Quantitative analysis of chimerism after allogeneic stem cell transplantation by PCR amplification of microsatellite markers and capillary electrophoresis with fluorescence detection: the Frankfurt experience

Monitoring of hematopoietic chimerism by real-time quantitative PCR of micro insertions/deletions in samples with low DNA quantities

BACKGROUND

Sensitive and accurate methods to detect hematopoietic chimerism after hematopoietic stem cell transplantation (HSCT) are essential to evaluate engraftment and to monitor response to therapeutic procedures such as donor lymphocyte infusion. Continuous long-term follow up, however, requires large amounts of pre-HSCT samples limiting the application of many widely used techniques for sensitive chimerism monitoring.

METHODS

DNAs from 42 normal healthy donors and 16 HSCT donor/recipient pairs were employed to validate the use of allele-specific insertion/deletion (indel) quantitative real-time polymerase chain reaction (qPCR) to quantify chimerism in samples with low amounts of DNA. Consequently, indel-qPCR analyses of samples from 16 HSCT patients were compared to short-tandem repeat (STR) specific PCR analyses.

RESULTS

Typing with reduced amounts of input DNA (15 vs. 60 ng) allowed for the reliable distinction of positive (mean threshold cycle (ct) 28.05) and negative (ct >36) signals. The high informativity of primer/probe sets, with 12 out of 19 markers exceeding 20% informativity, was confirmed in our cohort (n = 74). Importantly, a fourfold reduction of input DNA compared to published protocols did not alter PCR efficiencies and allowed for a more sensitive detection of chimerism in 7 of 16 HSCT patients compared to results obtained by STR-PCR.

CONCLUSIONS

Our data suggest that indel-qPCR is a more sensitive technique for the detection of hematopoietic chimerism compared to STR-PCR and works efficiently for samples with low amounts of DNA.

Post-transplant monitoring of chimerism by lineage-specific analysis

Molecular surveillance of hematopoietic chimerism is an important part of the routine diagnostic program in patients after allogeneic stem cell transplantation. Chimerism testing permits early prediction and documentation of successful engraftment and facilitates early risk assessment of impending graft rejection. In patients transplanted for treatment of malignant hematologic disorders, monitoring of chimerism can provide an early indication of incipient disease relapse. The investigation of chimerism has therefore become an indispensable tool for the management of patients during the post-transplant period. Increasing use of reduced-intensity conditioning, which is associated with prolonged duration of mixed hematopoietic chimerism, has further increased the clinical importance of chimerism analysis. At present, the most commonly used technical approach to the investigation of chimerism is microsatellite analysis by polymerase chain reaction. The investigation of chimerism within specific leukocyte subsets isolated from peripheral blood or bone marrow samples by flow sorting- or magnetic bead-based techniques provides more specific information on processes underlying the dynamics of donor/recipient chimerism. Moreover, cell subset-specific analysis permits the assessment of impending complications at a significantly higher sensitivity, thus providing a basis for earlier treatment decisions.

Detection and quantification of chimerism by droplet digital PCR

Accurate quantification of chimerism and microchimerism is proving to be increasingly valuable for hematopoietic cell transplantation as well as non-transplant conditions. However, methods that are available to quantify low-level chimerism lack accuracy. Therefore, we developed and validated a method for quantifying chimerism based on digital PCR technology. We demonstrate accurate quantification that far exceeds what is possible with analog qPCR down to 0.01% with the potential to go even lower. Also, this method is inherently more informative than qPCR. We expect the advantages of digital PCR will make it the preferred method for chimerism analysis.

Chimerism analysis in transplant patients: a hypothesis-free approach in the absence of reference genotypes

INTRODUCTION

During routine analysis of chimerism in bone marrow transplant patients pre-transplant genotype of the recipient or the donor might lack. We aimed to develop a new method to analyze DNA results suitable when reference genotypes are not available.

