Use of immobilized platelet membrane glycoproteins for the detection of platelet-specific alloantibodies in solid-phase ELISA

Factors influencing moderate to severe reactions to PLT transfusions: experience of the TRAP multicenter clinical trial

BACKGROUND

During the Trial to Reduce Alloimmunization to Platelets (TRAP Trial), data were prospectively collected for 8769 PLT transfusions regarding the frequency of moderate to severe PLT transfusion reactions.

STUDY DESIGN AND METHODS

At seven centers, 598 patients were randomly assigned to receive unmodified pooled random-donor PLT concentrates (PCs), UV-B-irradiated PCs, filtered PCs, or filtered random-donor apheresis PLTs.

RESULTS

Moderate to severe transfusion reactions were an increase in temperature of at least 2 degrees C, chills with rigors, extensive urticaria, dyspnea, cyanosis, or bronchospasm. These reactions occurred with 2.2 percent of the transfusions in 22 percent of the patients. Transfusion reactions were associated with WBC counts of more than 5 x 10(6) per transfusion (p = 0.002) and transfusions stored for more than 48 hours (p = 0.02). PLT counts before transfusion were significantly lower for transfusions associated with reactions (p = 0.005). Neither UV-B irradiation nor apheresis PLTs independently influenced reaction rates. The PLT increment at 1 hour after transfusion was lower for transfusions associated with reactions (p = 0.004), and the frequency of reactions was higher in PLT refractory patients (p < 0.001).

CONCLUSIONS

The provision of either fresh and/or WBC-reduced PLTs may decrease the frequency of PLT transfusion reactions and improve PLT transfusion efficacy.

A sensitive micro-platelet ELISA technique for screening anti-HLA antibodies

An enzyme-linked immunosorbant assay (ELISA) in a Terasaki plate (Micro-Platelet ELISA), using 30000 platelets per well, 2 microL primary antibody (anti HLA antiserum) and 5 microL of secondary antibody (1:2000) are described. Platelets from 30 selected HLA tissue typed cell panel individuals were used to characterize anti HLA A and B antibodies. The first half of the Terasaki tray had platelets in sequence to characterize anti HLA antibodies, while the second half contained anti HLA B antibodies. Results revealed that the HLA specificities of the sera identified by micro-platelet ELISA and microlymphocytotoxicity were concordant. Moreover, split antigens of broader specificities were identified in the Platelet ELISA technique. The advantages of micro-platelet ELISA technique were: (i) it does not require viable/frozen lymphocytes, (ii) reading is very simple and macroscopic, (iii) specificity of the serum is identified with accuracy within the same day, (iv) avoids inter-cell, inter-day variations, (v) complement is not required, (vi) requires only 1/50 volume of the reagents required by conventional ELISA in microtitre plates, and (vii) using platelets isolated from 5 mL of peripheral blood, fifteen thousand sera can be tested. This technique is, thus, very simple, cost effective, and very much suitable for any developing HLA laboratory, which is in the process of developing indigenous HLA reagents.

Leukocyte reduction and ultraviolet B irradiation of platelets to prevent alloimmunization and refractoriness to platelet transfusions

BACKGROUND

We conducted a multi-institutional, randomized, blinded trial to determine whether the use of platelets from which leukocytes had been removed by a filter or that had been treated with ultraviolet B irradiation would prevent the formation of antiplatelet alloantibodies and refractoriness to platelet transfusions.

METHODS

Patients who were receiving induction chemotherapy for acute myeloid leukemia were randomly assigned to receive one of four types of platelets transfusions: unmodified, pooled platelet concentrates from random donors (control); filtered, pooled platelet concentrates from random donors (F-PC); ultraviolet B-irradiated, pooled platelet concentrates from random donors (UVB-PC); or filtered platelets obtained by apheresis from single random donors (F-AP). All patients received transfusions of filtered, leukocyte-reduced red cells.

RESULTS

Of 530 patients with no alloantibodies at base line, 13 percent of those in the control group produced lymphocytotoxic antibodies and their thrombocytopenia became refractory to platelet transfusions, as compared with 3 percent in the F-PC group, 5 percent in the UVB-PC group, and 4 percent in the F-AP group (P< or =0.03 for each treated group as compared with the controls; there were no significant differences among the treated groups). Lymphocytotoxic antibodies were found in 45 percent of the controls, as compared with 17 to 21 percent in the treated groups (P<0.001 for each treated group as compared with the controls; there were no significant differences among the treated groups). Antibodies against platelet glycoproteins developed in 6 to 11 percent of the patients, with no significant differences among the four groups.

CONCLUSIONS

Reduction of leukocytes by filtration and ultraviolet B irradiation of platelets are equally effective in preventing alloantibody-mediated refractoriness to platelets during chemotherapy for acute myeloid leukemia. Platelets obtained by apheresis from single random donors provided no additional benefit as compared with pooled platelet concentrates from random donors.

Detection of equine antiplatelet and antineutrophil antibodies by enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay (ELISA) was standardised and applied for the detection of antiplatelet and antineutrophil antibodies using a heterologous system consisting of equine platelets or neutrophils and antisera raised in rabbits. The standardised technique consisted of using Immulon type 3 plate, 1 per cent gelatine as a blocking solution, poly-L-lysine buffer as a coating solution, unfixed antigen, 90 microliters test serum, horseradish peroxidase conjugated antibody and o-phenylenediamine dihydrochloride as a substrate. The number of unfixed platelets or neutrophils required for optimum detection of antibodies was 250,000 per well. Unfixed cellular antigens were as good as their extracts and superior to paraformaldehyde-fixed antigens in detecting specific antibodies. Microtitre plates coated with platelet or neutrophil antigens could be stored at 4 degrees and -70 degrees C for four to five weeks without significant loss of antigenicity. The ELISA was very sensitive in that antiplatelet antibody was detected up to a titre of 1:204,800 and antineutrophil antibody to a titre of 1:51,200. Some cross-reactivity (1:1600) was detected in antiplatelet and antineutrophil sera for neutrophil and platelet antigens, respectively. Platelet-associated antibody was also detected in extracts from platelets pretreated with 1:2 and 1:8 dilutions of antiplatelet serum. Standardised ELISA detected antiplatelet antibodies in nine and antineutrophil antibodies in three of 100 isologous equine blood typing sera.

Immunology of platelet disorders

Immune-mediated thrombocytopenias (IMTPs) are frequently-occurring haemostatic disorders in clinical medicine. They may be caused by allo-immunity, autoimmunity, or by drug-induced immune mechanism. All IMTPs are caused by antibodies, which may induce increased platelet destruction but may also hinder platelet function. Many different platelet membrane molecules (i.e. antigens) are involved in the immune processes that play a role in IMTP. Much is already known about the structure of these molecules. Notably the alloantigens involved in alloimmune-mediated thrombocytopenia have been quite well studied. Many of these antigens appear to be polymorphic determinants of adhesion molecules of the integrin superfamily, and are also present on cells other than platelets (endothelial cells, fibroblasts, smooth muscle cells). The methodology for studying platelet antigens and antibodies involved in IMTPs has markedly improved in recent years. This has not only led to better diagnostic tests but also to a better understanding of the immunopathogenesis of these diseases. Platelet immunology is scientifically very much alive, and is expected to remain so in the coming years.