Chimerism analysis within 6 months of allogeneic stem cell transplantation predicts relapse in acute myeloid leukemia

cfDNA Chimerism and Somatic Mutation Testing in Early Prediction of Relapse After Allogeneic Stem Cell Transplantation for Myeloid Malignancies

Background: Disease relapse is a primary cause of treatment failure after hematopoietic stem cell transplantation in the treatment of malignancy. Consolidation therapy early after transplantation may reduce this risk, but it is difficult to administer in the setting of various post-transplant complications. We proposed that testing donor cell chimerism and for persistent minimal residual disease (MRD) with next-generation sequencing (NGS) of plasma cell-free DNA (cfDNA) early after transplantation would identify those patients at higher risk of relapse who would possibly benefit from consolidation therapy. Methods: We enrolled 20 subjects with known tumor-associated somatic mutations into this prospective pilot study, testing plasma samples before and at 28, 56, and 84 days after transplantation. Pre- and post-transplant bone marrow samples were also analyzed. All samples were subjected to an agnostic, commercially available panel covering 302 genes. Results: Significantly more mutations (p < 0.0001) were detected in the plasma cfDNA than in the bone marrow cells in pre-transplant testing (92 versus 61 mutations, respectively), most likely reflecting sampling variation when bone marrow was used. Two subjects were negative for MRD in staging studies immediately before transplants. Most (19/20) subjects had intermittent or sustained MRD detected in post-transplant plasma cfDNA testing, albeit with much lower average variant allele frequencies (VAFs). Six out of 20 subjects suffered relapses within 12 months after transplantation, and all 6 could be identified by adverse-risk driver mutations that persisted after transplantation. No patients who cleared the adverse-risk mutations relapsed. Donor chimerism using cfDNA fell for all relapsed patients and contributed to the identification of patients at early risk for relapse. Conclusions: These data demonstrate that testing plasma cfDNA for persistent leukemia-associated somatic mutations and donor chimerism as early as 28 days after transplantation will identify a subset of patients with high-risk mutations who are at high risk of relapse. This early assessment of relapse risk may facilitate modifications to the treatment plan, reducing the risk of treatment failure.

Sensitivity and Specificity of Chimerism Tests in Predicting Leukemia Relapse Using Increasing Mixed Chimerism

Chimerism test was evaluated to predict leukemia relapse. Increasing mixed chimerism (IMC), defined as recipient increase ≥0.1% in peripheral blood total cell chimerism, was used as a surrogate of disease activity. Combination of quantitative PCR and short-tandem repeat method was applied to achieve high assay sensitivity. Total of 184 patients received stem cell transplant for acute myeloid leukemia (N = 110), acute lymphocytic leukemia (N = 41), myelodysplastic syndrome (N = 30), and 2389 chimerism tests (median follow-up, 1054 days). Sixty-six patients relapsed, and 118 patients did not. Cumulative incidence of relapse increased after 1 IMC or ≥2 consecutive IMCs (hazard ratios, 9.9 and 44.4, respectively). Predicted percentage relapse by day 30 after IMC was 0% (0 IMC), 10% (1 IMC), and 40% (≥2 IMCs). The last chimerism results before relapse detected IMC in 57 of 66 relapsed patients (sensitivity, 86.4%). Nine patients had no IMC before relapse (false negative) because of rapidly evolving relapse (N = 4) or extramural relapse (N = 5). In 118 patients without relapse, 158 of 1873 tests detected IMC (false positive, 8.4%; specificity, 91.6%). The false-positive rates increased with higher percentage recipient T-cell chimerism levels, indicating T-cell contamination as a cause. Chimerism monitoring predicts relapse. However, caution must be taken for false-positive or false-negative IMCs. T-cell removal can improve chimerism test specificity in patients with mixed T-cell chimerism.

Ultrasensitive chimerism enhances measurable residual disease testing after allogeneic hematopoietic cell transplantation

Increasing mixed chimerism (reemerging recipient cells) after allogeneic hematopoietic cell transplant (allo-HCT) can indicate relapse, the leading factor determining mortality in blood malignancies. Most clinical chimerism tests have limited sensitivity and are primarily designed to monitor engraftment. We developed a panel of quantitative polymerase chain reaction assays using TaqMan chemistry capable of quantifying chimerism in the order of 1 in a million. At such analytic sensitivity, we hypothesized that it could inform on relapse risk. As a proof-of-concept, we applied our panel to a retrospective cohort of patients with acute leukemia who underwent allo-HCT with known outcomes. Recipient cells in bone marrow aspirates (BMAs) remained detectable in 97.8% of tested samples. Absolute recipient chimerism proportions and rates at which these proportions increased in BMAs in the first 540 days after allo-HCT were associated with relapse. Detectable measurable residual disease (MRD) via flow cytometry in BMAs after allo-HCT showed limited correlation with relapse. This correlation noticeably strengthened when combined with increased recipient chimerism in BMAs, demonstrating the ability of our ultrasensitive chimerism assay to augment MRD data. Our technology reveals an underappreciated usefulness of clinical chimerism. Used side by side with MRD assays, it promises to improve identification of patients with the highest risk of disease reoccurrence for a chance of early intervention.

