survival were 67.77% ± 4.21% and 66.34% ± 3.82%, respectively, and the cumulative incidence of relapse (CIR) and non-relapse mortality were 33.82% ± 0.70% and 12.65% ± 0.46% respectively. According to the multivariate Cox regression analysis, CD34 cells, minimal residual disease (MRD) ≥0.01% before HSCT, chronic graft-versus-host disease (cGvHD) and cytomegalovirus were associated with relapse (p < 0.05). To develop a scoring system for stratifying patients, we combined the variables and stratified them into low (0-2 points) and high (3, 4) groups. Consequently, the 2-year CIR in low and high groups were 23.76% ± 1.83% and 48.22% ± 2.42% (p = 0.009), respectively. Children with T-ALL have poor long-term survival, and haplo-HSCT is a potent and safe treatment; however, the incidence of relapse is high. Eliminating pre-HSCT MRD, guaranteeing sufficient CD34 cells infusion and the occurrence of cGvHD and cytomegalovirus reactivation may benefit from relapse.

Chimerism analysis by ABO blood group genotyping with digital droplet PCR

OBJECTIVE

Chimerism analysis is an important post-transplant assessment for allogeneic hematopoietic stem cell transplant (HCT) recipients. Although various chimerism analysis techniques are already established, they are limited in terms of sensitivity, versatility, and turnaround time. Our objective was to develop a digital droplet polymerase chain reaction (ddPCR) assay for chimerism analysis using ABO gene polymorphisms as markers.

METHODS

Our new chimerism analysis method utilizes ddPCR to assess the ABO gene polymorphisms that encode the ABO blood genotype. ABO genotypes were determined in blood samples from 15 HCT recipients using the O panel (rs8176719) and B panel (rs8176746 and rs8176747).

RESULTS

The two panels distinguished six ABO genotypes (AA, AO, BB, BO, AB, and OO). The results of chimerism analysis using ABO genotypes with ddPCR were compatible with those of established methods, such as SRY gene analysis and the use of short tandem repeat markers via standard PCR. Our method could distinguish chimerism in 77% of donor and recipient combinations in the Japanese population.

CONCLUSIONS

We developed a sensitive and rapid chimerism analysis method for HCT using ABO gene polymorphisms in ddPCR.

Long-Term Outcomes of Allogeneic Stem Cell Transplantation for Relapsed/Refractory Hodgkin and Non-Hodgkin Lymphoma: Multi-center Experience from Turkey

This multicenter retrospective study evaluated the efficacy of allogeneic hematopoietic stem cell transplantation (allo-HSCT) on survival and safety in patients with relapsed/refractory (R/R) Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL). A total of 110 patients with R/R HL or NHL who underwent allo-HSCT between July 2007 and October 2022 at 7 adult stem cell transplant centers were evaluated. Progression-free survival (PFS), graft versus host disease-free survival (GRFS), and overall survival (OS) were the primary endpoints, and NRM was the secondary endpoint. Forty-one (37.3%) of the total patients were diagnosed with HL, 69 (62.7%) with NHL. The median age at the time of allo-HCT was 39.5 years (16-67), of which 66 (60%) were male. The median follow-up was 67.5 ± 8.1 months, and the rates of 5-year OS, PFS, and GRFS were 38.4%, 37%, and 34.8%, respectively. On multivariate analysis, CR/PR disease status after allo-HCT was significantly associated with longer PFS (HR: 13.47, 95% CI: 5.80-31.26, p = 0.000) and OS (HR: 5.23, 95% CI: 2.93-9.34, p = 0.000). CR/PR disease status after allo-HCT (HR: 5.79, 95% CI: 3.22-10.40, p = 0.000) and grade 1-2 acute GvHD (HR: 2.33, 95% CI: 1.25-4.35, p = 0.008) were significantly associated with longer GRFS. The 5-year cumulative incidence of NRM was 24.8% (95% CI, 12.5-36.7). The most common conditioning regimen was reduced intensity. Transplant outcomes are not influenced by disease subtype. However, the achievement of a CR/PR response after allo-HCT significantly prolongs OS, PFS and GRFS. In addition, the presence of acute grade 1-2 GvHD was found to be another factor prolonging GRFS. These results support the feasibility of allo-HCT, especially in heavily treated patients.

Mixed T-Cell Chimerism Following Hematopoietic Cell Transplantation for Non-Malignant Disorders Is Common, Facilitates Anti-Viral Immunity, and Is Not Associated with Graft Failure in Pediatric Patients

Myeloid chimerism better reflects donor stem cell engraftment than whole-blood chimerism in assessing graft function following allogeneic hematopoietic stem cell transplant (HCT). We describe our experience with 130 patients aged younger than 18 years, treated with allogeneic HCT using bone marrow or PBSC from HLA-matched donors for non-malignant diseases, whose pre-transplant conditioning therapy included alemtuzumab and who were monitored with lineage-specific chimerism after transplant. At 6 years post-transplant, overall survival (OS) was 91.1% and event-free survival (EFS) was 81.5%, with no grade III-IV acute GvHD or chronic GVHD observed. Recipient T-cells did not contribute to graft loss. Mixed T-cell chimerism (MC) did not affect EFS, and there was no connection between T-cell chimerism and myeloid chimerism in patients with MC or graft loss. MC significantly correlated with virus infection; more children with MC were CMV seropositive than those with complete chimerism (CC). Additionally, MC was more common in patients with CMV viramia post-transplant. CD8 T-cell reconstitution was affected by viral reactivation, including CMV, with CD8 T-cell counts higher in the MC group than in the CC group. Mixed T-cell chimerism is due to autologous, virus-specific, predominantly CD8, T-cell expansion, and is protective and not deleterious to the recipient.

The unnecessary use of short tandem repeat testing on bone marrow samples in patients after 1 year following allogeneic hematopoietic stem cell transplant

OBJECTIVES

To determine whether the information provided by short tandem repeat (STR) testing and bone marrow (BM) biopsy specimens following hematopoietic stem cell transplant (HSCT) provides redundant information, leading to test overutilization, without additional clinical benefit.

METHODS

Cases with synchronous STR and flow cytometric immunophenotyping (FCI) testing, as part of the BM evaluation, were assessed for STR/FCI concordance.

RESULTS

Of 1199 cases (410 patients), we found the overall concordance between STR and FCI was 93%, with most cases (1063) classified as STR-/FCI-. Of all discordant cases, 75 (6%) were STR+/FCI-, with only 5 (6.7%) cases best explained as identification of disease relapse. Eight cases were STR-/FCI+, representing relapsed/residual disease. Analysis of cases 1 year or more from transplant (54% of all cases) indicated only 9 (1.5%) were STR+/FCI-, and none uniquely identified relapse.

