How do I save cell therapy product's storage space?

BACKGROUND: Availability of liquid nitrogen (LN2) freezer storage space is a major challenge for many transplant programs as they continue to grow and accumulate products. The recent trend of allogeneic grafts cryopreservation that started during the COVID-19 pandemic, made the situation even worse requiring an increase in storage capacity. Multi-compartment cryopreservation bags can help save storage space but can be tricky to use. Here, we describe the validation of muti-compartment cryopreservation bags for the purpose of donor lymphocyte infusion (DLI) aliquots. METHODS: We validated the use of five compartment cryobags for cryopreservation of cell therapy products. Four products were cryopreserved using these bags and each compartment was tested post-thaw for product volume distribution, total cell count recovery, and viability. Additionally, the integrity of both bag compartments and labels was assessed as well. RESULTS: All tested specimens met post-thaw viability and TNC recovery acceptability criteria. Fill volume was optimized at 24-25 mL for acceptable volume distribution between aliquots. With proper heat sealing between compartments, all aliquots retain their integrity and cryopreservation labels were adherent and legible. CONCLUSIONS: Muti-compartment bags can be used successfully for cryopreservation of cell therapy products and increase storage capacity.

Cryopreserved hematopoietic stem/progenitor cells stability program-development, current status and recommendations: A brief report from the AABB-ISCT joint working group cellular therapy product stability project team.

BACKGROUND: The AABB-ISCT Joint Working Group Stability Project Team (SPT) was assigned to roadmap a path toward standardization of cryopreserved hematopoietic stem/progenitor cell (HSPC) stability programs. HSPC stability encompasses a broad scope of conditions including non-frozen ("fresh") and cryopreserved cell products, and varying methods for storage, thaw, and administration. This report assessed current practices and focused solely on cryopreserved HSPC cell therapy products to establish preliminary recommendations for a stability program roadmap. METHODS: A survey was prepared by the SPT and distributed to ISCT and AABB members. Survey results were summarized and recommendations were outlined based on the responses from the survey. This report highlights current practices for cryopreserved HSPC stability programs, including additional considerations and recommendations. RESULTS AND DISCUSSION: Eighty-two (82) centers worldwide participated in the survey. Survey results indicate variability across programs. HSPC stability depends on multiple factors within the processing facility (e.g., cryopreservation techniques, reagents used, and storage temperature) and independent variables (e.g., donor-related factors and starting material variability). While retention of hematopoietic engraftment potential is the primary goal for cryopreserved HSPC stability, engraftment results should not be used as the sole metric for stability programs. Based on the survey results, the SPT provides recommendations for consideration. CONCLUSIONS: The SPT recommendations for best practices are not intended to replace existing standards. The survey results emphasize the need for the community to optimize best practices and consider initiating collaborative projects to improve the standardization of cryopreserved HSPC stability programs for cell therapy products.

Cryopreserved hematopoietic stem/progenitor cells stability program-development, current status and recommendations: A brief report from the AABB-ISCT joint working group cellular therapy product stability project team.

BACKGROUND: The AABB-ISCT Joint Working Group Stability Project Team (SPT) was assigned to roadmap a path toward standardization of cryopreserved hematopoietic stem/progenitor cell (HSPC) stability programs. HSPC stability encompasses a broad scope of conditions including non-frozen ("fresh") and cryopreserved cell products, and varying methods for storage, thaw, and administration. This report assessed current practices and focused solely on cryopreserved HSPC cell therapy products to establish preliminary recommendations for a stability program roadmap. METHODS: A survey was prepared by the SPT and distributed to ISCT and AABB members. Survey results were summarized and recommendations were outlined based on the responses from the survey. This report highlights current practices for cryopreserved HSPC stability programs, including additional considerations and recommendations. RESULTS AND DISCUSSION: Eighty-two (82) centers worldwide participated in the survey. Survey results indicate variability across programs. HSPC stability depends on multiple factors within the processing facility (e.g., cryopreservation techniques, reagents used, and storage temperature) and independent variables (e.g., donor-related factors and starting material variability). While retention of hematopoietic engraftment potential is the primary goal for cryopreserved HSPC stability, engraftment results should not be used as the sole metric for stability programs. Based on the survey results, the SPT provides recommendations for consideration. CONCLUSIONS: The SPT recommendations for best practices are not intended to replace existing standards. The survey results emphasize the need for the community to optimize best practices and consider initiating collaborative projects to improve the standardization of cryopreserved HSPC stability programs for cell therapy products.