Posttransplant MRD and T-cell chimerism status predict outcomes in patients who received allografts for AML/MDS

Allogeneic stem-cell transplant allows for the delivery of curative graft-versus-leukemia (GVL) in patients with acute myeloid leukemia/myelodysplasia (AML/MDS). Surveillance of T-cell chimerism, measurable residual disease (MRD) and blast HLA-DR expression may inform whether GVL effectiveness is reduced. We report here the prognostic impact of these biomarkers in patients allografted for AML/MDS. One hundred eighty-seven patients from FIGARO, a randomized trial of reduced-intensity conditioning regimens in AML/MDS, were alive and relapse-free at the first MRD time-point and provided monitoring samples for flow cytometric MRD and T-cell chimerism, requested to month+12. Twenty-nine (15.5%) patients had at least 1 MRD-positive result posttransplant. MRD-positivity was associated with reduced overall survival (OS) (hazard ratio [HR], 2.18; P = .0028) as a time-varying Cox variable and remained significant irrespective of pretransplant MRD status in multivariate analyses (P < .001). Ninety-four patients had sequential MRD with T-cell chimerism results at months+3/+6. Patients with full donor T-cell chimerism (FDTC) had an improved OS as compared with patients with mixed donor T-cell chimerism (MDTC) (adjusted HR=0.4; P = .0019). In patients with MDTC (month+3 or +6), MRD-positivity was associated with a decreased 2-year OS (34.3%) vs MRD-negativity (71.4%) (P = .001). In contrast, in the group with FDTC, MRD was infrequent and did not affect the outcome. Among patients with posttransplant MRD-positivity, decreased HLA-DR expression on blasts significantly reduced OS, supporting this as a mechanism for GVL escape. In conclusion, posttransplant MRD is an important predictor of the outcome in patients allografted for AML/MDS and is most informative when combined with T-cell chimerism results, underlining the importance of a GVL effect in AML/MDS.

Competitive Repopulation and Allo-Immunologic Pressure Determine Chimerism Kinetics after T Cell-Depleted Allogeneic Stem Cell Transplantation and Donor Lymphocyte Infusion

After allogeneic stem cell transplantation (alloSCT), patient-derived stem cells that survived the pretransplantation conditioning compete with engrafting donor stem cells for bone marrow (BM) repopulation. In addition, donor-derived alloreactive T cells present in the stem cell product may favor establishment of complete donor-derived hematopoiesis by eliminating patient-derived lymphohematopoietic cells. T cell-depleted alloSCT with sequential transfer of potentially alloreactive T cells by donor lymphocyte infusion (DLI) provides a unique opportunity to selectively study how competitive repopulation and allo-immunologic pressure influence lymphohematopoietic recovery. This study aimed to determine the relative contribution of competitive repopulation and donor-derived anti-recipient alloimmunologic pressure on the establishment of lymphohematopoietic chimerism after alloSCT. In this retrospective cohort study of 281 acute leukemia patients treated according to a protocol combining alemtuzumab-based T cell-depleted alloSCT with prophylactic DLI, we investigated engraftment and quantitative donor chimerism in the BM and immune cell subsets. DLI-induced increase of chimerism and development of graft-versus-host disease (GVHD) were analyzed as complementary indicators for donor-derived anti-recipient alloimmunologic pressure. Profound suppression of patient immune cells by conditioning sufficed for sustained engraftment without necessity for myeloablative conditioning or development of clinically significant GVHD. Although 61% of the patients without any DLI or GVHD showed full donor chimerism (FDC) in the BM at 6 months after alloSCT, only 24% showed FDC in the CD4⁺ T cell compartment. In contrast, 75% of the patients who had received DLI and 83% of the patients with clinically significant GVHD had FDC in this compartment. In addition, 72% of the patients with mixed hematopoiesis receiving DLI converted to complete donor-derived hematopoiesis, of whom only 34% developed clinically significant GVHD. Our data show that competitive repopulation can be sufficient to reach complete donor-derived hematopoiesis, but that some alloimmunologic pressure is needed for the establishment of a completely donor-derived T cell compartment, either by the development of GVHD or by administration of DLI. We illustrate that it is possible to separate the graft-versus-leukemia effect from GVHD, as conversion to durable complete donor-derived hematopoiesis following DLI did not require induction of clinically significant GVHD.

Peripheral Blood CD34 Donor Chimerism has Greater Clinical Utility Than CD3 for Detecting Relapse after Allogeneic Stem Cell Transplantation for Acute Myeloid Leukemia or Myelodysplastic Syndrome

Monitoring of donor chimerism (DC) may detect early relapse following allogeneic hematopoietic stem cell transplantation (allo-SCT) for acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS). Most centers use unfractionated peripheral blood or T cells to monitor DC, although CD34⁺ DC may be more predictive. The limited adoption of CD34⁺ DC may be due to the lack of detailed, comparative studies. To address this knowledge gap, we compared peripheral blood CD34⁺ and CD3⁺ DC in 134 patients who underwent allo-SCT for AML or MDS. In July 2011, the Alfred Hospital Bone Marrow Transplantation Service adopted routine monitoring of DC in the lineage-specific CD34⁺ and CD3⁺ cell subsets from peripheral blood at 1, 2, 3, 4, 6, 9, and 12 months post-transplantation for AML or MDS. Immunologic interventions, including rapid withdrawal of immunosuppression, azacytidine, and donor lymphocyte infusion, were prespecified for CD34⁺ DC ≤80%. Overall, CD34⁺ DC ≤80% detected 32 of 40 relapses (positive predictive value [PPV], 68%; negative predictive value [NPV], 91%), compared with 13 of 40 relapses for CD3⁺ DC ≤80% (PPV, 52%; NPV, 75%). Receiver operating characteristic analysis showed the superiority of CD34⁺ DC, with the greatest value at day 120 post-transplantation. CD3⁺ DC provided additional value in only 3 cases, preceding CD34⁺ DC ≤80% by 1 month. We further show that the CD34⁺ DC sample can be used to detect NPM1^mut, with the combination of CD34⁺ DC ≤80% and NPM1^mut identifying the highest risk of relapse. Among the 24 patients in morphologic remission at the time of CD34⁺ DC ≤80%, 13 (54%) responded to immunologic interventions (rapid withdrawal of immunosuppression, azacitidine, or donor lymphocyte infusion) with recovery of CD34⁺ DC >80%, and 11 of these patients remained in complete remission for a median of 34 months (range, 28 to 97 months). In contrast, the other 9 patients did not respond to the clinical intervention and relapsed within a median of 59 days after detecting CD34⁺ DC ≤80%. The CD34⁺ DC was significantly higher in responders than in nonresponders (median, 72% versus 56%; P = .015, Mann-Whitney U test). Overall, monitoring of CD34⁺ DC was considered clinically useful (with early diagnosis of relapse enabling preemptive therapy or predicting low risk of relapse) in 107 of 125 evaluable patients (86%). Our findings show that peripheral blood CD34⁺ DC is feasible and superior to CD3⁺ DC for predicting relapse. It also provides a source of DNA for measurable residual disease testing, which may further stratify the risk of relapse. If validated by an independent cohort, our results suggest that CD34⁺ should be used in preference to CD3⁺ DC for detecting early relapse and guiding immunologic interventions following allo-SCT for AML or MDS.

