Immunofluorescence microphotometry for the detection of platelet antibodies. III. Demonstration of autoantibodies against platelets

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.

Pattern of platelet-associated immunoglobulin (classes and IgG subclasses) in childhood immune thrombocytopenic purpura

Platelet-associated Ig classes and IgG subclasses were studied by a semiquantitative platelet ELISA test in 17 children with immune thrombocytopenic purpura (ITP). An elevation of PAIg was found in 94% of the children. In nearly all cases increased amounts of PAIgG of subclass G1 was seen, and in half of the cases increased amounts of PAIgM were also seen. No statistical difference in the composition of PAIg classes and PAIgG subclasses in acute and in chronic ITP was found. However, a correlation of increased amount of PAIgG3 and very low platelet count (20 x 10(9)/l) was observed.

Platelet-associated IgG assay using flow cytometric analysis

In this study we describe an immunofluorescent assay system to measure platelet-associated immunoglobulin G levels using flow cytometric analysis. This semi-quantitative system allows ready distinction of immune from non-immune related thrombocytopenias. It is simple to perform, highly reproducible and has the advantage of requiring a minimum concentration of 5000 platelets/microliter per assay sample.

Splenic pulp, plasma cells and foamy histiocytes in immune thrombocytopenia: combined morphometric immunohistochemical and ultrastructural studies

Spleens from 16 splenectomized patients with immune thrombocytopenia (ITP) were compared with 10 controls (accidental and incidental splenectomy, staging laparotomy). Fresh unfixed material was secured for smears and cryostat sections, and splenic tissue was fixed by perfusion for histomorphometric, immunohistochemical and ultrastructural examination. The volumes of white pulp, including germinal centres, red pulp, and trabeculae and vessels were estimated. The ratio plasma cells containing various Ig-classes/total nucleated cells and the ratio immature/mature plasma cells were determined and the occurrence and character of foamy cells examined. There were slightly increased absolute volumes of white pulp, red pulp and trabeculae + major vessels in ITP, but the differences were not statistically significant. There was a statistically significant raised number of IgG-, IgA- and IgM-containing plasma cells in the germinal centres in ITP. More IgG-containing cells seemed to be immature than in controls, but the difference was not statistically significant. Light microscopy showed foamy histiocytes in 7 of 16 cases, but electron microscopy showed them in all cases of ITP. Foamy histiocytes reacted with antithrombocyte serum.

Platelet antibodies of the IgM class in immune thrombocytopenic purpura

The clinical course and response to therapy of patients with immune thrombocytopenic purpura (ITP) are not completely determined by the level of IgG present on the platelet surface. It is possible that antibodies of other immunoglobulin classes also play a role in platelet destruction in some of these patients. Therefore, we studied 175 patients with ITP for the presence of IgM anti-platelet antibodies using radiolabeled polyclonal or monoclonal anti-IgM. We observed that 57% of patients with clinical ITP had increased levels of IgM on their platelets, compared with normal controls and patients with thrombocytopenia who did not have ITP (less than 10%), (P less than 0.01). We obtained similar results using either radiolabeled polyclonal or monoclonal anti-IgM, reagents whose integrity was first characterized using erythrocytes coated with defined amounts of IgM antibody. Among patients with increased platelet-IgM there was a significant correlation both with the presence of increased platelet-C3 as well as the amount of platelet-C3 (P less than 0.01, r = 0.53). We demonstrated the presence of warm-reacting IgM anti-platelet antibodies in the plasma of two of these patients who were further studied. The isolated IgM fraction from these two plasmas was able to activate complement and place 3H-C3 on normal platelets. These studies demonstrate the presence of warm-reacting IgM anti-platelet antibodies in some patients with ITP. They suggest that the binding of complement to platelets by IgM antibodies may initiate platelet clearance as well as enhance the effect of IgG antibodies in ITP.

