Reduced-intensity conditioning regimen preserves thymic function in the early period after hematopoietic stem cell transplantation

Prognostic impact of HLA supertype mismatch in single-unit cord blood transplantation

The "human leukocyte antigen (HLA) supertype" is a functional classification of HLA alleles, which was defined by structural features and peptide specificities, and has been reportedly associated with the clinical outcomes of viral infections and autoimmune diseases. Although the disparity in each HLA locus was reported to have no clinical significance in single-unit cord blood transplantation (sCBT), the clinical significance of the HLA supertype in sCBT remains unknown. Therefore, we retrospectively analyzed clinical data of 1603 patients who received sCBT in eight institutes in Japan between 2000 and 2017. Each HLA allele was categorized into 19 supertypes, and the prognostic effect of disparities was then assessed. An HLA-B supertype mismatch was identified as a poor prognostic factor (PFS: hazard ratio [HR] = 1.23, p = 0.00044) and was associated with a higher cumulative incidence (CI) of relapse (HR = 1.24, p = 0.013). However, an HLA-B supertype mismatch was not associated with the CI of acute and chronic graft-versus-host-disease. The multivariate analysis for relapse and PFS showed the significance of an HLA-B supertype mismatch independent of allelic mismatches, and other previously reported prognostic factors. HLA-B supertype-matched grafts should be selected in sCBT.

Allogeneic stem cell transplantation in the treatment of acute myeloid leukemia: An overview of obstacles and opportunities

As an important treatment for acute myeloid leukemia, allogeneic hematopoietic stem cell transplantation (allo-HSCT) plays an important role in reducing relapse and improving long-term survival. With rapid advancements in basic research in molecular biology and immunology and with deepening understanding of the biological characteristics of hematopoietic stem cells, allo-HSCT has been widely applied in clinical practice. During allo-HSCT, preconditioning, the donor, and the source of stem cells can be tailored to the patient’s conditions, greatly broadening the indications for HSCT, with clear survival benefits. However, the risks associated with allo-HSCT remain high, i.e. hematopoietic reconstitution failure, delayed immune reconstitution, graft-versus-host disease, and post-transplant relapse, which are bottlenecks for further improvements in allo-HSCT efficacy and have become hot topics in the field of HSCT. Other bottlenecks recognized in the current treatment of individuals diagnosed with acute myeloid leukemia and subjected to allo-HSCT include the selection of the most appropriate conditioning regimen and post-transplantation management. In this paper, we reviewed the progress of relevant research regarding these aspects.

Dynamics of recent thymic emigrants in pediatric recipients of allogeneic hematopoetic stem cell transplantation

After allogeneic hematopoietic stem cell transplantation (allo-HSCT), the recurrence of recent thymic emigrants (RTE) and self-tolerant T cells indicate normalized thymic function. From 2008 to 2019, we retrospectively analyzed the RTE-reconstitution rate and the minimal time to reach normal age-specific first percentiles for CD31⁺CD45RA⁺CD4⁺T cells in 199 pediatric patients after allo-HSCT for various malignant and non-malignant diseases. The impact of clinically significant graft-versus-host disease (GvHD), age at transplantation, underlying disease and cumulative area under the curve of busulfan on RTE-reemergence was assessed in multivariable longitudinal analysis. RTE-reconstitution (coefficient -0.24, 95% CI -0.33 to -0.14, p < 0.001) was slowed down by GvHD and the time to reach P1 was significantly longer (Event Time Ratio 1.49, 95% CI 1.25 to 1.78, p < 0.001). Older age at transplantation was also associated with a slower RTE-reconstitution (coefficient -0.028, 95% CI -0.04 to -0.02, p < 0.001) and time to reach P1 was significantly longer (Event Time Ratio 1.03, 95% CI 1.02 to 1.05, p < 0.001). RTE-reconstitution velocity was not influenced by underlying disease or cumulative busulfan exposure. In summary, duration until thymic reactivation was independent of both conditioning intensity and underlying disease and was negatively influenced by older age and GvHD.

Evaluating Thymic Function After Human Hematopoietic Stem Cell Transplantation in the Personalized Medicine Era

Hematopoietic stem cell transplantation (HSCT) is an effective treatment option for several malignant and non-malignant hematological diseases. The clinical outcome of this procedure relies to a large extent on optimal recovery of adaptive immunity. In this regard, the thymus plays a central role as the primary site for de novo generation of functional, diverse, and immunocompetent T-lymphocytes. The thymus is exquisitely sensitive to several insults during HSCT, including conditioning drugs, corticosteroids, infections, and graft-vs.-host disease. Impaired thymic recovery has been clearly associated with increased risk of opportunistic infections and poor clinical outcomes in HSCT recipients. Therefore, better understanding of thymic function can provide valuable information for improving HSCT outcomes. Recent data have shown that, besides gender and age, a specific single-nucleotide polymorphism affects thymopoiesis and may also influence thymic output post-HSCT, suggesting that the time of precision medicine of thymic function has arrived. Here, we review the current knowledge about thymic role in HSCT and the recent work of genetic control of human thymopoiesis. We also discuss different transplant-related factors that have been associated with impaired thymic recovery and the use of T-cell receptor excision circles (TREC) to assess thymic output, including its clinical significance. Finally, we present therapeutic strategies that could boost thymic recovery post-HSCT.

