Spinal cord injury (SCI) provokes an inflammatory response that generates substantial secondary damage within the cord but also may contribute to its repair. Anti-inflammatory treatment of human SCI and its timing must be based on knowledge of the types of cells participating in the inflammatory response, the time after injury when they appear and then decrease in number, and the nature of their actions. Using post-mortem spinal cords, we evaluated the time course and distribution of pathological change, infiltrating neutrophils, monocytes/macrophages and lymphocytes, and microglial activation in injured spinal cords from patients who were 'dead at the scene' or who survived for intervals up to 1 year after SCI. SCI caused zones of pathological change, including areas of inflammation and necrosis in the acute cases, and cystic cavities with longer survival (Zone 1), mantles of less severe change, including axonal swellings, inflammation and Wallerian degeneration (Zone 2) and histologically intact areas (Zone 3). Zone 1 areas increased in size with time after injury whereas the overall injury (size of the Zones 1 and 2 combined) remained relatively constant from the time (1-3 days) when damage was first visible. The distribution of inflammatory cells correlated well with the location of Zone 1, and sometimes of Zone 2. Neutrophils, visualized by their expression of human neutrophil alpha-defensins (defensin), entered the spinal cord by haemorrhage or extravasation, were most numerous 1-3 days after SCI, and were detectable for up to 10 days after SCI. Significant numbers of activated CD68-immunoreactive ramified microglia and a few monocytes/macrophages were in injured tissue within 1-3 days of SCI. Activated microglia, a few monocytes/macrophages and numerous phagocytic macrophages were present for weeks to months after SCI. A few CD8(+) lymphocytes were in the injured cords throughout the sampling intervals. Expression by the inflammatory cells of the oxidative enzymes myeloperoxidase (MPO) and nicotinamide adenine dinucleotide phosphate oxidase (gp91(phox)), and of the pro-inflammatory matrix metalloproteinase (MMP)-9, was analysed to determine their potential to cause oxidative and proteolytic damage. Oxidative activity, inferred from MPO and gp91(phox) immunoreactivity, was primarily associated with neutrophils and activated microglia. Phagocytic macrophages had weak or no expression of MPO or gp91(phox). Only neutrophils expressed MMP-9. These data indicate that potentially destructive neutrophils and activated microglia, replete with oxidative and proteolytic enzymes, appear within the first few days of SCI, suggesting that anti-inflammatory 'neuroprotective' strategies should be directed at preventing early neutrophil influx and modifying microglial activation.

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Inflammation is a major contributor to the pathogenesis of cerebral ischemia and stroke. In the peripheral immune response, caspase-1 activation involves the formation of a macromolecular complex termed the inflammasome. We determined whether nucleotide-binding, leucine-rich repeat, pyrin domain containing 1 (NLRP1), molecular platform consisting of capase-1, apoptosisassociated speck-like protein containing a caspase-activating recruitment domain (ASC), and NLRP1, is expressed in the normal and postischemic brain. Mice underwent thromboembolic stroke to investigate the formation of the inflammasome and subsequent activation of downstream inflammatory responses. Western blot analysis showed expression and activation of interleukin (IL) IL-1b and IL-18 at 24 h after stroke. Size-exclusion chromatography and coimmunoprecipitation analysis showed protein association between NLRP1, ASC, caspase-1, and the X-linked inhibitor of apoptosis protein (XIAP). After ischemia, immunohistochemical analysis revealed inflammasome proteins in neurons, astrocytes, and microglia/macrophages. The potential of the inflammasome as an antiinflammatory target was showed by interference of inflammasome activation resulting in reduced cytokine levels in mice treated after ischemia with a neutralizing antibody against NLRP1. These findings show that the inflammasome complex forms after focal brain ischemia and may be a novel therapeutic target for reducing the detrimental consequences of postischemic inflammation.

