Abstract: Provided herein are methods and compositions for modulating T-cell activity by altering DNA methylation status. Altering the methylation status of CD8+ T cells can prevent T-cell exhaustion and maintain effector functions during sustained antigen exposure. The methods and compositions can be used to treat symptoms of chronic infections and cancer. Further, the methods and compositions relate to predicting T-cell activity by measuring the methylation status of specific memory cell methylation markers and using the markers to identify and separate populations of CD8 T cell having desired T cell activity. The memory cell methylation markers can further be used to identify subjects with chronic infections or cancer that would benefit from personalized therapy, including immune checkpoint blockade therapy.