Flow cytometric analysis of platelet surface antigens

Flow cytometry for evaluating platelet immunophenotyping and function in patients with thrombocytopenia

Platelets play an essential role in primary hemostasis through bleeding and thromboembolism. Thus, the diagnosis or evaluation of impaired hereditary, acquired, and drug-related platelet dysfunction has become imperative. The assessment of the platelet function is too complex for routine platelet function study. The major methods involved in platelet function study include platelet function analyzer testing, thromboelastography, thromboelastometry, light transmission aggregometry, and flow cytometry. The current review article focuses on the methods with flow cytometry for immunophenotyping of platelet and evaluating platelet function for platelet disorders, especially in patients with thrombocytopenia. According to the consensus published by the International Society on Thrombosis and Haemostasis, for inherited and acquired platelet disorders, the two major measures by which flow cytometry determines platelet function are glycoprotein IIb/IIIa/P-selectin (CD62p) expression and percentage of leukocyte-platelet aggregates. Using flow cytometry to determine platelet function has several advantages, including good sensitivity to low platelet counts, small blood volume required, and the nonnecessity of centrifugation. However, flow cytometry has still many limitations and challenges, with standardization for routine laboratory testing also proving difficult. Although flow cytometry is available for multipurpose and sensitive study of platelet functions at the same time, the challenging analysis gradually increases and needs to be addressed before reality.

Screening and diagnosis of inherited platelet disorders

Inherited platelet disorders are important conditions that often manifest with bleeding. These disorders have heterogeneous underlying pathologies. Some are syndromic disorders with non-blood phenotypic features, and others are associated with an increased predisposition to developing myelodysplasia and leukemia. Platelet disorders can present with thrombocytopenia, defects in platelet function, or both. As the underlying pathogenesis of inherited thrombocytopenias and platelet function disorders are quite diverse, their evaluation requires a thorough clinical assessment and specialized diagnostic tests, that often challenge diagnostic laboratories. At present, many of the commonly encountered, non-syndromic platelet disorders do not have a defined molecular cause. Nonetheless, significant progress has been made over the past few decades to improve the diagnostic evaluation of inherited platelet disorders, from the assessment of the bleeding history to improved standardization of light transmission aggregometry, which remains a "gold standard" test of platelet function. Some platelet disorder test findings are highly predictive of a bleeding disorder and some show association to symptoms of prolonged bleeding, surgical bleeding, and wound healing problems. Multiple assays can be required to diagnose common and rare platelet disorders, each requiring control of preanalytical, analytical, and post-analytical variables. The laboratory investigations of platelet disorders include evaluations of platelet counts, size, and morphology by light microscopy; assessments for aggregation defects; tests for dense granule deficiency; analyses of granule constituents and their release; platelet protein analysis by immunofluorescent staining or flow cytometry; tests of platelet procoagulant function; evaluations of platelet ultrastructure; high-throughput sequencing and other molecular diagnostic tests. The focus of this article is to review current methods for the diagnostic assessment of platelet function, with a focus on contemporary, best diagnostic laboratory practices, and relationships between clinical and laboratory findings.

Consensus recommendations on flow cytometry for the assessment of inherited and acquired disorders of platelet number and function: Communication from the ISTH SSC Subcommittee on Platelet Physiology

