Cre Driver Efficiency in Myeloid Lineages Across Tissues and in a Model of Myocardial Injury.

Myeloid cell subpopulations are critically implicated in homeostasis and disease. The Cre-lox system offers powerful tools for dissecting their functions; however, interpretation of the findings is dependent on characterization of the efficacy and specificity of the Cre drivers employed. We systematically assessed the recombination efficiency and cellular specificity of two widely used myeloid Cre drivers, LysM-Cre, and CX3CR1CreER, using flow cytometry and histological analyses in healthy tissues and in a model of myocardial infarction. LysM-Cre achieved near-complete recombination (98%-100%) in neutrophils and macrophages, showed moderate efficiency in monocytes, but also labeled a fraction of non-myeloid leukocytes and approximately 10% of septal cardiomyocytes. CX3CR1CreER had high efficiency in macrophages (except for hepatic macrophages), lower sensitivity in monocytes and negligible activity in neutrophils. In myocardial infarction, a second course of tamoxifen after injury significantly enhanced macrophage targeting in the infarct zone. In conclusion, both the macrophage-specific inducible CX3CR1CreER and the highly sensitive, but less specific constitutively active LysM-Cre represent valuable tools for studying myeloid cell biology. Limitations of the CX3CR1CreER include limited recombination efficiency in macrophages with low CX3CR1 expression (e.g., liver macrophages) and the need for extended tamoxifen induction protocols in conditions associated with extensive recruitment of monocyte-derived macrophages.