Enumeration of CD34-positive stem cells: evaluation and comparison of three methods

Australasian CD34+ quality assurance program and rationale for the clinical utility of the single-platform method for CD34+ cell enumeration

BACKGROUND

Enumeration of CD34(+) cells should be accurate and comparable between institutions, particularly when making clinical decisions, evaluating data, and in clinical trials. An Australasian CD34(+) quality assurance program (QAP) has been established to compare CD34(+) cell results and method (Part 1). Unexpected variation in WBCCs led to Part 2 of this report.

METHODS

Part 1: Methods reagents and results were evaluated for 12 QAP samples analyzed by 36-43 centers. Part 2: The effects of different anticoagulants on WBCC of 12 peripheral blood samples (PBs) were compared using three cell counters. To test the validity of applying the conclusions to clinical samples, the WBCCs of leukapheresed products and BM harvest were also compared.

RESULTS

Part 1: In some samples, WBCCs determined by certain cell-counter groups were significantly different. Results for percentage of CD34(+) and CD34(+)/microL suggest that standardization on the lyse-no-wash and single platform (SP) method reduces variation of results between institutions. Part 2: Using different counters, PB WBCC in ACD-A showed greater variation than the same PB in EDTA. For PB in different anticoagulants, the extent of difference in WBCC for the same PB is dependent on the counter used.

DISCUSSION

This CD34 QAP has identified ACD-A as an additional factor that contributes to the disparate WBCCs, which may further compromise the accuracy of CD34(+) cell counts obtained by the dual platform (DP) method, especially for leukapheresed products. In order to achieve greater accuracy within individual institutions, as well as permitting more reliable inter-institutional comparisons, our data supports the adoption of the SP as the standard method for CD34(+) cell enumeration.

Comparison of three methods of CD34+ cell enumeration in peripheral blood: dual-platform ISHAGE protocol versus single-platform, versus microvolume fluorimetry

BACKGROUND

Quantitation of peripheral blood (PB) CD34(+) cells is now an established method for timing PBPC harvesting. Recent refinements to the dual-platform ISHAGE gating strategy for CD34(+) cells has seen the introduction of microbeads to enable absolute counting of cells on a single instrument platform. This eliminates the need for total WBCC performed on an automated hematology analyzer and potentially increases the analytical precision of the methodology. At the same time, alternative methods for CD34(+) cell enumeration have started to emerge, notably microvolume fluorimetry, which forms the basis of the fully-automated STELLer CD34 method using the Imagn 2000.

METHODS

We performed a three-way evaluation of these methods. Sixty-eight samples of PB from 42 patients undergoing PBPC mobilization were analyzed by all three methods and correlations between all three calculated. The two-platform ISHAGE method was used as the reference method.

RESULTS

Precision and linearity of the single-platform and STELLer CD34 assays were excellent. Correlation with the dual-platform reference method was also excellent (single-platform method slope = 1.03, intercept = -0.03 and R(2) = 0.9325, STELLer CD34 assay slope = 0.827, intercept = 4.27, R(2) =0.8215). Bias, determined by Bland-Altman analysis, was 1.16 and -1.62 for single platform and STELLer CD34 assay respectively.

CONCLUSION

The three methods of CD34(+) cell enumeration gave equivalent results. The single-platform methodology negated the need for a separate white cell analyzer, while the STELLer CD34 methodology was technically the simplest.

Comparison of single- and dual-platform approaches to enumerate CD34(+) cells in bone marrow and mobilized peripheral blood stem cells

BACKGROUND

Different flow cytometric methods have been developed to derive absolute CD34(+) cells in predicting transplant outcome. Two techniques for preparing cells for quantification of CD34(+) cells were compared.

METHODS

Enumeration of CD34(+) cells in 16 samples of bone marrow (BM) and in 29 samples of mobilized peripheral blood stem cells (PBSC) obtained by leukapheresis was assessed simultaneously by single-platform (ProCOUNT kit) and dual-platform (Milan protocol) approaches within the first 3 h of collection.

RESULTS

Absolute number of CD34(+) cells obtained in PBSC and BM using single- and dual-platform methods showed high determination coefficients as follows: for PBSC, slope = 1.0515 +/- 0.048, y-intercept = 88.638 +/- 52.45, and r(2) = 0.941, and for BM, slope = 1.0203 +/- 0.093, y-intercept = 122.25 +/- 20.65, and r(2) = 0.878. There were no statistically significant differences in absolute number of CD34(+) cells from PBSC between single-platform (mean 575/microL, range 70-3683/microL) and dual-platform (786/microL, range 51-3804/microL) assays. In contrast, absolute number of CD34(+) cells from BM was significantly lower (p = 0.0002) when enumerated by ProCount kit (135/microL, 14-758/microL) than with dual-platform method (260/microL, 74-889/microL).