METHODS

The method was based on the balance between heterozygotes. It was implemented in a standard computer spreadsheet, and considered the hypothetical donor-recipient genotype combinations. Hypotheses with peak height ratios and allele sharing tendency above a critical threshold were accepted. The results were compared with those obtained with prior knowledge of reference genotypes.

RESULTS

The algorithm predicted correctly the proportion of donor/recipient chimerism, even in the absence of reference genotypes. In fact, the predicted values were closely correlated (r(2)>0.98) and free of systematic bias (slope 0.98-1.04), in comparison with the reference values obtained with prior knowledge of the donor and recipient genetic profiles.

CONCLUSIONS

This study constitutes a proof-of-concept of the application of the heterozygote balance for the quantitative study of chimerism. The algorithm computes post-transplant chimerism in an easy and time-efficient way, even when the donor and recipient reference genotypes are unavailable. Therefore, it can be a useful tool for laboratories involved in chimerism analysis.

Pre-emptive immunotherapy with purified natural killer cells after haploidentical SCT: a prospective phase II study in two centers

Adoptive immunotherapy with allogeneic purified natural killer (NK) cell products might exert graft-versus-tumor alloreactivity with little risk of GVHD. In a prospective phase II study in two centers, we administered purified NK cell products to high-risk patients treated with haploidentical T-cell-depleted SCT. Sixteen patients received a total of 29 NK cell infusions on days +3, +40 and +100 after transplantation. Median doses (and ranges) of infused NK- and T-cells per product were 1.21 (0.3-3.8) × 10(7)/kg and 0.03 (0.004-0.72) × 10(5)/kg, respectively. With a median follow-up of 5.8 years 4/16 patients are alive. Cause of death was relapse in five, GVHD in three, graft failure in three, and transplant related neurotoxicity in one patient. Four patients developed acute GVHDgrade II, all receiving a total of 0.5 × 10(5) T cells/kg. Compared with historical controls, NK cell infusions had no apparent effect on the rates of graft failure or relapse. Adoptive transfer of allogeneic NK cells is safe and feasible, but further studies are needed to determine the optimal dose and timing of NK cell therapy. Moreover, NK cell activation/expansion may be required to attain clinical benefit, while careful consideration must be given to the number of T cells infused.

Evaluation of the sensitivity of two recently developed STR multiplexes for the analysis of chimerism after haematopoietic stem cell transplantation

Forensic-oriented kits analysing short tandem repeat (STR) polymorphisms are widely used to determine the proportions of donor and recipient cells after haematopoietic stem cell transplantation. The sensitivity of this technology is crucial for the early detection of relapse and, in consequence, the adjustment of the treatment to enhance donor-origin haematopoiesis in transplant recipients. The objective of this study was to compare the performance of two recently developed STR multiplex kits, AmpFℓSTR(®) Identifiler(®) Plus PCR Amplification Kit (Applied Biosystems) and Investigator™ IDplex(®) (Qiagen), in the analysis of chimerism. Fifteen STR loci were amplified with both kits in 26 peripheral blood samples of transplantated patients showing chimerism. Peak amplitude threshold, detection limit (%DL), per cent donor chimerism and efficacy of each multiplex and STR were determined, and the results with both kits were compared. The %DL and the estimated per cent donor chimerism were similar with both kits. On the other hand, Identifiler(®) Plus kit allowed chimerism identification only in 24 (92%) of the 26 cases with chimerism detected by using the Investigator™ IDplex(®) when only 'type 5' allelic constellations (i.e. without potential interference by stutter peaks) were taken into account. However, IDplex(®) efficacy was somewhat lower than that of Identifiler Plus when only the most informative loci (D2S1338, D21S11, D18S51 and FGA) were considered. Therefore, although each system had some particular advantages and disadvantages, overall both STR multiplexes showed similar performance in qualitative and quantitative chimerism analysis.

