Specific inhibition of interferon signal transduction pathways by adenoviral infection.

Adenoviral evolution has generated strategies to resist host cell antiviral systems, but molecular mechanisms for evasion of interferon (IFN) effects by adenoviruses during late-phase infection are poorly defined. In this study, we examined adenovirus type 5 (AdV) effects on IFN-gamma-dependent gene expression and Janus family kinase-signal transducer and activator of transcription signaling components in human tracheobronchial epithelial cells. We found that AdV infection specifically inhibited IFN-gamma-dependent gene expression in airway epithelial cells without evidence of epithelial cell injury or generation of a soluble extracellular inhibitor. Furthermore, infection with AdV for 18-24 h blocked phosphorylation/activation of the Stat1 transcription factor that regulates IFN-gamma-dependent genes. Although AdV also inhibited IFN-alpha-dependent phosphorylation of Stat1 and Stat2, interleukin-4-dependent phosphorylation of the related transcription factor Stat6 was not affected, indicating that the virus selectively affected specific signaling pathways. Our results indicate that AdV inhibition of the IFN-gamma signal transduction cascade occurs through loss of ligand-induced receptor complex assembly and consequent component phosphorylation and suggest that lack of complex assembly is due to decreased expression of the IFN-gammaR2 chain of the IFN-gamma receptor. IFN-gammaR2 is required at an early step in Janus family kinase-signal transducer and activator of transcription pathway activation and is expressed at low levels in airway epithelial cells, supporting the concept that adenoviral down-regulation of the level of this IFN-gamma receptor component allows for persistent modulation of IFN-gamma-dependent gene expression.

Haemophilus influenzae stimulates ICAM-1 expression on respiratory epithelial cells.

Epithelial cells interact directly with bacteria in the environment and play a critical role in airway defense against microbial pathogens. In this study, we examined the response of respiratory epithelial cells to infection with nontypable Haemophilus influenzae. Using an in vitro cell culture model, we found that epithelial cell monolayers released significant quantities of IL-8 and expressed increased levels of ICAM-1 mRNA and surface protein in response to H. influenzae. In contrast, levels of IL-1beta, TNF-alpha, and MHC class I were not significantly affected, suggesting preferential activation of a specific subset of epithelial genes directed toward defense against bacteria. Induction of ICAM-1 required direct bacterial interaction with the epithelial cell surface and was not reproduced by purified H. influenzae lipooligosaccharide. Consistent with a functional role for this response, induction of ICAM-1 by H. influenzae mediated increased neutrophil adherence to the epithelial cell surface. Furthermore, in an in vivo murine model of airway infection with H. influenzae, increased epithelial cell ICAM-1 expression coincided with increased chemokine levels and neutrophil recruitment in the airway. These results indicate that ICAM-1 expression on human respiratory epithelial cells is induced by epithelial cell interaction with H. influenzae and suggest that an ICAM-1-dependent mechanism can mediate neutrophil adherence to these cells independent of inflammatory mediator release by other cell types. Direct induction of specific epithelial cell genes (such as ICAM-1 and IL-8) by bacterial infection may allow for rapid and efficient innate defense in the airway.

Surface components of HeLa cells that inhibit cytadherence of Chlamydia trachomatis.

Isolated HeLa plasma membrane (PM) preparations and extracts containing either cell-surface proteins or lipids were examined for inhibition of adherence of radiolabeled Chlamydia trachomatis serovar E elementary bodies to glutaraldehyde-fixed HeLa monolayers. A dose-dependent adherence-inhibitory activity could be demonstrated with the PM. A urea extract as well as lipids from HeLa cells also inhibited chlamydial cytadherence. The inhibitory activity of the PM was trypsin-sensitive. It was absent when the urea extract was prepared from trypsin-treated HeLa cells. The urea extract was subjected to electrophoresis and protein blotting using a native gel system. Probing with radiolabeled chlamydial cytadhesin showed a single protein present in the urea extract that could represent a HeLa cell protein receptor for the chlamydiae.

Further characterization of an outer membrane protein of Chlamydia trachomatis with cytadherence properties.

To further characterize the chlamydial cytadhesin (CCA), we have examined it for saturability of binding to HeLa cells that were grown as monolayers and in suspension. The CCA exhibited specific cytadherence properties of binding to HeLa cells that appeared to be saturable. The CCA showed a substantial decrease in binding to trypsin-treated HeLa cells in suspension. This finding, together with the fact that the CCA itself is known to be trypsin-sensitive, suggested a protein-protein type of interaction between CCA and HeLa cells. Periodate treatment of the CCA did not result in significant reduction in cytadherence, which implies that sugar moieties were probably not involved in CCA binding to HeLa cells. Whilst attempts to produce antibodies to the CCA in rabbits was unsuccessful, the CCA reacted with antibodies in a human serum known to contain high titer antibodies to Chlamydia trachomatis, suggesting it can be immunogenic, and is possibly expressed during human infection.

A heat-labile protein of Chlamydia trachomatis binds to HeLa cells and inhibits the adherence of chlamydiae.

From highly purified elementary bodies (EBs) of Chlamydia trachomatis, we have identified a protein of 38 kDa that selectively binds to monolayer cultures of HeLa cells. This protein, which we have named the chlamydial cytadhesin (CCA), is present on the surface of the EBs of three C. trachomatis serovars (B, E, and L1) that were examined. Localization of the CCA at the surface was confirmed by its ability to be labeled when viable EBs were iodinated and by its absence in preparations from trypsin-treated EBs. Viable EBs, but not heated or trypsin-treated EBs, inhibited the binding of the CCA to HeLa cells, indicating competition for a common receptor on the host cell membrane. A dose-dependent inhibition of adherence of radioactive EBs to HeLa cells was effected by extracts containing the CCA. This inhibition occurred even with extracts prepared from the EB of heterologous serovars. However, no inhibition could be demonstrated with extracts prepared from heat-treated EBs. Heat treatment of the extract resulted in the loss of ability of the CCA to bind to the host cells. HeLa cells preincubated with CCA-containing chlamydial extract showed reduced ability to bind labeled EBs and to develop cytoplasmic inclusions after infection. This protective activity was lost after exposure of the extract to heat. These findings indicate that the CCA is a thermolabile surface-exposed chlamydial adhesin; it may be useful in the development of vaccines for diseases caused by the pathogenic bacterium.