Incomplete chimerism following myeloablative and anti-thymocyte globulin-conditioned hematopoietic cell transplantation is a risk factor for relapse and chronic graft-versus-host disease

BACKGROUND AIMS

The value of routine chimerism determination after myeloablative hematopoietic cell transplantation (HCT) is unclear, particularly in the setting of anti-thymocyte globulin (ATG)-based graft-versus-host disease (GVHD) prophylaxis.

METHODS

Blood samples were collected at 3 months post-HCT from 558 patients who received myeloablative conditioning and ATG-based GVHD prophylaxis. Chimerism was assessed using multiplex polymerase chain reaction of short tandem repeats in sorted T cells (CD3+) and leukemia lineage cells (CD13+CD33+ for myeloid malignancies and CD19+ for B-lymphoid malignancies). ATG exposure was determined using a flow cytometry-based assay. The primary outcomes of interest were relapse and chronic GVHD (cGVHD).

RESULTS

Incomplete (<95%) T-cell chimerism and leukemia lineage chimerism were present in 17% and 4% of patients, respectively. Patients with incomplete T-cell chimerism had a significantly greater incidence of relapse (36% versus 22%, subhazard ratio [SHR] = 2.03, P = 0.001) and lower incidence of cGVHD (8% versus 25%, SHR = 0.29, P < 0.001) compared with patients with complete chimerism. In multivariate modeling, patients with high post-transplant ATG area under the curve and any cytomegalovirus (CMV) serostatus other than donor/recipient seropositivity (non-D+R+) had an increased likelihood of incomplete T-cell chimerism. Patients with incomplete leukemia lineage chimerism had a significantly greater incidence of relapse (50% versus 23%, SHR = 2.70, P = 0.011) and, surprisingly, a greater incidence of cGVHD (45% versus 20%, SHR = 2.64, P = 0.003).

CONCLUSIONS

High post-transplant ATG exposure and non-D+R+ CMV serostatus predispose patients to incomplete T-cell chimerism, which is associated with an increased risk of relapse. The increased risk of cGVHD with incomplete B-cell/myeloid chimerism is a novel finding that suggests an important role for recipient antigen-presenting cells in cGVHD pathogenesis.

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.

Lineage-specific early complete donor chimerism and risk of relapse after allogeneic hematopoietic stem cell transplantation for acute myeloid leukemia

Recipient–donor chimerism is routinely analyzed after allogeneic hematopoietic stem cell transplantation (HSCT) to monitor engraftment and graft rejection. For malignancies, chimerism can also be used to screen for disease relapse post-HSCT but methodology and interpretation of results are not standardized and likely depend on underlying diagnosis. We have implemented highly sensitive and accurate methodologies for chimerism analysis for the purpose of improving relapse prediction. Here, we report an exploratory retrospective analysis of clinical routine chimerism results from all 154 HSCTs for acute myeloid leukemia (AML) performed at our center during the years 2015–2020 with the aim of suggesting a clinically useful threshold at which risk of relapse is high. Relapse was not reliably predicted based on single elevated chimerism values obtained before time of overt relapse. However, early complete donor chimerism, here defined as recipient DNA < 0.2% in CD33⁺ cells in any blood or bone marrow sample taken during the first 60 days after HSCT, correlated inversely with relapse during the observation time (log-rank test P = 0.033). We propose that achievement of complete chimerism determined early after HSCT using sensitive methods can be used for risk-stratification of AML patients.

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.

Low-Dose Decitabine Monotherapy Reverses Mixed Chimerism in Adult Patients After Allogeneic Hematopoietic Stem Cell Transplantation With Myeloablative Conditioning Regimen: A Pilot Phase II Study

T cell mixed chimerism (MC) after allogeneic hematopoietic stem cell transplantation (allo-HSCT) with myeloablative conditioning for hematological malignancies may indicate engraftment failure or disease relapse. Immune modulation, such as donor lymphocyte infusion (DLI) or the rapid tapering-off or stopping of immunosuppressive treatment, can reverse MC to full donor chimerism (FDC). However, the development or aggravation of graft-versus-host disease (GvHD) and the related mortality remain major concerns with immune modulation. In this prospective, single-arm study (NCT03663751), we tested the efficacy and safety of low-dose decitabine (LD-DAC, 5 mg/m² daily for 5 days and repeated every 6–8 weeks) without immune modulation in the treatment of patients with MC to prevent MC-associated relapse and/or graft failure. A total of 14 patients were enrolled. All the patients received myeloablative conditioning regimens, and MC was documented from day +30 to day +180 after allo-HSCT with a donor chimerism level ranging from 59 to 97% without detectable measurable residual disease (MRD). Eleven patients (78.6%) responded favorably to treatment, showing increased levels of donor chimerism (≥95%), while nine achieved FDC. All of these patients maintained their responses for a median of 11 months (3–22). The three patients who failed to respond favorably eventually either relapsed or experienced graft failure. All three were alive and in remission at the last follow-up after the second allo-HSCT. LD-DAC monotherapy was well tolerated and exerted limited hematological and nonhematological toxicities. New-onset GvHD symptoms were observed only in two patients. Overall, the estimated 2-year overall survival (OS) and event-free survival (EFS) after allo-HSCT were 90.9 ± 8.7% and 67.0 ± 13.7%, respectively. In conclusion, LD-DAC alone could reverse MC in most patients after allo-HSCT with myeloablative conditioning, while those who achieved FDC enjoyed long-term EFS without major complications. Further prospective studies with larger sample sizes are warranted to confirm the benefits of LD-DAC.

