Figure 2.
PP response induced by VITT plasma is suppressed by heparin, is antibody mediated, and corresponded with clinical response to therapy. Healthy donor whole blood was treated with platelet agonist 5 µM SFLLRN and plasma from patients with VITT (n = 23) (A) or HIT (n = 8) (B) in the presence of low-dose (0.5 U/mL) or high-dose (100 U/mL) unfractionated heparin and assessed for PP formation by using flow cytometry. Blue lines represent patients with VITT who generated a heparin-enhancing PP response at low-dose heparin. Friedman test with Dunn’s correction for multiple comparisons was performed. Donor blood was pretreated with the FcγRIIa-blocking antibody IV.3 (10 µg/mL) (C) or IVIg (10 mg/mL) (D) for 15 minutes before exposure to VITT plasma and 5 µM SFLLRN. Wilcoxon matched-pairs signed rank test was performed. (E) PP response induced by VITT plasma (n = 4) collected pre-IVIg treatment was compared with the procoagulant response within 5 days’ post-IVIg therapy. The reduction in the PP response corresponded to the suppressive effect of exogenous IVIg (10 mg/mL) on the pre-IVIg sample. Results are presented as fold-change in PP proportion relative to no plasma control. Each line represents a unique patient. (F) Plasma samples from 6 patients with VITT collected before initiation of fondaparinux treatment was tested in the presence of fondaparinux (1.2 µg/mL) in vitro. PP response is normalized to no fondaparinux control. Four patients were subsequently found to be clinically responsive to fondaparinux, and 2 patients were fondaparinux resistant. Healthy donor whole blood was treated with platelet agonist 5 µM SFLLRN and VITT plasma in the presence of the ChAdOx1 nCoV-19 vaccine (AZD1222, 1:2000 [vol/vol], n = 23) (G) or recombinant SARS-CoV-2 spike protein (HexaPro 20 µg/mL, n = 10) (H) before flow cytometric assessment for PP formation. Wilcoxon matched-pairs signed rank test was performed. *P < .05, **P < .01, ***P < .001,****P <.0001. ns, not significant.

PP response induced by VITT plasma is suppressed by heparin, is antibody mediated, and corresponded with clinical response to therapy. Healthy donor whole blood was treated with platelet agonist 5 µM SFLLRN and plasma from patients with VITT (n = 23) (A) or HIT (n = 8) (B) in the presence of low-dose (0.5 U/mL) or high-dose (100 U/mL) unfractionated heparin and assessed for PP formation by using flow cytometry. Blue lines represent patients with VITT who generated a heparin-enhancing PP response at low-dose heparin. Friedman test with Dunn’s correction for multiple comparisons was performed. Donor blood was pretreated with the FcγRIIa-blocking antibody IV.3 (10 µg/mL) (C) or IVIg (10 mg/mL) (D) for 15 minutes before exposure to VITT plasma and 5 µM SFLLRN. Wilcoxon matched-pairs signed rank test was performed. (E) PP response induced by VITT plasma (n = 4) collected pre-IVIg treatment was compared with the procoagulant response within 5 days’ post-IVIg therapy. The reduction in the PP response corresponded to the suppressive effect of exogenous IVIg (10 mg/mL) on the pre-IVIg sample. Results are presented as fold-change in PP proportion relative to no plasma control. Each line represents a unique patient. (F) Plasma samples from 6 patients with VITT collected before initiation of fondaparinux treatment was tested in the presence of fondaparinux (1.2 µg/mL) in vitro. PP response is normalized to no fondaparinux control. Four patients were subsequently found to be clinically responsive to fondaparinux, and 2 patients were fondaparinux resistant. Healthy donor whole blood was treated with platelet agonist 5 µM SFLLRN and VITT plasma in the presence of the ChAdOx1 nCoV-19 vaccine (AZD1222, 1:2000 [vol/vol], n = 23) (G) or recombinant SARS-CoV-2 spike protein (HexaPro 20 µg/mL, n = 10) (H) before flow cytometric assessment for PP formation. Wilcoxon matched-pairs signed rank test was performed. *P < .05, **P < .01, ***P < .001,****P <.0001. ns, not significant.

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