Volume 13, Issue 7 , Pages 822-830, July 2007
Hematopoietic Engraftment in Recipients of Unrelated Donor Umbilical Cord Blood Is Affected by the CD34+ and CD8+ Cell Doses
Article Outline
- Abstract
- Introduction
- Patients, Materials, and Methods
- Results
- Discussion
- Acknowledgments
- References
- Copyright
Abstract
Umbilical cord blood (UCB) transplantation is limited by the low number of hematopoietic stem cells in UCB units, which results in a low engraftment rate in transplant recipients. Here, we measured the total nucleated cell count and CD34+, CD3+, CD4+, CD8+, CD14+, and CD16+/56+ cell doses in each UCB unit and evaluated their influence on engraftment and other outcomes in 146 recipients. Multivariate analysis showed a significant association between a higher incidence of successful engraftment and a dose of CD34+ and CD8+ cells above the median (1.4 × 105 and 15.7 × 105 cells/kg, respectively). Engraftment occurred 4 days earlier in patients who received UCB with more than the median dose of CD34+ cells than those receiving UCB at or below the median. Stratification of the group according to CD34+ cell dose revealed a significant influence of the CD8+ cell dose on the time to achieve neutrophil engraftment in patients receiving a lower CD34+ cell dose, whereas there was no significant influence in the patients receiving a higher CD34+ cell dose. These results suggest that consideration of CD34+ and CD8+ cell doses in UCB units may improve the engraftment in recipients of UCB transplantation.
Key Words: Cord blood transplantation, Engraftment, CD34, CD8, T lymphocyte
Introduction
Umbilical cord blood (UCB) from unrelated donors has been established as an alternative source for hematopoietic stem cell (HSC) transplantation in patients who lack a human leukocyte antigen (HLA)-matched bone marrow or a peripheral blood stem cell (PBSC) donor [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. UCB transplantation (UCBT) is limited, however, by the low number of HSCs per unit of UCB, resulting in a low engraftment rate and slow hematopoietic recovery after transplantation. There is a consensus that higher total nucleated cell count (TNCC) per recipient body weight and fewer differences in HLAs between the donor and recipient promote engraftment in recipients of UCBT [3, 11, 12, 13, 14]. A higher dose of CD34+ cells in UCB is also thought to be associated with better engraftment, lower treatment-related mortality (TRM), and higher survival rate in the recipients [13].
The contribution of accessory cells, which are cryopreserved along with HSCs in UCB units, to engraftment in UCBT recipients has not been well investigated. In bone marrow transplantation (BMT), it is well known that the numbers of donor T cells as well as HSCs can affect the engraftment rate. In humans, when compared with unmanipulated grafts, T cell-depleted bone marrow prevents severe acute graft-versus-host disease (aGVHD), but results in a lower probability of engraftment [15, 16, 17]. A phase I-II clinical trial of HLA-mismatched bone marrow transplantation demonstrated that the removal of CD8+ cells but not CD4+ cells from donor marrow increases the risk of rejection [18]. Also, in a transplant model, in which mice were lethally irradiated and subsequently reconstituted with a mixture of T cell-depleted syngeneic bone marrow cells and untreated major histocompatibility complex (MHC)-mismatched allogeneic bone marrow cells, all mice developed complete chimera of the MHC-mismatched donor mice [19]. Another MHC-mismatched mouse transplant model showed that donor CD8+ T cells are at least 5-fold more effective than donor CD4+ T cells for preventing marrow graft rejection [20]. Thus, CD8+ T cells in the graft facilitate optimal engraftment of HSCs in BMT.
In this study, we prospectively measured the TNCC and CD34+, CD3+, CD4+, CD8+, CD14+, CD16+/CD56+ cell doses in each UCB unit shipped from a single cord blood registry, and we evaluated how these accessory cell doses affect the engraftment and other outcomes in 146 recipients of UCBT. The analyses demonstrated that a higher incidence of successful engraftment was associated with a dose of CD34+ and CD8+ cells above the median (1.4 × 105 and 15.7 × 105/kg of recipient body weight, respectively). Stratification of the group according to CD34+ cell dose revealed a significant influence of the CD8+ cell dose on the time to achieve neutrophil engraftment in the patients receiving less than the median dose of CD34+ cells, but there was no significant influence in the patients receiving more than the median dose of CD34+ cells. These findings and previous data [12] suggest the number of CD34+ cells in the UCB units affects the engraftment in recipients. Additionally, the CD8+ cell dose may have a beneficial effect on engraftment in recipients of UCBT.
