Patients with both acute and chronic autoimmune thrombocytopenic purpura (AITP) have in vitro lymphocyte defects in the form of platelet- stimulated proliferation and cytokine secretion. A blinded study was performed to determine if these defects are related to serum cytokine levels and/or platelet antigen expression. Compared with controls, 53% of children with chronic AITP, but only 9% of those with acute AITP, had increased serum interleukin-2 (IL-2), interferon-gamma, and/or IL- 10; however, none of the patients had detectible serum levels of IL-4 or IL-6, cytokine patterns suggesting and early CD4+ Th0 and Th1 cell activation. In children with chronic AITP, the levels of serum IL-2 correlated with in vitro platelet-stimulated IL-2 production. Few (17%) patients with AITP showed platelet activation, as measured by CD62 expression, or abnormal expression levels of platelet membrane glycoprotein (GP) IIbIIIa, but abnormal GPIb levels were observed in one-third of children with AITP. In contrast to normal controls and patients with nonimmune thrombocytopenia, a significant number of children with acute (80%), chronic (71%), or chronic-complex (55%) AITP and GPIb+ peripheral blood cells expressing HLA-DR. HLA-DR was variably coexpressed on distinct smaller and larger-sized GPIb+ cell populations with CD41, CD45, CD14, CD80, and/or glycophorin molecules. GPIb+ cells isolated from spleens of patients with chronic AITP had high expression (49% +/- 30%) of HLA-DR and splenic T cells had a high level of in vitro platelet-stimulated IL-2 secretion compared with controls. Platelet HLA-DR expression correlated inversely with platelet count, but not with therapy, serum cytokines, or in vitro lymphocyte antiplatelet reactivity. The results indicate that platelet HLA-DR expression is a common occurrence in patients with immune thrombocytopenia, whereas a large subpopulation of children with chronic AITP can be identified by increased serum cytokine levels and in vitro platelet-stimulated IL-2 secretion by lymphocytes, suggesting that differences exist in the immune pathogenesis of acute and chronic AITP, particularly at the level of platelet reactive T cells.

Summary. The pathophysiology of platelet dysfunction in the Wiskott-Aldrich immune deficiency syndrome (WAS) remains unclear. Using flow cytometry, we have characterized the functional properties of platelets from 10 children with WAS. Patients with WAS had thrombocytopenia, small platelets, increased platelet-associated IgG and reduced platelet-dense granule content. Levels of reticulated 'young' platelets were normal in the WAS patients. Although the mean numbers of platelet glycoprotein (GP) Ib, GPIIbIIIa and GPIV molecules per platelet appeared lower in WAS patients than in healthy controls, analysis of similar-sized platelets revealed the mean number of GPIb molecules per platelet to be comparable in patients and normal controls. Surface GPIIbIIIa and GPIV expression was, however, significantly lower on the WAS platelets than on normal platelets. Compared with normal platelets, WAS platelets showed a reduced ability to modulate GPIIbIIIa expression following thrombin stimulation. In addition, thrombin-and ADPinduced expression of CD62P and CD63 was defective in WAS platelets. Phallacidin staining of the WAS platelets revealed less F-actin content than in normal platelets. Together, these data suggest that the reduced platelet number and function in WAS reflects, at least in part, a defect in bone marrow production as well as an intrinsic platelet abnormality.

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Accurate assessment of in vivo or in vitro platelet activation requires optimal preanalytical conditions to prevent artefactual in vitro activation of the platelets. The choice of anticoagulant is one of the critical preanalytical conditions as anticoagulants exert different effects on the activation of platelets ex vivo. We tested the effectiveness of Diatube-H (also known as CTAD; sodium citrate, theophylline, adenosine and dipyridamole) and citrate vacutainer tubes in preventing artefactual activation of platelets and preserving functional reserve. Platelet surface expression of the CD62P (reflecting alpha granule release), CD63 (reflecting lysosomal release) and modulation of normal platelet membrane glycoproteins CD41a and CD42b, were measured in whole blood and in isolated platelets immediately after collection and at 6, 24 and 48 h after venipuncture. Samples taken into Diatube-H showed less spontaneous platelet activation than did those taken into citrate. To measure in vitro platelet functional reserve, thrombin was added as agonist to blood stored for varying periods up to 48 h. Although Diatube-H suppressed in vitro platelet activation for up to 4 h, in samples kept for 6-24 h before thrombin addition, the inhibitory effect was lost and platelets responded fully to agonist activation. Hence, Diatube-H preserved platelets and allowed for measurement of in vivo platelet activation as well as thrombin-induced in vitro platelet activation after 6-24 h, in both whole blood and isolated platelets.