Antibodies reactive with surface membranes of cellular elements in the blood

Autoreactive antibodies or immune complexes may accelerate clearance of mature erythrocytes, leukocytes, and platelets from the circulation in patients with rheumatologic and immunologic disorders. The most compelling evidence for immune injury to hematopoietic cells exists in patients with systemic lupus erythematosus and patients with Felty's syndrome and its variants. These disorders may also cause tissue inflammation, which in turn commonly results in underproduction of erythrocytes and development of thrombocytosis. However, recent evidence indicates that underproduction of hematopoietic cells may also result from immune injury to cellular elements in the bone marrow. In many laboratories, sensitive techniques are now clinically available for the detection of cell-associated immunoglobulin and complement. These assays have helped confirm the role of antibody in the pathogenesis of autoimmune hemolytic anemia and idiopathic thrombocytopenic purpura. However, recent data indicate that there is probably a continuum between the amount of immunoglobulin and complement found on normal cells and that found in a variety of disease states. In several of these disorders, additional evidence will be required to establish that the increase in cell-bound immunoglobulin leads to a decrease in the life-span of the cell. In order to provide significant help to the clinician managing an individual patient, these serologic tests must be capable of identifying the portion of the cell-associated protein actually involved in the destructive process. The availability of monoclonal reagents capable of identifying restricted regions on cell-bound immunoglobulin may help identify molecules bound specifically as antibody and may help identify the antigens involved in autoimmune disorders.

Simultaneous enzymo-immunologic assays of platelet associated IgG, IgM and C3. A useful tool in assessment of immune thrombocytopenias

A solid phase enzymo-immunologic assay (EIA) has been developed to measure platelet associated (PA) IgG, IgM and C3. Washed platelets are mixed with a goat anti-IgG (IgM or C3) antiserum and then incubated in serum coated polystyrene plates. There is an inverse relationship between the level of PA IgG (IgM or C3) and the amount of goat antibodies binding to the IgG (IgM or C3) coating the plates. These goat antibodies are detected by addition of a phosphatase-labelled sheep anti-goat immunoglobulins antibody followed by a substrate giving a colour reaction. Using this technique, platelets from normal donors gave values of 2.04 +/- 1.10 fg IgG/platelet (mean +/- SD), 8.66 +/- 2.61 x 10(-16) g IgM/platelet and 5.67 +/- 2.63 x 10(-16) g C3/platelet. In 35 thrombocytopenic ITP patients we observed an increased level of PA IgG in 25, of PA IgM in 24 and of PA C3 in 23, and all of them had at least an increase of one of the 3 components. In 10 ITP patients in remission, 2 had an increased level of PA IgG while PA IgM and C3 values were normal in all. On the other hand, 2 non thrombocytopenic patients with systemic lupus erythematosus had an increased level of PA IgM. Compared to the standard antiglobulin consumption assay (ACA), the EIA was simpler to perform and more sensitive.

Enzyme linked immunoassay for the detection of platelet associated IgG

An enzyme linked immunoassay incorporating antihuman globulin coupled with alkaline phosphatase has been developed to measure platelet associated IgG (PAIgG). Using a method in which platelet IgG is extracted into the fluid phase after appropriate procedures, we were able to bind the 'solubilized' PAIgG to commercially obtained antihuman IgG (AHG) which had previously been coated onto polystyrene. The amount of PAIgG thus bound was subsequently measured by the addition of the enzyme reagent using p-nitro phenyl phosphate as substrate. With this technique platelets from normal donors were found to have 2.6-17.4 ng/10(6) platelets (mean +/-2 SD). These values are higher than those obtained when assay systems using intact platelets are employed. Platelets from patients with immune thrombocytopenia had PAIgG values of 8.2-98.0 ng/10(6) platelets. In a few patients with disorders other than autoimmune thrombocytopenia (AITP) increased levels of PAIgG were also demonstrated. The assumption that increased PAIgG always represents platelet autoantibody may not be valid. The relevance of PAIgG as a parameter in the diagnosis and clinical management of patients with AITP is discussed.