Patients, Materials, and Methods
Patients
Between September 1997 and August 2005, a total of 240 UCB units were shipped from the Tokai Cord Blood Bank to transplant institutions. A total of 146 patient and UCB graft pairs were selected according to the following criteria: (1) patients had acute leukemia, other malignancies, severe aplastic anemia, severe combined immunodeficiency syndrome, or a hereditary metabolic disorder; (2) at least 2 agents were used as a preconditioning regimen; (3) at least 1 immunosuppressant agent other than antithymocyte globulin was used for GVHD prophylaxis; (4) an unmanipulated single UCB graft was transplanted; (5) test vials were available for flow cytometry analysis; and (6) clinical outcome data were available.
The characteristics of the patients are summarized in Table 1. One hundred thirty-seven patients (93.8%) were treated for malignant diseases and 100 (68.5%) were classified as having advanced diseases, including acute lymphoblastic leukemia (ALL) beyond first relapse (n = 26), acute myelogenous leukemia (AML) beyond first relapse (n = 29), acute type adult T cell lymphoma/leukemia (n = 7), chronic myelogenous leukemia (CML) in accelerated or blast phase (n = 6), myelodysplastic syndrome (MDS) with excess blast (n = 8), non-Hodgkin’s lymphoma refractory or beyond second remission (n = 14), and multiple myeloma (n = 3), solid tumor (n = 2), or other malignancies (n = 5) not in complete remission (CR). The preconditioning regimen varied according to disease or stage of disease at transplantation. Thirty-seven patients (25.3%) received a regimen including fludarabine with reduced total body irradiation (TBI) (<10 Gy) or without TBI, which was defined as a fludarabine-based reduced intensity conditioning (RIC) regimen. As a prophylaxis for GVHD, 95 patients (65.1%) received a cyclosporine A-based regimen, 44 (30.0%) received a tacrolimus-based regimen, and 84 (57.5%) received short-term methotrexate with or without other immunosuppressive agents. One hundred forty (95.9%) patients received granulocyte colony-stimulating factor (G-CSF) after transplantation.
Table 1. Patient and Transplant Characteristics
| Patient Characteristics | n = 146 |
|---|---|
| Age, median, years (range) | 15.0 (0.2-74.0) |
| Body weight, median, kg (range) | 41.0(4.8-85.0) |
| Recipient sex, n (%) | |
| Male | 89(61.0) |
| Female | 57(39.0) |
| Diagnosis, n (%) | |
| Malignant disease | |
| Acute lymphoblastic leukemia | 49(33.5) |
| Acute myelogenous leukemia | 39(26.7) |
| Adult T cell lymphoma/leukemia | 8(5.5) |
| Chronic myelogenous leukemia | 6(4.1) |
| Myelodysplastic syndrome | 11(7.5) |
| Non-Hodgkin’s lymphoma | 14(9.6) |
| Multiple myeloma | 3(2.1) |
| Solid tumor | 2(1.4) |
| Other malignancies | 5(3.4) |
| Nonmalignant disease | |
| Severe aplastic anemia | 3(2.1) |
| Severe combined immunodeficiency syndrome | 2(1.4) |
| Other nonmalignancies | 4(2.7) |
| Disease states at transplant, n (%) | |
| Standard | 46(31.5) |
| Advanced | 100(68.5) |
| Degree of HLA match in rejection vector, n(%) | |
| 6/6 antigens | 35(24.0) |
| 5/6 antigens | 65(44.5) |
| 4/6 antigens | 45(30.8) |
| 3/6 antigens | 1(0.7) |
| Degree of HLA match in GVH vector, n (%) | |
| 6/6 antigens | 34(23.3) |
| 5/6 antigens | 72(49.3) |
| 4/6 antigens | 38(26.0) |
| 3/6 antigens | 2(1.4) |
| ABO blood type match, n (%) | |
| Match | 48(32.9) |
| Minor mismatch | 42(28.8) |
| Major mismatch | 56(38.3) |
| Conditioning agents, n (%)⁎ | |