Validation of automated fluorescent-based technology for measuring total nucleated cell viability of hematopoietic progenitor cell products.

BACKGROUND: A reliable rapid method for measuring total nucleated cell (TNC) viability is essential for cell-based products manufacturing. The trypan blue (TB) exclusion method, commonly used to measure TNC viability of hematopoietic progenitor cell (HPC) products, is a subjective assay, typically uses a microscope, and includes a limited number of cells. The NucleoCounter NC-200 is an automated fluorescent-based cell counter that uses pre-calibrated cartridges with acridine orange and DAPI dyes to measure cell count and viability. This study describes the validation of the NC-200 for testing HPC's viability. METHODS: Samples from 189 fresh and 60 cryopreserved HPC products were included. Fresh products were tested for viability after collection by both TB and NC-200. 7-aminoactinomycin D (7AAD) CD45+ cell viability results were obtained from a flow cytometry test. Cryopreserved products thawed specimens were tested for viability by both TB and NC-200. The NC-200 viability results were compared with the other methods. Acceptability criteria were defined as ≤10% difference between the NC-200 method and the other methods for at least 95% of the samples. RESULTS: Fresh products' mean viability difference between NC-200 and TB or 7AAD CD45+ method was 4.9% (95%CI 4.6-5.4) and 2.8% (95%CI 2.2-3.4), respectively. Thawed products' mean viability difference between NC-200 and TB was 3.0% (95%CI 0.4-5.6). CONCLUSION: The NC-200 automated fluorescent-based method can be used effectively to determine HPC's viability for both fresh and cryopreserved products. It can help eliminate human bias and provide consistent data and operational ease.

How do I: Evaluate the safety and legitimacy of unproven cellular therapies?

BACKGROUND: Unproven cellular therapies are being offered to patients for a variety of conditions and diseases for which other treatments have failed. The use of untested cellular therapies is a worldwide problem. Practitioners (e.g., physicians, scientists, QA/QI facility managers, and policy advocates) are perhaps unaware of the risks involved with such therapies. Therefore, a critical need exists to bring attention to the potential limitations and adverse effects of these therapies to inform and limit misinformation. STUDY DESIGN AND METHODS: We describe the extent of the unproven cellular therapy problem through a search of scientific literature and social media coverage. We also describe the regulatory framework that can be used by the practitioner to review and evaluate both proven and unproven cellular therapies. RESULTS: We report on the current state of unproven cellular therapies across the globe. A workflow to facilitate an understanding of the regulatory processes involved in the approval of cellular therapies is provided as well as a list of warnings required by regulatory agencies on various products. It is hoped that this article will serve as a tool kit to educate the practitioner on navigating the field of unproven cellular therapy products. DISCUSSION: Increasing awareness of the issues associated with unproven therapies through education is important to help in reducing misinformation and risks to patients.

How do I transfer liquid nitrogen freezers to a new home safely?

BACKGROUND: Our cell processing facility was planning to transfer more than 20 Liquid Nitrogen (LN2) freezers to a new location. Moving LN2 freezers is a complex task that can pose potential risk to the storage units' integrity as well as to the products that they hold. Careful planning is required, especially when moving multiple freezer units at once. METHODS: To achieve the task, we put together a detailed project plan, collaborated with all the involved partners, hired qualified professionals to perform the project-specific tasks, and put together a detailed risk assessment and risk mitigation plan. RESULTS: A facility was chosen and prepared according the project plan and safety department recommendations. Risk mitigation strategies were developed and implemented, and all freezers were transfered uneventfully to a new location. CONCLUSIONS: By performing detailed planning and engaging the appropriate partners, LN2 freezers can be successfully transferred to a new home.

Combined Haploidentical and Umbilical Cord Blood Allogeneic Stem Cell Transplantation for High-Risk Lymphoma and Chronic Lymphoblastic Leukemia.

