Nonmyeloablative regimen preserves "niches" allowing for peripheral expansion of donor T-cells

Clinical Relevance of Immunobiology in Umbilical Cord Blood Transplantation

Umbilical cord blood transplantation (UCBT) has been an important donor source for allogeneic hematopoietic stem cell transplantation, especially for patients who lack suitable matched donors. UCBT provides unique practical advantages, such as lower risks of graft-versus-host-disease (GVHD), permissive HLA mismatch, and ease of procurement. However, there are clinical challenges in UCBT, including high infection rates and treatment-related mortality in selected patient groups. These clinical advantages and challenges are tightly linked with cell-type specific immune reconstitution (IR). Here, we will review IR, focusing on T and NK cells, and the impact of IR on clinical outcomes. Better understanding of the immune biology in UCBT will allow us to further advance this field with improved clinical practice.

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.

Umbilical cord blood graft engineering: challenges and opportunities

We are entering a very exciting era in umbilical cord blood transplantation (UCBT), where many of the associated formidable challenges may become treatable by ex vivo graft manipulation and/or adoptive immunotherapy utilizing specific cellular products. We envisage the use of double UCBT rather than single UCBT for most patients; this allows for greater ability to treat larger patients as well as to manipulate the graft. Ex vivo expansion and/or fucosylation of one cord will achieve more rapid engraftment, minimize the period of neutropenia and also give certainty that the other cord will provide long-term engraftment/immune reconstitution. The non-expanded (and future dominant) cord could be chosen for characteristics such as better HLA matching to minimize GvHD, or larger cell counts to enable part of the unit to be utilized for the development of specific cellular therapies such as the production of virus-specific T-cells or chimeric-antigen receptor T-cells which are reviewed in this study.

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.

Umbilical cord blood transplantation using non-myeloablative conditioning: The Mexican experience

Twenty-eight children and adults underwent transplantation with allogeneic umbilical cord blood cells (UCB) using a non-ablative conditioning regimen; there were 15 males and 13 females. Seven patients were grafted because a non-malignant condition and 21 for a malignant disease. The median age was 8 years (range 4 months-72 years). Ten UCB were obtained from Mexican cord bloods banks, five cords were from compatible siblings and the remaining 13 cords were obtained from abroad. Median time to recover >0.5 x 10(9)/l granulocytes was 24 days (range 8-32 days), whereas median time to recover >20 x 10(9)/l platelets was 26 days (range 12-50 days). Twelve recipients never engrafted and recovered subsequently endogenous hematopoiesis. The non-engraftment rate was significantly higher in patients allografted for benign conditions (71%) than in those allografted for malignant conditions (28%). The median overall post-transplant survival (SV) was 33 months and the 73-months overall SV was 39%. The cumulative incidence of grade II-IV acute GVHD and grade III-IV GVHD for the entire cohort of patients were 14 and 7%, respectively. Additional studies are needed to define if non-myeloablative conditioning is preferable over conventional conditioning in the case of UCB allografting.

Immune Reconstitution and Early Infectious Complications Following Nonmyeloablative Hematopoietic Stem Cell Transplantation

Non-myeloablative stem cell transplantation (NMT) has been increasingly used in compromised patients who would otherwise have been unable to undergo allotransplant. There is little understanding of the kinetics of immune reconstitution and its influence on infective complications following NMT. The aim of present study was to evaluate lymphocyte subset reconstitution over the first 12 months post-transplant in 15 adult patients receiving NMT with comparison to that of 30 patients grafted with a conventional hemopoietic stem cell transplantation (HSCT). NMT recipients were conditioned with fludarabine-based conditioning regimens. Peripheral blood stem cell (PBSC) was the source of stem cells in 13 NMT recipients and in 24 conventional HSCT recipients. Absolute numbers of helper (CD4+) T cells, naive (CD4+ CD45RA+) and memory (CD4+ CD45RO+) T cells as well as suppressor (CD8+) T cells, CD19+ B cells and NK cells were comparable in the two groups at all time points after transplantation. A median value of 200 CD4+ T cells/microl was achieved at 2 months post-transplant by the NMT and HSCT recipients. The CD4:CD8 ratio remained severely depressed throughout the study period. Almost all CD4+ lymphocytes expressed CD45RO antigen in the both groups of patients B lymphocytes showed low counts throughout the entire study period in both groups. Bacteremia and CMV antigenemia occurred respectively in 13 and 36% of the patients in the NMT group and in 15 and 39% of the patients in the HSCT group. Our preliminary data indicate that patients receiving a NMT have a lymphocyte reconstitution similar to that observed in patients who received a conventional HSCT. The incidence of bacteremia and CMV infection were not significantly different between the groups. Nevertheless, due to the small sample size, these results should be considered suggestive rather than definitive.

