Systemic Interleukin-4 Application Promotes Functional Recovery and Reprograms Neuroinflammatory and Molecular Responses after Spinal Cord Injury in Rats

Rationale: Traumatic spinal cord injury (SCI) initiates a cascade of local and systemic inflammatory events that exacerbate tissue damage, hinder regeneration, and impair functional recovery. Interleukin-4 (IL-4) is an anti-inflammatory cytokine that promotes M2-macrophage polarization, but its functional benefit in SCI and the underlying mechanisms remain incompletely defined. We evaluated whether systemic IL-4 therapy can enhance recovery and modulate neuroinflammation in a rat model of SCI, and examined the translational relevance of key cytokine signatures in human SCI. Methods: Female Wistar rats (n = 120) were randomized to sham surgery, SCI with vehicle, or SCI with IL-4 treatment. SCI was induced at T10 by clip contusion-compression; IL-4 (0.5 µg/kg) or vehicle was administered intraperitoneally twice daily for up to 7 days post-injury (dpi). Functional recovery was assessed with the Basso-Beattie-Bresnahan (BBB) scale, CatWalk XT gait analysis, and gridwalk testing. Spinal cords collected at 1, 3, 7, 14, and 28 dpi underwent immunohistochemistry, RNA sequencing, and proteomic profiling. Serum cytokines were quantified in rats by bead-based multiplex assays and compared with longitudinal cytokine profiles from SCI patients. Results: IL-4-treated rats demonstrated significantly improved BBB scores and multiple CatWalk XT gait parameters by 14 dpi versus vehicle. RNA-seq and proteomics identified upregulation of pathways related to axonogenesis, tissue repair, and reduced TNF-α-mediated pro-inflammatory signaling. Immunohistochemistry confirmed increased IBA1⁺/ARG1⁺ and IBA1⁺/CD206⁺ M2-macrophages, reduced IBA1⁺/iNOS⁺ M1-macrophages, smaller cystic cavity area, and higher APC⁺ oligodendrocyte counts in IL-4-treated animals. Serum profiling showed suppression of acute/subacute pro-inflammatory cytokine surges (1-7 dpi) with IL-4. In SCI patients, lower circulating levels of these cytokines were associated with better neurological outcomes. Conclusions: Repeated systemic IL-4 administration after SCI promotes functional recovery, shifts macrophage polarization toward a regenerative phenotype, reduces astrogliosis and oligodendrocyte loss, and suppresses systemic inflammation. Multi-omics integration together with patient data suggests IL-4 targets convergent pathways of neuroprotection and immune modulation, supporting its further development as a therapeutic candidate for SCI.