CONCLUSIONS

Both approaches can be used indistinctly to estimate absolute number of CD34(+) cells in PBSC but not in BM.

New approaches to hematopoietic cell transplantation in oncology

The use of high-dose chemotherapy followed by autologous HCT and the use of allogeneic HCT in children and adolescents with high-risk ALL, AML, and NBL has successfully improved outcomes. For other diseases, however, the role of HCT in treatment remains a subject of further research. The availability of HCT was significantly expanded by developing alternative graft sources that currently include BM, peripheral blood, and UCB from autologous and allogeneic related or unrelated donors. Progress in autologous HCT has been achieved by the identification of more effective and less toxic preparative regimens and by ex vivo purging of stem cell products. In allogeneic HCT, graft-versus-leukemia or graft-versus-tumor effects are being exploited increasingly to lower relapse rates. In addition, immunomodulation to promote tolerance, as well as allogeneic antitumor reactions have been achieved by antibody therapy, cytokine therapy, or cell-based immunotherapy. Future improvements are likely, as evidenced by promising preliminary results in the development of stem cell collection techniques, in vitro stem cell expansion, and purging techniques of stem cell grafts. The development of less intensive or nonmyeloablative preparative regimens may further reduce regimen-related morbidity and mortality Specific immunotherapy may facilitate tolerance induction in mismatched allogeneic HCT and support allogeneic HCT in the setting of donor-host HLA disparity. Ultimately, advances in cytokine therapy, tumor-specific vaccines, and gene therapy may decrease or even eradicate recurrence of the malignant disease after HCT.

Detecting bacteria in platelet concentrates by use of reagent strips

BACKGROUND

Iatrogenic infection of immunosuppressed or immunocompromised hosts secondary to receipt of blood components containing bacteria may result in serious adverse outcomes. Measurement of pH and glucose by use of inexpensive reagent strips has been proposed as a practical means of screening for bacteria in platelet concentrate (PC) units.

STUDY DESIGN AND METHODS

Glucose and pH were measured in 3093 PC units by use of reagent strips (Multistix, Bayer Corp.) to screen for bacterial content. Any PC classified by the reagent strip method as containing bacteria was subsequently cultured to confirm the presence and quantity of bacteria present.

RESULTS

Thirty of 3093 PC units were classified as containing bacteria by the reagent strip method. Two of the 30 PC units positive by the reagent strip method were also positive by standard bacterial culture technique. Bacillus cereus was isolated from both units.

CONCLUSION

Screening PC units by the reagent strip method resulted in 9.7 units per 1000 being wasted, but prevented two patients from receiving a PC unit containing B. cereus.

A single-step colony-forming unit assay for unseparated mobilized peripheral blood, cord blood, and bone marrow

The colony-forming unit (CFU) assay is exposed to a lot of variation, part of which is introduced by several enrichment strategies that are routinely performed before assessment of clonogenic capacity in mobilized peripheral blood (PB), bone marrow (BM), or cord blood (CB). We investigated the possibility to perform a single-step CFU assay by direct plating of PB, BM, or CB into CFU culture medium to obtain more reproducible results than after a standard Ficoll or lysis procedure. Direct plating implies the presence of red blood cells (RBC), white blood cells (WBC), and plasma in the CFU assay, which could possibly influence the outcome of the assay. Of all components, only the RBC was found to negatively influence CFU-GM growth if a concentration of > 0.02 x 10(9)/ml was present in the CFU culture medium. Subsequently, depending on the RBC concentration PB, BM, and CB samples were prediluted in triplicate or quadruplicate and plated into CFU medium. Lysis and/or Ficoll procedures were also performed in triplicate or quadruplicate on the same samples, and the mean colony number and coefficient of variation (CV) of the three techniques were compared. Significantly smaller CV values were found using the direct plating technique (all assays, mean 7.5%, range 1.6-15.6%) than after Ficoll separation (mean 18.0%, range 2.2-62.5%). Intermediate results were obtained with the lysis method (mean CV 11.6%, range 3.3-29%). In most samples, and especially in those with a very low number of clonogenic cells per milliliter, more colonies were detected with the direct plating method than with either the lysis or Ficoll method. In conclusion, the single-step direct plating method significantly enhances reproducibility of the CFU assay for PB, BM, and CB samples in comparison with standard techniques by circumvention of loss of colony formation and by decreasing variability. Furthermore, the direct plating technique is a timesaving assay.

