Chlamydial inclusions: Accumulation of fibers bearing an intermediate filament epitope

Reorganization of the host cytoskeleton by the intracellular pathogen Chlamydia trachomatis

Chlamydiae are obligate intracellular pathogens that cause a wide range of human diseases. Chlamydia resides in a membrane bound vacuole ("inclusion") that expands to accommodate replicating bacteria. We recently reported that Chlamydia remodels and recruit two major cytoskeletal components of the host cell- F-actin and Intermediate filaments-to form a dynamic scaffold that provides structural stability to the inclusion. As the inclusion expands, a secreted chlamydial protease progressively modifies the intermediate filaments scaffold, presumably to increase the inclusion's flexibility and accommodate the increased bacterial load. This represents a unique mechanism employed by an intracellular pathogen to support its intracellular niche and may be linked to immune evasion by this pathogen. Here, we discuss the potential consequences of Chlamydia-mediated alteration of host cytoskeletal dynamics on the pathogenesis of chlamydial infections.

Uptake and intra-inclusion accumulation of exogenous immunoglobulin by Chlamydia-infected cells

Background

Obligate intracellular pathogens belonging to the Chlamydiaceae family possess a number of mechanisms by which to manipulate the host cell and surrounding environment. Such capabilities include the inhibition of apoptosis, down-regulation of major histocompatability complex (MHC) and CD1/d gene expression, and the acquisition of host-synthesized nutrients. It is also documented that a limited number of host-derived macromolecules such as β-catenin and sphingomyelin accumulate within the inclusion.

Results

This report provides evidence that immunoglobulin, inherently present in the extracellular environment in vivo and in vitro, enters infected cells and accumulates within the chlamydial inclusion. Using epi-fluorescent and confocal microscopy, this selective uptake of Ig is shown to occur among human leukocytes in vivo as well as cells cultured in vitro. These findings were confirmed by detection of IgG in the lysate of infected cells by western blot hybridization. Sequestered antibodies appear to be present during the entire course of the chlamydial developmental cycle and are distributed throughout this compartment. IgG pre-labeled with fluorescein, when added to the supernatant of infected cell cultures, was also imported and readily visualized. Accumulation of these molecules within the inclusion and the failure of bovine serum albumin or F(ab')₂ fragments to accumulate in a similar manner suggests the process of entry is specific for intact IgG molecules and not a result of pinocytosis, diffusion, or any other mass endocytic event.

Conclusion

Sequestration of a host cell-derived protein within the chlamydial inclusion, although unexpected, is not an unprecedented occurrence. However, selective accumulation of an exogenous host protein, such as extracellular IgG, has not been previously reported in connection with chlamydial infections. The selectivity of this process may indicate that this uptake plays an important role in pathogen physiology or virulence during infection and the phenomenon itself may give rise to novel diagnostic and therapeutic approaches.

Association of caveolin with Chlamydia trachomatis inclusions at early and late stages of infection

The mechanism by which the intracellular bacterial pathogen Chlamydia trachomatis enters eukaryotic cells is poorly understood. There are conflicting reports of entry occurring by clathrin-dependent and clathrin-independent processes. We report here that C. trachomatis serovar K enters HEp-2 and HeLa 229 epithelial cells and J-774A.1 mouse macrophage/monocyte cells via caveolin-containing sphingolipid and cholesterol-enriched raft microdomains in the host cell plasma membranes. First, filipin and nystatin, drugs that specifically disrupt raft function by cholesterol chelation, each impaired entry of C. trachomatis serovar K. In control experiments, filipin did not impair entry of the same organism by an antibody-mediated opsonic process, nor did it impair entry of BSA-coated microspheres. Second, the chlamydia-containing endocytic vesicles specifically reacted with antisera against the caveolae marker protein caveolin. These vesicles are known to become the inclusions in which parasite replication occurs. They avoid fusion with lysosomes and instead traffic to the Golgi region, where they intercept Golgi-derived vesicles that recycle sphingolipids and cholesterol to the plasma membrane. We also report that late-stage C. trachomatis inclusions continue to display high levels of caveolin, which they likely acquire from the exocytic Golgi vesicles. We suggest that the atypical raft-mediated entry process may have important consequences for the host-pathogen interaction well after entry has occurred. These consequences include enabling the chlamydial vesicle to avoid acidification and fusion with lysosomes, to traffic to the Golgi region, and to intercept sphingolipid-containing vesicles from the Golgi.