Evaluation of EuroFlow minimal residual disease measurement and donor chimerism monitoring following tandem auto-allogeneic transplantation for multiple myeloma

Prognostic factors for multiple myeloma (MM) after allogeneic haemopoietic stem cell transplantation (alloHSCT) are poorly characterised. Two potential factors include minimal residual disease (MRD) and CD3⁺ donor-specific chimerism. We retrospectively examined 93 consecutive patients who received upfront or deferred tandem auto-alloHSCT. Bone marrow (Euroflow) MRD was assessed pre-alloHSCT and 3-monthly post-alloHSCT. CD3⁺ donor chimerism was assessed at D30, D60, D90, 6 m and 12 m post-alloHSCT. There was no statistical difference between upfront and deferred transplants in progression free survival (PFS) (34 m vs. 15 m respectively, p = 0.20) and overall survival (OS) (75.5 m vs. 62.7 m respectively, p = 0.56). Patients who were MRD-positive post-alloHSCT had inferior PFS to MRD-negative patients from 6 m (6 m HR 3.32, p = 0.02; 9 m HR 4.08, p = 0.003; 12 m HR 4.47, p = 0.008). Attainment or maintenance of MRD-negativity predicted reduced relapse risk (23.5% vs. 62.5%, p = 0.04). However, there was no significant difference in OS between the MRD-negative and positive groups. Full CD3⁺ donor chimerism at early time points (D30 and D90) was associated with increased risk of acute GVHD (D30 p < 0.001, D90 p = 0.006) and extensive chronic GVHD (D90 p = 0.04), but not PFS or OS. These data support the use of sequential MRD evaluation post-alloHSCT to inform intervention to eradicate persistent or emergent MRD-positive disease.

[Prognostic value of donor chimerism at +90 days after allogeneic hematopoietic stem cell transplantation in young patients with intermediate-risk acute myeloid leukemia]

Objective: To investigate the relationship between donor chimerism and relapse after allogeneic hematopoietic stem cell transplantation (allo-HSCT) . Methods: The clinical data of 105 patients with acute myeloid leukemia (AML) who underwent allo-HSCT and recurrence-free survival>90 days from January 2010 to January 2019 were retrospectively analyzed. The bone marrow samples were collected at 15, 30, 60, 90, 180, 270, 360 days after transplantation. Donor chimerism was detected by single nucleotide polymorphism (SNP) -PCR. Results: Of the 105 patients, 43 cases were male and 62 cases were female, with a median age of 38 (16-60) years. Till April 2019, the median follow-up was 843 (94-3 261) days. Ninety days after transplantation, 18 cases relapsed, 33 cases died, and 72 cases survived. The 3-year overall survival (OS) rate was (66.8±5.1) %, and the recurrence-free survival (RFS) rate was (65.1±5.0) %. Pre-transplant disease status, pre-transplant minimal residual disease (MRD) , and 90 day post-transplantation chimerism were independent risk factors related to RFS. The risk of recurrence was significantly increased in patients with a donor chimerism rate ≤97.24% at 90 days after transplantation[HR=6.921 (95%CI 2.669-17.950) , P<0.001], which was considered as a sign of early relapse. Conclusion: SNP-PCR is an applicable method for detecting donor chimerism in patients after allo-HSCT. Chimerism rate equal or less than 97.24% at 90 days after transplantation predicts a higher risk of relapse.

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.

Cytomegalovirus-specific CD8+ T-cells are associated with a reduced incidence of early relapse after allogeneic stem cell transplantation

Leukemia relapse is the main cause for mortality after allogeneic stem cell transplantation (allo-SCT). Donor-derived allo-immune responses eliminate the residual host hematopoiesis and protect against relapse. Cytomegalovirus (CMV) reactivation (CMV-R) after allo-SCT may trigger anti-leukemic effects. The impact of CMV-specific CD8+ T-cells (CMV-CTLs) on the outcome after allo-SCT is currently unknown. Here, we studied the relationship between CMV-CTLs, overall T-cell reconstitution and relapse incidence in 103 patients with acute leukemia (n = 91) or myelodysplastic syndrome (n = 12) following CMV-seropositive recipient/donor (R+/D+) allo-SCT. Patients were subdivided based on the presence or absence of CMV-CTLs at 3 months after allo-SCT. Presence of CMV-CTLs was associated with preceding CMV-R and a fast T-cell reconstitution. Univariate analysis showed a significantly lower 1-, 2- and 5-year cumulative incidence of relapse (CIR) in patients with CMV-CTLs compared to those without CMV-CTLs. Multivariable regression analysis of the outcome performed with other relevant parameters chosen from univariate analysis revealed that presence of CMV-CTLs and chronic graft-versus-host disease (cGvHD) were the only independent factors associated with a low CIR. Onset of relapse was significantly later in patients with CMV-CTLs (median 489 days) than in in those without (median 152 days, p = 0.041) during a five-year follow-up. Presence of CMV-CTLs was associated with a lower incidence of early relapses (1 and 2-years), while cGvHD lead to a lower incidence of late relapses (2 to 5-years). In conclusion, our data show that CMV-CTLs indicate a functional immune-reconstitution protective against early relapse.

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.

Haplotype Counting for Sensitive Chimerism Testing: Potential for Early Leukemia Relapse Detection

Fields of forensics, transplantation, and paternity rely on human identity testing. Currently, this is accomplished through amplification of microsatellites followed by capillary electrophoresis. An alternative and theoretically better approach uses multiple single-nucleotide polymorphisms located within a small region of DNA, a method we initially developed using HLA-A and called haplotype counting. Herein, we validated seven additional polymorphic loci, sequenced a total of 45 individuals from three of the 1000 Genomes populations (15 from each), and determined the number of haplotypes, heterozygosity, and polymorphic information content for each locus. In addition, we developed a multiplex PCR that amplifies five of these loci simultaneously. Using this strategy with a small cohort of leukemic patients who underwent allogeneic bone marrow transplantation, we first attempted to define a threshold (0.26% recipient) by examining seven patients who tested all donor and did not relapse. Although this initial threshold will need to be confirmed in a larger cohort, we detected increased recipient DNA above this threshold 90 to 145 days earlier than microsatellite positivity, and 127 to 142 days before clinical relapse in four of eight patients (50%). Haplotype counting using these novel loci may be useful for ultrasensitive detection in fields such as bone marrow transplantation, solid organ transplant rejection, patient identification, and forensics.

