Lipoproteins: pieces of the puzzle of Brucella-elicited inflammation

GUILLERMO H. GIAMBARTOLOMEI

Congreso; 6th International Research Conference on Brucellosis.; 2011

Despite the diversity of clinical signs and symptoms, inflammation is a hallmark of human brucellosis. Inflammation is present both in the acute and chronic phases of the disease and virtually in all of the organs affected. Studies conducted to date have revealed that B. abortus can induce in a variety of cell types the release of pro-inflammatory cytokines such as IL-1b, IL-6, IL-12 and TNF-a. These pro-inflammatory cytokines generally have potent effector functions that overlap extensively with each other to bring about the many components of inflammation, e.g., tissue necrosis, formation of cellular infiltrates, induction of collagenase and matrix metalloproteinases (MMP) by sinovial fibroblasts and chondrocytes, bone resorption, cartilage destruction, etc. Thus, cytokine-stimulatory properties possessed by B. abortus may explain the correlation between tissue invasion and localized inflammation. The most likely molecular basis of the inflammatory effectiveness of B. abortus has recently become apparent. Although B. abortus, a Gram-negative bacterium, possesses lipopolysaccharide (LPS), the generic endotoxin of Gram-negative organisms, it may derive its inflammatory capacity from lipoproteins. At variance with the LPS from the Enterobacteriaceae, Brucella LPS has thus far found to be virtually devoid of pro-inflammatory activity. Moreover, we have recently shown that the production of pro-inflammatory cytokines by monocytes/macrophages and dendritic cells is induced by Brucella lipoproteins rather than LPS. In this opportunity we will present data demonstrating the role of Brucella lipoproteins in the inflammatory pathogenesis of neurobrucellosis and osteoarticular brucellosis, the most morbid and the more frequent focalizations of the disease, respectively. Our results indicate that the direct presence of B. abortus or outer membrane protein 19 (Omp19), a model B. abortus lipoprotein, in the brain of normal mice induces an inflammatory response leading to astrogliosis. Accordingly, infection of astrocytes with B. abortus induced a pro-inflammatory response that leads to astrocyte apoptosis and proliferation, two features observed during astrogliosis. TNF-á signaling via TNFR1 through the coupling of caspases determines apoptosis. Also, our results unraveled an immune mechanism for inflammatory bone loss and joint damage in response to infection by Brucella abortus. We established a requirement for MyD88 and TLR2 in TNF-a-elicited osteoclastogenesis in response to B. abortus infection. Also, infection of human synoviocytes results in the production of MMP-2 and proinflammatory mediators. Finally, B. abortus increases the production of inflammatory mediators and adhesion molecules in endothelial cells, leading to transmigration of neutrophils. In all these models, Omp19 recapitulated the cytokine production and the subsequent pathological phenomena induced by the whole bacterium. Our results provide proof of the principle that Brucella lipoproteins could be key factors in disease pathogenesis. We envision a model in which at least three factors contribute to the development of cytokine-mediated inflammation in brucellosis. These factors are bacterial tissue-invasion and persistence, interaction of lipoproteins with effector cells, and regulation of this effect by modulatory cytokines produced in the microenvironment of the invaded tissues.is present both in the acute and chronic phases of the disease and virtually in all of the organs affected. Studies conducted to date have revealed that B. abortus can induce in a variety of cell types the release of pro-inflammatory cytokines such as IL-1b, IL-6, IL-12 and TNF-a. These pro-inflammatory cytokines generally have potent effector functions that overlap extensively with each other to bring about the many components of inflammation, e.g., tissue necrosis, formation of cellular infiltrates, induction of collagenase and matrix metalloproteinases (MMP) by sinovial fibroblasts and chondrocytes, bone resorption, cartilage destruction, etc. Thus, cytokine-stimulatory properties possessed by B. abortus may explain the correlation between tissue invasion and localized inflammation. The most likely molecular basis of the inflammatory effectiveness of B. abortus has recently become apparent. Although B. abortus, a Gram-negative bacterium, possesses lipopolysaccharide (LPS), the generic endotoxin of Gram-negative organisms, it may derive its inflammatory capacity from lipoproteins. At variance with the LPS from the Enterobacteriaceae, Brucella LPS has thus far found to be virtually devoid of pro-inflammatory activity. Moreover, we have recently shown that the production of pro-inflammatory cytokines by monocytes/macrophages and dendritic cells is induced by Brucella lipoproteins rather than LPS. In this opportunity we will present data demonstrating the role of Brucella lipoproteins in the inflammatory pathogenesis of neurobrucellosis and osteoarticular brucellosis, the most morbid and the more frequent focalizations of the disease, respectively. Our results indicate that the direct presence of B. abortus or outer membrane protein 19 (Omp19), a model B. abortus lipoprotein, in the brain of normal mice induces an inflammatory response leading to astrogliosis. Accordingly, infection of astrocytes with B. abortus induced a pro-inflammatory response that leads to astrocyte apoptosis and proliferation, two features observed during astrogliosis. TNF-á signaling via TNFR1 through the coupling of caspases determines apoptosis. Also, our results unraveled an immune mechanism for inflammatory bone loss and joint damage in response to infection by Brucella abortus. We established a requirement for MyD88 and TLR2 in TNF-a-elicited osteoclastogenesis in response to B. abortus infection. Also, infection of human synoviocytes results in the production of MMP-2 and proinflammatory mediators. Finally, B. abortus increases the production of inflammatory mediators and adhesion molecules in endothelial cells, leading to transmigration of neutrophils. In all these models, Omp19 recapitulated the cytokine production and the subsequent pathological phenomena induced by the whole bacterium. Our results provide proof of the principle that Brucella lipoproteins could be key factors in disease pathogenesis. We envision a model in which at least three factors contribute to the development of cytokine-mediated inflammation in brucellosis. These factors are bacterial tissue-invasion and persistence, interaction of lipoproteins with effector cells, and regulation of this effect by modulatory cytokines produced in the microenvironment of the invaded tissues.