| Summary: | The role of dietary probiotic strains on host anti-cancer immune responses against experimental colon carcinoma was investigated. We have previously shown that <i>Lactobacillus casei</i> administration led to tumor growth suppression in an experimental colon cancer model. Here, we investigated the underlying immune mechanisms involved in this tumor-growth inhibitory effect. BALB/c mice received daily live lactobacilli <i>per os</i> prior to the establishment of a syngeneic subcutaneous CT26 tumor. Tumor volume, cytokine production, T cell differentiation and migration, as well as tumor cell apoptosis were examined to outline potential immunomodulatory effects following <i>L. casei</i> oral intake. Probiotic administration in mice resulted in a significant increase in interferon gamma (IFN-γ), Granzyme B and chemokine production in the tumor tissue as well as enhanced CD8<sup>+</sup> T cell infiltration, accompanied by a suppression of tumor growth. Cytotoxic activity against cancer cells was enhanced in probiotic-fed compared to control mice, as evidenced by the elevation of apoptotic markers, such as cleaved caspase 3 and poly (ADP-ribose) polymerase 1 (PARP1), in tumor tissue. Oral administration of <i>Lactobacillus casei</i> induced potent Th1 immune responses and cytotoxic T cell infiltration in the tumor tissue of tumor-bearing mice, resulting in tumor growth inhibition. Thus, the microorganism may hold promise as a novel dietary immunoadjuvant in raising protective anti-cancer immune responses.
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