Preferential activation of an IL-2 regulatory sequence transgene in TCRγδ and NKT cells:: Subset-specific differences in IL-2 regulation

A transgene with 8.4-kb of regulatory sequence from the murine IL-2 gene drives consistent expression of a green fluorescent protein (GFP) reporter gene in all cell types that normally express IL-2. However, quantitative analysis of this expression shows that different T cell subsets within the same mouse show divergent abilities to express the transgene as compared with endogenous IL-2 genes. TCR-gammadelta cells, as well as alphabetaTCR-NKT cells, exhibit higher in vivo transgene expression levels than TCRalphabeta cells. This deviates from patterns of normal IL-2 expression and from expression of an IL-2-GFP knock-in. Peripheral TCRgammadelta cells accumulate GFP RNA faster than endogenous IL-2 RNA upon stimulation, whereas TCRalphabeta cells express more IL-2 than GFP RNA. In TCRgammadelta cells, IL-2-producing cells are a subset of the GFP-expressing cells, whereas in TCRalphabeta cells, endogenous IL-2 is more likely to be expressed without GFP. These results are seen in multiple independent transgenic lines and thus reflect functional properties of the transgene sequences, rather than copy number or integration site effects. The high ratio of GFP: endogenous IL-2 gene expression in transgenic TCRgammadelta cells may be explained by subset-specific IL-2 gene regulatory elements mapping outside of the 8.4-kb transgene regulatory sequence, as well as accelerated kinetics of endogenous IL-2 RNA degradation in TCRgammadelta cells. The high levels and percentages of transgene expression in thymic and splenic TCRgammadelta and NKT cells, as well as skin TCRgammadelta-dendritic epidermal T cells, indicate that the IL-2-GFP-transgenic mice may provide valuable tracers for detecting developmental and activation events in these lineages.