Effect of Asymptomatic Natural Infections due to Common Mouse Pathogens on the Metastatic Progression of B16 Murine Melanoma in C57BL/6 Mice

To investigate whether the presence of infections in C57BL/6 mice influences the metastatic ability of B16 melanoma (B16M) cells, we compared the susceptibility to metastasis development of pathogen-free mice with that of mice from a colony endemically infected with several mouse pathogens. We found that, compared to seronegative controls, mice that were seropositive at least to Mouse Hepatitis Virus (MHV) and Mycoplasma pulmonis: (i) exhibited a higher interindividual variability in all the parameters quantifying metastatic progression; (ii) had elevated serum levels of proinflammatory cytokines both before and at the end of the experiment; (iii) were more susceptible to hepatic metastasis. Interestingly, final levels of tumor necrosis factor (TNF)-α and interleukin (IL)-18 correlated with the extent of hepatic colonization by the melanoma cells. To confirm the metastasis-enhancing effect of MHV and M. pulmonis we measured the ability of B16M cells to metastasize in pathogen-free animals housed for increasing time-intervals in the vicinity of MHV(+) animals. Notably, susceptibility to metastasis was lower in animals seronegative to MHV than in MHV(+) mice, whereas the latter were less susceptible to metastasis than MHV(+) M. pulmonis(+) mice. Seropositive animals had increased levels of TNF-α and IL-18 suggesting that MHV and M. pulmonis enhance the metastatic ability of melanoma cells by inducing the release of proinflammatory cytokines. While our results highlight the importance of using pathogen-free animals in metastasis studies, they emphasize the need for a comprehensive health monitoring of the mice used in such studies, particularly in case of using facilities lacking appropriate containment measures.