Immune recovery in adult patients after myeloablative dual umbilical cord blood, matched sibling, and matched unrelated donor hematopoietic cell transplantation

Immunologic reconstitution after allogeneic hematopoietic cell transplantation is a critical component of successful outcome. Umbilical cord blood (UCB) transplantation in adult recipients is associated with slow and often inadequate immune recovery. We characterized the kinetics and extent of immune recovery in 95 adult recipients after a dual UCB (n = 29) and matched sibling donor (n = 33) or matched unrelated donor (n = 33) transplantation. All patients were treated with myeloablative conditioning. There were no differences in the immune recovery profile of matched sibling donor and matched unrelated donor recipients. Significantly lower levels of CD3+, CD4+, and CD8+ T cells were observed in UCB recipients until 6 months after transplantation. Lower levels of regulatory T cells persisted until 1 year after transplantation. Thymopoiesis as measured by TCR rearrangement excision circle was comparable among all recipients by 6 months after transplantation. In a subset of patients 1 year after transplantation with similar levels of circulating T cells and TCR rearrangement excision circle, there was no difference in TCR diversity. Compared to HLA-identical matched sibling donor and matched unrelated donor adult hematopoietic cell transplantation recipients, quantitative lymphoid recovery in UCB transplantation recipients is slower in the first 3 months, but these differences disappeared by 6 to 12 months after transplantation.

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.

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.

Hematopoietic stem cell transplantation for hemoglobinopathies: current practice and emerging trends

Despite improvements in the management of thalassemia major and sickle cell disease, treatment complications are frequent and life expectancy remains diminished for these patients. Hematopoietic stem cell transplantation (HSCT) is the only curative option currently available. Existing results for HSCT in patients with hemoglobinopathy are excellent and still improving. New conditioning regimens are being used to reduce treatment-related toxicity and new donor pools accessed to increase the number of patients who can undergo HSCT.

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.

CCL25 increases thymopoiesis after androgen withdrawal

Although studies have demonstrated that androgen withdrawal increases thymic size, molecular mechanisms underlying this expansion remain largely unknown. We show that decreased androgen signaling leads to enhanced immigration of bone marrow T-cell precursors, as manifested by both an early increase of early thymic progenitors (ETP) and improved uptake of adoptively transferred quantified precursors into congenic castrated hosts. We provide evidence that the ETP niche is enhanced after androgen withdrawal by proliferation of UEA(+) thymic epithelial cells (TEC) and increased TEC production of CCL25, a ligand critical for ETP entry. Moreover, the greatest increase in CCL25 production is by UEA(+) TEC, linking function of this subset with the increase in ETP immigration. Furthermore, blockade of CCL25 abrogated the effects of castration by impairing ETP entry, retarding immature thymocyte development, limiting increase of thymic size, and impairing increase of thymopoiesis. Taken together, these findings describe a cohesive mechanism underlying increased thymic productivity after androgen withdrawal.

