A simultaneous three-color T cell subsets analysis with single laser flow cytometers using T cell gating protocol. Comparison with conventional two-color immunophenotyping method

Laser Sources for Traditional and Spectral Flow Cytometry

Lasers for light scattering measurement and fluorescence excitation are essential components of all flow cytometers. Flow cytometers now typically rely on multiple laser wavelengths allowing excitation of a constantly increasing variety of fluorescent probes. The expanding use of spectral flow cytometry to increase the magnitude of multiparametric analysis is also changing the significance of laser choice in cytometry. In this chapter, we review the lasers available for flow cytometry and provide guidance in choosing laser wavelengths and characteristics to best match the needs of modern cell analysis by both conventional and spectral cytometry. We also discuss the recent advances in laser technology as the push to expand the palette of laser wavelength for cytometry continues.

Thirty-five years of CD4 T-cell counting in HIV infection: From flow cytometry in the lab to point-of-care testing in the field

CD4 T-cell counting was introduced in clinical laboratories shortly after the discovery of the human immune deficiency virus (HIV) in the early eighties. In western clinical laboratories, improvements in the CD4 T-cell counting methods were mainly driven by progress in the field of flow cytometry and immunology. In contrast, the development of dedicated CD4 T-cell counting technologies were needs driven. When antiretroviral treatment (ART) was made available on a large scale by international Acquired Immune Deficiency Syndrome (AIDS) relief programs to HIV+ patients living in low income countries in 2003, there was a distinct need for simplified and affordable CD4 T-cell counting technologies. The first decade of 2000, several compact flow cytometers appeared on the market, mainly to the benefit of low income countries with limited resources. More recently, however, portable point-of-care (POC) CD4 T-cell counting devices have been developed especially to improve access to affordable monitoring of HIV+ patients in low income countries. The accuracy of these POC instruments is not yet very well documented as many are still under development and clinical validation but preliminary evidence is encouraging. The new HIV treatment guidelines released by the World Health Organization in 2016 give CD4 T-cell counting a less central role in the management of HIV infection. It is, therefore, to be expected that CD4 T-cell counting will be phased out as a tool to assess eligibility of HIV+ patients for ART in the future. However, CD4 T-cell counting will remain a valuable tool for directing treatment against opportunistic infections. © 2016 International Clinical Cytometry Society.

A model for continuous quality control incorporating sample-to-sample assessment of optical alignment, fluorescence sensitivity, and volumetric operation of flow cytometers

BACKGROUND

Bead count rate (BCR) monitoring successfully identifies pipetting error during single platform CD4 enumeration. Despite rigorous prescribed quality control performed, preliminary data suggested that BCR outliers could also be attributed to occasional failure of flow cytometric volumetric operation. The aim of this report was to use counting beads in a model of continuous quality control (CQC) to monitor overall flow cytometric performance (laser alignment, fluorescence stability and volumetric operation).

METHODS

The proposed CQC model used FlowCheck and IMMUNOTROL blood controls daily. Extended monitoring of fluidics (FPV; beads and sheath only) and sample preparation (SPV; blood, IMMUNOPREP and beads) was done daily on five flow cytometers over five consecutive days prior to testing patient samples. Sample-to-sample CQC included monitoring BCR, selected time/fluorescence histograms (Time vs. Count; Time vs. Fluorescence and Forward Scatter vs. Fluorescence) and full peak coefficient of variation (FPCV) for 2000 samples tested.

RESULTS

Prescribed quality controls showed Half Peak CV values of <2% (FlowCheck) with Immunotrol within 0.5SD of the target means. Laser stability was confirmed (FPCV values <2%). However, fluidics (volumetric operation) fluctuated as indicated by a 3.2% BCR outlier rate of 2,000 samples tested (minus pipetting error) despite optimal fluidics performance verified at start-up (FPV CV < 3%).

CONCLUSIONS

Sustained laser stability was confirmed with Time vs. Fluorescence histograms, but Time vs. Count histograms were insufficient to detect intermittent volumetric failure. The proposed CQC model, incorporating BCR monitoring with time/fluorescence histograms and FPCV monitoring can identify all volumetric inconsistencies in real-time.

Fc receptor-like 3 protein expressed on IL-2 nonresponsive subset of human regulatory T cells

Fc receptor-like 3 (FCRL3) is a cell surface protein homologous to Fc receptors. The FCRL3 gene is present in humans but not in mice. We found that FCRL3 protein is expressed on 40% of human naturally occurring CD4(+) regulatory T (nTreg) cells (CD4(+)CD25(+)CD127(low)). Sorted nTreg cells with the surface phenotype FCRL3(+) and FCRL3(-) were both hypoproliferative to TCR stimulation and both suppressive on proliferation of conventional T cells (CD4(+)CD25(-)) in vitro. They both expressed forkhead box p3 (Foxp3) protein, the intracellular regulatory T cell marker. However, in contrast to FCRL3(-) nTreg cells, FCRL3(+) nTreg cells were not stimulated to proliferate by the addition of exogenous IL-2. In addition, Foxp3(+) cells induced from conventional T cells by TGF-beta treatment did not exhibit FCRL3 expression. These results suggest that the FCRL3(+) subset of human nTreg cells identified in this study arise in vivo and Foxp3 expression alone is not sufficient to induce FCRL3 expression. FCRL3 may be involved in human-specific mechanisms to control the generation of nTreg cells.

