Purpose: The lysozyme 2 (Lyz2 or LysM) cre mouse is extensively used to achieve genetic manipulation in myeloid cells and it has been widely employed in retinal research. However, LysM has been recently described to be expressed in brain neurons and there is a debate on whether it is also expressed by resident microglia in addition to infiltrating macrophages.
Methods: We examined LysM-cre recombination in retinal tissue using a LysM-cre/tdTomato reporter mouse together with immunolabeling for several retinal cell markers. We further compared LysM-cre tdTomato recombination with that of Cdh5-cre driver, which is expressed in both endothelial and hematopoietic cells.
Results: LysM-cre was strongly expressed in most microglia/resident macrophages in neonatal retinas (P8) and to a lesser extent in microglia of adult retinas. In addition, there was some neuronal recombination (8 %) of LysM-cre specifically in adult retinal ganglion cells and amacrine cells. After retinal ischemia-reperfusion injury, LysM-cre was strongly expressed in microglia/infiltrating macrophages. Cdh5-cre was expressed in endothelial and myeloid cells of P8 pups retinas. Unexpectedly, Cdh5 showed additional expression in adult mouse retinal ganglion cells and brain neurons.
Conclusions: LysM-cre is expressed in macrophages and a subset of microglia together with a small but significant recombination of LysM-cre in the retinal neurons of adult mice. Cdh5 also showed some neuronal expression in both retina and brain of adult mice. These findings should be taken into consideration when interpreting results from central nervous system research using LysM-cre and Cdh5-cre mice.