Traumatic brain injury elicits acute inflammation that in turn exacerbates primary brain damage. A crucial part of innate immunity in the immune privileged central nervous system involves production of proinflammatory cytokines mediated by inflammasome signaling. Here, we show that the nucleotide-binding, leucine-rich repeat pyrin domain containing protein 1 (NLRP1) inflammasome consisting of NLRP1, caspase-1, caspase-11, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), the X-linked inhibitor of apoptosis protein, and pannexin 1 is expressed in neurons of the cerebral cortex. Moderate parasagittal fluid-percussion injury (FPI) induced processing of interleukin-1b, activation of caspase-1, cleavage of X-linked inhibitor of apoptosis protein, and promoted assembly of the NLRP1 inflammasome complex. Anti-ASC neutralizing antibodies administered immediately after fluid-percussion injury to injured rats reduced caspase-1 activation, X-linked inhibitor of apoptosis protein cleavage, and processing of interleukin-1b, resulting in a significant decrease in contusion volume. These studies show that the NLRP1 inflammasome constitutes an important component of the innate central nervous system inflammatory response after traumatic brain injury and may be a novel therapeutic target for reducing the damaging effects of posttraumatic brain inflammation.

Pattern recognition receptors (PRRs) are part of the innate immune response and were originally discovered for their role in recognizing pathogens by ligating specific pathogen associated molecular patterns (PAMPs) expressed by microbes. Now the role of PRRs in sterile inflammation is also appreciated, responding to endogenous stimuli referred to as “damage associated molecular patterns” (DAMPs) instead of PAMPs. The main families of PRRs include Toll-like receptors (TLRs), Nod-like receptors (NLRs), RIG-like receptors (RLRs), AIM2-like receptors (ALRs), and C-type lectin receptors. Broad expression of these PRRs in the CNS and the release of DAMPs in and around sites of injury suggest an important role for these receptor families in mediating post-injury inflammation. Considerable data now show that PRRs are among the first responders to CNS injury and activation of these receptors on microglia, neurons, and astrocytes triggers an innate immune response in the brain and spinal cord. Here we discuss how the various PRR families are activated and can influence injury and repair processes following CNS injury.

The central nervous system (CNS) is an active participant in the innate immune response to infection and injury. In these studies, we show embryonic cortical neurons express a functional, deoxyribonucleic acid (DNA)-responsive, absent in melanoma 2 (AIM2) inflammasome that activates caspase-1. Neurons undergo pyroptosis, a proinflammatory cell death mechanism characterized by the following: (a) oligomerization of apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC); (b) caspase-1 dependency; (c) formation of discrete pores in the plasma membrane; and (d) release of the inflammatory cytokine interleukin-1b (IL-1b). Probenecid and Brilliant Blue FCF, inhibitors of the pannexin1 channel, prevent AIM2 inflammasome-mediated cell death, identifying pannexin1 as a cell death effector during pyroptosis and probenecid as a novel pyroptosis inhibitor. Furthermore, we show activation of the AIM2 inflammasome in neurons by cerebrospinal fluid (CSF) from traumatic brain injury (TBI) patients and oligomerization of ASC. These findings suggest neuronal pyroptosis is an important cell death mechanism during CNS infection and injury that may be attenuated by probenecid.

The inflammasome is an intracellular multiprotein complex involved in the activation of caspase-1 and the processing of the proinflammatory cytokines interleukin-1b (IL-1b) and IL-18. The inflammasome in the central nervous system (CNS) is involved in the generation of an innate immune inflammatory response through IL-1 cytokine release and in cell death through the process of pyroptosis. In this review, we consider the different types of inflammasomes (NLRP1, NLRP2, NLRP3, and AIM2) that have been described in CNS cells, namely neurons, astrocytes, and microglia. Importantly, we focus on the role of the inflammasome after brain and spinal cord injury and cover the potential activators of the inflammasome after CNS injury such as adenosine triphosphate and DNA, and the therapeutic potential of targeting the inflammasome to improve outcomes after CNS trauma.

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