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Background and the purpose of the studySelenium enriched Lactobacillus has been reported as an immunostimulatory agent which can be used to increase the life span of cancer bearing animals. Lactic acid bacteria can reduce selenium ions to elemental selenium nanoparticles (SeNPs) and deposit them in intracellular spaces. In this strategy two known immunostimulators, lactic acid bacteria (LAB) and SeNPs, are concomitantly administered for enhancing of immune responses in cancer bearing mice.MethodsForty five female inbred BALB/c mice were divided into three groups of tests and control, each containing 15 mice. Test mice were orally administered with SeNP-enriched Lactobacillus brevis or Lactobacillus brevis alone for 3 weeks before tumor induction. After that the administration was followed in three cycles of seven days on/three days off. Control group received phosphate buffer saline (PBS) at same condition. During the study the tumor growth was monitored using caliper method. At the end of study the spleen cell culture was carried out for both NK cytotoxicity assay and cytokines measurement. Delayed type hypersensitivity (DTH) responses were also assayed after 72h of tumor antigen recall. Serum lactate dehydrogenase (LDH) and alkaline phosphatase (ALP) levels were measured, the livers of mice were removed and prepared for histopathological analysis.ResultsHigh level of IFN-γ and IL-17 besides the significant raised in NK cytotoxicity and DTH responses were observed in SeNP-enriched L. brevis administered mice and the extended life span and decrease in the tumor metastasis to liver were also recorded in this group compared to the control mice or L.brevis alone administered mice.ConclusionOur results suggested that the better prognosis could be achieved by oral administration of SeNP-enriched L. brevis in highly metastatic breast cancer mice model.