| Two | 51(34.9) |
| Three | 95(65.1) |
| Total body irradiation dose, n (%) | |
| Full (≥10 Gy) | 75(51.4) |
| Reduced (<10 Gy) | 33(22.6) |
| No | 38(26.0) |
| GVHD prophylaxis, n (%) | |
| Cyclosporine A alone | 30(20.6) |
| Cyclosporine A + short-term methotrexate | 55(37.7) |
| Cyclosporine A + prednisolone | 10(6.8) |
| Tacrolimus alone | 12(8.2) |
| Tacrolimus + short-term methotrexate | 28(19.2) |
| Tacrolimus + predonisolone | 4(2.7) |
| Short-term methotrexate alone | 1(0.7) |
| Others | 6(4.1) |
| G-CSF administration after transplantation, n(%) | |
| Yes | 140(95.9) |
| No | 6(4.1) |
⁎Total body irradiation was considered a single agent. |
HLA Typing and Donor Matching
Confirmatory HLA typing and matching of HLA-A, -B, and -DRB1 of the selected UCB unit and recipient were performed at the Tokai Cord Blood Bank prior to transplantation. HLA-A and -B were typed using the standard serologic method, and HLA-DRB1 type was determined by polymerase chain reaction sequencing-based typing. The HLA differences between the patients and the UCB units are summarized in Table 1.
Flow Cytometry of UCB Units
Flow cytometry was performed for the cells in UCB test vials when they were thawed for confirmatory HLA typing. The doses of each cell type from the UCB units actually used for transplantation were estimated by multiplying the TNCC measured before cryopreservation by the percentage of each cell fraction. For immunophenotyping, cells were incubated for 20 minutes at 4°C with 1 of the following monoclonal antibodies: fluorescein isothiocyanate-conjugated mouse anti-CD34, -CD3, -CD4, or -CD45 (BD Biosciences, Mountain View, CA), or phycoerythrin-conjugated mouse anti-CD8, -CD14, -CD16, or -CD56 (BD Biosciences). The numbers of each cell were calculated as the percentage with a low side-scatter and using the CD45+ lymphocyte gate, adjusted for live cells by 7-amino actinomycin D vital dye exclusion. Counting of CD34+ cells was performed according to the International Society of Hematotherapy and Graft Engineering guidelines [21]. CD3+, CD4+, CD8+, CD14+, and CD16+/CD56+ cell numbers were determined using a protocol based on the guidelines from the Centers for Disease Control and Prevention [22]. Stained samples were analyzed on a FACScan flow cytometer (BD Biosciences) as described perviously [23]. Approval of the study was obtained from the institutional review board of Mie University Hospital.
Study Definitions
The primary study endpoints were the engraftment rate and its kinetics. The secondary study endpoints were GVHD, TRM, relapse-free survival, and overall survival (OS). Hematologic recovery was defined as the time in days to an absolute neutrophil count ≥500/μL (first of 3 consecutive days) and a platelet count ≥20,000/μL (without transfusion support). Failure of engraftment was defined as the absence of an absolute neutrophil count recovery at day 100. Patients who survived at least 14 days after transplantation were evaluated for aGVHD, and patients who survived at least 100 days after transplantation were evaluated for chronic GVHD (cGVHD). aGVHD and cGVHD were graded according to standard criteria [24, 25]. TRM was defined as any death that occurred while the patient was in remission. Relapse-free survival was defined as the number of days from transplantation to disease progression or death from any causes other than disease progression. OS was defined as the number of days from transplantation to death from any cause.
