AimsThis study aimed to identify the main active component of Lactobacillus brevis KB290 (KB290) that is responsible for enhanced cell-mediated cytotoxic activity of mouse splenocytes Live KB290, a probiotic strain derived from a Japanese traditional pickle, was previously reported to modulate innate immune responses as affecting on cell-mediated cytotoxic activity of mouse splenocytes. Methods and ResultsWe used live KB290, heat-killed KB290, a derivative strain (Lact.brevis KB392) with different amounts of cell-bound exopolysaccharide (EPS-b), and a crude extract of EPS-b from KB290 cell surface. Female BALB/c mice were fed a diet containing 10(10)CFU live KB290, 10(10)CFU live KB392, 15mg heat-killed KB290 or 600g crude extract of EPS-b for 1day. Live KB290 (P<001), heat-killed KB290 (P<005) and crude EPS-b at 600g (P<005) per mouse significantly enhanced cytotoxic activity; however, live KB392 had no effect. ConclusionsBoth live and heat-killed KB290 and crude EPS-b significantly enhanced cytotoxic activity of mouse splenocytes. Significance and Impact of the StudyWe demonstrated that EPS-b produced by KB290 has a critical role in enhancing cell-mediated cytotoxic activity in mouse spleen.