We have demonstrated in the rat sciatic nerve model of PNI that regenerating axons have an intrinsic preference to grow along TENG axons, thus facilitating vigorous host axon regeneration. For these studies, allogeneic TENGs (encased in FDA-approved NGTs) were used to repair 1.2-1.5cm PNI lesions. By several weeks post-implant, dense host axon growth was visualized throughout the TENG and beyond. Conversely, host axon growth was limited and substantially delayed in control groups receiving an empty NGT or a NGT seeded with unstretched DRG neurons.
Importantly, host axons closely intertwined with the TENG axons demonstrating axon-induced axon growth, which promoted regeneration through the lesion. Moreover, axons from each end of the transplanted TENG penetrated longitudinally into host tissue providing an extended guidance.
Complete recapitulation of lost nerve anatomy using a TENG was demonstrated at 4 months, including re-vascularization and myelination. Moreover, functional recovery was demonstrated at this time-point via electrophysiological conduction, hindlimb contraflexion and angle board tasks. Remarkably, TENG neurons/axons survived over several months and no immunological response to allogeneic TENGs was observed. These data and others suggest that neurons are immunologically inert, thereby paving the way for the potential use of allogeneic TENGs without immunosuppressive therapy.
Another important aspect of the mechanism of action for TENGs is their ability to prolong the pro-regenerative environment of the distal nerve structure, which is necessary to facilitate and guide axon regeneration to an appropriate target. In particular, degeneration of the axon segments distal to an injury site is an inevitable consequence of transection; however, the supporting Schwann cells (SCs) survive and switch to a pro-regenerative phenotype to support axon growth.
Unfortunately, this natural pro-regenerative environment degrades after several months without the presence of axons, thus depriving regenerating axons of their “road map” to an end target. This occurs when the time it takes regenerating axons to infiltrate is greater than the time this distal pathway will remain – which is often the case following long or proximal PNI – and is primarily responsible for incomplete functional recovery (e.g., regaining elbow but not hand function following upper-arm PNI). The effectiveness of viable axons in maintaining this distal pathway is also evidenced by greater recovery following nerve crush injury where only a portion of the axons at the site of injury degenerate.
We have shown that TENGs have the unique ability, via their axons, to “babysit” the distal pathway following nerve transection. In particular, resident SCs in the distal sheath after TENG implantation maintained their pro-regenerative phenotype and alignment over extended time periods compared to an NGT alone. Therefore, beyond creating a suitable environment for axon regeneration, TENGs maintain the efficacy of the distal pathway to provide a complete guide for regenerating host axons to reach long-distance targets.