Characterization of the Complement Cascade in Human Traumatic Brain Injury

Neuroinflammation is known to contribute to worse patient outcomes following severe traumatic brain injury (TBI). Specifically, complement activation as part of the innate immune response has been explored in animal models and post-mortem human tissue. However, it has not been well-characterized in a substantial clinical cohort. We aimed to investigate the complement cascade, specifically focusing on the lectin pathway initiators, in patients with TBI compared with healthy controls. We describe temporal profiles of complement proteins, and investigate their association with outcome, and clinical variables following TBI. Plasma blood samples collected from 64 patients with TBI (on days 1-7, 42, and 365) and 17 healthy controls (single samples) were analyzed for 10 complement proteins using single and multiplexing protein assays. We quantified initiator (MBL, MASP2, ficolin3), effector (C4b, C2, factor D, factor I), and downstream (C5, C5a, C5b9) complement proteins. Clinical variables included injury severity score, Glasgow Coma Scale motor score, pupil reactivity, and Glasgow Outcome Scale Extended at 6 months post-injury. 7 out of 10 measured complement proteins had median concentrations that were significantly different in patients with TBI compared with healthy controls (all <i>p</i> values < 0.05). Ficolin3, an initiator, was particularly lower in TBI versus controls (0.22 vs. 1.84 ng/mL, <i>p</i> < 0.001), and consistently lower over time, showing depressed levels even 1 year post-injury (<i>p</i> < 0.001). Initiators MBL and MASP2, effector C2 and downstream proteins C5, C5a, and factor I demonstrate a steady rise in the first week post-injury, and are consistently elevated compared with control levels. Patients with unfavorable outcome had higher levels of C4b, C5, and factor I, with C5 demonstrating this trend over time (OR: 1.53 [1.52-1.53], <i>p</i> value < 0.001), and showing additional prognostic benefit over and above IMPACT clinical outcome predictors. This study characterized the complement cascade in human TBI with high temporal resolution in initiator, effector, and downstream proteins. Seven out of the 10 measured proteins were significantly different in patients with TBI compared with healthy controls. Notably, C5 has an independent and robust association with outcome in both univariate and linear mixed regression, where a higher C5 concentration was associated with unfavorable outcome 6 months post-injury. Further investigation is required to explore the potential of complement as a therapeutic target in TBI.