To investigate whether the inhibition of tumor growth in CCR5−/− mice is related to tumor-specific immune responses, we analyzed the distribution patterns of CD8+ cytotoxic T cell and CD57+ Natural Killer cells in tumor and spleen tissues. By staining the tumor sections from CCR5−/− mice, we found that there was a significant influx of CD8+ cells into the tumor, as well as an increase in the number of infiltrating NK cells (Figure 5A). CD8+ T cells and CD57+ NK cells were spread diffusely throughout the entire sections. To further investigate the differences in the numbers of CD8+ T cells and CD57+ NK cells between CCR5+/+ mice and CCR5−/− mice in immunity-related organ, we also analyzed the CD8 and CD57 reactive cell number in spleen tissues. The number of CD8+ T cells and CD57+ NK cells were higher in the spleen of the CCR5−/− mice than in the spleen of the CCR5+/+ mice (Figure 5B). These data suggest that by promoting the infiltration of CD8 T cells and NK cells, which are potent cytotoxic effectors for tumors, could play a role in an anti-tumor effect in CCR5−/− mice.