CONCLUSIONS

These data suggest that STR analysis performed 1 year or more post-HSCT does not identify unknown cases of relapse. Furthermore, while STR testing is critical for identifying graft failure/rejection within the first year posttransplant, FCI appears superior to STR at detecting late relapses with low-level disease. Therefore, STR testing from patients 1 year or more post-HSCT may be unnecessary, as BM biopsy evaluation is sufficient to identify disease relapse.

Can novel methods replace the gold standard chimerism method after allogeneic hematopoietic stem cell transplantation?

After hematopoietic stem cell transplantation, chimerism assay is a useful approach to monitor the success of the transplant and to select the appropriate treatment strategy, such as donor leukocyte infusion or immunosuppressive drug dosage. Short tandem repeat PCR is the method that has been accepted as the gold standard for chimerism. However, it has not yet been sufficient to detect mixed chimerism in patients with minimal residual disease. Simultaneously, recent years have been marked by developing sensitive, high-throughput, and accurate molecular genetic assays. These novel methods have subsequently been adapted for the analysis of post-transplant chimerism. In this review, we discuss the technical features of both novel and conventional gold standard chimerism assays. We also discuss their advantages and disadvantages.

Chimerism Testing by Next Generation Sequencing for Detection of Engraftment and Early Disease Relapse in Allogeneic Hematopoietic Cell Transplantation and an Overview of NGS Chimerism Studies

Chimerism monitoring after allogenic Hematopoietic Cell Transplantation (allo-HCT) is critical to determine how well donor cells have engrafted and to detect relapse for early therapeutic intervention. The aim of this study was to establish and detect mixed chimerism and minimal residual disease using Next Generation Sequencing (NGS) testing for the evaluation of engraftment and the detection of early relapse after allo-HCT. Our secondary aim was to compare the data with the existing laboratory method based on Short Tandem Repeat (STR) analysis. One hundred and seventy-four DNA specimens from 46 individuals were assessed using a commercially available kit for NGS, AlloSeq HCT NGS (CareDx), and the STR-PCR assay. The sensitivity, precision, and quantitative accuracy of the assay were determined using artificially created chimeric constructs. The accuracy and linearity of the assays were evaluated in 46 post-transplant HCT samples consisting of 28 levels of mixed chimerism, which ranged from 0.3-99.7%. There was a 100% correlation between NGS and STR-PCR chimerism methods. In addition, 100% accuracy was attained for the two external proficiency testing surveys (ASHI EMO). The limit of detection or sensitivity of the NGS assay in artificially made chimerism mixtures was 0.3%. We conducted a review of all NGS chimerism studies published online, including ours, and concluded that NGS-based chimerism analysis using the AlloSeq HCT assay is a sensitive and accurate method for donor-recipient chimerism quantification and minimal residual disease relapse detection in patients after allo-HCT compared to STR-PCR assay.

Immune Cell Lineage-Specific Chimerism Testing by Next-Generation Sequencing for Engraftment Monitoring After Allogeneic Hematopoietic Stem Cell Transplantation

Chimerism is an unusual state in which a person's body comprises cells from genetically different people. Chimerism testing allows monitoring for the relative proportion of recipient and donor-derived cell subsets in recipient blood and bone marrow. In the bone marrow transplant setting, chimerism testing is the standard diagnostic tool for early detection of graft rejection and the risk of malignant disease relapse. Chimerism testing enables the identification of patients with increased risk for recurrence of the underlying disease. Herein, we describe a step-by-step technical procedure of a novel, commercially available, next-generation sequencing-based chimerism testing method for use in the clinical laboratory.

Toxic epidermal necrolysis-like acute graft-versus-host disease in pediatric bone marrow transplant patients: Case series and review of the literature

BACKGROUND/OBJECTIVES

Complications of hematopoietic stem cell transplant (HSCT) include acute graft-versus-host disease (aGVHD). Severe cutaneous aGVHD can present with generalized erythroderma, desquamation, and bullae which can mimic toxic epidermal necrolysis (TEN). TEN occurs in response to a culprit medication. Transplant patients are often on many medications, making it difficult to distinguish between the two conditions. Given that TEN-like aGVHD is rare, we describe a case series of pediatric patients and review the literature.

METHODS

This is a multi-institutional case series of children who developed TEN-like aGVHD following bone marrow transplantation. Demographic, clinical, and treatment information was collected.

RESULTS

Ten patients were identified. Median age at transplantation was 8.5 years (range 0.12-17 years). Median time from transplant to first skin symptoms was 35 days (range 6-110 days) and to first TEN-like symptoms was 40 days (range 16-116 days). 7/10 had other organ GVHD involvement. All patients were on concurrent medications at time of first skin symptoms including immunosuppression for GVHD prophylaxis, infection prophylaxis or treatment, and pain medication. Treatments for TEN-like aGVHD included immunosuppression.

CONCLUSIONS

We observe that patients with > or equal to 50% BSA involvement of their skin with TEN-like aGVHD, extracutaneous GVHD, and lack of reepithelization tend to have poor outcomes. Given the rarity of this condition, multidisciplinary care of these patients is important for accurate and timely diagnosis and treatment.

Assessment of donor cell engraftment after hematopoietic stem cell transplantation for sickle cell disease: A review of current and future methods

Hematopoietic stem cell transplantation (HSCT) is the only established curative treatment for sickle cell disease (SCD), a debilitating red blood cell (RBC) disorder with significant prevalence worldwide. Accurate assessment of RBC engraftment following HSCT is essential to evaluate the status of the graft and can enable early intervention to treat or prevent graft rejection. Currently, chimerism measurement is performed on whole blood samples, which mainly reflect white blood cell (WBC) chimerism. This approach has limitations in assessing engraftment in patients with SCD because RBCs engraft non-linearly with WBCs. Direct measures of RBC chimerism exist but are not routinely used. In this review, we critically examine the current methodologies for assessing donor engraftment; highlight the limitations of these different methods, and present emerging and novel technologies with the potential to improve clinical monitoring of RBC engraftment post-HSCT for SCD. Promising alternative methodologies include RBC-specific flow cytometry, RBC-specific RNA analysis, and quantification of plasma cell-free DNA derived specifically from nucleated RBCs.

Mixed T cell CHIMERISM in bone marrow at day+30 after allogeneic stem cell transplantation: Correlation with acute GVHD grades III-IV and no impact on relapse and overall survival

OBJECTIVES

Early chimerism analysis is important to assess engraftment in allogeneic hematopoietic stem cell transplantations.

