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OBJECTIVES:To identify triggering receptor expressed in myeloid cells-like transcript-1 positive (TLT-1⁺) microparticles (MPs) and evaluate if their presence is associated with clinical outcomes and/or disease severity in acute respiratory distress syndrome (ARDS). DESIGN:Retrospective cohort study. SETTING:ARDS Network clinical trials. PATIENTS:A total of 564 patients were diagnosed with ARDS. INTERVENTIONS:None. MEASUREMENTS AND MAIN RESULTS:Using flow cytometry, we demonstrated the presence of TLT-1⁺platelet-derived microparticles (PMP) that bind fibrinogen in plasma samples from fresh donors. We retrospectively quantified TLT-1, glycoprotein (Gp) 1b, or α_IIbβ_IIIaimmunopositive microparticles in plasma samples from patients with ARDS enrolled in the ARMA, KARMA, and LARMA (Studies 01 and 03 lower versus higher tidal volume, ketoconazole treatment, and lisofylline treatment Clincial Trials) ARDS Network clinical trials and evaluated the relationship between these measures and clinical outcomes. No associations were found between Gp1b⁺MPs and clinical outcomes for any of the cohorts. When stratified by quartile, associations were found for survival, ventilation-free breathing, and thrombocytopenia with α_IIbβ_IIIa⁺and TLT-1⁺MPs (χ²p< 0.001). Notably, 63 of 64 patients in this study who failed to achieve unassisted breathing had TLT⁺PMP in the 75th percentile. In all three cohorts, patients whose TLT⁺MP counts were higher than the median had higher Acute Physiology and Chronic Health Evaluation III scores, were more likely to present with thrombocytopenia and were 3.7 times (p< 0.001) more likely to die than patients with lower TLT⁺PMP after adjusting for other risk factors. CONCLUSIONS:Although both α_IIbβ_IIIa⁺and TLT⁺microparticles (α_IIbβ_IIIa, TLT-1) were associated with mortality, TLT-1⁺MPs demonstrated stronger correlations with Acute Physiology and Chronic Health Evaluation III scores, unassisted breathing, and multiple system organ failure. These findings warrant further exploration of the mechanistic role of TLT-1⁺PMP in ARDS or acute lung injury progression.