Unique features of vaccine-induced immune thrombotic thrombocytopenia; a new anti-platelet factor 4 antibody-mediated disorder

Thrombosis and Bleeding in Patients with Vaccine-Induced Immune Thrombotic Thrombocytopenia: A Systematic Review of Published Cases

INTRODUCTION

 Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a highly prothrombotic reaction to COVID-19 (coronavirus disease 2019) adenoviral vector vaccines. Its distinct bleeding and thrombotic patterns compared with other platelet consumptive disorders remain unclear.

METHODS

 We performed a systematic review of the literature (PubMed and Embase) up to July 31, 2022, including case reports and case series providing nonaggregate data of VITT patients. Accurate VITT diagnosis required fulfillment of the following criteria: (1) endorsement by the authors, (2) consistent vaccine type and timing, (3) presence of thrombocytopenia and thrombosis, (4) detection of anti-platelet factor 4 antibodies. Data are presented as frequencies with 95% confidence intervals (CIs) calculated with the exact binomial method.

RESULTS

 We retrieved 143 eligible studies, describing 366 patients. Of 647 thrombotic events, 53% (95% CI: 49-56) were venous thromboses at unusual sites and 30% (95% CI: 27-34) were cerebral venous sinus thromboses (CVSTs). The ratio of venous-to-arterial events was 4.1. Thromboses in most sites were associated with at least another thrombotic event, with the exception of CVST and CNS arterial thrombosis (isolated in 49 and 39% of cases, respectively). Bleeding occurred in 36% (95% CI: 31-41) of patients; 68% (95% CI: 59-75) of bleeding events were intracranial hemorrhages (ICHs). Overall mortality was 24% (95% CI: 19-29), and 77% (95% CI: 58-90) in patients with isolated CVST complicated by ICH.

CONCLUSION

 VITT displays a venous-to-arterial thrombosis ratio comparable to heparin-induced thrombocytopenia. However, VITT is characterized by a higher prevalence of CVST and ICH, which contribute to the increased bleeding frequency and mortality.

The immunology of PF4 polyanion interactions

PURPOSE OF REVIEW

Platelet factor 4 (PF4, CXCL4), the most abundant α-granule platelet-specific chemokine, forms tetramers with an equatorial ring of high positive charge that bind to a wide range of polyanions, after which it changes conformation to expose antigenic epitopes. Antibodies directed against PF4 not only help to clear infection but can also lead to the development of thrombotic disorders such as heparin-induced thrombocytopenia (HIT) and vaccine-induced thrombocytopenia and thrombosis (VITT). This review will outline the different mechanisms through which PF4 engagement with polyanions combats infection but also contributes to the pathogenesis of inflammatory and thrombotic disease states.

RECENT FINDINGS

Recent work has shown that PF4 binding to microbial polyanions may improve outcomes in infection by enhancing leukocyte-bacterial binding, tethering pathogens to neutrophil extracellular traps (NETs), decreasing the thrombotic potential of NET DNA, and modulating viral infectivity. However, PF4 binding to nucleic acids may enhance their recognition by innate immune receptors, leading to autoinflammation. Lastly, while HIT is induced by platelet activating antibodies that bind to PF4/polyanion complexes, VITT, which occurs in a small subset of patients treated with COVID-19 adenovirus vector vaccines, is characterized by prothrombotic antibodies that bind to PF4 alone.

SUMMARY

Investigating the complex interplay of PF4 and polyanions may provide insights relevant to the treatment of infectious disease while also improving our understanding of the pathogenesis of thrombotic disorders driven by anti-PF4/polyanion and anti-PF4 antibodies.