O-linked sialic acid residues on platelet membrane glycoprotein IIb mask the human HPA-9b alloepitope

Sialic acids occupy the terminal position of glycan chains and have the potential to influence the antigenicity of glycoproteins (GP). The polymorphisms of human platelet alloantigens (HPA)-3 and HPA-9, located near the C-terminus of the extracellular domain of platelet membrane GPIIb, are adjacent to sialyl-core 1 O-glycans emanating from serines 845 and 847. Whether the nearby O-glycans affect the antigenicity of HPA-9b or influence the binding of anti-HPA-9b alloantibodies in clinically significant cases of neonatal alloimmune thrombocytopenia is unknown. To address this issue, we generated a series of O-glycan mutant HPA-9 allele-specific induced pluripotent stem cell lines, differentiated them to megakaryocytes (MKs), and examined their ability to bind HPA-9b-specific alloantibodies. We found that both wild-type MKs treated with neuraminidase and those genetically modified to lack the sialidases ST3GAL1 and ST3GAL2 dramatically increased anti-HPA-9b alloantibody binding, indicating that the HPA-9b epitope is partially masked by terminal sialic acids on nearby O-glycans of GPIIb. Interestingly, mutating the serine residues that carry these glycan chains to alanine actually reduced the binding of anti-HPA-9b alloantibodies, indicating that these 2 O-glycan chains contribute to the presentation of the HPA-9b epitope-perhaps by stabilizing the conformation of the GP in this region. Collectively, our data suggest that detection of anti-HPA-9b alloantibodies may be enhanced through the use of HPA-9b-specific MKs that have been genetically altered to lack nearby terminal sialic acid residues but retain the glycan chains to which they are attached.

Complement C3d/C4d Deposition on Platelets Correlates with 2'-O-Methoxyethyl Antisense Oligonucleotide-Induced Thrombocytopenia in Monkeys

2'-O-Methoxyethyl antisense oligonucleotide (2'-MOE ASO)-induced severe thrombocytopenia (TCP) [platelet (PLT) count <50 K/μL] was observed in the Asian-sourced cynomolgus monkeys with low incidence (2%-4% at doses >5 mg/kg/week). The potential mechanisms for TCP were studied using the Mauritian-sourced cynomolgus monkeys, which were shown to be more susceptible to ASO-induced TCP, along with the Asian-sourced animals. ISIS 405879, a 2'-MOE ASO, induced severe TCP (PLT <50 K/μL) in seven of nine Mauritian-sourced monkeys but not in the Asian-sourced monkeys after 16 weeks of treatment at 40 mg/kg/week. Marked increases in PLT-bound C3d/C4d were detected in all thrombocytopenic Mauritian-sourced monkeys but not in the unaffected Mauritian- or Asian-sourced monkeys, suggesting increased PLT clearance due to complement deposition on the PLTs. However, this effect was independent of the ASO-mediated fluid-phase alternative complement activation. A correlation was also observed between serum antiglycoprotein (GP) IIb/IIIa immunoglobulin G (IgG) and PLT reduction. In addition, increases in total serum IgM, anti-PLT IgM, and anti-PLT factor 4 IgM levels were observed in monkeys from both sources but were more evident in the Mauritian-sourced monkeys. These data suggest an enhanced innate immune cell activation to ISIS 405879, leading to increased PLT destruction through complement fixation on the PLTs or PLT crossreacting polyclonal antibody production.

HLA antibodies in fetal and neonatal alloimmune thrombocytopenia

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by antibodies against human platelet antigens (HPA). However, in many cases that meet clinical criteria for the condition, maternal sera do not have HPA antibodies. In studies examining whether human leukocyte antigen (HLA) antibodies cause FNAIT, the results are limited and inconclusive. This study sought to examine whether clinically suspected FNAIT cases with absent maternal HPA antibodies had different HLA antibody strength and specificity compared to controls.

STUDY DESIGN AND METHODS

A retrospective case-control study assessed class I HLA antibody strength and specificity in cases submitted for testing to Versiti, Wisconsin. There were 813 cases that met initial screening criteria, but written consent could only be obtained for 50. After review of medical records and expert panel review, 31 cases with clinical criteria of FNAIT and maternal HLA but not HPA antibodies were included. Each case was matched for maternal age, gestational age at delivery, parity, and race/ethnicity to two controls from unaffected pregnancies that had maternal serum HLA antibodies.

RESULTS

FNAIT cases were found to have both significantly higher HLA antibody strength, measured by mean fluorescence index (MFI), and broader HLA antibody specificity at antigen epitope level, compared to matched controls (p < .001). p-values remained significant after controlling for parity and gestational age at delivery.

DISCUSSION

Additional studies are needed to further examine whether the strong HLA antibodies identified in HPA-antibody-negative cases directly cause neonatal thrombocytopenia and whether prenatal treatment may be warranted in select cases to prevent recurrence.

Anchoring IgG-degrading enzymes to the surface of platelets selectively neutralizes antiplatelet antibodies

Immune thrombocytopenia (ITP) is an acquired bleeding disorder characterized by immunoglobulin G (IgG)-mediated platelet destruction. Current therapies primarily focus on reducing antiplatelet antibodies using immunosuppression or increasing platelet production with thrombopoietin mimetics. However, there are no universally safe and effective treatments for patients presenting with severe life-threatening bleeding. The IgG-degrading enzyme of Streptococcus pyogenes (IdeS), a protease with strict specificity for IgG, prevents IgG-driven immune disorders in murine models, including ITP. In clinical trials, IdeS prevented IgG-mediated kidney transplant rejection; however, the concentration of IdeS used to remove pathogenic antibodies causes profound hypogammaglobulinemia, and IdeS is immunogenic, which limits its use. Therefore, this study sought to determine whether targeting IdeS to FcγRIIA, a low-affinity IgG receptor on the surface of platelets, neutrophils, and monocytes, would be a viable strategy to decrease the pathogenesis of antiplatelet IgG and reduce treatment-related complications of nontargeted IdeS. We generated a recombinant protein conjugate by site-specifically linking the C-terminus of a single-chain variable fragment from an FcγRIIA antibody, clone IV.3, to the N-terminus of IdeS (scIV.3-IdeS). Platelets treated with scIV.3-IdeS had reduced binding of antiplatelet IgG from patients with ITP and decreased platelet phagocytosis in vitro, with no decrease in normal IgG. Treatment of mice expressing human FcγRIIA with scIV.3-IdeS reduced thrombocytopenia in a model of ITP and significantly improved the half-life of transfused platelets expressing human FcγRIIA. Together, these data suggest that scIV.3-IdeS can selectively remove pathogenic antiplatelet IgG and may be a potential treatment for patients with ITP and severe bleeding.

