Dysregulation of stress erythropoiesis and enhanced susceptibility to Salmonella Typhimurium infection in AhR-deficient mice.

BACKGROUND: By acting as an environmental sensor, the ligand-induced transcription factor aryl hydrocarbon receptor (AhR) regulates acute innate and adaptive immune responses against pathogens. Here, we analyzed the function of AhR in a model for chronic systemic infection with attenuated Salmonella Typhimurium (STM). METHODS: WT and AhR-deficient mice were infected with the attenuated STM strain TAS2010 and analyzed for bacterial burden, host defense functions and inflammatory stress erythropoiesis. RESULTS: AhR-deficient mice were highly susceptible to TAS2010 infection compared with WT mice demonstrated by reduced bacterial clearance and increased mortality. STM infection resulted in macrocytic anemia and enhanced splenomegaly along with destruction of the splenic architecture in AhR-deficient mice. In addition, AhR-deficient mice displayed a major expansion of splenic immature red blood cells, indicative of infection-induced stress erythropoiesis. Elevated serum levels of erythropoietin and interleukin-6 upon infection as well as increased numbers of splenic stress erythroid progenitors already in steady state probably drive this effect and might cause the alterations in splenic immune cell compartments, thereby preventing an effective host defense against STM in AhR-deficient mice. CONCLUSIONS: AhR-deficient mice fail to clear chronic TAS2010 infection due to enhanced stress erythropoiesis in the spleen and accompanying destruction of the splenic architecture.

CCL17-expressing dendritic cells in the intestine are preferentially infected by Salmonella but CCL17 plays a redundant role in systemic dissemination.

INTRODUCTION: Salmonella spp. are a recognized and global cause of serious health issues from gastroenteritis to invasive disease. The mouse model of human typhoid fever, which uses Salmonella enterica serovar Typhimurium (STM) in susceptible mouse strains, has revealed that the bacteria gain access to extraintestinal tissues from the gastrointestinal tract to cause severe systemic disease. Previous analysis of the immune responses against Salmonella spp. revealed the crucial role played by dendritic cells (DCs) in carrying STM from the intestinal mucosa to the mesenteric lymph nodes (mLNs), a key site for antigen presentation and T cell activation. In this study, we investigated the influence of chemokine CCL17 on the dissemination of STM. METHODS: WT, CCL17/EGFP reporter, or CCL17-deficient mice were infected orally with STM (SL1344) or mCherry-expressing STM for 1-3 days. Colocalization of STM with CCL17-expressing DCs in Peyer's patches (PP) and mLN was analyzed by fluorescence microscopy. In addition, DCs and myeloid cell populations from naïve and Salmonella-infected mice were analyzed by flow cytometry. Bacterial load was determined in PP, mLN, spleen, and liver 1 and 3 days after infection. RESULTS: Histological analysis revealed that CCL17-expressing cells are located in close proximity to STM in the dome area of PP. We show that, in mLN, STM were preferentially located within CCL17+ rather than CCL17- DCs, besides other mononuclear phagocytes, and identified the CD103+ CD11b- DC subset as the main STM-carrying DC population in the intestine. STM infection triggered upregulation of CCL17 expression in specific intestinal DC subsets in a tissue-specific manner. The dissemination of STM from the gut to the mLN, however, was only moderately influenced by the presence of CCL17. CONCLUSION: CCL17-expressing DCs were preferentially infected by Salmonella in the intestine in comparison to other DC. Nevertheless, the production of CCL17 was not essential for the early dissemination of Salmonella from the gut to systemic organs.

Accumulation of novel glycolipids and ornithine lipids in Mesorhizobium loti under phosphate deprivation.

Glycolipids are found mainly in photosynthetic organisms (plants, algae, and cyanobacteria), Gram-positive bacteria, and a few other bacterial phyla. They serve as membrane lipids and play a role under phosphate deprivation as surrogates for phospholipids. Mesorhizobium loti accumulates different di- and triglycosyl diacylglycerols, synthesized by the processive glycosyltransferase Pgt-Ml, and two so far unknown glycolipids, which were identified in this study by mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy as O-methyl-digalactosyl diacylglycerol (Me-DGD) and glucuronosyl diacylglycerol (GlcAD). Me-DGD is a novel glycolipid, whose synthesis depends on Pgt-Ml activity and the involvement of an unknown methyltransferase, while GlcAD is formed by a novel glycosyltransferase encoded by the open reading frame (ORF) mlr2668, using UDP-glucuronic acid as a sugar donor. Deletion mutants lacking GlcAD are not impaired in growth. Our data suggest that the different glycolipids in Mesorhizobium can mutually replace each other. This may be an adaptation mechanism to enhance the competitiveness in natural environments. A further nonphospholipid in Mesorhizobium was identified as a hydroxylated form of an ornithine lipid with the additional hydroxy group linked to the amide-bound fatty acid, introduced by the hydroxylase OlsD. The presence of this lipid has not been reported for rhizobia yet. The hydroxy group is placed on the C-2 position of the acyl chain as determined by NMR spectroscopy. Furthermore, the isolated ornithine lipids contained up to 80 to 90% d-configured ornithine, a stereoform so far undescribed in bacteria.

A bifunctional glycosyltransferase from Agrobacterium tumefaciens synthesizes monoglucosyl and glucuronosyl diacylglycerol under phosphate deprivation.

Glycolipids are mainly found in phototrophic organisms (like plants and cyanobacteria), in Gram-positive bacteria, and a few other bacterial phyla. Besides the function as bulk membrane lipids, they often play a role under phosphate deprivation as surrogates for phospholipids. The Gram-negative Agrobacterium tumefaciens accumulates four different glycolipids under phosphate deficiency, including digalactosyl diacylglycerol and glucosylgalactosyl diacylglycerol synthesized by a processive glycosyltransferase. The other two glycolipids have now been identified by mass spectrometry and nuclear magnetic resonance spectroscopy as monoglucosyl diacylglycerol and glucuronosyl diacylglycerol. These two lipids are synthesized by a single promiscuous glycosyltransferase encoded by the ORF atu2297, with UDP-glucose or UDP-glucuronic acid as sugar donors. The transfer of sugars differing in their chemistry is a novel feature not observed before for lipid glycosyltransferases. Furthermore, this enzyme is the first glucuronosyl diacylglycerol synthase isolated. Deletion mutants of Agrobacterium lacking monoglucosyl diacylglycerol and glucuronosyl diacylglycerol or all glycolipids are not impaired in growth or virulence during infection of tobacco leaf discs. Our data suggest that the four glycolipids and the nonphospholipid diacylglyceryl trimethylhomoserine can mutually replace each other during phosphate deprivation. This redundancy of different nonphospholipids may represent an adaptation mechanism to enhance the competitiveness in nature.