Mechanisms of Immunothrombosis in Vaccine-Induced Thrombotic Thrombocytopenia (VITT) Compared to Natural SARS-CoV-2 Infection.

Herein, we consider venous immunothrombotic mechanisms in SARS-CoV-2 infection and anti-SARS-CoV-2 DNA vaccination. Primary SARS-CoV-2 infection with systemic viral RNA release (RNAaemia) contributes to innate immune coagulation cascade activation, with both pulmonary and systemic immunothrombosis - including venous territory strokes. However, anti-SARS-CoV-2 adenoviral-vectored-DNA vaccines -initially shown for the ChAdOx1 vaccine-may rarely exhibit autoimmunity with autoantibodies to Platelet Factor-4 (PF4) that is termed Vaccine-Induced Thrombotic Thrombocytopenia (VITT), an entity pathophysiologically similar to Heparin-Induced Thrombocytopenia (HIT). The PF4 autoantigen is a polyanion molecule capable of independent interactions with negatively charged bacterial cellular wall, heparin and DNA molecules, thus linking intravascular innate immunity to both bacterial cell walls and pathogen-derived DNA. Crucially, negatively charged extracellular DNA is a powerful adjuvant that can break tolerance to positively charged nuclear histone proteins in many experimental autoimmunity settings, including SLE and scleroderma. Analogous to DNA-histone interactons, positively charged PF4-DNA complexes stimulate strong interferon responses via Toll-Like Receptor (TLR) 9 engagement. A chain of events following intramuscular adenoviral-vectored-DNA vaccine inoculation including microvascular damage; microbleeding and platelet activation with PF4 release, adenovirus cargo dispersement with DNA-PF4 engagement may rarely break immune tolerance, leading to rare PF4-directed autoimmunity. The VITT cavernous sinus cerebral and intestinal venous territory immunothrombosis proclivity may pertain to venous drainage of shared microbiotal-rich areas of the nose and in intestines that initiates local endovascular venous immunity by PF4/microbiotal engagement with PF4 autoantibody driven immunothrombosis reminiscent of HIT. According to the proposed model, any adenovirus-vectored-DNA vaccine could drive autoimmune VITT in susceptible individuals and alternative mechanism based on molecular mimicry, vaccine protein contaminants, adenovirus vector proteins, EDTA buffers or immunity against the viral spike protein are secondary factors. Hence, electrochemical DNA-PF4 interactions and PF4-heparin interactions, but at different locations, represent the common denominator in HIT and VITT related autoimmune-mediated thrombosis.

COVID-19 vaccine-induced immune thrombotic thrombocytopenia: A review of the potential mechanisms and proposed management.

With over 600 million coronavirus (COVID-19) vaccine doses administered globally, adverse events are constantly monitored. Recently however, reports of thrombosis and thrombocytopenia following vaccination with the ChAdOx1 nCoV-19 vaccine have emerged. This paper aims to review the available literature and guidelines pertaining to vaccine-induced immune thrombotic thrombocytopenia (VITT) and the proposed guidelines, while offering a potential approach that unifies the available evidence. While the risk of VITT remains extremely low and the benefits outweigh the risks, experimental studies are needed to clarify the pathophysiology behind VITT and possibly decrease the risk of thrombosis and other adverse events occurring. However, treatment should not be delayed in suspected cases, and IV immunoglobulin and non-heparin anticoagulation should be initiated.

Exacerbation of immune thrombocytopenia following COVID-19 vaccination.

There is concern that COVID-19 vaccination may adversely affect immune thrombocytopenia (ITP) patients. Fifty-two consecutive chronic ITP patients were prospectively followed after COVID-19 vaccination. Fifteen percent had no worsening of clinical symptoms but no post-vaccination platelet count; 73% had no new symptoms and no significant platelet count decline. However, 12% had a median platelet count drop of 96% within 2-5 days post vaccination with new bleeding symptoms; after rescue therapy with corticosteroids +/- intravenous immunoglobulin (IVIG), platelets recovered to >30 × 109 /l a median three days later. ITP exacerbation occurred independently of remission status, concurrent ITP treatment, or vaccine type. Safety of a second vaccine dose needs careful assessment.

Perspectives on vaccine induced thrombotic thrombocytopenia.

As the novel SARS-CoV-2 continues to infect numerous individuals worldwide, one of the leading approaches in dealing with the global health crisis is vaccination against the COVID-19. Due to recent reports, vaccination with ChAdOx1 nCov-19 (developed by Oxford and AstraZeneca) may result in a vaccine-induced catastrophic thrombotic thrombocytopenia disorder. Thus, as of March 16 of 2021, vaccination programs in 18 countries had been suspended until further examination, including Sweden, Germany and France. This disorder presents as extensive thrombosis in atypical sites, primarily in the cerebral venous, alongside thrombocytopenia and the production of autoantibody against platelet-factor 4 (PF4). PF4 autoantibody has the ability to binds the human FcRγIIA receptor of platelets and contribute to their aggregation. This rare adverse effect extremely resembles the clinical presentation of the classical immune-mediated HIT disorder, which occurs following exposure to heparin. Surprisingly, none of these patients had been pre-exposed to heparin before disease onset, leading to the hypothesis that a viral antigen from the vaccine had triggered the response. Importantly, COVID-19 had been associated with numerous autoimmune manifestations, including the production of pathogenic autoantibodies, new onset of autoimmune diseases and disorders. As the ChAdOx1 nCov-19 vaccination leads to the synthesis of specific SARS-CoV-2-proteins, they may trigger a production of PF4 autoantibody though molecular mimicry phenomena, while vaccination compounds lead to a rigorous bystander activation of immune cells. If existing, removing such homological sequences from the vaccine may eliminate this phenomenon. In contrast, it needs to be emphasized that the ChAdOx1 nCoV-19 vaccine was found to be safe and efficacious against symptomatic COVID-19 in randomized controlled trials, which included 23,848 participants from the UK, Brazil and South Africa.