Autophagy induction by pathogenic leptospira in human and murine macrophages

Abstract

Leptospirosis is a zoonotic disease with a worldwide distribution, caused by pathogenic Leptospira species. Pathogenic leptospires may evoke severe systemic infection in humans but only cause mild chronic disease or asymptomatic carrier state in several animals. The reasons for this difference in infection and related natural defense mechanisms remain unclear. In this study, we aimed to study autophagy induction by pathogenic Leptospira in murine and human macrophages. L. interrogans serovar Pomona was used to infect RAW264.7 murine macrophage cell line and THP-1 human monocytic cell line. Autophagy formation in murine and human macrophages was detected as the appearance of autophagic markers, LC3-II, in Western blot. Furthermore, the leptospires in the murine macrophages were observed under transmission electron microscopy to be located within double membrane vesicles, which is the characteristic of autophagosomes. Leptospires in human macrophages, however, were found in the single membrane compartment and/or in the cytosol. In addition, in contrast to murine macrophages, co-localization of leptospires with lysosome was not observed by confocal microscopy in human macrophages at 8 hour post infection. Treatment with an autophagy inhibitor, 3-methyladenine, the autophagy process seemed to participate in the killing of pathogenic Leptospira in murine macrophages but was not the major mechanism in the clearance of pathogenic Leptospira in human macrophages. Furthermore, TLR4 may contribute to autophagy after pathogenic Leptospira infection in murine macrophages. Therefore, the distinct autophagic process between human and murine cells may explain the nature of infection in different hosts of Leptospira.