Molecular Mechanisms of Immune Dysregulation during Acute Orientia tsutsugamushi Infection in Mice

Abstract

Abstract Orientia tsutsugamushi is a Gram- and LPS-negative bacterium that can cause severe scrub typhus in the Asia-Pacific region. While bacteria replicate freely in endothelial cell (EC) and phagocyte cytosol, little is known regarding pathogen recognition and cellular responses. Our newly developed O. tsutsugamushi infection mouse models are valuable for mechanistic studies. Here, we show that a hallmark of lethal infection is Th1-skewed, but Th2-impaired, immune responses, accompanied by severe endothelial damage and acute tissue injury. While tissue bacteria peak at disease onset (D6), neutrophil and CD8 T cell influx/activation reach peaks around D10 (prior to mouse death). We further show the pathological characteristics of vascular activation and tissue damage: 1) a significant increase of ICAM-1, VEGFR2, and angiopoietin-2 proteins in infected tissues and lung-derived ECs, 2) a loss of endothelial quiescent/junction proteins (angiopoietin-1, VE-cadherin), and 3) a functional loss of Tie2 receptor. Pulmonary macrophages predominantly display an M1-like (CD80+CD64+CD11b+) phenotype at D6 and D9, which correlated with the significant loss of CD31+CD45− ECs and M2-like (CD206+CD64+CD11b+−) cells. NanoString analyses of infected lung and spleen tissues and in vitro studies suggested a role for Clec4e in pathogen recognition and the IL-12/IL-27 axis in driving proinflammatory immune responses and cellular injury. This is the first detailed investigation of lung cellular immune responses during acute O. tsutsugamushi infection. It uncovers specific biomarkers for vascular dysfunction and M1-skewed inflammatory responses, highlighting future therapeutic research for the control of this neglected tropical disease.