Statistical Analysis
Prospective collection of patient characteristics and outcomes were performed by the Tokai Cord Blood Bank Data Management Center. Variables related to the patients, diseases, grafts, and transplant procedure were compared among the groups by chi-square analysis for categoric variables and the Mann-Whitney test for continuous variables. The Kaplan-Meier method was used to obtain estimates of relapse-free survival and OS, and the log-rank test was used to evaluate differences between patient group strata [26]. Cumulative incidence curves for hematopoietic recovery, GVHD, TRM, and relapse were used, taking account of competing risks. To calculate the probability of neutrophil and platelet recovery and GVHD, death was the competing event; for TRM, relapse was the competing event; for relapse, TRM was the competing event [27]. Cox-proportional hazards univariate and multivariate regression models were applied to identify predictors of study endpoints [28]. All variables with P < .10 were entered into the multivariate logistic regression using a backward, stepwise method. Day 100 posttransplant was applied as a landmark for analysis of cGVHD as a correlate of study endpoints. P-values of <.05 were regarded as indicating statistical significance, and those between .05 and .20 as suggesting a trend. Statistical analyses were performed using STATA version 8.2 software (STATA Corp., College Station, TX).
Results
UCB Graft Composition
We found that the TNCC, CD34+, CD3+, CD4+, CD8+, CD14+, and CD16+/CD56+ cell doses in the individual UCB units varied greatly (Figure 1). The median TNCC was 3.2 × 107/kg (range: 1.1-30.4 × 107/kg), and UCB containing >3.2 × 107/kg of TNCC was used in 61 (81.3%) of 75 child patients (≤15 years old) and 12 (16.9%) of 71 adult patients (>15 years old). UCB containing >2.5 × 107/kg of TNCC, which is regarded as a minimum dose for successful engraftment [9] was used in 70 (93.3%) of 75 child patients and 27 (38.0%) of 71 adult patients. The median number of CD34+ cells was 1.4 × 105/kg (range: 0.4-8.2 × 105/kg), and UCB containing >1.4 × 105/kg of CD34+ cell was used in 53 (70.7%) of 75 child patients and 18 (25.4%) of 71 adult patients. UCB containing >1.7 × 105/kg of CD34+ cells, which has been proposed by Wagner et al. [13] as the minimum for engraftment, was used in 42 (56.0%) of 75 child patients and 8 (11.3%) of 71 adult patients. The median numbers and ranges of CD3+, CD4+, CD8+, CD14+, and CD16+/CD56+ cells are summarized in Figure 1.
Figure 1.
UCB graft cell dose. Box plot for cell doses of each cell fraction. The horizontal bar within each box plot indicates the median of the contained cell number. The top and bottom of the box indicates the 25th and 75th percentiles, respectively. The top and bottom of the whisker indicates the 5th and 95th percentiles, respectively.
Engraftment and Hematopoietic Recovery Kinetics
The cumulative incidence of engraftment at day 60 was 0.81 (95% confidence interval [CI], 0.74-0.87). A total of 26 patients (17.8%) did not exhibit an engraftment. Of these patients, 15 died on or before day 28 and 3 died after day 28 from neutropenia. Three patients died of early progression of their disease, and the remaining 5 patients were censored on the day of the second transplantation because of primary engraftment failure. For the 120 patients who exhibited engraftment, the median time to achieve neutrophil engraftment was 22 days (range: 12-49 days), and the median time to achieve platelet engraftment was 44 days (range: 12-135 days).
Univariate analysis of engraftment revealed a significant association with a higher incidence of engraftment failure with higher patient age and body weight as a continuous variable and with a median or lower TNCC and CD34+, CD3+, CD4+, CD8+, and CD16+/CD56+ dose (Table 2). A trend for a higher incidence of engraftment failure (P-value between 0.05 and 0.20) was found in the patients with advanced disease, an HLA mismatch in the rejection vector in 1 or more loci, and a fludarabine-based reduced intensity conditioning (RIC) regimen. Multivariate analysis showed a significant association between a higher incidence of engraftment failure and a lower dose of CD34+ cells (≤1.4 × 105/kg) and CD8+ cells (≤15.7 × 105/kg).