Flow cytometry is increasingly used in the study of platelets in inherited and acquired disorders of platelet number and function. However, wide variation exists in specific reagents, methods, and equipment used, making interpretation and comparison of results difficult. The goal of the present study was to provide expert consensus guidance on the use of flow cytometry for the evaluation of platelet disorders. A modified RAND/UCLA survey method was used to obtain a consensus among 11 experts from 10 countries across four continents, on the appropriateness of statements relating to clinical utility, pre-analytical variables, instrument and reagent standardization, methods, reporting, and quality control for platelet flow cytometry. Feedback from the initial survey revealed that uncertainty was sometimes due to lack of expertise with a particular test condition rather than unavailable or ambiguous data. To address this, the RAND method was modified to allow experts to self-identify statements for which they could not provide expert input. There was uniform agreement among experts in the areas of instrument and reagent standardization, methods, reporting, and quality control and this agreement is used to suggest best practices in these areas. However, 25.9% and 50% of statements related to pre-analytical variables and clinical utility, respectively, were rated as uncertain. Thus, while citrate is the preferred anticoagulant for many flow cytometric platelet tests, expert opinions differed on the acceptability of other anticoagulants, particularly heparin. Lack of expert consensus on the clinical utility of many flow cytometric platelet tests indicates the need for rigorous multicenter clinical outcome studies.

Platelet immunophenotyping in health and inherited bleeding disorders, a review and practical hints

Inherited platelet function disorders are rare hemorrhagic diseases. The gold standard for their exploration is optical aggregometry; however, investigations by flow cytometry (FCM) are being increasingly used. In this review, the physiology of platelets is first recalled, setting the stage for the compartments of platelets that can be apprehended by specific and appropriate labeling. As this requires some pre-analytical precautions and specific analytical settings, a second part focuses on these characteristic aspects, based on literature and on the authors' experience in the field, for qualitative or quantitative explorations. Membrane labeling with antibodies to CD42a or CD41, respectively, useful to assess the genetic-related defects of Glanzmann thrombocytopenia and Bernard Soulier syndrome are then described. Platelet degranulation disorders are detailed in the next section, as they can be explored, upon platelet activation, by measuring the expression of surface P-Selectin (CD62P) or CD63. Mepacrin uptake and release after activation is another test allowing to explore the function of dense granules. Finally, the flip-flop anomaly related to Scott syndrome is depicted. Tables summarizing possible FCM assays, and characteristic histograms are provided as reference for flow laboratories interested in developing platelet exploration.

Calcium indirectly regulates immunochemical reactivity and functional activities of the N-domain of thrombospondin-1

Conformational changes induced in thrombospondin-1 by removal of calcium regulate interactions with some ligands of its N-modules. Because calcium binds primarily to elements of the C-terminal signature domain of thrombospondin-1, which are distant from the N-modules, such regulation was unexpected. To clarify the mechanism for this regulation, we compared ligand binding to the N-modules of thrombospondin-1 in the full-length protein and recombinant trimeric thrombospondin-1 truncated prior to the signature domain. Three monoclonal antibodies were identified that recognize the N-modules, two of which exhibit calcium-dependent binding to native thrombospondin-1 but not to the truncated trimeric protein. These antibodies or calcium selectively modulate interactions of fibronectin, heparin, sulfatide, alpha3beta1 integrin, tumor necrosis factor-alpha-stimulated gene-6 protein, and, to a lesser extent, alpha4beta1 integrin with native thrombospondin-1 but not with the truncated protein. These results indicate connectivity between calcium binding sites in the C-terminal signature domain and the N-modules of thrombospondin-1 that regulates ligand binding and functional activities of the N-modules.

The clinical utility of flow cytometry in the study of platelets

Flow cytometry has emerged in the past few years as an important technology for the study of platelets. It offers the ability to make measurements on platelets with little or no isolation or manipulation. Most flow cytometric platelet studies can be carried out on whole blood, thus eliminating a host of artifacts. In addition, flow cytometric techniques have been developed that allow the measurement of nearly all of the functional capabilities of platelets, such as activation and aggregation and to identify new functions by permitting observation of platelets interacting with leukocytes and measurement of platelet microparticles. Several of these measurements have already reached the stage of clinical utility and others offer considerable promise for practical applications. This review describes each of the flow cytometric techniques used to study platelets and summarizes their current state of clinical utility. Semin Hematol 38:160-168.

The Effect of Hemodialysis on the Expression of Platelet Glycoproteins Ib, IIb/IIIa and P-Selectin

To determine the effect of hemodialysis on expression of platelet receptors in patients with chronic renal failure.

