Analysis of human primary macrophages after live Lactobacillus rhamnosus GG (LGG) or LC705 stimulation for 6h and 24h. The results reveal novel mechanisms for probiotics-induced activation of the healthy human innate immune system. Macrophages are phagocytic cells of the innate immune system that perform sentinel functions to initiate appropriate responses to surrounding stimuli. Macrophages that reside at mucosa encounter ingested bacteria. Our understanding of the responses elicited by nonpathogenic bacteria in human innate immune system is limited. Lactobacillus are nonpathogenic bacteria commonly used in food and as supplements with health-promoting probiotic potential. In this study, we have utilized global gene expression profiling to compare the responses of human primary macrophages to two closely related, well-characterized L. rhamnosus strains LGG and LC705. Our results demonstrate that live LGG and LC705 induced quantitatively different gene expression in macrophages. A gene ontology analysis revealed functional similarities and differences in responses to LGG and LC705. Both LGG and LC705 induced IL-1b production in macrophages that required caspase-1 activity. LC705 but not LGG induced a strong type I IFN-dependent gene activation that correlated with the ability of LC705 to prevent influenza A virus replication and production of viral proteins in macrophages. Differentiated 7d human primary macrophages from 18 healthy individuals were stimulated with either live L. rhamnosus GG (LGG) or LC705 for 6 h and 24 h in RPMI medium containing penicillin and streptomycin. As a control, macrophages were stimulated with the medium only. The experiment was performed three times, each time with cells from six different individuals (samples 1-3). For each experiment, macrophages were stimulated separately as described, and macrophages from different donors were pooled after each stimulation experiment. Extracted RNA was hybridized to Affymetrix® U133-plus2.0 GeneChip® arrays.