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Flow cytometry
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  Flow Cytometry: Principles and ClinicalApplications in Hematology Michael Brown  and  Carl Wittwer * The use of flow cytometry in the clinical laboratory hasgrown substantially in the past decade. This is attribut-able in part to the development of smaller, user-friendly, less-expensive instruments and a continuousincrease in the number of clinical applications. Flowcytometry measures multiple characteristics of individ-ual particles flowing in single file in a stream of fluid.Light scattering at different angles can distinguish dif-ferences in size and internal complexity, whereas lightemitted from fluorescently labeled antibodies can iden-tify a wide array of cell surface and cytoplasmic anti-gens. This approach makes flow cytometry a powerfultool for detailed analysis of complex populations in ashort period of time. This report reviews the generalprinciples in flow cytometry and selected applicationsof flow cytometry in the clinical hematology laboratory. © 2000 American Association for Clinical Chemistry Flow cytometry provides rapid analysis of multiple char-acteristics of single cells. The information obtained is bothqualitative and quantitative. Whereas in the past flowcytometers were found only in larger academic centers,advances in technology now make it possible for commu-nity hospitals to use this methodology. Contemporaryflow cytometers are much smaller, less expensive, moreuser-friendly, and well suited for high-volume operation.Flow cytometry is used for immunophenotyping of avariety of specimens, including whole blood, bone mar-row, serous cavity fluids, cerebrospinal fluid, urine, andsolid tissues. Characteristics that can be measured includecell size, cytoplasmic complexity, DNA or RNA content,and a wide range of membrane-bound and intracellularproteins. This review will describe the basic principles of flow cytometry and provide an overview of some appli-cations to hematology. General Principles Flow cytometry measures optical and fluorescence char-acteristics of single cells (or any other particle, includingnuclei, microorganisms, chromosome preparations, andlatex beads). Physical properties, such as size (represented by forward angle light scatter) and internal complexity(represented by right-angle scatter) can resolve certain cellpopulations. Fluorescent dyes may bind or intercalatewith different cellular components such as DNA or RNA.Additionally, antibodies conjugated to fluorescent dyescan bind specific proteins on cell membranes or insidecells. When labeled cells are passed by a light source, thefluorescent molecules are excited to a higher energy state.Upon returning to their resting states, the fluorochromesemit light energy at higher wavelengths. The use of multiple fluorochromes, each with similar excitationwavelengths and different emission wavelengths (or “col-ors”), allows several cell properties to be measured simul-taneously. Commonly used dyes include propidium io-dide, phycoerythrin, and fluorescein, although manyother dyes are available. Tandem dyes with internalfluorescence resonance energy transfer can create evenlonger wavelengths and more colors. Table 1 lists clinicalapplications and cellular characteristics that are com-monly measured. Several excellent texts and reviews areavailable  (1–6) .Inside a flow cytometer, cells in suspension are drawninto a stream created by a surrounding sheath of isotonicfluid that creates laminar flow, allowing the cells to passindividually through an interrogation point. At the inter-rogation point, a beam of monochromatic light, usuallyfrom a laser, intersects the cells. Emitted light is given off in all directions and is collected via optics that direct thelight to a series of filters and dichroic mirrors that isolateparticular wavelength bands. The light signals are de-tected by photomultiplier tubes and digitized for com-puter analysis. Fig. 1 is a schematic diagram of the fluidicand optical components of a flow cytometer. The resultinginformation usually is displayed in histogram or two-dimensional dot-plot formats. Department of Pathology, University of Utah, ARUP Laboratories, Inc.,Salt Lake City, UT 84132.