Abstract
Purpose :
Axon regenerative failure in the mature CNS contributes to functional deficits following traumatic injuries, ischemic injuries and neurodegenerative diseases. Although neuro-immune interactions modulate critical CNS functions, potential contributions of the classical complement cascade and myeloid cells to CNS axon regeneration or its failure are largely unknown. Building on our recent finding that classical complement proteins C1q and C3 and CR3+ (CD11b+) myeloid cells each increase locally following experimental mouse optic nerve injury and are each required for retinal ganglion cell (RGC) axon regeneration, we tested the hypothesis that complement and phagocytic myeloid cells support RGC axon regeneration by altering the local environment of the injured optic nerve.
Methods :
We quantified RGC axon regeneration (GAP43 immunolabeled axons 0.5mm past the injury site) in adult mice 14 days after optic nerve crush plus intravitreal injection of pro-regenerative treatment (zymosan + CPT-cAMP) and localized (either intravitreal, intra-nerve, or intraperitoneal) injection of C1q function-blocking antibody or IgG control (N = 8-16/group). We characterized the local environment of the injured optic nerve with regard to inflammatory response and myelin debris in optic nerve sections collected 1, 5, 7 and 14 days after injury and immunolabeled for CR3, CD68, and myelin basic protein (MBP). We assessed MBP+ myelin debris levels 5 and 14 days after optic nerve injury in CR3-/- mice and CR3+/+ controls, both with and without pro-regenerative treatment (zinc chelation; N = 6-8/group).
Results :
Whereas intravitreal anti-C1q injection did not alter RGC axon regeneration, intra-nerve and intra-peritoneal injections of anti-C1q each decreased axon regeneration by ~50%. We observed an MBP-negative area that expanded from the injury site over time and that closely corresponded to a dense CR3+ and CD68+ myeloid cell area, as well as MBP localized inside these myeloid cells. CR3-/- mice exhibited a 3.3-fold increase in myelin debris remaining in the vicinity of the injury site compared to CR3+/+ controls.
Conclusions :
Our data demonstrate that classical complement proteins and myeloid cells act within the optic nerve to support RGC axon regeneration, primarily through phagocytic removal of growth-inhibitory myelin debris. These results point to the importance of the innate immune response for CNS repair.
This is a 2021 ARVO Annual Meeting abstract.