Research Article: Mononuclear-macrophages but not neutrophils act as major infiltrating anti-leptospiral phagocytes during leptospirosis

Date Published: July 11, 2017

Publisher: Public Library of Science

Author(s): Xu Chen, Shi-Jun Li, David M. Ojcius, Ai-Hua Sun, Wei-Lin Hu, Xu’ai Lin, Jie Yan, Yung-Fu Chang.


To identify the major infiltrating phagocytes during leptospirosis and examine the killing mechanism used by the host to eliminate Leptospira interrogans.

Major infiltrating phagocytes in Leptospira-infected C3H/HeJ mice were detected by immunohistochemistry. Chemokines and vascular endothelial cell adhesion molecules (VECAMs) of Leptospira-infected mice and leptospirosis patients were detected by microarray and immunohistochemistry. Leptospira-phagocytosing and -killing abilities of human or mouse macrophages and neutrophils, and the roles of intracellular ROS, NO and [Ca2+]i in Leptospira-killing process were evaluated by confocal microscopy and spectrofluorimetry.

Peripheral blood mononuclear-macrophages rather than neutrophils were the main infiltrating phagocytes in the lungs, liver and kidneys of infected mice. Levels of macrophage- but not neutrophil-specific chemokines and VECAMs were significantly increased in the samples from infected mice and patients. All macrophages tested had a higher ability than neutrophils to phagocytose and kill leptospires. Higher ROS and NO levels and [Ca2+]i in the macrophages were involved in killing leptospires. Human macrophages displayed more phagolysosome formation and a stronger leptospire-killing ability to than mouse macrophages.

Mononuclear-macrophages but not neutrophils represent the main infiltrating and anti-leptospiral phagocytes during leptospirosis. A lower level of phagosome-lysosome fusion may be responsible for the lower Leptospira-killing ability of human macrophages.

Partial Text

Leptospirosis is a global zoonotic infectious disease caused by pathogenic Leptospira species [1]. The disease is endemic in Asia, South America, and Oceania [2–5]. Moreover, in recent years, human leptospirosis has been considered as an emerging infectious disease in Europe and North America [6–8]. Over one million human leptospirosis cases have been reported annually, and the mortality rate ranges from 5 to 20% worldwide [9].

Macrophages and neutrophils are central mediators of the host innate immune system and play a key role in phagocytosis and killing of invaded pathogens at early stages of microbial infection [49]. However, the two phagocytes also have distinctive functional properties during infection by different prokaryotic pathogens. Many pathogens, such as Staphylococcus aureus and Pseudomonas aeruginosa, are killed by neutrophils and cause a typical clinical symptom at the site of infection characterized by formation of pus, a mass of killed pathogens and dead neutrophils [50,51]. On the contrary, some bacterial pathogens, such as Salmonella species and Mycobacterium tuberculosis, are killed by mononuclear-macrophages and cause a nonpyogenic infection [18,52]. Leptospirosis is well known as a nonpyogenic infection, but the infiltrated phagocytic cell types had not been documented carefully. C3H/HeJ mice are TLR4 gene deficient, but more susceptible to L. interrogans and display typical histopathological changes of leptospirosis compared to other species of mice [39,40,53]. Moreover, TLR2 but not TLR4 is responsible for recognizing leptospiral lipopolysaccharide (LPS), the major inducer of inflammatory reactions and immune responses during leptospirosis [54,55]. In the present study, we confirmed that peripheral blood mononuclear-macrophages but not neutrophils are the main infiltrating phagocytes in all the examined internal organs from L. interrogans-infected mice, which could help to explain leptospirosis as a nonpyogenic infectious disease.




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