Real-time biallelic polymorphism-polymerase chain reaction for chimerism monitoring of hematopoietic stem cell transplantation relapsed patients

BACKGROUND

An accurate analysis of chimerism kinetics permits early detection of hematopoietic stem cell transplantation (HSCT) in patients with high risks of graft-versus-host disease or those liable to relapse. Although short tandem repeats-PCR (STR-PCR) is the golden standard for quantitative chimerism analysis in most of the clinical laboratories, it has a relatively low sensitivity of 5% and the detection of low percentage in mixed chimerism is usually delayed. In this study, we developed a real-time PCR for chimerism analysis based on the informative biallelic polymorphisms (BP).

METHODS

The allele frequencies of 19 selective biallelic polymorphic markers were analyzed using the genomic DNA from 100 healthy Taiwanese volunteers. The informative biallelic polymorphic markers with high discrimination power in the Taiwanese population were identified. The TaqMan probe-based real-time BP-PCR for amplification of the informative loci was designed and the detection sensitivity was determined. Clinical application of real-time BP-PCR in chimerism monitoring was evaluated and was compared with the conventional STR-PCR by analyzing the DNA samples obtained at different time points post-HSCT from 4 relapsed and 10 non-relapsed patients.

RESULTS

Allele distribution analysis revealed that the loci of S01a, S03, S04a, S05b, S06, S07b, S08b, S09b, S10b and S11a had a relatively high discrimination power and were the informative BP for chimerism monitoring in the Taiwanese population. Real-time BP-PCRs for these 10 BP loci were set up with the detection sensitivity equivalent to 0.003-0.006%. Real-time BP-PCR of the 4 HSCT patients revealed the presence of recipient-specific DNA at early time point than STR-PCR for 3 of the patients, whereas real-time BP-PCR was as effective as STR-PCR in uncovering the sign of relapse for one of the patients. In addition, the baseline value for the patients with no sign of relapse was 0.127 ± 0.193% of recipient DNA.

CONCLUSION

We conclude that real-time BP-PCR is a sensitive and reliable method for chimerism monitoring and is superior to the STR-PCR in identifying patients who are at high risk for relapse after transplantation.

IL-2-driven regulation of NK cell receptors with regard to the distribution of CD16+ and CD16- subpopulations and in vivo influence after haploidentical NK cell infusion

To characterize natural killer (NK) cell subpopulations during activation, we analyzed the NK cell receptor repertoire and functionality of purified clinical scale CD56CD3 donor NK cells during stimulation with 1000 U/mL interleukin (IL)-2 for up to 14 days. In a phase I/II trial, we investigated the efficacy and feasibility of nonidentical NK cell infusion in patients with neuroblastoma after haploidentical stem cell transplantation. After IL-2 stimulation, large differences in the distribution of CD16 and CD16 subpopulations were found in 12 donors. Thereby, surface expression for all natural cytotoxicity receptors (NCRs) and NKG2D increased. In addition, killer cell immunoglobulin-like receptor (KIR) NK cells were overgrown by KIR proportion and the homing receptor CD62L was lost during stimulation. NK cell cytotoxicity against K562 and neuroblastoma cells increased and significantly higher cytokine secretion (eg, interferon-gamma, tumor necrosis factor-beta, macrophage inflammatory protein-1alpha, macrophage inflammatory protein-1beta) was observed after IL-2 stimulation compared with freshly isolated NK cells. However, NK cells of donors showing an initially enhanced cytotoxicity combined with NCR and CD69 expression, seemed to be exhausted and did not favor a stimulation period over 9 days. When IL-2-stimulated NK cells were given to transplant recipients, they induced a decrease of peripheral blood NK, in particular of CD56-NK cells. Our data indicate that IL-2 stimulation increases the expression of activating receptors and emphasizes mechanisms beside KIR/human leukocyte antigen. Furthermore, the results suggest that the expansion period of purified NK cells has to be individualized to optimize NK cell immunotherapy.