CD4 immunophenotyping in HIV infection

The ability to rapidly identify immune cell subsets such as CD4 cells, which became possible around the same time as the onset of the HIV/AIDS pandemic, was one of the greatest advances in clinical and diagnostic immunology. The evolution of this global pandemic and the subsequent development of treatment strategies to prolong the life of infected individuals mean that it is now more crucial than ever that we develop affordable, reliable and accurate methods for the enumeration of CD4 cells. Here, we provide an overview of the historical developments in CD4 enumeration technologies that are related to HIV infection, and summarize the current technological challenges that must be overcome to meet the needs of those living with HIV infection.

Principles of gating

Few aspects of flow cytometry are common to every application. Gating principles is one. This unit provides an excellent overview as well as detailed conceptual material on how gating works, when certain types of gating should or should not be applied, and the advantages and disadvantages of various techniques. This unit should be required reading for every technician using a flow cytometer and every graduate student starting out in the area, and is an excellent refresher for those more advanced in the technology. It brings out the historical development of our understanding of gating applications as well as providing a high level of technical accuracy.

African regional external quality assessment for CD4 T-cell enumeration: development, outcomes, and performance of laboratories

BACKGROUND

An independent African Regional External Quality Assessment Scheme (AFREQAS) was implemented from Johannesburg. The aim was to establish a network of CD4 laboratories supporting HIV/AIDS anti-retroviral therapy programs and improve the quality of regional CD4 testing with EQA assessment, feedback, remedial action, and technical training. The overall performance from 2002 to 2006 (Trials 1-20) is reported, together with cumulative longitudinal performance of the different CD4 methods used.

METHODS

Stabilized blood samples with "normal" and/or "low" CD4 values were shipped over 20 Trials. Data was analyzed for each trial including trimmed mean, standard deviation, and percentage coefficient of variation (%CV); "Residual" and SDI values were also calculated for each participating laboratory for both absolute CD4 counts (CD4abs) and CD4 percentage of lymphocytes values (CD4%/Ly). Standardized individual laboratory SDI values across 20 trials were analyzed according to CD4 method.

RESULTS

Average participation was 91.5%. Overall AFREQAS between-laboratory reproducibility (trimmed %CV) was 10.5% and 9.1% for absolute CD4 and CD4%/Ly, respectively. For the respective CD4abs and CD4%/Ly values in the trials where "normal" material was shipped trimmed %CV of 10.9 and 7.3% were noted, and in "low" value shipments %CV of 13.8% and 12.4% were noted. Cumulative absolute CD4 SDI analysis revealed the best between-laboratory precision amongst FACSCount and PanLeucogating (PLG-CD4) users (both SD of SDI = <1.2 and %CV of <<8%). Dual Platform or Single Platform algorithm-based systems and certain volumetric methods (laboratories who used Partec CyFlow instruments) had higher numbers of outlying laboratories (>12-25%CV and SD(SDI) > 2.2 noted), indicating that additional technical training and/or manufacturer support was required.

CONCLUSIONS

Participation in an AFREQAS with feedback and remedial action improves the quality of CD4 testing. African laboratory professionals can easily master CD4 counting technologies. However, the introduction of the simplest and most cost-effective methodologies is required to take ownership, and enable the delivery of quality CD4 counts in vast numbers necessary to support expansion of African ART programs.

Flow rate calibration. III. The use of stabilized biostandards to calibrate the flow rate and calculate absolute CD4+ T-cell counts

BACKGROUND

We have previously reported a flow rate calibration method for the determination of absolute CD4(+) T-lymphocyte counts that removes the need for the addition of latex beads to each sample. However, a limitation with this approach is that a calibration factor (CF) needs to be applied to adjust for differences in viscosity between latex bead suspensions and biological specimens. We have also demonstrated the value of using stabilized whole blood samples in external quality assessment (EQA) studies; such samples have a stable absolute lymphocyte count for over 1 year, at 4 degrees C. It was successfully demonstrated that this material can be used as a flow rate biocalibration (FRB) material for use as a flow cytometric control to provide a sample with a known CD4(+) T-lymphocyte count. Such material has advantages over latex bead technology as it can act as a full process control as well as having the same matrix and viscosity characteristics as the test material, thus removing the need for a CF.

METHODS

In this study, we have analyzed 268 consecutive normal, abnormal, and HIV(+) samples using FRB, incorporating the PanLeucoGating approach and compared this to the MultiSet method, defined as the predicate.

