Padmanabhan, Anand

ABSTRACT Background Monoclonal gammopathy of thrombotic significance (MGTS) is a recently described chronic prothrombotic condition characterized by monoclonal anti-PF4 antibodies that are detected above the polyclonal antibody background in patient sera (i.e. present as monoclonal gammopathy of undetermined significance, MGUS). Due to conflicting data in the published literature on antibody clonality in heparin-induced thrombocytopenia (HIT), we evaluated clonality and abundance of anti-PF4 antibodies in HIT, including investigating whether an MGUS, if present in HIT, represents the causative anti-PF4 antibody. Methods Blood samples from 15 patients with HIT were subject to Platelet Factor 4-dependent antigen-based and functional tests. The unmanipulated serum antibody repertoire and isolated anti-PF4 antibodies were subjected to mass spectrometric evaluation. Results Two of the 15 HIT patients had an IgG MGUS. Notably, anti-PF4 antibodies were not synonymous with the MGUS antibody in either of the two patients. Eight of the 15 patients demonstrated monoclonal anti-PF4 antibodies, however, none of the anti-PF4 antibodies were detectable as an MGUS upon evaluation of the entire serum antibody repertoire, reflecting their low abundance. In the seven patients with multiple anti-PF4 antibodies, non-monoclonality was confirmed by analysis of deglycosylated antibody heavy chains. Conclusions Anti-PF4 HIT antibodies are monoclonal in approximately 50% of HIT patients, however, antibody abundance is low such that they are not detectable over the polyclonal IgG background (i.e. are MGUS-negative), differentiating HIT from MGTS. This observation helps explain the transient nature of HIT relative to the persistent prothrombotic state seen in MGTS.

ABSTRACT Background Platelet factor 4‐polyanion enzyme‐linked immunosorbent assays (ELISAs) are considered highly sensitive for diagnosing heparin‐induced thrombocytopenia (HIT), such that current practice guidelines recommend use of ELISA‐negative results to exclude HIT. Once HIT is ruled out, alternative, non‐heparin‐based anticoagulant treatments are ceased, and heparin reintroduction frequently occurs. Methods Antigen‐based and PF4‐dependent functional testing were used to study PF4/polyvinylsulfonate ELISA‐negative platelet‐activating antibodies in HIT‐suspected patients. Results Three patients with clinical presentations consistent with HIT tested negative in an ELISA using PF4‐polyvinylsulfonate (PF4/PVS), an antigenic target very commonly used for HIT antibody screening. All three patients demonstrated PF4‐dependent platelet activation in functional testing that was sensitive to blockade of platelet FcγRIIa receptors and inhibited by high concentrations of heparin, consistent with pathogenic HIT antibodies. Functional testing‐based screening of 500 ELISA‐negative patients identified three additional patients whose sera activated platelets in a PF4‐ and FcγRIIa‐dependent manner and had clinical histories consistent with HIT. Five of the six ELISA‐negative HIT patients were re‐exposed to heparin, which precipitated a decrease in platelet counts in all re‐exposed patients, and one patient developed a new thrombus. Conclusions Recognition of ELISA‐negative HIT is critical to avoid harm due to the cessation of alternative anticoagulation therapy and re‐exposure of these patients to heparin.

Abstract The hallmark of heparin-induced thrombocytopenia (HIT) is the presence of immunoglobulin G (IgG) antibodies against platelet factor 4/heparin (PF4/H) complexes, typically detected by PF4/H enzyme-linked immunosorbent assay (ELISA); thus, negative ELISA results are commonly used to exclude this diagnosis. Here, we report a prevalent yet previously unrecognized subset of antibodies that are undetectable by PF4/H ELISA (ELISA−) but activate platelets in the PF4-dependent P-selectin expression assay (PEA+). In 11 patients with clinically confirmed HIT who tested positive in both PF4/H ELISA and platelet activation assays, ELISA−PEA+ antibodies accounted for 65% ± 19% of total platelet-activating IgG activity and coexisted with ELISA+PEA+ antibodies. Consistent with this finding, single-cell cloning from 7 patients with HIT identified 23 PEA+ antibody-producing B-cell clones, of which 17 were ELISA−, outnumbering the ELISA+ clones. Functionally, ELISA−PEA+ antibodies closely resembled ELISA+PEA+ antibodies: platelet binding and activation required exogenous PF4 and were inhibited by FcγRIIA blockade, high-dose heparin, or Fab fragments made from ELISA+PEA+ antibodies. Importantly, these antibodies induced thrombocytopenia in a humanized mouse model of HIT. Despite lacking PF4/H reactivity in ELISAs, they recognize PF4 on platelets and showed no appreciable binding to neutrophil-activating peptide-2, interleukin-8, or PF4 alone. Structurally, these antibodies were heterogeneous, with a subset sharing heavy-chain features with ELISA+PEA+ antibodies. Collectively, our findings demonstrate that ELISA−PEA+ antibodies are a common, previously unrecognized feature of HIT, with functional relevance, supporting the possibility that they play an important, perhaps even central, role in HIT pathogenesis. Defining their prevalence, kinetics, and clinical impact deserves high priority for further investigation.