Thymic activity in immune recovery after allogeneic hematopoietic stem cell transplantation in children

Thymic output was studied prospectively in 52 children who underwent allogeneic hematopoietic stem cell transplantation (allo-HSCT). Thymic activity was assessed by quantification of recent thymic emigrants (RTE) discriminated from the rest of naive T cells by immunophenotype CD3+/CD4+/CD31+/CD45RA+. Thymic output was analyzed in correlation with the kinetics of immune recovery and in relation to other potential risk factors that may influence thymopoiesis: underlying disease, type of HSCT, source of stem cells, age of recipient and donor, type of conditioning, implemented graft versus host disease (GvHD) prophylaxis, viral reactivations (herpes viruses cytomegalovirus - CMV, Epstein-Barr virus - EBV, adenovirus - ADV, BK virus - BKV), occurrence and grade of both acute and chronic graft versus host disease (aGvHD, cGvHD) and number of transplanted CD34 cells/kg. The absolute count of RTE in peripheral blood was evaluated at 6 time points: before the conditioning and on days +15, +30, +60 , +90 and +180 after HSCT. Occurrence of grade II-IV aGvHD was the most important factor associated with low RTE counts after HSCT. History of malignant disease, and transplantation from matched unrelated donor were risk factors for lower thymic output. We found a weak inverse correlation between the age of the recipient and thymic output on post-HSCT day +180. Source of stem cells, type of conditioning, viral reactivations, occurrence of chronic GvHD, age of the donor and the number of transplanted CD34 cells/kg did not affect thymopoiesis in our study group. These preliminary findings and identification of risk factors for deterioration of thymic activity may in the future help in selecting candidates for thymus rejuvenation strategies.

Clinical significance of T cell receptor excision circle (TREC) quantitation after allogenic HSCT

Background

Hematopoietic stem cell transplantation (HSCT) is a well-established treatment modality for a variety of diseases. Immune reconstitution is an important event that determines outcomes. The immune recovery of T cells relies on peripheral expansion of mature graft cells, followed by differentiation of donor-derived hematopoietic stem cells. The formation of new T cells occurs in the thymus and as a byproduct, T cell receptor excision circles (TRECs) are released. Detection of TRECs by PCR is a reliable method for estimating the amount of newly formed T cells in the circulation and, indirectly, for estimating thymic function. The aim of this study was to determine the role of TREC quantitation in predicting outcomes of human leucocyte antigen (HLA) identical allogenic HSCT.

Methods

The study was conducted on 100 patients receiving allogenic HSCT from an HLA identical sibling. TREC quantification was done by real time PCR using a standard curve.

Results

TREC levels were inversely related to age (P=0.005) and were significantly lower in patients with malignant diseases than in those with benign diseases (P=0.038). TREC levels could predict relapse as an outcome but not graft versus host disease (GvHD) and infections.

Conclusion

Age and nature of disease determine the TREC levels, which are related to relapse.

Immune Reconstitution and Infection Patterns after Early Alemtuzumab and Reduced Intensity Transplantation for Nonmalignant Disorders in Pediatric Patients

Hematopoietic stem cell transplantation (HSCT) is a therapeutic option for many nonmalignant disorders (NMD) and is curative or prevents disease progression. Reduced-intensity conditioning (RIC) in HSCT for NMD may reduce regimen-related acute toxicities and late complications. Myeloablation is often replaced by immune suppression in RIC regimens to support donor engraftment. The pace of immune reconstitution after immune suppression by RIC regimens is influenced by agents used, donor source, and graft-versus-host disease prophylaxis/treatment. In a multicenter trial (NCT 00920972) of HSCT for NMD, a RIC regimen consisting of alemtuzumab, fludarabine, and melphalan was substituted for myeloablation. Alemtuzumab was administered early (days -21 to -19) to mitigate major lymphodepletion of the incoming graft and the risk of graft rejection. Immune reconstitution and infectious complications were prospectively monitored for 1-year post-HSCT. Seventy-one patients met inclusion criteria for this report and received marrow or peripheral blood stem cell transplants. Immune reconstitution and infections are reported for related donor (RD) and unrelated donor (URD) transplants at 3 time-points (100days, 6 months, and 1 year post-HSCT). Natural killer cell recovery was rapid, and numbers normalized in both cohorts by day +100. Mean CD3, CD4, and CD8 T-lymphocyte numbers normalized by 6 months after RD HSCT and by 1 year in the URD group. CD4 and CD8 T-lymphocyte counts were significantly higher in patients who received RD HSCT at 6 months and at 1 year, respectively, post-HSCT compared with patients who received URD HSCT. The pace of CD19 B-cell recovery was markedly different between RD and URD cohorts. Mean B-cell numbers were normal by day 100 after RD HSCT but took 1 year post-HSCT to normalize in the URD cohort. Despite these differences in immune reconstitution, the timing and nature of infections did not differ between the groups, presumably because of comparable T-lymphocyte recovery. Immune reconstitution occurred at a faster pace than in prior reports using RIC with T-cell depletion. The incidence of infections was similar for both cohorts and occurred most frequently in the first 100days post-HSCT. Viral and fungal infections occurred at a lower incidence in this cohort, with "early" alemtuzumab compared with regimens administering serotherapy in the peritransplantation period. Patients were susceptible to bacterial infections primarily in the first 100days irrespective of donor source and had no increase in mortality from the same. The overall mortality rate from infections was 1.4% at 1 year. Close monitoring and prophylaxis against bacterial infections in the first 100days post-HSCT is necessary but is followed by robust immune reconstitution, especially in the T-cell compartment.

