Adherence of multiple serovars of Chlamydia trachomatis to a common receptor on HeLa and McCoy cells is mediated by thermolabile protein(s)

All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function

The bacterial pathogens Chlamydia trachomatis and C. pneumoniae are obligate intracellular parasites, cause a number of serious diseases, and can infect various cell types in humans. Chlamydial infections are probably initiated by binding of the bacterial outer membrane protein OmcB to host cell glycosaminoglycans (GAGs). Here, we show that all nine members of the polymorphic membrane protein (Pmp) family of C. trachomatis mediate adhesion to human epithelial and endothelial cells. Importantly, exposure of infectious particles to soluble recombinant Pmps blocks subsequent infection, thus implicating an important function of the entire protein family in the infection process. Analogous experiments with pairs of recombinant Pmps or a combination of Pmp and OmcB revealed that all Pmps probably act in an adhesion pathway that is distinct from the OmcB-GAG pathway. Finally, we provide evidence that the Pmps of C. trachomatis and C. pneumoniae exhibit species and tissue specificity. These findings argue for the involvement of C. trachomatis Pmps in the initial phase of infection and suggest that they may interact with a receptor other than the epidermal growth factor receptor recently identified for their counterparts in C. pneumoniae.

Mechanisms of Chlamydia trachomatis entry into nonphagocytic cells

The mechanisms of entry for the obligate intracellular bacterium C. trachomatis were examined by functional disruption of proteins essential for various modes of entry. RNA interference was used to disrupt proteins with established roles in clathrin-mediated endocytosis (clathrin heavy chain, dynamin-2, heat shock 70-kDa protein 8, Arp2, cortactin, and calmodulin), caveola-mediated endocytosis (caveolin-1, dynamin-2, Arp2, NSF, and annexin II), phagocytosis (RhoA, dynamin-2, Rac1, and Arp2), and macropinocytosis (Pak1, Rac1, and Arp2). Comparative quantitative PCR analysis was performed on small interfering RNA-transfected HeLa cells to accurately determine the extent of C. trachomatis entry after these treatments. Key structural and regulatory factors associated with clathrin-mediated endocytosis were found to be involved in Chlamydia entry, whereas those for caveola-mediated endocytosis, phagocytosis, and macropinocytosis were not. Thus, clathrin and its coordinate accessory factors were required for entry of C. trachomatis, although additional, uncharacterized mechanisms are also utilized.

Entry of the lymphogranuloma venereum strain of Chlamydia trachomatis into host cells involves cholesterol-rich membrane domains

Chlamydiae are bacterial pathogens which develop strictly inside the epithelial cells of their hosts. The mechanism used by chlamydiae to enter cells is not well characterized; however, it is thought to consist of a receptor-mediated process. In addition, the formation of clathrin-coated pits appears to be dispensable for chlamydiae to be internalized by host cells. Clathrin-independent endocytosis has recently been shown to occur through cholesterol-rich lipid microdomains, which are characterized by detergent insolubility. In the present study, we investigated whether these lipid domains play a role in Chlamydia trachomatis serovar L2 internalization by host cells. Our results show that after binding to HeLa cells, chlamydiae are associated with detergent-resistant lipid microdomains (DRMs), which can be isolated by fractionation of infected HeLa cells and flotation on a sucrose gradient. After internalization by HeLa cells, chlamydiae were still found in DRMs. In addition, extraction of plasma membrane cholesterol inhibited infection of HeLa cells by C. trachomatis. Many of the proteins associated with DRMs are glycosylphosphatidylinositol (GPI)-anchored proteins; however, our results could not identify a role for GPI-anchored proteins in the entry process. The same results were obtained for Chlamydia psittaci strain GPIC. We propose that cholesterol-rich domains participate in the entry of chlamydiae into host cells. Chlamydia binding to cholesterol-rich domains may lead to coalescence of the bacterial cells, which could trigger internalization by host cells.

Selection of mutant cell lines resistant to infection by Chlamydia spp [corrected]

The lytic outcome of natural infection by Chlamydia trachomatis was exploited to select CHO (Chinese hamster ovary) cells, following chemical mutagenesis, that were deficient in their ability to sustain productive chlamydial infection. Four distinct mutant cell phenotypes with defects in either attachment or postattachment mechanisms that are required for infection by C. trachomatis and Chlamydia pneumoniae were characterized.