Multicentre evaluation of 5B9, a monoclonal anti-PF4/heparin IgG mimicking human HIT antibodies, as an internal quality control in HIT functional assays: Communication from the ISTH SSC Subcommittee on Platelet Immunology

BACKGROUND

Functional tests for the diagnosis of heparin-induced thrombocytopenia (HIT) exhibit variable performance.

OBJECTIVES

We evaluated in a multicenter study whether 5B9, a monoclonal anti-PF4/heparin IgG mimicking human HIT antibodies, could be used as an internal quality control.

METHODS

5B9 was sent to 11 laboratories in seven countries, and six initial concentrations ranging from 10 to 400 μg/mL were tested by heparin-induced platelet activation assay (HIPA), serotonin release assay (SRA), platelet aggregation test (PAT), flow cytometry (FC), or heparin-induced multiple-electrode aggregometry (HIMEA). Each method was evaluated in three different laboratories using experimental procedures identical to those usually applied for the diagnosis of HIT by testing platelets from 10 different healthy donors.

RESULTS

The procedures used varied among the laboratories, particularly when platelet-rich plasma and whole blood were used. Nevertheless, positive results were obtained with at least 100 μg/ml of 5B9 for most donors tested by all centers (except one) performing HIPA, SRA, or HIMEA. FC and PAT results were more heterogeneous. FC results from one center that used washed platelets preincubated with PF4 were positive with all donors at 50 µg/ml 5B9, but at least 200 μg/ml of 5B9 were required to activate cells with most donors tested using PAT.

CONCLUSION

This study confirms that HIT functional tests are not well standardized and exhibit variable sensitivity for the detection of platelet-activating antibodies. However, 5B9 is a potentially useful tool to standardize functional tests, to select responding platelet donors, and consequently to improve the performance of these assays and comparability between laboratories.

Generation of 'designer erythroblasts' lacking one or more blood group systems from CRISPR/Cas9 gene-edited human-induced pluripotent stem cells

Despite the recent advancements in transfusion medicine, red blood cell (RBC) alloimmunization remains a challenge for multiparous women and chronically transfused patients. At times, diagnostic laboratories depend on difficult-to-procure rare reagent RBCs for the identification of different alloantibodies in such subjects. We have addressed this issue by developing erythroblasts with custom phenotypes (Rh null, GPB null and Kx null/Kell low) using CRISPR/Cas9 gene-editing of a human induced pluripotent stem cell (hiPSC) parent line (OT1-1) for the blood group system genes: RHAG, GYPB and XK. Guide RNAs were cloned into Cas9-puromycin expression vector and transfected into OT1-1. Genotyping was performed to select puromycin-resistant hiPSC KOs. CRISPR/Cas9 gene-editing resulted in the successful generation of three KO lines, RHAG KO, GYPB KO and XK KO. The OT1-1 cell line, as well as the three KO hiPSC lines, were differentiated into CD34+ CD41+ CD235ab+ hematopoietic progenitor cells (HPCs) and subsequently to erythroblasts. Native OT1-1 erythroblasts were positive for the expression of Rh, MNS, Kell and H blood group systems. Differentiation of RHAG KO, GYPB KO and XK KO resulted in the formation of Rh null, GPB null and Kx null/Kell low erythroblasts, respectively. OT1-1 as well as the three KO erythroblasts remained positive for RBC markers-CD71 and BAND3. Erythroblasts were mostly at the polychromatic/ orthochromatic stage of differentiation. Up to ~400-fold increase in erythroblasts derived from HPCs was observed. The availability of custom erythroblasts generated from CRISPR/Cas9 gene-edited hiPSC should be a useful addition to the tools currently used for the detection of clinically important red cell alloantibodies.

Severe Platelet Transfusion Refractoriness in Association with Antibodies Against CD36

Platelet-transfusion refractoriness (PTR) is common in patients with hematological malignancies. The etiology of immune PTR is typically human leukocyte antigen (HLA) antibodies (Abs) from pregnancy or previous transfusion. Herein, we report PTR in the setting of induction chemotherapy for acute myelogenous leukemia (AML) from Abs against CD36/glycoprotein (GP)IV. A 66-year-old African American woman presented with anemia and thrombocytopenia. She was found to have transfusion-dependent AML, and a 7 + 3 regimen (7 days of standard-dose cytarabine and 3 days of an anthracycline antibiotic or an anthracenedione, most often daunorubicin) was initiated. The patient developed profound thrombocytopenia, with platelet nadir of 0 by day 13. The results of HLA antibody screening were negative. However, the results of a screening test for platelet-specific antibodies screen showed Abs against cluster of differentiation (CD)36. The platelets of the patient lacked expression of CD36, and DNA analysis showed mutations in the CD36 gene. HLA Ab-mediated PTR is common in patients with hematological malignancies. However, once HLA Abs are excluded, other less-frequent Abs should be considered, particularly in patient populations of Asian, African, or Middle Eastern descent.