Joint Modeling of Immune Reconstitution Post Haploidentical Stem Cell Transplantation in Pediatric Patients With Acute Leukemia Comparing CD34+-Selected to CD3/CD19-Depleted Grafts in a Retrospective Multicenter Study

Rapid immune reconstitution (IR) following stem cell transplantation (SCT) is essential for a favorable outcome. The optimization of graft composition should not only enable a sufficient IR but also improve graft vs. leukemia/tumor effects, overcome infectious complications and, finally, improve patient survival. Especially in haploidentical SCT, the optimization of graft composition is controversial. Therefore, we analyzed the influence of graft manipulation on IR in 40 patients with acute leukemia in remission. We examined the cell recovery post haploidentical SCT in patients receiving a CD34⁺-selected or CD3/CD19-depleted graft, considering the applied conditioning regimen. We used joint model analysis for overall survival (OS) and analyzed the dynamics of age-adjusted leukocytes; lymphocytes; monocytes; CD3⁺, CD3⁺CD4⁺, and CD3⁺CD8⁺ T cells; natural killer (NK) cells; and B cells over the course of time after SCT. Lymphocytes, NK cells, and B cells expanded more rapidly after SCT with CD34⁺-selected grafts (P = 0.036, P = 0.002, and P < 0.001, respectively). Contrarily, CD3⁺CD4⁺ helper T cells recovered delayer in the CD34 selected group (P = 0.026). Furthermore, reduced intensity conditioning facilitated faster immune recovery of lymphocytes and T cells and their subsets (P < 0.001). However, the immune recovery for NK cells and B cells was comparable for patients who received reduced-intensity or full preparative regimens. Dynamics of all cell types had a significant influence on OS, which did not differ between patients receiving CD34⁺-selected and those receiving CD3/CD19-depleted grafts. In conclusion, cell reconstitution dynamics showed complex diversity with regard to the graft manufacturing procedure and conditioning regimen.

The Transcription Factor Hif-1 Enhances the Radio-Resistance of Mouse MSCs

Mesenchymal stromal cells (MSCs) are multipotent progenitors supporting bone marrow hematopoiesis. MSCs have an efficient DNA damage response (DDR) and are consequently relatively radio-resistant cells. Therefore, MSCs are key to hematopoietic reconstitution following total body irradiation (TBI) and bone marrow transplantation (BMT). The bone marrow niche is hypoxic and via the heterodimeric transcription factor Hypoxia-inducible factor-1 (Hif-1), hypoxia enhances the DDR. Using gene knock-down, we have previously shown that the Hif-1α subunit of Hif-1 is involved in mouse MSC radio-resistance, however its exact mechanism of action remains unknown. In order to dissect the involvement of Hif-1α in the DDR, we used CRISPR/Cas9 technology to generate a stable mutant of the mouse MSC cell line MS5 lacking Hif-1α expression. Herein, we show that it is the whole Hif-1 transcription factor, and not only the Hif-1α subunit, that modulates the DDR of mouse MSCs. This effect is dependent upon the presence of a Hif-1α protein capable of binding to both DNA and its heterodimeric partner Arnt (Hif-1β). Detailed transcriptomic and proteomic analysis of Hif1a KO MS5 cells leads us to conclude that Hif-1α may be acting indirectly on the DNA repair process. These findings have important implications for the modulation of MSC radio-resistance in the context of BMT and cancer.

Differential Response of Mouse Thymic Epithelial Cell Types to Ionizing Radiation-Induced DNA Damage

Thymic epithelial cells (TECs) are the main components of the thymic stroma that support and control T-cell development. Preparative regimens using DNA-damaging agents, such as total body irradiation and/or chemotherapeutic drugs, that are necessary prior to bone marrow transplantation (BMT) have profound deleterious effects on the hematopoietic system, including the thymic stroma, which may be one of the main causes for the prolonged periods of T-cell deficiency and the inefficient T cell reconstitution that are common following BMT. The DNA damage response (DDR) is a complex signaling network that allows cells to respond to all sorts of genotoxic insults. Hypoxia is known to modulate the DDR and play a role affecting the survival capacity of different cell types. In this study, we have characterized in detail the DDR of cortical and medullary TEC lines and their response to ionizing radiation, as well as the effects of hypoxia on their DDR. Although both mTECs and cTECs display relatively high radio-resistance, mTEC cells have an increased survival capacity to ionizing radiation (IR)-induced DNA damage, and hypoxia specifically decreases the radio-resistance of mTECs by upregulating the expression of the pro-apoptotic factor Bim. Analysis of the expression of TEC functional factors by primary mouse TECs showed a marked decrease of highly important genes for TEC function and confirmed cTECs as the most affected cell type by IR. These findings have important implications for improving the outcomes of BMT and promoting successful T cell reconstitution.

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.

