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Distinction between 'inflammatory' and 'immune' macrophages killing Listeria monocytogenes in murine infection
1Department of Microbiology, University of Melbourne, Parkville, Victoria, Australia.
Abstract:
Two populations of efficiently phagocytic and bacteriolytic cells have been defined in the peritoneal cavity following infection of mice with Listeria monocytogenes. One was the result of a transient inflammatory response 2 days after intraperitoneal (i.p.) infection. It consisted of a mixture of monocyte/macrophages and neurotrophils which, when separated on Percoll gradients or by adherence, were both highly bacteriolytic compared with normal resident peritoneal macrophages. It was rich in recently divided cells as evidenced by in vivo labelling with tritiated thymidine. Although having the enlarged, vacuolated appearance of 'activated' macrophages, three-quarters of the monocyte/macrophages stained positive for myeloperoxidase (MPO), characteristic of monocytes rather than mature macrophages. In contrast, intravenous (i.v.) infection, which localizes in spleen and liver, did not produce this early response in the peritoneal cavity. However, 8 days after either i.v. or i.p. infection there existed in the peritoneal cavity a highly active population of cells comprising chiefly macrophages of typical foamy appearance which did not stain for MPO+. They were actively phagocytic and bacteriolytic and, like the early inflammatory exudate, produced increased amounts of oxygen degradative products. They appear to typify the concept of macrophages activated by T cell mediated immunity. Two day peritoneal exudates induced in these previously infected mice by i.p. rechallenge with L. monocytogenes organisms comprised mostly MPO- macrophages.
Insights
Mice infected with Listeria monocytogenes develop two distinct populations of immune cells in their peritoneal cavity. These cells show enhanced phagocytic and bacteriolytic activity, crucial for combating infection.
Area of Science:
- Immunology
- Cell Biology
- Microbiology
Background:
- Listeria monocytogenes infection triggers immune responses in mice.
- The peritoneal cavity harbors key immune cells involved in host defense.
Purpose of the Study:
- To characterize distinct phagocytic and bacteriolytic cell populations in the mouse peritoneal cavity following Listeria monocytogenes infection.
- To differentiate early inflammatory cells from later-activated macrophages.
Main Methods:
- Intraperitoneal (i.p.) and intravenous (i.v.) infection models in mice using Listeria monocytogenes.
- Cell separation using Percoll gradients and adherence properties.
- In vivo cell proliferation assessment via tritiated thymidine labeling.
- Myeloperoxidase (MPO) staining to distinguish monocyte/macrophage populations.
- Assessment of phagocytic and bacteriolytic activity.
- Measurement of oxygen degradative products.
Main Results:
- Two distinct populations of phagocytic and bacteriolytic cells were identified in the peritoneal cavity.
- An early (2-day) transient inflammatory response included monocyte/macrophages and neutrophils, with many monocytes expressing MPO.
- A later (8-day) response consisted of MPO-negative, foamy macrophages, indicative of T cell-mediated activation.
- Both cell populations exhibited enhanced bacteriolytic activity compared to resident macrophages.
- Rechallenge of infected mice induced MPO-negative macrophages.
Conclusions:
- Listeria monocytogenes infection induces distinct early inflammatory and later adaptive immune cell populations in the peritoneal cavity.
- These peritoneal cell populations display potent phagocytic and bacteriolytic capabilities.
- The study differentiates between inflammatory monocytes and T cell-activated macrophages based on MPO expression and morphology.