Fig. 4: ITP Patient polyclonal IgG platelet-associated (PA) antibodies are directed towards epitopes the integrin α_IIbβ₃ and modulate platelet force in a confirmation-dependent manner.

a We performed contraction cytometry on the platelets from ITP patients and extracted polyclonal IgG antibodies from the plasma for further analysis. b In those patients, the extent of clinical bleeding severity scores correlated with the lack of highly contractile platelets. c We found that isolated IgG antibodies (10 μg/mL) from those ITP patients with bleeding modulated the forces of platelets from healthy donors (n = 3,3,2,3,3 donors/condition with 379, 369, 338, 393, 390 antibody treated platelets, respectively and 532 untreated control platelets analyzed as shown in Supplementary Fig. 9). d As platelet contractile force is transmitted via the α_IIbβ₃ integrin, we used negative stain electron microscopy to study those specific antibody-integrin interactions. We found that autoantibodies from ITP patients with higher bleeding scores stabilized integrins in the bent and extended closed conformations. For (c), all antibody-treated platelets were compared to the untreated condition, and statistical significance was determined by mixed effects model, to account for within-subject variation, followed by the Benjamini-Hochberg’s posthoc test to account for multiple comparisons. Antibody treated platelet contraction force difference for (c) is shown as mean ± the minimum and maximum differences from the untreated control mean. *P ≤ 0.05, **P ≤ 0.01, ****P ≤ 0.0001 and specific p-values are shown in Supplementary Table 5. Source data are provided as a Source Data file.