T Cell Receptor Excision Circle (TREC) Monitoring after Allogeneic Stem Cell Transplantation; a Predictive Marker for Complications and Clinical Outcome

Allogeneic hematopoietic stem cell transplantation (HSCT) is a well-established treatment modality for a variety of malignant diseases as well as for inborn errors of the metabolism or immune system. Regardless of disease origin, good clinical effects are dependent on proper immune reconstitution. T cells are responsible for both the beneficial graft-versus-leukemia (GVL) effect against malignant cells and protection against infections. The immune recovery of T cells relies initially on peripheral expansion of mature cells from the graft and later on the differentiation and maturation from donor-derived hematopoietic stem cells. The formation of new T cells occurs in the thymus and as a byproduct, T cell receptor excision circles (TRECs) are released upon rearrangement of the T cell receptor. Detection of TRECs by PCR is a reliable method for estimating the amount of newly formed T cells in the circulation and, indirectly, for estimating thymic function. Here, we discuss the role of TREC analysis in the prediction of clinical outcome after allogeneic HSCT. Due to the pivotal role of T cell reconstitution we propose that TREC analysis should be included as a key indicator in the post-HSCT follow-up.

Thymic Activity and T Cell Repertoire Recovery after Autologous Hematopoietic Stem Cell Transplantation Preceded by Myeloablative Radiotherapy or Chemotherapy

It was previously postulated that pretransplant myeloablative treatment may impair thymopoiesis, contributing in this way to delayed reconstitution of T cells after hematopoietic stem cell transplantation (HSCT). On the other hand, de novo generation of T cells after HSCT requires a competent thymus. Various myeloablative conditioning regimens (total body irradiation [TBI] or high-dose chemotherapy) routinely used in clinical practice may have potentially different impacts on the thymus. However, no comparative study on thymic output and T cell repertoire in autologous (auto)HSCT model has been presented so far. Here we evaluated thymic output and TCR diversity in 45 lymphoma patients submitted to autoHSCT differing in respect to conditioning regimen: high-dose chemotherapy as monotherapy (BEAM, n = 22) or combination of total body irradiation with cyclophosphamide chemotherapy: Cy/TBI (n = 23). Thymic output was assessed before and on days +100, +180, and +365 after autoHSCT by flow cytometric counts of recent thymic emigrant (RTE) cells (CD31(+) CD62L(+) CD45RA(+) CD4(+)) and quantification of signal joint TCR receptor excision circles (sjTRECs) by quantitative PCR. T cell repertoire diversity was analyzed on day +365 after autoHSCT by spectra-typing of the CDR3 region in the TCRVβ chain. The BEAM group, in contrast to the Cy/TBI group, manifested significantly higher proportions of RTE cells and sjTREC copy numbers on days +100 and +180. Analysis of TCRVβ spectra-types on day +365 revealed more restricted (monoclonal or oligoclonal) T cell repertoires in the Cy/TBI versus BEAM group (48.8% versus 18.2%, P = .0002). In conclusion, the conditioning scheme based on BEAM chemotherapy may be performed with lower risk of thymic destruction and T cell repertoire distortion than Cy/TBI scheme. This finding may help to potentially improve conditioning schemes to efficiently perform myeloablation and maintain active thymopoiesis.

Feasible outcomes of T cell-replete haploidentical stem cell transplantation with reduced-intensity conditioning in patients with myelodysplastic syndrome

Even with the recent optimization of haploidentical stem cell transplantation (SCT), its role for patients with myelodysplastic syndrome (MDS) or acute myeloid leukemia evolving from MDS (sAML) should be validated. We analyzed the outcomes of consecutive 60 patients with MDS or sAML who received T cell-replete haploidentical SCT after reduced-intensity conditioning with fludarabine, busulfan, and rabbit antithymocyte globuline ± 800 cGy total body irradiation. Patients achieved a rapid neutrophil engraftment after a median of 12 days (range, 8 to 23) and an early immune reconstitution without high incidences of acute graft-versus-host disease (GVHD) II to IV and chronic GVHD (36.7% and 48.3%, respectively). After a median follow-up of 4 years, incidence of relapse and nonrelapse mortality and rate of overall survival and disease-free survival was 34.8%, 23.3%, 46.8%, and 41.9%, respectively. In multivariate analysis, the disease status at peak was a significant predictor for relapse (lower-risk MDS versus higher-risk MDS or sAML; hazard ratio [HR], 5.69; 95% confidence interval [CI], 1.45 to 22.29; P = .013) and disease-free survival (HR, 4.44; 95% CI, 1.14 to 17.34; P = .032). Chronic GVHD was an additional significant predictor for relapse (no versus yes; HR, 2.87; 95% CI, 1.03 to 7.51; P = .043). Our T cell-replete haploidentical SCT may be a feasible option for patients with MDS and sAML without conventional donors.

The role of the thymus in T-cell immune reconstitution after umbilical cord blood transplantation

Umbilical cord blood (UCB) is an alternative source of hematopoietic stem cells for patients without HLA-matched adult donors. UCB contains a low number of nucleated cells and mostly naive T cells, resulting in prolonged time to engraftment and lack of transferred T-cell memory. Although the first phase of T-cell reconstitution after UCB transplantation (UCBT) depends on peripheral expansion of transferred T cells, permanent T-cell reconstitution is mediated via a central mechanism, which depends on de novo production of naive T lymphocytes by the recipient's thymus from donor-derived lymphoid-myeloid progenitors (LMPs). Thymopoiesis can be assessed by quantification of recent thymic emigrants, T-cell receptor excision circle levels, and T-cell receptor repertoire diversity. These assays are valuable tools for monitoring posttransplantation thymic recovery, but more importantly they have shown the significant prognostic value of thymic reconstitution for clinical outcomes after UCBT, including opportunistic infections, disease relapse, and overall survival. Strategies to improve thymic entry and differentiation of LMPs and to accelerate recovery of the thymic stromal microenvironment may improve thymic lymphopoiesis. Here, we discuss the mechanisms and clinical implications of thymic recovery and new approaches to improve reconstitution of the T-cell repertoire after UCBT.

