The clinical picture of moderate to severe thrombocytopenia and thrombotic complications at unusual sites starting about 1 to 2 weeks after vaccination against SARS-CoV-2 with ChAdOx1 nCov-19 indicates a disorder that is clinically similar to severe heparin-induced thrombocytopenia. a known prothrombotic disorder caused by platelet-activating antibodies that recognize multimolecular complexes between cationic PF4 and anionic heparin.6 However, in contrast to the usual situation in heparin-induced thrombocytopenia, these vaccinated patients received no heparin to explain the subsequent occurrence of thrombosis and thrombocytopenia.
In recent years it has been known that triggers other than heparin can cause a prothrombotic disorder that on clinical and serological grounds strongly resembles heparin-induced thrombocytopenia, including certain polionic drugs (e.g. pentosan polysulfate,7 anti-angiogenic agent PI-88,8 and hypersulfate chondroitin sulfate8). Such a prothrombotic syndrome has also been observed in the absence of prior exposure to any polionionic medication, such as after viral and bacterial infections.9.10 and knee replacement surgeries.11.12 These different clinical scenarios with apparent non-pharmacological triggers are classified under the term autoimmune heparin-induced thrombocytopenia.13 Unlike patients with classic heparin-induced thrombocytopenia, patients with autoimmune heparin-induced thrombocytopenia have extremely severe thrombocytopenia, an increased frequency of disseminated intravascular coagulation, and atypical thrombotic events. Serum from these patients activates platelets strongly in the presence of heparin (0.1 to 1.0 IU per milliliter), but also in the absence of heparin (heparin-independent platelet activation). When these unusual antibodies are observed in patients with thrombocytopenia without prior exposure to heparin, the term “spontaneous” treparositis syndrome is induced by heparin13.14 has been used. Occasionally, patients who develop heparin-induced thrombocytopenia may develop post-exposure to heparin with atypical clinical features, such as the onset of thrombocytopenia that begins a few days after discontinuation of heparin (heparin-induced thrombocytopenia)15.16) or thrombocytopenia lasting several weeks despite discontinuation of heparin (persistent or refractory heparin-induced thrombocytopenia17.18). Serum from these patients also shows the phenomenon of heparin-independent platelet activating properties.
These clinical features, similar to those of autoimmune heparin-induced thrombocytopenia, have been observed in patients with vaccine-induced immune thrombotic thrombocytopenia. The serum usually shows strong reactivity to the PF4-heparin ELISA. In addition, serum showed varying degrees of platelet activation in the presence of buffer which in most cases was greatly improved in the presence of PF4 (Figures 1A and 1B). More strikingly, most serum inhibition shows rather than increased activation in the presence of low-dose low-molecular-weight heparin (0.2 U per milliliter of anti-factor Xa). In addition, antibodies from two patients affinity-purified on immobilized PF4 or immobilized PF4-heparin activated platelets strongly, but only in the presence of PF4. Enhancement of platelet activation by PF4 is also a feature of heparin-induced thrombocytopenia19.20 and has been used to enhance the detection of platelet-activating antibodies during diagnostic tests for this adverse drug reaction.21 Whether these antibodies are autoantibodies to PF4 caused by the strong inflammatory stimulation of vaccination, or antibodies induced by the vaccine that cross-reacts with PF4 and platelets, needs to be further studied.
Although we found an increased reactivity of patient serum with platelets in the presence of ChAdOx1 nCov-19, it is likely an in vitro artifact. It is known that adenovirus binds to platelets22 and causes platelet activation.22.23 Furthermore, the amount of adenovirus in a 500 microliter vaccine injection administered 1 or 2 weeks earlier is unlikely to be similar to the subsequent platelet activation observed in these patients. However, interactions between the vaccine and platelets or between the vaccine and PF4 may play a role in pathogenesis. One possible trigger of these PF4-reactive antibodies may be the free DNA in the vaccine. We have previously shown that DNA and RNA form multimolecular complexes with PF4, which bind antibodies of patients with heparin-induced thrombocytopenia and also induce antibodies to PF4-heparin in a murine model.24 Unfortunately, other Covid-19 vaccines were not available for us to test.
Our findings have several important clinical implications. First, clinicians should note that venous or arterial thrombosis in some patients may develop in unusual places such as the brain or abdomen, becoming clinically visible approximately 5 to 20 days after vaccination. If such a reaction is associated with thrombocytopenia, it may be a detrimental effect of the previous vaccination against Covid-19. To date, this response has only been reported with the ChAdOx1 nCov-19 vaccine, which has been used in approximately 25% of vaccine recipients in Germany and in 30% of those in Austria.
Second, ELISA to detect PF4 heparin antibodies in patients with heparin-induced thrombocytopenia is widely available and can be used to screen patients for possible thrombocytopenia after vaccination or thrombosis related to antibodies to PF4.25 A strongly positive ELISA result obtained in a patient who has not been recently exposed to heparin is a striking disorder.
Third, we showed that these antibodies recognize PF4 and that the addition of PF4 significantly improves their observability in a platelet activation test. Since vaccination of millions of people will be hampered by a background of non-vaccination-related thrombotic events, a PF4-dependent ELISA or a PF4-enhanced platelet activation test can be used to diagnose vaccine-induced immune thrombotic thrombocytopenia by confirming this novel. mechanism of post-vaccination platelet-activating antibodies against PF4. Although treatment decisions such as the administration of intravenous immunoglobulin and the initiation of anticoagulation are not necessary, laboratory diagnosis should be awaited, but the detection of these unusual platelet-activating antibodies is very relevant for the identification of cases and future risk benefit assessment of these and other vaccines.
Potential diagnostic and therapeutic strategies for the management of suspected vaccine-induced immune thrombotic thrombocytopenia.
Shown is a decision tree for the evaluation and treatment of patients who had symptoms of thrombocytopenia or thrombosis within 20 days of receiving the ChAdOx1 nCov-19 vaccine and who had no exposure to heparin. The diagnostic and therapeutic strategies in such patients differ from those in patients with autoimmune heparin-induced thrombocytopenia (HIT).13 DIC indicates disseminated intravascular coagulation, INR international normalized ratio, PF4 platelet factor 4, and PTT partial thromboplasty time.
Figure 2 demonstrates a possible diagnostic and therapeutic strategy for the management of this new prothrombotic thrombocytopenic disorder. One consideration is to administer a high dose of intravenous immunoglobulin to inhibit Fcγ receptor-mediated platelet activation. This recommendation is consistent with the emerging experience in the treatment of severe autoimmune heparin-induced thrombocytopenia in which high dose intravenous immunoglobulin has led to rapid increases in platelet count and de-escalation of hypereroagulability.12.26 We found that the addition of immunoglobulin in doses that could be easily achieved clinically was effective in inhibiting platelet activation by antibodies of patients. The reluctance of clinicians to initiate anticoagulation can be reduced by administering high-dose intravenous immunoglobulin to increase platelet counts, especially if a patient has severe thrombocytopenia and thrombosis, such as cerebral venous thrombosis.
Given the parallels with autoimmune heparin-induced thrombocytopenia, anticoagulant options should use non-heparin anticoagulants used to treat heparin-induced thrombocytopenia.27 unless a functional test ruled out the heparin-dependent improvement in platelet activation. Finally, we propose that this new entity be named induced immune thrombotic thrombocytopenia (VITT) to avoid confusion with heparin-induced thrombocytopenia.