Enumeration of CD34+ cells in cord blood: a variation on a single-platform flow cytometric method based on the ISHAGE gating strategy

Single-platform flow cytometric absolute cell counting protocols provide increased robustness for CD34+ cell enumeration by limiting potential sources of imprecision. However, samples with any cellular fragmentation or debris, such as cord blood samples, provide challenges for these assays. We describe a simple, robust absolute CD34+ cell counting protocol, suitable for cord blood, using TRUCOUNT absolute count tubes (BD Biosciences, San Jose, CA) and a modified ISHAGE (International Society for Hematotherapy and Graft Engineering) gating strategy. An advantage of TRUCOUNT tubes is that each tube is supplied with a known number of lyophilized fluorescent beads. The method includes no-wash fixative-free ammonium chloride red blood cell lysis and the viability dye, 7-amino actinomycin D, to exclude dead cells. The threshold was set on CD45 expression in the FL1 channel and an exclusion gate in the forward scatter channel reduced debris. No manual adjustment of the gating regions was required, even for samples in less than optimal condition. Comparison of the TRUCOUNT-ISHAGE protocol with the original dual-platform ISHAGE assay (n = 30) and the single-platform ISHAGE protocol using Flow-Count Fluorospheres (Beckman Coulter, Fullerton, CA; n = 22) showed high correlation (R(2) = 0.949 and 0.989, respectively) and no significant difference or bias for samples ranging from 22 to 600 CD34+ cells per microliter. Results are presented that demonstrate the detrimental effect of a fixative-containing lysis reagent when used in a lyse-and-wash procedure. The TRUCOUNT-ISHAGE protocol combines the attributes of TRUCOUNT tubes and the ISHAGE gating strategy to provide a single-platform protocol capable of achieving readily standardization of CD34+ cell enumeration.

Cytofluorometric methods for assessing absolute numbers of cell subsets in blood. European Working Group on Clinical Cell Analysis

The enumeration of absolute levels of cells and their subsets in clinical samples is of primary importance in human immunodeficiency virus (HIV)+ individuals (CD4+ T- lymphocyte enumeration), in patients who are candidates for autotransplantation (CD34+ hematopoietic progenitor cells), and in evaluating leukoreduced blood products (residual white blood cells). These measurements share a number of technical options, namely, single- or multiple-color cell staining and logical gating strategies. These can be accomplished using single- or dual-platform counting technologies employing cytometric methods. Dual-platform counting technologies couple the percentage of positive cell subsets obtained by cytometry and the absolute cell count obtained by automated hematology analyzers to derive the absolute value of such subsets. Despite having many conceptual and technical limitations, this approach is traditionally considered as the reference method for absolute cell count enumeration. As a result, the development of single-platform technologies has recently attracted attention with several different technical approaches now being readily available. These single-platform approaches have less sources of variability. A number of reports clearly demonstrate that they provide better coefficients of variation (CVs) in multicenter studies and a lower chance to generate aberrant results. These methods are therefore candidates for the new gold standard for absolute cell assessments. The currently available technical options are discussed in this review together with the results of some cross-comparative studies. Each analytical system has its own specific requirements as far as the dispensing precision steps are concerned. The importance of precision reverse pipetting is emphasized. Issues still under development include the establishment of the critical error ranges, which are different in each test setting, and the applicability of simplified low-cost techniques to be used in countries with limited resources.

Antibiotic-labeled probes and microvolume fluorimetry for the rapid detection of bacterial contamination in platelet components: a preliminary report

BACKGROUND

Approximately 1 platelet in 2000 components is bacterially contaminated. Most commonly, contaminating organisms are gram positive skin saprophytes (such as Staphylococcus sp. or Bacillus sp.). A novel approach to the rapid diagnosis of gram positive contamination by the use of a fluorescence-labeled antibiotic probe with affinity for the gram positive cell was investigated.

STUDY DESIGN AND METHODS

Two isolates of Staphylococcus epidermidis were inoculated into bags of Day 0 platelets. Quantitative cultures along with a semi-automated screening assay on a microvolume fluorimeter employing a fluorescence-conjugated vancomycin probe was performed for each day of storage. In addition, serial dilutions of the bacteria were added to sterile platelets to achieve a range spanning 10(1) to 10(8) CFUs per mL.

RESULTS

All samples with a bacterial contamination of > or =10(5) CFU per mL were detected. Sterile samples were nonreactive. The entire procedure requires three pipetting steps and took less than 1 hour to perform.

CONCLUSION

These preliminary results with the use of fluorescence-labeled antibiotics as probes combined with microvolume fluorimetry for the rapid detection of bacterial contamination of platelet components suggest that this is a promising approach. Further studies with additional organisms and alternative conjugates, bacteria, and antibiotics are underway.

Semi-automated flow cytometric analysis of CD34-expressing hematopoietic cells in peripheral blood progenitor cell apheresis products

BACKGROUND

The measurement of CD34+ cells is the most important step in the quality control of peripheral blood progenitor cell apheresis products. For this purpose, flow cytometry is applied. Recently, a new test kit has been introduced for the enumeration of CD34-expressing cells, in combination with software support for semi-automation of data acquisition and analysis.

