Cognitive dysfunction and reactive microglia are hallmarks of traumatic brain injury (TBI), yet whether these cells contribute to cognitive deficits and secondary inflammatory pathology remains poorly understood. Here, we show that removal of microglia from the mouse brain has little effect on the outcome of TBI, but inducing the turnover of these cells through either pharmacologic or genetic approaches can yield a neuroprotective microglial phenotype that profoundly aids recovery. The beneficial effects of these repopulating microglia are critically dependent on interleukin-6 (IL-6) trans-signaling via the soluble IL-6 receptor (IL-6R) and robustly support adult neurogenesis, specifically by augmenting the survival of newborn neurons that directly support cognitive function. We conclude that microglia in the mammalian brain can be manipulated to adopt a neuroprotective and pro-regenerative phenotype that can aid repair and alleviate the cognitive deficits arising from brain injury.

Neutrophil infiltration after insult is a prominent feature of both human and experimental traumatic spinal cord injury (SCI) (8-10), and presence of these cells at the lesion site is thought to contribute considerably to secondary inflammatory pathology and thus worse outcomes (11-14). Relatively little is known, however, about the molecular mechanisms orchestrating neutrophil mobilization and recruitment to the injured spinal cord, and to the best of our knowledge no study to date has explored a role for C3aR1 in this pathology. Here, we first delineated a role for C3a/C3aR1 in SCI by comparing the recovery of wild-type (WT) and C3ar1-/-(i.e., knockout) mice from blunt spinal cord trauma, one of the most common types of SCI in humans (15). Because C3aR1 is expressed by cells of myeloid (16, 17) and central nervous system (CNS) origin (18, 19), we also used BM chimera approaches to disentangle peripheral from central C3a/C3aR1 roles in relation to SCI outcomes. We then employed a variety of genetic and pharmacological approaches, including in vitro and in vivo functional assays, antibody-mediated neutrophil depletion, and chemotaxis assays to demonstrate that C3aR1 engages phosphatase and tensin homolog (PTEN) to negatively regulate granulocyte egress from the BM into the circulation in response to inflammatory stimuli. These findings are significant from a therapeutic perspective, as a greater number of circulating neutrophils in the blood was associated with worse outcomes in both mouse and human SCI. Results SCI leads to C3a generation, leukocyte infiltration, and elevated C3aR1 expression. To begin exploring a role for C3a in SCI, we first assessed the time course of its generation. C3a levels in the mouse spinal cord rapidly increased following injury (Figure 1A), and they were significantly elevated over sham-operated controls at 6, 12, and 24 hours after surgery (>5-fold increase; P < 0.001). Plasma C3a levels also rose sharply within 30 minutes of SCI, and remained elevated over sham-operated controls for at least 1 day after SCI (Figure 1B). Select key comparisons of C3a levels in plasma and spinal cord samples of C3ar1-/mice yielded similar results, suggesting a similar magnitude of complement system activation between genotypes (2 hours after surgery: C3ar1-/plasma 7.93 ± 1.63 μg/ml vs. WT plasma 6.94 ± 0.90 μg/ml, n = 4-5 per genotype, P = 0.51; C3ar1-/spinal cord 0.76 ± 0.10 pg/μg vs. WT spinal cord 0.68 ± 0.08 pg/μg, n = 4 per genotype, P = 0.58). Widespread C3aR1 staining was observed at and around the site of SCI, and on a variety of cell types. In the acute phase, C3aR1-expressing Ly6B.2 + and CD11b + cells were abundant at and around the lesion site at 1 day after injury (Figure 1, C and D), a time point that coincides with peak neutrophil recruitment (20). The majority of infiltrating Ly6B.2 + cells were genuine neutrophils, as little overlap was observed between Ly6B.2 staining and GFP + cells of monocytic lineage in Cx3cr1 gfp/+ mice at this time point (Figure 1E). Overall, these findings ar...

Background The acute phase response (APR) to CNS insults contributes to the overall magnitude and nature of the systemic inflammatory response. Aspects of this response are thought to drive secondary inflammatory pathology at the lesion site, and suppression of the APR can therefore afford some neuroprotection. In this study, we examined the APR in a mouse model of traumatic spinal cord injury (SCI), along with its relationship to neutrophil recruitment during the immediate aftermath of the insult. We specifically investigated the effect of IL-1 receptor antagonist (IL-1RA) administration on the APR and leukocyte recruitment to the injured spinal cord. Methods Adult female C57BL/6 mice underwent either a 70kD contusive SCI, or sham surgery, and tissue was collected at 2, 6, 12, and 24 hours post-operation. For IL-1RA experiments, SCI mice received two intraperitoneal injections of human IL-1RA (100mg/kg), or saline as control, immediately following, and 5 hours after impact, and animals were sacrificed 6 hours later. Blood, spleen, liver and spinal cord were collected to study markers of central and peripheral inflammation by flow cytometry, immunohistochemistry and qPCR. Results were analysed by two-way ANOVA or student’s t-test, as appropriate. Results SCI induced a robust APR, hallmarked by elevated hepatic expression of pro-inflammatory marker genes and a significantly increased neutrophil presence in the blood, liver and spleen of these animals, as early as 2 hours after injury. This peripheral response preceded significant neutrophil infiltration of the spinal cord, which peaked 24 hours post-SCI. Although expression of IL-1RA was also induced in the liver following SCI, its response was delayed compared to IL-1β. Exogenous administration of IL-1RA during this putative therapeutic window was able to suppress the hepatic APR, as evidenced by a reduction in CXCL1 and SAA-2 expression as well as a significant decrease in neutrophil infiltration in both the liver and the injured spinal cord itself. Conclusions Our data indicate that peripheral administration of IL-1RA can attenuate the APR which in turn reduces immune cell infiltration at the spinal cord lesion site. We propose IL-1RA treatment as a viable therapeutic strategy to minimise the harmful effects of SCI-induced inflammation.

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