Chimerism in Myeloid Malignancies following Stem Cell Transplantation Using FluBu4 with and without Busulfan Pharmacokinetics versus BuCy

In the era of precision medicine, the impact of personalized dosing of busulfan is not clear. We undertook a retrospective analysis of 78 patients with myeloid malignancies who received fludarabine and busulfan (FluBu4) with or without measuring Bu pharmacokinetics (Bu PK) and those who received busulfan with cyclophosphamide (BuCy). Fifty-five patients received FluBu4, of whom 21 had Bu PK measured, and 23 patients received BuCy. Total donor cell chimerism showed that the percentage of patients maintaining 100% donor chimerism on day 100 was 66.7%, 38.2%, and 73.9% in the FluBu4 with PK, FluBu4 with no PK, and BuCy, respectively (P = .001). Patients who had decreasing donor chimerism by day 100 were 23.8%, 52.9%, and 26.1% in the FluBu4 with PK, FluBu4 with no PK, and BuCy, respectively (P = .04). Bu PK group had fewer patients with less than 95% donor chimerism on day 30, which was not statistically significant, 5% (FluBu4 PK), 31% (FluBu4 with no PK), and 21% (BuCy) (P = .18). Survival distributions were not statistically significant (P = .11). Thus, personalized drug dosing can impact donor chimerism in myeloid malignancies. This will need to be examined in larger retrospective multicenter studies and prospective clinical trials.

Usefulness of Post-Transplant Hematopoietic Chimera Monitoring by Use of the Quantitative Fluorescence Polymerase Chain Reaction Method

BACKGROUND

In the light of an increasing number of hematopoietic stem cell transplantations and more frequent use of reduced-intensity conditioning as preparative regimens for hematopoietic stem cell transplantation, post-transplant cell chimera monitoring is considered a necessity.

METHODS

The quantitative fluorescence polymerase chain reaction method, along with the commercial AmpFSTR SGMPlus kit, was applied in research on hematopoietic chimeras.

RESULTS

The total of 102 patients who had undergone allogenic transplantations were investigated. Chimerism monitoring was commenced on the seventh day after transplantation and lasted up to 12 years in some cases, according to the instituted schedule.

CONCLUSIONS

The kit has been shown to be fully sufficient for determining genetic profiles of recipients and donors and selecting informative markers. The method has been proven effective and satisfactory for assessing quantitative chimeras.

Possible Impact of Cytomegalovirus-Specific CD8+ T Cells on Immune Reconstitution and Conversion to Complete Donor Chimerism after Allogeneic Stem Cell Transplantation

Complete donor chimerism is strongly associated with complete remission after allogeneic stem cell transplantation (allo-SCT) in patients with hematologic malignancies. Donor-derived allo-immune responses eliminate the residual host hematopoiesis and thereby mediate the conversion to complete donor chimerism. Recently, cytomegalovirus (CMV) reactivation was described to enhance overall T cell reconstitution, to increase graft-versus-host disease incidence, and to reduce the leukemia relapse risk. However, the link between CMV and allo-immune responses is still unclear. Here, we studied the relationship between CMV-specific immunity, overall T cell reconstitution, and residual host chimerism in 106 CMV-seropositive patients transplanted after reduced-intensity conditioning including antithymocyte globulin. In accordance with previous reports, the recovery of CMV-specific cytotoxic T cells (CMV-CTLs) was more frequent in CMV-seropositive recipients (R) transplanted from CMV-seropositive than from seronegative donors (D). However, once CMV-CTLs were detectable, the reconstitution of CMV-specific CTLs was comparable in CMV R+/D- and R+/D+ patients. CD3⁺ and CD8⁺ T cell reconstitution was significantly faster in patients with CMV-CTLs than in patients without CMV-CTLs both in the CMV R+/D- and R+/D+ setting. Moreover, CMV-CTL numbers correlated with CD3⁺ and CD8⁺ T cell numbers in both settings. Finally, presence of CMV-CTLs was associated with low host chimerism levels 3 months after allo-SCT. In conclusion, our data provide a first indication that CMV-CTLs in CMV-seropositive patients might trigger the reconstitution of T cells and allo-immune responses reflected by the conversion to complete donor chimerism.

Allogeneic Hematopoietic Cell Transplantation in Multiple Myeloma: Focus on Longitudinal Assessment of Donor Chimerism, Extramedullary Disease, and High-Risk Cytogenetic Features

Although generally not applied as first-line treatment of multiple myeloma, allogeneic hematopoietic cell transplantation (allo-SCT) can still be chosen as ultimate escalation approach in high-risk patients, preferentially within the framework of clinical trials. In this study, we investigated whether decreasing donor chimerism (DC) is predictive for relapse. In addition, we comprehensively determined the impact of several other disease- and treatment-related factors on outcome. One hundred fifty-five multiple myeloma patients whose DC status was followed serially by the short tandem repeat-based techniques at a single lab were included in this retrospective study. Outcome variables were studied in univariate and multivariable analyses. Available were 2.324 DC samples (median, 12 per patient). Loss of full DC was associated with shorter progression-free survival (PFS) (HR, 1.7; 95% CI, 1.1 to 2.6) but did not impact overall survival. Two-thirds of patients with International Myeloma Working Group-defined relapses still displayed a full DC in peripheral blood or bone marrow. Extramedullary manifestations were observed in 33% of patients, accounting for the discrepancy between DC analysis and the actual disease status. In multivariable analysis, the 2 most relevant variables for an unfavorable PFS were progressive disease before allo-SCT (HR, 3.0; 95% CI, 1.5 to 5.9) and allo-SCT at least the second relapse (HR, 2.8; 95% CI, 1.5 to 4.9), whereas for overall survival progressive disease or partial response before allo-SCT had the strongest negative effects (HR, 4.2; 95% CI, 1.9 to 9, and HR, 2.0; 95% CI, 1.0 to 3.8, respectively). Adverse cytogenetics such as del17p, t(4,14) or amp(1q21) were not associated with shorter survival after allo-SCT. Extensive DC sampling beyond robust engraftment does not appear to provide additional information helpful for disease management in most patients and is challenged by a significant incidence of extramedullary disease. In our series, allo-SCT overcame unfavorable cytogenetics.