Immune reconstitution after allogeneic hematopoietic stem cell transplantation in children: a single institution study of 59 patients

Purpose

Lymphocyte subset recovery is an important factor that determines the success of hematopoietic stem cell transplantation (HSCT). Temporal differences in the recovery of lymphocyte subsets and the factors influencing this recovery are important variables that affect a patient's post-transplant immune reconstitution, and therefore require investigation.

Methods

The time taken to achieve lymphocyte subset recovery and the factors influencing this recovery were investigated in 59 children who had undergone HSCT at the Department of Pediatrics, The Catholic University of Korea Seoul St. Mary's Hospital, and who had an uneventful follow-up period of at least 1 year. Analyses were carried out at 3 and 12 months post-transplant. An additional study was performed 1 month post-transplant to evaluate natural killer (NK) cell recovery. The impact of pre- and post-transplant variables, including diagnosis of Epstein-Barr virus (EBV) DNAemia posttransplant, on lymphocyte recovery was evaluated.

Results

The lymphocyte subsets recovered in the following order: NK cells, cytotoxic T cells, B cells, and helper T cells. At 1 month post-transplant, acute graft-versus-host disease was found to contribute significantly to the delay of CD16⁺/56⁺ cell recovery. Younger patients showed delayed recovery of both CD3⁺/CD8⁺ and CD19⁺ cells. EBV DNAemia had a deleterious impact on the recovery of both CD3⁺ and CD3⁺/CD4⁺ lymphocytes at 1 year post-transplant.

Conclusion

In our pediatric allogeneic HSCT cohort, helper T cells were the last subset to recover. Younger age and EBV DNAemia had a negative impact on the post-transplant recovery of T cells and B cells.

Impact of lymphocyte and monocyte recovery on the outcomes of allogeneic hematopoietic SCT with fludarabine and melphalan conditioning

We have recently shown that lymphocyte and monocyte recovery by day +100 are associated with survival post myeloablative allogeneic hematopoietic transplant for acute leukemia. We hypothesized that lymphocyte and monocyte recovery would have a similar impact on survival in the reduced intensity setting. To test this hypothesis, we analyzed clinical data from 118 consecutive fludarabine/melphalan-conditioned patients by correlating peripheral blood absolute lymphocyte counts and monocyte counts (ALC and AMC, respectively) at days +15, +30, +60 and +100 with the outcomes. Multivariate analysis revealed that day +100 AMC (risk ratio (RR) 0.22, 95% confidence interval (CI) 0.07-0.73, P=0.01) and mild chronic GVHD (RR 0.09, 95% CI 0.005-0.43, P=0.008) were independently associated with survival. To explore whether the patterns of lymphocyte and monocyte recovery had a prognostic value, we performed unsupervised hierarchical clustering on the studied hematopoietic parameters and identified three patient clusters, A-C. Patient clusters A and B both had improved OS compared with cluster C (77.8 months vs not reached vs 22.3 months, respectively, P<0.001). No patient in cluster C had a day +100 AMC >300. Both severe acute GVHD and relapse occurred more frequently in cluster C. Our data suggest that patients with low AMC by day +100 post fludarabine/melphalan-conditioned allogeneic hematopoietic SCT may be at risk for poor outcomes.

A comparison of immune reconstitution and graft-versus-host disease following myeloablative conditioning versus reduced toxicity conditioning and umbilical cord blood transplantation in paediatric recipients

Immune reconstitution appears to be delayed following myeloablative conditioning (MAC) and umbilical cord blood transplantation (UCBT) in paediatric recipients. Although reduced toxicity conditioning (RTC) versus MAC prior to allogeneic stem cell transplantation is associated with decreased transplant-related mortality, the effects of RTC versus MAC prior to UCBT on immune reconstitution and risk of graft-versus-host disease (GVHD) are unknown. In 88 consecutive paediatric recipients of UCBT, we assessed immune cell recovery and immunoglobulin reconstitution at days +100, 180 and 365 and analysed risk factors associated with acute and chronic GVHD. Immune cell subset recovery, immunoglobulin reconstitution, and the incidence of opportunistic infections did not differ significantly between MAC versus RTC groups. In a Cox model, MAC versus RTC recipients had significantly higher risk of grade II-IV acute GVHD [Hazard Ratio (HR) 6·1, P = 0·002] as did recipients of 4/6 vs. 5-6/6 HLA-matched UCBT (HR 3·1, P = 0·03), who also had significantly increased risk of chronic GVHD (HR 18·5, P = 0·04). In multivariate analyses, MAC versus RTC was furthermore associated with significantly increased transplant-related (Odds Ratio 26·8, P = 0·008) and overall mortality (HR = 4·1, P = 0·0001). The use of adoptive cellular immunotherapy to accelerate immune reconstitution and prevent and treat opportunistic infections and malignant relapse following UCBT warrants further investigation.