Autoimmune thrombocytopenia in dogs and cats

Autoimmune thrombocytopenia has been recognized as a distinct entity in the dog and cat. It is characterized by: (1) clinical signs of thrombocytopenia, such as hemorrhages into the skin and tissues and from body orifices, (2) coagulation defects related to thrombocytopenia, such as prolonged bleeding time and poor clot retraction, (3) hematologic changes such as severe to moderate thrombocytopenia, often blood loss anemia, and signs of increased erythropoiesis, and (4) an absence or decreased number of megakaryocytes in the bone marrow during early phase and increased number during compensatory phase. Megakaryocytes may also show morphologic abnormalities. Serologic diagnosis of AITP in the dog and cat currently involves demonstration of antiplatelet antibody in serum by PF-3 test and/or associated with marrow megakaryocytes by a technique of direct immunofluorescence. Circumstances leading to formation of antiplatelet antibody remain unknown. Immune-mediated platelet destruction is believed to occur in the reticuloendothelial system, primarily in the spleen. Treatment consists primarily of corticosteroids and other immunosuppressive drugs. Dogs with primary AITP generally respond favorably to such therapy.

In vitro tests for platelet compatibility

Four methods were investigated to determine their suitability as platelet compatibility procedures: leukoagglutination, lymphocytotoxicity, platelet suspension immunofluorescence and platelet enzyme-linked immunosorbant assay. None of the tests were found to reliably predict the 24-hour-posttransfusion platelet increment in 8 refractory thrombocytopenic patients judged refractory to random donor platelet therapy.

A simple method for detecting complement-fixation by autologous platelets in autoimmune thrombocytopenic purpura

A new modification of the microtitre complement fixation test, (CFT), is described for the detection of platelet-bound antibodies (PBA). The test was positive in 12 out of 16 patients, (75%), with active autoimmune thrombocytopenic purpura (AITP). It was negative in four patients who were in remission of AITP when tested, in 10 patients with non-immune thrombocytopenia and in 51 normal blood donors. This is a semi-quantitative method in which suspensions of the patients' own platelets consume complement and therefore prevent the lysis of sensitised sheep red cells (SRBC). Sera from some of these cases were also tested for serum anti-platelet antibody (SPA) and immune complexes. The possible mechanisms and the relevance of positive results are discussed.

Detection of platelet antibody using rosette technique with anti-IgG antibody-coated polyacrylamide gel: a preliminary report

Paraformaldehyde-fixed platelets from normal donors were used for detection of antibody to platelet by in vitro sensitization (indirect method) utilizing rabbit anti-human IgG heavy chain specific antibody-coated polyacrylamide gels (Immunobeads). The sensitized platelets formed rosettes with Immunobeads and the positive rosette count was over 30%, while control showed less than 8% when normal sera were used. This method was also applicable for detecting antibody-sensitized platelets in vivo (direct method) in patients with autoimmune thrombocytopenia. This method is simple, rapid and reproducible for clinical use. Direct and indirect immunofluorescent antibody tests and a blocking test with anti-human serum also supported the results of Immunobead rosetting technique.

The clinical significance of platelet-associated IgG: a study on 298 patients with various disorders

Platelet-associated IgG (PAIgG) was studied by a quantitative platelet radioactive anti-IgG test (PRAT) in 298 patients. At the time of investigation, 171 patients were thrombocytopenic (platelet count < 100 X 10(9)/l), 127 had normal platelet counts. Patients fell into the following disease categories: Idiopathic thrombocytopenic purpura (ITP) (N = 81), possible ITP (19), acute ITP (9), systemic lupus erythematosus (22), autoimmune haemolytic anaemia of warm-type (18), systemic blood disease (65), liver diseases (35), other (49). A significant elevation of PAIgG was found in all disease categories. There was a significant correlation between PAIgG and the reciprocal values of platelet counts for most disease groups. No relationship was discernible between PAIgG and hypergammaglobulinaemic states (serum IgG > 1.8 g/l). Platelet survival studies (N = 30) revealed that normal and increased values of PAIgG were associated with normal or shortened platelet mean life span. It is concluded that an elevated PAIgG is only one of several factors involved in the development of immunologically medicated thrombocytopenia.

