Platelet transfusions in hematological disorders

Abstract

Platelets are important for clot formation and vascular integrity. Thrombocytopenic patients with hematological disorders are at risk of bleeding and platelet transfusions are important in supportive care and treatment of lite-threatening bleedings. Transfusion response in terms of platelet increment is highly variable and hard to predict in the individual patient. Platelet transfusion refractoriness is often due to non-immune factors such as fever, bleeding or infection. Of the immune-causes, antibodies against Human Leukocyte Antigen (HLA) class I is the most common, and these patients can receive HLA-matched platelet transfusions. In a more experimental approach HLA deficient platelets are created by acid treatment.

The overall aim of this thesis was to gain a better knowledge of factors affecting platelet transfusion outcome and platelet function, and to improve transfusion practice to patients with platelet refractoriness due to anti-HLA antibodies. In the included research papers, we focused on platelet function and clot formation in patients before and after platelet transfusion (paper I), response to HLA matched platelet transfusions (paper II) and function and survival of HLA deficient platelets (paper III and IV).

In paper I, we evaluated the effect of platelet transfusions in 40 hematology patients, measuring platelet count increment, clot formation and response to agonist activation. Platelet count increment correlated with improved clot formation, but the response was highly variable and 34 % of the patients did not respond to the transfusion. In responder patients, efficient clot formation was predicted by good platelet responsiveness to agonist stimulation, but transfusion did not, however, restore platelet function to that of healthy controls. In paper II we evaluated HLA matched platelet transfusions to platelet refractory hematology patients. We found that selection of platelets with either a complete HLA match, or an acceptable HLA mismatch, was highly predictable for a successful transfusion response. For HLA- mismatched transfusions, the degree of matching correlated with the fraction of successful transfusions. Many transfusions were successful despite the presence of donor-specific antibodies (DSAs). In paper III we evaluated the function of HLA deficient platelets after acid treatment and found that they were viable with a normal upregulation of activation markers. The HLA deficient platelets aggregated to a similar extent as untreated platelets in response to stimulation with agonists. Acid treatment removed 70 to 90% of all HLA Class I complexes, which was followed by protection from anti-HLA antibody-mediated complement lysis and reduced phagocytosis by monocytes in vitro. In paper IV we further evaluated the function of HLA deficient platelets in whole transfusion units and investigated the survival of HLA deficient platelets in a mouse model. Recovery, HLA reduction, and viability in acid-treated platelet bags were comparable to the small-scale treatment protocol, and platelets contributed normally to clot formation. The HLA deficient platelets also contributed to endothelial integrity. Acid-treated platelets showed a lower in vivo recovery compared to untreated platelets in the mouse model. They were protected from rejection by single HLA allele-specific antibodies, but still cleared by a pan-HLA antibody.

In summary, the results and conclusions presented in this thesis provide a basis for adjusted transfusion practices and several hypotheses for future research. Our results also take HLA deficient platelets closer to a clinical trial.