A phase I study in adults of clofarabine combined with high-dose melphalan as reduced-intensity conditioning for allogeneic transplantation

Clofarabine is a novel purine nucleoside analog with immunosuppressive and antileukemia activity. We performed a phase I study of the combination of clofarabine plus melphalan as a reduced-intensity conditioning regimen for allogeneic stem cell transplantation in patients with acute myelogenous leukemia. Patients over age 18 in complete remission or with active disease (up to 50% marrow blasts) who had a matched related or unrelated donor were eligible. The conditioning regimen consisted of escalating doses of clofarabine plus melphalan, followed by allogeneic stem cell transplantation. Sixteen patients (median age, 63 years) were treated at 3 dose levels; 4 of these patients had primary induction failure, and 3 were in first relapse. One patient at dose level 2 and 1 patient at dose level 3 died of multiorgan toxicity; no other dose-limiting toxicities were seen. All other patients at both doses of clofarabine studied demonstrated complete engraftment by day 30, with a median time to absolute neutrophil count recovery of 14 days, and 16 days for platelet recovery. With a median follow-up of 17 months, only 2 patients relapsed, and 4 patients died. Clofarabine plus melphalan at dose level 2 is a well-tolerated conditioning regimen with activity in patients with advanced acute myelogenous leukemia.

Reduced intensity versus myeloablative allogeneic stem cell transplantation for the treatment of acute myeloid leukemia, myelodysplastic syndrome and acute lymphoid leukemia

PURPOSE OF REVIEW

Use of a reduced intensity conditioning (RIC) regimen has now become standard practice among older or more infirmed stem cell transplantation candidates. Encouraging outcome in this population has led to the question of whether RIC should replace standard myeloablative conditioning (MAC) regimens. This review will summarize the available outcomes data comparing RIC and MAC approaches to stem cell transplantation in adult patients with acute myeloid leukemia, myelodysplastic syndrome (MDS) and acute lymphoid leukemia.

RECENT FINDINGS

There are currently no completed prospective randomized controlled studies comparing outcomes of RIC to MAC. The best insight into differences in outcome comes from large registry-based retrospective studies. These studies demonstrate that the use of RIC is associated with a reduction in transplant-related mortality but an increased risk of disease relapse. As a result, for patients undergoing stem cell transplantation in remission, disease free and overall survival are similar.

SUMMARY

The current retrospective data provide justification for the use of RIC regimens in all adult stem cell transplant candidates with acute leukemia in remission and MDS. More definitive conclusions regarding differences between the MAC and RIC approach to stem cell transplantation await results of ongoing prospective randomized trials.

Thymic T-cell development in allogeneic stem cell transplantation

Cytoreductive conditioning regimens used in the context of allogeneic hematopoietic cell transplantation (HCT) elicit deficits in innate and adaptive immunity, which predispose patients to infections. As such, transplantation outcomes depend vitally on the successful reconstruction of immune competence. Restoration of a normal peripheral T-cell pool after HCT is a slow process that requires the de novo production of naive T cells in a functionally competent thymus. However, there are several challenges to this regenerative process. Most notably, advanced age, the cytotoxic pretransplantation conditioning, and posttransplantation alloreactivity are risk factors for T-cell immune deficiency as they independently interfere with normal thymus function. Here, we discuss preclinical allogeneic HCT models and clinical observations that have contributed to a better understanding of the transplant-related thymic dysfunction. The identification of the cellular and molecular mechanisms that control regular thymopoiesis but are altered in HCT patients is expected to provide the basis for new therapies that improve the regeneration of the adaptive immune system, especially with functionally competent, naive T cells.

Immune reconstitution is preserved in hematopoietic stem cell transplantation coadministered with regulatory T cells for GVHD prevention

Recipient-specific regulatory T cells (rsTreg) can prevent graft-versus-host disease (GVHD) by inhibiting donor T-cell expansion after hematopoietic stem cell transplantation (HSCT) in mice. Importantly, in adult humans, because of thymus involution, immune reconstitution during the first months after HSCT relies on the peripheral expansion of donor T cells initially present in the graft. Therefore, we developed a mouse model of HSCT that excludes thymic output to study the effect of rsTreg on immune reconstitution derived from postthymic mature T cells present within the graft. We showed that GVHD prevention with rsTreg was associated with improvement of the limited immune reconstitution compared with GVHD mice in terms of cell numbers, activation phenotype, and cytokine production. We further demonstrated a preserved in vivo immune function using vaccinia infection and third-party skin-graft rejection models, suggesting that rsTreg immunosuppression was relatively specific of GVHD. Finally, we showed that rsTreg extensively proliferated during the first 2 weeks and then declined. In turn, donor Treg proliferated from day 15 on. Taken together, these results suggest that rsTreg GVHD prevention is associated with improved early immune reconstitution in a model that more closely approximates the biology of allogeneic HSCT in human adults.

Comparison of reduced intensity and myeloablative conditioning regimens for stem cell transplantation in patients with malignancies: a meta-analysis

OBJECTIVE

The reduced intensity conditioning (RIC) stem cell transplantation is widely employed for the treatment of many hematologic malignancies, but the survival effectiveness is still unclear. This study conducted an updated meta-analysis to determine whether any significant difference could be found by using RIC vs. myeloablative conditioning (MAC) regimen for transplantation in patients with malignancies.

METHODS

We electronically searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, and relevant articles (1987.01-2009.12). Comparative studies were carried out on clinical therapeutic effect of RIC and MAC on the survival outcomes and the transplantation-related complications.

