CD11b+GR1+ Myeloid Cells Promote Trigeminal Ganglion Neurite Growth: Implications for Corneal Nerve Regeneration

Abstract

Purpose: It is becoming increasingly clear that the nervous and inflammatory (myeloid) systems display considerable overlap in their molecular and cellular repertoire. Bone marrow-derived YFP+ cells infiltrate the cornea during nerve regeneration in thy1-YFP mouse. We characterized these cells and determined whether they promote trigeminal ganglion (TG) cell neurite growth.

Methods: Excimer laser annular keratectomy was performed in thy1-YFP+ mice and corneas were imaged to visualize regenerating nerves and infiltrating cells. Bone marrow cells (BMCs) were harvested from naive thy1-YFP mice and a flow cytometry analyzer was used to assess the expression levels of cell surface markers in the total BMC population. Compartmental cultures of dissociated TG cells were performed and sorted YFP+ cells (>95% pure) were co-cultured in the side compartments with neurites to determine their neurotrophic effect.

Results: Following annular keratectomy, YFP+ cells infiltrate the cornea and localize adjacent to transected nerves. These are bone marrow-derived cells that share surface markers (CD11b+Gr1+Ly6C+Ly6G-F4/80low) with monocytic myeloid-derived suppressor cells (MDSCs); thus, we call them YFP-MDSCs. They are CD11c, CD3e and MHC-II negative. YFP-MDSCs significantly increase the growth of TG neurites and make physical contact with neurites. When cultured in a transwell with TG neurites, YFP-MDSCs express neurotrophins and secrete NGF and BDNF in the conditioned media.

Conclusions: CD11b+Gr1+ myeloid cells promote neurite growth in TG cells. These cells infiltrate the cornea after nerve transection surgery and interact with the regenerating nerves; thus they may promote reinnervation by their neurotrophic action. One mechanism by which these cells promote neurite growth is by secretion of neurotrophins.

Keywords: 480 cornea: basic science • 687 regeneration