Brian R. Curtis

Platelet CD36 links hyperlipidemia, oxidant stress and a prothrombotic phenotype.

Dyslipidemia is associated with a prothrombotic phenotype; however, the mechanisms responsible for enhanced platelet reactivity remain unclear. Proatherosclerotic lipid abnormalities are associated with both enhanced oxidant stress and the generation of biologically active oxidized lipids, including potential ligands for the scavenger receptor CD36, a major platelet glycoprotein. Using multiple mouse in vivo thrombosis models, we now demonstrate that genetic deletion of Cd36 protects mice from hyperlipidemia-associated enhanced platelet reactivity and the accompanying prothrombotic phenotype. Structurally defined oxidized choline glycerophospholipids that serve as high-affinity ligands for CD36 were at markedly increased levels in the plasma of hyperlipidemic mice and in the plasma of humans with low HDL levels, were able to bind platelets via CD36 and, at pathophysiological levels, promoted platelet activation via CD36. Thus, interactions of platelet CD36 with specific endogenous oxidized lipids play a crucial role in the well-known clinical associations between dyslipidemia, oxidant stress and a prothrombotic phenotype.

Malaria susceptibility and CD36 mutation.

A critical step in infection by Plasmodium falciparum, the microorganism that causes the most severe form of malaria, is the adhesion of parasitized red blood cells to capillary endothelium. The human protein CD36 is a major receptor for P. falciparum-infected red blood cells and may contribute to the disease by sequestering infected red blood cells and inhibiting the immune response to the parasite. We have found that African populations contain an exceptionally high frequency of mutations in CD36. Unexpectedly, these mutations that cause CD36 deficiency are associated with susceptibility to severe malaria, suggesting that the presence of distinct CD36 mutations in Africans and Asians is due to some selection pressure other than malaria.

TRALI due to granulocyte‐agglutinating human neutrophil antigen‐3a (5b) alloantibodies in donor plasma: a report of 2 fatalities

BACKGROUND : TRALI is usually an immunologic reaction to WBC antibodies in infused plasma and ranks second only to ABO mismatch as a cause of transfusion‐associated death. Implicated donors are usually multiparous women (≥3 pregnancies).

Drug-induced immune thrombocytopenia: pathogenesis, diagnosis, and management.

Summary.  Drug‐induced immune thrombocytopenia (DITP) can be triggered by a wide range of medications. Although many cases of DITP are mild, some are characterized by life‐threatening bleeding symptoms. The pathogenesis of DITP is complex, in that at least six different mechanisms have been proposed by which drug‐induced antibodies can promote platelet destruction. It is possible in many cases to identify antibodies that react with platelets in the presence of the sensitizing drug, but the required testing is technically demanding and not widely available. Therefore, a decision on whether to discontinue an implicated medication in a patient suspected of having DITP must be made on clinical grounds. An algorithm is available that can be helpful in assessing the likelihood that a particular drug caused thrombocytopenia, but the most important aspects of patient management are a high index of suspicion and a careful history of drug exposure in an individual who presents with acute, often severe thrombocytopenia of unknown etiology. How drugs induce platelet‐reactive antibodies and how, once formed, the antibodies cause platelet destruction following exposure to the drug is poorly understood. Further studies to address these issues and characterize more completely the range of drugs and drug metabolites that can cause DITP are needed.

Human platelet antigen-specific alloantibodies implicated in 1162 cases of neonatal alloimmune thrombocytopenia

BACKGROUND:  Neonatal alloimmune thrombocytopenia (NATP) caused by fetomaternal mismatch for human platelet (PLT) alloantigens (HPAs) complicates approximately 1 in 1000 to 1 in 2000 pregnancies and can lead to a serious bleeding diathesis, intracranial hemorrhage, and sometimes death of the fetus or neonate. As a national reference center for NATP investigations, our experience with this entity over a 12‐year period was reviewed.

Population genetics: Malaria susceptibility and CD36 mutation

A critical step in infection by Plasmodium falciparum, the microorganism that causes the most severe form of malaria, is the adhesion of parasitized red blood cells to capillary endothelium. The human protein CD36 is a major receptor for P. falciparum-infected red blood cells and may contribute to the disease by sequestering infected red blood cells and inhibiting the immune response to the parasite. We have found that African populations contain an exceptionally high frequency of mutations in CD36. Unexpectedly, these mutations that cause CD36 deficiency are associated with susceptibility to severe malaria, suggesting that the presence of distinct CD36 mutations in Africans and Asians is due to some selection pressure other than malaria.