Table 2. Univariate and Multivariate Analyses for Engraftment
| Significant Factor | Hazard Ratio (95% CI)⁎⁎⁎ | P-value |
|---|---|---|
| Univariate analysis | ||
| Background | ||
| Age⁎ | 1.01(1.00-1.02) | .007 |
| Body weight⁎ | 1.01(1.00-1.02) | .001 |
| Advanced disease | 1.34(0.91-1.98) | .14 |
| HLA mismatch (one or more)⁎⁎ | 1.47(0.97-2.17) | .067 |
| Conditioning | ||
| Two conditioning agents | 1.24(0.85-1.82) | .26 |
| Fludarabine-based RIC regimen | 1.46(0.92-2.30) | .10 |
| Total body irradiation dose⁎ | 1.00(0.97-1.03) | .97 |
| Immunosuppression | ||
| Tacrolimus (vs. cyclosporine) | 0.93(0.63-1.38) | .73 |
| The use of methotrexate | 0.84(0.58-1.22) | .37 |
| G-CSF administration | ||
| Yes | 0.76(0.31-1.87) | .55 |
| Graft cell dose | ||
| TNCC >3.2 × 107/kg | 0.60(0.41-0.87) | .006 |
| CD34 >1.4 × 105/kg | 0.46(0.31-0.66) | <.0001 |
| CD3 >31.2 × 105/kg | 0.63(0.44-0.91) | .015 |
| CD4 >23.8 × 105/kg | 0.59(0.41-0.86) | .005 |
| CD8 >15.7 × 105/kg | 0.50(0.34-0.72) | .0002 |
| CD14 >14.0 × 105/kg | 1.21(0.84-1.75) | .31 |
| CD16/CD56 >16.0 × 105/kg | 0.64(0.45-0.92) | .017 |
| Multivariate analysis | ||
| CD34 dose >1.4 × 105/kg | 0.56(0.37-0.85) | .007 |
| CD8 dose >15.7 × 105/kg | 0.65(0.43-0.99) | .047 |
⁎Continuous variable. |
⁎⁎HLA mismatch in rejection vector. |
⁎⁎⁎A hazard ratio over 1.00 indicates that the category is a risk factor for an engraftment failure. |
The time to achieve neutrophil engraftment in the patients receiving more than a median dose of CD34+ cells (median: 21 days; range: 12-49 days) was significantly earlier than that in patients receiving a median or lower dose of CD34+ cells (25 days; 12-48 days) (P = .0001 at day 21 and P < .0001 at day 28). The overall incidences of engraftment in each patient group were 90.4% and 72.3% (P = .004), respectively.
We next evaluated the influence of the CD8+ cell dose on the cumulative incidence of neutrophil engraftment in each patient group according to CD34+ cell dose (Figure 2). In the group receiving more than the median dose of CD34+ cells, the time to achieve neutrophil engraftment was similar (median: 21 days; range: 12-49 days) in the patients receiving more than the median dose of CD8+ cells (15.7 × 105/kg) than in those receiving this dose or less (median: 22 days; range: 15-43 days; P = .11 on day 21); however, in the group receiving less than the median dose of CD34+ cells, the time to achieve neutrophil engraftment was significantly earlier in the patients receiving more than the median dose of CD8+ cells (median: 21 days; range: 17-33 days) than in those receiving less than the median dose (median: 25 days; range: 12-48 days) (P = .0047 at day 21).
Figure 2.
Cumulative incidence of engraftment. A, Cumulative incidence of engraftment in the patients who received more than median dose of CD34+ cells (>1.4 × 105/kg). The solid line indicates the patients receiving more than the median dose of CD8+ cells (>15.7 × 105/kg), and the dotted line indicates the patients receiving a median or lower dose of CD8+ cells (<15.7 × 105/kg). The overall incidences of engraftment in each patient group were 0.93 (n = 53) and 0.85 (n = 20), respectively. B, Cumulative incidence of engraftment in the patients who received a median or lower dose of CD34+ cells (≤1.4 × 105/kg). The solid line indicates the patients receiving more than the median dose of CD8+ cells, and the dotted line indicates the patients receiving a median or lower dose of CD8+ cells. The overall incidences of engraftment in each patient group were 0.90 (n = 20) and 0.66 (n = 53), respectively.
