Investigating endothelial cell transduction and hexon: platelet factor 4 binding of ChAdOx1 in the context of vaccine-induced thrombotic thrombocytopenia.

BACKGROUND

Vaccines against SARS-CoV-2 have been essential in controlling COVID-19-related mortality and have saved millions of lives. Adenoviral (Ad)-based vaccines have been an integral part of this vaccine campaign, with licensed vaccines based on the simian Y25 isolate (Vaxzevria; AstraZeneca) and human Ad type 26 (Jcovden; Janssen) widely adopted. As part of the largest global vaccination program ever undertaken, cases of vaccine-induced thrombotic thrombocytopenia have been described in approximately 1:200 000 vaccinees administered with Ad-based SARS-CoV-2 vaccines.

OBJECTIVES

The mechanism underpinning these adverse events remains to be completely delineated but is characterized by elevated autoantibodies against platelet factor 4 (PF4), which, when complexed with PF4, cluster and bind FcγRIIa on platelets and induce thrombus formation. Here, we investigated the ability of ChAdOx1 to transduce and activate endothelial cells (ECs).

METHODS

Using protein sequence alignment tools and in vitro transduction assays, the ability of ChAdOx1 to infect ECs was assessed. Furthermore, the ability of ChAdOx1 infection to activate ECs was determined. Finally, using surface plasmon resonance, we assessed the electrostatic interactions between the ChAdOx1 hexon and PF4.

RESULTS

Despite lacking the primary cell entry receptor, coxsackie and adenovirus receptor, ChAdOx1 efficiently transduced ECs in a coxsackie and adenovirus receptor-independent manner. This transduction did not result in EC activation. Purified hexon protein from ChAdOx1 preparations did, however, bind PF4 with a similar affinity to that previously reported for the whole ChAdOx1 capsid.

CONCLUSION

These data confirm the need to develop non-PF4 binding Ad capsids and assess their potential to mitigate adverse events associated with vaccine-induced thrombotic thrombocytopenia.