DESIGN

Blood sampling performed in chronic HD patients prior to the dialysis session, then 15 and 180 minutes into HD. Both dialysis machine inlet and outlet samples were taken at 15 minutes. Control subjects had a single blood sample taken.

PATIENTS

Thirteen adult males on chronic hemodialysis and 20 age and sex matched healthy controls.

MEASUREMENTS

Flow-cytometric analysis of platelet GP-Ib, GP-IIb/IIIa, and P-selectin. Plasma vWF multimers were analyzed by SDS-polyacrylamide gel electrophoresis and immunoblotting.

RESULTS

Mean channel fluorescence (MCF) for GP lb was significantly (P < 0.05) decreased in pre-dialysis patients compared to controls and decreased further 3 hours into the dialysis session compared to the start of the dialysis treatment (P < 0.01). MCF for GP IIb/IIIa between predialysis patients and controls was similar, but decreased after a single dialysis session (P < 0.01). MCF for P-selectin on platelets was similar in patients and controls, but fewer platelets from the patients expressed P-selectin compared to controls (P < 0.05). Qualitative multimeric analysis of the vWF in patients, pre- and post-HD was unchanged.

CONCLUSIONS

Alterations in platelet surface expression of GPIb, GPIIb/IIIa, and P-selectin may, partially contribute to the changes in platelet function seen in patients on hemodialysis. It is unlikely that alterations in the surface expression of these receptors alone can adequately account for the complex platelet and hemostatic changes associated with uremia and the HD procedure.

Standardization of the flow cytometric determination of HLA class I antigens, 'platelet-specific' glycoproteins and activation markers

Flow cytometry (FC) provides a reproducible investigation of cell surface antigens on platelets. The aim of this study was to elaborate appropriate protocols and to compare them with other techniques that have already been published. (1) Venipuncture with tubes containing citrate was better for the preservation of the antigenicity than using ACD tubes. The isolated platelets could not be completely distinguished from detritus and protein aggregates. Therefore a platelet concentration between 10(7) and 10(8)/ml measurement buffer was necessary to obtain a sufficient resolution by FC. (2) Isolation methods using either differential centrifugation or diluted Ficoll-Hypaque as a flotation medium provided platelets of equal purity. The method with Ficoll-Hypaque resulted in a higher number of isolated platelets than differential centrifugation. The demonstration of platelets and their antigens in whole blood without isolation gave good results provided the platelets were not activated. Activation of platelets with 1 NIH-U thrombin/l resulted in the loss of a part of the highly activated platelets because of their aggregation. (3) Comparing different concentrations of paraformaldehyde in PBS, fixation with 1% for 15 min provided the best antigen preservation for most of the antigens investigated. Isolation induced platelet activation. In order to avoid this effect, the whole anticoagulated blood was fixed with 1% paraformaldehyde for 15 min immediately after venipuncture. Then the platelets were isolated using diluted Ficoll-Hypaque. In this way, systemic activation of platelets can be detected with antibodies against glycoproteins which are translocated from the alpha-granules or lysosomes to the cell membrane. These activation markers can be determined on immediately fixed platelets (already in the whole blood) without any interference due to unspecific activation caused by the isolation procedure. (4) Platelet treatment with citric acid at pH 3, in order to remove the antigenicity of HLA-class I molecules, was sensitive to immediate fixation with paraformaldehyde in the whole blood. Fixation after isolating the platelets made it possible to demonstrate antigen stripping, and the free heavy chain, devoid of the beta 2-microglobulin, could be clearly demonstrated. (5) Using standardization beads, the average number of antigenic sites per platelet could be determined for the investigated specificities. It was shown that antibodies which have been directly conjugated or biotinylated and combined with streptavidin-phycoerythrin yielded similar results in terms of the number of antigenic binding sites while unconjugated antibodies in combination with FITC-conjugated anti-mouse-IgG led to overestimation of antigenic binding sites.