Finally, no significant factor was identified by either univariate or multivariate analysis for platelet recovery.
GVHD
Of 130 patients that could be evaluated, aGVHD was scored as grade 0 (n = 36; 27.7%), I (n = 41; 31.5%), II (n = 37; 28.5%), III (n = 10; 7.7%), or IV (n = 6; 4.6%). The cumulative incidences of grades II-IV and III-IV aGVHD were 0.41 (95% CI, 0.32-0.50) and 0.14 (95% CI, 0.08-0.22), respectively. Cox regression analysis showed a significant association between a higher incidence for grade II-IV aGVHD and an HLA mismatch in the graft-versus-host (GVH) vector in 1 or more loci and a higher CD8+ cell dose (>15.7 × 105/kg) (Table 3). A trend for a higher incidence of grades III-IV aGVHD was found in the patients who received >15.7 × 105/kg of CD8+ cells (hazard ratio: 2.15; 95% CI, 0.78-5.96; P = .14).
Table 3. Multivariate Analyses for GVHD, Relapse, Treatment-Related Mortality, Relapse-Free Survival, and Overall Survival
| Outcome and Significant Factor | Hazard Ratio (95% CI) | P-value |
|---|---|---|
| Grade II-IV acute GVHD | ||
| HLA mismatch (one or more)⁎ | 2.70(1.25-5.88) | .012 |
| CD8 >15.7 × 105/kg | 2.02(1.16-3.55) | .014 |
| Relapse | ||
| Advanced disease | 2.71(1.25-5.88) | .012 |
| Treatment-related mortality | ||
| HLA mismatch (one or more)⁎ | 8.33(2.13-33.3) | .003 |
| Fludarabine-based RIC regimen | 2.29(1.24-4.23) | .008 |
| Use of short-term methotrexate | 0.50(0.27-0.93) | .027 |
| Relapse-free survival | ||
| Advanced disease | 1.73(1.05-2.83) | .031 |
| HLA mismatch (one or more)⁎ | 2.08(1.19-3.70) | .010 |
| Use of short-term methotrexate | 0.55(0.36-0.83) | .005 |
| Overall survival | ||
| HLA mismatch (one or more)⁎ | 2.63(1.39-4.76) | .003 |
| Fludarabine-based RIC regimen | 1.81(1.09-3.00) | .021 |
| Use of short-term methotrexate | 0.52(0.32-0.83) | .006 |
⁎HLA mismatch in GVH vector. |
Of 116 patients that could be evaluated, the cumulative incidence of cGVHD was 0.16 (95% CI, 0.10-0.23). Cox regression analysis showed a trend for a higher incidence of cGVHD in the patients who received UCBT from an HLA mismatched donor in the GVH vector in 1 or more loci (hazard ratio: 3.23; 95% CI, 0.93-11.1; P = .065). There was no significant association between each cell dose and the incidence of cGVHD.
Relapse
Relapse was observed in 41 of 137 patients treated for malignant disease (median: day 110; range: day 7-655). The cumulative incidence of relapse at 2 years was 0.31 (95% CI, 0.24-0.40) for all patients with malignant disease, 0.31 (0.18-0.45) for patients with ALL (n = 49), and 0.33 (0.19-0.49) for patients with AML (n = 39). Cox regression analysis revealed a significant association between advanced disease at transplantation and a higher incidence of relapse (Table 3). There was no significant association between each cell dose and the incidence of relapse.
Treatment-Related Mortality
The cumulative incidences of TRM at day 100 and 365 were 0.25 (95% CI, 0.18-0.32) and 0.35 (0.28-0.43), respectively. Cox regression analysis showed a significant association between a higher TRM and an HLA mismatch in the GVH vector in 1 or more loci, fludarabine-based RIC regimen, and not using short-term methotrexate (Table 3). There was no significant association between each cell dose and TRM.
