Interferon-gamma induced resistance to Legionella pneumophila in susceptible A/J mouse macrophages
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T W Klein1, Y Yamamoto, H K Brown
1Department of Medical Microbiology and Immunology, University of South Florida College of Medicine, Tampa 33612.
Abstract:
Legionella pneumophila (Lp) grow in cultures in human, guinea pig, and mouse macrophages from A/J strain mice. Because exudate macrophages from this strain of mice have been reported deficient in responsiveness to lymphokines, we thought it of interest to document the extent of responsiveness to interferon-gamma in the context of growth restriction of Lp. Peritoneal exudate macrophages were obtained from A/J mice and cultured in either the presence or absence of recombinant interferon-gamma. These cultures were then infected with Lp and the extent of bacterial growth estimated 48 hr later by means of a colony-forming unit (CFU) assay and electron microscopy. Interferon-gamma treatment significantly restricted the number of CFUs in the culture at concentrations as low as 20 U/ml, but did not affect the uptake of bacteria by macrophages. Furthermore, treatment with interferon induced morphological changes consistent with activated macrophages. The involvement of oxygen-dependent mechanisms in phagocyte killing and growth restriction was examined by the use of inhibitors such as superoxide dismutase (SOD) and catalase. Neither one of these inhibitors of toxic oxygen metabolites affected the interferon-gamma-induced suppression of Lp growth. These results suggest that although thioglycolate-induced exudate macrophages from A/J mice support the growth of Lp, these cells readily respond to the activating influence of interferon-gamma. Furthermore, lymphokine treatment does not inhibit Lp uptake by macrophages and apparently restricts the growth of bacteria by mechanisms independent of the activity of toxic oxygen metabolites.
Insights
Interferon-gamma activates macrophages to restrict Legionella pneumophila growth, independent of bacterial uptake or oxygen-dependent killing mechanisms. This study highlights interferon-gamma
Area of Science:
- Immunology
- Microbiology
- Cell Biology
Background:
- Legionella pneumophila (Lp) infects macrophages.
- A/J strain mouse macrophages show reduced lymphokine responsiveness.
- Interferon-gamma (IFN-γ) is a key immune signaling molecule.
Purpose of the Study:
- To investigate IFN-γ's effect on Lp growth in A/J macrophages.
- To determine if IFN-γ activates macrophages against Lp.
- To explore the mechanisms of IFN-γ-mediated Lp restriction.
Main Methods:
- Culturing A/J mouse peritoneal exudate macrophages.
- Treating macrophages with recombinant IFN-γ.
- Infecting macrophages with Lp and quantifying bacterial growth via CFU assay and electron microscopy.
- Assessing the role of oxygen metabolites using superoxide dismutase and catalase inhibitors.
Main Results:
- IFN-γ significantly restricted Lp colony-forming units (CFUs) at low concentrations (≥20 U/ml).
- IFN-γ treatment did not alter Lp uptake by macrophages.
- Morphological changes indicated macrophage activation by IFN-γ.
- Inhibitors of toxic oxygen metabolites did not affect IFN-γ's growth restriction of Lp.
Conclusions:
- A/J macrophages, despite supporting Lp growth, are responsive to IFN-γ.
- IFN-γ restricts Lp growth via mechanisms independent of bacterial uptake.
- The growth restriction is not mediated by toxic oxygen metabolites.