A prospective, blinded study of a PF4-dependent assay for HIT diagnosis

Heparin-induced thrombocytopenia (HIT) is a life-threatening, prothrombotic, antibody-mediated disorder. To maximize the likelihood of recovery, early and accurate diagnosis is critical. Widely available HIT assays, such as the platelet factor 4 (PF4) heparin enzyme-linked immunosorbent assay (ELISA) lack specificity, and the gold-standard carbon 14-labeled serotonin release assay (SRA) is of limited value for early patient management because it is available only through reference laboratories. Recent studies have demonstrated that pathogenic HIT antibodies selectively activate PF4-treated platelets and that a technically simpler assay, the PF4-dependent P-selectin expression assay (PEA), may provide an option for rapid and conclusive results. Based upon predefined criteria that combined 4Ts scores and HIT ELISA results, 409 consecutive adults suspected of having HIT were classified as disease positive, negative, or indeterminate. Patients deemed HIT indeterminate were considered disease negative in the primary analysis and disease positive in a sensitivity analysis. The ability of PEA and SRA to identify patients judged to have HIT was compared using receiver operating characteristic curve statistics. Using these predefined criteria, the diagnostic accuracy of PEA was high (area under the curve [AUC], 0.94; 95% confidence interval [CI], 0.87-1.0) and similar to that of SRA (AUC, 0.91; 95% CI, 0.82-1.0). In sensitivity analysis, the AUCs of PEA and SRA were also similar at 0.88 (95% CI, 0.78-0.98) and 0.86 (95% CI, 0.77-0.96), respectively. The PEA, a technically simple nonradioactive assay that uses ∼20-fold fewer platelets compared with the SRA, had high accuracy for diagnosing HIT. Widespread use of the PEA may facilitate timely and more effective management of patients with suspected HIT.

Underlying Immune Disorder May Predispose Some Transthyretin Amyloidosis Subjects to Inotersen-Mediated Thrombocytopenia

Inotersen, a 2'-O-methoxyethyl (2'-MOE) phosphorothioate antisense oligonucleotide, reduced disease progression and improved quality of life in patients with hereditary transthyretin amyloidosis with polyneuropathy (hATTR-PN) in the NEURO-TTR and NEURO-TTR open-label extension (OLE) trials. However, 300 mg/week inotersen treatment was associated with platelet count reductions in several patients. Mean platelet counts in patients in the NEURO-TTR-inotersen group remained ≥140 × 10⁹/L in 50% and ≥100 × 10⁹/L in 80% of the subjects. However, grade 4 thrombocytopenia (<25 × 10⁹/L) occurred in three subjects in NEURO-TTR trial, and one of these suffered a fatal intracranial hemorrhage. The two others were treated successfully with corticosteroids and discontinuation of inotersen. Investigations in a subset of subjects in NEURO-TTR (n = 17 placebo; n = 31 inotersen) and OLE (n = 33) trials ruled out direct myelotoxicity, consumptive coagulopathy, and heparin-induced thrombocytopenia. Antiplatelet immunoglobulin G (IgG) antibodies were detected at baseline in 5 of 31 (16%) inotersen-treated subjects in NEURO-TTR, 4 of whom eventually developed grade 1 or 2 thrombocytopenia while on the drug. In addition, 24 subjects in the same group developed treatment-emergent antiplatelet IgG antibodies, of which 2 developed grade 2, and 3 developed grade 4 thrombocytopenia. Antiplatelet IgG antibodies in two of the three grade 4 thrombocytopenia subjects targeted GPIIb/IIIa. Plasma cytokines previously implicated in immune dysregulation, such as interleukin (IL)-23 and a proliferation-inducing ligand (APRIL) were often above the normal range at baseline. Collectively, these findings suggest an underlying immunologic dysregulation predisposing some individuals to immune-mediated thrombocytopenia during inotersen treatment.

Strategies to develop a prophylaxis for the prevention of HPA-1a immunization and fetal and neonatal alloimmune thrombocytopenia

Anti-HPA-1a-antibodies are the main cause of fetal and neonatal alloimmune thrombocytopenia (FNAIT) which may result in intracranial hemorrhage (ICH) and death among fetuses and newborns. Advances in understanding the pathogenesis of FNAIT and proof of concept for prophylaxis to prevent immunization suggest that development of hyperimmune anti-HPA-1a IgG aimed at preventing immunization against HPA-1a and FNAIT is feasible. Anti-HPA-1a IgG can be obtained either by isolating immunoglobulin from already-immunized women or by development of monoclonal anti-HPA-1a antibodies. Here we discuss recent advances that may lead to the development of a prenatal and postnatal prophylactic treatment for the prevention of HPA-1a-associated FNAIT and life-threatening FNAIT-induced complications.

Post-Transfusion Purpura: Current Perspectives

Post transfusion purpura (PTP) is an uncommonly reported post transfusion adverse event that can present with severe thrombocytopenia; sometimes resulting in significant bleeding and hemorrhage. Its diagnosis can be elusive given its substantial symptomatic overlap with other thrombocytopenic syndromes. Underdiagnosis and underreporting make the true incidence of disease difficult to define. While clinical suspicion is key, laboratory evidence of platelet-targeted antibodies and identification of the antigen(s) they recognize are necessary to confirm the diagnosis. A curious aspect of PTP is paradoxical destruction of both transfused and autologous platelets. Although the first case was reported over 50 years ago, this aspect of PTP pathogenesis is still not fully understood and is widely debated. Several theories exist, but conclusive evidence to support most is lacking. Despite limited understanding of disease incidence and etiology, treatment with IVIG (Intravenous Immunoglobulin) has become standard practice and can be highly effective. Although recurrence is rare, precautions should be taken if patients with a history of PTP require transfusions in the future.