Limited studies have reported on outcomes for lymphoid malignancy patients receiving alternative donor allogeneic stem cell transplants. We have previously described combining CD34-selected haploidentical grafts with umbilical cord blood (haplo-cord) to accelerate neutrophil and platelet engraftment. Here, we examine the outcome of patients with lymphoid malignancies undergoing haplo-cord transplantation at the University of Chicago and Weill Cornell Medical College. We analyzed 42 lymphoma and chronic lymphoblastic leukemia (CLL) patients who underwent haplo-cord allogeneic stem cell transplantation. Patients underwent transplant for Hodgkin lymphoma (n = 9, 21%), CLL (n = 5, 12%) and non-Hodgkin lymphomas (n = 28, 67%), including 13 T cell lymphomas. Twenty-four patients (52%) had 3 or more lines of therapies. Six (14%) and 1 (2%) patients had prior autologous and allogeneic stem cell transplant, respectively. At the time of transplant 12 patients (29%) were in complete remission, 18 had chemotherapy-sensitive disease, and 12 patients had chemotherapy-resistant disease. Seven (17%), 11 (26%), and 24 (57%) patients had low, intermediate, and high disease risk index before transplant. Comorbidity index was evenly distributed among 3 groups, with 13 (31%), 14 (33%), and 15 (36%) patients scoring 0, 1 to 2, and ≥3. Median age for the cohort was 49 years (range, 23 to 71). All patients received fludarabine/melphalan/antithymocyte globulin conditioning regimen and post-transplant graft-versus-host disease (GVHD) prophylaxis with tacrolimus and mycophenolate mofetil. The median time to neutrophil engraftment was 11 days (range, 9 to 60) and to platelet engraftment 19.5 days (range, 11 to 88). Cumulative incidence of nonrelapse mortality was 11.6% at 100 days and 19 % at one year. Cumulative incidence of relapse was 9.3% at 100 days and 19% at one year. With a median follow-up of survivors of 42 months, the 3-year rates of GVHD relapse free survival, progression-free survival, and overall survival were 53%, 62%, and 65%, respectively, for these patients. Only 8% of the survivors had chronic GVHD. In conclusion, haplo-cord transplantation offers a transplant alternative for patients with recurrent or refractory lymphoid malignancies who lack matching donors. Both neutrophil and platelet count recovery is rapid, nonrelapse mortality is limited, excellent disease control can be achieved, and the incidence of chronic GVHD is limited. Thus, haplo-cord achieves high rates of engraftment and encouraging results.

Validating and implementing the use of an infusion pump for the administration of thawed hematopoietic progenitor cells-a single-institution experience.

BACKGROUND: Direct thaw and administration of previously cryopreserved peripheral blood stem cell products is a commonly used practice and should be performed rapidly to reduce cellular damage caused by dimethyl sulfoxide exposure. Cells are typically thawed at the bedside and infused by gravity through a high-flow-rate central venous catheter. An existing nontunneled catheter is occasionally used instead and often results in a slower infusion rate. To ensure expedient and consistent infusions, we validated and implemented the use of an infusion pump for thawed peripheral blood stem cells. STUDY DESIGN AND METHODS: Validation was performed in two phases: in vitro simulation and in vivo clinical assessment. Total nucleated cell recovery and viability plus progenitor cell viability and potency were compared in vitro between two cryopreserved peripheral blood stem cell units that were either passed through a preset infusion pump or drained by gravity. The infusion rate, adverse events, and engraftment times were retrospectively compared between patients who received infusions by infusion pump (n = 35) and by gravity (n = 38). RESULTS: No significant differences were observed in vitro between the infusion methods for all measured variables. Overall infusion rates were similar in vivo for both groups but were significantly lower for patients who had nontunneled catheters that delivered the infusion by gravity. The time to neutrophil and platelet engraftment was similar for both groups. CONCLUSION: This is the first study to assess the use of an infusion pump for stem cell transplant. The use of an infusion pump for peripheral blood stem cell infusion is safe, provides a reliable and consistent infusion method, and can mitigate the effect of the type of venous access line used.

Engraftment for CD34 selected stem cell products is not compromised by cryopreservation.