Recent advances in quantitative chimerism analysis

Quantitative chimerism analysis is a diagnostic tool used to monitor engraftment kinetics after allogeneic stem cell transplantation. It reflects the proportion of recipient and donor genotypes and is based on the identification of genetic markers characteristic to a given transplant pair. Currently, PCR amplification of short tandem repeats and single-nucleotide polymorphism-specific quantitative real-time PCR are the most widely used techniques for this purpose. In this review, we will address advances as well as technology-specific imperfections, of both techniques that have emerged over the recent years. We will discuss new principles that may simplify assay design, and improve its robustness and reliability. A better chimerism assay could then guide clinical interventions and may, eventually, improve the outcome of allogeneic stem cell transplantation.

Quantitative monitoring of multi-donor chimerism: a systematic, validated framework for routine analysis

Despite therapeutic advantages, double-donor (DD) HSCTs present technical problems for molecular chimerism (CHM) monitoring. These DD chimeras contain three matched DNAs, so that the genomes of donor(s) and recipient often share the same alleles. In the STR assay, shared recipient/donor alleles are common and have identical physico-chemical properties. As a consequence of the latter, they co-migrate in the same band ('shared peak'), which prevents measuring each allele separately. Without individual allelic measurements, the direct calculation of the chimeric recipient/donor DNA ratio is precluded. This is the first study to document and systematically examine these problems. Its goal was to provide a validated framework for accurate, routine monitoring based on a stepwise analytic paradigm for approximating percent CHM (%CHM) from shared STR-alleles. Analysis of STR-DNA from DD loci showed that at least four of six alleles were typically shared. Despite such extensive allelic sharing, we show how simple arithmetic procedures can be applied for standardized calculation of %CHM based on peak measurements. Criteria for selecting loci suitable for such analysis are provided. Validation of the computational results required analyzing 18 'informative' loci with pre-established reference values for %CHM. In all cases, the results for %CHM, calculated from peak measurements, were +/-5% of the reference value. The conclusions of the study are as follows: (1) Multi-donor chimeras, with shared alleles, can be accurately and simply analyzed within the usual limits of STR measurement error; (2) by examining these various facets of DD CHM analysis, this novel study has provided a basis for standardized, routine quantitative monitoring using the STR/VNTR assay.

Direct magnetic separation of immune cells from whole blood using bacterial magnetic particles displaying protein G

Direct separation of target cells from mixed population, such as peripheral blood, umbilical cord blood, and bone marrow, is an essential technique for various therapeutic or diagnosis applications. In this study, novel particles were fabricated, and direct magnetic separation of immune cells from whole blood using such particles was performed. The magnetotactic bacterium Magnetospirillum magneticum AMB-1 synthesizes intracellular bacterial magnetic particles (BacMPs), and protein G was expressed on the surface of the BacMPs by gene fusion techniques with anchor proteins isolated from BacMP membrane. The BacMPs displaying protein G (protein G-BacMPs) had high binding capabilities to a wide range of antibody types, and various versions of protein G-BacMPs binding with different anti-CD monoclonal antibodies were constructed. Consequently, direct magnetic separation of immune cells from whole blood using protein G-BacMPs binding with anti-CD monoclonal antibodies was demonstrated. B lymphocytes (CD19+ cells) or T lymphocytes (CD3+ cells), which represent less than 0.05% in whole blood cells, were successfully separated at a purity level of more than 96%. This level was superior to that from previous reports using other magnetic separation approaches. The results of this study demonstrate the utility of protein G-BacMP and this particle may become a powerful tool for various therapeutic or diagnosis applications.

Assessing quantitative chimerism longitudinally: technical considerations, clinical applications and routine feasibility

In this review, we describe the current laboratory approach to quantitative chimerism testing based on short tandem repeats (STRs), focusing on a longitudinal analysis. The latter is based on relative changes appearing in the course of sequential samples, and as such exploits the ultimate potential of this intrinsically semiquantitative platform. Such an analysis is more informative than single static values, less likely to be confused with platform artifacts, and is individualized to the particular patient. It is particularly useful with non-myeloablative conditioning, where mixed chimerism is common. Importantly, longitudinal monitoring is a routinely feasible laboratory option because multiplex STR-polymerase chain reaction kits are available commercially, and modern software can be used to perform computation, reliability testing and longitudinal tracking in a rapid, easy to use format. The ChimerTrack application, a shareware, user friendly program developed for this purpose, produces a report that automatically summarizes and illustrates the quantitative temporal course of the patient's chimeric status. Such a longitudinal perspective enhances the value of quantitative chimerism monitoring for decisions regarding immunomodulatory post transplant therapy. This information also provides unique insights into the biological dynamics of engraftment underlying the fluctuations in the temporal course of a patient's chimeric status.

