Cooperation between Monocyte-Derived Cells and Lymphoid Cells in the Acute Response to a Bacterial Lung Pathogen.
PLoS Pathog 2016;
12:e1005691. [PMID:
27300652 PMCID:
PMC4907431 DOI:
10.1371/journal.ppat.1005691]
[Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2016] [Accepted: 05/17/2016] [Indexed: 01/12/2023] Open
Abstract
Legionella pneumophila is the causative agent of Legionnaires’ disease, a potentially fatal lung infection. Alveolar macrophages support intracellular replication of L. pneumophila, however the contributions of other immune cell types to bacterial killing during infection are unclear. Here, we used recently described methods to characterise the major inflammatory cells in lung after acute respiratory infection of mice with L. pneumophila. We observed that the numbers of alveolar macrophages rapidly decreased after infection coincident with a rapid infiltration of the lung by monocyte-derived cells (MC), which, together with neutrophils, became the dominant inflammatory cells associated with the bacteria. Using mice in which the ability of MC to infiltrate tissues is impaired it was found that MC were required for bacterial clearance and were the major source of IL12. IL12 was needed to induce IFNγ production by lymphoid cells including NK cells, memory T cells, NKT cells and γδ T cells. Memory T cells that produced IFNγ appeared to be circulating effector/memory T cells that infiltrated the lung after infection. IFNγ production by memory T cells was stimulated in an antigen-independent fashion and could effectively clear bacteria from the lung indicating that memory T cells are an important contributor to innate bacterial defence. We also determined that a major function of IFNγ was to stimulate bactericidal activity of MC. On the other hand, neutrophils did not require IFNγ to kill bacteria and alveolar macrophages remained poorly bactericidal even in the presence of IFNγ. This work has revealed a cooperative innate immune circuit between lymphoid cells and MC that combats acute L. pneumophila infection and defines a specific role for IFNγ in anti-bacterial immunity.
Legionnaires’ Disease, a leading cause of community-acquired pneumonia resulting in significant morbidity and death, develops after infection with Legionella bacteria that replicate inside specialised sentinel cells of the lung. Although some factors that help combat Legionella infection are known, an overall view of the early immune events that are triggered by infection were unclear and we have addressed this issue here using recently developed methods. Our study implicates a number of new cells in the defence against Legionella infection and identifies key molecules that participate in a feedback circuit required for eradication of bacteria. In particular, we find that specific immune cells derived from blood monocytes invade the infected lung and trigger other blood-derived cells to produce the potent inflammatory mediator IFNγ. In turn IFNγ stimulates monocyte-derived cells to destroy bacteria. Surprisingly, IFNγ did not influence the behaviour of other abundant immune cells. The reported mechanism provides a basis for future investigation into the host response to combat intracellular bacteria, particularly in lung, and for assessing the risk to individuals infected with lung pathogens.
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