METHODS

We retrospectively investigated the impact of T-cell chimerism at day 30 in bone marrow on acute graft-versus-host disease (aGVHD), relapse, and overall survival in 142 adult allo-transplanted patients.

RESULTS

The majority of patients (89%) received myeloablative conditioning and 90% have undergone T-cell replete donor graft. At day 30, 103 patients showed T-complete chimerism with prevalence in haploidentical transplants, whereas 39 cases had CD3+ mixed chimerism, including 30 patients transplanted with HLA identical donors, and 21 with T-cell donors<90%. T-cell chimerism at day 30 was weakly inversely related to aGVHD grades II-IV (p = .078) with no cases of grades III-IV aGVHD in patients with CD3+ <95%. Mixed T-cell chimerism did not impact on relapse (p = .448) and five of the seven patients who relapsed had T-cell chimerism ≤90%. Older age and active disease at transplant had a statistically significant negative effect on overall survival (p = .01 and p = .0001, respectively), whereas mixed CD3+ chimerism did not.

CONCLUSIONS

T lymphocyte chimerism analysis at day +30 in bone marrow could identify allo-transplanted patients at major risk of aGVHD grades III-IV (CD3+ donors >95%) mainly post-myeloablative conditioning regimen.

Digital Droplet PCR in Hematologic Malignancies: A New Useful Molecular Tool

Digital droplet PCR (ddPCR) is a recent version of quantitative PCR (QT-PCR), useful for measuring gene expression, doing clonality assays and detecting hot spot mutations. In respect of QT-PCR, ddPCR is more sensitive, does not need any reference curve and can quantify one quarter of samples already defined as "positive but not quantifiable". In the IgH and TCR clonality assessment, ddPCR recapitulates the allele-specific oligonucleotide PCR (ASO-PCR), being not adapt for detecting clonal evolution, that, on the contrary, does not represent a pitfall for the next generation sequencing (NGS) technique. Differently from NGS, ddPCR is not able to sequence the whole gene, but it is useful, cheaper, and less time-consuming when hot spot mutations are the targets, such as occurs with IDH1, IDH2, NPM1 in acute leukemias or T315I mutation in Philadelphia-positive leukemias or JAK2 in chronic myeloproliferative neoplasms. Further versions of ddPCR, that combine different primers/probes fluorescences and concentrations, allow measuring up to four targets in the same PCR reaction, sparing material, time, and money. ddPCR is also useful for quantitating BCR-ABL1 fusion gene, WT1 expression, donor chimerism, and minimal residual disease, so helping physicians to realize that "patient-tailored therapy" that is the aim of the modern hematology.

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.

Therapeutic Drug Monitoring of Conditioning Agents in Pediatric Allogeneic Stem Cell Transplantation; Where do We Stand?

Allogeneic hematopoietic stem cell transplantation (HSCT) is an established curative treatment that has significantly improved clinical outcome of pediatric patients with malignant and non-malignant disorders. This is partly because of the use of safer and more effective combinations of chemo- and serotherapy prior to HSCT. Still, complications due to the toxicity of these conditioning regimens remains a major cause of transplant-related mortality (TRM). One of the most difficult challenges to further improve HSCT outcome is reducing toxicity while maintaining efficacy. The use of personalized dosing of the various components of the conditioning regimen by means of therapeutic drug monitoring (TDM) has been the topic of interest in the last decade. TDM could play an important role, especially in children who tend to show greater pharmacokinetic variability. However, TDM should only be performed when it has clear added value to improve clinical outcome or reduce toxicity. In this review, we provide an overview of the available evidence for the relationship between pharmacokinetic parameters and clinical outcome or toxicities of the most commonly used conditioning agents in pediatric HSCT.

Analysis of Subset Chimerism for MRD-Detection and Pre-Emptive Treatment in AML

Allogeneic hematopoietic stem cell transplantation (alloHCT) represents the only potentially curative treatment in high-risk AML patients, but up to 40% of patients suffer from relapse after alloHCT. Treatment of overt relapse poses a major therapeutic challenge and long-term disease control is achieved only in a minority of patients. In order to avoid post-allograft relapse, maintenance as well as pre-emptive therapy strategies based on MRD-detection have been used. A prerequisite for the implementation of pre-emptive therapy is the accurate identification of patients at risk for imminent relapse. Detection of measurable residual disease (MRD) represents an effective tool for early relapse prediction in the post-transplant setting. However, using established MRD methods such as multicolor flow cytometry or quantitative PCR, sensitive MRD monitoring is only applicable in about half of the patients with AML and advanced MDS undergoing alloHCT. Donor chimerism analysis, in particular when performed on enriched leukemic stem and progenitor cells, e.g. CD34+ cells, is a sensitive method and has emerged as an alternative option in the post alloHCT setting. In this review, we will focus on the current strategies for lineage specific chimerism analysis, results of pre-emptive treatment using this technology as well as future developments in this field.

Chimerism analysis for clinicians: a review of the literature and worldwide practices

This review highlights literature pertinent to chimerism analysis in the context of hematopoietic cell transplantation (HCT). We also conducted a survey of testing practices of program members of CIBMTR worldwide. Questions included testing methods, time points, specimen type, cell lineage tested and testing indications. Recent literature suggests that detection of low level mixed chimerism has a clinical utility in predicting relapse. There is also increasing recognition of HLA loss relapse to potentially guide rescue decisions in cases of relapse. These developments coincide with wider access to high sensitivity next generation sequencing (NGS) in clinical laboratories. Our survey revealed a heterogeneity in practices as well as in findings and conclusions of published studies. Although the most commonly used method is STR, studies support more sensitive methods such as NGS, especially for predicting relapse. There is no conclusive evidence to support testing chimerism in BM over PB, particularly when using a high sensitivity testing method. Periodic monitoring of chimerism especially in diagnoses with a high risk of relapse is advantageous. Lineage specific chimerism is more sensitive than whole blood in predicting impending relapse. Further studies that critically assess how to utilize chimerism testing results will inform evidence based clinical management decisions.

