Traumatic Brain Injury in the Omics Era: Plasma and Extracellular Vesicle Proteomic Signatures in Polytrauma

Background/Objectives: Clinical outcomes after traumatic brain injury (TBI) remain difficult to predict, highlighting the need for more sensitive diagnostic and prognostic biomarkers, particularly in polytrauma. This study aimed to identify TBI-specific proteomic signatures in plasma and extracellular vesicles (EV)-enriched fractions of critically injured trauma patients. Methods: Seventy-five severely injured adult trauma patients (ISS ≥ 16) were included: isolated severe TBI (TBI; AIS_head ≥ 4, other AIS ≤ 1, n = 23), polytrauma with TBI (PT-TBI; AIS_head ≥ 4, n = 22), and polytrauma without TBI (PT; AIS_head = 0, n = 30). 24 age- and sex-matched healthy volunteers served as controls. Neat plasma and EV-enriched fractions were profiled using HPLC-MS/MS. Differentially expressed proteins (DEP) were analyzed bioinformatically, and associations with clinical parameters were assessed using Spearman's correlation. Results: The EV-enriched plasma fraction yielded more DEPs than neat plasma (846 vs. 258). Most DEPs were linked to polytrauma, with plasma reflecting metabolism and EVs showing translation and protein catabolism signatures. Among TBI-context proteins, EV-associated NRCAM and AQR and plasma IGHV1-69D, MASP1, PON1, and IGFBP7 remained significantly associated with TBI after adjustment for confounder. Notably, only EV-associated proteins correlated with injury-related clinical parameters. AQR negatively correlated with GCS (r = -0.51, p < 0.0001), while elevated NRCAM, AQR, and plasma MASP1 were associated with neurological deterioration and neurosurgical intervention. Conclusions: EV-enriched plasma proteomics enhances biomarker discovery in polytrauma, revealing distinct pathways and greater sensitivity than whole plasma. While most changes reflect systemic injury, EV-associated NRCAM and AQR, along with plasma MASP1, show TBI-specific associations with neurological status, highlighting their potential as candidate TBI biomarkers for further study.