RESULTS

Percentage similarity statistics revealed the following: 0-3,000 CD4(+) cells/mul mean percentage difference (MPD; bias) 1.2%, 95% CI of 5.6-8%; 0-200 CD4(+) cells/microl MPD of 1.25%, 95% CI of 11.63-14.13%; 201-500 CD4(+) cells/microl MPD of 1%, 95% CI of 4.6-6.6%.

DISCUSSION

This study demonstrates that stabilized whole blood can be used for FRB. It has the advantage of being a full process control, in addition to costing less than latex beads with highly comparable results. As bench top flow cytometers are extremely stable, this is a low cost and robust alternative to bead based methods for generating absolute CD4 counts.

Reference values for peripheral blood lymphocyte phenotypes applicable to the healthy adult population in Switzerland

OBJECTIVES

Use of domestic reference values is known to improve the accuracy of flow cytometric analysis by integrating local variation due to race, gender, and age. In the absence of previously published estimates, we now report establishment of reference values for a wide range of peripheral blood lymphocyte phenotypes applicable to the healthy adult population in Switzerland and other regions with similar demographic characteristics.

METHODS

A representative sample population was recruited from among well characterized local blood donors (n = 70) and quantitative multiparametric flow cytometry used to estimate absolute and proportional values for a range of T-, B-, and NK-cell subsets, including those associated with activation and maturity. Distribution-free methods were then applied to generate 95% reference values and to estimate the significance of gender and age-related differences.

RESULTS

Reference values were obtained for the absolute and proportional levels of total CD3(+) T cells (536-1787 cells/microL, 54.90-84.03%), helper CD4(+) T cells (309-1139 cells/microL, 32.53-62.88%), cytotoxic CD8(+) T cells (137-823 cells/microL, 11.55-38.60%), activated CD3(+) T cells expressing CD25 (7-94 cells/microL, 0.50-5.95%), CD38 (102-554 cells/microL, 5.98-26.80%), HLA-DR (18-186 cells/microL, 1.25-9.68%) or CD38/HLA-DR (4-52 cells/microL, 0.30-2.30%), activated CD4(+) T cells expressing CD25 (7-52 cells/microL, 0.33-2.80%), CD38 (69-547 cells/microL, 6.13-32.20%), HLA-DR (11-55 cells/microL, 0.80-4.43%) or CD38/HLA-DR (4-22 cells/microL, 0.30-1.35%), activated CD8(+) T cells expressing CD25 (0-12 cells/microL, 0.00-0.69%), CD38 (13-124 cells/microL, 0.93-7.03%), HLA-DR (6-108 cells/microL, 0.33-6.38%) or CD38/HLA-DR (2-47 cells/microL, 0.13-2.68%), naive CD4(+) T cells expressing CD45RA(+)/CD62L(+) (84-761 cells/microL, 9.48-41.88%), naive CD8(+) T cells expressing CD45RA(+)/CD62L(+) (42-360 cells/microL, 3.68-19.23%), memory CD4(+) T cells expressing CD45RO(+) (247-807 cells/microL, 16.50-42.15%), memory CD8(+) T cells expressing CD45RO(+) (72-377 cells/microL, 3.78-22.80%), B-cells expressing CD19 (72-460 cells/microL, 4.70-19.13%) or CD20 (66-529 cells/microL, 4.63-21.00%), total CD3(-)/(CD16(+)/CD56(+)) NK-cells (77-427 cells/microL, 5.35-30.93%), and activated NK-cells expressing CD25 (0-10 cells/microL, 0-0.50%) or HLA-DR (3-99 cells/microL, 0.20-7.28%).

CONCLUSION

It is anticipated that availability of localized reference values for an extended range of peripheral blood lymphocyte phenotypes should supplement previously published reference values and enhance the utility of flow cytometric analysis undertaken in Switzerland.

Flow rate calibration II: a clinical evaluation study using PanLeucoGating as a single-platform protocol

BACKGROUND

CD4(+) T-lymphocyte enumeration is vital for monitoring disease progression in individuals positive for the human immunodeficiency virus (HIV), and as a result, there is a need to develop cost-effective protocols that provide accuracy, precision, and affordability. Recently, PanLeucoGating has been shown to fulfill these requirements; however, although comparable to state-of-the-art single-platform protocols (SP), there is still a requirement for an accurate total white cell count. To overcome this limitation, we recently developed a flow-rate based calibration method that enables the PanLeucoGating protocol to be used as a SP approach, and in this study show that this approach can be used for CD4(+) T-lymphocyte enumeration.

METHODS

A total of 113 HIV samples were analyzed using three protocols: (a) state-of-the art SP bead-based method (MultiSet; predicate protocol), (b) PanLeucoGating protocol used as a dual-platform (DP) approach, and (c) the newly developed flow rate-based SP approach. We demonstrate that flow rate calibration can be achieved easily and that the method is highly comparable to the state-of-the-art SP method.

