Exopolysaccharides From Lactobacillus delbrueckii subsp. bulgaricus OLL1073R‐1 Enhance Macrophage Bactericidal Activity Through IL‐12‐Dependent Macrophage–NK Cell Crosstalk

Exopolysaccharides (EPS) produced by Lactobacillus delbrueckii subsp. bulgaricus OLL1073R-1 have been reported to exert immunomodulatory effects, primarily in antiviral responses; however, their role in antibacterial host defense remains unclear. Here, we investigated whether EPS enhance macrophage-mediated killing of Streptococcus pyogenes, a facultative intracellular pathogen, using an in vitro co-culture system with NK-like cells. EPS alone did not enhance the intracellular bactericidal activity of macrophages. In contrast, EPS significantly increased bacterial killing when macrophages were co-cultured with NK-like cells. Mechanistically, EPS induced interleukin (IL)-12 expression in macrophages, while interferon (IFN)-γ production was not detected when macrophages or NK-like cells were cultured separately, but was significantly induced in the co-culture system. Neutralization of IL-12 or IFN-γ significantly abrogated the EPS-mediated enhancement of bactericidal activity, indicating that both cytokines are required for this effect. These findings support a model in which EPS stimulate macrophages to produce IL-12, leading to NK cell activation and subsequent IFN-γ-dependent enhancement of macrophage bactericidal function. Our results reveal a previously uncharacterized role of probiotic-derived EPS in promoting antibacterial immunity through macrophage-NK cell crosstalk.