Estimation of intrafamilial DNA contamination in family trio genome sequencing using deviation from Mendelian inheritance

With the increasing number of sequencing projects involving families, quality control tools optimized for family genome sequencing are needed. However, accurately quantifying contamination in a DNA mixture is particularly difficult when genetically related family members are the sources. We developed TrioMix, a maximum likelihood estimation (MLE) framework based on Mendel's law of inheritance, to quantify DNA mixture between family members in genome sequencing data of parent-offspring trios. TrioMix can accurately deconvolute any intrafamilial DNA contamination, including parent-offspring, sibling-sibling, parent-parent, and even multiple familial sources. In addition, TrioMix can be applied to detect genomic abnormalities that deviate from Mendelian inheritance patterns, such as uniparental disomy (UPD) and chimerism. A genome-wide depth and variant allele frequency plot generated by TrioMix facilitates tracing the origin of Mendelian inheritance deviations. We showed that TrioMix could accurately deconvolute genomes in both simulated and real data sets.

Use of chimerism analysis after allogeneic stem cell transplantation: Belgian guidelines and review of the current literature

Background: Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative treatment option in both adult and pediatric patients with malignant and non-malignant hematological diseases.  Chimerism analysis, which determines the donor or recipient origin of hematopoietic cells in HSCT recipients, is an essential aspect of post-HSCT follow-up.Objectives: To review the current literature and develop Belgian consensus guidelines for the use of chimerism analysis in the standard of care after allogeneic HSCT.Methods: Non-systematic review of the literature in consultancy with the members of the BHS transplantation committee.Results: Clinical application with regards to prediction of graft failure or relapse as well as cell source are reviewed. A consensus guideline on the use of chimerism analysis after HSCT is presented.Conclusion: Monitoring of the dynamics or kinetics of a patient's chimerism status by serial analysis at fixed time points, as well as on suspicion of relapse or graft failure, is needed to monitor engraftment levels, as well as disease control and possible relapse.

Performance characteristics of chimerism testing by next generation sequencing

Chimerism testing provides informative clinical data regarding the status of a biological sample mixture. For years, this testing was achieved by measuring the peaks of informative short tandem repeat (STR) loci using capillary electrophoresis (CE). With the advent of next generation sequencing (NGS) technology, the quantification of the percentage of donor/recipient mixtures is more easily done using sequence reads in large batches of samples run on a single flow cell. In this study, we present data on using a FORENSIC NGS chimerism platform to accurately measure the percentage of donor/recipient mixtures. We were able to detect chimerism to a limit threshold of 1% using both STR and single nucleotide polymorphism (SNP) informative loci. Importantly, a significant correlation was observed between NGS and CE chimerism methods when compared at donor detection ranges from 1% to 10%. Furthermore, 100% accuracy was achieved through proficiency testing over six surveys. Its usefulness was expanded beyond this to help identify suitable donors for solid organ transplant patients using ancestry SNP profiles. In summary, the NGS method provides a sensitive and reliable alternative to traditional CE for chimerism testing of clinical samples.

Differential effects of donor lymphocyte infusion upon treatment response and GVHD according to relapse level and donor sources in patients with myelodysplastic syndrome

INTRODUCTION

Donor lymphocyte infusion (DLI) is one of the effective options for post-transplant disease control of myelodysplastic syndrome (MDS). Its success or failure depends on the induction of antitumor immune reactions, durability of clinical responses, and severity of unwanted toxicities mainly from graft-versus-host disease (GVHD).

METHODS

By analyzing 61 patients receiving DLI for post-transplant MDS relapse, we assessed treatment outcomes and affecting factors, especially focusing on the level of relapse (hematological, molecular, and imminent relapse).

RESULTS

The response rate (42.1%, 36.4%, 72.7%), and overall survival (OS) at 2 years (27.8%, 45.5%, 70.1%) were different for each relapse level with imminent relapse group showing the most promising results. For OS, response to DLI or pre-DLI chemotherapy, and time to relapse were independent prognostic factors. Meanwhile, post-DLI GVHD and time to relapse were independently predictive for DLI response; post-DLI GVHD was predictive for DLI response, but not for OS, suggesting a potential detrimental impact of GVHD on survival. The incidence of GVHD and GVHD-related deaths were 37.7% and 10.0%, respectively, and CD3+ cell doses triggering GVHD tended to be lower in cases with haploidentical donor or imminent relapse.

CONCLUSION

Despite being limited by small number of cases and its retrospective nature, this study again demonstrated the therapeutic effects of DLI in relapsed MDS, and that earlier detection and intervention at lower level relapse might possibly be associated with better results. Furthermore, we propose that tailored cell dosing schedule based on relapse level and donor source may be helpful in minimizing fatal GVHD.

Impact of mother donor, peripheral blood stem cells and measurable residual disease on outcomes after haploidentical hematopoietic cell transplantation with post-transplant cyclophosphamide in children with acute leukaemia

Haploidentical hematopoietic-cell transplantation using post-transplant cyclophosphamide(Haplo-PTCy) is a feasible procedure in children with haematologic malignancies. However, data of a large series of children with acute leukaemia(AL) in this setting is missing. We analysed 144 AL Haplo-PTCy paediatric recipients; median age was 10 years. Patients had acute lymphoblastic(ALL; n = 86) or myeloblastic leukaemia(AML; n = 58) and were transplanted in remission(CR1: n = 40; CR2: n = 57; CR3+: n = 27) or relapse (n = 20). Bone marrow was the graft source in 57%; donors were father (54%), mother (35%), or sibling (11%). Myeloablative conditioning was used in 87%. Median follow-up was 31 months. At day +100, cumulative incidence (CI) of neutrophil recovery and acute GVHD (II-IV) were 94% and 40%, respectively. At 2-years, CI of chronic GVHD and relapse, were 31%, 40%, and estimated 2-year overall survival (OS), leukaemia-free survival (LFS) and graft-versus-host-relapse-free survival (GRFS) were 52%, 44% and 34% respectively. For patients transplanted in remission, positive measurable residual disease (MRD) prior to transplant was associated with decreased LFS (p = 0.05) and GRFS (p = 0.003) and increased risk of relapse (p = 0.02). Mother donor was associated with increased risk of chronic GVHD (p = 0.001), decreased OS (p = 0.03) and GRFS (p = 0.004). Use of PBSC was associated with increased risk of chronic GVHD (p = 0.04). In conclusion, achieving MRD negativity pre-transplant, avoiding use of mother donors and PBSC as graft source may improve outcomes of Haplo-PTCy in children with AL.

CAR-T after Stem Cell Transplantation in B-Cell Lymphoproliferative Disorders: Are They Really Autologous or Allogenic Cell Therapies?

