Myelin Induces Inflammation environment for axons but also prolong inflammation, thereby amplifying secondary injury and inhibiting axonal regeneration. Therefore, clearance of myelin debris in CNS may be critical for allowing regeneration. Myelin can induce macrophages to produce pro-inflammatory cytokines and NO in vitro by van der Laan et al [13]. However, there were lack of in vivo data and cellular mechanism study about how myelin stimulates cytokine production. Understanding the molecular underlying myelin stimulation of inflammatory response should prove vital insights into indentifying new therapies that reduce the secondary damage and enhance regeneration. Our results indicate that myelin strongly activates FAK/PI3K/Akt/ NF-kB pathway in macrophages and increases the expression of inflammatory mediators in WT mice but not in CR3 KO mice, suggesting that NF-kB activation by myelin may be CR3- dependent (schematized in Figure 10). Myelin-induced cytokine production is more likely a direct effect as NF-kB can be activated within 5 min, suggesting myelin interacts with macrophages directly rather than the consequence of myelin phagocytosis by macrophages or cytokines released in response to myelin binding to CR3 working in an autocrine manner to trigger NF-kB. Treatment with inhibitors of PI3K, Akt and NF-kB or deletion of CR3 reversed myelin-induced NF-kB activation and also inhibited myelin–mediated production of pro-inflammatory mediators. Therefore, myelin induces cytokine production by activating FAK/PI3K/Akt/NF-kB through CR3-dependent mechanisms. Further studies are needed to identify or rule out other signaling pathways contributing to the myelin-induced cytokine expression. This study suggests a few strategies to reduce inflammatory response and secondary damage and to promote neuronal regeneration: 1) enhance clearance of myelin debris such as increasing phagocytotic uptake of myelin; 2) inactivation of PI3K/ Akt/NF-kB pathway; and 3) targeting CR3-myelin interaction. Although inhibition of NF-kB activity has been shown to reduce inflammation and improves function recovery after SCI [37], enhancement of myelin debris clearance and targeting CR3- myelin interaction after SCI should be further investigated. Leukemia inhibitory factor (LIF) not only acts as a neurotrophic factor [38] in CNS but also inhibits the production of oxygen radicals and TNF-a, and stimulates myelin uptake by macro- phages [39], suggesting that LIF may be useful for treating CNS trauma or other demyelinating diseases. In addition, phagocytosis Figure 10. Schematic diagram depicting the possible mechanism that myelin induces inflammatory responses. Myelin debris generated in injured CNS or demyelinating diseases may bind to CR3 and then activate NF-kB through FAK/PI3K/Akt pathway, therefore initiating gene expression of pro-inflammatory mediators. doi:10.1371/journal.pone.0009380.g010 PLoS ONE | www.plosone.org 13 February 2010 | Volume 5 | Issue 2 | e9380