The present invention relates to a method for activating a cell and a cell activated thereby and a use thereof, more particularly, to an in vitro method of enhancing, recovering of immune response of CD8 T cells in exhaustion and proliferating the CD8 T cells comprising the step of inducing overexpression of Klf4 protein in CD8 T cells, a cell population containing the CD8 T cells or transduced CAR-CD8 T cells whose anticancer activity is enhanced by overexpressing Klf4 protein and use thereof.