Detection of impending graft rejection and relapse by lineage-specific chimerism analysis

Molecular surveillance of hematopoietic chimerism has become part of the routine diagnostic program in patients after allogeneic stem cell transplantation. Chimerism testing permits early prediction and documentation of successful engraftment, and facilitates early detection of impending graft rejection. In patients transplanted for treatment of malignant hematological disorders, monitoring of chimerism can provide an early indication of incipient disease relapse. The investigation of chimerism has therefore become an indispensable tool for the management of patients during the posttransplant period. Growing use of nonmyeloablative conditioning, which is associated with prolonged duration of mixed hematopoietic chimerism, has further increased the clinical importance of chimerism analysis. At present, the most commonly used technical approach to the investigation of chimerism is microsatellite analysis by PCR. The investigation of chimerism within specific leukocyte subsets isolated from peripheral blood or bone marrow samples by flow-sorting or magnetic beads-based techniques provides more specific information on processes underlying the dynamics of donor/recipient chimerism. Moreover, cell subset-specific analysis permits the assessment of impending complications at a significantly higher sensitivity, thus providing a basis for earlier treatment decisions.

Clinical outcomes and direct hospital costs of reduced-intensity allogeneic transplantation in chronic myeloid leukemia

A reduced-intensity conditioning allogeneic stem cell transplantation was given to 19 patients (aged 15-59 years) in the first chronic phase and one patient in the accelerated phase with chronic myeloid leukemia (CML) after a regimen consisting of fludarabine (Flu), busulfan (Bu) and ATG Fresenius. The median follow-up was 27 months. Until day +100, no transplant-related mortality was recorded. The incidence of acute and chronic graft-versus-host disease (GvHD) was 55 and 75%, respectively. Two patients (10%) died from GvHD. Fourteen (70%) patients achieved molecular remission. Additional post-transplant intervention (donor lymphocyte infusion, imatinib) was necessary, however, in 10 patients (50% of the patients; non-achievement of stable molecular remission or later relapses). The total direct cost of the transplantation treatment for all of the patients came to 1,572,880 euro. If the patients had been treated with imatinib and followed-up with the same time period as they were following a transplantation, the direct cost of the imatinib treatment would have been 2,005,117 euro. The transplantation treatment appears to be less expensive after approximately 2 years of follow-up. Flu+Bu+ATG is a low-toxicity regimen for patients with CML. However, a close follow-up is necessary and about 50% of the patients require further therapeutic intervention.

Quantification of chimerism within peripheral blood, bone marrow and purified leukocyte subsets: comparison of singleplex and multiplex PCR amplification of short tandem repeat (STR) loci

OBJECTIVE AND METHODS

Chimerism analysis has become a routine diagnostic procedure after haematopoietic allogeneic stem cell transplantation for early detection of relapse of disease or graft failure. Whereas some centres developed individual in-house short tandem repeat (STR) systems, others prefer commercial multiplex PCR systems. However, little is known about inter-assay variation, which could have a significant impact on treatment decision. We therefore compared two commercial multiplex PCR kits with our in-house STR system using different sample sources, such as peripheral blood (PB), bone marrow (BM) and specific leukocyte subsets.

RESULTS

Fifty samples of eighteen paediatric patients were analysed. For neither material, PB, BM and leukocyte subtypes, a significant difference between the STR systems tested was observed. Chimerism analyses of each single STR primer, which is component of both the in-house and the commercial STR system, did not reveal significant differences.

CONCLUSION

Our analysis demonstrates that similar results can be obtained with both assays, even when using various sample sources. Further evaluation of different test systems will help to increase interlaboratory standardisation of chimerism analyses for early clinical intervention.

