Macrophages and microglia play a key role in the maintenance of nervous system homeostasis. However, upon different challenges, they can adopt several phenotypes, which may lead to divergent effects on tissue repair. After spinal cord injury (SCI), microglia and macrophages show predominantly pro-inflammatory activation and contribute to tissue damage. However, the factors that hamper their conversion to an anti-inflammatory state after SCI, or to other protective phenotypes, are poorly understood. Here, we show that IL-4 protein levels are undetectable in the spinal cord after contusion injury, which likely favors microglia and macrophages to remain in a pro-inflammatory state. We also demonstrate that a single delayed intraspinal injection of IL-4, 48 hours after SCI, induces increased expression of M2 marker in microglia and macrophages. We also show that delayed injection of IL-4 leads to the appearance of resolution-phase macrophages, and that IL-4 enhances resolution of inflammation after SCI. Interestingly, we provide clear evidence that delayed administration of IL-4 markedly improves functional outcomes and reduces tissue damage after contusion injury. It is possible that these improvements are mediated by the presence of macrophages with M2 markers and resolution-phase macrophages. These data suggest that therapies aimed at increasing IL-4 levels could be valuable for the treatment of acute SCI, for which there are currently no effective treatments. GLIA 2016;64:2079–2092

Inflammation is a common neuropathological feature in several neurological disorders, including amyotrophic lateral sclerosis (ALS). We have studied the contribution of CSF1R signalling to inflammation in ALS, as a pathway previously reported to control the expansion and activation of microglial cells. We found that microglial cell proliferation in the spinal cord of SOD1^G93A transgenic mice correlates with the expression of CSF1R and its ligand CSF1. Administration of GW2580, a selective CSF1R inhibitor, reduced microglial cell proliferation in SOD1^G93A mice, indicating the importance of CSF1-CSF1R signalling in microgliosis in ALS. Moreover, GW2580 treatment slowed disease progression, attenuated motoneuron cell death and extended survival of SOD1^G93A mice. Electrophysiological assessment revealed that GW2580 treatment protected skeletal muscle from denervation prior to its effects on microglial cells. We found that macrophages invaded the peripheral nerve of ALS mice before CSF1R-induced microgliosis occurred. Interestingly, treatment with GW2580 attenuated the influx of macrophages into the nerve, which was partly caused by the monocytopenia induced by CSF1R inhibition. Overall, our findings provide evidence that CSF1R signalling regulates inflammation in the central and peripheral nervous system in ALS, supporting therapeutic targeting of CSF1R in this disease.

IL-1β and IL-18 are pro-inflammatory cytokines that are linked to inflammation. Activation of the NOD-like receptor protein 3 (NLRP3) inflammasome is required for the maturation and secretion of IL-1β and IL-18 and, thus, plays a key role in the pathogenesis of many inflammatory conditions, including multiple sclerosis (MS). OLT1177™ (dapansutrile) is a newly developed drug that is safe in humans and inhibits specifically the NLRP3 inflammasome. In the present study, we investigated whether OLT1177 exerts therapeutic effects in experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. We found that EAE mice fed an OLT1177-enriched diet prophylactically were significantly protected against functional deficits and demyelination in the spinal cord. We also demonstrated that prophylactic oral administration of OLT1177 led to marked reduction (~2-3 fold) in the protein levels of IL-1β and IL-18, as well as, IL-6 and TNFα, in the spinal cord of EAE mice. Moreover, prophylactic oral administration of OLT1177 significantly attenuated the infiltration of CD4 T cells and macrophages in the spinal cord. We also demonstrated that oral administration of OLT1177, starting at disease onset, resulted in significant amelioration of the clinical signs of EAE. Overall, these first data suggest that OLT1177 could have clinical benefit for the treatment of MS in humans.

Resolution of inflammation is defective after spinal cord injury (SCI), which impairs tissue integrity and remodeling and leads to functional deficits. Effective pharmacological treatments for SCI are not currently available. Maresin 1 (MaR1) is a highly conserved specialized proresolving mediator (SPM) hosting potent anti-inflammatory and proresolving properties with potent tissue regenerative actions. Here, we provide evidence that the inappropriate biosynthesis of SPM in the lesioned spinal cord hampers the resolution of inflammation and leads to deleterious consequences on neurological outcome in adult female mice. We report that, after spinal cord contusion injury in adult female mice, the biosynthesis of SPM is not induced in the lesion site up to 2 weeks after injury. Exogenous administration of MaR1, a highly conserved SPM, propagated inflammatory resolution after SCI, as revealed by accelerated clearance of neutrophils and a reduction in macrophage accumulation at the lesion site. In the search of mechanisms underlying the proresolving actions of MaR1 in SCI, we found that this SPM facilitated several hallmarks of resolution of inflammation, including reduction of proinflammatory cytokines (CXCL1, CXCL2, CCL3, CCL4, IL6, and CSF3), silencing of major inflammatory intracellular signaling cascades (STAT1, STAT3, STAT5, p38, and ERK1/2), redirection of macrophage activation toward a prorepair phenotype, and increase of the phagocytic engulfment of neutrophils by macrophages. Interestingly, MaR1 administration improved locomotor recovery significantly and mitigated secondary injury progression in a clinical relevant model of SCI. These findings suggest that proresolution, immunoresolvent therapies constitute a novel approach to improving neurological recovery after acute SCI.SIGNIFICANCE STATEMENTInflammation is a protective response to injury or infection. To result in tissue homeostasis, inflammation has to resolve over time. Incomplete or delayed resolution leads to detrimental effects, including propagated tissue damage and impaired wound healing, as occurs after spinal cord injury (SCI). We report that inflammation after SCI is dysregulated in part due to inappropriate synthesis of proresolving lipid mediators. We demonstrate that the administration of the resolution agonist referred to as maresin 1 (MaR1) after SCI actively propagates resolution processes at the lesion site and improves neurological outcome. MaR1 is identified as an interventional candidate to attenuate dysregulated lesional inflammation and to restore functional recovery after SCI.