RESULTS

A high correlation was observed between the predicate protocol and the SP PanLeucoGating approach over the whole range of CD4 counts tested (r(2) = 0.9928; bias 8 cells/microl), including the clinically relevant range (e.g., 0-200 CD4 cells/microl; bias 0 cells/microl). For batched samples, the cost of providing a CD4(+) T-lymphocyte count was reduced to approximately US $1.

CONCLUSIONS

The SP PanLeucoGating is a cost-effective approach to CD4(+) T-lymphocyte enumeration that maintains accuracy and precision.

Affordable CD4(+)-T-cell counting by flow cytometry: CD45 gating for volumetric analysis

The flow cytometers that are currently supported by industry provide accurate CD4(+)-T-cell counts for monitoring human immunodeficiency virus disease but remain unaffordable for routine service work under resource-poor conditions. We therefore combined volumetric flow cytometry (measuring absolute lymphocyte counts in unit volumes of blood) and simpler protocols with generic monoclonal antibodies (MAbs) to increase cost efficiency. Volumetric absolute counts were generated using CD45/CD4 and CD45/CD8 MAb combinations in two parallel tubes. The percentage values for the various subsets were also determined within the leukocyte and lymphocyte populations utilizing a fully automated protocol. The levels of agreement between the newly developed method and the present industry standards, including both volumetric and bead-based systems using a full MAb panel for subset analysis, were tested by Bland-Altman analyses. The limits of agreement for CD4 counts generated by the volumetric methods using either CD45/CD4 (in a single tube) or the full Trio MAb panel (in three tubes) on the CytoronAbsolute flow cytometer were between -29 and +46 cells/mm(3) with very little bias for CD4 counts (in favor of the Trio method: +8 CD4(+) lymphocytes/mm(3); 0.38% of lymphocytes). The limits of agreement for absolute CD4 counts yielded by the volumetric CD45/CD4 method and the bead-based method were between -118 and +98 cells/mm(3), again with a negligible bias (-10 CD4(+) lymphocytes/mm(3)). In the volumetric method using CD45/CD8, the strongly CD8(+) cells were gated and the levels of agreement with the full Trio showed a minor bias (in favor of the Trio; +40 CD8(+) cells/mm(3); 5.2% of lymphocytes) without a significant influence on CD4/CD8 ratios. One trained flow cytometrist was able to process 300 to 400 stained tubes per day. This workload extrapolates to a throughput of >30,000 samples per year if both CD45/CD4 and CD45/CD8 stainings are performed for each patient or a throughput of >60,000 samples if only CD45/CD4 counts are tested in a single tube. Thus, on the basis of the high efficiency and excellent agreement with the present industry standards, volumetric flow cytometers with automated gating protocols and autobiosamplers, complemented by generic CD45, CD4, and CD8 MAbs used in two-color immunofluorescence, represent the most suitable arrangements for large regional laboratories in resource-poor settings.

Reduction of variation in T-cell subset enumeration among 55 laboratories using single-platform, three or four-color flow cytometry based on CD45 and SSC-based gating of lymphocytes

BACKGROUND

Enumeration of CD4(+) and CD8(+) T-cell subsets provides relevant information for diagnosis and monitoring of patients with cellular immunodeficiencies. As a result, an external quality assurance scheme was implemented in Belgium, The Netherlands, and Luxembourg in 1995. A workshop was held to train the participants in state-of-the art technology for assessment of absolute T-cell subset counts (i.e., a three or four-color, single-platform assay with lymphocyte gating based on CD45 and sideward light scatter) with the aim to achieve between-site coefficients of variation (CVs) <10% and within-site CVs <5% for > or =75% of the participants.

METHODS

Three send-outs of stabilized blood from a healthy donor were distributed to 55 laboratories, each with the request to perform the standard assay on three occasions. For comparison, each laboratory performed its local technique in parallel.

RESULTS

With the standard technique, between-site CVs of approximately 8% (CD3+ T cells), approximately 9% (CD4+ T cells), and approximately 10% (CD8+ T cells) were achieved. Within-site CVs were <5% for 82% (CD3+ T cells) and approximately 70% (CD4+ and CD8+ subsets) of the participants. Local techniques yielded between-site CVs of 13%-17% for CD3+, CD4+, and CD8+ T cells.

CONCLUSIONS

The state-of-the-art technology for T-cell subset enumeration was implemented successfully among 55 Belgian-Dutch laboratories and resulted in significant reductions of between-site variation of absolute CD3+, CD4+, and CD8+ T-cell counts.