Allogenic hematopoietic stem cell transplantation (allo-HSCT) is one of the standard treatments for B-cell lymphoproliferative disorders; however, deep relapses are common after an allo-HSCT, and it is associated with poor prognosis. A successful approach to overcome these relapses is to exploit the body's own immune system with chimeric antigen receptor (CAR) T-cells. These two approaches are potentially combinatorial for treating R/R B-cell lymphoproliferative disorders. Several clinical trials have described different scenarios in which allo-HSCT and CAR-T are successively combined. Further, for all transplanted patients, assessment of chimerism is important to evaluate the engraftment success. Nonetheless, for those patients who previously received an allo-HSCT there is no monitorization of chimerism before manufacturing CAR T-cells. In this review, we focus on allo-HSCT and CAR-T treatments and the different sources of T-cells for manufacturing CAR T-cells.

A practical guide to chimerism analysis: Review of the literature and testing practices worldwide

BACKGROUND AND PURPOSE

Currently there are no widely accepted guidelines for chimerism analysis testing in hematopoietic cell transplantation (HCT) patients. The objective of this review is to provide a practical guide to address key aspects of performing and utilizing chimerism testing results. In developing this guide, we conducted a survey of testing practices among laboratories that are accredited for performing engraftment monitoring/chimerism analysis by either the American Society for Histocompatibility & Immunogenetics (ASHI) and/or the European Federation of Immunogenetics (EFI). We interpreted the survey results in the light of pertinent literature as well as the experience in the laboratories of the authors.

RECENT DEVELOPMENTS

In recent years there has been significant advances in high throughput molecular methods such as next generation sequencing (NGS) as well as growing access to these technologies in histocompatibility and immunogenetics laboratories. These methods have the potential to improve the performance of chimerism testing in terms of sensitivity, availability of informative genetic markers that distinguish donors from recipients as well as cost.

SUMMARY

The results of the survey revealed a great deal of heterogeneity in chimerism testing practices among participating laboratories. The most consistent response indicated monitoring of engraftment within the first 30 days. These responses are reflective of published literature. Additional clinical indications included early detection of impending relapse as well as identification of cases of HLA-loss relapse.

Performance Assessment of the Devyser High-Throughput Sequencing-Based Assay for Chimerism Monitoring in Patients after Allogeneic Hematopoietic Stem Cell Transplantation

Chimerism analysis is widely used to aid in the clinical management of patients after allogeneic hematopoietic stem cell transplantation. Many laboratories currently use assays based on polymerase chain reaction followed by capillary electrophoresis, with a limit of quantification of 1% to 5%. Assays with a lower limit of quantification could allow for earlier relapse detection, resulting in improved patient care. This study investigated the analytical, clinical, technical, and practical performance of the Devyser NGS chimerism assay, a commercial high-throughput sequencing-based assay for chimerism analysis. Performance of this assay was compared with that of the Promega PowerPlex 16 HS assay, a commercial capillary electrophoresis-based assay. A limit of quantification of 0.1% was achievable with the Devyser assay. The repeatability, reproducibility, trueness, and linearity of the Devyser assay were acceptable. The Devyser assay showed potential for earlier relapse detection compared with the Promega assay. Conclusive analysis was not possible for 3% of donor-recipient pairs with the Devyser assay due to an insufficient number of informative markers; this factor was not an issue for the Promega assay. Further improvements in assay design or data analysis may allow the assay's applicability to be extended to all donor-recipient pairs studied. Technical performance criteria for chimerism analysis by high-throughput sequencing were suggested and evaluated. Both assays were found to be practical for use in a clinical diagnostics laboratory.

Mixed chimerism after allogeneic hematopoietic stem cell transplantation for severe aplastic anemia

A retrospective study on 287 patients with SAA who underwent allo-HSCT between October 2012 and January 2020 was conducted to explore the outcomes, risk factors and treatment options for MC. Among 287 AA patients who excluded Fanconi anemia (FA), Congenital dyskeratosis (DKC), Paroxysmal nocturnal hemoglobinuria (PNH), etc.112 underwent matched sibling donor (MSD)-HSCT, 91 matched unrelated donor-HSCT and 84 haploidentical-HSCT. Patients were divided into the following 4 groups: group 1: Donor chimerism (DC); group 2: MC without cytopenia; group 3: MC with cytopenia; group 4: secondary graft failure (SGF).

Compared with the other three groups, SGF predicted a poor prognosis of SAA (P< 0.001). In addition, SGF was associated with the early (within 3 months after transplantation) presence of MC and the high levels of MC. Uni- and multivariate logistic regression analysis showed that donor/recipient sex-mismatching and CTX + ATG regimen were high-risk factors for MC. Of note, in MC patients with cytopenia (group 3), the effective response rate reached 55% (6/11) following enhanced immunosuppression combined with cellular therapy, while only one of the four was effective who received enhanced immunosuppression alone.

SGF was associated with poor prognosis, early presence of MC and increased levels of recipient chimerism. The donor/recipient sex-mismatching and CTX + ATG regimen based MSD-HSCT were risk factors for MC. Cellular therapy could improve the effective response rate of patients with progressive MC.

Chimerism Monitoring Techniques after Hematopoietic Stem Cell Transplantation: An Overview of the Last 15 Years of Innovations

Chimerism analysis is a well-established method for monitoring the state of hematopoietic stem cell transplantation (HSCT) over time by analyzing peripheral blood or bone marrow samples of the recipient in several malignant and non-malignant hematologic diseases. From a clinical point of view, a continuous monitoring is fundamental for an effective early therapeutic intervention. This paper provides a comparative overview of the main molecular biology techniques which can be used to study chimerism after bone marrow transplantation, focusing on their advantages and disadvantages. According to the examined literature, short tandem repeats (STR) analysis through simple PCR coupled with capillary electrophoresis (STR-PCR) is the most powerful method which guarantees a high power of differentiation between different individuals. However, other methods such as real-time quantitative PCR (qPCR), digital PCR (dPCR), and next-generation sequencing (NGS) technology were developed to overcome the technical limits of STR-PCR. In particular, these other techniques guarantee a higher sensitivity, which allows for the detection of chimerism at an earlier stage, hence expanding the window for therapeutic intervention. After a comparative evaluation of the various techniques, it seems clear that STR-PCR still remains the gold standard option for chimerism study, even if it is likely that both dPCR and NGS could supplement or even replace the common methods of STR analysis.

Chimerism Assay Using Single Nucleotide Polymorphisms Adjacent and in Linkage-Disequilibrium Enables Sensitive Disease Relapse Monitoring after Hematopoietic Stem-Cell Transplantation

BACKGROUND

Short tandem repeat (STR)-based chimerism analysis has been widely used for chimerism monitoring after hematopoietic stem-cell transplantation (HSCT), but technical artifacts can be problematic. We designed a chimerism assay using single nucleotide polymorphisms (SNPs) adjacent and in linkage-disequilibrium (CASAL), which doubly checked for SNP pairs, and thus could reduce background errors and increase analytical sensitivity.