Long-term persistence of host cells detected by X-chromosome gene-based assay in patients undergoing gender-mismatched hematopoietic stem cell transplantation

Using our recently developed human androgen receptor (HUMARA) gene-based chimerism assay, long-term chimerism was investigated in female patients who underwent hematopoietic stem cell transplantation (HCT) with cells from male donors. After restriction digestion of samples, we detected a small number of female-derived cells within a large population of male-derived cells, with a sensitivity from 0.1% to 0.05%. Chimerism was examined in four patients with myeloid malignancies: two patients with acute myeloid leukemia (AML) from myelodysplastic syndrome (MDS), and one patient each with AML M3 and AML M4. All patients underwent myeloablative conditioning regimens and exhibited good clinical results during a median follow-up period of 6.6 years (range, 3.4-7.5 years). Female-derived cells were detected throughout the entire follow-up period in all bone marrow samples, but they became undetectable in the peripheral blood samples of 3 patients. Moreover, the HUMARA band pattern suggests that these residual host cells were normal cells. This study confirms the usefulness of the HUMARA gene-based assay, which showed that patients undergoing HCT frequently show mixed chimerism (MC) for a long period, especially in bone marrow, although the possibility of contamination by host stromal cells cannot be excluded.

IL-2 activated NK cell immunotherapy of three children after haploidentical stem cell transplantation

Natural killer (NK) cells are thought to be of benefit in HLA-mismatched hematopoietic transplantation (H-SCT). Therefore, we developed a protocol for clinical-use expansion of highly enriched and IL-2-stimulated NK cells. Purification of unstimulated leukaphereses by a two-step T cell depletion with a final CD56 enrichment procedure leads to a mean purity of 95% CD56(+)CD3- NK cells with a four- to five-log depletion of T cells. So far, three pediatric patients with multiply relapsed acute lymphoblastic leukemia (ALL) or acute myelogenous leukemia (AML) were treated with repeated transfusions post-H-SCT. Directed killer immunoglobulin-like receptor (KIR) mismatches were demonstrated in all three cases. Although all patients showed blast persistence at the time of transplant, they reached complete remission and complete donor chimerism within 1 month post-H-SCT. NK cell therapy was tolerated well without graft-versus-host disease (GvHD) induction or other adverse events. The AML patient died of early relapse on day +80, while the ALL patients died of thrombotic-thrombocytopenic purpura and atypical viral pneumonia on days +45 and +152, respectively. This initial trial showed the feasibility of good manufacturing practice (GMP)-compliant NK cell isolation and expansion for clinical applications. We now launch a clinical phase I trial with activated NK cells post-H-SCT.

A novel four-colour flow cytometric assay to determine natural killer cell or T-cell-mediated cellular cytotoxicity against leukaemic cells in peripheral or bone marrow specimens containing greater than 20% of normal cells

To be able to determine the cytotoxic activity of NK cells or T cells against leukaemic cells in patient samples containing >20% of normal peripheral blood cells, we have developed a four-colour flow cytometric cytotoxicity assay. The assay is based on differential immunostaining of both leukaemic cells and effector cells in combination with propidium iodide (PI). The cytometer is set for measuring the fluorescence of the monoclonal antibody (mAb) bound fluorochromes, with moderate overcompensation of the third and fourth fluorescence signals. PI-positive events were excluded from analysis by their characteristic uncompensated signal on these two detectors. Thus, all four fluorescence ranges can be used for detection of mAb-derived signals and this allows discrimination between various populations contained in effector and target cell samples. The cytotoxic activity in our assay is calculated by the absolute loss of vital leukaemic cells. For this purpose, fluorescent beads are included as an internal standard. When calculating the effector concentrations after coculture, characteristic changes can be seen which yield additional information about the presence of cytotoxic activity and the active effector cell subset. With this assay, we present a versatile tool that combines minimum cell manipulation before coculture with maximum information from a sample. The assay is suitable for the analysis of complex samples with regard to different cell subsets, their decrease or increase, and conjugate formation.