*Address correspondence to this author at: Department of Pathology,University of Utah, 50 North Medical Dr., Salt Lake City, UT 84132. E-mailcarl_wittwer@hlthsci.med.utah.edu.Received February 15, 2000; accepted April 10, 2000. Clinical Chemistry  46:8(B)1221–1229 (2000) Beckman Conference 1221  DNA Content Analysis The measurement of cellular DNA content by flow cytom-etry uses fluorescent dyes, such as propidium iodide, thatintercalate into the DNA helical structure. The fluorescentsignal is directly proportional to the amount of DNA in thenucleus and can identify gross gains or losses in DNA.Abnormal DNA content, also known as “DNA contentaneuploidy”, can be determined in a tumor cell population.DNA aneuploidy generally is associated with malignancy;however, certain benign conditions may appear aneuploid (7–12) . DNA aneuploidy correlates with a worse prognosisin many types of cancer but is associated with improvedsurvival in rhabdomyosarcoma, neuroblastoma, multiplemyeloma, and childhood acute lymphoblastic leukemia(ALL) 1 (11 ,  13–16) . In multiple myeloma, ALL, and myelo-dysplastic syndromes, hypodiploid tumors cells portend apoor prognosis. In contrast, hyperdiploid cells in ALL havea better prognosis  (11 ,  13) . For many hematologic malignan-cies, there are conflicting reports regarding the independentprognostic value of DNA content analysis. Although con- 1 Nonstandard abbreviations: ALL, acute lymphoblastic leukemia; PNH,paroxysmal nocturnal hemoglobinuria; and RBC, red blood cell. Table 1. Common clinical uses of flow cytometry. Field Clinical application Common characteristic measured Immunology Histocompatibility cross-matching IgG, IgMTransplantation rejection CD3, circulating OKT3HLA-B27 detection HLA-B27Immunodeficiency studies CD4, CD8Oncology DNA content and S phase of tumors DNAMeasurement of proliferation markers Ki-67, PCNA a  Hematology Leukemia and lymphoma phenotyping Leukocyte surface antigensIdentification of prognostically important subgroups TdT, MPOHematopoietic progenitor cell enumeration CD34Diagnosis of systemic mastocytosis CD25, CD69Reticulocyte enumeration RNAAutoimmune and alloimmune disordersAnti-platelet antibodies IgG, IgMAnti-neutrophil antibodies IgGImmune complexes Complement, IgGFeto-maternal hemorrhage quantification Hemoglobin F, rhesus DBlood banking Immunohematology Erythrocyte surface antigensAssessment of leukocyte contamination of blood products Forward and side scatter, leukocytesurface antigensGenetic disorders PNH CD55, CD59Leukocyte adhesion deficiency CD11/CD18 complex a  PCNA, proliferating cell nuclear antigen; TdT, terminal deoxynucleotidyltransferase; MPO, myeloperoxidase. Fig. 1. Schematic of a flow cytometer. A single cell suspension is hydrodynamically focused withsheath fluid to intersect an argon-ion laser. Signals arecollected by a forward angle light scatter detector, a side-scatter detector ( 1 ), and multiple fluorescence emissiondetectors ( 2–4  ). The signals are amplified and converted todigital form for analysis and display on a computer screen. 1222  Brown and Wittwer: Flow Cytometry  ventional cytogenetics can detect smaller DNA content dif-ferences, flow cytometry allows more rapid analysis of alarger number of cells. Immunophenotyping Applications in Hematology  The distributed nature of the hematopoietic system makesit amenable to flow cytometric analysis. Many surfaceproteins and glycoproteins on erythrocytes, leukocytes,and platelets have been studied in great detail. Theavailability of monoclonal antibodies directed againstthese surface proteins permits flow cytometric analysis of erythrocytes, leukocytes, and platelets. Antibodies againstintracellular proteins such as myeloperoxidase and termi-nal deoxynucleotidyl transferase are also commerciallyavailable and permit analysis of an increasing number of intracellular markers. erythrocyte analysis The use of flow cytometry for the detection and quantifi-cation of fetal red cells in maternal blood has increased inrecent years. Currently in the