T-cell subset counting and the fight against AIDS: reflections over a 20-year struggle

The story of T-lymphocyte subset immunophenotyping technology is reviewed on the occasion of the 20th anniversary of CD4 T-cell enumeration. Over time, immunophenotyping has evolved into precise, reliable, but complicated and expensive technology requiring fresh blood samples. The gating technologies that were universally adapted for clinical flow cytometry for the past decade relied on rapidly deteriorating morphological scatter characteristics of leukocytes. This special issue dedicated to CD4 T-cell enumeration features most of the available new options that will have a significant impact on how this technology will be implemented within the first decade of the 21st century. In a series of original publications, including the new NIH guideline for T-cell subset enumeration, contemporary gating protocols that use immunologically logical parameters are presented as part of the more reliable and affordable immunophenotyping alternative. Some of the improvements addressed here include the costs of the assays and the capacity to monitor interlaboratory and intralaboratory performances. It is clear that an effective attack on the human immunodeficiency virus (HIV) epidemic has to embrace resource-poor regions. Reducing the cost of the assay while improving reliability and durability is a move in the right direction.

Selection of lymphocyte gating protocol has an impact on the level of reliability of T-cell subsets in aging specimens

BACKGROUND

In the past decade, human immunodeficiency virus (HIV) lymphocyte immunophenotyping has evolved significantly. New fluorochromes, new multicolor reagents, enhanced instruments, and the capacity to provide absolute cell counts using the single-platform technique have all contributed to the reliability of T-cell subset measurements. In this study, four gating protocols were evaluated to select the most robust method for T-cell subset enumeration.

METHODS

Peripheral blood specimens from 21 HIV(+) and 20 HIV(-) individuals were monitored up to 96 h. Aliquots of specimens were stored at room temperature and analyzed at 6 (baseline), 48, 72, and 96 h. Aliquots were stained with CD45-fluorescein isothiocyanate (FITC)/CD3PC5/CD4RD1/CD8ECD. Data analysis was performed with all four gating protocols.

RESULTS

Only with fresh blood did all protocols provide similar results. From samples that were 48 h old, the choice of gating strategy had a dramatic impact on immunophenotyping results. The largest deviations from baseline values occurred at 96 h and gating protocols that included dual light scatter gates provided the greatest shift of T-cell subset values over time. The gating protocols that were based exclusively on cell lineage-specific gates gave the most robust T-cell values up to 96 h.

CONCLUSION

By selecting the appropriate gating protocol, the temporal integrity of specimens can be extended up to 4 days.

CD45-assisted PanLeucogating for accurate, cost-effective dual-platform CD4+ T-cell enumeration

BACKGROUND

North American and European guidelines for dual-platform (DP) flow cytometry recommend absolute CD4 T-cell counts to be calculated from two parameters: the absolute lymphocyte counts obtained on a hematology analyzer and the percentages of CD4+ cells among lymphocytes (CD4%/lympho) obtained by flow cytometry. Nevertheless, the identification of lymphocytes is error-prone: a poor match between these common denominators in the two systems is the main source of inaccuracy. In contrast, total leucocyte counts (white cell counts [WCC]) and CD4% among the gated CD45+ leucocytes (CD4%/leuco) can be determined with greater accuracy.

METHODS

We introduced "PanLeucogating," i.e., we used total leucocytes as the common denominator for improving the precision of DP absolute CD4 counting. Correlations and Bland-Altman tests were used for statistical analysis.

RESULTS

First, 22 stabilized blood product samples were provided by U.K. National External Quality Assessment Scheme (NEQAS) and a higher accuracy and precision of CD4 counts were documented using PanLeucogating compared with lymphocyte gating. Next, 183 fresh and 112 fixed (TransFix) whole blood samples were used to compare DP methods and single-platform (SP) methodology, including both volumetric and bead-based techniques. A particularly high correlation and comparable precision of absolute CD4 counts were observed between the SP volumetric method and DP PanLeucogating (R(2) = 0.990; bias 6 +/- SD 17%). The SP volumetric method showed lower levels of agreement with the DP lymphocyte gating (R(2) = 0.758; bias 14 +/- SD 51%) and with the SP bead-based method (R(2) = 0.923; bias 4 +/-SD 31%).

CONCLUSIONS

These observations show that DP leucocyte counts (WCC) should replace lymphocyte counts as the "common denominator" although CD4%/lympho values can, as an extra step, be also provided readily if requested. When coupled with quality control for WCC on hematology analyzers, the DP method with CD45 PanLeucogating represents a robust CD4 T-cell assay that is as accurate as the SP volumetric technique. This DP method uses only two, CD45 and CD4, antibody reagents and can be run on any pair of hematological analyzer plus flow cytometer.

Quality control of CD4+ T-lymphocyte enumeration: results from the last 9 years of the United Kingdom National External Quality Assessment Scheme for Immune Monitoring (1993-2001)