Summary This nutshell review discusses the pathophysiology, diagnostic features, treatment strategies and avenues for future research in a recently identified thrombotic entity, monoclonal gammopathy of thrombotic significance (MGTS). MGTS is an anti‐platelet factor 4 (PF4) antibody‐mediated disorder distinct from heparin‐induced thrombocytopenia (HIT) and vaccine‐induced thrombotic thrombocytopenia (VITT), as it involves the production of persistent, monoclonal anti‐PF4 antibodies. Several subtypes of MGTS, including ‘HIT‐like’, ‘VITT‐like’ and ‘non‐HIT/VITT‐like’, exhibit unique serological profiles that can complicate diagnosis. Recognition of ‘HIT‐like’ MGTS is critical to avoid heparin exposure in these patients, which can result in adverse effects like those seen in heparin‐exposed HIT patients. ‘Non‐HIT/VITT‐like’ MGTS antibodies have only been detected in PF4‐enhanced functional testing; therefore, we recommend using these platelet‐based tests as the front‐line assay for detecting MGTS antibodies. Mass profiling using mass spectrometry is critical for the ‘fingerprinting’ of monoclonal antibodies to establish a diagnosis of MGTS. Treatments used in HIT and VITT, such as non‐heparin anticoagulants and intravenous immunoglobulin G, are typically insufficient for managing MGTS. Emerging MGTS therapies, such as plasma cell‐directed drugs and Bruton tyrosine kinase inhibitors, are discussed. The review concludes with an emphasis on areas of future research in MGTS.

Abstract Vaccine‐induced immune thrombotic thrombocytopenia (VITT) is a new disorder that emerged in the wake of coronavirus disease 2019 (COVID‐19) vaccination. It is a rare but life‐threatening condition that requires an aggressive course of treatment to improve patient outcomes. Currently, the diagnosis of VITT relies on a comprehensive panel of criteria, including a history of recent vaccination, platelet count, positive enzyme linked immunosorbent assays (ELISA) result for a closely related thrombotic disorder, heparin‐induced thrombocytopenia (HIT), and platelet factor 4 (PF4)‐dependent functional assays. The study describes a technically simple antigenic assay designed to detect autoimmune antibodies associated with VITT. It is first shown that cross‐linked platelet factor 4 (c‐PF4) represents an antigenic target specific for VITT/VITT‐like Abs. This antigenic target is then incorporated into a microfluidic electrochemical biosensor and specific and sensitive detection of VITT/VITT‐like antibodies is demonstrated in a fully automated manner while using microliter volumes of patient sera. Fifty‐five serum samples are tested using the microfluidic electrochemical biosensor and demonstrated high accuracy for the detection of VITT sera and VITT‐like monoclonal gammopathy of thrombotic ignificance (MGTS) antibodies compared to healthy controls and HIT patients. It is envisioned that the microfluidic electrochemical biosensor will be used in combination with other clinical criteria to enable timely, sensitive, and specific diagnosis of VITT.

