Composite Fibrin and Carbon Microfibre Implant to Modulate Postraumatic Inflammation after Spinal Cord Injury

Abstract

Poor functional recovery after spinal cord injury (SCI) drives the development of novel strategies to manage this devastating condition. We recently showed promising immunomodulatory and pro-regenerative actions of bio-functionalized carbon microfibres (MFs) implanted in a rodent model of SCI. In order to maximize tissue repair while easing MF implantation, we produced a composite implant based on the embedding of several MFs within a fibrin hydrogel. We used intravital imaging of fluorescent reporter mice at the early stages and spinal sections of the same animals 3 months later to characterize the neuroinflammatory response to the implant and its impact on axonal regeneration. Whereas fibrin alone was inert in the first week, its enzymatic degradation drove the chronic activation of microglial cells and axonal degeneration within 3 months. However, the presence of MFs inside the fibrin hydrogel slowed down fibrin degradation and boosted the early recruitment of immune cells. Noteworthy, there was an enhanced contribution of monocyte-derived dendritic cells (moDCs), preceding a faster transition toward an anti-inflammatory environment with increased axonal regeneration over 3 months. The inclusion of MF here ensured the long-term biocompatibility of fibrin hydrogels, which would otherwise preclude successful spinal cord regeneration.

Composite Fibrin/Carbon Microfiber Implants for Bridging Spinal Cord Injury: A Translational Approach in Pigs

“…Differently to the dense meningeal scar, which was largely impenetrable to axons and glial cells in all treatment groups, the spongy scar contained numerous longitudinally aligned axons in the implant group. Thus, as previously demonstrated in rodents [3,24,25], the multimolecular complex of PLL/heparin/bFGF/fibronectin attached to the MFs was able to facilitate the growth of some axonal types in the porcine spinal cord, even in the inhibitory environment provided by the scarring cells. Neural tracers can be applied in swine [26,29] to study the origin and termination of spinal axonal tracts.…”

“…Fibrin has a crucial role in blood clotting and wound healing, and fibrin hydrogels have demonstrated safety and usefulness in SCI repair strategies [ 53 , 54 ]. Like our previous studies in mice [ 25 ], embedding multiple MFs in fibrin facilitated their handling, alignment, and implantation in the porcine spinal cord. Additionally, fibrin partially filled the tissue gap, thus reducing accumulation of blood products in the cavity after myelotomy and lesion debridement.…”

“…The latter biofunctionalized MFs were implanted in spinal-cord-transected rats, providing compelling evidence that they can enhance and guide blood vessel growth, cell migration, and axonal extension across the lesion [3]. Similar effects together with a prohealing immunomodulatory response were obtained in transgenic mice [24,25]. Remarkably, a single biofunctionalized microfiber was able to promote the adhesion and growth of numerous axons and glial cells both in vitro [20,21] and in vivo [3,24], with hundreds of axons following the pioneering ones in a process resembling developmental axon fasciculation.…”

“…Moreover, the distribution of nanoparticles aimed at drug delivery has been studied in a contusion model of porcine SCI [ 13 ], as well as the feasibility of introducing scaffolds made of a copolymer of lactic-co-glycolic acid (PLGA) and poly(L-lysine) into the lesion [ 4 ]. Biofunctionalized electroconducting MFs have been shown to enhance and guide axonal and blood vessel growth after SCI in rodents [ 3 , 24 , 25 ] and can be advantageously used for sensing [ 49 ] and stimulating [ 50 ] neuronal activity. To our knowledge, the present study represents the first attempt to promote spinal axon regeneration in a large animal by implanting an MFs-based electroconductive scaffold.…”

“…For this, we produced contusion/compression SCI in pigs, obtained a baseline of histology and behavior, and studied the consequences of a dorsal myelotomy at one day post-injury, alone or followed by the implantation of biofunctionalized carbon MFs embedded in fibrin gel. Based on the abovementioned studies implanting those MFs in rodents [ 3 , 24 , 25 ], we considered that an implant with a small number of MFs (about one or two MFs per mm 2 of transverse spinal cord section) might be sufficient to support collective cell regrowth after SCI in pigs. Besides characterizing the porcine SCI, we obtained preliminary evidence showing that the biofunctionalized MFs can also favor neural regrowth in this translational context.…”