Complement mediates neuroinflammation and cognitive decline at extended chronic time points after traumatic brain injury

Abstract

Traumatic brain injury (TBI) can result in progressive cognitive decline occurring for years after the initial insult, and for which there is currently no pharmacological treatment. An ongoing chronic inflammatory response after TBI is thought to be an important factor in driving this cognitive decline. Here, we investigate the role of complement in neuroinflammation and cognitive decline for up to 6 months after murine TBI. Male C57BL/6 mice were subjected to open head injury using a controlled cortical impact device. At 2 months post TBI, mice were moved to large cages with an enriched environment to simulate rehabilitation therapy, and assigned to one of three treatment groups: 1. vehicle (PBS), 2. CR2Crry (3 doses over 1 week), 3. CR2Crry (continuous weekly dose until the end of the study). The study was terminated at 6 months post-TBI for all groups. Motor and cognitive function was analyzed, with histopathological analysis of brain tissue. Measured at 6 months after TBI, neither of the complement inhibition paradigms improved motor performance. However, mice receiving continuous CR2Crry treatment showed improved spatial learning and memory compared to both mice receiving only 3 doses and to mice receiving vehicle control. Analysis of brain sections at 6 months after injury revealed ongoing complement activation in the control group, with reduced complement activation and C3 deposition in the continuous CR2Crry treatment group. The ipsilateral hemisphere of continuously treated animals also showed a decrease in microglia/macrophage and astrocyte activation compared to vehicle. There was also increased astrocytosis in the contralateral hippocampus of vehicle treated vs. naïve mice, which was reduced in mice continuously treated with CR2Crry. This study demonstrates continued complement mediated neuroinflammation at extended chronic time points after TBI, and extends the potential treatment window for complement inhibition, which has previously been shown to improve outcomes after murine TBI.

Chronic complement dysregulation drives neuroinflammation after traumatic brain injury: a transcriptomic study

“…CR2-Crry is the combined product of the genes, Cr2 and Cr1l , where CR2 acts as an injury site-targeting moiety by binding to C3 activation products at the site of brain injury. To date, CR2-Crry is the only complement inhibitor shown to confer neuroprotection when administered at both acute and chronic time points after TBI [ 12 , 16 ].…”

“…For example, the knockout of lectin pathway initiators, MBL and MASP-2, was recently shown to improve chronic motor outcomes after controlled cortical impact [ 13 ], and the inhibition or knockout of complement factor B improved acute histological outcomes after weight-drop TBI [ 49 , 52 ]. Conversely, the local production of complement proteins may play a more prominent role chronically after TBI [ 15 , 16 ], or after mild closed head injuries.…”

“…Preparation and quality control of CR2-Crry was as previously described [ 17 ]. Dosing (16 mg/Kg) and route of administration (intravenous) was based on our previous studies showing improvement in chronic behavioral and histological outcomes after injury using this paradigm [ 12 , 15 , 16 ]. Animals were treated 1 h after TBI, which is a short interval from injury, but nevertheless represents a clinically relevant scenario in certain settings.…”

“…Animals were treated 1 h after TBI, which is a short interval from injury, but nevertheless represents a clinically relevant scenario in certain settings. Subsequent doses of CR2-Crry, as performed in the 28 day study, were administered via intraperitoneal injection due to difficulty in repeated tail vein injection, and as we have previously reported [ 16 ].…”

“…In accordance with these findings, several animal studies have shown that complement inhibition following open head or closed head brain injury is neuroprotective, and improves behavioral and histological outcomes [ 11 – 14 ]. Notably, recent studies have shown ongoing complement activation up to 6 months after a single controlled cortical impact (CCI), and with an ongoing cognitive decline that was reversed with complement inhibition, even when administered 2 months after injury [ 15 , 16 ]. Nevertheless, the complement system comprises more than 50 proteins, and the complexity associated with dissecting the role of complement in pathogenic and protective mechanisms of TBI has limited progress towards clinical application.…”