Nlrp6 promotes recovery after peripheral nerve injury independently of inflammasomes

Background: NOD-like receptors (Nlrs) are key regulators of immune responses during infection and autoimmunity. A subset of Nlrs assembles inflammasomes, molecular platforms that are activated in response to endogenous danger and microbial ligands and that control release of interleukin (IL)-1 beta and IL-18. However, their role in response to injury in the nervous system is less understood. Methods: In this study, we investigated the expression profile of major inflammasome components in the peripheral nervous system (PNS) and explored the physiological role of different Nlrs upon acute nerve injury in mice. Results: While in basal conditions, predominantly members of NOD-like receptor B (Nlrb) subfamily (NLR family, apoptosis inhibitory proteins (NAIPs)) and Nlrc subfamily (ICE-protease activating factor (IPAF)/NOD) are detected in the sciatic nerve, injury causes a shift towards expression of the Nlrp family. Sterile nerve injury also leads to an increase in expression of the Nlrb subfamily, while bacteria trigger expression of the Nlrc subfamily. Interestingly, loss of Nlrp6 led to strongly impaired nerve function upon nerve crush. Loss of the inflammasome adaptor apoptosis-associated speck-like protein containing a CARD (ASC) and effector caspase-1 and caspase-11 did not affect sciatic nerve function, suggesting that Nlrp6 contributed to recovery after peripheral nerve injury independently of inflammasomes. In line with this, we did not detect release of mature IL-1 beta upon acute nerve injury despite potent induction of pro-IL-1 beta and inflammasome components Nlrp3 and Nlrp1. However, Nlrp6 deficiency was associated with increased pro-inflammatory extracellular regulated MAP kinase (ERK) signaling, suggesting that hyperinflammation in the absence of Nlrp6 exacerbated peripheral nerve injury. Conclusions: Together, our observations suggest that Nlrp6 contributes to recovery from peripheral nerve injury by dampening inflammatory responses independently of IL-1 beta and inflammasomes

Profiling peripheral nerve macrophages reveals two macrophage subsets with distinct localization, transcriptome and response to injury

The authors identify two subsets of peripheral nerve macrophages residing in the endoneurium and the epineurium and displaying a distinct transcriptome and response to injury. These cells lack the main microglia identity and have a distinct origin. While CNS microglia have been extensively studied, relatively little is known about macrophages populating the peripheral nervous system. Here we performed ontogenic, transcriptomic and spatial characterization of sciatic nerve macrophages (snMacs). Using multiple fate-mapping systems, we show that snMacs do not derive from the early embryonic precursors colonizing the CNS, but originate primarily from late embryonic precursors and become replaced by bone-marrow-derived macrophages over time. Using single-cell transcriptomics, we identified a tissue-specific core signature of snMacs and two spatially separated snMacs: Relm alpha(+)Mgl1(+) snMacs in the epineurium and Relm alpha(-)Mgl1(-) snMacs in the endoneurium. Globally, snMacs lack most of the core signature genes of microglia, with only the endoneurial subset expressing a restricted number of these genes. In response to nerve injury, the two resident snMac populations respond differently. Moreover, and unlike in the CNS, monocyte-derived macrophages that develop during injury can engraft efficiently in the pool of resident peripheral nervous system macrophages

Acute injury in the peripheral nervous system triggers an alternative macrophage response

Background: The activation of the immune system in neurodegeneration has detrimental as well as beneficial effects. Which aspects of this immune response aggravate the neurodegenerative breakdown and which stimulate regeneration remains an open question. To unravel the neuroprotective aspects of the immune system we focused on a model of acute peripheral nerve injury, in which the immune system was shown to be protective. Methods: To determine the type of immune response triggered after axotomy of the sciatic nerve, a model for Wallerian degeneration in the peripheral nervous system, we evaluated markers representing the two extremes of a type I and type II immune response (classical vs. alternative) using real-time quantitative polymerase chain reaction (RT-qPCR), western blot, and immunohistochemistry. Results: Our results showed that acute peripheral nerve injury triggers an anti-inflammatory and immunosuppressive response, rather than a pro-inflammatory response. This was reflected by the complete absence of classical macrophage markers (iNOS, IFN gamma, and IL12p40), and the strong up-regulation of tissue repair markers (arginase-1, Ym1, and Trem2). The signal favoring the alternative macrophage environment was induced immediately after nerve damage and appeared to be established within the nerve, well before the infiltration of macrophages. In addition, negative regulators of the innate immune response, as well as the anti-inflammatory cytokine IL-10 were induced. The strict regulation of the immune system dampens the potential tissue damaging effects of an over-activated response. Conclusions: We here demonstrate that acute peripheral nerve injury triggers an inherent protective environment by inducing the M2 phenotype of macrophages and the expression of arginase-1. We believe that the M2 phenotype, associated with a sterile inflammatory response and tissue repair, might explain their neuroprotective capacity. As such, shifting the neurodegeneration-induced immune responses towards an M2/Th2 response could be an important therapeutic strategy

The neuroinflammatory role of Schwann cells in disease

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Antibody profiling identifies novel antigenic targets in spinal cord injury patients

Contains fulltext : 171493.pdf (publisher's version ) (Open Access)BACKGROUND: Recent evidence implicates antibody responses as pivotal damaging factors in spinal cord injury (SCI)-induced neuroinflammation. To date, only a limited number of the antibody targets have been uncovered, and the discovery of novel targets with pathologic and clinical relevance still represents a major challenge. METHODS: In this study, we, therefore, applied an unbiased, innovative and powerful strategy, called serological antigen selection (SAS), to fully identify the complex information present within the antibody repertoire of SCI patients. RESULTS: We constructed a high-quality cDNA phage display library derived from human spinal cord tissue to screen for antibody reactivity in pooled plasma samples from traumatic SCI patients (n = 10, identification cohort). By performing SAS, we identified a panel of 19 antigenic targets to which the individual samples of the plasma pool showed antibody reactivity. Sequence analysis to identify the selected antigenic targets uncovered 5 known proteins, to which antibody reactivity has not been associated with SCI before, as well as linear peptides. Immunoreactivity against 9 of the 19 novel identified targets was validated in 41 additional SCI patients and an equal number of age- and gender-matched healthy subjects. Overall, we found elevated antibody levels to at least 1 of the 9 targets in 51 % of our total SCI patient cohort (n = 51) with a specificity of 73 %. By combining 6 of these 9 targets into a panel, an overall reactivity of approximately half of the SCI patients could be maintained while increasing the specificity to 82 %. CONCLUSIONS: In conclusion, our innovative high-throughput approach resulted in the identification of previously unexplored antigenic targets with elevated immunoreactivity in more than 50 % of the SCI patients. Characterization of the validated antibody responses and their targets will not only provide new insight into the underlying disease processes of SCI pathology but also significantly contribute to uncovering potential antibody biomarkers for SCI patients