Cre Driver Mice Targeting Macrophages

Abstract

The Cre/loxP system is a widely applied technology for site-specific genetic manipulation in mice. This system allows for deletion of the genes of interest in specific cells, tissues, and whole organism to generate a diversity of conditional knockout mouse strains. Additionally, the Cre/loxP system is useful for development of cell- and tissue-specific reporter mice for lineage tracing, and cell-specific conditional depletion models in mice. Recently, the Cre/loxP technique was extensively adopted to characterize the monocyte/macrophage biology in mouse models. Compared to other relatively homogenous immune cell types such as neutrophils, mast cells, and basophils, monocytes/macrophages represent a highly heterogeneous population which lack specific markers or transcriptional factors. Though great efforts have been made toward establishing macrophage-specific Cre driver mice in the past decade, all of the current available strains are not perfect with regard to their depletion efficiency and targeting specificity for endogenous macrophages. Here we overview the commonly used Cre driver mouse strains targeting macrophages and discuss their major applications and limitations.

Keywords: Cre/loxP; Macrophage reporter mice; Macrophage-specific conditional knockout; Macrophages; Monocytes.

Figure 1. Generation of LysM⁺ cell-specific conditional knockout mice using Cre/loxP technology.

LysM-Cre transgenic mouse is first crossed with the mouse homozygous for loxP-flanked gene A. Approximately 50% of the offspring will be heterozygous for the loxP-flanked gene A and hemizygous/heterozygous for the Cre transgene. In this F1 generation, the gene A is heterozygous knockout specific for LysM⁺ cells. A further cross of this F1 generation with the parental mice homozygous for loxP-flanked gene A will lead to generation of about 25% of the progeny (experimental mice) homozygous for the loxP-flanked gene A and hemizygous/heterozygous for the Cre transgene. In this F2 generation of mice, Cre-mediated excision causes the homozygous knockout for gene A specifically in LysM⁺ cells. Other F2 littermates, such as mice homozygous for loxP with no Cre transgene, and mice heterozygous for loxP and hemizygous/heterozygous for the Cre transgene, can serve as controls.

Figure 2. Application of Cre/loxP system for generation of mouse lines with gene conditional activation.

The gene B-loxP transgenic mouse is created by inserting a loxP-flanked transcriptional “STOP” sequence between the promoter and the gene B coding sequence, which prevents the expression of gene B. To cross this gene B-loxP transgenic mouse with LysM-Cre mouse will generate the progeny with gene B specifically expressed in LysM⁺ cells. Such a system helps to develop cell-specific reporter mice, gene activation/overexpression and conditional depletion mouse lines.