United States, rhesus D-negative women receive prophylactic Rh-immune globu-lin at 28 weeks and also within 72 h of delivery  (17) . Thestandard single dose is enough to prevent alloimmuniza-tion from   15 mL of fetal rhesus D   red cells. If feto-maternal hemorrhage is suspected, the mother’s blood istested for the presence and quantity of fetal red cells, andan appropriate amount of Rh-immune globulin is admin-istered. The quantitative test most frequently used inclinical laboratories is the Kleihauer-Betke acid-elutiontest. This test is fraught with interobserver and interlabo-ratory variability, and is tedious and time-consuming (18) . The use of flow cytometry for the detection of fetalcells is much more objective, reproducible, and sensitivethan the Kleihauer-Betke test  (19–21) . Fluorescently la- beled antibodies to the rhesus (D) antigen can be used, ormore recently, antibodies directed against hemoglobin F (19–27) . This intracellular approach, which uses perme-abilization of the red cell membrane and an antibody tothe     chain of human hemoglobin, is precise and sensitive (21) . This method has the ability to distinguish fetal cellsfrom F-cells (adult red cells with small amounts of hemo-globin F). Fig. 2 is a histogram of a positive test forfeto-maternal hemorrhage. Although the flow cytometrymethod is technically superior to the Kleihauer-Betke test,cost, instrument availability, and stat access may limit itspractical utility.Paroxysmal nocturnal hemoglobinuria (PNH) is anacquired clonal stem cell disorder that leads to intravas-cular hemolysis with associated thrombotic and infectiouscomplications. PNH can arise in the setting of aplasticanemia and may be followed by acute leukemia  (28) . Thedisease is caused by deficient biosynthesis of a glyco-sylphosphatidylinositol linker that anchors several com-plement and immunoregulatory surface proteins onerythrocytes, monocytes, neutrophils, lymphocytes, andplatelets  (28–31) . On erythrocytes, deficiencies of decay-accelerating factor and membrane-inhibitor of reactivelysis render red cells susceptible to complement-mediatedlysis  (30 ,  31) . Conventional laboratory tests for the diag-nosis of PNH include the sugar water test and the Ham’sacid hemolysis test  (32) . Problems associated with thesetests include stringent specimen requirements and limitedspecificity. Antibodies to CD55 and CD59 are specific fordecay-accelerating factor and membrane-inhibitor of re-active lysis, respectively, and can be analyzed by flowcytometry to make a definitive diagnosis of PNH  (29 ,  33– 35) . In affected patients, two or more populations of erythrocytes can be readily identified, with different de-grees of expression of CD55 and CD59 (Fig. 3)Reticulocyte counts are based on identification of re-sidual ribosomes and RNA in immature nonnucleated red blood cells (RBCs). Traditionally, a blood smear is stainedwith a dye that precipitates the nucleic acid, and the cellsare counted manually  (36) . This method is subjective,imprecise, labor-intensive, and tedious. The flow cytomet-ric enumeration of reticulocytes uses fluorescent dyes that bind the residual RNA, such as thiazole orange  (37 ,  38) .This method provides excellent discrimination betweenreticulocytes and mature RBCs, with greater precision,sensitivity, and reproducibility than the traditionalmethod  (37 ,  38) . However, Howell-Jolley bodies (a rem-nant of nuclear DNA) are not distinguished from reticu-locytes  (39) . Because the fluorescence intensity is directlyproportional to the amount of RNA and related to theimmaturity of the RBC, a reticulocyte maturity index has been used clinically to assess bone marrow engraftmentand erythropoietic activity and to help classify anemias (34 ,  38 ,  40 ,  41) . Some current automated cell counters usesimilar technology to estimate reticulocyte counts  (42) .In the blood bank, flow cytometry can be used as acomplementary or replacement test for red cell immunol-ogy, including RBC-bound immunoglobulins and red cellantigens  (43) . In multiply transfused patients, determin-ing the recipient’s blood type can be very difficult. Flowcytometry has