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Ultrastructural study of mode of entry of Chlamydia psittaci into L-929 cells
Abstract:
The entry of Chlamydia psittaci into L-929 cells was studied morphologically by transmission electron microscopy and quantitatively by a method that discriminates between attachment and uptake. Upon adsorption of 3H-labeled elementary bodies (EBs) to host cells at 4 degrees C, the EBs bound efficiently to the L-cell surface. Binding reached an equilibrium level of 55% in 3 h. Ultrastructural analysis revealed that EBs were bound preferentially to the tips and sides of microvilli at this temperature. The EBs were also observed in coated pits located at the bases of microvilli and along smooth surfaces of the host cell. No internalization was observed at 4 degrees C. When cells with prebound 3H-labeled EBs were warmed to 37 degrees C, the EBs rapidly became resistant to proteinase K removal (half time = 5 min), indicating ingested chlamydiae. At 37 degrees C, the EBs were internalized within tightly bound vesicles surrounded by an electron-dense coat of fibrillar material. EBs were also present in smooth-surfaced pits and vesicles of the host cell. Using alpha 2-macroglobulin coupled to colloidal gold (a known marker for receptor-mediated endocytosis), we observed that the entry of EBs into cells via coated pits was identical in appearance to the internalization of alpha 2-macroglobulin. Also, when the two ligands were mixed together, they could be seen within the same coated pits and were cointernalized within endocytic vesicles of the host cell. These results suggest that C. psittaci can enter nonprofessional phagocytic cells by a pathway which is similar to that of receptor-mediated endocytosis of many physiologically important macromolecules, bacterial toxins, and viruses.
Insights
Chlamydia psittaci elementary bodies (EBs) attach to host cells and are internalized via a process resembling receptor-mediated endocytosis. This bacterial entry mechanism involves coated pits and vesicles at 37°C.
Area of Science:
- Cell Biology
- Microbiology
- Infectious Diseases
Background:
- Chlamydia psittaci is an obligate intracellular bacterium that infects various host cells.
- Understanding the entry mechanism of C. psittaci into non-professional phagocytic cells is crucial for developing therapeutic strategies.
Purpose of the Study:
- To investigate the morphological and quantitative aspects of Chlamydia psittaci entry into L-929 cells.
- To elucidate the specific cellular pathway utilized by C. psittaci for host cell invasion.
Main Methods:
- Transmission electron microscopy (TEM) for morphological analysis.
- Quantitative assays using 3H-labeled elementary bodies (EBs) to differentiate attachment from uptake.
- Comparison with alpha 2-macroglobulin coupled to colloidal gold as a marker for receptor-mediated endocytosis.
Main Results:
- C. psittaci EBs efficiently bound to L-929 cell surfaces, preferentially to microvilli, at 4°C.
- Internalization of EBs occurred rapidly at 37°C, involving coated pits and vesicles, similar to receptor-mediated endocytosis.
- Co-localization studies with alpha 2-macroglobulin confirmed the involvement of coated pits and cointernalization.
Conclusions:
- C. psittaci enters non-professional phagocytic cells through a mechanism analogous to receptor-mediated endocytosis.
- The entry pathway involves specific interactions with host cell surface structures and endocytic machinery.
- This finding provides insights into the pathogenesis of Chlamydia infections and potential targets for intervention.