Abstract

Background: Immune suppression in the tumour microenvironment remains a major limitation to successful immunotherapy of cancer. In the current study, we analysed whether the natural killer T cell-activating glycolipid alpha-galactosylceramide could overcome immune suppression and improve vaccination against metastatic breast cancer. Methods: Mice with metastatic breast cancer (4T1 model) were therapeutically treated with a Listeria monocytogenes-based vaccine expressing tumour-associated antigen Mage-b followed by alpha-galactosylceramide as separate agents, or as a complex of alpha-galactosylceramide stably incorporated into Listeria-Mage-b. Effects on metastases, tumour weight, toxicity and immune responses were determined. Results: Sequential treatments of mice with established 4T1 breast carcinomas using Listeria-Mage-b followed by alpha-galactosylceramide as a separate agent was highly effective at reducing metastases, but was accompanied by severe liver toxicity. In contrast, combined therapy using Listeria-Mage-b modified by incorporation of alpha-galactosylceramide resulted in nearly complete elimination of metastases without toxicity. This was associated with a significant increase in the percentage of natural killer T cells in the spleen, and an increase in natural killer cell activity and in T cell responses to Mage-b. Conclusions: Our results suggest that direct incorporation of alpha-galactosylceramide into a live bacterial vaccine vector is a promising non-toxic new approach for the treatment of metastatic breast cancer.