Rationale: New vaccine approaches are needed for Pseudomonas aeruginosa, which continues to be a major cause of serious pulmonary infections. Although Th17 cells can protect against gram-negative pathogens at mucosal surfaces, including the lung, the bacterial proteins recognized by Th17 cells are largely unknown and could be potential new vaccine candidates. Objectives: We describe a strategy to identify Th17-stimulating protein antigens of Pseudomonas aeruginosa to assess their efficacy as vaccines against pneumonia.Methods: Using a library of in vitro transcribed and translated P. aeruginosa proteins, we screened for Th17-stimulating antigens by coculturing the library proteins with splenocytes from mice immunized with a live-attenuated P. aeruginosa vaccine that is protective via Th17-based immunity. We measured antibody and Th17 responses after intranasal immunization of mice with the purified proteins mixed with the Th17 adjuvant curdlan, and we tested the protective efficacy of vaccination in a murine model of acute pneumonia. Measurements and Main Results: The proteins PopB, FpvA, FptA, OprL, and PilQ elicited strong IL-17 secretion in the screen, and purified versions of PopB, FpvA, and OprL stimulated high IL-17 production from immune splenocytes. Immunization with PopB, which is a highly conserved component of the type III secretion system and a known virulence factor, elicited Th17 responses and also enhanced clearance of P. aeruginosa from the lung and spleen after challenge. PopB-immunized mice were protected from lethal pneumonia in an antibody-independent, IL-17-dependent manner. Conclusions: Screening for Th17-stimulating protein antigens identified PopB as a novel and promising vaccine candidate for P. aeruginosa.Keywords: vaccine; pneumonia; Th17; IL-17; Pseudomonas aeruginosa Pseudomonas aeruginosa is a major cause of serious, often antibiotic-resistant, lung infections in humans, particularly in patients receiving mechanical ventilation and those with cystic fibrosis (1, 2). Most P. aeruginosa vaccines developed to date, including those based on the LPS O antigen (3), the outer membrane proteins F and I (4, 5), or the type III secretion system component PcrV (6), have relied on conventional protective mechanisms-namely, antibody-mediated opsonophagocytic killing and/or antibody-mediated toxin inhibition. Although LPS O antigen-based vaccines can mediate high levels of immunity to P. aeruginosa, the protection is limited to strains having the same LPS serogroup (3). The failure of the Federal Hyperimmune Immunoglobulin Trial (7), which found no benefit in critically ill adults passively immunized with P. aeruginosa LPS O antigenspecific IgG, perhaps best illustrates that antibody-mediated protective mechanisms are not sufficient.Th17 cells have recently been shown to mediate antibodyindependent host defense against Klebsiella pneumoniae (8), although the bacterial proteins recognized by the Th17 cells in those studies were not fully characterized. In our own evaluations of live-attenuated P. ...

