(SS#V) Charles Tator Spinal Cord Injury Resident Research Award (2023 Award Winner): Systemic Application of IL-4 Attenuates Inflammation and Confers Enhanced Functional Recovery After Traumatic Spinal Cord Injury in Rats

Introduction: Traumatic spinal cord injury (SCI) initiates a local and systemic inflammation limiting neuro-regeneration. To this end, we aimed to decipher the effect of anti-inflammatory interleukin-4 (IL-4) on functional recovery and local spinal cord and systemic inflammation upon its systemic application after SCI in rats.

Methods: 120 female Wistar rats were randomized for a laminectomy without(sham) or with thoracic(Th10) clip compression SCI. SCI animals then received IL-4 or placebo intraperitoneally for 7 days. Neurological function was assessed 3, 7-, 14-, 21- and 28-days post injury (dpi) via the CatWalk-XT gait analysis, the Basso, Beattie, Bresnahan(BBB) openfield rating scale. Rats were sacrificed at said timepoints and immunohistochemistry (IHC) was used to assess neuroinflammation, neurodegeneration, and astrogliosis of explanted spinal cords. In addition, protein and RNA levels of cytokines were measured in the serum and peripheral organs via flow cytometry/RT-PCR to assess systemic inflammation.

Results: IL-4-treated rats showed a higher recovery of BBB scores compared to placebo-treated rats at 14 and 28 dpi. The CatWalk-XT gait analysis showed higher recovery of hindlimb function in IL-4-treated rats, climaxing at 14 dpi. In IHC analyses, a significantly higher ratio of anti-inflammatory M2- to pro-inflammatory M1-macrophages was observed in IL-4 compared to placebo rats 3 and 7 dpi. Furthermore, astrogliosis was reduced under IL-4 at 28 dpi. While placebo-treated SCI rats showed significantly higher levels of pro-inflammatory serum cytokines compared to sham rats, a strong reduction of pro-inflammatory and an increase in anti-inflammatory cytokines was observed under IL-4 in the subacute post-injury phase.

Conclusion : Our findings establish an association between systemic IL-4 application and improved functional recovery after SCI in rats, possibly due to its local and systemic sub-acute immune-modulatory effect. With neuroinflammation and tissue scarring effectively reduced by the systemic application of IL-4, further preclinical and translational studies on IL-4 in the context of SCI might be warranted.