Tlr4deletion modulates cytokine and extracellular matrix expression in chronic spinal cord injury, leading to improved secondary damage and functional recovery

Toll-like receptors (TLRs) play an important role in the innate immune response after CNS injury. Although TLR4 is one of the best characterized, its role in chronic stages after spinal cord injury (SCI) is not well understood. We examined the role of TLR4 signaling in injury-induced responses at 1day, 1 and 8 weeks after spinal cord contusion injury in adult female TLR4 null and wildtype mice. Analyses include secondary damage, a range of transcriptome and protein analyses of inflammatory, cell death and extracellular matrix (ECM) molecules; as well as immune cell infiltration; changes in axonal sprouting and locomotor recovery. Lack of TLR4 signaling results in reduced neuronal and myelin loss; reduced activation of NF-κB and decreased expression of inflammatory cytokines and necroptotic cell death pathway at a late time point (8 weeks) after injury. TLR4 null mice also showed reduction of scar-related ECM molecules at 8 weeks after SCI, accompanied by increase in ECM molecules associated with perineuronal nets, increased sprouting of serotonergic fibers and improved locomotor recovery. These findings reveal novel effects of TLR4 signaling in chronic SCI. We show that TLR4 influences inflammation, cell death, ECM deposition at late-stage post-injury when secondary injury processes are normally considered to be over. This highlights the potential for late stage targeting of TLR4 as a potential therapy for chronic SCI.Significance StatementSpinal cord injury often results in life-long paralysis and sensory loss of the limbs. Much of the research on biological mechanisms in SCI is focused on the acute period after injury. However, most SCI patients have been living with their injuries for months or years. We now show a delayed effect of Toll-like receptor 4 (TLR4) mediated inflammation several months after injury that induces changes in cytokines, neuronal cell death and extracellular matrix deposition, and axonal sprouting that can influence functional recovery. This work addresses a question of much interest in the field of spinal cord injury and could also be of wider interest for recovery after brain trauma.