A case of platelet transfusion refractoriness due to anti-CD36 with a successful treatment outcome

Antibodies (Abs) against antigens on platelets (PLTs), including glycoprotein IV (CD36), can cause PLT refractoriness. Transfusing PLTs to patients with anti-CD36 is challenging because of the rarity of CD36-negative (CD36-) donors and the possibility of additional HLA Abs. We report a case of PLT refractoriness due to anti-CD36 and HLA Abs. A 21-year-old man (group O, D+) with assumed drug-induced aplastic anemia received multiple PLT transfusions and developed severe PLT refractoriness. He was found to have anti-CD36 as well as HLA class I Abs, with a CD36- phenotype on both PLTs and monocytes. He was diagnosed with type 1 CD36 deficiency and received intravenous immunoglobulin (IVIG) and rituximab to decrease future Ab production. The PLT corrected count increment (CCI) improved significantly with subsequent transfusions of flow crossmatchcompatible as well as uncrossmatched PLTs. He eventually received a bone marrow transplant and has been doing well since. The mean CCI before and after IVIG/rituximab treatment was 0.2 and 6.2, respectively. Soon after IVIG started, the patient's CCI after receiving CD36-, group AB, D+, and HLA untested PLTs was 0.8, but his CCI after receiving flow crossmatch-compatible PLTs was 12.6. Two months after IVIG was started, the mean CCIs for uncrossmatched apheresis PLTs and crossmatch-compatible PLTs were comparable (6.1 versus 6.0, respectively). Desensitization treatment with IVIG and rituximab lowered anti-CD36 and HLA Ab levels, and the CCI of PLT transfusion improved significantly. This case demonstrates that immune suppression is effective for successful PLT transfusion of patients with anti-CD36.

Investigation into the Mechanism(s) That Leads to Platelet Decreases in Cynomolgus Monkeys During Administration of ISIS 104838, a 2'-MOE-Modified Antisense Oligonucleotide

ISIS 104838, a 2'-O-methoxyethyl (2'-MOE)-modified antisense oligonucleotide (ASO), causes a moderate, reproducible, dose-dependent, but selflimiting decrease in platelet (PLT) counts in monkeys and humans. To determine the etiology of PLT decrease in cynomolgus monkeys, a 12-week repeat dose toxicology study in 5 cynomolgus monkeys given subcutaneous injections of ISIS 104838 (30-60 mg/kg/week). Monkeys were also injected intravenously with 111Indium(In)-oxine-labeled PLTs to investigate PLT sequestration. In response to continued dosing, PLT counts were decreased by 50%-90% by day 30 in all monkeys. PLT decreases were accompanied by 2- to 4.5-fold increases in immunoglobulin M(IgM), which were typified by a 2- to 5-fold increase in antiplatelet factor 4 (antiPF4) IgM and antiPLT IgM, respectively. Monocyte chemotactic protein 1 increased upon dosing of ISIS 104838, concomitant with a 2- to 6-fold increase in monocyte-derived extracellular vesicles (EVs), indicating monocyte activation but not PLT activation. Despite a 2- to 3-fold increase in von Willebrand factor antigen in all monkeys following ASO administration, only 2 monkeys showed a 2- to 4-fold increase in endothelial EVs. Additionally, a ∼60 - 80%% increase in PLT sequestration in liver and spleen was also observed. Collectively, these results suggest the overall increase in total IgM, antiPLT IgM and/or antiPF4 IgM, in concert with monocyte activation contributed to increased PLT sequestration in spleen and liver, leading to decreased PLTs in peripheral blood.

High-resolution mapping of the polyclonal immune response to the human platelet alloantigen HPA-1a (PlA1)

Antibodies to platelet-specific antigens are responsible for 2 clinically important bleeding disorders: posttransfusion purpura and fetal/neonatal alloimmune thrombocytopenia (FNAIT). The human platelet-specific alloantigen 1a/1b (HPA-1a/1b; also known as PlA1/A2) alloantigen system of human platelet membrane glycoprotein (GP) IIIa is controlled by a Leu33Pro polymorphism and is responsible for ∼80% of the cases of FNAIT. Local residues surrounding polymorphic residue 33 are suspected to have a profound effect on alloantibody binding and subsequent downstream effector events. To define the molecular requirements for HPA-1a alloantibody binding, we generated transgenic mice that expressed murine GPIIIa (muGPIIIa) isoforms harboring select humanized residues within the plexin-semaphorin-integrin (PSI) and epidermal growth factor 1 (EGF1) domains and examined their ability to support the binding of a series of monoclonal and polyclonal HPA-1a-specific antibodies. Humanizing the PSI domain of muGPIIIa was sufficient to recreate the HPA-1a epitope recognized by some HPA-1a-specific antibodies; however, humanizing distinct amino acids within the linearly distant but conformationally close EGF1 domain was required to enable binding of others. These results reveal the previously unsuspected complex heterogeneity of the polyclonal alloimmune response to this clinically important human platelet alloantigen system. High-resolution mapping of this alloimmune response may improve diagnosis of FNAIT and should facilitate the rational design and selection of contemplated prophylactic and therapeutic anti-HPA-1a reagents.

A patient with oxaliplatin immune-induced syndrome (OIIS) who also developed leucovorin and palonosetron-associated thrombocytopenia

OBJECTIVES

We report a case of an 83 year old man who developed oxaliplatin immune-induced syndrome (OIIS) after his 19th cycle of FOLFOX (5FU, leucovorin, oxaliplatin). When oxaliplatin was omitted from his next cycle of chemotherapy he continues to show signs of drug-induced immune thrombocytopenia (DITP) and was found to have drug-dependent, platelet-reactive antibodies (DDPA) to leucovorin and palonosetron as well as oxaliplatin.

METHODS

The patient was admitted for monitoring but required no transfusions and thrombocytopenia resolved without treatment during his first admission. Drug-dependent antibody testing was performed on his blood by the Blood Center of Wisconsin (Diagnostic Laboratories; Milwaukee, WI).

RESULTS

No RBC or platelet IgG or IgM antibodies were detected in the absence of any drugs, but upon addition of palonosetron, leucovorin, or oxaliplatin, the tests became strongly positive for anti-RBC IgG and anti-platelet IgG antibodies.