The scaffold protein CARD9 plays an essential role in antifungus immunity and is implicated in mediating Dectin-1/Sykinduced NF-B activation in response to Candida albicans infection. However, the molecular mechanism by which CARD9 mediates C. albicans-induced NF-B activation is not fully characterized. Here we demonstrate that CARD9 is involved in mediating NF-B activation induced by the hyphal form of C. albicans hyphae (Hyphae) but not by its heat-inactivated unicellular form. Our data show that inhibiting Dectin-2 expression selectively blocked Hyphae-induced NF-B, whereas inhibiting Dectin-1 mainly suppressed zymosan-induced NF-B, indicating that Hyphae-induced NF-B activation is mainly through Dectin-2 and not Dectin-1. Consistently, we find that the hyphae stimulation induces CARD9 association with Bcl10, an adaptor protein that functions downstream of CARD9 and is also involved in C. albicans-induced NF-B activation. This association is dependent on Dectin-2 but not Dectin-1 following the hyphae stimulation. Finally, we find that although both CARD9 and Syk are required for Hyphae-induced NF-B activation, they regulate different signaling events in which CARD9 mediates IB␣ kinase ubiquitination, whereas Syk regulates IB␣ kinase phosphorylation. Together, our data demonstrated that CARD9 is selectively involved in Dectin-2-induced NF-B activation in response to C. albicans hyphae challenging.Candida albicans is a major opportunistic fungal pathogen that predominantly causes infection to cancer patients and immunocompromised individuals. During C. albicans infection, macrophages and dendritic cells recognize components from the fungal cell wall through their pattern recognition receptors (1, 2), which triggers a series of signaling cascades leading to activation of various transcription factors including NF-B (1). The activation of NF-B and other transcription factors further induce the expression of various cytokines and chemokines and inflammatory responses. However, the pattern recognition receptors that recognize fungal cell wall components are not fully defined (3).NF-B is a family of transcription factors that control the expression of pro-inflammatory genes in immune cells (4). In resting cells, the activity of NF-B is tightly controlled by the IB family of proteins, which bind to NF-B dimers and keep these dimers in the cytoplasm. The canonical NF-B activation pathway by most of NF-B-inducing stimuli activates the IB␣ kinase (IKK) 2 complex. The IKK complex is controlled by signal-induced phosphorylation of IKK␣ and IKK␤ subunits (5) and signal-induced K63-linked ubiquitination of the regulatory subunit NEMO (6). The activated IKK complex in turn phosphorylates IB␣ proteins on N-terminal conserved serine residues to target them for ubiquitination-dependent degradation (5). This process releases NF-B and allows its translocation into the nucleus for the activation of its target genes (4). Although it has been shown that bacterial and viral infections induce IKK activation by Toll-like receptors (TLR...

The type III secretion system (T3SS) of Pseudomonas aeruginosa is an important virulence factor. The T3SS of P. aeruginosa can be induced by a low calcium signal or upon direct contact with the host cells. The exact pathway of signal sensing and T3SS activation is not clear. By screening a transposon insertion mutant library of the PAK strain, mutation in the mucA gene was found to cause repression of T3SS expression under both type III-inducing and -noninducing conditions. Mutation in the mucA gene is known to cause alginate overproduction, resulting in a mucoid phenotype. Alginate production responds to various environmental stresses and plays a protective role for P. aeruginosa. Comparison of global gene expression of mucA mutant and wild-type PAK under T3SS-inducing conditions confirmed the down regulation of T3SS genes and up regulation of genes involved in alginate biosynthesis. Further analysis indicated that the repression of T3SS in the mucA mutant was AlgU and AlgR dependent, as double mutants mucA/algU and mucA/algR showed normal type III expression. An algR::Gm mutant showed a higher level of type III expression, while overexpression of the algR gene inhibited type III gene expression; thus, it seems that the AlgR-regulated product inhibits the expression of the T3SS genes. It is likely that P. aeruginosa has evolved tight regulatory networks to turn off the energy-expensive T3SS when striving for survival under environmental stresses.

During initial colonization and chronic infection, pathogenic bacteria encounter distinct host environments. Adjusting gene expression accordingly is essential for the pathogenesis. Pseudomonas aeruginosa has evolved complicated regulatory networks to regulate different sets of virulence factors to facilitate colonization and persistence. The type III secretion system (T3SS) and motility are associated with acute infections, while biofilm formation and the type VI secretion system (T6SS) are associated with chronic persistence. To identify novel regulatory genes required for pathogenesis, we screened a P. aeruginosa transposon (Tn) insertion library and found suhB to be an essential gene for the T3SS gene expression. The expression of suhB was upregulated in a mouse acute lung infection model, and loss of suhB resulted in avirulence. Suppression of T3SS gene expression in the suhB mutant is linked to a defective translation of the T3SS master regulator, ExsA. Further studies demonstrated that suhB mutation led to the upregulation of GacA and its downstream small RNAs, RsmY and RsmZ, triggering T6SS expression and biofilm formation while inhibiting the T3SS. Our results demonstrate that an in vivo-inducible gene, suhB, reciprocally regulates genes associated with acute and chronic infections and plays an essential role in the pathogenesis of P. aeruginosa.