METHODS

CASAL targeted 84 SNP pairs within 10 bp distance and in perfect linkage-disequilibrium. Using undiluted and serially diluted samples, baseline error rates, and linearity was calculated. Clinical performance of CASAL was evaluated in comparison with a conventional STR assay, using 191 posttransplant samples from 42 patients with HSCT.

RESULTS

CASAL had ∼10 times lower baseline error rates compared to that of ordinary next-generation sequencing. Limit of detection and quantification of CASAL were estimated to be 0.09 and 0.39%, respectively, with a linear range of 0.1-100%. CASAL correlated well with STR assay (r2 = 0.99) and the higher sensitivity enabled detection of low-level recipient chimerism and earlier prediction of relapse.

CONCLUSIONS

CASAL is a simple, analytically sensitive and accurate assay that can be used in clinical samples after HSCT with a higher performance compared to that of traditional assays. It should also be useful in other forensic and archeological testing.

Impaired Hematopoiesis after Allogeneic Hematopoietic Stem Cell Transplantation: Its Pathogenesis and Potential Treatments

Impaired hematopoiesis is a serious complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Bone marrow aplasia and peripheral cytopenias arise from primary and secondary graft failure or primary and secondary poor graft function. Chimerism analysis is useful to discriminate these conditions. By determining the pathogenesis of impaired hematopoiesis, a timely and appropriate treatment can be performed. Hematopoietic system principally consists of hematopoietic stem cells and bone marrow microenvironment termed niches. Abnormality in hematopoietic stem and progenitor cells and/or abnormality in the relevant niches give rise to hematological diseases. Allo-HSCT is intended to cure each hematological disease, replacing abnormal hematopoietic stem cells and bone marrow niches with hematopoietic stem cells and bone marrow niches derived from normal donors. Therefore, treatment for graft failure and poor graft function after allo-HSCT is required to proceed based on determining the pathogenesis of impaired hematopoiesis. Recent progress in this area suggests promising treatment manipulations for graft failure and poor graft function.

A quantitative hematopoietic stem cell reconstitution protocol: Accounting for recipient variability, tissue distribution and cell half-lives

Hematopoietic stem and progenitor cell (HSPC) transplantation is the paradigm for stem cell therapies. The protocol described here enables quantitative assessment of the body-wide HSPC reconstitution of different mature hematopoietic cells in mice based on their presence in circulating blood. The method determines donor-derived mature cell populations per mouse, over time, by quantitatively obtaining their absolute numbers in the peripheral blood and utilizing previously assessed tissue-distribution factors. A Markov-based birth/death computational model accounts for the drastic differences in mature cell half-lives. By quantifying the number of cells produced and eliminating host variability, the protocol can be used to directly compare the lineage output of different types of HSPCs on a per cell basis, thereby clarifying the lineage potential and expansion capacity of different cell populations. These protocols were developed for hematopoiesis, but can readily be extended to other contexts by simply replacing the cell types and distributions.

Short Tandem Repeats (STRs) as Biomarkers for the Quantitative Follow-Up of Chimerism after Stem Cell Transplantation: Methodological Considerations and Clinical Application

Chimerism refers to the relative proportion of donor and recipient DNA after hematopoietic stem cell transplantation (HSCT) and its quantitative follow-up is of great clinical utility in this setting. PCR of short tandem repeats (STR-PCR) constitutes the gold standard method for chimerism quantification, although more sensitive PCR techniques (such as qPCR) have recently arisen. We compared the sensitivity and the quantification capacity of both techniques in patient samples and artificial mixtures and demonstrated adequate performance of both methods, with higher sensitivity of qPCR and better quantification skills of STR-PCR. By qPCR, we then prospectively followed up 57 patients that were in complete chimerism (CC) by STR-PCR. Twenty-seven patients (59%) showed 0.1–1% recipient DNA in the bone marrow. Only 4 patients presented 0.1–1% recipient DNA in peripheral blood (PB), and one of them relapsed. Finally, by qPCR, we retrospectively studied the last sample that showed CC by STR-PCR prior to relapse in 8 relapsed patients. At a median of 59 days prior to relapse, six patients presented mixed chimerism by qPCR in PB. Since both approaches have complementary characteristics, we conclude that different techniques should be applied in different clinical settings and therefore propose a methodological algorithm for chimerism follow-up after HSCT.

Allogeneic Peripheral Blood Stem Cell Transplant: Correlation of Donor Factors with Yield, Engraftment, Chimerism, and Outcome: Retrospective Review of a Single Institute During a 3-Year Period

BACKGROUND

Donor factors have a variable correlation with cluster of differentiation (CD)34+ cell dose in allogeneic peripheral blood stem cell (PBSC) harvests. CD34+ cell dose affects the speed of hematopoietic recovery and percentage of donor chimerism in the recipient.

METHODS

A total of 25 allogeneic PBSC transplants performed during a 3-year period were included. All donors underwent mobilization with filgrastim. Leukapheresis, flowcytometric CD34+ cell enumeration, and chimerism analysis were performed and correlated with recipient outcome.

RESULTS

Besides age, all other donor parameters had a positive correlation with CD34+ cell count. Engraftment kinetics and chimerism had a positive correlation with the CD34+ yield of the PBSC product. Acute graft-vs-host disease (GVHD) was observed in patients with complete chimerism at day 30 after transplantation.

CONCLUSION

Adequate CD34+ cell yield happens in healthy donors, independent of donor demographic patterns with G-CSF only. A diverse population of donors can thus be approached for Matched Unrelated Donor (MUD) transplants. An accurate quantitative analysis of early donor chimerism in the recipient (at day 30) is an excellent tool for post-transplant monitoring for acute GvHD.

Quantitative polymerase chain reaction technology in chimerism monitoring after hematopoietic stem cell transplantation: One center experience

Chimerism status evaluation is a routine test performed in post-hematopoietic stem cell transplantation (HSCT) period. The aim of the study was to evaluate a quantitative polymerase chain reaction (qPCR) method (GenDx, Utrecht, the Netherlands) applicability for this purpose. The study included 74 recipient/donor pairs tested for informative markers: median of four and six informative markers was found for patients (related and unrelated donor, respectively). Higher sensitivity of qPCR method was confirmed by analysis of recipient post-HSCT samples (N = 800) among which microchimerism (0.1%-1% recipient DNA) was detected in 21.8% of cases. The ability to detect less than 1% of minor population, as opposed to the short tandem repeat (STR) method for which 1% is the limit, translated into earlier identification of a disease relapse for four patients in our study sample.