Early phase mixed chimerism in bone marrow does not affect long-term outcomes of myeloablative single-unit cord blood transplantation for adult patients with hematological malignancies

Coexisting hematopoiesis from donor and recipient origin, called a mixed chimerism status, can occur in patients after myeloablative allogeneic hematopoietic stem cell transplantation. However, its impact on the outcomes of cord blood transplantation (CBT) has yet to be clarified. We retrospectively analyzed 150 adult patients who received myeloablative single-unit CBT for hematological malignancies in our institute. At the median time of first bone marrow analysis of 41 days after CBT, mixed chimerism was observed in 16 of the 150 patients. Among patients with mixed chimerism, 4 patients relapsed. The remaining 12 patients were alive and in remission at a median follow-up of 50 months. Bone marrow-mixed chimerism did not have a significant impact on the incidences of disease-free survival, relapse, or transplant-related mortality after CBT. These data show that early phase mixed chimerism did not have a significant impact on long-term outcomes after myeloablative single-unit CBT for hematological malignancies.

Complete donor chimerism is a prerequisite for the effect of Predicted Indirectly ReCognizable HLA Epitopes (PIRCHE) on acute graft-versus-host disease

Predicted indirectly recognizable HLA epitopes (PIRCHE) computationally predict donor T-cell recognition of mismatched-HLA derived peptides following allogeneic haematopoietic stem-cell transplantation (allo-HSCT), as is evidenced by the correlation between presence of HLA-DPB1-derived PIRCHE and the occurrence of graft-vs.-host disease (GVHD). Complete donor T-cell chimerism associates with an increased GVHD risk compared to mixed patient and donor chimerism. If the correlation between the presence of PIRCHE and GVHD occurrence is indeed mediated by donor T cells, the presence of donor T cells should be required to observe such a correlation. This study was initiated to investigate whether the effect of PIRCHE is different in patients with complete chimerism compared to those with mixed chimerism. Indeed, the correlation between PIRCHE and GVHD is present in patients with complete chimerism, whereas it is absent in those with mixed chimerism. The data presented here suggest that chimerism status is important for the detection of potential GVHD epitopes.

Very Long Term Stability of Mixed Chimerism after Allogeneic Hematopoietic Stem Cell Transplantation in Patients with Hematologic Malignancies

The objective of this study is to analyze the evolution of chimerism of all patients transplanted for hematologic malignancies in our unit during a 20-year period, alive without relapse at 1 year after allogeneic hematopoietic stem cell transplantation (HSCT). Chimerism was tested using short tandem repeat polymorphisms after separation into mononuclear cells and granulocytes by Ficoll density gradient centrifugation. Of 155 patients studied, 89 had full chimerism (FC), 36 mononuclear cells mixed chimerism (MNC-MC), and 30 granulocytic MC with or without mononuclear cells MC (Gran-MC). Survival was significantly better in MNC-MC than in Gran-MC patients, with FC patients being intermediate. There was more disease relapse in the Gran-MC group but not in the MNC-MC group as compared to FC. MC was stable up to 21 years in the MNC-MC group and up to 19 years in the Gran-MC group. Of MC patients alive at 10 years, MC persisted in 83% in the MNC-MC and 57% in the Gran-MC groups. In conclusion, mixed chimerism may remain stable over a very long time period. In survivors without relapse at 1 year after HSCT, determining lineage specific chimerism may be useful as outcome differs, MNC-MC being associated with better outcome than Gran-MC.

Chimerism analysis in clinical practice and its relevance for the detection of graft rejection and malignant relapse in pediatric hematopoietic stem cell transplant patients

Chimerism and clinical outcome data from 244 hematopoietic stem cell transplants in 218 children were retrospectively analyzed to assess their relevance for the detection of graft rejection and malignant relapse. Patients transplanted for a non-malignant disease had significantly higher proportions of residual recipient T cells in peripheral blood at one, three, and six months compared with patients transplanted for malignant disease. Recipient T-cell levels were below 50% at one month after transplantation in most patients (129 of 152 transplants). Graft rejection occurred more frequently in the group of patients with high levels of recipient cells at one month (10 graft rejections in the 23 patients with recipient T cells >50% at one month as compared to seven graft rejections occurred in 129 patients with recipient T cells <50% (p < 0.001). Multilineage chimerism data in 87 children with leukemia at one, three, and six months after transplantation were not correlated with subsequent relapse of malignant disease. In conclusion, early analysis of lineage-specific chimerism in peripheral blood can be used to identify patients who are at high risk of graft rejection. However, the efficacy of early chimerism analysis for predicting leukemia relapse was limited.