been used to accurately identify and phe- Fig. 2. Hemoglobin F test for feto-maternal hemorrhage. Most adult RBCs do not have any hemoglobin F and are included in the largepeak on the  left  . A few adult red cells have a small amount of hemoglobin F andare called  F cells  . Higher quantities of hemoglobin F in  fetal cells   yield a higherfluorescence signal and allow discrimination between fetal cells and adult F cells. Clinical Chemistry  46, No. 8(B), 2000  1223  notype the recipient’s red cells  (44) . Flow cytometry is being used increasingly in the blood bank to assessleukocyte contamination in leukocyte-reduced bloodproducts  (45 ,  46) . leukocyte analysis Immunologic monitoring of HIV-infected patients is amainstay of the clinical flow cytometry laboratory. HIVinfects helper/inducer T lymphocytes via the CD4 anti-gen. Infected lymphocytes may be lysed when new viri-ons are released or may be removed by the cellularimmune system. As HIV disease progresses, CD4-positiveT lymphocytes decrease in total number. The absoluteCD4 count provides a powerful laboratory measurementfor predicting, staging, and monitoring disease progres-sion and response to treatment in HIV-infected individu-als. Quantitative viral load testing is a complementary testfor clinical monitoring of disease and is correlated in-versely to CD4 counts  (47 ,  48) . However, CD4 countsdirectly assess the patient’s immune status and not justthe amount of virus. It is likely that both CD4 T-cellenumeration and HIV viral load will continue to be usedfor diagnosis, prognosis, and therapeutic management of HIV-infected persons.Perhaps the best example of simultaneous analysis of multiple characteristics by flow cytometry involves theimmunophenotyping of leukemias and lymphomas. Im-munophenotyping as part of the diagnostic work-up of hematologic malignancies offers a rapid and effectivemeans of providing a diagnosis. The ability to analyzemultiple cellular characteristics, along with new antibod-ies and gating strategies, has substantially enhanced theutility of flow cytometry in the diagnosis of leukemiasand lymphomas. Different leukemias and lymphomasoften have subtle differences in their antigen profiles thatmake them ideal for analysis by flow cytometry. Diagnos-tic interpretations depend on a combination of antigenpatterns and fluorescence intensity. Several recent reviewarticles are available  (49–60) . Flow cytometry is veryeffective in distinguishing myeloid and lymphoid lin-eages in acute leukemias and minimally differentiatedleukemias. Additionally, CD45/side scatter gating oftencan better isolate the blast population for more definitivephenotyping than is possible with forward scatter/sidescatter gating. Fig. 4 is an example of CD45/side scattergating for an acute myeloid leukemia. Although mostacute myeloid leukemias are difficult to classify by phe-notype alone, flow cytometry can be useful in distinguish-ing certain acute myeloid leukemias, such as acute pro-myelocytic leukemia  (61 ,  62) . Flow cytometry can also beused to identify leukemias that may be resistant to ther-apy  (63) . In ALL, phenotype has been shown to correlatestrongly with outcome  (64 ,  65) .The B-cell lymphoproliferative disorders often havespecific antigen patterns. The use of a wide range of antibodies allows clinicians to make specific diagnoses based on patterns of antigen expression. Table 2 lists someof the common phenotypes expressed by various B-celllymphoproliferative disorders. Not only is the presence orabsence of antigens useful in making specific diagnoses,the strength of antigen expression can also aid in diagno-sis. One example is the weak expression of CD20 and Fig. 3. Diagnosis of PNH. Control individuals ( A ) show high expression of CD55 and CD59 on all red cells. In PNH ( B  ), some stem cell clones produce RBCs with decreased expression of CD55and CD59. In the PNH patient ( B  ), two distinct populations are present: normal red cells with high CD55 and CD59 expression and a second population with low CD55and CD59 expression. 1224  Brown and Wittwer: Flow Cytometry
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