Abstract Introduction: Monoclonal gammopathy of thrombotic significance (MGTS) is a recently recognized thrombophilia where monoclonal platelet factor 4 (PF4)-specific IgG antibodies initiate a chronic prothrombotic state characterized by recurrent and potentially life-threatening thrombosis. Studies from large data sets demonstrate that patients with monoclonal gammopathy of undetermined significance (MGUS) have a heightened risk of experiencing thromboembolic events without a precise explanatory mechanism. The prevalence of MGTS in MGUS-related thrombosis is unknown; however, the recent identification of MGTS antibodies as a cause of thrombosis suggests a potentially causative mechanism. Emerging evidence indicates that detecting pathogenic MGTS antibodies presents diagnostic challenges due to the insensitivity of FDA-approved diagnostic assays adopted from HIT (HIT ELISAs) and gold-standard functional tests that use heparin-treated platelets, such as the serotonin release assay (SRA). Only PF4-enhanced functional assays, including the PF4-dependent P-selectin expression assay (PEA) and PF4-SRA, have proven sensitive for detecting MGTS antibodies. In this study, the PEA was utilized to better define the incidence of pathogenic anti-PF4 antibodies among a cohort of MGUS patients with a history of thrombosis. Methods: A search of the Mayo Clinic Dysproteinemia Bank was conducted to identify two cohorts of IgG MGUS patients, one with venous thromboembolism-associated diagnosis code(s) and the other without. A diagnosis of multiple myeloma was exclusionary. Each serum sample was assessed using the PEA. Mayo Clinic's Institutional Review Board approved the research study. Results: Among the 134 patients studied in the thrombosis cohort, 17 (13%) stimulated platelet activation in the PEA (positive cut-off ≥19%), some weakly. Of these, four patients (Patients 1-4) were confirmed to have PF4-dependent platelet-activating antibodies inhibitable with high concentrations of heparin and sensitive to blockade of the IgG receptor on platelets, FcGRIIa. The thrombotic complications experienced included recurrent deep venous thrombosis (DVT) over several years (Patient 1), a single DVT (Patient 2), a thrombotic cerebrovascular accident (CVA) (Patient 3), and a series of thrombotic events, including DVTs, left atrial thrombosis, pulmonary embolism, and acute limb ischemia (Patient 4). Notably, longitudinal samples spanning twelve years (Patient 1), six years (Patient 2), three years (Patient 3), and on a single sample date (Patient 4) were confirmed positive by PEA. The prolonged persistence of platelet-activating anti-PF4 antibodies in Patients 1-3 supported a diagnosis of MGTS in these patients. Strikingly, all four patients tested negative in two FDA-approved HIT ELISAs (Immucor PF4 IgG and Zymutest HIA IgG). SRA was negative for Patients 1 and 2, weakly positive for Patient 3 (33%), and positive for Patient 4 (72%). Of the 166 patients in the MGUS minus thrombosis cohort, one patient demonstrated a non-heparin induced HIT-like antibody that was HIT ELISA (Optical Density >1.0) and PEA-positive, and inhibitable using high concentrations of heparin and FcGRIIa blockade. No follow-up samples were available to assess antibody longevity. The patient had no history of thrombosis, and testing demonstrated that the anti-PF4 antibody was not synonymous with the MGUS antibody.Conclusions: Among a cohort of 134 MGUS patients with thrombosis, the prevalence of MGTS was found to be 2.2% (3 patients with persistent platelet-activating anti-PF4 antibodies). The identification of anti-PF4 antibodies among MGUS patients provides a mechanism contributing to the increased thrombotic risk associated with monoclonal gammopathies. Antibody negativity in HIT ELISA and variable positivity in the SRA highlight the nonclassical serology associated with anti-PF4 antibodies in MGTS and support performing PF4-enhanced functional testing. In addition, some patients appear to produce coincidental (non-monoclonal) platelet-activating anti-PF4 antibodies without evidence of thrombosis, although the longevity of such antibodies could not be examined in the one patient where it was identified. These findings highlight the need for larger, prospective MGUS studies to validate these observations, further define the interplay between monoclonal gammopathies and thrombotic risk, and to better understand the long-term outcomes of patients with MGTS.

