IL-4 expressing cells are recruited to nerve after injury and promote regeneration

Interleukin-4 (IL-4) has garnered interest as a cytokine that mediates regeneration across multiple tissues including peripheral nerve. Within nerve, we previously showed endogenous IL-4 was critical to regeneration across nerve gaps. Here, we determined a generalizable role of IL-4 in nerve injury and regeneration. In wild-type (WT) mice receiving a sciatic nerve crush, IL-4 expressing cells preferentially accumulated within the injured nerve compared to affected sites proximal, such as dorsal root ganglia (DRGs), or distal muscle. Immunohistochemistry and flow cytometry confirmed that eosinophils (CD45+, CD11b+, CD64-, Siglec-F+) were sources of IL-4 expression. Examination of targets for IL-4 within nerve revealed macrophages, as well as subsets of neurons expressed IL-4R, while Schwann cells expressed limited IL-4R. Dorsal root ganglia cultures were exposed to IL-4 and demonstrated an increased proportion of neurons that extended axons compared to cultures without IL-4 (control), as well as longer myelinated axons compared to cultures without IL-4. The role of endogenous IL-4 during nerve injury and regeneration in vivo was assessed following a sciatic nerve crush using IL-4 knockout (KO) mice. Loss of IL-4 affected macrophage accumulation within injured nerve compared to WT mice, as well as shifted macrophage phenotype towards a CD206- phenotype with altered gene expression. Furthermore, this loss of IL-4 delayed initial axon regeneration from the injury crush site and subsequently delayed functional recovery and re-innervation of neuromuscular junctions compared to wild-type mice. Given the role of endogenous IL-4 in nerve regeneration, exogenous IL-4 was administered daily to WT mice following a nerve crush to examine regeneration. Daily IL-4 administration increased early axonal extension and CD206+ macrophage accumulation but did not alter functional recovery compared to untreated mice. Our data demonstrate IL-4 promotes nerve regeneration and recovery after injury.