posted on 2023-11-11, 03:29authored byDaniil
G. Ivanov, Nikola Ivetic, Yi Du, Son N. Nguyen, S. Hung Le, Daniel Favre, Ishac Nazy, Igor A. Kaltashov
The
massive COVID-19 vaccine roll-out campaign illuminated a range
of rare side effects, the most dangerous of whichvaccine-induced
immune thrombotic thrombocytopenia (VITT)is caused by adenoviral
(Ad)-vectored vaccines. VITT occurrence had been linked to the production
of pathogenic antibodies that recognize an endogenous chemokine, platelet
factor 4 (PF4). Mass spectrometry (MS)-based evaluation of the ensemble
of anti-PF4 antibodies obtained from a VITT patient’s blood
indicates that the major component is a monoclonal antibody. Structural
characterization of this antibody reveals several unusual characteristics,
such as the presence of an N-glycan in the Fab segment
and high density of acidic amino acid residues in the complementarity-determining
regions. A recombinant version of this antibody (RVT1) was generated
by transient expression in mammalian cells based on the newly determined
sequence. It captures the key properties of VITT antibodies such as
their ability to activate platelets in a PF4 concentration-dependent
fashion. Homology modeling of the Fab segment reveals a well-defined
polyanionic paratope, and the docking studies indicate that the polycationic
segment of PF4 readily accommodates two Fab segments, cross-linking
the antibodies to yield polymerized immune complexes. Their existence
was verified with native MS by detecting assemblies as large as (RVT1)3(PF4)2, pointing out at FcγRIIa-mediated
platelet activation as the molecular mechanism underlying VITT clinical
manifestations. In addition to the high PF4 affinity, RVT1 readily
binds other polycationic targets, indicating a polyreactive nature
of this antibody. This surprising promiscuity not only sheds light
on VITT etiology but also opens up a range of opportunities to manage
this pathology.