The vitronectin receptor (alpha v beta 3) is implicated, in cooperation with P-selectin and platelet-activating factor, in the adhesion of monocytes to activated endothelial cells

In this study we have investigated the presence on endothelial cells of potential glycoprotein receptors, other than P-selectin, which are involved in the adhesion of monocytes at the early stages of activation. We report that the majority of cells binding to thrombin-activated endothelial cells from a peripheral blood mononuclear cell (PBMC) preparation are monocytes. The adhesion of PBMC to thrombin-activated, but not resting, endothelial cells was inhibited (66%) by a monoclonal antibody (mAb) directed against alpha v beta 3. Elutriated monocytes or a monocytic cell line (U937) were also inhibited by this antibody, its F(ab)'2 fragments and three other anti-(alpha v beta 3) mAbs. alpha v beta 3 isolated from endothelial-cell lysates significantly inhibited the adhesion of monocytes and U937 cells to endothelial cells. A peptide motif (RGDF) known to interact with alpha v beta 3 inhibited U937 cell adhesion to activated endothelial cells by 53%. Finally, an anti-(P-selectin) mAb (LYP20) or a platelet-activating factor (PAF)-receptor antagonist (WEB 2086) inhibited monocyte adhesion to activated endothelial cells. This study shows for the first time that alpha v beta 3 is implicated, in addition to P-selectin and PAF, in the adhesion of monocytes to activated endothelial cells.

Bernard-Soulier syndrome: quantitative characterization of megakaryocytes and platelets by flow cytometric and platelet kinetic measurements

Platelets and megakaryocytes have been characterized in a Bernard-Soulier syndrome (BSS) kindred with respect to glycoprotein (GP) membrane receptors and measurements of thrombocytopoiesis. The index patient exhibited lifelong bleeding tendency, moderate thrombocytopenia (35 x 10(9)/l), giant platelets (mean platelet volume 12.5 microns 3 compared to 7.5 +/- 1.5 microns 3 in normals), absent ristocetin-induced platelet agglutination and absent binding of von Willebrand factor (vWF). Flow-cytometric analysis revealed absent platelet binding (0-2%) of monoclonal antibodies (mAb, LJ-P3, LJ-Ib1 and LJ-Ib10) directed against distinct epitopes on membrane GPIb alpha of the GPIb-IX complex, and normal binding of LJ-P4 mAb directed against GPIIb/IIIa complex (relative to increased platelet surface area). Marrow megakaryocytes also failed to express GPIb-IX complex, but demonstrated normal expression of GPIIb/IIIa. Among 6 asymptomatic family members, the patient's mother and 2 of his 4 children exhibited approximately 50% binding of anti-GPIb alpha mAb to their platelets by both flow cytometry and direct binding studies using 125I-vWF, 125I-LJ-Ib1 and 125I-LJ-Ib10 mAb. Marrow megakaryocytes were increased in the average cell volume and cytoplasmic granularity with a corresponding increase in ploidy (46% > 16N compared to 22 +/- 5% in normal individuals), a pattern typical of megakaryocytes stimulated by thrombocytopenia. Autologous 111In-platelet life span was shortened to 4.1 days (compared with 9.5 +/- 0.5 days in normal subjects), and the turnover of platelet mass in the circulation was near normal. The data directly demonstrate that the platelet membrane GPIb-IX defect in BSS originates in megakaryocytes at all levels of cell maturation, and exclude the possibility that the receptor abnormality is acquired during cell maturation or after platelets are released into the circulation. Since marrow megakaryocytes exhibited cellular changes consistent with stimulated megakaryocytopoiesis, these results also suggest that thrombocytopenia in this kindred of BSS is a consequence of both decreased platelet survival and ineffective platelet production.

