CX₃CR1 deficiency exacerbates neuronal loss and impairs early regenerative responses in the target-ablated olfactory epithelium

The olfactory epithelium is a site of sustained adult neurogenesis where olfactory sensory neurons are continuously replaced from endogenous stem/progenitor cells. Epithelial macrophages have been implicated in the phagocytosis of degenerating cells but the molecular mechanisms allowing for their recruitment and activation while maintaining a neurogenic microenvironment are poorly understood. We have previously shown that the chemokine fractalkine (CX₃CL1) is expressed by olfactory sensory neurons and ensheathing cells in the olfactory epithelium. In turn, the fractalkine receptor, CX₃CR1, is expressed on macrophages and dendritic cells within the olfactory epithelium. We report that a selective cell death of olfactory sensory neurons in the epithelium of CX₃CR1-deficient mice via target ablation (i.e. olfactory bulbectomy) results in an exacerbated loss of olfactory sensory neurons compared to wild-type mice. In addition, reduced proliferation of intraepithelial stem/progenitor cells was observed in lesioned CX₃CR1-deficient mice, suggesting an impaired regenerative response. Importantly, a lack of CX₃CL1-signaling caused increased recruitment of macrophages into the olfactory epithelium, which in turn contained higher levels of pro-inflammatory cytokines (e.g. TNF-α and IL-6) as determined by qPCR. We also present novel data showing that, relative to wild-type, CX₃CR1-deficient macrophages have diminished phagocytic activity following stimulation with CX₃CL1. Collectively, our data indicate that signaling through the CX₃CR1 receptor modulates macrophage activity, resulting in an environment conducive to olfactory sensory neuron clearance and targeted replacement from endogenous stem/progenitor cells.