Relapse-Free Survival and Overall Survival
The cumulative incidences of relapse-free survival and OS at 3 years after transplantation were 0.33 (95% CI, 0.24-0.41) and 0.37 (0.28-0.46), respectively. The median follow-up time for surviving patients was 24 months (range: 2-62). Cox regression analysis revealed a significant association between a lower relapse-free survival rate with advanced disease, an HLA mismatch in the GVH vector in 1 or more loci, and not using short-term methotrexate (Table 3). There was also a significant association between a lower OS and an HLA mismatch in the GVH vector in 1 or more loci, a fludarabine-based RIC regimen, and not using short-term methotrexate (Table 3). There was no significant association between each cell dose and relapse-free survival or OS.
Discussion
In UCBT, engraftment in the recipients is an important surrogate marker because of there is a lower engraftment rate and a delayed blood recovery compared to bone marrow or PBSC transplantation [6, 7]. We analyzed the influence of CD34+ and other accessory cell doses in UCB grafts on the engraftment rate and kinetics in recipients, and we demonstrated that a higher CD34+ cell dose is associated with successful engraftment. In patients receiving more than 1.4 × 105/kg of CD34+ cells, the engraftment occurred 4 days earlier than in patients receiving 1.4 × 105/kg or less CD34+ cells. Wagner et al. [13] also reported that the CD34+ cell dose is associated with engraftment in recipients of UCBT. They showed that the rate and probability of engraftment were markedly lower in patients receiving <1.7 × 105/kg of CD34+ cells compared with those receiving a higher dose of these cells. These findings suggest that the CD34+ cell dose in UCB grafts is a definitive predictor of the engraftment rate and kinetics after UCBT and that the threshold dose for assured and rapid engraftment is approximately 1.4-1.7 × 105/kg, although differences in methodologies for measuring CD34+ cells could influence the exact threshold dose.
A few studies have attempted to show an influence of the CD34+ cell dose on engraftment after UCBT, but most of them have not been able to confirm that there is an effect [11, 29]. These studies have determined the CD34+ cell dose using different techniques and institutions [11] or in small populations (<30 patients) [29]. In contrast, Wagner et al. [13] estimated the infused CD34+ cell dose by flow cytometry of an aliquot of the infused UCB cells under a fixed condition at a single institution and in a larger study population (n = 102). We also estimated the cell dose by measuring the percentage of CD34+ cells in UCB test vials at a single institution, although there is no assurance that the compositions of the test vial and the unit actually thawed and infused into the patients were identical. It is well known that the measurement of CD34+ cells cannot be used for comparative studies between transplant centers because of the absence of standard thawing, sampling, and counting methods. The use of a standardized technique with sufficient reproducibility is essential for accurately evaluating the association between cell components in the UCB graft and their role in UCBT.
We showed for the first time that a higher dose of CD8+ cells (>15.7 × 105/kg) in UCB grafts is associated with successful engraftment. Stratification of the group according to CD34+ cell dose revealed that there is a significant influence of CD8+ cells on the median time to achieve neutrophil engraftment in the patients receiving lower CD34+ cell doses (P = .0047), whereas there was no significant influence in the patients receiving a higher CD34+ cell dose (P = .11). These results suggest CD8+ cells have a supportive role in the engraftment of UCB containing fewer HSCs.
The mechanism of action by which CD8+ T cells facilitate engraftment has been extensively studied in BMT [30]. Earlier studies demonstrated that donor CD8+ T cells eliminate residual immune cells of the recipient via perforin or a Fas ligand-dependent mechanism [20, 31]. Several subpopulations of CD8+ T cells have been reported to facilitate engraftment. Kaufman et al. [32] reported that CD8+ T cells expressing T cell receptor (TCR)-associated CD3 molecules but not TCRs can facilitate marrow engraftment without causing GVHD. Gandy et al. [33] later reported that TCR+ T cells also facilitate engraftment of highly enriched HSCs in allogeneic recipients. It remains possible that other mechanisms such as “veto activity” as defined by Miller [34] contribute to the graft-enhancing effect. Veto cells can induce apoptosis of cytotoxic T cells that are directed against antigens of the veto cells via a Fas-Fas ligand interaction [35]. Some of the most potent veto cells are of T cell origin, and in particular, CD8+ cytotoxic T lymphocytes have a very high veto activity [36, 37]. We showed here that each UCB unit contains a considerable number of CD8+ cells (median, 15.7 × 105/kg), which is 1 to 2 orders of magnitude more than cells obtained from T cell-depleted bone marrow by CD34+ positive selection (typically <105 CD8+ cells/kg). T cells in UCB are more naive than adult lymphocytes [38], but, as described above, expression of the TCR is not necessarily required for facilitation of engraftment by T cells. Overall, these results are consistent with the idea that CD8+ T cells play an important role in facilitating HSC engraftment in transplantation of UCB.
