Modulation of Anti-Tumour Immune Responses by Probiotic Bacteria

Abstract

There is a growing amount of evidence to support the beneficial role of a balanced intestinal microbiota, or distinct members thereof, in the manifestation and progression of malignant tumours, not only in the gastrointestinal tract but also in distant tissues as well. Intriguingly, bacterial species have been demonstrated to be indispensable modulatory agents of widely-used immunotherapeutic or chemotherapeutic regiments. However, the exact contribution of commensal bacteria to immunity, as well as to neoplasia formation and response to treatment, has not been fully elucidated, and most of the current knowledge acquired from animal models has yet to be translated to human subjects. Here, recent advances in understanding the interaction of gut microbes with the immune system and the modulation of protective immune responses to cancer, either naturally or in the context of widely-used treatments, are reviewed, along with the implications of these observations for future therapeutic approaches. In this regard, bacterial species capable of facilitating optimal immune responses against cancer have been surveyed. According to the findings summarized here, we suggest that strategies incorporating probiotic bacteria and/or modulation of the intestinal microbiota can be used as immune adjuvants, aiming to optimize the efficacy of cancer immunotherapies and conventional anti-tumour treatments.

Keywords: Lactobacillus; animal models; cancer; cancer immunotherapy; immune responses; microbiota; probiotic bacteria.

Figure 1

Effect of probiotic bacteria on the cancer–immunity cycle. Probiotic bacteria affect and can modulate different stages in the cancer–immunity cycle, such as cancer antigen presentation, the priming and activation of T cells, the trafficking of T cells to tumours and the infiltration of CD8+ cells into tumours, as well as the killing of cancer cells and release of tumour antigens. Several molecules are upregulated (green-blue color) or downregulated [orange color] following the administration of probiotic bacteria. The cancer–immunity cycle is adapted from [4] and was designed with Biorender (www.biorender.com).