B16 melanoma tumor growth is delayed in mice in an age-dependent manner

A major risk factor for cancer is increasing age, which suggests that syngeneic tumor implants in old mice would grow more rapidly. However, various reports have suggested that old mice are not as permissive to implanted tumor cells as young mice. In order to determine and characterize the age-related response to B16 melanoma, we implanted 5×10(5) tumor cells into 8, 16, 24, and 32-month-old male C57BL/6 (B6) and C57BL/6×BALB/c F1 (CB6 F1) mice subcutaneously in the inguinal and axillary spaces, or intradermally in the lateral flank. Results showed decreased tumor volume with increasing age, which varied according to mouse genetic background and the implanted site. The B6 strain showed robust tumor growth at 8 months of age at the inguinal implantation site, with an average tumor volume of 1341.25 mm(3). The 16, 24, and 32-month age groups showed a decrease in tumor growth with tumor volumes of 563.69, 481.02, and 264.55 mm(3), respectively (p≤0.001). The axillary implantation site was less permissive in 8-month-old B6 mice with an average tumor volume of 761.52 mm(3). The 24- and 32-month age groups showed a similar decrease in tumor growth with tumor volumes of 440 and 178.19 mm(3), respectively (p≤0.01). The CB6F1 strain was not as tumor permissive at 8 months of age as B6 mice with average tumor volumes of 446.96 and 426.91 mm(3) for the inguinal and axillary sites, respectively. There was a decrease in tumor growth at 24 months of age at both inguinal and axillary sites with an average tumor volume of 271.02 and 249.12 mm(3), respectively (p≤0.05). The strain dependence was not apparent in 8-month-old mice injected intradermally with B16 melanoma cells, with average tumor volumes of 736.82 and 842.85 mm(3) for B6 and CB6 F1, respectively. However, a strain difference was seen in 32-month-old B6 mice with an average decrease in tumor volume of 250.83 mm(3) (p≤0.01). In contrast, tumor growth significantly decreased earlier in CB6 F1 mice with average tumor volumes of 417.62 and 216.34 mm(3) in the 16- and 24-month age groups, respectively (p≤0.005). Histologically, implanted tumors in young mice exhibited characteristics of aggressive, rapidly growing tumor cells including high vascularity, mitosis, and invasiveness compared to tumors in old mice. We contend that the decrease in B16 melanoma tumor growth seen with increasing age in B6 and CB6 F1 mice represents a biological process, which we are calling age-dependent cancer resistance (ADCR). Our data provide a detailed description of conditions necessary to use the model to investigate the mechanisms of ADCR and determine its biological and clinical relevance.