Revising CX3CR1 expression on murine classical and non-classical monocytes

Abstract

In mice, monocytes (Mo) are conventionally described as CX3CR1^low classical Mo (CMo) and CX3CR1^high non-classical Mo (NCMo) based on the expression level of EGFP in Cx3cr1^+/EGFP mice and by analogy with human CX3CR1 expression. Although this terminology is widely used, very few works assessed the strict correlation between EGFP reporter and CX3CR1 expression. Using an unsupervised multiparametric analysis of blood Mo in steady state and after sterile peritonitis, we observed no difference in CX3CR1 expression between CMo and NCMo. Our results highlight that despite being a reliable reporter to discriminate Mo subpopulations, EGFP level in Cx3cr1^+/EGFP mice does not reflect CX3CR1 expression measured by a fluorescently-labeled CX3CL1 chemokine and a CX3CR1 specific antibody. In conclusion, authors should be cautious not to identify murine classical and non-classical Mo as CX3CR1^low and CX3CR1^high but rather use alternative markers such as the combination of Ly6C and CD43.