Abstract Introduction: Antisense oligonucleotides (ASOs) technology has emerged as a transformative approach in molecular therapeutics, offering precise regulation of gene expression. ASOs are short, synthetic single-stranded nucleic acids that bind to target RNA sequences via Watson–Crick base pairing. Depending on their chemical modifications (e.g., phosphorothioate linkages) and sequence design (e.g., aptamer, gapmer), ASOs can mediate RNA degradation via RNase H, modulate pre-mRNA splicing, or inhibit translation. This targeted mechanism of action positions ASOs as promising treatments for a variety of genetic, inflammatory, and neoplastic diseases. More than 10 ASOs have received regulatory approval. However, safety concerns like injection site reactions, thrombocytopenia, renal toxicity, and liver enzyme elevations have somewhat limited their broader clinical application. Notably, Inotersen and Volanesorsen were associated with severe thrombocytopenic events, including fatalities in phase 3 trials of Volanesorsen. This clinical profile of ASO-associated thrombocytopenia mirrors some aspects of heparin-induced thrombocytopenia (HIT), an immune-mediated condition triggered by anti-platelet factor 4 (PF4) antibodies recognizing PF4–heparin complexes. Hypothesis & Methods: This study investigated the hypothesis that PF4, a positively charged protein, interacts with negatively charged ASOs to form immunogenic complexes that expose neoepitopes, triggering recognition by pathogenic antibodies from HIT patients. Twelve ASOs, Inotersen, Eplontersen, Olezarsen, Fomivirsen, Mipomersen, Tofersen, Nusinersen, Eteplirsen, Golodirsen, Viltolarsen, Casimersen (all FDA approved) and Volanesorsen (EMA approved) were evaluated in this study. All twelve ASOs had lengths ranging from 18 to 30 nucleotides. The association of HIT patient antibodies with ASO-PF4 complexes was evaluated by ELISA and analytical ultracentrifugation. Functional testing was performed with donor platelets treated with ASOs. Results: HIT antibodies recognized PF4–ASO complexes in 7 out of 12 ASOs in a stoichiometry-dependent manner, similar to PF4-heparin complexes, with maximal binding at specific molar ratios. HIT antibodies activated platelets incubated with Inotersen and Volanesorsen, analogous to HIT antibody-mediated activation of heparin-treated platelets in assays such as the serotonin release assay. Interestingly, the addition of Fomivirsen and Eteplirsen induced platelet activation independent of HIT antibodies. Many ASOs have phosphorothioate [PS] linkages, a modification that enhances stability to nuclease digestion and confers additional pharmacokinetic advantages. All ASO-PF4 complexes recognized by HIT antibodies in ELISA and those that supported HIT antibody-mediated platelet activation contained PS linkages. Removal of PS linkages from a PS-containing ASO, Inotersen, abolished antibody binding to PF4-Inotersen complexes, and HIT antibody-mediated platelet activation of Inotersen-treated platelets. Conversely, introducing PS linkages into a non-PS containing ASO, Viltolarsen conferred immunogenic properties to the modified ASO, supporting both recognition by HIT antibodies in ELISA and activation of platelets by HIT antibodies. A platelet binding assay revealed nearly five-fold greater HIT antibody binding to human platelets pretreated with PS-modified ASOs compared to their PS-free counterparts. Analytical ultracentrifugation confirmed that PF4 formed large aggregates immediately upon interaction with PS-containing ASOs. Additionally, we isolated and recombinantly expressed a human monoclonal HIT-like antibody that recognizes PF4–ASO complexes in ELISA and activates ASO-treated platelets in functional assays which could serve as a valuable reagent to investigate ASO-mediated thrombocytopenia.Conclusion: With two ASOs, Fomivirsen and Eteplirsen, direct activation of platelets was noted. Studies with additional ASOs revealed a novel immune mechanism involving ASO-PF4 complex formation and anti-PF4 antibody recognition that can plausibly mediate ASO-induced thrombocytopenia. These findings highlight the key role PS linkages may play in ASO immunogenicity and provide a mechanistic framework for risk mitigation in ASO drug design, supporting the safer development and broader application of ASO therapeutics.

