Plasma OPN levels are low during TRALI, increase upon TRALI prevention by OPN blockade with levels remaining low when macrophages are depleted, and treatment with cystamine but not with anti-CD44 antibody prevents antibody-mediated PMN-dependent TRALI. Plasma OPN levels in C57BL/6 WT mice primed with LPS and/or depleted of CD4⁺ T cells, injected with anti-MHC class I antibodies (clones 34-1-2s and AF6-88.5.5.3), injected with either isotype control antibody or anti-OPN antibody, and depleted or not of macrophages (A). Lung W/D weight ratios (B,D) and percentage of pulmonary PMNs (C) in C57BL/6 WT mice primed with LPS, depleted of CD4⁺ T cells, injected with anti-MHC class I antibodies (clones 34-1-2s and AF6-88.5.5.3) and either cystamine or not (B,C) or anti-CD44 antibody or isotype antibody (D). Statistical analysis was performed with 1-way ANOVA with a Tukey post hoc test (A) or with 1-tailed unpaired Student t test (B-D). Each dot represents 1 mouse, and error bars represent SD. *P < .05, **P < .01, ****P < .0001.