Direct Cell-Cell Contact with Chondrocytes Is a Key Mechanism in Multipotent Mesenchymal Stromal Cell-Mediated Chondrogenesis

Using a combination of articular chondrocytes (ACs) and mesenchymal stromal cells (MSCs) has shown to be a viable option for a single-stage cell-based treatment of focal cartilage defects. However, there is still considerable debate whether MSCs differentiate or have a chondroinductive role through trophic factors. In addition, it remains unclear whether direct cell-cell contact is necessary for chondrogenesis. Therefore, the aim of this study was to investigate whether direct or indirect cell-cell contact between ACs and MSCs is essential for increased cartilage production in different cellular environments and elucidate the mechanisms behind these cellular interactions. Human ACs and MSCs were cultured in a 10:90 ratio in alginate beads, fibrin scaffolds, and pellets. Cells were mixed in direct cocultures, separated by a Transwell filter (indirect cocultures), or cultured with conditioned medium. Short tandem repeat analysis revealed that the percentages of ACs increased during culture, while those of MSCs decreased, with the biggest change in fibrin glue scaffolds. For alginate, where the lack of cell-cell contact could be confirmed by histological analysis, no difference was found in matrix production between direct and indirect cocultures. For fibrin scaffolds and pellet cultures, an increased glycosaminoglycan production and type II collagen deposition were found in direct cocultures compared with indirect cocultures and conditioned medium. Positive connexin 43 staining and transfer of cytosolic calcein indicated communication through gap junctions in direct cocultures. Taken together, these results suggest that MSCs stimulate cartilage formation when placed in close proximity to chondrocytes and that direct cell-cell contact and communication through gap junctions are essential in this chondroinductive interplay.

Mixed T Lymphocyte Chimerism after Allogeneic Hematopoietic Transplantation Is Predictive for Relapse of Acute Myeloid Leukemia and Myelodysplastic Syndromes

Chimerism testing after allogeneic hematopoietic stem cell transplantation (allo-HSCT) in patients with acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) represents a promising tool for predicting disease relapse, although its precise role in this setting remains unclear. We investigated the predictive value of T lymphocyte chimerism analysis at 90 to 120 days after allo-HSCT in 378 patients with AML/MDS who underwent busulfan/fludarabine-based myeloablative preparative regimens. Of 265 (70%) patients with available T lymphocyte chimerism data, 43% of patients in first or second complete remission (CR1/CR2) at the time of transplantation had complete (100%) donor T lymphocytes at day +90 to +120 compared with 60% of patients in the non-CR1/CR2 cohort (P = .005). In CR1/CR2 patients, donor T lymphocyte chimerism ≤ 85% at day +90 to +120 was associated with a higher frequency of 3-year disease progression (29%; 95% confidence interval [CI], 18% to 46% versus 15%; 95% CI, 9% to 23%; hazard ratio [HR], 2.1; P = .04). However, in the more advanced, non-CR1/CR2 cohort, mixed T lymphocyte chimerism was not associated with relapse (37%; 95% CI, 20% to 66% versus 34%; 95% CI, 25% to 47%; HR, 1.3; P = .60). These findings demonstrate that early T lymphocyte chimerism testing at day +90 to +120 is a useful approach for predicting AML/MDS disease recurrence in patients in CR1/CR2 at the time of transplantation.

Is there a role for B lymphocyte chimerism in the monitoring of B-acute lymphoblastic leukemia patients receiving allogeneic stem cell transplantation?

OBJECTIVE

To determine the sensitivity and significance of B-cell chimerism for the detection of early engraftment, transplant rejection, and disease relapse.

METHODS

The dynamic monitoring of lineage-specific cell subtypes (B, T, and NK cells) was made in 20 B-cell acute lymphoblastic leukemia (B-ALL) patients following allogeneic hematopoietic stem cell transplantation (allo-HSCT). In the early period after allo-HSCT, the latest establishment of B-cell complete chimerism (CC) was observed in a majority of patients.

RESULTS

The percentage of donor cells of B-cell lineage was lower than the percent of T-cell lineage in most of the mixed chimerism (MC) patients. During graft rejection, the frequency of patients with decreasing MC of B-, T- and NK-cell lineage were 5/5, 2/5, and 2/5. When disease relapsed, five patients showed a faster decrease of the donor percent of B-cells than of T- or NK-cells. Only one patient displayed a more rapid decrease in NK-cells than in T- or B-cells.

CONCLUSION

Monitoring of B-cell chimerism after HSCT seems to be valuable for insuring complete engraftment, anticipating graft rejection, and relapse in B-ALL patients.

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.

Donor chimerism early after reduced-intensity conditioning hematopoietic stem cell transplantation predicts relapse and survival

The impact of early donor cell chimerism on outcomes of T cell-replete reduced-intensity conditioning (RIC) hematopoietic stem cell transplantation (HSCT) is ill defined. We evaluated day 30 (D30) and 100 (D100) total donor cell chimerism after RIC HSCT undertaken between 2002 and 2010 at our institution, excluding patients who died or relapsed before D30. When available, donor T cell chimerism was also assessed. The primary outcome was overall survival (OS). Secondary outcomes included progression-free survival (PFS), relapse, and nonrelapse mortality (NRM). We evaluated 688 patients with hematologic malignancies (48% myeloid and 52% lymphoid) and a median age of 57 years (range, 18 to 74) undergoing RIC HSCT with T cell-replete donor grafts (97% peripheral blood; 92% HLA-matched), with a median follow-up of 58.2 months (range, 12.6 to 120.7). In multivariable analysis, total donor cell and T cell chimerism at D30 and D100 each predicted RIC HSCT outcomes, with D100 total donor cell chimerism most predictive. D100 total donor cell chimerism <90% was associated with increased relapse (hazard ratio [HR], 2.54; 95% confidence interval [CI], 1.83 to 3.51; P < .0001), impaired PFS (HR, 2.01; 95% CI, 1.53 to 2.65; P < .0001), and worse OS (HR, 1.50; 95% CI, 1.11 to 2.04, P = .009), but not with NRM (HR, .76; 95% CI, .44 to 2.27; P = .33). There was no additional utility of incorporating sustained D30 to D100 total donor cell chimerism or T cell chimerism. Low donor chimerism early after RIC HSCT is an independent risk factor for relapse and impaired survival. Donor chimerism assessment early after RIC HSCT can prognosticate for long-term outcomes and help identify high-risk patient cohorts who may benefit from additional therapeutic interventions.