DISCUSSION

Repeated administration of oxaliplatin can result in drug-induced immune thrombocytopenia (DITP) or autoimmune hemolytic anemia (AIHA). This phenomenon has recently been termed OIIS and may additionally include Evan's syndrome or thrombotic microangiopathy (TMA). Here we describe a patient who developed OIIS with drug-dependent, platelet-reactive antibodies (DDPA) to leucovorin and palonosetron. To our knowledge, these two drugs have never been described in the literature as a cause of DDPA. We suggest that OIIS in addition to oxaliplatin-induced thrombocytopenia may be associated with the development of DDPAs to other drugs causing clinically significant thrombocytopenia which is important to recognize and manage with discontinuation of provoking agents.

Non-chemotherapy drug-induced neutropenia: key points to manage the challenges

Non-chemotherapy idiosyncratic drug-induced neutropenia (IDIN) is a relatively rare but potentially fatal disorder that occurs in susceptible individuals, with an incidence of 2.4 to 15.4 cases per million population. Affected patients typically experience severe neutropenia within several weeks to several months after first exposure to a drug, and mortality is ∼5%. The drugs most frequently associated with IDIN include metamizole, clozapine, sulfasalazine, thiamazole, carbimazole, amoxicillin, cotrimoxazole, ticlopidine, and valganciclovir. The idiosyncratic nature of IDIN, the lack of mouse models and diagnostic testing, and its low overall incidence make rigorous studies to elucidate possible mechanisms exceptionally difficult. An immune mechanism for IDIN involving neutrophil destruction by hapten (drug)-specific antibodies and drug-induced autoantibodies is frequently suggested, but strong supporting evidence is lacking. Although laboratory testing for neutrophil drug-dependent antibodies is rarely performed because of the complexity and low sensitivity of tests currently in use, these assays could possibly be enhanced by using reactive drug metabolites in place of the parent drug. Patients typically experience acute, severe neutropenia, or agranulocytosis (<0.5 × 10⁹ neutrophils/L) and symptoms of fever, chills, sore throat, and muscle and joint pain. Diagnosis can be difficult, but timely recognition is critical because if left untreated, there is an increase in mortality. Expanded studies of the production and mechanistic role of reactive drug metabolites, genetic associations, and improved animal models of IDIN are essential to further our understanding of this important disorder.

A Platelet Factor 4-Dependent Platelet Activation Assay Facilitates Early Detection of Pathogenic Heparin-Induced Thrombocytopenia Antibodies

Heparin-induced thrombocytopenia (HIT) is a dangerous complication of heparin therapy. HIT diagnosis is established by recognizing thrombocytopenia and/or thrombosis in an affected patient and from the results of serological tests such as the platelet factor 4 (PF4)/heparin immunoassay (PF4 ELISA) and serotonin release assay (SRA). Recent studies suggest that HIT antibodies activate platelets by recognizing PF4 in a complex with platelet glycosaminoglycans (and/or polyphosphates) and that an assay based on this principle, the PF4-dependent P-selectin expression assay (PEA), may be even more accurate than the SRA for HIT diagnosis. Here, we demonstrate that the PEA detected pathogenic antibodies before the SRA became positive in two patients with HIT studied serially, in one case even before seropositivity in the PF4 ELISA. In one of the patients treated with plasma exchange, persistent dissociation between the PEA and SRA test results was observed. These results support a role for the PEA in early HIT diagnosis.

Detection and identification of platelet antibodies using a sensitive multiplex assay system-platelet antibody bead array

BACKGROUND

Tests for platelet-specific antibodies are important in the diagnosis of immune platelet disorders. Monoclonal antibody glycoprotein capture assays have been the gold standards in platelet antibody detection for almost 30 years. However, such assays are complex and cumbersome to perform, which limits their routine use. We report the performance of a newly developed, easy to perform platelet antibody bead array (PABA) for the detection of platelet-specific antibodies.

STUDY DESIGN AND METHODS

PABA is the equivalent of the monoclonal antigen capture enzyme-linked immunosorbent assay (ELISA) (MACE) on a bead and instead with fluorescent detection of immunoglobulin (Ig)G platelet antibodies by Luminex. Antibodies against platelet glycoproteins (GP) GPIIb/IIIa, GPIb/IX, GPIa/IIa, GPIV, and class I human leukocyte antigen (HLA) could be detected in a patient's serum simultaneously in a single well of a microplate. Results from 80 patient samples and 20 normal donor samples were compared using PABA, MACE, and a commercial ELISA kit.

RESULTS

PABA detected all antibodies, with specificity for human platelet antigens (HPAs) HPA-1a, HPA-1b, HPA-2a, HPA-2b, HPA-3a, HPA-3b, HPA-4a, HPA-4b, HPA-5a, HPA-5b, HPA-15b, and HLA. Overall, compared with MACE, the sensitivity and specificity of PABA were 99% and 97%, respectively, and both were significantly better than those of the commercial ELISA. PABA had greater analytic sensitivity than MACE for HPA-1a, HPA-3a, and HPA-5b antibodies. In addition, PABA detected HPA-5b and HPA-3b antibodies that were missed by MACE. The overall false-positive rate of PABA compared with MACE was 2.7%.

CONCLUSIONS

The PABA is a rapid, highly sensitive and specific, multiplex bead-based assay for detecting human platelet antibodies. Such a simple yet high-throughput platform will facilitate practical, routine testing for the identification of platelet-specific antibodies.

IVIg for Treatment of Severe Refractory Heparin-Induced Thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) complicated by severe thrombocytopenia and thrombosis can pose significant treatment challenges. Use of alternative anticoagulants in this setting may increase bleeding risks, especially in patients who have a protracted disease course. Additional therapies are lacking in this severely affected patient population.

METHODS

We describe three patients with HIT who had severe thromboembolism and prolonged thrombocytopenia refractory to standard treatment but who achieved an immediate and sustained response to IVIg therapy. The mechanism of action of IVIg was evaluated in these patients and in five additional patients with severe HIT. The impact of a common polymorphism (H/R 131) in the platelet IgG receptor FcγRIIa on IVIg-mediated inhibition of platelet activation was also examined.

