FcγRIIA amplifies chronic platelet activation in SLE. (A) Platelet count in B/WF1.FcγRIIA^Null and B/WF1.FcγRIIA^TGN female mice between age 12 and 28 weeks (n = 5 to 8 per group). Dashed line indicates platelet count in 42-week-old C57BL/6.FcγRIIA^TGN control mice. (B) Analysis of the proportions of megakaryocyte progenitors (MK; selected as lineage-negative, DNA-positive cells expressing CD41 and CD9) in the bone marrow of female B/WF1.FcγRIIA mice at age 9 and 28 weeks (n = 7-9 per group). (C) Quantification of platelet-neutrophil aggregates in the blood of female B/WF1.FcγRIIA^Null and B/WF1.FcγRIIA^TGN mice, (n = 11 Null and 10 TGN). Quantification of platelet activation (CD62P+) (D) and CD41⁺CD62P⁺ EV (E) in the blood of female B/WF1.FcγRIIA^Null and B/WF1.FcγRIIA^TGN mice aged 28 weeks (n = 6 per group). Dashed (B/WF1.FcγRIIA^TGN) and solid (B/WF1.FcγRIIA^Null) lines indicate mean of values at 9 weeks (9 w). (F) Quantification of plasma serotonin in female B/WF1.FcγRIIA^Null and B/WF1.FcγRIIA^TGN mice at 28 weeks (n = 6). (G) Quantification of platelet serotonin content in female B/WF1.FcγRIIA^Null and B/WF1.FcγRIIA^TGN mice with SLE (n = 4-5 per group). Data are presented as the mean ± SEM. Statistical analyses, 2-way analysis of variance with Šídák’s multiple comparisons test (A-B), Mann-Whitney U test (C,D,F), and Student t test (E,G). *P < .05, **P < .01.