Influence of Mixed Chimerism on Outcome in Children With Anaemia After Haematopoietic Stem Cell Transplantation

BACKGROUND/AIM

In patients with non-malignant diseases, mixed chimerism is not a rare phenomenon. The clinical impacts of chimerism following allogeneic haematopoietic stem cell transplantation (allo-HSCT) in children with congenital anaemia (CA) and severe aplastic anaemia (SAA) were analysed.

PATIENTS AND METHODS

We studied twenty-seven consecutive children with congenital and acquired anaemia who had undergone allogeneic haematopoietic stem cell transplantations. In the observed group of patients, the median of the follow-up was 6.12 years (2.00-14.8 years).

RESULTS

Overall survival (OS) did not depend on the type of disease p=0.1. OS did not significantly differ in patients who received more than 5x10⁶/kg stem cells (91%) and those who received less than 5x10⁶/kg (85%) (p=0.61). Two patterns of stable mixed chimerism (SMC) were observed: SMC (95-97% cells of the donor), and SMC with a fluctuation between 50-90% of the cells of the donor. None of the surviving patients received immunosuppression treatments of chronic Graft-versus-Host Disease (cGvHD).

CONCLUSION

Our results showed that mixed chimerism did not influence the survival of children with congenital and aplastic anaemia following allo-HSCT.

[Expirience introduction of quantitative analysis of chimerism after allogenic stem cell transplantation by real-time PCR with InDel polymorphism.]

Using data obtained from domestic and foreign sources, we formed a set of primers and fluorogenic probes for analyzing twentysix specific sequence polymorphisms and one reference gene. In the course of evaluating the effectiveness of real-time PCR, using the example of one of the markers (S01a), we obtained the optimal amount of DNA per reaction (70 ng), providing a resolution of at least 0.1% of the method with the ability to estimate linear chimerism. Formed panel of primers for genetic polymorphisms - InDel has a high degree of informational content for donor-recipient pairs of Russia. From January 2018 to June 2019, a quantitative assessment of the level of linear (CD3 +, CD34 +) and general chimerism was carried out for 28 patients of the clinic of the Institution. Finally, we analyzed patients who received allografts and present 4 different clinical situations that illustrate the informativity level of this method.

Monitoring minimal residual/relapsing disease after allogeneic haematopoietic stem cell transplantation in adult patients with acute lymphoblastic leukaemia

Relapse after allogeneic haematopoietic stem cell transplantation (SCT) is a major cause of death in patients with acute lymphoblastic leukaemia (ALL). Here, we retrospectively analysed the contributions of lineage-sorted donor cell chimerism (sDCC) and quantitative PCR (qPCR) targeting disease-specific genetic rearrangements to detect minimal residual/relapsing disease (MRD) and predict impending relapse in 94 adult ALL patients after SCT. With a median follow-up of surviving patients (n = 61) of 3.3 years, qPCR and/or sDCC measurements turned positive in 38 patients (40%). Of these, 22 patients relapsed and 16 remained in complete remission. At 3 years, qPCR and/or sDCC positive patients showed an increased incidence of relapse (50% vs. 4%, p < 0.0001), decreased relapse-free survival (RFS, 40% vs. 85%, p < 0.0001), and decreased overall survival (OS, 47% vs. 87%, p 0.004). Both, qPCR and sDCC pre-detected 11 of 21 relapses occurring within the first two years after SCT and, overall, complemented for each other method in four of the relapsing and four of the non-relapsing cases. Patients receiving pre-emptive MRD-driven interventions (n = 11) or not (n = 10) showed comparable median times until relapse, RFS, and OS. In our single centre cohort, qPCR and sDCC were similarly effective and complementary helpful to indicate haematological relapse of ALL after SCT.

Simultaneous Monitoring of Mutation and Chimerism Using Next-Generation Sequencing in Myelodysplastic Syndrome

Monitoring minimal residual disease (MRD) provides important information during treatment of hematologic malignancies. Chimerism analysis also provides key information after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Recent advances in next-generation sequencing (NGS) have enabled identification of various mutations and quantification of mutant allele burden. In this study, we developed a new analytic algorithm to monitor chimerism applicable to NGS multi-gene panel in use to identify mutations of myelodysplastic syndrome (MDS). We enrolled patients who were diagnosed with MDS and received allo-HSCT and their corresponding donors. Monitoring MRD by NGS assay was performed using 53 DNA samples by calculating mutant allele burden after treatment. For monitoring chimerism by NGS, we selected 121 single nucleotide polymorphisms (SNPs) after careful stepwise evaluation and calculated average donor allele burden. Data obtained from NGS were compared with bone marrow findings, chromosome analysis and short tandem repeat (STR)-based chimerism. SNP-based NGS chimerism analysis was accurate and even superior to conventional STR method by overcoming the various technical limitations of STR. In addition, simultaneous monitoring of mutation and chimerism using NGS could implement comprehensive pre- and post-HSCT monitoring of various clinical conditions such as complete donor chimerism, persistent mixed chimerism, early relapse, and even donor cell-derived diseases.

Impact of early chimerism status on clinical outcome in children with acute lymphoblastic leukaemia after haematopoietic stem cell transplantation

Background

The significance of very early chimerism assessment before day + 28, which is considered the moment of engraftment, is still unclear. In this retrospective study, we evaluated the clinical impact of very early chimerism on the clinical outcome after allogeneic haematopoietic stem cell transplantation (allo-HSCT) in children with acute lymphoblastic leukaemia (ALL).

Methods

The study group included 38 boys and 18 girls. Very early chimerism was evaluated on days + 7, + 14, + 21 and + 28 after the transplant. Short tandem repeat polymerase chain reaction (STR PCR) was used to analyse chimerism.

Results

Overall survival (OS) and event-free survival (EFS) were 84 and 80%, respectively. The OS in the group of 24 patients with complete donor chimerism on day + 14 was 83%, and it did not differ statistically compared to the 32 patients with mixed chimerism on day + 14 (OS was 84%). In our cohort of patients, the matched unrelated donor, male gender of donor, number of transplanted cells above 4.47 × 10⁶ kg and no serotherapy with anti-thymocyte globulin (ATG) were statistically related to a higher level of donor chimerism. The immunophenotypes of disease, age of patient at time HSCT, recipient sex, stem cell source (peripheral blood/bone marrow) and conditioning regimen had no impact on early chimerism. Acute graft versus host disease grades II-IV was diagnosed in 23 patients who presented with donor chimerism levels above 60% on day 7.

