Polymicrobial sepsis alters the adaptive immune response and induces T cell suppression and Th2 immune polarization. We identify a GR-1+CD11b+ population whose numbers dramatically increase and remain elevated in the spleen, lymph nodes, and bone marrow during polymicrobial sepsis. Phenotypically, these cells are heterogeneous, immature, predominantly myeloid progenitors that express interleukin 10 and several other cytokines and chemokines. Splenic GR-1+ cells effectively suppress antigen-specific CD8+ T cell interferon (IFN) γ production but only modestly suppress antigen-specific and nonspecific CD4+ T cell proliferation. GR-1+ cell depletion in vivo prevents both the sepsis-induced augmentation of Th2 cell–dependent and depression of Th1 cell–dependent antibody production. Signaling through MyD88, but not Toll-like receptor 4, TIR domain–containing adaptor-inducing IFN-β, or the IFN-α/β receptor, is required for complete GR-1+CD11b+ expansion. GR-1+CD11b+ cells contribute to sepsis-induced T cell suppression and preferential Th2 polarization.

SummaryThe single polar flagellum of Pseudomonas aeruginosa is an important virulence and colonization factor of this opportunistic pathogen. In this study, the annotation of the genes belonging to the fla regulon was updated and their organization was analysed in strains PAK and PAO1, representative type-a and type-b strains of P. aeruginosa respectively. The flagellar genes are clustered in three non-contiguous regions of the chromosome.

More judicious use of third-generation cephalosporins may decrease the incidence of nosocomial multiresistant Enterobacter spp., which in turn may result in a lower mortality for Enterobacter bacteremia. When Enterobacter organisms are isolated from blood, it may be prudent to avoid third-generation cephalosporin therapy regardless of in-vitro susceptibility.

Pseudomonas aeruginosa is an opportunistic Gram-negative human pathogen that is responsible for a broad range of infections in individuals with a variety of predisposing conditions. After infection, P. aeruginosa induces a marked inflammatory response in the host. However the mechanisms involved in bacterium recognition and induction of immune responses are poorly understood. Here we report that the Nod-like receptor family member Ipaf is required for optimal bacterial clearance in an in vivo model of P. aeruginosa lung infection. Further analysis showed that bacterial flagellin was essential for caspase-1 and IL-1b and this activity depended on Ipaf and the adaptor ASC but not TLR5. Notably, P. aeruginosa induced macrophage cell death and this event relied on flagellin and Ipaf but not on ASC. Analysis of Pseudomonas mutants revealed that different amino acid residues of flagellin were critical for sensing by Ipaf and TLR5. Finally, activation of caspase-1 and IL-1b secretion by P. aeruginosa required a functional type III secretion system, but not the effector molecules ExoS, ExoT and ExoY. These results provide new insight into the interaction of P. aeruginosa with host macrophages and suggest that distinct regions of flagellin are sensed by Ipaf and TLR5. Introduction IL-1b plays an important role in the induction of immune responses and in the development of inflammatory disease, fever and septic shock [1]. In response to proinflammatory stimuli including pathogenic bacteria, the IL-1b precursor is induced in monocytes and macrophages and processed into the biologically active IL-1b molecule by caspase-1 [2][3][4][5]. The protease caspase-1 is expressed in monocytes/macrophages as an inactive zymogen that is activated by self cleavage in large multi-protein complexes named 'inflammasomes' [6].The mechanism responsible for activation of caspase-1 in response to microbial stimuli has remained poorly understood. Recent studies have revealed members of the Nod-like receptor (NLR) family as critical components of the inflammasomes by linking microbial sensing to caspase-1 activation [7,8]. For example, Ipaf, an NLR family member and the adaptor ASC have been implicated in activation of caspase-1 in response to Salmonella and Legionella through the cytosolic sensing of flagellin [7,9,10]. Notably, flagellin is also recognized by TLR5 [11]. However, it is unclear whether Ipaf and TLR5 sense identical or distinct regions of flagellin. Similarly, Cryopyrin/Nalp3 is critical for caspase-1 activation and secretion of IL-1b and IL-18 in response to microbial RNA, synthetic purine-like compounds and endogenous urate crystals [12][13][14]. In addition, Cryopyrin regulates caspase-1 activation triggered by exogenous ATP or pore-forming toxins in macrophages stimulated with several TLR agonists [15,16]. Pseudomonas aeruginosa is a flagellated opportunistic Gram-negative human pathogen that is responsible for a broad range of infections in individuals with a variety of predisposing conditions including cystic fibrosis, immu...