The human immunodeficiency virus (HIV) global epidemic has necessitated the routine enumeration of T-lymphocyte subsets, which has created a need for external quality assurance (EQA). The United Kingdom National External Quality Assessment Scheme (UK NEQAS) for Immune Monitoring provides EQA for 296 laboratories in 40 countries. In 1993, UK NEQAS developed and incorporated into its program stabilized whole blood that enables the accurate monitoring of laboratory performance. Overall, the mean interlaboratory coefficient of variation (CV) for percentage CD4(+) T-lymphocyte subset enumeration has fallen from 15% to less than 5%, as a direct result of the increased use of CD45/ side scatter (SSC) gating. Laboratories using alternative gating strategies (i.e., CD45/CD14 or forward scatter [FSC]/SSC) were about 7.4 times more likely to fail an EQA exercise. Furthermore, the adoption of single-platform technology resulted in a reduction of the overall mean interlaboratory CV for absolute CD4(+) T lymphocytes from 56% (prior to the widespread use of single-platform technology) to 9.7%. Individual laboratory deficiencies were also identified using a performance monitoring system and, through re-education by collaboration with the coordinating center, satisfactorily resolved. In conclusion, during the last 9 years, the UK NEQAS for Immune Monitoring program has highlighted the significant technological advances made by laboratories worldwide that undertake lymphocyte subset enumeration.

BD Biosciences contributions in CD4 counting and immune status for HIV/AIDS

BD Biosciences is a leader in the use of flow cytometry for determining immune system status and for counting CD4 cells in patients with human immunodeficiency virus (HIV) infection. The company has gained this position through many years of basic research and product development in immunology and cell biology, dye chemistry, immunoassays, instrumentation, and software. Some of the highlights of these developments and their historical perspective are described in this review.

Affordable CD4+ T cell counts by flow cytometry. II. The use of fixed whole blood in resource-poor settings

We tested the feasibility and precision of affordable CD4+ T cell counting in resource-poor settings using a recently standardised fixative, TransFix in whole blood (WB) by flow cytometry (FCM). The precision of the assays was established under optimal conditions for single-platform FCM such as the volumetric CytoronAbsolute and the bead-based FACSCan. Fresh WB samples from HIV-seropositive and seronegative patients were tested in Tanzania and South Africa, fixed and sent to the UK for reanalysis 7 days later. Correlation, bias and limits of agreements were analysed by linear regression and the Bland-Altman test. Absolute CD4+ T cell counts remained stable for at least 10 days when TransFix was added to WB in 1:10 dilution at 20-25 degrees C, and for 7 days when added in 1:10 or 1:5 dilution to samples stored to mimic 'tropical' conditions at 37 degrees C. Higher temperatures such as 42 degrees C were tolerated for only short periods since the recovery had decreased to 63% by day 3. The reproducibility of lymphocyte subset analysis remained unchanged by TransFix with coefficient of variations <6% for all T cell subsets. Absolute CD4+ T cell counts and CD4+ T cell % values on fixed samples in the UK showed a high correlation with the results using fresh samples in Tanzania (r=0.993 and 0.969, respectively) and with the samples handled in Johannesburg (r=0.991 and 0.981) with minimal bias. Primary CD4 gating using only a single CD4 antibody also remained accurate in TransFixed samples (r=0.999). Thus, TransFix permits optimal fixation and transport of WB samples in the developing world for FCM to local regional laboratories and for quality assurance in international centres. When used together with inexpensive primary CD4 gating, TransFix will allow reliable and affordable CD4+ T cell counting by FCM in resource-poor settings.

Multiparameter data acquisition and analysis of leukocytes by flow cytometry

Each laboratory has to establish its own experience base and standard operating procedures. The intent of this discussion has been to illustrate the procedures that will lead to good flow cytometry data acquisition and analysis and to illustrate problematic areas. The most important rule of all is to recognize when there is a problem and find the correct solution. It is hoped the information provided herein will be of help in the recognition process.

Affordable CD4(+) T-cell counts on 'single-platform' flow cytometers I. Primary CD4 gating

Here, we demonstrate the flow cytometric concept of 'primary CD4 gating' utilizing three different CD4 monoclonal antibodies (mAbs) conjugated with five different fluorochromes. CD4(+) lymphocytes were defined by an autogate in a single histogram of CD4 fluorescence intensity (FI) (y-axis) vs. side light scatter (x-axis). A wide range of absolute counts for > 600 individuals, including HIV(+) patients, were compared with those obtained by 'state-of-the-art' single-platform flow cytometers such as the volumetric Ortho CytoronAbsolute and the Becton Dickinson FACSCalibur using TruCount beads. The correlation between CD4 counts obtained with primary CD4 gating and the full test panel on the Ortho Cytoron was excellent (R(2) = 0.999). Bland-Altman statistics showed a mean difference of -2 cells/mm(3) [confidence interval (CI) 95% = -3 to -1; limits of agreement -27 to +23]. In addition to absolute CD4 counts, CD4% values and CD4/CD8 ratios are also frequently requested. To obtain these, lymphocytes need to be counted using scatter gates, and a second tube stained with a CD8 mAb to count CD8(++) lymphocytes can be incorporated. We conclude that primary CD4 gating on single-platform volumetric flow cytometers is one of the most economical and flexible technologies for routine cost-conscious service work, particularly during the follow-up of patients undergoing anti-HIV therapy and/or vaccination in the developing world.