Influence of chloroquine or acid treatment of human platelets on the antigenicity of HLA and the 'thrombocyte-specific' glycoproteins Ia/IIa, IIb, and IIb/IIIa

The influence of treatment of platelets with citrate buffer (pH 7.2), chloroquine, or citric acid at pH 3 on the expression of HLA class I antigens and 'thrombocyte-specific' glycoproteins was investigated by means of flow cytometry. After treatment with citric acid at pH 3 and chloroquine, the expression of HLA class I was significantly reduced, while the density of the molecules GPIa/IIa, GPIIb, and GPIIb/IIIa (GP = glycoprotein) carrying 'thrombocyte-specific' antigens was not or only weakly decreased on the surface of the platelets. The use of two monoclonal antibodies (HC-10 and HC-A2) against the native heavy chain of the HLA class I molecule revealed that 'antigen stripping' with chloroquine or citric acid does not affect the entire molecule: only the beta 2-microglobulin is cleaved, or only some epitopes on the heavy chain are altered by this procedure. The treatment with citric acid yielded better results with respect to the removal of HLA class I activity and the preservation of 'thrombocyte-specific' glycoproteins. The presence of the heavy chain of HLA class I molecules on the surface of platelets after treatment with citric acid and chloroquine confirms the hypothesis that platelets--like nucleated cells--bear HLA class I antigens inserted in the cell by a cytoplasmic and a transmembrane domain.

A flow-cytometric approach to quantitative estimation of platelet surface immunoglobulin G

Flow cytometry (FC) was used to estimate platelet-surface IgG (PSIgG) by quantifying the fluorescence of platelets incubated with a fluorescein isothiocyanate (FITC)-labelled polyclonal goat anti-human IgG antibody or FITC-labelled non-immune goat IgG. Results were expressed as relative fluorescence intensity (RFI) defined as the ratio of specific fluorescence (mean fluorescence of platelets incubated with the FITC anti-IgG) over non-specific fluorescence (mean fluorescence of platelets incubated with FITC non-immune goat IgG). A normal range was formed by analysing platelets from 71 healthy subjects. Platelets from 16 patients with a firm clinical diagnosis of immune-mediated thrombocytopenia had a mean RFI significantly higher (p < 0.001) than the controls, whereas platelets from 9 patients thought to have non-immune thrombocytopenia had an RFI not significantly different from the normal controls. From a prospectively studied group of 62 patients with no clinically obvious cause for their thrombocytopenia or impaired platelet function 35.5% had raised PSIgG. In order to express the results as number of IgG molecules per platelet, reference curves were created by using FC to measure PSIgG of platelets coated with known amounts of a chimeric IgG (human IgG with murine hypervariable region) monoclonal antibody to the glycoprotein IIb-IIIa complex. Normal platelets had an average 1,463 (SD = 927) molecules of PSIgG. In patients with immune-mediated thrombocytopenia the levels ranged from 690 to 32,328 (mean 11,535) molecules per platelet. Flow-cytometric PSIgG estimation was sensitive, fast and easy to perform and therefore suitable for both research and clinical service purposes.

Studies of the platelet fibrinogen receptor in Glanzmann patients and uremic patients

The purpose of the present study was to investigate the binding of fibrinogen to the platelet fibrinogen receptor. Glycoprotein (GP) IIIa was measured utilizing a fluorescein isothiocyanate (FITC)-labelled monoclonal antibody and an Ortho Spectrum III flow cytometer. The number of binding sites per platelet was calculated to be 30,200. Using this technique it appears possible not only to diagnose Glanzmann's thrombasthenia but also to identify carriers. In uremic patients a slightly lower number of GPIIIa molecules per cell than in control subjects was found. Treatment with erythropoietin had no significant effect on the expression of GPIIIa. Thrombin, and to a less extent ADP, increased the binding of FITC-conjugated fibrinogen to normal platelets but had no significant effect on the expression of GPIIIa.