Abstract Introduction: Only 30% of patients (pts) presenting with abnormal bleeding symptoms receive a clear diagnosis, with many classified as having bleeding disorder of unknown cause, or BDUC (Baker et al. 2021). Currently, the recommended diagnostic workup for BDUC by the International Society on Thrombosis and Haemostasis (ISTH) includes plasmatic assays for coagulation factors, von Willebrand factor (vWF) antigen/activity, and platelet function testing (PFT), specifically platelet function analyzer-100 (PFA-100) and light transmission aggregometry (LTA). However, PFTs are not widely available and cannot be mailed to reference laboratories. We studied the diagnostic yield of an expanded testing panel which includes quantitative platelet glycoprotein flow cytometry (GpFL) and platelet electron microscopy (PTEM), which we have optimized for specimen mailing. Methods: We conducted a minimal risk, retrospective study by reviewing the electronic health records of pts (18 years and older) who were seen in the Coagulation Clinic of a tertiary academic medical center for evaluation of a bleeding disorder between October 2017 and June 2025. Testing included plasma-based assays including PT, APTT, vWF assay, FVIII, FIX, FXIII screen, and thrombin time (and additional reflex assays as needed). At the discretion of the treating clinician, PFTs (PFA-100 and LTA), GpFL, PTEM, and fibrinolysis assays (plasminogen activator inhibitor-1 antigen [PAI-1 Ag] and alpha-2 plasmin inhibitor) were included. We abstracted clinical data on pt demographics, ISTH bleeding scores, and the results of coagulation testing. Pts with an established diagnosis of a bleeding disorder, underlying malignancy, and those who did not have plasma-based coagulation testing were excluded from this study. Results: 109 pts met study criteria, and 82.6% were females (n = 90) with a median age of 45 years (range = 18-88 years). The median ISTH-BAT score was 7 (range: 2-21). About half of the pts, 49.5% (n = 54), had normal coagulation profiles and PFTs. All 54 had PTEM performed, of which 14.8% (n = 8) were abnormal (alpha and/or dense granule deficiencies). 85.2% (n = 46) had GpFL testing performed, with one showing reduced Gp2b/CD42 and Gp3a/CD61 expression, four showing reduced Gp1a/CD49b expression, and three showing reduced GpVI expression. Half of the pts, 50.5% (n = 55) had an abnormal coagulation profile from plasma and/or PFTs, of which 20% (n = 11) had a clinically significant coagulation factor deficiency (FV, FVII, FX). Roughly 9.1% (n = 5) had type 1 vWD of which 1 also had PAI-1 deficiency. One pt had isolated PAI-1 deficiency. Abnormal coagulation profiles were noted in 53 pts who had PFA-100 and LTA done, and both tests were normal in 22.6% (n = 12). Within the abnormal coagulation profile group, 10.9% (n = 6) had ultrastructural abnormalities on PTEM (dense and alpha granule abnormalities). Majority of the pts (n = 46) had GpFL done, and one had abnormal reductions in Gp1b-aCD42b, three in Gp1aCD49b, and seven in GPVI. Comparison of the ISTH BAT score between the normal and abnormal coagulation profiles/PFTs groups by non-parametric (Mann-Whitney U test) and parametric (Welch's t-test) showed no significant difference (p = 0.56 and p = 0.87) respectively. Conclusion: In this cohort of pts with abnormal bleeding, standard recommended testing resulted in a diagnosis in 66.4% pts. The remainder would have been classified as BDUC. The addition of GpFL and PTEM resulted in establishing a diagnosis that explains the bleeding phenotype in 14.8% of pts with an otherwise normal coagulation profile and PFT. Although abnormal PFTs indicate a platelet function defect, GpFL and PTEM may further define the abnormalities particularly differentiating syndromic from non-syndromic inherited platelet disorders. Additional studies are needed to confirm our findings and explore the possibility of addition of GpFL and PTEM for patients with BDUC.

ABSTRACT BACKGROUND Platelet factor 4-polyanion enzyme-linked immunosorbent assays (ELISAs) are considered highly sensitive for diagnosing heparin-induced thrombocytopenia (HIT), such that current practice guidelines recommend use of ELISA-negative results to exclude HIT. Once HIT is ruled out, alternative, non-heparin-based anticoagulant treatments are ceased, and heparin reintroduction frequently occurs. METHODS Antigen-based and PF4-dependent functional testing were used to study PF4/polyvinyl sulfonate ELISA-negative platelet-activating antibodies in HIT-suspected patients and mice immunized with PF4/heparin. RESULTS Three patients with clinical presentations consistent with HIT tested negative in an ELISA using PF4-polyvinylsulfonate (PF4/PVS), an antigenic target very commonly used for HIT antibody detection. All three patients demonstrated PF4-dependent platelet activation in functional testing that was sensitive to blockade of platelet FcγRIIa receptors and inhibited by high concentrations of heparin, consistent with pathogenic HIT antibodies. Functional testing-based screening of 500 ELISA-negative patients identified three patients whose sera activated platelets in a PF4- and FcγRIIa-dependent manner, and had clinical histories consistent with HIT. Five of the six ELISA-negative HIT patients were re-exposed to heparin, which precipitated a decrease in platelet counts in all re-exposed patients, and one patient developed a new thrombus. To advance the study of ELISA-negative HIT antibodies, mice were immunized with PF4/heparin, and functional and antigenic assays were simultaneously used to successfully identify an ELISA-negative, PF4-dependent platelet-activating murine monoclonal antibody that recapitulated the serological characteristics of ELISA-negative HIT patients. CONCLUSIONS Recognition of ELISA-negative HIT is critical to avoid harm due to the cessation of alternative anticoagulation therapy and re-exposure of these patients to heparin.