Selection of cord blood unit(s) for transplantation

Selection of cord blood (CB) units for transplantation involves combining both cell dose and HLA matching as independent yet overlapping variables. Cell dose and cell yield at the time of transplant are critical given that the transplants are being performed with minimal cells for reliable engraftment. In transplants for malignant disorders, the greater allogenicity and lower relapse rate associated with the less well-matched units balance any benefit of better HLA matching on TRM. The only factor that has repeatedly been associated with improved outcome post CB transplant is cell dose. The CB inventories are rapidly increasing in size and the quality of CB units being banked (larger, better characterized) is improving. With this, some of our current limitations in CB availability will soon become moot. Explorations into CB expansion and multiple CB unit transplants are addressing the limited cell doses attainable with a single CB collection. At this point, one must conclude that bigger is better when selecting CB units for transplantation.

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.

Hematopoietic cell transplantation for sickle cell disease: state of the art

Sickle cell disease is associated with considerable morbidity and premature mortality. Hematopoietic cell transplantation offers the possibility of cure and is associated with excellent results in pediatric patients receiving stem cell transplantation from a matched sibling donor. A reduced-intensity conditioning regimen has the potential to further reduce regimen-related morbidity and mortality. Improved understanding of the natural history of complications, such as stroke and pulmonary hypertension, effects of treatments such as hydroxyurea and blood transfusions, as well as the impact of transplantation on organ damage, are likely to influence the timing and indication of transplantation. Improvements in preparative regimens may enable the safe use of an alternative source of stem cells, such as unrelated matched donors, and further improve the applicability and acceptability of this treatment.

Recent thymic output function in patients with hematological malignancy

Thymic function is important for the generation of T-cell diversity in the periphery of both children and adults during both health and disease. Until recently, thymic function could not be monitored, as a consequence of the absence of adequate technology to differentiate recent thymic emigrants (RTEs) from naïve T cells. The generation of TCR diversity occurs in the thymus through recombination of gene segments encoding the variable parts of the TCR alpha and beta chains. During these processes, by-products of the rearrangements are generated in the form of signal joint T-cell receptor excision circles (sjTRECs). As sjTRECs are stable extrachromosomal DNA fragments, they are not replicated during mitosis and thus diluted with each round of cell division, and are therefore most frequent in naïve T cells that have recently left the thymus, their quantification is actually considered as a valuable tool to estimate thymic function. Therefore, quantitative sjTRECs content have been recently used to assess thymic output during both health and disease. In this review, we summarize recent data on the recent thymic output function feature in patients with hematological malignancy and the immune reconstitution after stem cell transplantation and we also characterize factors that may improve the thymic output function.

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

OBJECTIVE

To compare T-cell reconstitution in two groups of patients submitted to allogeneic stem cell transplantation (SCT): those receiving reduced-intensity conditioning (RIC, n = 24) and those receiving myeloablative conditioning (MA, n = 27).

METHODS

Fifty-one consecutive patients undergoing SCT were evaluated. Serial assessments of lymphocyte subsets and T cell receptor excision circles (TRECs) levels were performed using multiparametric flow cytometry and real-time PCR, respectively.

RESULTS

During the first 6 months posttransplant, total and naïve CD4(+) T cell counts were higher after RIC-SCT than after MA-SCT (total CD4(+): p = 0.04, p = 0.08, and p = 0.058; naïve CD4(+): p = 0.14, p = 0.05, and p = 0.01 at 1, 3, and 6 months, respectively). In both groups of patients, TRECs levels were low or undetectable in the first 3 months after SCT and progressively increased during the study. However, a higher proportion of patients with detectable levels of TRECs was observed in RIC-SCT at 1 and 3 months and more patients in this group reached normal levels of TRECs at 6 months post-SCT. In the multivariate analysis, including factors such as type of donor (sibling vs unrelated), dose of CD34(+) cells infused with the graft, patient age, and graft-vs-host disease (GVHD), the most important factor influencing TRECs recovery in the early period after SCT was the type of conditioning regimen.

CONCLUSIONS

In this study, the pattern of immune reconstitution after RIC-SCT was different from that of MA-SCT and was characterized by higher posttransplant naïve CD4(+) T cell counts and TRECs levels in the early period after transplant.

