Figure 3. CCR7 introduces microenvironmental niche for memory CD8+ T cells. The schematic model indicates the trafficking patterns of CCR7^hi or CCR7^lo CD8+ T_M cells to find individual niches for their survival. CCR7^hi CD8+ T_M cells migrate toward high concentration of CCL19 or CCL21, where IL-7 is also expressed. In the contrary, CCR7^lo T_M cells migrate toward IL-15-rich microenvironment, even though the homing receptors for this migration have not been identified yet. Altogether, CCR7^hi and CCR7^lo CD8+ T_M cells do not share their survival cytokines, and CCR7 biases the survival of CD8+ T_M cells toward IL-7 niche.