Conclusions

The data presented in this study provide valuable insight into the analysis of very early chimerism in children with ALL treated with HSCT.

Quantitative chimerism in CD3-negative mononuclear cells predicts prognosis in acute myeloid leukemia patients after hematopoietic stem cell transplantation

Relapse is a major complication of acute myeloid leukemia (AML) after allogeneic hematopoietic stem cell transplantation (SCT). The objective of our study was to evaluate chimerism monitoring on the CD3-negative mononuclear cells by RQ-PCR to predict relapse of patients allografted for AML and to compare its performance with WT1 quantification. A cohort of 100 patients undergoing allogenic SCT for AML was retrospectively analyzed in a single institution. Patients without complete chimerism, defined as less than 0.01% of recipient's DNA in CD3-negative cells, had a significantly higher risk of relapse and a lower overall survival (p < 0.001). An increase in the percentage of recipient DNA in CD3-negative cells was associated with an increased risk of relapse (p < 0.001) but not with overall survival. Comparable performances between monitoring of CD3-negative cell chimerism and WT1 expression to predict relapse was observed up to more than 90 days before hematological relapse, with sensitivity of 82% and 78%, respectively, and specificity of 100% for both approaches. Quantitative specific chimerism of the CD3-negative mononuclear fraction, enriched in blastic cells, is a new and powerful tool for monitoring measurable residual disease and could be used for AML patients without available molecular markers.

Current Trends in Applications of Circulatory Microchimerism Detection in Transplantation

Primarily due to recent advances of detection techniques, microchimerism (the proportion of minor variant population is below 1%) has recently gained increasing attention in the field of transplantation. Availability of polymorphic markers, such as deletion insertion or single nucleotide polymorphisms along with a vast array of high sensitivity detection techniques, allow the accurate detection of small quantities of donor- or recipient-related materials. This diagnostic information can improve monitoring of allograft injuries in solid organ transplantations (SOT) as well as facilitate early detection of relapse in allogeneic hematopoietic stem cell transplantation (allo-HSCT). In the present review, genetic marker and detection platform options applicable for microchimerism detection are discussed. Furthermore, current results of relevant clinical studies in the context of microchimerism and SOT or allo-HSCT respectively are also summarized.

Digital droplet PCR-based chimerism analysis for monitoring of hematopoietic engraftment after allogeneic stem cell transplantation

INTRODUCTION

Allogeneic hematopoietic stem cell transplantation (alloHSCT) is a curative approach for multiple hematologic diseases. The success of alloHSCT is evaluated by analyzing the proportion of living donor cells in blood and bone marrow samples of the recipient (chimerism analysis). To monitor the engrafted cells, donor's individual genetic markers are analyzed in peripheral blood and bone marrow samples, usually by using short tandem repeat (STR) analysis. An alternative method to measure chimerism is based on insertion and deletion markers (InDels) analyzed by digital droplet PCR (ddPCR); however, this approach is rarely evaluated in clinical practice.

METHODS

In this study, we examined the usefulness of ddPCR-based chimerism analysis against the standard STR analysis in samples around day+30 after alloHSCT in clinical practice using peripheral blood and bone marrow samples.

RESULTS

The median absolute difference between ddPCR and STR analysis was 0.55% points for bone marrow chimerisms and 0.25% points for peripheral blood chimerisms, respectively, including variation in the range of maximum 2% for both methods. The results of every single sample gave the same clinical message.

CONCLUSION

According to our data, chimerism analysis by ddPCR has an excellent correlation with STR-based analyses. Due to its fast and easy applicability, the ddPCR technique is suitable for chimerism monitoring in clinical practice.

Multiplex STR panel for assessment of chimerism following hematopoietic stem cell transplantation (HSCT)

Short tandem repeat (STR) analysis is used in chimerism monitoring after allogeneic hematopoietic stem cell transplantation (HSCT) for patients with various hematologic malignancies. Commercial forensic STR kits often contain loci with huge differences in power of discrimination (PD) across populations, causing some loci to be less informative for chimerism analysis in certain populations. This study aimed to construct a new STR multiplex panel with highly informative loci for efficient chimerism analysis. Thirteen STR markers which exhibit high PD (> 0.9) in at least 80% of 50 populations globally were selected to form a new panel and used in STR analysis of 253 Malaysian subjects. Cumulative power of discrimination (CPD) and combined power of exclusion (CPE) were determined from 253 Malaysian individuals. Loci informativity was assessed and compared to the commercial AmpFLSTR Identifiler PCR Amplification kit in 14 donor-recipient pairs. The new panel had detected 202 unique alleles including five novel alleles from the 253 individuals with high CPD and CPE (> 0.99999999999999999 and > 0.999999997 respectively). All loci from the new panel in the donor-recipient pair analysis showed higher than 50% informativity, while five loci from the commercial kit demonstrated lower than 50% informativity. Four loci from the new panel ranked the highest informativity. A sequenced allelic ladder which consists of 202 unique alleles from the 253 subjects was also developed to ensure accurate allele designation. The new 13-loci STR panel, thus, could serve as an additional powerful, accurate, and highly informative panel for chimerism analysis for HSCT patients.

Personalized Chimerism Test that Uses Selection of Short Tandem Repeat or Quantitative PCR Depending on Patient's Chimerism Status

Chimerism testing is used to monitor engraftment and risk of relapse after allogeneic hematopoietic stem cell transplantation for hematologic malignancies. Although short tandem repeat (STR) method is widely used among clinical laboratories, quantitative PCR (qPCR) provides better sensitivity (0.1%) than STR (1% to 5%) but is less accurate than STR for patients in mixed chimerism. qPCR chimerism allows evaluation of residual recipient cells as a surrogate of measurable residual disease. To achieve higher sensitivity and accuracy, we applied qPCR or STR based on patient chimerism status (recipient alleles <5% or ≥5%, respectively). Of the 230 patients tested by STR in a 1-year period, excluding 10 deceased patients, 30 qPCR markers were genotyped and 167 patients converted to qPCR chimerism (76%), including eight patients undergoing multiple-donor transplantation. STR was continued on 53 patients (24%) for the following reasons: mixed chimerism (n = 23), lack of donor or pretransplantation DNA (n = 22), and insufficient qPCR informative markers [8 of 60 patients with related donors (13.3%)]. qPCR detected residual recipient chimerism in 85.5% of patients with complete chimerism by STR (<5% recipient). Selecting STR or qPCR testing based on each patient's chimerism status facilitates sensitive and accurate chimerism testing in clinical settings. In addition, we discuss clinical relevance of chimerism testing for measurable residual disease detection in various hematologic malignancies.