BACKGROUND: The coinfusion of haploidentical CD34+ selected peripheral blood stem cell products with umbilical cord blood (UCB) provides early neutrophil recovery, long-term UCB engraftment, and a lower incidence of graft-versus-host disease; however, this complex transplant presents a scheduling challenge for both the cellular therapy laboratory and the clinical team. Cryopreservation of the haploidentical product can facilitate scheduling, but has been previously shown to be associated with infusion reactions and delayed platelet (PLT) engraftment in allogeneic hematopoietic progenitor cell transplant. STUDY DESIGN AND METHODS: To test whether cryopreservation of the CD34+ selected product compromises the graft, we compared neutrophil and PLT engraftment kinetics for patients receiving freshly infused or cryopreserved products. Seventy-two products collected from haploidentical related donors were CD34+ selected and infused in a combined transplant with UCB: 32 were cryopreserved before infusion and 40 were infused fresh. RESULTS: No adverse infusion events were reported in either group and there was no difference in neutrophil and PLT engraftment time between fresh and cryopreserved products. CONCLUSION: Cryopreservation of a CD34+-selected product can be safely used in a combined transplant with UCB and does not affect engraftment time.

How we handled the dextran shortage: an alternative washing or dilution solution for cord blood infusions.

Dextran 40 is the main component of the solution used to wash or dilute thawed cord blood unit (CBU) products for stem cell transplant. Dextran 40 became unavailable in the United States as of April 2014. Like many other cellular therapy laboratories in the United States, we found ourselves with limited dextran 40 inventory, a growing CBU transplant requirement, and no alternative solution. Since there are no published alternative washing solutions for cryopreserved CBU we had to develop and validate a new solution rapidly. We chose to validate hydroxyethyl starch (HES) due to its similar ability to stabilize red blood cells and reduce sudden changes in osmolality that occur during thawing. For the validation we used 3 CBUs and thawed and washed each unit with both dextran 40- and HES-based solutions; thus, each CBU served as its own control. We observed no significant differences between the two wash solutions for all the monitored variables including cell viability, cell recovery, or potency measured by colony-forming cell assay. Based on this initial validation we began using HES-albumin for CBU washing after our supply was exhausted. Our initial experience with the first 16 CBU transplants after validation indicates safe infusion and preliminary cord engraftment.

Sustained Hematopoietic Engraftment Potential after Prolonged Storage of Cryopreserved Hematopoietic Stem Cells Used in Salvage Autologous Stem Cell Transplantation.

Salvage autologous hematopoietic stem cell transplantation (HSCT) is an effective treatment for patients with relapsed multiple myeloma (MM). Peripheral blood stem cells (PBSCs), a source of hematopoietic stem cells (HSCs), are collected before the first transplantation, and adequate quantities of PBSCs can be collected and stored potentially for years to support at least 2 transplantations for eligible patients. To ensure the safety of salvage HSCT in the treatment of patients in subsequent relapse, PBSCs must retain the potential to engraft even after several years of cryopreservation. Although PBSC viability has been studied extensively using in vitro techniques, few publications describe the most rigorous functional potency measure, of patients receiving a myeloablative conditioning regimen. This study describes a large single-institution experience evaluating the engraftment kinetics of PBSCs used in salvage transplantation after multiple years of storage compared with first transplantation for the same patients in the treatment of MM. A retrospective chart review of patients with MM undergoing HSCT between 2000 and 2021 identified 89 patients who received salvage autologous PBSCs stored for >1 year after first HSCT. PBSCs were cryopreserved and stored in vapor-phase liquid nitrogen refrigerators at ≤-150°C. All patients received a PBSC product from the same collection cycle for both transplantations. Differences in CD34+ cell doses and days to engraftment between the first and salvage transplantations were tested using the paired 2-tailed t-test and Wilcoxon signed-rank test. Univariate and multivariable linear regressions were used to determine the association between storage time and days to engraftment, adjusting for CD34+ cell dose and conditioning regimen in the multivariable model. The median duration of storage between the day of initial collection and salvage transplant was 5.4 years (range, 1.0 to 19.7 years). Engraftment kinetics demonstrated a sustained neutrophil engraftment (absolute neutrophil count >0.5 × 109 cells/L) at a median of 11 days after both the first and salvage transplantations (range, 8 to 15 days and 8 to 19 days, respectively; P < .05). The median time to sustained platelet engraftment (>20 × 109 cells/L without transfusion support) was 13.5 days after the first HSCT and 14 days after salvage HSCT (range, 9 to 27 days and 10 to 56 days, respectively; P = .616). After adjusting for CD34+ cell doses and conditioning regimens, there was no association between the duration of cryopreservation and days to neutrophil engraftment (r = 0.178, P = .130) or platelet engraftment (r = 0.244, P = .100). Engraftment kinetics of the salvage HSCT are comparable to those of the first HSCT even when products are stored in vapor-phase nitrogen refrigerators for a median of 5.4 years. There is no association between the duration of storage and time to engraftment when controlling for CD34+ cell dose and conditioning regimen. Prolonged storage of cryopreserved HSC products is a safe practice for MM patients undergoing salvage autologous HSCT.