RESULTS

We obtained 1776 records, and 29 studies totaling 6235 patients have been assessed. Compared with MAC regimen, the RIC regimen had a higher overall survival (OS) at one-yr and no difference at two-yr later after transplantation. RIC regimen had significantly lower rates of disease-free survival (DFS) after two-yr follow-up, lower incidences of ≥ II degree acute graft-versus-host disease (aGVHD), and lower TRM [OR, 0.61, 95% CI (0.53, 0.69)], but with a higher relapse rate [OR, 1.88(1.41, 2.51)]. No significant difference was found in rates of cytomegalovirus (CMV) infection and chronic GVHD between the regimens.

CONCLUSIONS

This meta-analysis confirmed that compared with MAC condition regimen, the RIC regimen had a consistently equivalent or even better rate in OS, but with lower DFS at longer follow-up.

Post-transplant immune reconstitution after unrelated allogeneic stem cell transplant in patients with acute myeloid leukemia

We evaluated immune recovery in 67 patients with acute myeloid leukemia (AML) with a median age of 40 years (4-69) following allo-SCT after reduced (n = 35) or myeloablative (n = 32) conditioning. The following lymphocyte populations were determined on days +30, +90, +180, +270, and +365 by flow associated cell sorting: CD3+, CD3+CD4+, CD3+CD8+, CD3+CD4+/CD3+CD8+ ratio, CD3-CD56+, and CD19+ cells. Peripheral blast count >5% was related to lower number of CD3+CD4+ (day +30) and NK cells (day +180; p = 0.02). Intensity of conditioning did not have any significant impact on the kinetics of immune recovery. Patients with normal CD3+CD4+/CD3+CD8+ ratio (day +30) and NK cell count (day +90; p <0.05) experienced better survival than those with decreased parameters. Post-transplant sepsis/severe infections impaired CD3+CD8+ (day +90; p = 0.015) and CD19+ (day +90; p = 0.02) recovery. Relapse in patients following allo-SCT showed an association with decreased numbers of CD19+ (day +270) and NK cells (day +365). Acute GvHD (II-IV) was accompanied by reduced CD19+ and CD3+CD4+ cells. Thus, the evaluation of post-transplant immune reconstitution in patients with AML might improve risk stratification concerning either relapse or TRM and remains to be further explored.

Single cell analysis of complex thymus stromal cell populations: rapid thymic epithelia preparation characterizes radiation injury

Thymic epithelial cells (TECs) and dendritic cells are essential for the maintenance of thymopoiesis. Because these stromal elements define the progenitor niche, provide critical survival signals and growth factors, and direct positive and negative selection, detailed study of these populations is necessary to understand important elements for thymic renewal after cytotoxic injury. Study of TEC is currently hindered by lengthy enzymatic separation techniques with decreased viability. We present a new rapid separation technique that yields consistent viable TEC numbers in a quarter of the prior preparation time. Using this new procedure, we identify changes in stroma populations following total body irradiation (TBI). By flow cytometry, we show that TBI significantly depletes UEA+ medullary TEC, while sparing Ly51+ CD45- cells. Further characterization of the Ly51+ subset reveals enrichment of fibroblasts (CD45- Ly51+ MHCII-), while cortical TECs (CD45- Ly51+ MHCII+) were markedly reduced. Dendritic cells (CD11lc+ CD45+) were also decreased following TBI. These data suggest that cytotoxic preparative regimens may impair thymic renewal by reducing critical populations of cortical and medullary TEC, and that such thymic damage can be assessed by this new rapid separation technique, thereby providing a means of assessing optimal conditioning pretransplantfor enhancing thymic-dependent immune reconstitution posttranspiant.

Immune reconstitution and implications for immunotherapy following haematopoietic stem cell transplantation

Recovery of a fully functional immune system is a slow and often incomplete process following allogeneic stem cell transplantation. While innate immunity reconstitutes quickly, adaptive B- and especially T-cell lymphopoeisis may be compromised for years following transplantation. In large part, these immune system deficits are due to the decrease, or even absence, of thymopoiesis following transplantation. Thereby, T-cell reconstitution initially relies upon expansion of mature donor T cells; a proliferation driven by high cytokine levels and the presence of allo-reactive antigens. This peripheral mechanism of T-cell generation may have important clinical consequences. By expanding tumouricidal T cells, it may provide a venue to enhance T-cellular immunotherapy following transplantation. Alternatively, decreased thymic function may impair long-term anti-tumour immunity and increase the likelihood of graft-versus-host disease.

Immune reconstitution following hematopoietic stem-cell transplantation

Hematopoietic stem-cell transplantation is associated with a profound immune deficiency manifested as an increased propensity to develop infections and probably also malignancies. Innate immunity, including epithelial barriers and phagocytes, typically recovers within weeks after grafting, and B-cell counts and CD8 T-cell counts recover within months. CD4 T-cell counts are low for years, and their recovery is particularly slow in older patients with poor thymic function. Therapies to improve immune function include vaccinations, immunoglobulins for recurrent infections, cytokines, and antigen-specific donor lymphocyte infusions.