Insights into heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is characterized by the presence of an antibody which can activate platelets in the presence of heparin. The in vitro Ca2+ mobilization induced by purified IgG from patients with HIT was evaluated to elucidate the platelet activation mechanism in this syndrome. HIT-IgG induced platelet activation in a heparin-dependent manner. This activation was inhibited by high viscosity, consistent with a previously documented cell-cell-mediated mechanism. We found that F(ab')2 fragments from an anti-GPIIb/IIIa monoclonal antibody in some cases, and dextran sulphate or salmon sperm DNA in all cases, could substitute for heparin, suggesting that heparin exposes a neo-antigen on the platelet surface rather than serving as the primary antigen in the process. Furthermore, heparin, dextran sulphate and salmon sperm DNA all augmented the platelet activation induced by aggregated IgG, suggesting an additional charge shielding effect. These data suggest the following mechanism for platelet activation in HIT: heparin binds to the platelet surface, exposing a neoantigen which elicits an antibody response in some patients. Antibody binding in the presence of heparin leads to platelet activation through Fc receptor mediated platelet-platelet interaction, a process augmented by the charge shielding effect of heparin.

Endogenous platelet fibrinogen: its modulation after surface expression is related to size-selective access to and conformational changes in the bound fibrinogen

Platelet stimulation results in the release of endogenous platelet fibrinogen which binds to the platelet surface. Previous studies have demonstrated that plasma fibrinogen bound to activated platelets becomes inaccessible to a variety of probes. We have studied endogenous platelet fibrinogen binding to activated platelets by employing an immunopurified polyclonal anti-fibrinogen antibody and F26, a monoclonal anti-fibrinogen antibody, which recognizes fibrinogen only when it is bound to a surface. Employing the Ig or F(ab')2 of the poly- or monoclonal antibody we found a marked decrease of fibrinogen accessibility 30-60 min after platelet activation. In contrast, platelet-bound fibrinogen remains accessible to the Fab fragment of F26 at a constant level for 30 min and increases at 60 min. The reduction of the polyclonal Fab fragment binding at 30 and 60 min is similar to the F26 Ig. These results indicate that the decreased accessibility of bound fibrinogen is related to two mechanisms; (1) that the access route to fibrinogen in size selective for the antibody probes and only small antibody probes, e.g. Fab fragments, can gain access to fibrinogen and (2) fibrinogen undergoes a conformational change(s) after binding which exposes at least one neo-epitope in the D domain of fibrinogen and which may decrease or mask the reactivity of other fibrinogen domains. Only the F26 Fab probe has full access to and identifies fibrinogen present on the platelet surface 60 min after stimulation.

Flow cytometric assessment of platelet function in patients with peripheral arterial occlusive disease

This study compared new and traditional measures of platelet function in 16 patients with severe peripheral arterial occlusive disease and 15 age-matched controls. Circulating platelets were characterized by the use of fluorescence flow cytometry to assess platelet aggregate formation and expression of the secretion-dependent alpha granule membrane protein GMP-140, by measurement of plasma beta-thromboglobulin (beta-TG), and by performance of platelet-rich plasma aggregation studies. In addition, blood samples were treated with graded concentrations of adenosine diphosphate (ADP; 0 to 10 mumols/L) to characterize by fluorescence flow cytometry the secretory and aggregatory responses to mild stimulation. No differences were detected between the two groups with regard to platelet function in unstimulated circulating blood by use of these techniques. Values (mean +/- SEM) observed were: GMP-140-positive platelets, 11% +/- 3% versus 13% +/- 2%; platelet aggregates in circulating whole blood, 4% +/- 1% versus 9% +/- 3%; plasma beta-TG, 92 +/- 12 versus 94 +/- 22 ng/ml; and ED50 (concentration of ADP required to produce half maximal aggregation), 3.8 +/- 1.1 versus 3.1 +/- 0.5 mumol/L in the patients with peripheral arterial occlusive disease and controls, respectively. Treatment with ADP caused a dose-related increase in GMP-140 expression in both groups, without significant differences in this parameter between the groups at any given concentration. However, stimulation with ADP concentrations greater than 1 mumol/L resulted in more frequent aggregate formation in the control than in the peripheral arterial occlusive disease group (25% +/- 4% versus 11% +/- 2%, respectively at 5.0 mumols/L, p = 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-GPIIb/IIIa (CD41) monoclonal antibody-induced platelet activation requires Fc receptor-dependent cell-cell interaction

