Local Injection of Lenti–BDNF at the Lesion Site Promotes M2 Macrophage Polarization and Inhibits Inflammatory Response After Spinal Cord Injury in Mice

Abstract

There is much evidence to suggest that brain-derived neurotrophic factor (BDNF) is a prominent candidate in promoting neuroprotection, axonal regeneration, and synaptic plasticity following spinal cord injury (SCI). Although some evidence indicates that BDNF has potent anti-oxidative effects and may be involved in the regulation of the immune response, the effects of BDNF in the inflammatory response during the course of secondary damage after SCI is still unclear. The present study was designed to investigate the effects of BDNF with a special focus on their effect on macrophage polarization after SCI. Adult C57 mice underwent T10 spinal cord clip compression injury and received lenti-BDNF vector injections at the epicenter of the lesion site. Four days later, total BDNF levels were greatly increased in animals that received lenti-BDNF injections. Confocal imaging showed that more than 80 % of the lenti-virus infected cells were CD11b-positive macrophages. In addition, the expression of arginase-1 and CD206 (associated with M2 macrophage phenotype) significantly increased in the animals that received lenti-BDNF injections compared with those that received lenti-EGFP injections. On the contrary, the expression of CD16/32 and inducible nitric oxide synthase (M1 phenotype marker) was down-regulated as demonstrated using flow cytometry and immunohistochemistry. Furthermore, the production of interleukin 1β and tumor necrosis factor alpha was significantly reduced whereas the levels of interleukin 10 and interleukin 13 were elevated in subjects that received lenti-BDNF vector injections. The time course of functional recovery revealed that gradual recovery was observed in the subacute phase in lenti-BDNF group, little improvement was observed in lenti-EGFP group. At the axonal level, significant retraction of the CST axons were observed in lenti-EGFP injected animals relative to lenti-BDNF group by biotinylated dextran amine tracing. In addition, compared to lenti-BDNF group markedly demyelination was observed in the lenti-EGFP group using luxol fast blue staining. In conclusion, we found that BDNF could promote the shift of M1 to M2 phenotype and ameliorate the inflammatory microenvironment. Furthermore, the roles of BDNF in immunity modulation may enhance neuroprotective effects and partially contribute to the locomotor functional recovery after SCI.

Regulation of Inflammatory Cytokines for Spinal Cord Injury Repair Through Local Delivery of Therapeutic Agents

“…In addition to inflammatory cytokines, local lentivirus‐mediated regulation of other proteins also affected inflammation and facilitated neurorepair. Ji et al edited a lentivirus BDNF‐overexpressing vector and injected it into the SCI epicenter 229. The injection resulted in reduced IL‐1β and TNF‐α, elevated IL‐10 and IL‐13, and promotion of M2 polarization.…”

Neurotrophic Factors Used to Treat Spinal Cord Injury

“…Evidence also indicates that BDNF has potent antioxidative effects and may be involved in the regulation of the immune response after SCI (Ji et al, 2015). The importance of the location of BDNF administration as well as the timing of delivery has been specifically highlighted by Lu et al and several other groups (Blesch & Tuszynski, 2007;Kwon et al, 2002;Novikova et al, 2000).…”

“…However, adult SCI can alter TrkB expression in and around the lesion site (Liebl et al, 2001) and it has been reported that this receptor is expressed primarily on the soma and dendrites but not on the axons of adult CST neurons . Interestingly, adult SCI is similarly known to alter p75 expression (Ji et al, 2015;King et al, 2000;Liebl et al, 2001), which could potentially affect the action of exogenously applied BDNF. In contrast to these inconsistent effects on CST regeneration, many studies have reported regrowth of 5HT fibers in response to BDNF administration (Boyce et al, 2007;Brock et al, 2010;Coumans et al, 2001;Golden et al, 2007;Koda et al, 2007;Lu et al, 2005;Tobias et al, 2001), which has the potential to facilitate locomotor recovery (Slawinska, Miazga, & Jordan, 2014).…”

“…2.2.1 Neuroprotection, Sparing, and Growth With NT-3 NT-3 applied using minipumps or via cellular or extracellular matrix delivery can enhance the sprouting and local growth of CST axons Iarikov et al, 2007;Ji et al, 2015;Ruitenberg et al, 2005;Schnell et al, 1994) although others reported no effect (Vavrek et al, 2006) or even reduced sprouting (Hagg et al, 2005) of such axons. Overexpression of NT-3 in lumbar spinal cord using an adenoviral vector 2 weeks but not 4 months after unilateral lesion of the medullary pyramids resulted in enhanced sprouting of intact CST axons (Chen, Zhou, & Shine, 2006).…”

“…Other axonal populations also respond to exogenous application of NT-3 after SCI (Bradbury et al, 1999;Bradbury, King, Simmons, Priestley, & McMahon, 1998;Hou et al, 2012;Lu et al, 2007;Taylor, Jones, et al, 2006), although in contrast to BDNF, NT-3 has no impact on rubrospinal neurons or their axons (Ruitenberg et al, 2003), and reduced growth of axons from brainstem neurons (Tsai et al, 2006). Positive effects on myelination have been documented (Han et al, 2015;Hwang et al, 2011;Ji et al, 2015;Yang et al, 2015), perhaps associated with infiltrating Schwann cells (Thomas et al, 2014).…”