Protein disulfide isomerase, a component of the estrogen receptor complex, is associated with Chlamydia trachomatis serovar E attached to human endometrial epithelial cells

Chlamydia trachomatis serovar E, the leading bacterial agent responsible for sexually transmitted diseases, is required to invade genital epithelial cells for its growth and survival, yet little is known about the adhesin-receptor interactions promoting its entry. In contrast, much has been published on the heparan sulfate receptor for binding C. trachomatis L2 elementary bodies (EBs) prior to entry into HeLa cells. Using a different experimental approach in which a biotinylated apical membrane protein receptor(s) attached to EB at 4 degrees C was stripped off the surface of polarized HEC-1B cells and immunoprecipitated with polyclonal anti-EB antibodies, an approximately 55-kDa protein was reproducibly detected by enhanced chemiluminescence and two-dimensional gel electrophoresis. Matrix-assisted laser desorption ionization mass-spectrometry sequence analysis revealed the 55-kDa protein to be protein disulfide isomerase (PDI), a member of the estrogen receptor complex which carries out thiol-disulfide exchange reactions at infected host cell surfaces. Exposure of HEC-1B cells during EB attachment (1.5 to 2 h) to three different inhibitors of PDI reductive reactions--(i) the thiol-alkylating reagent DTNB (5,5'-dithiobis[2-nitrobenzoic acid]), (ii) bacitracin, and (iii) anti-PDI antibodies--resulted in reduced chlamydial infectivity. Since (i) C. trachomatis serovar E attachment to estrogen-dominant primary human endometrial epithelial cells is dramatically enhanced and (ii) productive entry into and infectivity of EB in host cells is dependent on reduction of EB cross-linked outer membrane proteins at the host cell surface, these data provide some preliminary evidence for an intriguing new potential receptor candidate for further analysis of luminal C. trachomatis serovar E entry.

Isolation and characterization of a mutant Chinese hamster ovary cell line that is resistant to Chlamydia trachomatis infection at a novel step in the attachment process

Host factors involved in Chlamydia trachomatis pathogenesis were investigated by random chemical mutagenesis of Chinese hamster ovary (CHO-K1) cells followed by selection for clones resistant to chlamydial infection. A clonal mutant cell line, D4.1-3, refractory to infection by the C. trachomatis L2 serovar was isolated. The D4.1-3 cell line appears to be lacking in a previously undescribed temperature-dependent and heparin-resistant binding step that occurs subsequent to engagement of cell surface heparan sulfate by L2 elementary bodies. This novel binding step differentiates the lymphogranuloma venereum (LGV) serovar from other serovars and may contribute the different pathologies associated with LGV and non-LGV strains.

Identification of protective epitopes by sequencing of the major outer membrane protein gene of a variant strain of Chlamydia psittaci serotype 1 (Chlamydophila abortus)

Protective monoclonal antibodies (MAbs) to the major outer membrane protein (MOMP) of species of the family Chlamydiaceae, which is the primary vaccine candidate antigen, recognize nonlinear epitopes conferred by the oligomeric conformation of the molecule. Protective MAbs failed to recognize oligomeric MOMP of the variant strain LLG, which bears amino acid substitutions in variable segments (VSs) 1, 2, and 4, and competed with monomer-specific MAbs mapping to these VSs in reference strain 577. The results suggest that multiple sites located in the three VSs contribute to the epitope of protective MAbs.

Eukaryotic cell uptake of heparin-coated microspheres: a model of host cell invasion by Chlamydia trachomatis

Using polystyrene microspheres coated with heparin or heparan sulfate, it was shown that coated microspheres specifically bound eukaryotic cells and were endocytosed by nonprofessional phagocytic cells. Coated microspheres displayed properties of binding to eukaryotic cells that were similar to those of chlamydiae, and the microspheres were competitively inhibited by chlamydial organisms. Endocytosis of heparin-coated beads resulted in the tyrosine phosphorylation of a similar set of host proteins as did endocytosis of chlamydiae; however, unlike viable chlamydial organisms, which prevent phagolysosomal fusion, endocytosed beads were trafficked to a lysosomal compartment. These findings suggest that heparin-coated beads and Chlamydia trachomatis enter eukaryotic cells by similar pathways.

