Investigating the use of responsive hydrogels as therapeutic delivery vehicles for spinal cord injury

Spinal cord injury (SCI) is a devastating condition for which no curative therapy currently exists. Immunomodulation can be applied as a therapeutic strategy post-injury to drive alternative immune cell activation and promote regeneration. Locally injected hydrogels carrying immunotherapeutic agents directly to injured tissue represent an encouraging treatment approach from an immunopharmacological perspective. Interleukin(IL)-13 has shown potential as an immunotherapeutic candidate in preclinical SCI, as previous work from our lab has demonstrated that cell-based delivery of IL-13 improves both functional and histopathological recovery following SCI in mice. Here, we propose that IL-13 delivery via a hydrogel-based system offers a more translationally relevant approach towards sustained and localised immunotherapeutic delivery post-SCI. To this end, we have designed the ‘HGIL13’ delivery system consisting of IL-13 encapsulated poly(lactic-co-glycolic acid) (PLGA) microparticles embedded in a gelatin methacrylate (GelMA) hydrogel. Firstly, we extensively reviewed recent developments in the use of hydrogels as immunomodulatory therapies for SCI. We then analysed the potential of GelMA hydrogels as an immunotherapeutic platform for preclinical SCI, with particular focus on immunogenicity and biocompatibility in BV2 microglia and organotypic spinal cord slices as in vitro and ex vivo models of the central nervous system microenvironment. We identified 3% (w/v) GelMA, synthesised from type-A gelatin, as an optimal formulation from which to develop the HGIL13 system. The HGIL13 delivery system was achieved by combining IL-13/PLGA microparticles at 2% (w/v) and recombinant IL-13 at 1000 ng/mL within a 3% (w/v) GelMA hydrogel to achieve a multiphasic and sustained IL-13 delivery system. HGIL13 released IL-13 for up to six weeks and retained its bioactivity in vitro, as demonstrated by its ability to reduce LPS-induced inflammatory responses in microglial cultures. In a mouse contusion SCI model, HGIL13 significantly improved functional and histopathological recovery, as evidenced by increased Basso Mouse Scale scores and decreased lesion size, demyelinated area, astrogliosis and microglial infiltration. Mechanistically, HGIL13 exerted an immunomodulatory effect by decreasing the number of resident microglia at the lesion site, reducing CD86 expression in microglia and increasing Arginase-1 expression in both microglia and infiltrating macrophages. Temporal gene expression analysis revealed a multiphasic immunomodulatory effect, reducing proinflammatory markers at several distinct timepoints post-injury. Further transcriptomic analysis of microglia at seven days post-injury highlighted regulation of apoptotic signalling and innate immune responses as contributors to recovery. Looking towards future therapeutic avenues, we reviewed motor rehabilitation, the current gold standard SCI treatment in humans, and its effect on neuroinflammation. In doing so we discussed how motor rehabilitation can be optimised as an immunotherapeutic regime, thus presenting a roadmap towards a novel treatment approach for SCI whereby current rehabilitative measures may be combined with state-of-the-art hydrogel delivery systems to offer a multi-component approach to revolutionise the clinical SCI field and significantly improve outcomes for SCI patients worldwide.