Platelet-released serotonin, but not platelet-activating factor, is responsible for the FcγRIIA/CD32A–induced aggravation of antibody-mediated TRALI. (A) The platelet-activating factor receptor antagonist ABT-491 (1.25 mg/kg, intravenous [IV]) or saline was administered 5 minutes before injection of the mAb hIgG1-34-1-2S (1.5 mg/kg, IV) or vehicle into LPS-sensitized (0.1 mg/kg, intraperitoneal [IP]) CD32A^– or CD32A⁺ mice, and protein concentrations in BALs were analyzed 15 minutes later (CD32A^–, n = 3-8; CD32A⁺, n = 3-7). Plasma concentrations of platelet factor 4 (PF4) (CD32A^–, n = 3-12; CD32A⁺, n = 5-12) (B) and serotonin (CD32A^–, n = 3-12; CD32A⁺, n = 5-12) (C) 15 minutes after injection of the mAb hIgG1-34-1-2S (1.5 mg/kg, IV) or vehicle into LPS-sensitized (0.1 mg/kg, IP) CD32A^– or CD32A⁺ mice. (D) Serotonin concentrations in the serum of CD32A^– or CD32A⁺ mice treated or not with the serotonin transporter inhibitor fluoxetine (160 mg/L) in drinking water for 3 weeks (CD32A^–, n = 11-19; CD32A⁺, n = 5-18). (E) CD32A^– or CD32A⁺ mice, treated or not with fluoxetine, were sensitized with LPS (0.1 mg/kg, IP) and, 24 hours later, injected with the mAb hIgG1-34-1-2S (1.5 mg/kg, IV) or vehicle. Protein concentrations in BALs were evaluated 15 minutes afterward (CD32A^–, n = 3-16; CD32A⁺, n = 3-16). (F) Circulating platelet counts before and 15 minutes after injection of the mAb hIgG1-34-1-2S (1.5 mg/kg, IV) or vehicle into LPS-sensitized (0.1 mg/kg, IP) CD32A^– or CD32A⁺ mice treated or not with fluoxetine (n ≥ 10). (G) Plasma concentrations of serotonin before and 15 minutes after injection of the mAb hIgG1-34-1-2S (1.5 mg/kg, IV) or vehicle into LPS-sensitized (0.1 mg/kg, IP) CD32A^– or CD32A⁺ mice treated or not with clodronate (CD32A^–, n = 4-6; CD32A⁺, n = 4-6). (H) Protein concentrations in BALs were evaluated 15 minutes after injection of the mAb hIgG1-34-1-2S (1.5 mg/kg, IV) or vehicle into LPS-sensitized (0.1 mg/kg, IP) CD32A^– or CD32A⁺ mice treated or not with fluoxetine or clodronate (CD32A^–, n = 5-9; CD32A⁺, n = 3-9). (I) Circulating platelet counts before and 15 minutes after injection of the mAb hIgG1-34-1-2S (1.5 mg/kg, IV) or vehicle into LPS-sensitized (0.1 mg/kg, IP) CD32A^– or CD32A⁺ mice treated or not with clodronate (CD32A^–, n = 7-12; CD32A⁺, n = 4-12). Results are presented as the mean ± standard error of the mean. P > .05 (not significant [ns]), *P < .05, **P < .01, ***P < .001 by one-way analysis of variance.