Differences in the association of Chlamydia trachomatis serovar E and serovar L2 with epithelial cells in vitro may reflect biological differences in vivo

Chlamydia trachomatis serovar E is one of the most common bacterial sexually transmitted pathogens. Since it is an obligate intracellular bacterium, efficient colonization of genital mucosal epithelial cells is crucial to the infectious process. Serovar E elementary bodies (EB) metabolically radiolabeled with 35S-Cys-Met and harvested from microcarrier bead cultures, which significantly improves the infectious EB-to-particle ratio, provided a more accurate picture of the parameters of attachment of EB to human endometrial epithelial cells (HEC-1B) than did less infectious 14C-EB harvested from flask cultures. Binding of serovar E EB was (i) equivalent at 35 and 4 degrees C, (ii) decreased by preexposure of EB to heat or the topical microbicide C31G, (iii) comparable among common eukaryotic cell lines (HeLa, McCoy), and (iv) significantly increased to the apical surfaces of polarized cells versus nonpolarized cells. In parallel experiments with C. trachomatis serovar L2, serovar E attachment was not affected by heparin or heparan sulfate whereas these glucosaminoglycans dramatically reduced serovar L2 attachment. These data were confirmed by competitive inhibition of serovar E binding and infectivity by excess unlabeled live and UV-inactivated serovar E EB but not by excess serovar L2 EB. The noninvasive serovar E strains in the lumen of the genital tract enter and exit the apical domains of target columnar epithelial cells to spread canalicularly in an ascending fashion from the lower to the upper genital tract. In contrast, the invasive serovar L2 strains are primarily submucosal pathogens and likely use the glucosaminoglycans concentrated in the extracellular matrix to colonize the basolateral domains of mucosal epithelia to perpetuate the infectious process.

Interaction of outer envelope proteins of Chlamydia psittaci GPIC with the HeLa cell surface

The chlamydial life cycle involves the intimate interaction of components of the infectious elementary body (EB) surface with receptors on the susceptible eukaryotic cell plasma membrane. We have developed an in vitro ligand binding assay system for the identification and characterization of detergent-extracted EB envelope proteins capable of binding to glutaraldehyde-fixed HeLa cell surfaces. With this assay, the developmentally regulated cysteine-rich envelope protein Omp2 of Chlamydia psittaci strain guinea pig inclusion conjunctivitis was shown to bind specifically to HeLa cells. HeLa cells bound Omp2 selectively over other cell wall-associated proteins, including the major outer membrane protein, and the binding of Omp2 was abolished under conditions which alter its conformation. Furthermore, trypsin treatment, which reduces EB adherence, resulted in the proteolytic removal of a small terminal peptide of Omp2 at the EB surface and inactivated Omp2 in the ligand binding assay, while having a negligible effect on the major outer membrane protein. Collectively, our results suggest that Omp2 possesses the capacity to engage in a specific interaction with the host eukaryotic cell. We speculate that, since Omp2 is present only in the infectious EB form, the observed in vitro interaction may be representative of a determining step of the chlamydial pathogenic process.

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.

Characterization of the cytochalasin D-resistant (pinocytic) mechanisms of endocytosis utilized by chlamydiae

The cytochalasin D-resistant (pinocytic) portion of the entry of two chlamydia strains (Chlamydia trachomatis L2/434/Bu and Chlamydia psittaci GPIC [guinea pig inclusion conjunctivitis]) was examined. By ultrastructural criteria, few organisms of either strain were observed in association with coated host-cell plasma membrane during entry into McCoy cells; this argues against a coated-pit mechanism of entry. When association with a coated membrane was seen, coat material appeared to pinch off ahead of internalizing chlamydiae. However, entry of both strains was substantially reduced by cytosol acidification, a procedure shown to prevent coated-pit vesiculation (K. Sandvig, S. Olsnes, O. W. Petersen, and B. van Deurs, J. Cell Biol. 105:679-689, 1987). No conclusive evidence of displacement of the fluid-phase marker [3H]sucrose from constitutively forming endocytic vesicles was found. Indeed the entry of strain 434 (but not strain GPIC) was accompanied by the influx of a large volume of fluid, suggesting an inducible mechanism. Additionally, entry of strain 434 (but not strain GPIC) was partially inhibitable by amiloride, yet the drug had no effect on the entry of transferrin, a ligand known to enter solely via coated pits. Our findings endorse the view that chlamydial entry can occur via a pathway involving coated pits. However, the unusual morphology of entry and lack of fluid exclusion are consistent with a process whereby although chlamydiae are not fully enclosed by coat material, their entry is dependent on the vesiculation of coated pits. Furthermore, the data support the proposition that a significant proportion of the entry of strain 434 occurs via an inducible pathway independent of coated-pit uptake.