Diagnostic chimerism analysis after allogeneic stem cell transplantation: new methods and markers

Analysis of chimerism after allogeneic hematopoietic cell transplantation is important for assessing engraftment and the early detection of graft failure. In addition, the monitoring of minimal residual disease and early detection of imminent relapse has also become an important issue. Novel transplant procedures, for example dose-reduced conditioning protocols, rely on chimerism analysis to guide intervention, i.e. the reduction of immunosuppression or infusion of donor lymphocytes. During the last 30 years, several methods for the analysis of chimerism after hematopoietic cell transplantation have been published. Currently, fluorescent in situ hybridization (XY-FISH) analysis of sex chromosomes after transplantation from a sex-mismatched donor or analysis of polymorphic DNA sequences, i.e. short tandem repeats (STR) or variable number of tandem repeats (VNTR), are the most widely used procedures used in the assessment of chimerism. Two major diagnostic fields can be defined for chimerism analysis: the period of engraftment and the detection of minimal residual disease. Although STR-PCR and FISH analysis are very useful in the diagnosis of engraftment and graft failure, they are only of limited use in the monitoring of minimal residual disease, largely because of its limited level of sensitivity (1-5% for the minor population). Several novel procedures to improve this level of detection have been reported in recent years. One focus has been the use of real-time PCR techniques based on analysis of the Y-chromosome or, more recently, single nucleotide polymorphism (SNPs). These procedures combine quantitative analysis with high sensitivity (10(-4) to 10(-6)), and hold great potential for the future. In addition, the combination of cell sorting based on leukemia-specific immunophenotype and STR-PCR has been successfully used for minimal residual disease detection. First clinical data using these procedures indicate that intervention (e.g. the reduction of immunosuppression or donor lymphocyte infusion) may be effective in the minimal residual disease situation, even in high risk diseases like acute myeloid leukemia and acute lymphoblastic leukemia. The optimal timing of these diagnostic interventions is a critical issue and has to be further optimized. Whether this will ultimately improve the survival of patients with leukemia after transplantation has to be shown in prospective studies.

Strategies and clinical implications of chimerism diagnostics after allogeneic hematopoietic stem cell transplantation

Analysis of donor chimerism has become a routine method for the documentation of engraftment after allogeneic hematopoietic stem cell transplantation (HSCT). In recent years several groups have also focused on the application of this technique for the detection of relapsing disease after allogeneic HSCT. This review addresses technical issues (sensitivity, specificity) and discusses the advantages and limitations of methods currently used for chimerism analysis and their usefulness for the detection of MRD. In addition, the potential impact of novel procedures, e.g. subset chimerism or real-time PCR-based procedures, is discussed.

Assessing hematopoietic chimerism after allogeneic stem cell transplantation by multiplexed SNP genotyping using microarrays and quantitative analysis of SNP alleles

Single-nucleotide polymorphisms (SNPs) have the potential to be particularly useful as markers for monitoring of chimerism after stem cell transplantation (SCT) because they can be analyzed by accurate and robust methods. We used a two-phased minisequencing strategy for monitoring chimerism after SCT. First, informative SNPs with alleles differing between donor and recipient were identified using a multiplex microarray-based minisequencing system screening 51 SNPs to ensure that multiple informative SNPs were detected in each donor-recipient pair. Secondly, the development of chimerism was followed up after SCT by sensitive, quantitative analysis of individual informative SNPs by applying the minisequencing method in a microtiter plate format. Using this panel of SNPs, we identified multiple informative SNPs in nine unrelated and in 16 related donor-recipient pairs. Samples from nine of the donor-recipient pairs taken at time points ranging from 1 month to 8 years after transplantation were available for analysis. In these samples, we monitored the allelic ratios of two or three informative SNPs in individual minisequencing reactions. The results agreed well with the data obtained by microsatellite analysis. Thus, we conclude that the two-phased minisequencing strategy is a useful approach in the following up of patients after SCT.