Modulation of the regenerative microenvironment within acellular nerve allografts using tacrolimus improves nerve regeneration

Acellular nerve allografts have been used as an alternative to reconstruct nerve gaps. However, regeneration and recovery using long acellular nerve allografts (> 3 cm) is poor in comparison to short acellular nerve allografts (< 3 cm). To understand why long acellular nerve allografts have limited regeneration, we focused on identifying differences in the microenvironment of short (2 cm) and long (4 cm) acellular nerve allografts by comparing the transcriptional profile of these acellular nerve allografts. After repairing the sciatic nerve of Lewis rats using either short or long acellular nerve allografts, we found that the proximal and mid-distal graft regions of long acellular nerve allografts are characterized by an upregulation of metabolic and immune pathways and downregulation of regenerative processes in comparison to the short acellular nerve allografts. Based on these results, we modulated the regenerative and immune microenvironment of long acellular nerve allografts using tacrolimus (FK506). Histomorphometric and muscle force analysis revealed that FK506 increases the number of axons and improves recovery of motor function across long acellular nerve allografts. Transcriptome analysis of the mid-distal graft region of long acellular nerve allografts from animals treated with FK506 revealed upregulation of regenerative pathways and downregulation of immune processes, specifically related to T cell activity. Additionally, FK506 altered the number of macrophages and Schwann cells in the long acellular nerve allografts. From the transcriptome analysis, we identified FK506 upregulates expression of Spp1 (osteopontin) which promotes regeneration of motor neurons after injury. Experiments on cultured Schwann cells revealed that FK506 increases mRNA expression of Spp1. Our data show the development of a degenerative and immune microenvironment within long acellular nerve allografts and demonstrate that FK506 can modulate this microenvironment to improve nerve regeneration across these long acellular nerve allografts.