Flow cytometric analysis of intracellular IFN-gamma, IL-4 and IL-10 in CD3(+)4(+) T-cells from rat spleen

The application of multi-parameter flow cytometry for the assessment of T-cell and cytokine functioning has been used by several groups for studying human and mouse samples, although little has been reported for the rat. Here we report the optimisation of immunofluorescent staining for cell surface and intracellular antigens using three-colour flow cytometric analysis to measure the frequency of rat CD3(+)4(+) T-cells that produce IFN-gamma, IL-4 and IL-10. In vitro stimulation of IFN-gamma production required incubation of splenocytes with PMA and ionomycin in the presence of the protein transport inhibitor brefeldin A for 6 h. Three stimulation protocols for IL-4 and IL-10 production were evaluated. In vitro priming of splenic T-cells with antibodies against CD3 and CD28 and recombinant cytokines (IL-2 and IL-4) for 5 days followed by restimulation with PMA and ionomycin was required to stimulate cells to produce either IL-4 or IL-10. Brefeldin A was found to be a more suitable protein transport inhibitor than monensin. This method will be useful for analysing the nature of individual rat cytokine-producing cells in a variety of experimental model systems.

Flow cytometric assessment of LDL receptor activity in peripheral blood mononuclear cells compared to gene mutation detection in diagnosis of heterozygous familial hypercholesterolemia

BACKGROUND

Studies indicate that human peripheral blood mononuclear cells mirror low-density lipoprotein (LDL) receptor activity of other cells in the body. To measure LDL receptor activity in patients with heterozygous familial hypercholesterolemia (FH), we prepared peripheral blood mononuclear cells from individuals with molecularly verified LDL receptor defective (Trp66-Gly mutation, n = 18) or receptor negative (Trp23-stop mutation, n = 17) heterozygous FH and from healthy individuals (n = 24).

METHODS

The cells were stimulated to express maximum LDL receptor by preincubation in lipoprotein-free medium. They were then incubated at 4 degrees or 37 degrees C with fluorescently conjugated LDL (DiI-LDL). T-lymphocytes and monocytes were identified by fluorescently conjugated monoclonal antibodies. DiI-LDL bound (at 4 degrees C) or internalized (at 37 degrees C) by the cells was measured using flow cytometry. Knowing the LDL receptor gene mutation of the FH patients allowed us to compare the diagnostic capability of our functional assay with the DNA diagnosis.

RESULTS

The diagnostic accuracy did not allow our assay to be used for diagnosis of individual cases of heterozygous FH.

CONCLUSIONS

We suggest that our two-color fluorescence flow cytometry assay can be used to characterize functionally gene mutations causing LDL receptor dysfunction in patients with heterozygous FH.

Phenotypic analysis of receptor-ligand pairs on B-cells in B-chronic lymphocytic leukemia

Whole-blood three-color immunofluorescence analysis was used to investigate the role of CD5/CD72 and CD21/CD23 receptor-ligand pair formation on B-chronic lymphocytic leukemia (B-CLL) cells as well as sCD23 and bcl-2 oncoprotein expression in disease progression and activity and total tumor mass in B-cell chronic leukemia (B-CLL) patients. Thirty-four patients with B-CLL and 19 controls were included in the study. The majority of B-cells in B-CLL patients coexpressed CD5 and CD72 as well as the CD23 antigen. Unlike B-cells in B-CLL patients, B-cells in all healthy controls tested had high expression of CD21 antigen. We identified two groups of B-CLL patients according to high (n = 20) or low levels (n = 14) of CD21 expression on CD19+CD23+ B-cells. Only in the patients with high CD21 expression, were sCD23 levels positively correlated with factors known to have prognostic significance in B-CLL (Rai stage and TTM) and could, therefore, be used as a prognostic parameter for these B-CLL patients. Bcl-2 oncoprotein expression did not differ between these patient groups. We presumed that in patients with a lower expression of CD21 antigen, the contribution of the CD21 molecule to homotypic adhesion was lacking. Further studies are necessary to determine the possible association of higher expression of the CD21 antigen with disease progression and the aggressive character of the B-CLL.

Cytokines and cell surface markers in prediction of cardiac allograft rejection

Endomyocardial biopsy is generally used to quantify heart allograft rejection and guide immunotherapy. Biopsy, however, is invasive, costly, and risky. Since rejection requires lymphocyte activation, the purpose of this study was to assess alternative methods to evaluate rejection dynamics by investigating serum levels of cytokines and cell surface markers after heart transplantation. Interleukin-2-receptor bearing CD4+T (IL-2R/CD4) cell levels were higher in the peripheral blood of human transplant recipients with rejection grade 2 (p < 0.02). HLA-DR/CD3 levels were somewhat higher in rejection grade 2. There was no correlation between biopsy scores and serum levels of tumor necrosis factor (TNF-alpha), IL-2, or percentage of T cell, NK cell, B cell, CD4+T cell, CD8+T cell, HLA-DR/CD4, HLA-DR/CD8, IL-2R/CD3, IL-2R/CD8. Interleukin-1 (IL-1 beta) was not detectable in all of the samples. The current studies suggest that monitoring lymphocyte IL-2R/CD4 and HLA-DR/CD3 levels is useful in predicting cardiac transplant rejection.

