Abstract Background CD4 + and CD8 + regulatory T cells (Treg) in diverse species include different subsets from different origins. In all species, CD8 + Treg have been poorly characterized. CD4 + and CD8 + Treg in rats have only partially been characterized and there is no rat model in which FOXP3 + Treg are genetically tagged. Results We generated a rat transgenic line using the CRISPR/Cas9 system in which EGFP was inserted in frame on the 3’ end of the Foxp3 gene using a 2A self-cleaving peptide. EGFP was exclusively expressed by CD4 + and CD8 + T cells in similar proportion as observed with anti-FOXP3 antibodies. CD4 + EGFP + Treg were 5-10 times more frequent than CD8 + EGFP + Treg. CD4 + and CD8 + EGFP + Treg expressed both the CD25 high CD127 low CD45RC low/- markers. The suppressive activity of CD4 + and CD8 + Treg was largely confined to EGFP + cells. RNAseq analyses showed similarities but also differences among CD4 + and CD8 + EGFP + cells and provided the first description of the natural FOXP3 + CD8 + Treg transcriptome. In vitro culture of CD4 + and CD8 + EGFP - cells with TGFbeta and IL-2 resulted in the induction of EGFP + Treg. Preferential expansion of CD4 + and CD8 + EGFP + Treg could be detected upon in vivo administration of a low dose of IL-2. Conclusions This new and unique Foxp3-EGFP rat line constitutes a useful model to identify and isolate viable natural and induced CD4 + and CD8 + Treg. Additionally, it allows to identify new molecules expressed in CD8 + Treg that may allow to better define their phenotype and function not only in rats but also in other species.

CD4+and CD8+regulatory T cells characterization in the rat using a unique transgenicFoxp3-EGFPmodel