For detailed description of laboratory findings see Table S1 in Supplementary Appendix.
D-Dimer levels were elevated in both patients at admission. Coagulation parameters were normal. No signs of hemolysis were evident.
A peripheral blood smear showed giant platelets in patient 1 and platelet anisocytosis in patient 2.
Patient 1 developed a progressive severe normochromic normocytic anemia with nadir hemoglobin level of 5,4 gr/dL on day 4 when red blood cell transfusion was performed. Patient received a second transfusion on day 14 for a subsequent reduction of hemoglobin level (nadir 7,4 gr/dL).
Screening for thrombophilia was negative in both patients. Lupus anticoagulant, anticardiolipin and beta-2 glycoprotein antibodies and antinuclear antibodies tested negative.
Coagulation Factor VIII was found increased in both the patients. Factor XIII was markedly decreased in patient 1 and not dosed in patient 2. We observed increased circulating levels of VWF:Ag and an increase in the levels of VWF:RCo.
Serum of both patients showed high levels of pan antibodies (IgG, IgM and IgA) to PF4-polyanion complexes. However, patient 1 was initially not positive but high levels of antibodies were found at day 15 (>1 optical density - OD405). Patient 2 resulted positive since day 1 (>1 OD405). The two patients did not receive heparin.
Functional activity test showed that platelets from healthy donors were clearly activated from patient 1 and 2 sera in the absence of added heparin (saline), with a percent of ATP release after 20 minutes of 22% and 14% respectively. Platelet activation was not inhibited efficiently by high-dose heparin in both the patients (percent of ATP release after 20 minutes of 19% and 9% respectively). Serum from healthy volunteers did not induce platelet secretion.
COVID-19 serological testing.
Serum antibodies to spike protein of Sars-CoV-2 (IgG) were positive in patient 1 and negative in patient 2. Both the patients had negative COVID-19 rRT-PCR test on nasopharyngeal swab. Patient 2 BAL tested on day 1 was also negative.
Clot collected from patient 1 (Fig.S1) during the first thrombectomy was mainly composed of platelets (85% of the total material examined) and was massively infiltrated by neutrophils with scarce evidence of karyorrhexis. Histological features consistent with the presence of neutrophil DNA Extracellular Traps (NETs) were observed.
Clot collected during the second endovascular procedure was a red blood cell-rich thrombus (90% of red blood cells and 10% fibrin and platelets) with scarce neutrophils (not shown).
The data of this report are available from the corresponding author, upon reasonable request.