The role of PF4-specific B cells in patients with Vaccine-induced Immune Thrombotic Thrombocytopenia (VITT)

To date, the induction of autoimmunity and in particular autoreactive B cell
responses is incompletely understood. During the corona pandemic, protection
against the SARS-Cov-2 virus was generated by the administration of vaccines. 4
different vaccines were administered to the majority of the Dutch population.
Two of these, both based on an adenoviral vector, appeared to induce a severe
side effect called Vaccine-induced Immune Thrombotic Thrombocytopenia (VITT).
VITT occurred in a very small number of individuals and was found to be
strongly associated with high-titer IgG autoantibodies directed against a
cationic platelet chemokine, platelet factor 4 (PF4). It has been hypothesised
that components from the vaccine could interact with PF4, thereby leading to
the activation of PF4-specific B cells. A similar mechanism has been described
for heparin-induced thrombocytopenia (HIT) and for spontaneous heparin-induced
thrombocytopenia (HIT) syndrome, a form of *autoimmune* HIT (aHIT). Both HIT
and aHIT share clinical features with VITT. Little is known, however, on how
VITT is induced precisely and how it is different, on the B-cell and antibody
level, from HIT or aHIT. Of note, also 2-6% of healthy individuals harbour IgG
anti-PF4 antibodies. Here, we intend to study the antibody response and the B
cell-receptor profile of PF4-specific B cells in patients that have been
diagnosed with VITT or HIT/aHIT to better understand the molecular details and
reactivity patterns of PF4-directed antibodies. Using VITT and the PF4-specific
B cell response as a model, we expect that important insights can be generated
on how autoreactive B cell responses are induced, which has direct relevance
for systemic autoimmune diseases in other fields, amongst which rheumatology.

To enhance our knowledge of the molecular nature, induction and regulation of
the autoantibody and B cell response involved in/responsible for the induction
of VITT

Observational study, single centercentre

The primary procedure for study participants is a single blood draw and
completion of a questionnaire. VITT and aHIT/HIT patients will be identified
through database records available at Sanquin, Amsterdam. These (former)
patients will be contacted by letter. To respond, patients will be provided
with a return envelope or can actively make contact via email or phone.
Eligible (former) patients are expected to live in different parts of the
Netherlands. For patients who live at >25 km distance from the LUMC, a trained
member of the research team will visit study participants at home to draw blood
once (100mL total). Patients who live within a distance of <25km from the LUMC
will be asked to come to the LUMC. Age- and sex-matched healthy donors will be
recruited by advertising within the LUMC (e.g. local blackboards at the
restaurant and the shopping area *Leidse Plein*, information screens at the
outpatient clinic of the rheumatology departments). In all cases, the risk
for study participation relates to a single blood draw by routine venous
puncture. Possible side-effects are local hematoma and discomfort due to the
puncture. These risks are considered negligible.