CD4(+) and CD8(+) regulatory T cell characterization in the rat using a unique transgenic Foxp3-EGFP model

BACKGROUND: Regulatory T cells (Treg) in diverse species include CD4(+) and CD8(+) T cells. In all species, CD8(+) Treg have been only partially characterized and there is no rat model in which CD4(+) and CD8(+) FOXP3(+) Treg are genetically tagged. RESULTS: We generated a Foxp3-EGFP rat transgenic line in which FOXP3 gene was expressed and controlled EGFP. CD4(+) and CD8(+) T cells were the only cells that expressed EGFP, in similar proportion as observed with anti-FOXP3 antibodies and co-labeled in the same cells. CD4(+)EGFP(+) Treg were 5–10 times more frequent than CD8(+)EGFP(+) Treg. 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 generated induced EGFP(+) Treg. CD4(+) and CD8(+) EGFP(+) Treg were expanded upon in vivo administration of a low dose of IL-2. CONCLUSIONS: This new and unique rat line constitutes a useful model to identify and isolate viable CD4(+) and CD8(+) FOXP3(+) Treg. Additionally, it allows to identify molecules expressed in CD8(+) Treg that may allow to better define their phenotype and function not only in rats but also in other species. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12915-022-01502-0.