The lack of anti‐PF4 antibodies in convalescent plasma from COVID‐19 infected blood donor

Dear Editor, Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is rare but can be life-threatening and may cause cerebral venous sinus thrombosis (CVST), splanchnic, pulmonary and cardiac thrombosis. Raised level of antibodies binding PF4 (anti-PF4) has been detected in patients who developed VITT syndrome after COVID-19 vaccination. This mechanism is believed to sustain VITT pathophysiology in a similar fashion to heparin induced thrombocytopenia (HITT). We report on anti-PF4 level in a large cohort of blood donors post COVID-19 infection. The aim of the study was to clarify if raised anti-PF4 level is induced by SARS COVID-2 viral infection or if it is a consequence of vaccination. During COVID-19 pandemic the Northern Ireland Blood Transfusion Service (NIBTS) in Belfast started a national campaign to recruit and collect convalescent plasma (CP) from healthy blood donor post-COVID-19 infection. Consent for the study was not required as it was already included in the NIBTS donation form. All donors were: (i) 28 days post COVID-19 infection, (ii) fully recovered, recorded as either positive PCR test or antibody test, (iii) met all NIBTS other eligibility criteria, and (iv) antibody titre level >6. Antibody testing was performed with EuroImmune© assay by Public Health England. The collection of CP occurred (a) via whole blood donation or (b) via plasmapheresis collection. PF4 testing was performed using a lab kit from IMMUCOR© company. A total of 152 units of CP was tested. Of these 28 were whole blood collected (18%) and 124 from apheresis collected (82%). All blood components were leucodepleted by filtration. The anticoagulant used in whole blood packs was Citrate-phosphate-dextrose solution (CPD). Of those, 30 units were from male donors and 122 from female donors. Age range was 17–64 years and median age was 43 years. Overall, 117 donors were new donors and 35 were regular donors. The CP units were bled between 7 October 2020 and 24 February 2021. Collection of CP was carried out with plasmapheresis in 29 cases only and with whole blood donations in 123 cases. None of the donors were vaccinated with any COVID-19 vaccine. Titre levels of units for research ranged from 1.0 to 31 EuroImmune© (EI) test result by PHE. 115 of these units for research had EI < 6 and were therefore classified as medium or low titre. Plasma from female donors was permitted to be bled during CP trial but was only of clinical use if HLA testing was found to be negative. Of the 122 units of female plasma issued to this research, 52 were negative for HLA antibodies, the remaining 70 units were positive for HLA antibodies or not tested due to titre levels being too low. Interestingly, anti-PF4 level was within normal range for all samples tested, optical density range being 0.099–0.274 nm (median 0.108 nm). A previous observation from Greinacher A and colleagues showed that anti-PF4 induced by vaccination do not crossreact with the SARS-CoV-2 spike protein, suggesting that SARSCoV-2 spike protein is not the trigger of VITT. This finding is in line with our study. Furthermore, the absence of PF4 antibodies and VITT in our donors, indirectly confirms (negative control) that the presence of PF4 antibodies is the conditio sine qua non in order to develop VITT. Our results show that within the natural course of COVID-19 disease the development of antiPF4 is not seen, at least in patients (blood donors) recovered from post COVID-19 infection, who do not develop any thrombotic complications. The availability of large stocks of CP at blood bank centres should be made available for conducting further investigational research studies on the immunological response to COVID-19.