Immunofluorescent staining of platelet suspensions and detection of antiplatelet antibody in patients with idiopathic thrombocytopenic purpura

Direct immunofluorescent staining of 31 specimens of platelets obtained from 13 cases of idiopathic thrombocytopenic purpura (ITP) revealed positive staining on the surface of platelets for both immunoglobulins (Igs) and human B1C globulin in 9 specimens, for only Igs in 1 specimen and for human B1C alone in 5 specimens. The pattern of the positive immunofluorescent staining was granular. Indirect immunofluorescent staining of normal platelets in serum obtained from patients with ITP was positive for antiplatelet antibody in 9 out of 31 specimens. This suggests that platelets in patients with ITP may be damaged by an antiplatelet autoantibody acting directly on the platelet surface and/or by antigen antibody complexes binding via Fc IgG receptors on the surface of the platelets.

Autoimmune granulocytopenia: the detection of granulocyte autoantibodies with the immunofluorescence test

Neutropenia may be caused by neutrophil autoantibodies. The detection of such antibodies has always been difficult. Recently, we developed a sensitive indirect immunofluorescence technique applicable to granulocytes which proved to be of value in the detection of granulocyte alloantibodies. We have now used this method to investigate the serum and cells of 29 patients with idiopathic or secondary neutropenia. In four patients neutrophil-antigen-specific autoantibodies were detected in the serum and the patient's own granulocytes showed direct fluorescence. Furthermore, differences in the immunoglobulin composition and the temperature of optimal activity of the autoantibodies were found. Direct fluorescence was also demonstrated with the granulocytes from five other neutropenic patients and with the granulocytes from five non-neutropenic patients mainly with infectious disease. However, no granulocyte antibodies could be eluted or shown to be present in the serum. This indicates that a positive direct granulocyte fluorescence test may not be considered as proof that autoantibodies are present.

A simple immunofluorescence test for the detection of platelet antibodies

Immunofluorescence tests on platelets have always been hampered by nonspecific fluorescence caused by non-immunological binding of plasma proteins to the platelet membrane. It was found that this could be easily overcome by fixation of the cells with paraformaldehyde (PFA). By using PFA-fixed platelets, a simple method for the detection of platelet antibodies, the platelet suspension immunofluorescence test (PSIFT) was developed. PFA fixation did not alter or inactivate the platelet antigens tested. Platelet-reactive antibodies detected specifically with the PSIFT included platelet-specific agglutinins of the IgM class, non-agglutinating platelet-specific antibodies of the IgG class, drug-dependent platelet antibodies, HLA antibodies, as well as anti-A and anti-B antibodies. The sensitivity of the new test was satisfactory, as was its reproducibility. Measurement of platelet immunofluorescence was possible in a continuous flow microfluorometer, making a principle, quantitation of platelet antibodies and antigens possible.

The problem of platelet autoantibodies. I. Evaluation of the platelet factor 3 availability test for their detection

To assess two modifications of the platelet factor 3 (PF3) test for platelet antibody detection, an analysis of test conditions was performed with normal test material (serum, plasma, globulin fraction), defined HLA-specific, complement-fixing antisera and a quinidine-induced antiserum. It was shown that, under standardized conditions, the PF3 test revealed known platelet antibodies confirming earlier results. Its reproducibility and, to a lesser degree, its sensitivity were inferior to platelet complement fixation. In contrast, the test did not permit the reliable determination of platelet autoantibodies in sera of 89 thrombocytopenic patients including 36 cases of idiopathic thrombocytopenic purpura. Positive results loosely corresponded to the presence of HLA antibodies in sera.