Age-related thymic activity in adults following chemotherapy-induced lymphopenia

BACKGROUND

The potential role of the adult thymus in T-cell homeostasis subsequent to lymphopenia remains the subject of debate. We examined whether thymic activity contributes to reconstitution of the peripheral T-cell pool, a critical process for patients recovering from antineoplastic therapy.

METHODS

In selected patients with various neoplastic diseases we assessed peripheral blood lymphocyte subsets by flow-cytometry, including thymus-derived, CD4+ T cells expressing the CD45RA molecule, and thymic size rebound by CT scan before, and 3, 6 and 12 months after completion of cytotoxic therapy.

RESULTS

Adult patients (n = 21, mean age of 30 years, range 18-49) had higher baseline numbers of B and lower numbers of NK cells than elderly patients (n = 15, mean age of 79 years, range 70-91), while total T-cell numbers did not differ. Despite the reduction of lymphocyte counts being comparable in the adult (mean of 45%) and elderly (mean of 49%) groups, occurring at, or near, completion of treatment, an enlargement of the previously atrophic thymus was evident in 63% of the adult, but in none of the elderly, subjects. In 22 patients who remained active disease-free during the following year, B cells and NK cells recovered to pretreatment levels as soon as at 3 months, whereas overall T-cell recovery occurred at 6 months post-treatment. Thymic rebound, observed in 11 of 22 patients who were of younger age, correlated significantly with a faster and more complete recovery of CD45RA+ CD4+ (mainly helper-naïve) T cells.

CONCLUSION

The adult thymus appears capable of regeneration, at least up to middle age, contributing significantly to the reconstitution of the peripheral T-cell pool following chemotherapy-induced lymphopenia. In advanced age, however, although peripheral homeostatic pathways appear intact, regeneration of the naïve repertoire is incomplete.

Onset of thymic recovery and plateau of thymic output are differentially regulated after stem cell transplantation in children

Thymus-dependent T-cell regeneration is a major pathway for immune reconstitution after stem cell transplantation in children. Therefore, we prospectively assessed T-cell dynamics and thymic function in 164 pediatric patients between 1 and 124 months after transplantation by measuring T-cell receptor recombination excision circles and spontaneous expression of Ki67 in peripheral T-cell subsets. We analyzed the effect of recipient age, conditioning regimen, type of donor and graft, stem cell dose, and graft-versus-host disease on the onset and the plateau of thymic output. A high rate of spontaneous proliferation in early-reconstituting naive and memory T cells inversely correlated with total T-cell numbers. Accordingly, T-cell receptor recombination excision circle content was diminished in early-appearing naive T cells. A multivariate analysis revealed that the onset of thymic recovery was inversely correlated only with recipient age ( P < .0002), whereas the plateau of thymic output was higher in patients receiving increased stem cell numbers ( P < .0022). Donor type, stem cell source, and conditioning regimen influenced none of the analyzed parameters. In conclusion, lymphopenia-driven proliferation is important for T-cell homeostasis in children early after stem cell transplantation, but it might result in underestimation of thymic function. Onset and plateau of thymic activity are independently regulated by different transplant-related factors.

Thymopoiesis following allogeneic stem cell transplantation: new possibilities for improvement

Treatment related mortality (TRM) has restricted the application of allogeneic hematopoietic stem cell transplantation (allo-SCT) as a treatment modality for patients with a variety of malignant and non-malignant hematological disorders. TRM is mainly caused by severe opportunistic infections, due to an impaired immune reconstitution. The extreme slow recovery of newly developed, donor stem cell derived naive T-cells is currently considered to be the most important determinant of the impaired immune competence after allo-SCT. Therefore, enhancing naive T-cell recovery following allo-SCT by improving thymopoiesis has recently gained new interest. Possible strategies to improve thymopoiesis may include approaches to protect the nursing stromal compartment and approaches to directly stimulate the differentiation and proliferation of T-cell progenitors intra-thymically. Among the latter is interleukin-7 (IL-7), which has appeared promising in preclinical experimental settings and is expected to enter early clinical studies soon. Keratinocyte growth factor (KGF) is an epithelial growth factor that may protect the thymic epithelium and thereby may preserve it's support of thymopoiesis. KGF has been evaluated clinically in the setting of autologous stem cell transplantation and studies in the setting of allo-SCT are awaited in the near future.

