Genetic association and expression studies indicate a role of toll-like receptor 8 in pulmonary tuberculosis.

Despite high rates of exposure, only 5-10% of people infected with Mycobacterium tuberculosis will develop active tuberculosis (TB) disease, suggesting a significant role for genetic variation in the human immune response to this infection. Here, we studied TB association and expression of 18 genes involved in the Toll-like receptor (TLR) pathways. Initially, we genotyped 149 sequence polymorphisms in 375 pulmonary TB patients and 387 controls from Indonesia. We found that four polymorphisms in the TLR8 gene on chromosome X showed evidence of association with TB susceptibility in males, including a non-synonymous polymorphism rs3764880 (Met1Val; P = 0.007, odds ratio (OR) = 1.8, 95% c.i. = 1.2-2.7). We genotyped these four TLR8 polymorphisms in an independent collection of 1,837 pulmonary TB patients and 1,779 controls from Russia and again found evidence of association in males (for rs3764880 P = 0.03, OR = 1.2, 95% c.i. = 1.02-1.48). Combined evidence for association is P = 1.2x10(-3)-6x10(-4). In addition, a quantitative PCR analysis indicated that TLR8 transcript levels are significantly up-regulated in patients during the acute phase of disease (P = 9.36x10(-5)), relative to baseline levels following successful chemotherapy. A marked increase in TLR8 protein expression was also observed directly in differentiated macrophages upon infection with M. bovis bacille Calmette-Guérin (BCG). Taken together, our results provide evidence, for the first time, of a role for the TLR8 gene in susceptibility to pulmonary TB across different populations.

Serum amyloid A is an innate immune opsonin for Gram-negative bacteria.

Serum amyloid A (SAA) is the major acute-phase protein in man and most mammals. Recently we demonstrated that SAA binds to many Gram-negative bacteria including Escherichia coli and Pseudomonas aeruginosa through outer membrane protein A (OmpA) family members. Therefore we investigated whether SAA altered the response of innate phagocytic cells to bacteria. Both the percentage of neutrophils containing E coli and the number of bacteria per neutrophil were greatly increased by SAA opsonization, equivalent to the increase seen for serum opsonization. In contrast, no change was seen for Streptococcus pneumoniae, a bacteria that did not bind SAA. Neutrophil reactive oxygen intermediate production in response to bacteria was also increased by opsonization with SAA. SAA opsonization also increased phagocytosis of E coli by peripheral blood mononuclear cell-derived macrophages. These macrophages showed strong enhancement of TNF-alpha and IL-10 production in response to SAA-opsonized E coli and P aeruginosa. SAA did not enhance responses in the presence of bacteria to which it did not bind. These effects of SAA occur at normal concentrations consistent with SAA binding properties and a role in innate recognition. SAA therefore represents a novel innate recognition protein for Gram-negative bacteria.

Serum amyloid A protein binds to outer membrane protein A of gram-negative bacteria.

Serum amyloid A (SAA) is the major acute phase protein in man and most mammals. We observed SAA binding to a surprisingly large number of Gram-negative bacteria, including Escherichia coli, Salmonella typhimurium, Shigella flexneri, Klebsiella pneumoniae, Vibrio cholerae, and Pseudomonas aeruginosa. The binding was found to be high affinity and rapid. Importantly, this binding was not inhibited by high density lipoprotein with which SAA is normally complexed in serum. Binding was also observed when bacteria were offered serum containing SAA. Ligand blots following SDS-PAGE or two-dimensional gels revealed two major ligands of 29 and 35 kDa that bound SAA when probing with radiolabeled SAA or SAA and monoclonal anti-SAA. Following fractionation the ligand was found in the outer membrane fraction of E. coli and was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry to be outer membrane protein A (OmpA). OmpA-deficient E. coli did not bind SAA, and following purification of OmpA the protein retained binding activity. The ligands on other bacteria were likely to be homologues of OmpA because wild type, but not OprF-deficient, P. aeruginosa bound SAA.