Exploring mechanisms of Salmonella-induced reduction of cancer metastasis

Abstract

Metastasis is responsible for 90% death of cancer patients. Long-term dormancy, genetic heterogeneity, drug resistance and multiple growths all contribute to the lethal feature of metastasis. We have successfully engineered a tumor-hypoxia targeting Salmonella typhimurium strain YB1 for cancer therapy. Administration of YB1 to 4T1 tumor-bearing BALB/c mice partially inhibits the growth of primary tumor, but significantly reduces lung metastasis. Little is known about how bacterial treatment inhibits cancer metastasis. In this study, we aim at understanding the underlying mechanisms by investigating Salmonella-induced immune responses and their effects on metastasis.

The overall process of metastasis includes localized invasion, intravasation, circulation, extravasation and colonization. Only Salmonella treatments taken prior to colonization could reduce lung metastasis. However, later treatment could not eliminate established lung metastasis. Experimental metastasis model further demonstrates that YB1 treatment interferes with the colonization process of cancer cells in lung, thus reducing metastasis.

Both arms of immunity are investigated in this study. T cell frequency was quantified in tumor, metastatic tissue lung, spleen and tumor-draining lymph nodes after YB1 treatment. Surprisingly, dramatic reduction of CD3+ T cells in tumor was observed as early as one-day post YB1 treatment, while the number of CD3+ T cells in lung remains unchanged. In addition, significant decrease of CD4+ and CD8+ T cells was observed in spleen. A decrease of CD4+ T cells in tumor-draining lymph nodes was also detected, indicating that T cells may not be actively involved in reducing lung metastasis by functioning in tumor. Indeed, similar anti-metastasis efficacy can be observed in tumor-bearing immunodeficient NOD SCID mice treated with Salmonella, suggesting that the innate immunity in particular could be responsible for the reduction of lung metastasis.

Treatment with Salmonella induces strong systemic and tumor-localized pro-inflammatory responses, as indicated by highly elevated cytokine levels of IL-1, IL-6 and TNF-α. Administration of immunosuppressive drug prednisolone to mice treated with YB1 partially impaired the anti-metastasis effect of YB1, implicating an important role of immune responses in mediating the reduction of lung metastasis. Much higher levels of IL-6, IL-12, IL-18, TNF-α and IFN-γ were observed in YB1-treated mice than E. coli- or heat-killed YB1-treated mice, which show no anti-metastasis effect. Depletion of IFN-γ but not TNF-α abolishes the anti-metastasis effect of YB1. However, when administrated alone, IFN-γ fails to inhibit metastasis, suggesting that IFN-γ is not sufficient to reduce lung metastasis and other factors may be involved. Our study highlights the importance of innate immunity in mediating the reduction of lung metastasis. Another important implication from our study is that stimulating the immune system would be effective in the prevention of metastasis. A particular cytokine response profile induced by Salmonella treatment, especially IFN-γ response, may be needed to interfere the process of metastasis.