Platelet Antibodies in Thrombocytopenic Patients

Abstract

Platelet antibodies either bound to the surface of platelets or free in the serum were sought in patients who had low platelet counts for a variety of reasons. They were detected by finding excess IgG on the surface of washed platelets either directly or after incubation of the serum with normal platelets. The technique used was a modification of that described recently (Dixon et al, 1975) in which the greater the amount of anti-IgG consumed by the reaction with platelets the less the subsequent lysis of sheep red cells coated with IgG. This test could be calibrated by adding known quantities of IgG to the antisera and thus the amount of bound IgG could be measured. Platelets from normal donors and those with thrombocytopenia due to non-immunological causes such as aplastic anaemia or acute leukaemia were found to have 15-70 ng IgG/10(7) platelets (mean 53 ng). 37 out of 38 thrombocytopenic patients in whom immune destruction of platelets was suspected were found to have excess IgG on their platelets ranging from 70 to 720 ng/10(7) (mean 297 ng, P less than 0.001) and there was a significant inverse correlation between this amount and the platelet count (r = 0.85, P less than 0.001). Antibody in the serum was found in 14 of 22 patients with 'idiopathic' thrombocytopenic purpura (ITP), three of four patients with underlying lymphoma and in all five cases of systemic lupus erythematosus (SLE). Four non-thrombocytopenic patients with autoimmune haemolytic anaemia (AIHA) due to IgG on the red cells were also studied and were shown to have no increase in platelet-bound IgG. Our results confirm the work of Dixon et al (1975) that platelet antibody as excess IgG can be readily detected on the surface of platelets in patients with immune thrombocytopenia. The clinical implications of these findings are discussed.

Quantification of Platelet-Associated IgG with Competitive Solid-Phase Enzyme Immunoassay

“…On the other hand, raised levels of circulating immune complexes have been demonstrated in sera from patients with ITP and an inverse relationship was observed between the platelet count and level of im mune complexes in the patients [16,21], The findings suggest that immune complex es also participate in immune destruction of platelets. Our results, as shown by the pre vious reports [4,8,9], also showed elevated levels of PAIgG in patients with SLE and a decreased platelet count. It has been consid ered that the thrombocytopenia is caused by immune complexes [5].…”

“…The platelet counts and the PAIgG values showed the best inverse correlation in a loglog plot. The results obtained in this and other studies [4,8,10,15] imply that the amounts of PAIgG are an important factor determining the severity of ITP. The mech anism of reduction in a platelet count in ITP is interpreted as showing that platelets are injured by antiplatelet antibodies (ITP factor) and rapidly destroyed in the reticu loendothelial system.…”

“…Though normal PAIgG values which Dixon et al [4] re ported by means of complement lysis-inhibi tion assay were as great as 3,000 ± 500 ng/ 107 platelets, the values which Hedge et al [8], Hauch and Rosse [9] and Kelton et al [12] obtained later by the same method were approximately 50 ng/107 platelets. On the other hand, Luiken et al [15] who used Fab anti-Fab IgG assay reported 12 ng/ 107 platelets as an average normal level.…”

Reactions of immunoglobulin G-binding ligands with platelets and platelet-associated immunoglobulin G.

“…The stoichiometry is assumed to be the same for platelet-bound IgG as for fluid-phase IgG and the tests are therefore calibrated by assessing the consumption of anti-IgG by known amounts of fluid-phase IgG. By these techniques, 5,000-20,000 molecules of IgG are detected on the platelets of normal donors and values >300,000 molecules/platelet have been reported in patients with idiopathic thrombocytopenic purpura (1)(2)(3)(4)(5)(6). In addition, increased amounts of platelet-bound IgG have been found by these techniques on the platelets of thrombocytopenic patients with systemic lupus erythematosus, myeloproliferative disorders, sepsis and other infections, drug-induced purpura, various hematologic disorders, and cancer (7)(8)(9).…”

In vitro Platelet Phagocytosis in Idiopathic Thrombocytopenic Purpura

“…Quantitative assays of platelet-associat ed IgG (PAIgG), including our competi tive solid-phase enzyme immunoassay [3], have been reported and the results indicat ed that in ITP, increased PAIgG correlat ed inversely with both platelet survival [4] and the level of thrombocytopenia [3][4][5][6][7][8]. We thought the PAIgG assay to be the most valuable method for detecting plate let autoantibody in ITP and investigated the relationship between the results ob tained by the direct assay and PAIgG val ues in ITP patients.…”