Innate immune encounters of the (Type) 4th kind: Brucella

In humans, pathogenic Brucella species cause a febrile illness known as brucellosis. A key pathogenic trait of this group of organisms is their ability to survive in immune cells and persist in tissues of the reticuloendothelial system, a process that requires the function of a Type IV secretion system. In contrast to other well-studied Gram-negative bacteria, Brucella spp. do not cause inflammation at the site of invasion, but have a latency period of 2-4 weeks before the onset of symptoms. This review discusses several mechanisms that allow Brucella spp. both to evade detection by pattern recognition receptors of the innate immune system and suppress their signalling. In contrast to these stealth features, the VirB Type IV secretion system, which mediates survival within phagocytic cells, stimulates innate immune responses in vivo. The responses stimulated by this virulence factor are sufficient to check bacterial growth, but not to elicit sterilizing immunity. The result is a stand-off between host and pathogen that results in persistent infection.