Although both CD34+ cells and CD8+ cells had a beneficial effect on engraftment, neither cell population affected survival in UCBT recipients. This may result from the heterogeneity of the study population. The patients had a variety of diseases and severity of disease, and they received a variety of preconditioning regimens and prophylaxis for GVHD (Table 1). A study population with a more homogenous background is required to define the contribution of these cell populations on survival after transplantation. Thus, our results do not exclude the possibility that CD34+ and/or CD8+ cell doses influence survival in UCBT recipients.
Interestingly, the use of methotrexate was associated with lower rates of TRM and higher relapse-free survival and OS (Table 3). Although it was not associated with lower incidence of aGVHD in this study population (Table 2), we previously demonstrated that use of methotrexate has a significant favorable effect on post-UCBT immune reactions and OS in a different study population [39]. In contrast, Locatelli et al. [40] reported that the use of methotrexate is unfavorable for neutrophil engraftment and event-free survival in a study of 44 pediatric patients with thalassemia and sickle cell disease. Accordingly, the effect of methotrexate on UCBT outcome remains unclear. It might be worth further investigation to determine whether there is an explanation for the finding that the use of methotrexate is associated with a lower risk of TRM and higher rate of survival.
A fludarabine-based RIC regimen was a risk factor for TRM. Because we used registry data in this study, the patients who were at high risk for TRM might have tended to receive an RIC regimen rather than a myeloablative regimen. In addition, the RIC regimens used in this study greatly varied. Thus, another study is required to determine whether the RIC regimen decreases TRM and improves the survival rate in the patients who are at high risk for TRM.
We studied the effect of the dose of CD34+ and other cells on engraftment in patients receiving a single unmanipulated UCB graft and an immunosuppressant other than ATG. Antithymocyte globulin as a preconditioning or posttransplantation immunosuppressive treatment would considerably attenuate the influence of T cells, but the role of natural killer cells could be expected to become more important [41, 42]. Thus, the present results should be interpreted with caution for patients treated with ATG or other T cell depletion procedures.
In summary, the present study demonstrated that both CD34+ and CD8+ cells have a beneficial effect on engraftment in UCBT recipients, and our findings raise the possibility that CD8+ cells have a supportive role in engraftment of UCB units containing fewer HSCs. Further analysis in a larger population is warranted. If our findings are confirmed, if all of the UCB units accompany their test vial, and if the measurements of CD34+ and CD8+ cells are standardized, it may be worthwhile to consider the doses of both CD34+ and CD8+ cells as well as TNCC and HLA matching for UCB graft selection strategies.
Acknowledgments
This work was supported by a Research Grant on Human Genome, Tissue Engineering from the Ministry of Health, Labour and Welfare of Japan (to E.A. and M.M.). We would like to thank the staff of the Japan Cord Blood Network and its attendant transplant centers. We are especially grateful for the help of Drs. Kiyoaki Suzuki, Tadayuki Ishimaru, Kaoru Ishikawa, and Takami Inoue in collecting the UCB, as well as the assistance of Mses. Tomoko Ito, Makiko Shibayama, Hiromi Ogawa, and Nobuko Ishikawa in processing the UCB units.
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PII: S1083-8791(07)00215-7
doi:10.1016/j.bbmt.2007.03.006
© 2007 American Society for Blood and Marrow Transplantation. Published by Elsevier Inc. All rights reserved.
Volume 13, Issue 7 , Pages 822-830, July 2007