Quality monitoring of microbial contamination of cryopreserved parathyroid tissue.

Cryopreservation of parathyroid tissue (PT) provides patients undergoing parathyroidectomy with an option for delayed autologous heterotopic parathyroid transplantation. A standard protocol for quality monitoring of PT has not been established. This article describes a method for detecting the presence of bacterial contamination in PT tissue intended for autologous transplantation. PT was received in the tissue bank, processed under aseptic conditions, and placed into cryopreservation medium. Sterility testing was performed at 2 time points prior to cryopreservation. From January 2005 to October 2008, 47 PT samples were cryopreserved. The following bacteria were isolated from 11 PT specimens: Staphylococcus epidermidis, Staphylococcus capitis subspecies ureolyticus, Staphylococcus lugdunensis, Bacillus pumilus, and corynebacteria (diphtheroids). 23% of PTs were contaminated at the time of collection, predominantly with indigenous bacteria. Quality monitoring using this protocol is a useful tool to identify tissues contaminated with bacteria.

Poor graft function after T cell-depleted allogeneic hematopoietic stem cell transplant.

PGF implies persistent cytopenia in the presence of predominant donor chimerism. We examined contributors to PGF in 104 HCT recipients who survived ≥100 days without relapse or major complications. Surrogate parameters for PGF were: Hg <10 g/dl, RBC transfusion dependence, platelet count <20 × 109/L or ANC < 0.5 × 109/L. All patients received T cell depletion with alemtuzumab or ATG. The 2-year OS and PFS probabilities were 66%, 95%CI (56 - 75%) and 51%, 95%CI (41-60%) respectively. Fifty-four patients (52%) met one or more PGF criteria. There was significant association between major ABO incompatibility and platelet <20 × 109/L (OR = 4.7, 95%CI 1.05-21.26, p = .043), acute GVHD and Hg <10 g/dl (OR 3.7, 95%CI 1.4-9.6, p = .005) and CMV viremia and ANC < 0.5 × 109/L (OR 3.0, 95% CI 1.0, 8.7, p = .043). NRM was significantly higher in the PGF group compared to patients with adequate graft function (45.5% vs 16.7%, p = .014).

Gemfibrozil inhibits Legionella pneumophila and Mycobacterium tuberculosis enoyl coenzyme A reductases and blocks intracellular growth of these bacteria in macrophages.

We report here that gemfibrozil (GFZ) inhibits axenic and intracellular growth of Legionella pneumophila and of 27 strains of wild-type and multidrug-resistant Mycobacterium tuberculosis in bacteriological medium and in human and mouse macrophages, respectively. At a concentration of 0.4 mM, GFZ completely inhibited L. pneumophila fatty acid synthesis, while at 0.12 mM it promoted cytoplasmic accumulation of polyhydroxybutyrate. To assess the mechanism(s) of these effects, we cloned an L. pneumophila FabI enoyl reductase homolog that complemented for growth an Escherichia coli strain carrying a temperature-sensitive enoyl reductase and rendered the complemented E. coli strain sensitive to GFZ at the nonpermissive temperature. GFZ noncompetitively inhibited this L. pneumophila FabI homolog, as well as M. tuberculosis InhA and E. coli FabI.

Postthaw clotting of peripheral blood stem cell products due to insufficient anticoagulant.

The amount of acid citrate dextrose formula A (ACD-A), which is a commonly used anticoagulant in leukopheresis, has to ensure both the safety of the donor and guarantee the integrity of the peripheral blood stem cell (PBSC) product until its transplant. Two recent consecutive cases of postthaw PBSC product clotting initiated a look-back investigation of the ACD-A percentage in leukopheresis products collected in our facility. The data indicated a significant difference between the average amount of ACD-A in prefreezing products collected during 2006 (11.4%) and in products collected during 2007 and 2008 (8.8% and 8.7%, respectively). These findings and the fact that the two clotted products had less than 7% ACD-A indicated that insufficient amount of anticoagulant might contribute to their clotting. This investigation prompted us to modify our collection and thawing procedures to prevent similar events in the future.

