DESCRIPTION (provided by applicant): Brucella are intracellular facultative bacteria causing brucellosis in animals and humans. Four Brucella species are pathogenic to humans and classified as NIAID category B priority pathogens. However, no safe and effective Brucella vaccine is available for human use and the basic mechanisms by which Brucella successfully replicate intracellularly in host tissues remains unclear. B. melitensis, B. suis, and B. abortus are the most virulent to humans. The presence of the O-side chain in lipopolysaccharide (LPS) distinguishes smooth virulent Brucella strains from rough attenuated or avirulent strains. Brucella O-side chain has been shown to induce protective cell mediated immunity. Smooth Brucella infect and replicate within host macrophages whereas rough strains infect and replicate for a limited time. We propose to use high throughput DNA microarray technology to study differential and coordinated gene expression in the time course of murine macrophage responses to infection with smooth and rough Brucella strains. The role of Brucella O-side chain in stimulation of specific macrophage gene responses will be analyzed. The gene expression profiles detected by DNA microarray experiments will be confirmed by real-time RT-PCR. The proposed research will greatly help in understanding the genetic basis of host defenses against infection by virulent and attenuated Brucella strains and make possible development of novel strategies to treat and prevent infections.