RESULTS

At levels attained in vivo, IVIg inhibits HIT antibody-mediated platelet activation. The constant domain of IgG (Fc) but not the antigen-binding portion (Fab) is required for this effect. Consistent with this finding, IVIg had no effect on HIT antibody binding in a solid-phase HIT immunoassay (platelet factor 4 enzyme-linked immunoassay). The H/R131 polymorphism in FcγRIIa influences the susceptibility of platelets to IVIg treatment, with the HH131 genotype being most susceptible to IVIg-mediated inhibition of antibody-induced activation. However, at high doses of IVIg, activation of platelets of all FcγRIIa genotypes was significantly inhibited. All three patients did well on long-term anticoagulation therapy with direct oral anticoagulants.

CONCLUSIONS

These studies suggest that IVIg treatment should be considered in patients with HIT who have severe disease that is refractory to standard therapies.

Low yield of ventilation and perfusion imaging for the evaluation of pulmonary embolism after indeterminate CT pulmonary angiography

PURPOSE

Ventilation and perfusion (VQ) imaging is common following suboptimal CT pulmonary angiogram (CTPA) for pulmonary embolism (PE) evaluation; however, the results of this diagnostic pathway are unclear. The purpose of our study is to determine the incidence of PE diagnosed on VQ scans performed in patients with suboptimal CTPAs.

METHODS

One hundred twenty-two suboptimal CTPAs with subsequent VQ scans within 1 week were retrospectively identified. VQ reports utilizing modified ​prospective investigation of pulmonary embolism diagnosis (PIOPED) and prospective investigative study of acute pulmonary embolism diagnosis (PISAPED) criteria were evaluated for presence of PE; intermediate probability, high probability, and PE present were considered PE positive. Three hundred consecutive reports of each diagnostic CTPA and diagnostic VQ studies were reviewed to estimate baseline PE positive rates at our institution. These were compared to the positive VQ scan rate after suboptimal CTPA by Fisher's exact test. Reported reason for suboptimal CTPA was noted. When contrast bolus timing was suboptimal, we measured main pulmonary artery (mPA) Hounsfield units (HU). Potential alternative diagnoses in CTPA reports were noted.

RESULTS

97.5% (119/122) of VQ scans following suboptimal CTPA were negative for PE, and 2.5% (3/122) were positive for PE. This was significantly lower than baseline PE positive rate of 10.7% (32/300, p < 0.01) for VQ imaging, and 10.3% (31/300, p < 0.01) for CTPA at our institution. Most (79.5%) CTPAs were suboptimal due to contrast timing. Average mPA density in these cases was 164 ± 61 HU. Most of these studies ruled out central PE. Potential alternative diagnosis was reported in 34/122 (28%) of suboptimal CTPAs, for which pneumonia accounted 59%.

CONCLUSION

There is very low incidence of PE diagnosed on VQ imaging performed after suboptimal CTPA. This may be attributed to the ability of most suboptimal CTPAs to rule out central PE.

A Novel PF4-Dependent Platelet Activation Assay Identifies Patients Likely to Have Heparin-Induced Thrombocytopenia/Thrombosis

BACKGROUND

Almost without exception, patients with heparin-induced thrombocytopenia/thrombosis (HIT) have antibodies that recognize platelet factor 4 (PF4) in a complex with heparin; however, many heparin-treated patients without HIT are also antibody-positive. A platelet activation test, the serotonin release assay (SRA), is useful for identifying a subset of antibodies that are platelet-activating and most likely to cause HIT. However, this "gold standard" assay for HIT diagnosis is technically demanding and is routinely available only through referral laboratories, limiting its availability for timely diagnosis and management.

METHODS

We compared the diagnostic performance of the SRA with that of a technically simple platelet activation assay, the PF4-dependent P-selectin expression assay (PEA), which uses platelets pretreated with PF4 as targets for antibody detection. Archived serum samples from 91 patients for whom clinical information (HIT 4Ts [thrombocytopenia, timing of platelet count fall, thrombosis, and other causes of thrombocytopenia] score) was available were used. Patients with an intermediate 4Ts score and a PF4 ELISA (enzyme-linked immunosorbent assay) optical density ≥ 2.0, or a high 4Ts score and a PF4 ELISA optical density ≥ 1.0, were considered HIT positive; others were designated HIT negative.

RESULTS

The PEA had higher diagnostic accuracy (area under the curve, 0.92 vs 0.82; P = .02) than the SRA, using this definition of HIT. Eleven of 16 serum samples that were PEA positive and SRA negative were HIT positive. Studies done with identical target platelets and serially diluted samples from patients with HIT showed that the PEA is inherently more sensitive than the SRA for the detection of platelet-activating antibodies.

CONCLUSIONS

The PEA is technically less demanding than the SRA and may be more accurate for the diagnosis of HIT.

Recent progress in understanding the pathogenesis of fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs in c. 1 in 1000 births and is caused by maternal antibodies against human platelet alloantigens that bind incompatible fetal platelets and promote their clearance from the circulation. Affected infants can experience bleeding, bruising and, in severe cases, intracranial haemorrhage and even death. As maternal screening is not routinely performed, and first pregnancies can be affected, most cases are diagnosed at delivery of a first affected pregnancy. Unlike its erythrocyte counterpart, Haemolytic Disease of the Fetus and Newborn, there is no prophylactic treatment for FNAIT. This report will review recent advances made in understanding the pathogenesis of FNAIT: the platelet alloantigens involved, maternal exposure and sensitization to fetal platelet antigens, properties of platelet Immunoglobulin G antibodies, maternal-fetal antibody transport mechanisms and efforts to develop an effective FNAIT prophylaxis.