Umbilical cord blood transplantation in adults using myeloablative and nonmyeloablative preparative regimens

Unrelated umbilical cord blood (UCB) transplantation has recently been explored in an increasing number of adult patients. The relative ease of procurement and the lower-than-anticipated risk of severe acute graft-versus-host disease has made UCB transplantation an appealing alternative to bone marrow-derived hematopoietic stem cells. The use of reduced-intensity or nonmyeloablative preparative regimens to allow engraftment of UCB broadens the scope of patients who may benefit from allogeneic immunotherapy, including elderly and medically infirm patients with no matched sibling donor. This review summarizes the available data on the use of UCB as an alternative source of hematopoietic stem cell transplantation in adult patients.

Adult recipients of umbilical cord blood transplants after nonmyeloablative preparative regimens

We report the outcome of 13 patients with advanced malignancies who underwent nonmyeloablative conditioning therapy followed by infusion of partially matched unrelated cord blood cells. The median age of these patients was 49 years, and their median weight was 65.7 kg. The median nucleated cell dose infused was 2.07 x 10(7)/kg. Eight of the 13 patients demonstrated donor chimerism between 4 weeks and 6 months, and subsequent conversion to full donor chimerism was achieved in 5 patients. Three patients were alive and free of disease at 158 to 1054 days, with a median survival of 288 days after transplantation. The 100-day event-free survival is 69%, and overall survival is 77%. At 1 year, the event-free and overall survival was 43%. Treatment-related mortality observed within the first 100 days after transplantation was low: 1 previously extensively pretreated patient died of multiorgan failure. This result provides a basis for further exploring this potentially curative approach to selected patients who lack matched related or unrelated hematopoietic stem cell donors.

Hematopoietic stem cell dose correlates with the speed of immune reconstitution after stem cell transplantation

In the current study, we tested whether higher numbers of hematopoietic stem cells correlate with the speed of immune reconstitution in a congenic transplantation model (C57BL/Ka, CD45.1, Thy1.1→C57BL/6, CD45.2, Thy1.2) using purified hematopoietic stem cells (c-Kit+Thy1.1lowLin-/lowSca-1+). There were 3 different doses of stem cells used (400, 1000, and 5000). Phenotypic analyses in peripheral blood and spleen demonstrated that higher numbers of infused stem cells are associated with more rapid regeneration of T cells (CD4+, CD8+, naive CD4+, naive CD8+) and B cells at early time points. The numbers of T and B cells eventually became equivalent between different dose groups at late time points. Production of interleukin-2 and inter-feron-γ per T cell was similar regardless of stem cell dose even when tested at the time when there were significant differences in peripheral T-cell counts. The improved immune recovery was attributed to a more rapid regeneration of donor-type immune cells. Higher numbers of total thymocytes and signal joint T-cell receptor excision circles were observed in the higher dose stem cell recipients, suggesting that accelerated regeneration of T cells was due to enhanced thymopoiesis. (Blood. 2004;103:4344-4352)

Stem cell transplantation (cord blood transplants)