T-cell chimerism is valuable in predicting early mortality in steroid-resistant acute graft-versus-host disease after myeloablative allogeneic cell transplantation

The main aim of this study was to evaluate the impact of early T-cell chimerism status on the incidence and clinical course of acute graft-versus-host disease (aGVHD) in allogeneic transplant recipients after myeloablative conditioning. Of 62 patients, 38 (61%) had complete T-cell donor chimerism (DC), and 24 (39%) had mixed T-cell chimerism (MC). The incidence of early death (before day 100) with steroid-resistant aGVHD was significantly higher in patients with complete DC compared to patients with MC (6/38 vs. 0/24, p = 0.04). The time from aGVHD diagnosis to nonrelapse death was markedly different in patients with complete DC versus MC [53 days (range 12–183) vs. 238 days (range 135– 550), p = 0.005]. Conditioning with Etopophos/total body irradiation (TBI) resulted in significantly more patients with MC compared to conditioning with cyclophosphamide/TBI. In conclusion, early complete T-cell DC was associated with a high incidence of early death in patients receiving highdose steroids for the treatment of aGVHD.

New approaches to manipulate minimal residual disease after allogeneic stem cell transplantation

Minimal residual disease (MRD) is a complex topic that has been studied extensively in hematologic malignancies given its clinical implications related to prognosis. However, methods to monitor and treat MRD, especially after stem cell transplantation, are not well defined and vary in different disease processes. Alternative transplant strategies, such as reduced-intensity conditioning, have altered the way we assess and address MRD after transplantation. Development of new diagnostic tools have allowed for higher sensitivity and specificity of testing. Both targeted chemotherapeutic agents and immunotherapies have been developed to treat MRD in hopes of improving patient outcomes. This article aims to address ways to define and manipulate MRD specifically after stem cell transplantation.

Acute GVHD is a strong predictor of full donor CD3+ T cell chimerism after reduced intensity conditioning allogeneic stem cell transplantation

The monitoring of chimerism is a standard procedure to assess engraftment and achievement of full donor lymphoid cells after reduced intensity conditioning (RIC) stem cell transplantation (Allo-SCT). However, there is no consensus on when and how often to monitor post-transplant chimerism. We retrospectively analyzed our experience regarding the impact of acute graft versus host disease (GVHD) for the prediction of allograft chimerism. One-hundred-and-fifteen patients transplanted between 2001 and 2010 were identified. This group included 57 females and 58 males with a median age of 50 years (range: 26-68). Patients evaluated in this study were adult patients with hematologic malignancies, who received transplants from an HLA-matched sibling donor or matched unrelated donor (MUD) at allele level so-called 10/10, and received the RIC regimen including fludarabine/busulfan and anti-thymoglobulin (ATG). Mixed T-cell chimerism was defined as between 5 and 94% recipient cells, and full chimerism was defined as the presence of more than 95% donor T-cell chimerism (TCC). Full donor TCC was achieved in 93 patients (81%) at a median of 77 days (range: 30-120) post-transplant. The cumulative incidence of Grade 2-4 GVHD in our population was 25% (95% CI 17-34). The analysis of the population of patients with acute GVHD grade ≥2 showed that at day 120 after Allo-SCT they all had a total full donor TCC. On the other hand, 78 (68%) patients without acute GVHD grade ≥2 presented with mixed chimerism (p = 0.002) on day 120 post-transplant. Interestingly, patients who received ATG 5 mg/kg obtained a higher probability of complete chimerism compared with those receiving 2.5 mg/kg (p = 0.03). In conclusion, our study demonstrates that acute GVHD was predictive of full donor TCC after RIC Allo-SCT. Therefore, our data may challenge the concept of the frequent or close monitoring of donor chimerism in some patients with ongoing acute GVHD. However, chimerism testing could represent an attractive modality for minimal residual disease detection or for impeding relapse warranting further prospective studies.

Prevention and treatment of acute myeloid leukemia relapse after allogeneic stem cell transplantation

PURPOSE OF REVIEW

Relapse remains a major cause of treatment failure for acute myeloid leukemia (AML) patients treated with allogeneic hematopoietic stem cell transplantation (allo-HCT). Most patients that recur will perish due to low treatment efficacy, toxicity, or frailty issues. This review summarizes recent developments in clinical research and therapeutic applications for prevention and treatment of this complication of transplantation.

RECENT FINDINGS

Several groups have demonstrated that monitoring minimal residual disease (MRD) after allo-HCT is feasible and is predictive of impending hematologic recurrence. The introduction of novel antileukemia agents in the preparative regimen, maintenance of remission treatment posttransplant, and early MRD-based therapeutic interventions all have the potential to improve outcomes.

SUMMARY

Innovative basic and clinical investigation is urgently needed to improve treatment and prevention of AML recurrence after allogeneic transplantation.

Increasing hematopoietic microchimerism is a reliable indicator of incipient AML relapse

INTRODUCTION

The reoccurrence or increase in autologous hematopoiesis after allogeneic transplantation has been linked to incipient leukemia relapse. However, the importance of such an emergency regarding microchimerism (i.e. mixed chimerism below 1% of autologous cells) still remains controversial, as fluctuating microchimerism can be observed for a very long time after transplantation.

METHODS

Using real-time PCR (RQ-PCR), we compare peripheral blood samples obtained from patients with acute myeloid leukemia (AML) before hematological relapse and those taken during complete remission (i.e. either complete cytogenetic remission or complete molecular remission where applicable). By comparison of these two groups, we describe microchimerism dynamics clearly connected with imminent AML relapse. Additionally, we compare applicability of RQ-PCR and conventional PCR with fragment analysis.

RESULTS

Mere reappearance of autologous hematopoiesis within patients with complete donor chimerism is alarming, and another sample with further increase confirms ongoing relapse. In case of patients with continuous microchimerism, another two consecutive samples with increasing trend are required. RQ-PCR predicted a significantly higher number of hematological relapses (87%vs. 39%) with a median anticipation period of 33 days, 26 days earlier than conventional PCR (P= 0.0002). Moreover, the outcome of microchimerism dynamics was in complete agreement with monitoring of minimal residual disease when analyzed from the same cell compartment.

CONCLUSION

Within this paper, we emphasize the importance of microchimerism monitoring as a reliable indicator of incipient AML relapse, especially in patients where no other specific molecular marker is available.