Anti-platelet factor 4 (PF4) antibody-mediated disorders are a heterogeneous group of diseases characterized by the presence of highly pathogenic immunoglobulins G directed against PF4 and/or PF4/heparin complexes. These antibodies are able to activate platelets, neutrophils, and monocytes, thus resulting in thrombocytopenia and a hypercoagulable state. Five different forms of anti-PF4 antibody-mediated disorders have been identified: (1) classic heparin-induced thrombocytopenia (HIT) mediated by heparin and certain polyanionic drugs; (2) autoimmune HIT characterized by the presence of anti-PFA/polyanion antibodies that can strongly activate platelets even in the absence of heparin; (3) spontaneous HIT characterized by thrombocytopenia and thrombosis without proximate exposure to heparin, with two subtypes: (a) post-total knee arthroplasty and cardiac surgery using cardiopulmonary bypass or extracorporeal membrane oxygenation and (b) postinfections; (4) vaccine-induced immune thrombotic thrombocytopenia (VITT) characterized by thrombocytopenia, arterial and venous thrombosis, or secondary hemorrhage after receiving adenoviral vector vaccines for coronavirus disease 2019; (5) VITT-like disorders triggered by adenoviral infections. Although extremely rare and largely unknown, there has been growing interest in the VITT syndrome in recent years due to its clinical relevance. Timely detection of these antibodies is crucial for the diagnosis and treatment of anti-PF4 antibody-mediated disorders, via anti-PF4 antibody immunoassays using several antibody capture systems (e.g., enzyme-linked immunosorbent assay-based, particle gel, turbidimetry) and functional assays (e.g., serotonin release assay or heparin-induced platelet activation). We aimed to present the latest on laboratory findings, clinical characteristics, and therapeutic approaches for anti-PF4 antibody-mediated disorders.

Four platelet-activating anti-platelet factor 4 (PF4) disorders have been recognized: classic heparin-induced thrombocytopenia (cHIT), autoimmune heparin-induced thrombocytopenia (aHIT), spontaneous heparin-induced thrombocytopenia (SpHIT), and vaccine-induced immune thrombotic thrombocytopenia (VITT). All test immunoglobulin G (IgG) positive using solid-phase enzyme immunoassay (solid-EIA) against PF4/heparin (PF4/H) and/or PF4 alone. Fluid-phase EIA (fluid-EIA) should better discriminate between anti-PF4 and anti-PF4/H antibodies since conformationally altered PF4 bound to solid phase is avoided. To compare anti-PF4 vs anti-PF4/H antibody profiles for anti-PF4 disorders using solid- and fluid-EIA. We developed a novel fluid-EIA to measure anti-PF4 vs anti-PF4/H antibodies. Using fluid-EIA, 27 of 27 (100%) cHIT sera tested IgG positive with PF4/H, but only 4 of 27 (14.8%) tested positive against PF4 alone; all 27 exhibited heparin-enhanced binding. In contrast, 17 of 17 (100%) VITT sera tested IgG positive against PF4 alone, with markedly reduced binding against PF4/H; this distinct VITT antibody profile was not evident using solid-EIA. All 15 aHIT sera and all 11 SpHIT sera tested IgG positive against PF4 alone, with variable reactivity in PF4/H-EIA (heparin-enhanced binding in 14 of 15 and 10 of 11 aHIT and SpHIT sera, respectively). Remarkably, 1 SpHIT patient with a VITT-mimicking fluid-EIA profile (PF4 >> PF4/H) also clinically resembled patients with VITT (postviral cerebral vein/sinus thrombosis), with anti-PF4 reactivity correlating inversely with platelet count recovery; moreover, the single aHIT patient with a VITT-mimicking fluid-EIA profile also developed postviral cerebral vein/sinus thrombosis. cHIT and VITT sera showed opposite fluid-EIA profiles (cHIT: PF4/H >> PF4, with most testing negative against PF4 alone; VITT: PF4 >> PF4/H, with most testing negative against PF4/H). In contrast, all aHIT and SpHIT sera reacted against PF4 alone but with variable (usually enhanced) reactivity against PF4/H. VITT-mimicking clinical/serologic profiles occurred in only a minority of patients with SpHIT and aHIT.

Platelet factor 4 (PF4) is a highly cationic tetrameric protein that can be targeted by platelet-activating anti-PF4 antibodies of immunoglobulin G (IgG) class. Certain features of PF4, including its multivalent nature (duplicate antigen sites per tetramer), the ability of many PF4 tetramers to undergo close approximation through charge neutralization, and the dimeric binding of IgG molecules, results in formation of IgG-containing immune complexes in situ on platelets, neutrophils, and monocytes, resulting in Fcγ receptor-mediated pancellular activation that also activates hemostasis (potential for disseminated intravascular coagulation). This review discusses 4 anti-PF4 disorders: classic heparin-induced thrombocytopenia ([HIT]; triggered by heparin and certain other polyanionic pharmaceuticals, featuring predominantly heparin-dependent antibodies), autoimmune HIT (aHIT; severe subtype of HIT that features both heparin-dependent and heparin-independent platelet-activating antibodies), and spontaneous HIT (non-heparin triggers such as knee replacement surgery and infection; predominantly heparin-independent platelet-activating antibodies). Most recently, a novel fourth anti-PF4 disorder, vaccine-induced immune thrombotic thrombocytopenia (VITT), was identified as an ultrarare complication of adenovirus vector vaccines. VITT is characterized by thrombocytopenia, disseminated intravascular coagulation, a high frequency of thrombosis-including in unusual sites (cerebral veins, splanchnic veins)-and highly pathogenic anti-PF4 antibodies with heparin-independent platelet-activating properties.

Currently, the world is coping with the COVID-19 pandemic with a few vaccines. So far, the European Medicine Agency has approved four of them. However, following widespread vaccination with the recombinant adenoviral vector-based Oxford-AstraZeneca vaccine, available only in the United Kingdom and Europe, many concerns have emerged, especially the report of several cases of the otherwise rare cerebral sinus vein thrombosis and splanchnic vein thrombosis. The onset of thrombosis particularly at these unusual sites, about 5--14 days after vaccination, along with thrombocytopenia and other specific blood test abnormalities, are the main features of the vaccine side effects. The acronym vaccine-induced prothrombotic immune thrombocytopenia (VIPIT) has been coined to name this new condition, with the aim of highlighting the difference from the classic heparin-induced thrombocytopenia (HIT). VIPIT seems to primarily affect young to middle-aged women. For this reason, the vaccine administration has been stopped or limited in a few European countries. Coagulopathy induced by the Oxford-AstraZeneca vaccine (and probably by Janssen/Johnson & Johnson vaccine as well in the USA) is likely related to the use of recombinant vector DNA adenovirus, as experimentally proven in animal models. Conversely, Pfizer and Moderna vaccines use mRNA vectors. All vaccine-induced thrombotic events should be treated with a nonheparin anticoagulant. As the condition has some similarities with HIT, patients should not receive any heparin or platelet transfusion, as these treatments may potentially worsen the clinical course. Aspirin has limited rational use in this setting and is not currently recommended. Intravenous immunoglobulins may represent another potential treatment, but, most importantly, clinicians need to be aware of this new unusual postvaccination syndrome.