Constitutive expression of CX3CR1-BAC-Cre introduces minimal off-target effects in microglia

CX3CR1-Cre mouse lines have produced important advancements in our understanding of microglial biology. Recent studies have demonstrated the adverse effects of tamoxifen-induced CX3CR1-Cre expression during development, which include changes in microglial density, phenotype, and DNA damage, as well as anxiety-like behavior. However, the unintended effects of constitutive CX3CR1-BAC-Cre expression remain unexplored. Here, we characterized the effects of CX3CR1-BAC-Cre expression on microglia in CX3CR1-BAC-Cre+/- and CX3CR1-BAC-Cre-/-male and female littermates during early postnatal development and adulthood in multiple brain regions. Additionally, we performed anxiety-like behavior tests to assess changes caused by Cre expression. We found that CX3CR1-BAC-Cre expression causes subtle region- and sex-specific changes in microglial density, volume, and morphology during development, but these changes normalized by adulthood in all brain regions except the hippocampus. No behavioral effects were found. Our findings suggest that the constitutive-Cre model might be less detrimental than the inducible model, and highlight the need for proper controls.