We studied platelet activation by UR1, a murine IgG1 anti-CD41 mAb. Like thrombin and crosslinked anti-Fc gamma RII mAb IV3, UR1 initiates prompt aggregation and Ca2+ mobilization. UR1 F(ab')2 fragments failed to activate, yet inhibited UR1 IgG-mediated activation. UR1-induced activation was blocked by anti-Fc gamma RII mAb. High viscosity (15% dextran or Ficoll), which impedes cell-cell interaction, inhibited activation by UR1. Cell-cell interaction was confirmed by cell-mixing studies. UR1 binding to platelets of one pool was blocked with UR1 F(ab')2 allowing UR1 binding only to Fc gamma RII. IV3 Fab fragments blocked ligand binding to Fc gamma RII on platelets of a second pool; thus, UR1 could bind only its epitope. UR1 initiated an immediate [Ca2+]i increase in the intermixed pools at low ionic strength. These studies indicate that UR1 IgG binds CD41 on one platelet to form immune complexes which then crosslink and stimulate Fc gamma RII on nearby platelets. Two other anti-CD41 mAb, 6C9 and C17, and two anti-CD9 mAb, AG1 and mAb7, activated platelets in a UR1-like manner. We propose that platelet Fc gamma RII crosslinking that follows the interaction of IgG-opsonized platelets may be a common mechanism by which anti-platelet antibodies activate platelets.

Post-transfusion purpura as the main manifestation of a trilineal transfusion reaction, responsive to steroids: flow-cytometric investigation of granulocyte and platelet antibodies

We report a typical case of post-transfusion purpura (PTP) due to anti-PlA1 in a 65-year-old woman. Serological studies were carried out using flow cytometry (FCM). The patient also developed red cell alloantibodies that produced a delayed hemolytic transfusion reaction (DHTR) and broad HLA antibodies. Treatment with high-dose intravenous IgG (HDIgG; a first-generation preparation) was ineffective, but a course of steroids resulted in a rapid increase in the the platelet count.

Chlorophyll a fluorescence measurements of isolated spinach thylakoids obtained by using single-laser-based flow cytometry

Flow cytometry data of spinach thylakoid membrane preparations indicate the presence of a homogeneous thylakoid population. Fluorescence data from a flow cytometer and comparison with data from two other fluorometers show that chlorophyll a fluorescence detected with a flow cytometer has the character of maximum fluorescence (Fmax), not of the constant component (Fo). This conclusion is important since Fo measures fluorescence that is affected mostly by changes in excitation energy transfer and Fmax-Fo (the variable fluorescence) by changes in photochemistry. This was demonstrated by: 1) The light intensity as well as diffusion rate dependence of the quenching effect of various quinones (p-benzoquinone, phenyl-benzoquinone, and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, DBMIB) on fluorescence yield; quenching for the same concentration of these quinones was lower at the higher than at the lower light intensities. 2) Temperature dependence of the fluorescence yield; increasing the temperature from 20 to 70 degrees C did not show an increase in fluorescence yield using a flow cytometer in contrast to measurements with weak excitation light, but similar to those obtained for Fmax. 3) Addition of an inhibitor diuron up to 100 microM did not change the fluorescence intensity. A comparison of quenching of fluorescence by various quinones obtained by flow cytometry with those by other fluorometers suggests that the high intensity used in the cytometry produces unique results: the rate of reduction of quinones in much larger than the rate of equilibration with the bulk quinones.