Epigenetic modulations rendering cell-to-cell variability and phenotypic metastability

Tumor cells display phenotypic plasticity and heterogeneity due to genetic and epigenetic variations which limit the predictability of therapeutic interventions. Chromatin modifications can arise stochas- tically but can also be a consequence of environmental influences such as the microenvironment of cancer cells. A better understanding of the impact and dynamics of epigenetic modulation at defined chromosomal sites is required to get access to the underlying mechanisms. We investigated the epige- netic modulations leading to cell-to-cell heterogeneity in a tumor cell line model. To this end, we analyzed expression variance in 80 genetically uniform cell populations having a single-copy reporter randomly integrated in the genorne. Single-cell analysis showed high intraclonal heterogeneity. Epige- netic characterization revealed that expression heterogeneity was accompanied by differential histone marks whereas contribution of DNA methylation could be excluded. Strikingly, some clones revealed a highly dynamic, stochastically altered chromatin state of the transgene cassette which was accompanied with a metastable expression pattern. In contrast, other clones represented a robust chromatin state of the transgene cassette with a stable expression pattern. Together, these results elucidate locus-specific epigenetic modulation in gene expression that contributes to phenotypic heterogeneity of cells and might account for cellular plasticity.

Effective intrahepatic CD8＋ T-cell immune responses are induced by low but not high numbers of antigen-expressing hepatocytes

Liver infections with hepatotropic viruses, such as hepatitis B virus and hepatitis C virus are accompanied by viral persistence and immune failure. CD8＋ T cells are crucial mediators of the intrahepatic antiviral immune response. Chronic infections of the liver and other organs correlate with T-cell exhaustion. It was previously suggested that high antigen load could result in T-cell exhaustion. We aimed at elucidating the impact of different intrahepatic antigen loads on the quality of CD8＋ T-cell-mediated immunity by employing an infection-free transgenic mouse model expressing ovalbumin （Ova） as the target antigen. Adoptive transfer of OT-I cells induced a transient intrahepatic immune response toward both high and low Ova levels. However, antigen clearance was achieved only in mice expressing low antigen levels. In contrast, T cells exposed to high antigen levels underwent exhaustion and became depleted, causing antigen persistence. Moreover, when functional T cells were exposed to high intrahepatic antigen levels, a complete transition toward exhaustion was observed. Thus, this study shows that the antigen expression level in the liver correlates inversely with T-cell immunity in vivo and governs the efficiency of immune responses upon antigen presentation.