Application of pulsed-field gel electrophoresis to identify the source of bacterial contamination of peripheral blood progenitor cell products.

BACKGROUND: Positive microbial cultures of peripheral blood progenitor cell (PBPC) products, although estimated to be low, are serious events in the manufacture of hematopoietic progenitor cell (HPC) products that warrant a thorough investigation to determine the contamination source. STUDY DESIGN AND METHODS: Two patients underwent autologous PBPC collection. The first patient was admitted before the collection and was febrile intermittently throughout hospitalization. The second patient spiked a low-grade fever by the end of the procedure. The HPC products from each patient were cultured during processing and before infusion. Blood cultures were drawn during febrile episodes and before transplant. Bacterial identification and antimicrobial susceptibilities were performed on all positive cultures. All strains were typed using pulsed-field gel electrophoresis (PFGE) to determine their relatedness. RESULTS: The blood cultures from both patients and their corresponding HPC products grew Staphylococcus epidermidis. The PFGE pattern of the S. epidermidis recovered from each patient blood was indistinguishable from the one recovered from the corresponding HPC product. The gel pattern of the strains recovered from the first patient differed by four bands from the one recovered from the second. For each patient, the antibiotic susceptibility patterns of the blood cultures and the HPC products were identical. Infusion of the contaminated HPC had no adverse event, and the patients engrafted successfully. CONCLUSION: By use of PFGE technology, the contamination source of PBPC products was identified. It is concluded that the contamination resulted from intermittent bacteremia in the donors and was not introduced during laboratory manufacturing.

Comparison of time to engraftment between autologous patients receiving washed versus non-washed cryopreserved peripheral blood stem cell products.

Washing cryopreserved peripheral blood stem cell (PBSC) products can decrease infusion-related adverse reactions but can also result in cell loss and reduced cell viability. To assess the risk and benefit of washing products, we compared the time to neutrophil and platelet engraftment between autologous patients that received washed products (n = 201) and non-washed products (n = 89). The effect of the other variables, including age, gender, diagnosis, transplant dose, method of stem cell mobilization, and growth factor support regimen post-transplant, was assessed. In multivariate analysis, direct thaw and infusion of non-washed products resulted in significantly faster neutrophil engraftment (p = .003) and platelet engraftment (p = .017) than washed products. The mean neutrophil and platelet engraftment times were 1.07 days faster and 2.27 days faster, respectively. In conclusion, direct thaw and infusion of cryopreserved PBSC without washing results in significantly shorter time to recovery of neutrophils and platelets after autologous transplantation.

Cord blood chimerism and relapse after haplo-cord transplantation.

Haplo-cord stem cell transplantation combines the infusion of CD34 selected hematopoietic progenitors from a haplo-identical donor with an umbilical cord blood (UCB) graft from an unrelated donor and allows faster count recovery, with low rates of disease recurrence and chronic graft-versus-host disease (GVHD). But the contribution of the umbilical cord blood graft to long-term transplant outcome remains unclear. We analyzed 39 recipients of haplo-cord transplants with acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS), engrafted and in remission at 2 months. Median age was 66 (18-72) and all had intermediate, high, or very-high risk disease. Less than 20% UCB chimerism in the CD33 lineage was associated with an increased rate of disease recurrence (54% versus 11% p < 0.0001) and decrease in one year progression-free (20% versus 55%, p = 0.004) and overall survival (30% versus 62%, p = 0.02). Less than 100% UCB chimerism in the CD3 lineage was associated with increase rate of disease recurrence (46% versus 12%, p = 0.007). Persistent haplo-chimerism in the CD3 lineage was associated with an increased rate of disease recurrence (40% versus 15%, p = 0.009) Chimerism did not predict for treatment related mortality. The cumulative incidence of acute GVHD by day 100 was 43%. The cumulative incidence of moderate/severe chronic GVHD was only 5%. Engraftment of the umbilical cord blood grafts provides powerful graft-versus-leukemia (GVL) effects which protect against disease recurrence and is associated with low risk of chronic GVHD. Engraftment of CD34 selected haplo-identical cells can lead to rapid development of circulating T-cells, but when these cells dominate, GVL-effects are limited and rates of disease recurrence are high.