Drug-induced thrombotic microangiopathy: Experience of the Oklahoma Registry and the BloodCenter of Wisconsin

Many drugs have been reported to cause thrombotic microangiopathy (TMA), often described as thrombotic thrombocytopenic purpura (TTP) or hemolytic-uremic syndrome (HUS). We recently established criteria to evaluate the evidence for a causal association of a drug with TMA and then we systematically reviewed all published reports of drug-induced TMA (DITMA) to determine the level of evidence supporting a causal association of the suspected drug with TMA. On the basis of this experience, we used these evaluation criteria to assess the Oklahoma TTP-HUS Registry patients who had been previously categorized as drug-induced, 1989-2014. We also reviewed the experience of the BloodCenter of Wisconsin with testing for drug-dependent antibodies reactive with platelets and neutrophils in patients with suspected immune-mediated DITMA, 1988-2014. Among 58 patients in the Oklahoma Registry previously categorized as drug-induced (15 suspected drugs), 21 patients (three drugs: gemcitabine, pentostatin, quinine) had evidence supporting a definite association with TMA; 19 (90%) of the 21 patients had quinine-induced TMA. The BloodCenter of Wisconsin tested 40 patients with suspected DITMA (eight drugs); drug-dependent antibodies, supporting a definite association with TMA, were identified in 30 patients (three drugs: oxaliplatin, quinine, vancomycin); 28 (93%) of the 30 patients had quinine-induced TMA. Combining the data from these two sources, 51 patients (five drugs) have been identified with evidence supporting a definite association with TMA. DITMA was attributed to quinine in 47 (92%) of these 51 patients.

Bortezomib for refractory autoimmunity in pediatrics

Therapy of refractory autoimmunity remains challenging. In this study, we evaluated the therapeutic effect of bortezomib, a proteasome inhibitor, by targeting plasma cells in 7 patients (median age, 9.9 years). Four doses of bortezomib were administered at a dose of 1.3 mg/m(2) intravenously (n = 6) or subcutaneously (n = 1) every 72 hours. Bortezomib was administered at a median of 120 days from laboratory confirmation of autoantibodies. All patients had failed 2 or more standard therapies. Rituximab was administered on the first day if B cells were present, and all patients received plasmapheresis 2 hours before bortezomib administration. Six patients experienced resolution of cytopenias. Two of 6 patients experienced recurrence of cytopenias after initial response. Adverse effects include nausea (n = 1), thrombocytopenia (n = 2), Clostridium difficile colitis (n = 1)), febrile neutropenia (n = 1), and cellulitis at the subcutaneous injection site (n = 1). Our experience suggests that bortezomib may be beneficial in the treatment of refractory autoimmunity in children.

Transfusion-related acute lung injury-associated HNA-3a antibodies recognize complex determinants on choline transporter-like protein 2

BACKGROUND

HNA-3a-specific antibodies can cause severe, sometimes fatal, transfusion-related acute lung injury when present in transfused blood. The HNA3-a/b antigens are determined by an R154Q polymorphism in the first of five extracellular (EC) loops of the 10-membrane-spanning choline transporter-like protein 2 (CTL2) expressed on neutrophils, lymphocytes, and other tissues. Approximately 50% of HNA-3a antibodies (Type 1) can be detected using CTL2 Loop 1 peptides containing R154; the remaining 50% (Type 2) fail to recognize this target. Understanding the basis for this difference could guide efforts to develop practical assays to screen blood donors for HNA-3 antibodies.

STUDY DESIGN AND METHODS

Reactions of HNA-3a antibodies against recombinant versions of human, mouse, and human/mouse (chimeric) CTL2 were characterized using flow cytometry and various solid-phase assays.

RESULTS

The findings show that, for binding to CTL2, Type 2 HNA-3a antibodies require nonpolymorphic amino acid residues in the third, and possibly the second, EC loops of CTL2 to be in a configuration comparable to that found naturally in the cell membrane. In contrast, Type 1 antibodies require only peptides from the first EC loop that contain R154 for recognition.

CONCLUSION

Although Type 1 HNA-3a antibodies can readily be detected in solid-phase assays that use a CTL2 peptide containing R154 as a target, development of a practical test to screen blood donors for Type 2 antibodies will pose a serious technical challenge because of the complex nature of the epitope(s) recognized by this antibody subgroup.

Antiplatelet antibodies in oxaliplatin-induced immune thrombocytopenia

Drug-induced immune thrombocytopenia may be potentially fatal; here we report the development of severe thrombocytopenia with strong oxaliplatin-dependent antiplatelet antibodies.

A case of oxaliplatin-induced immune-mediated thrombocytopenia

Oxaliplatin is a platinum compound used in patients with gastrointestinal malignancies. It is known to evoke a drug-induced immune-mediated thrombocytopenia, which has not been reported in Korea. We describe a 53-year-old man who developed oxaliplatin-induced immune-mediated thrombocytopenia during chemotherapy for colon cancer. Oxaliplatin-dependent IgG platelet antibodies were detected in his serum on flow cytometry. He was treated with immunoglobulin and corticosteroids without any complications. Physicians should consider oxaliplatin-induced immune-mediated thrombocytopenia, when a sudden, isolated thrombocytopenia develops during chemotherapy with oxaliplatin.

Drug-induced immune thrombocytopenia: incidence, clinical features, laboratory testing, and pathogenic mechanisms

Drug-induced immune thrombocytopenia (DIIT) is a relatively uncommon adverse reaction caused by drug-dependent antibodies (DDAbs) that react with platelet membrane glycoproteins only when the implicated drug is present. Although more than 100 drugs have been associated with causing DIIT, recent reviews of available data show that carbamazepine, eptifibatide, ibuprofen, quinidine, quinine, oxaliplatin, rifampin, sulfamethoxazole, trimethoprim, and vancomycin are probably the most frequently implicated. Patients with DIIT typically present with petechiae, bruising, and epistaxis caused by an acute, severe drop in platelet count (often to <20,000 platelets/pL). Diagnosis of DIIT is complicated by its similarity to other non-drug-induced immune thrombocytopenias, including autoimmune thrombocytopenia, posttransfusion purpura, and platelet transfusion refractoriness, and must be differentiated by temporal association of exposure to a candidate drug with an acute, severe drop in platelet count. Treatment consists of immediate withdrawal of the implicated drug. Criteria for strong evidence of DIIT include (1) exposure to candidate drug-preceded thrombocytopenia; (2) sustained normal platelet levels after discontinuing candidate drug; (3) candidate drug was only drug used before onset of thrombocytopenia or other drugs were continued or reintroduced after resolution of thrombocytopenia, and other causes for thrombocytopenia were excluded; and (4) reexposure to the candidate drug resulted in recurrent thrombocytopenia. Flow cytometry testing for DDAbs can be useful in confirmation of a clinical diagnosis, and monoclonal antibody enzyme-linked immunosorbent assay testing can be used to determine the platelet glycoprotein target(s), usually GPIIb/IIIa or GPIb/IX/V, but testing is not widely available. Several pathogenic mechanisms for DIIT have been proposed, including hapten, autoantibody, neoepitope, drug-specific, and quinine-type drug mechanisms. A recent proposal suggests weakly reactive platelet autoantibodies that develop greatly increased affinity for platelet glycoprotein epitopes through bridging interactions facilitated by the drug is a possible mechanism for the formation and reactivity of quinine- type drug antibodies.

