Pseudomonas aeruginosa is an opportunistic bacterial pathogen that primarily infects immunocompromised individuals and patients with cystic fibrosis. Using a tissue culture system, invasive strains of P. aeruginosa were discovered to induce apoptosis at high frequency in HeLa and other epithelial and fibroblast cell lines. This apoptotic phenotype in the infected cells was determined by several criteria including (i) visual changes in cell morphology, (ii) induction of chromatin condensation and nuclear marginalization, (iii) the presence of a high percentage of cells with subG1 DNA content, and (iv) activation of caspase-3 activity. Induction of the type III secretion machinery, but not invasion of P. aeruginosa is required for induction of apoptosis. The apoptosis phenotype is independent of the cytoskeletal rearrangements that occur in the host cell early after infection. Mutants in P. aeruginosa exoS fail to induce apoptosis and complementation with wild-type exoS restored the apoptosis-inducing capacity, demonstrating that ExoS is the effector molecule. Analysis of exoS activity mutants shows that the ADP-ribosylating capacity of ExoS is essential for inducing the apoptotic pathway.

DsbA is a periplasmic thiol:disulfide oxidoreductase which contributes to the process of protein folding by catalyzing the formation of disulfide bonds. In this study, we demonstrate that the dsbA gene is required for the expression of the type III secretion system under low-calcium inducing conditions, intracellular survival of P. aeruginosa upon infection of HeLa cells, and twitching motility. The diverse phenotypes of the dsbA mutant are likely due to its defect in the folding of proteins that are involved in various biological processes, such as signal sensing, protein secretion, and defense against host clearing. In light of its effect on various virulence factors, DsbA could be an important target for the control of P. aeruginosa infections.

Transforming growth factor‐β (TGF‐β) family members are multifunctional growth factors involved in regulating diverse biological processes. Despite the critical role for TGF‐β in regulating cell proliferation, differentiation, migration and development, its role in regulating NF‐κB‐dependent inflammatory response still remains unclear. Here, we show that TGF‐β1 induces acetylation of NF‐κB p65 subunit to synergistically enhance bacterium nontypeable Haemophilus influenzae‐induced NF‐κB activation and inflammatory response in vitro and in vivo. The TGF‐β1‐induced acetylation of p65 is mediated via a Smad3/4‐PKA‐p300‐dependent signaling pathway. Acetylation of p65 at lysine 221 by TGF‐β1 is critical for synergistic enhancement of bacteria‐induced DNA‐binding activity, NF‐κB activation, NF‐κB‐dependent transcription of TNF‐α and IL‐1β and interstitial polymorphonuclear neutrophil infiltration in vitro and in vivo. These studies provide new insights into the novel regulation of NF‐κB by TGF‐β signaling.

Non-typeable Haemophilus influenza (NTHi) is an important human pathogen causing respiratory tract infections in both adults and children. NTHi infections are characterized by inflammation, which is mainly mediated by nuclear transcription factor kappaB (NF-κB)-dependent production of inflammatory mediators. The deubiquitinating enzyme cylindromatosis (CYLD), loss of which was originally reported to cause a benign human syndrome called cylindromatosis, has been identified as a key negative regulator for NF-κB in vitro. However, little is known about the role of CYLD in bacteria-induced inflammation in vivo. Here, we provided direct evidence for the negative role of CYLD in NTHi-induced inflammation of the mice in vivo. Our data demonstrated that CYLD is induced by NTHi in the middle ear and lung of mice. NTHi-induced CYLD, in turn, negatively regulates NTHi-induced NF-κB activation through deubiquitinating TRAF6 and 7 and down-regulates inflammation. Our data thus indicate that CYLD acts as a negative regulator for NF-κB-dependent inflammation in vivo, hence protecting the host against detrimental inflammatory response to NTHi infection.