STUDY DESIGN AND METHODS

This study evaluated the ProCOUNT kit. Ninety samples obtained from peripheral blood progenitor cell apheresis products from 39 patients with hemato-oncologic diseases were analyzed. For data acquisition and analysis, ProCOUNT software was used. Data comparison was performed with parallel measurements according to the International Society for Hematotherapy and Graft Engineering (ISHAGE) guidelines and the German reference protocol for analysis of CD34-expressing cells.

RESULTS

Correlation of the German and ISHAGE techniques was excellent (r2 = 0.99). The initial correlation coefficient of ProCOUNT analysis with the German protocol was r2 = 0.89. In 21 (23.3%) of 90 ProCOUNT analyses, a warning message was encountered from the ProCOUNT software. Following manual reevaluation of these data with CellQUEST software, a correlation of r2 = 0.96 with the German protocol and r2 = 0.97 with the ISHAGE analyses was obtained. ANOVA testing revealed significant differences between ProCOUNT and ISHAGE techniques (p<0.05) and between ProCOUNT and the German protocol (p<0.05). No statistically significant difference between ISHAGE and German protocol was observed (p = 0.19).

CONCLUSION

The ProCOUNT kit and software for semi-automated data acquisition and analysis represents a further step toward standardization of CD34 cell quantitation in peripheral blood progenitor cell apheresis products. However, the occurrence of software warnings is high, and analysis or data reevaluation by experienced staff is still mandatory. Therefore, currently there is no definite advantage of the kit and software over the existing guidelines for CD34+ analysis in peripheral blood progenitor cell grafts.

Comparison of volumetric capillary cytometry with standard flow cytometry for routine enumeration of CD34+ cells

BACKGROUND

This study assesses the feasibility of a new volumetric cytometry system for the enumeration of CD34+ cells in apheresis components, peripheral blood, and cord blood samples in routine laboratory work. This system is compared with the following flow cytometry protocols: Milan, ISHAGE, ISHAGE with 7-AAD, and flow-count fluorospheres.

STUDY DESIGN AND METHODS

Correlation, linearity, and reproducibility studies were performed for the various methods. Clonogenic cultures were performed, as an external control, to assess the correlation between the number of CD34+ cells per microL and the number of colony-forming units per microL.

RESULTS

The linear regression analysis demonstrated that the five methods were comparable (R2 ranged from 0.86 to 0.96 and slopes were close to 1). The CD34+ assay and the flow-count methods showed poor linearity for CD34+ cell counts below 10 cells per microL (R2 = 0.46 and 0.47). The reproducibility assay for a CD34+ count of 10 cells per microL showed a CV of 12 percent and 25 percent for the Milan and CD34+ assay methods, respectively. The mean CV among all five methods for the 46 evaluated samples was 20 percent. There was a strong correlation between the number of CD34+ cells per microL and colony-forming units per microL in cord blood and apheresis samples (r = 0.71-0.81).

CONCLUSION

The CD34+ assay is useful in CD34 enumeration in cord blood, leukapheresis samples, and peripheral blood samples and provides comparable results to the Milan, ISHAGE, ISHAGE with 7-AAD, and flow-count methods. Nevertheless, peripheral blood samples with low CD34 absolute counts (below 10 cells/microL) should be analyzed by alternative flow cytometry protocols. Even though the same operator performed the study in a single laboratory, the high inter-method CV suggests that differences in sample preparation and gating strategy are factors that increase variability. Protocols with fewer intermediate steps or fully automated protocols such as the CD34+ assay are expected to reduced intra- and inter-laboratory variability.

Circulating CD34-expressing cells: German Proficiency Testing Survey

Determination of absolute numbers of CD34-expressing cells is critical in the setting of peripheral blood stem and progenitor cell transplantation/reinfusion. The diagnostic value of the parameter, CD34-expressing cells/microliter, has been validated. A survey of CD34-expressing cells has been integrated into a series of flow cytometry proficiency testing surveys (reticulocytes, lymphocytes, leukemia, and lymphoma) that we have established in Germany. Commercially available, modified, stabilized myeloblastic leukemia cells (KG1a cell line) spiked at different numbers into two normal blood samples were sent out, and report forms were returned from 50 of 58 participants. With a predicted percentage of CD34-expressing cells of 0.5% (sample A) and of 0.25% (sample B), the respective mean values analyzing data from 44 participants returning the completed forms were 0.49% (sample A) and 0.29% (sample B). The coefficients of variation were 57% and 83%, respectively. Engineered samples based on normal blood and on commercially available stabilized modified KG1a cells seem to be reliable material for external quality assessment surveys of CD34-expressing cells.