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Rickettsia-macrophage interactions: host cell responses to Rickettsia akari and Rickettsia typhi
S Radulovic1, P W Price, M S Beier
1University of Maryland, School of Medicine, Department of Microbiology and Immunology, Baltimore, Maryland 21201, USA. sradu001@umaryland.edu
Abstract:
The existence of intracellular rickettsiae requires entry, survival, and replication in the eukaryotic host cells and exit to initiate new infection. While endothelial cells are the preferred target cells for most pathogenic rickettsiae, infection of monocytes/macrophages may also contribute to the establishment of rickettsial infection and resulting pathogenesis. We initiated studies to characterize macrophage-Rickettsia akari and -Rickettsia typhi interactions and to determine how rickettsiae survive within phagocytic cells. Flow cytometry, microscopic analysis, and LDH release demonstrated that R. akari and R. typhi caused negligible cytotoxicity in mouse peritoneal macrophages as well as in macrophage-like cell line, P388D1. Host cells responded to rickettsial infection with increased secretion of proinflammatory cytokines such as interleukin-1beta (IL-1beta) and IL-6. Furthermore, macrophage infection with R. akari and R. typhi resulted in differential synthesis and expression of IL-beta and IL-6, which may correlate with the existence of biological differences among these two closely related bacteria. In contrast, levels of gamma interferon (IFN-gamma), IL-10, and IL-12 in supernatants of infected P388D1 cells and mouse peritoneal macrophages did not change significantly during the course of infection and remained below the enzyme-linked immunosorbent assay cytokine detection limits. In addition, differential expression of cytokines was observed between R. akari- and R. typhi-infected macrophages, which may correlate with the biological differences among these closely related bacteria.
Insights
Rickettsia bacteria infect macrophages with minimal cell damage, triggering inflammatory cytokine release. Differential cytokine expression between Rickettsia akari and Rickettsia typhi suggests distinct bacterial biology.
Area of Science:
- Microbiology
- Immunology
- Cell Biology
Background:
- Rickettsiae are intracellular bacteria requiring host cell invasion, survival, and replication.
- While endothelial cells are primary targets, macrophages also play a role in rickettsial infections.
- Understanding macrophage-rickettsiae interactions is crucial for elucidating pathogenesis.
Purpose of the Study:
- To characterize the interaction between Rickettsia akari, Rickettsia typhi, and macrophages.
- To investigate how rickettsiae survive within phagocytic cells.
- To analyze the host immune response, specifically cytokine production, following infection.
Main Methods:
- Infection of mouse peritoneal macrophages and P388D1 cell line with R. akari and R. typhi.
- Assessment of cytotoxicity using lactate dehydrogenase (LDH) release assay.
- Flow cytometry and microscopic analysis to study cellular interactions.
- Quantification of cytokine levels (IL-1beta, IL-6, IFN-gamma, IL-10, IL-12) in cell supernatants.
Main Results:
- R. akari and R. typhi exhibited negligible cytotoxicity in macrophages.
- Rickettsial infection induced increased secretion of pro-inflammatory cytokines IL-1beta and IL-6.
- Differential expression of IL-1beta and IL-6 was observed between R. akari and R. typhi infections.
- Levels of IFN-gamma, IL-10, and IL-12 remained unchanged and below detection limits.
Conclusions:
- Macrophages are permissive to R. akari and R. typhi infection with low cytotoxicity.
- Rickettsial infection modulates host cytokine profiles, particularly IL-1beta and IL-6.
- Differential cytokine responses suggest distinct biological characteristics between R. akari and R. typhi within macrophages.