Temporal gene expression profiles of target-ablated olfactory epithelium in mice with disrupted expression of scavenger receptor A: Impact on macrophages

Target ablation [removal of the olfactory bulb (OBX)] induces apoptotic death of olfactory sensory neurons (OSNs) and an immune response in which activation and recruitment of macrophages (mφs) into the olfactory epithelium (OE) occupy a central role. Mφs phagocytose apoptotic neurons and secrete cytokines/growth factors that regulate subsequent progenitor cell proliferation and neurogenesis. Scavenger receptor A (SR-A) is a pattern recognition receptor that mediates binding of mφs to apoptotic cells and other relevant immune response functions. The aim of this study was to determine the impact of the absence of SR-A on the immune response to OBX. The immune response to OBX was evaluated in mice in which functional expression of the mφ scavenger receptor (MSR) was eliminated by gene disruption (MSR ^-/-) and wild-type (wt) mice of the same genetic background. OBX induced significant apoptotic death of mature OSNs in the two strains. However, subsequent mφ infiltration and activation and progenitor cell proliferation were significantly reduced in MSR^-/- vs. wt mice. Gene expression profiling at short intervals after OBX demonstrated significant differences in temporal patterns of expression of several gene categories, including immune response genes. Many immune response genes that showed different temporal patterns of expression are related to mφ function, including cytokine and chemokine secretion, phagocytosis, and mφ maturation and activation. These studies suggest that impairment of the immune response to OBX in the OE of MSR^-/- mice most likely resulted from decreased mφ adhesion and subsequent reduced infiltration and activation, with a resultant decrease in neurogenesis.