Allogeneic stem cell transplantation is an accepted treatment modality for selected malignant and non-malignant diseases. However, the ability to identify suitably matched related or unrelated donors can be difficult in some patients. Alternative sources of stem cells such as cord blood provide a readily available graft for such patients. Data accumulated over the past several years have demonstrated that the use of cord blood is an accepted source of stem cells for pediatric patients. Since the cell numbers of hematopoietic progenitors in cord blood is limited and the collection can occur only in a single occasion, its use in adult patients can be more problematic. Here, new developments in the use of cord blood for adults and studies aimed at expansion of cord blood cells and immune reconstitution are described. In Section I, Dr. Nelson Chao describes the early data in cord blood transplantation in adult patients. The patient outcomes are reviewed and analyzed for various factors such as cell dose, HLA typing, and patient selection that could have contributed to the final outcome of these adult patients. Myeloablative as well as nonmyeloablative approaches are presented. Discussion of the various benefits and risks are presented. More recent data from multiple single institutions as well as larger registry data comparisons are also provided. Analyses of these studies suggest methods to improve on the outcome. These newer data should lead to a logical progression in the use of cord blood cells in adult patients. In Section II, Dr. Stephen Emerson describes the historical efforts associated with expansion of hematopoietic stem cells, specifically with cord blood cells. These efforts to expand cord blood cells continue with novel methods. Moreover, a better understanding of stem cell biology and signaling is critical if we are to be able to effectively expand these cells for clinical use. An alternative, more direct, approach to expanding stem cells could be achieved by specific genetic pathways known or believed to support primitive HSC proliferation such as Notch-1 receptor activation, Wnt/LEF-1 pathway induction, telomerase or the Homeobox (Hox) gene products. The clinical experience with the use of expanded cord blood cells is also discussed. In Section III, Dr. Kenneth Weinberg describes immune reconstitution or lack thereof following cord blood transplantation. One of the hallmarks of successful hematopoietic stem cell transplantation is the ability to fully reconstitute the immune system of the recipient. Thus, the relationship between stem cell source and the development of T lymphocyte functions required for protection of the recipient from infection will be described, and cord blood recipients will be compared with those receiving other sources of stem cells. T cell development is described in detail, tracking from prethymic to postthymic lymphocytes with specific attention to umbilical cord blood as the source of stem cells. Moreover, a discussion of the placenta as a special microenvironment for umbilical cord blood is presented. Strategies to overcome the immunological defects are presented to improve the outcome of these recipients.

T-cell reconstitution after unmanipulated, CD8-depleted or CD34-selected nonmyeloablative peripheral blood stem-cell transplantation

BACKGROUND

We have previously shown that CD8 depletion or CD34 selection of peripheral blood stem cells (PBSC) reduced the incidence of acute graft-versus-host disease (GvHD) after nonmyeloablative stem-cell transplantation (NMSCT). In this study, we analyze the effect of CD8 depletion or CD34 selection of the graft on early T-cell reconstitution.

METHODS

Nonmyeloablative conditioning regimen consisted in 2 Gy total-body irradiation (TBI) alone, 2 Gy TBI and fludarabine, or cyclophosphamide and fludarabine. Patients 1 to 18 received unmanipulated PBSC, patients 19 to 29 CD8-depleted PBSC, and patients 30 to 35 CD34-selected PBSC.

RESULTS

T-cell counts, and particularly CD4+ and CD4CD45RA+ counts, remained low the first 6 months after nonmyeloablative stem-cell transplantation (NMSCT) in all patients. CD34 selection (P<0.0001) but not CD8 depletion of PBSC significantly decreased T-cell chimerism. Donor T-cell count was similar in unmanipulated compared with CD8-depleted PBSC recipients but was significantly lower in CD34-selected PBSC recipients (P=0.0012). T cells of recipient origin remained stable over time in unmanipulated and CD8-depleted PBSC patients but expanded in some CD34-selected PBSC recipients between day 28 and 100 after transplant. Moreover, whereas CD8 depletion only decreased CD8+ counts (P<0.047), CD34 selection reduced CD3+(P<0.001), CD8+(P<0.016), CD4+ (P<0.001), and CD4+CD45RA+ (P<0.001) cell counts. T-cell repertoire was restricted in all patients on day 100 after hematopoietic stem-cell transplantation but was even more limited after CD34 selection (P=0.002).