Drug-induced immune neutropenia/agranulocytosis

Neutrophils are the most abundant white blood cell in blood and play a critical role in preventing infections as part of the innate immune system. Reduction in neutrophils below an absolute count of 500 cells/pL is termed severe neutropenia or agranulocytosis. Drug-induced immune neutropenia (DIIN) occurs when drug-dependent antibodies form against neutrophil membrane glycoproteins and cause neutrophil destruction. Affected patients have fever, chills, and infections; severe infections left untreated can result in death. Treatment with granulocyte colony-stimulating factor can hasten neutrophil recovery. Cumulative data show that severe neutropenia or agranulocytosis associated with exposure to nonchemotherapy drugs ranges from approximately 1.6 to 15.4 cases per million population per year. Drugs most often associated with neutropenia or agranulocytosis include dipyrone, diclofenac, ticlopidine, calcium dobesilate, spironolactone, antithyroid drugs (e.g., propylthiouracil), carbamazepine, sulfamethoxazole- trimethoprim, [3-lactam antibiotics, clozapine, levamisole, and vancomycin. Assays used for detection of neutrophil drug-dependent antibodies (DDAbs) include flow cytometry, monoclonal antibody immobilization of granulocyte antigens, enzyme-linked immunosorbent assay, immunoblotting, granulocyte agglutination, and granulocytotoxicity. However, testing for neutrophil DDAbs is rarely performed owing to its complexity and lack of availability. Mechanisms proposed for DIIN have not been rigorously studied, but those that have been studied include drug- or hapten-induced antibody formation and autoantibody production against drug metabolite or protein adducts covalently attached to neutrophil membrane proteins. This review will address acute, severe neutropenia caused by neutrophil-reactive antibodies induced by nonchemotherapy drugs-DIIN

Drug-induced thrombocytopenia in children

BACKGROUND

Acute, immune-mediated thrombocytopenia may be caused by many different approved drugs as well as by other substances including vaccines, complementary and alternative medicines, herbal remedies, nutritional supplements, foods and beverages. All causes are described as drug-induced thrombocytopenia (DITP). Often the cause is not recognized, resulting in recurrent thrombocytopenia and inappropriate treatments. Systematic analysis of children (age less than 18 years) with suspected DITP has not been previously reported.

PROCEDURES

(1) We searched 15 databases to identify articles describing children with thrombocytopenia as an adverse effect of drugs and other substances. Articles were reviewed to assign levels of evidence for an association of the suspected substance with thrombocytopenia. (2) Data from the BloodCenter of Wisconsin were reviewed to identify reports of drug-dependent, platelet-reactive antibodies in children with suspected DITP.

RESULTS

Of 2,191 articles identified, 242 were selected for review. Seventy-two articles reporting 74 individual patients and nine groups of patients had evaluable data. Eleven individual patients and one group had definite evidence and 40 patients and three groups had probable evidence for an association of the suspected substance with thrombocytopenia. Thirty-two substances had a definite or probable association with thrombocytopenia. During 2008-2012, sera from 91 children with suspected DITP were tested and 21 had drug-dependent, platelet-reactive antibodies involving six substances.

CONCLUSIONS

Drugs and other substances must be considered as potential causes of thrombocytopenia. Evidence from published reports and data for drug-dependent, platelet-reactive antibodies can help clinicians evaluate of children with unexpected thrombocytopenia.

Low-frequency human platelet antigens as triggers for neonatal alloimmune thrombocytopenia

BACKGROUND

Twenty-four low-frequency human platelet antigens (LFHPAs) have been implicated as immunogens in neonatal alloimmune thrombocytopenia (NAIT). We performed studies to define more fully how often these antigens trigger maternal immunization leading to NAIT.

STUDY DESIGN AND METHODS

In a Phase 1 study, fathers of selected NAIT cases not resolved by serologic testing but thought to have a high likelihood of NAIT on clinical and serologic grounds were typed for LFHPAs by DNA sequencing. In a Phase 2 study, high-throughput methods were used to type fathers of 1067 consecutive unresolved NAIT cases for LFHPAs. Mothers of 1338 unresolved cases were also typed to assess the prevalence of LFHPAs in a population racially/ethnically similar to the fathers.

RESULTS

In Phase 1, LFHPAs were identified in 16 of 244 fathers (6.55%). In Phase 2, LFPAs were found in only 28 of 1067 fathers (2.62%). LFHPAs were identified in 27 of 1338 maternal samples (2.01%). HPA-9bw was by far the most common LFHPA identified in the populations studied and was the only LFHPA that was significantly more common in fathers than in mothers of affected infants (p = 0.02).

CONCLUSIONS

Maternal immunization against recognized LFHPAs accounts for only a small fraction of the cases of apparent NAIT not resolved by standard serologic testing. Typing of the fathers of such cases for LFHPAs is likely to be rewarding only when a maternal antibody specific for a paternal platelet glycoprotein is demonstrated and/or there is compelling clinical evidence for NAIT.