Legions Of Immune Cells In The Lung Keep Legionella At Bay

Monocyte-derived cells, which are part of the immune system, have been found to play a significant role in fighting off the bacterium that causes Legionnaire’s disease.

AsianScientist (Jun. 23, 2016) – Researchers in Australia believe they have found a major response that helps keep the Legionella infection at bay.

Immunologists and microbiologists from the University of Melbourne and Royal Melbourne Hospital have published a study in PLOS Pathogens that defines a new immune cell type responsible for destroying Legionella pneumophila, the bacterium that causes Legionnaire’s disease.

The bacterium usually grows within pond amoebae, but can ‘accidentally’ cause serious lung infections in susceptible humans. Legionella causes disease when it invades and destroys the amoeba-like macrophages in the lungs, and people contract the disease through inhaling contaminated water in the form of water vapor droplets from air-conditioning units, spas and other water sources.

PhD student Mr. Andrew Brown, who was the first author of the study, characterized immune cell populations in inflamed lung tissues infected with Legionella bacteria. To his surprise, he uncovered a new population of immune cells that was playing a significant role: the monocyte-derived cells (MCs).

Rather than macrophages, it was the MCs that responded to Legionella within 24 hours of infection and were present in over tenfold the numbers of macrophages in the lung by 48 hours after infection.

MCs are part of the immune system’s first line of defense against the bacteria, and in this case, responded to the infection by secreting a chemical messenger called interleukin-12. This, in turn, drove immune T cells to produce large amounts of interferon gamma, another powerful chemical messenger of the immune system that instructed the MCs to kill the engulfed Legionella bacteria.

“As immunologists, we usually look at what is happening in the immune organs, such as the bone marrow, lymph nodes and spleen, but in this study, we decided to look at what was happening in the tissue at the site of infection,” said Professor Ian van Driel from the University of Melbourne, who was the corresponding author on the study.

The article can be found at: Brown et al. (2016) Cooperation between Monocyte-Derived Cells and Lymphoid Cells in the Acute Response to a Bacterial Lung Pathogen.


Source: University of Melbourne; Photo: Shutterstock.
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