Determination of neutrophil antigen gene frequencies in five ethnic groups by polymerase chain reaction with sequence-specific primers

Background: The granulocyte antigens NA1 and NA2 are the two recognized allelic forms of Fcγ receptor IIIB. These antigens are clinically relevant, because they are the most frequent targets of neutrophil antibodies in alloimmune neonatal neutropenia, transfusion‐related acute lung injury, and chronic benign autoimmune neutropenia of infancy.

Identifying drugs that cause acute thrombocytopenia: an analysis using 3 distinct methods

Drug-induced immune thrombocytopenia (DITP) is often suspected in patients with acute thrombocytopenia unexplained by other causes, but documenting that a drug is the cause of thrombocytopenia can be challenging. To provide a resource for diagnosis of DITP and for drug safety surveillance, we analyzed 3 distinct methods for identifying drugs that may cause thrombocytopenia. (1) Published case reports of DITP have described 253 drugs suspected of causing thrombocytopenia; using defined clinical criteria, 87 (34%) were identified with evidence that the drug caused thrombocytopenia. (2) Serum samples from patients with suspected DITP were tested for 202 drugs; drug-dependent, platelet-reactive antibodies were identified for 67 drugs (33%). (3) The Food and Drug Administrations Adverse Event Reporting System database was searched for drugs associated with thrombocytopenia by use of data mining algorithms; 1444 drugs had at least 1 report associated with thrombocytopenia, and 573 (40%) drugs demonstrated a statistically distinctive reporting association with thrombocytopenia. Among 1468 drugs suspected of causing thrombocytopenia, 102 were evaluated by all 3 methods, and 23 of these 102 drugs had evidence for an association with thrombocytopenia by all 3 methods. Multiple methods, each with a distinct perspective, can contribute to the identification of drugs that can cause thrombocytopenia.

The neutrophil alloantigen HNA-3a (5b) is located on choline transporter-like protein 2 and appears to be encoded by an R>Q154 amino acid substitution

The molecular basis of the HNA-3a/b (5b/a) leukocyte antigen system has not yet been defined despite evidence that HNA-3a-specific antibodies are particularly prone to cause severe, often fatal, transfusion-related lung injury. We used genome-wide single nucleotide polymorphism scanning and sequencing of DNA from persons of different HNA-3a/b phenotypes to identify a single single nucleotide polymorphism in exon 7 of the CLT2 gene (SLC44A2) that predicts an amino acid substitution in the first extracellular loop of choline transporter-like protein 2, a member of the choline transporter-like protein family of membrane glycoproteins, and correlates perfectly with HNA-3a/b phenotypes (R154 encodes HNA-3a; Q154 encodes HNA-3b). Mass spectrometric analysis of proteins immunoprecipitated from leukocytes by anti-HNA-3a provided direct evidence that anti-HNA-3a recognizes choline transporter-like protein 2. These findings will enable large-scale genotyping for HNA-3a/b to identify blood donors at risk to have HNA-3a-specific antibodies and should facilitate development of practical methods to detect such antibodies and prevent transfusion-related lung injury.

Mechanisms of transfusion-related acute lung injury (TRALI): anti-leukocyte antibodies.

There is abundant evidence that leukocyte antibodies in blood donor products are somehow involved in transfusion-related acute lung injury (TRALI). Human leukocyte antigen (HLA) class I, HLA class II, and neutrophil-specific antibodies in the plasma of both blood donors and recipients have been implicated in the pathogenesis of TRALI. The case for a relationship between leukocyte antibodies and TRALI is more compelling if concordance between the antigen specificity of the leukocyte antibodies in the donor plasma and the corresponding antigen on the cells of the affected recipient is demonstrated. Such antibody-antigen concordance can be investigated by typing the recipient for the cognate leukocyte antigens or by cross-matching the donor plasma against the recipients leukocytes. Two proposed pathophysiologic mechanisms for TRALI have received the most attention: the antibody hypothesis and the two-event hypothesis. The final common pathway in all of the proposed pathogenic mechanisms of TRALI is increased pulmonary capillary permeability, which results in movement of plasma into the alveolar space causing pulmonary edema. A typical TRALI serologic workup consists of tests for HLA class I and II and neutrophil-specific antibodies. The use of flow cytometry and HLA-coated microbeads is recommended for detection of HLA antibodies in plasma of implicated blood donors and a combination of the granulocyte agglutination test and granulocyte immunofluorescence test for detection of neutrophil-specific antibodies. Genotyping for class I and II HLA and for a limited number of neutrophil antigens may also be helpful in establishing antibody-antigen concordance.