Immune reconstitution after allogeneic stem cell transplantation with reduced-intensity conditioning regimens

Reduced-intensity conditioning (RIC) regimens have been increasingly used as an alternative to conventional myeloablative conditioning (MAC) regimens for elderly patients, for patients medically infirm to qualify for conventional allogeneic stem cell transplantation (SCT), and for disorders in which traditional MAC-SCT are associated with high rates of non-relapse mortality. One of the theoretical advantages of RIC-SCT is that it might lend to better immune reconstitution after transplantation due to less damage of the thymus, allowing regeneration of naive T cells derived from prethymic donor stem cells, and due to the proliferation of immunologically competent host T cells that survive the conditioning regimen. Although limited, studies comparing immune recovery following RIC and MAC-SCT have been insightful. One of the main difficulties of these studies is the current spectrum of RIC protocols, which vary considerably in myeloablative and immunosuppressive potential, resulting in apparently contradictory findings. In spite of this, most reports have shown significant quantitative and/or qualitative differences in T- and B-cell reconstitution after RIC-SCT in comparison with conventional SCT. This paper will review current knowledge of immune reconstitution following RIC-SCT.

Lymphocyte reconstitution following allogeneic hematopoietic stem cell transplantation: a retrospective study including 148 patients

Here we investigated the influence of parameters known before hematopoietic stem cell transplantation (HSCT) as well as the relevance of graft-versus-host disease (GvHD) and cytomegalovirus (CMV) reactivation on post transplant lymphocyte reconstitution in 148 patients treated in our institution between 1996 and 2003. Median patient age was 42 (19-68) years, HSCT followed standard high dose (n=91) or reduced-intensity conditioning regimens (n=57) with bone marrow (BM, n=67) or peripheral blood stem cells (PBSC, n=81) from related (n=71) or unrelated (n=77) donors. In the first months, we observed a partially faster reconstitution of CD3+4+, CD3+8+ and CD4+45RA+ T cells in patients following peripheral blood stem cell transplantation when compared to bone marrow transplantation. Prolonged CD3+4+ and CD4+45RA+ lymphopenia was noted after unrelated donor HSCT and GvHD prophylaxis containing anti-T-lymphocyte globulin. Lymphocyte subset counts in patients older than the median age were comparable to those in patients transplanted at a younger age and not influenced by the conditioning regimen. CD3+8+ T cell reconstitution was strongly correlated with CMV reactivation, but not significantly affected by CMV serostatus before HSCT. Incidence or extent of GvHD did not significantly influence lymphocyte reconstitution. Therefore, the source of graft is the most predictive parameter in early lymphocyte reconstitution, but the differences in lymphocyte recovery completely resolved within the first year after HSCT.

Fludarabine vs cladribine plus busulfan and low-dose TBI as reduced intensity conditioning for allogeneic hematopoietic stem cell transplantation: a prospective randomized trial

Purine analogs are often used for conditioning preceding allogeneic hematopoietic stem cell transplantation (HCT). We prospectively tested fludarabine (Flu) 40 mg/m(2)/day x 5 days vs cladribine (Clad) 10 mg/m(2)/day x 5 days plus oral busulfan (1 mg/kg q6 h x 2 days) and total body irradiation 200 cGy in 32 recipients of matched sibling and unrelated donor (URD) HCT. Patients were similar in age (median 52 years), diagnosis, extensive pre-HCT therapy (56 vs 63%), and high-risk disease status (81 vs 93%). Neutrophil engraftment was prompt (median 11 vs 12 days), but early graft failure using Clad halted randomization. Platelet recovery was prompt (median Flu 18 vs Clad 24 days). Graft-versus-host disease (GVHD) after Flu vs Clad was similar; (acute grade II/IV 56 vs 69%, P=0.26; chronic 50 vs 31%, P=0.27). Nonrelapse mortality (Flu 25 vs Clad 38%, P=0.47) and progression-free survival at 3 years were similar as well. Multivariate analyses showed slightly, but not significantly lower relative risk (RR) of neutrophil engraftment with Clad (RR 0.6 (95% CI 0.2-1.3) P=0.16) and with URD RR 0.4 (0.2-1.0) P=0.04). Older patients with advanced hematologic malignancies achieve satisfactory outcomes using either of these reduced intensity conditioning regimens.

Clinical factors influencing T-cell receptor excision circle (TRECs) counts following allogeneic stem cell transplantation in adults

To ascertain the clinical factors involved in T-cell reconstitution after allogeneic stem cell transplantation (SCT), we evaluated serial assessments of lymphocyte subsets by flow cytometry and TRECs levels by quantitative PCR in 83 adult patients. Patient age >25 years, unrelated donor, CMV infection and acute graft-versus-host disease (GVHD) adversely affected CD3(+) and CD8(+) T-cell recovery after SCT (p < 0.05). TRECs were low or undetectable during the first months after transplant and progressively increased thereafter. However, median TRECs of patients did never achieve normal values compared to healthy donors (median follow-up 9 months, range 2-42). Presence and severity of chronic GVHD significantly affected TRECs counts: patients with chronic GVHD had lower TRECs than patients without GVHD at 9, 12 and 24 months after SCT (p = 0.002, p = 0.022, p = 0.015). Patients with limited chronic GVHD had higher TRECs compared to patients with extensive GVHD (p = 0.018). No relationship was observed between fungal or bacterial infections and TRECs. Nonetheless, CMV infection was associated with lower TRECs (p = 0.032). Our data support the concept that adult thymus contributes with a slow but continuous production of thymic T cells to immune reconstitution after SCT. Chronic GVHD is the main factor associated to a delay in TRECs counts recovery.