Pulmonary inflammation and Tie2/Angiopoietin mediated vascular dysregulation during lethal Orientia tsutsugamushi infection

Abstract Orientia tsutsugamushi infection can cause acute lung injury and high mortality in humans; however, the underlying mechanisms are unclear. Here, we tested a hypothesis that dysregulated pulmonary inflammation and Tie2-mediated endothelial malfunction contribute to lung damage. Using a murine model of lethal O. tsutsugamushi infection, we demonstrated pathological characteristics of lung damage: 1) a significant increase of ICAM-1, VEGFR2, and angiopoietin-2 (Ang2) proteins in inflamed tissues and lung-derived endothelial cells (EC), 2) a progressive loss of endothelial quiescent and junction proteins (Ang1, VE-cadherin), and 3) a profound impairment of Tie2 receptor at the transcriptional and functional levels. In vitro infection of primary human EC cultures and serum Ang2 proteins in scrub typhus patients support our animal studies, implying endothelial dysfunction in severe scrub typhus. Evidence for neutrophil-mediated inflammation and tissue damage included in vitro infection and activation of neutrophils, lung accumulation of activated (CD63+)neutrophils, co-localization of lung bacteria with neutrophils, and co-localization of MPO+ neutrophils with Ang2+ ECs. At the onset of disease, pulmonary macrophages (MF) were polarized toward an M1-like phenotype, with limited detection of M2-like cells. Comparative studies of LPS- versus IL-4-primed MFs revealed extensive bacterial replication in M2-type MFs that preferentially expressed the IL-10 and SOCS1 genes. This is the first detailed investigation of lung cellular immune responses during acute O. tsutsugamushi infection and highlights inflammatory biomarkers future therapeutic research for the control of this neglected tropical disease.