CONCLUSIONS

Despite of the persistence of a significant number of T cells of recipient origin, T-cell counts were low the first 6 months after NMSCT. Moreover, contrary with CD8 depletion of the graft that only affects CD8+ lymphocyte counts, CD34 selection dramatically decreased both CD8 and CD4 counts.

T-cell receptor excision circles: a novel prognostic parameter for the outcome of transplantation in multiple myeloma patients

This study investigated whether T-cell receptor excision circles (TRECs) are a prognostic marker for the outcome of myeloma patients undergoing a tandem autologous peripheral blood stem cell transplantation (PBSCT). Twenty-five patients were enrolled. Samples were obtained at study enrollment, after conventional therapy, between first and second transplantation and 3, 6, 12 and 24 months after the second PBSCT. TRECs were quantified using real-time polymerase chain reaction. A high variation in TREC levels was found at diagnosis (median TREC level 136/10(5) peripheral blood mononuclear cells (PBMCs); range 1-1729), suggesting individual differences in thymic output of naive T cells. Patients with more than 136 TRECs/10(5) P BMCs at diagnosis had a statistically significant better overall survival (P=0.05) and event-free survival (P=0.045), whereas low TREC levels correlated with a higher incidence of infectious complications. Median TREC values were lowest after the first PBSCT (52/10(5) PBMCs) and reached the baseline 12 months after the second transplantation. Patients with high TREC levels after the second PBSCT had a significantly higher probability of being in complete or partial remission 30 months after the second PBSCT. TREC levels were not correlated with beta2-microglobulin and C-reactive protein levels at diagnosis. These data suggest that TRECs could be a relevant prognostic factor for patients who receive high-dose chemotherapy and autologous PBSCT.

A comparison of T-, B- and NK-cell reconstitution following conventional or nonmyeloablative conditioning and transplantation with bone marrow or peripheral blood stem cells from human leucocyte antigen identical sibling donors

This retrospective study compares the reconstitution of T, B and NK cells in three groups of patients transplanted for haematological malignancies with grafts from their HLA-identical sibling donors. In all, 15 patients received PBSC after a nonmyeloablative conditioning regimen consisting of fludarabine and 200 cGy TBI, 13 patients received PBSC after myeloablative conditioning and 37 patients received BM after myeloablative conditioning. In the nonmyeloablative group, the NK cells normalised after 1 month, the CD8+ T cells normalised after 3 months, the CD4+ T cells reached near normal values after 9 months and the B cell values were reduced until 12 months after transplant. In the two myeloablative groups, recipients of PBSC had a significantly higher number of CD4+ T cells after 4 months (P=0.004) and after 12 months (P=0.001), than recipients of BM. We found no differences in the T cell reconstitution between the two PBSC groups. This was of interest as the recipients of nonmyeloablative conditioning were older (P<0.001) and had a higher occurrence of chronic GVHD (P<0.05) than the recipients of myeloablative conditioning. In contrast, the recipients of nonmyeloablative conditioning had a delayed B cell recovery when compared to the patients who received myeloablative conditioning (P=0.04).

Reconstitution of B cell function in murine models of immunodeficiency

Murine models of immunodeficiency were used to evaluate strategies that might allow B cell engraftment in patients with X-linked agammaglobulinemia. Mice with defects in Btk or mu heavy chain were given 2.5 x 10(6) bone marrow cells from wild-type congenic donors. In the absence of any preparative regimen or immunosuppression, Btk-deficient mice on the CBA background developed normal concentrations of serum IgM and IgG3 by 12 weeks posttransplant. By contrast, mu heavy chain-deficient mice on the C57BL/6 background required some immunosuppression to achieve engraftment. Treatment of these mice with anti-T-cell antibodies 2 and 4 days prior to transplant resulted in normal concentrations of serum immunoglobulins by 6 weeks posttransplant. These pretreated mice had only 10% of the normal number of splenic B cells and they had no evidence of donor T cell engraftment. These results suggest that myelotoxic drugs may not be needed to achieve B cell engraftment in B-cell-deficient subjects.