Determination of the antibody binding capacity of lymphocyte membrane antigens by flow cytometry in 58 blood donors

The relative density of lymphocyte CD3, CD4, CD5, CD8, CD20, CD23, CD28, CD38, CD45RA, CD45RO, CD57 and HLA-DR antigens was measured as antibody binding capacity (ABC) in 58 blood donors aged 19-66 years. The group was analysed in order to obtain reference values (percentages and absolute numbers) for routine, quantitative three-colour flow cytometry (FC) tests, and we included around ten males and females for each of the 15 year age intervals. Whole blood was stained (30 min on ice) with FITC, PE or PerCP conjugated MAbs. The analysis was performed with a FACScan equipped with LYSYS II and Paint-a-GatePlus software. The instrument was calibrated daily with QC3, QuickCal (FITC and PE) and Calibrite and monthly with QSC and stained cells (which included also the control for PerCP performance). The ABC was measured with QSC (Flow Cytometry Standards Corporation). The CD4+ lymphocytes expressed significantly more CD3, CD28 and HLA-DR antigens, and less CD45RA antigen than the CD8+ cells (p < 0.0001). A significant decrease with age was observed for CD3 and CD45RA on both CD4+ and CD8+ subsets (p < 0.05). The lymphocytes of women, compared with those of men, showed decreased ABC for CD8, CD20 and CD28 antigens. The results illustrate the necessity for close matching of control with case groups. They also illustrate the possibilities of modern FC methods based on quantitative quality control and three-colour analysis.

Immunologic differentiation of absolute lymphocyte count with an integrated flow cytometric system: a new concept for absolute T cell subset determinations

We describe a method to obtain results for immune status monitoring that uses a three-test panel, comprised of isotype control and 2 specific Mab tests (CD4/CD8/CD3 and CD16/CD19/CD3), in conjunction with a flow cytometer that directly measures absolute counts. Automated software is used for lineage-specific gating of three-color immunofluorescence to determine lymphocyte and lymphocyte subset counts. The autogating function of this software is shown to yield equivalent results to manual analysis by an expert user, and to be effective when as few as 25 target cells are present. The software is also shown to perform automatic quality control checks of the sample preparation, reagent, and automated analysis. We demonstrate that the sum of T (CD3+), B (CD19+), and natural killer (NK, CD16 + CD3-) cells, as a determination of all lymphocytes, correlates well with lymphocytes measured using a light scatter differential. Moreover, T + B + NK lymphocyte count is shown to be less error-prone than lymphocyte count from light scatter differential, and to minimize errors that arise from between-technician variation in sample preparation. Our data suggest that the new approach that we describe could offer an alternative to the traditional two-stage methods for measuring absolute counts of lymphocyte subsets for immune status monitoring. As such this method could reduce, through objective automated analysis, testing cost and complexity, without sacrificing the quality of results.

Three-colour flow cytometric immunophenotyping in HIV-patients; comparison to dual-colour protocols

Flow cytometric measurement of circulating CD4+ lymphocytes is important in the evaluation of disease progression in HIV-infected patients. Development of dyes that can be exited at 488 nm and have emission maximum in the far red area has made three-colour protocols, together with fluorescein isothiocyanate (FITC) and R-phycoerythrin (PE), possible in most clinical flow cytometers. We report here the comparison of a two-tube, three-colour protocol (including CD45/CD4/ CD3 and CD8/CD4/CD3) with our conventional dual-colour protocol. No significant differences were found between percentage of CD3+ lymphocytic cells determined with three different antibody combinations. When the CD8/CD4/CD3 combination was used a systematic overestimation of CD3+ CD4+% cells was found. This turned out to be caused by the formation of 'CD8-escapees'. These are clumps of CD8+ cells that fall outside the lymphocyte gating region, principally because of high side scatter. The problem can be overcome by rigorous vortexing to loosen aggregates. The lymphocyte gating principle used in this protocol (gating on a side scatter/CD45 dot plot) is readily applicable to other antibody combinations. This was demonstrated by measuring CD5+ B lymphocytes, a subset receiving increasing attention in the study of HIV-induced immune deviations. We conclude that our three-colour protocol for CD4+ T-lymphocyte determinations offers significant advantages to the conventional dual-colour method, and we suggest that when possible anti-CD45 be added to dual-colour combinations in order to improve lymphocyte gating.

Multiparameter analysis of leukocytes by flow cytometry

The flow cytometry described can be performed using a single laser. Each laboratory has to establish its own experience base and standard operating procedures. The intent of this discussion has been to illustrate the procedures that will lead to good flow cytometry data acquisition and analysis and to illustrate problematic areas. The most important rule of all is to recognize when there is a problem. It is hoped the information provided herein will be of help in the recognition process.