Neutrophils and M1 macrophages contribute to vascular injury and lung Pathogenesis during Orientia tsutsugamushi infection

Abstract

Abstract Acute lung injury (ALI) leading to acute respiratory distress syndrome is a prevailing pathologic manifestation during O. tsutsugamushi-induced severe scrub typhus in humans. In this study, we tested a hypothesis that lung pathology in scrub typhus is due in part to dysregulated activation of inflammatory responses, leading to vascular malfunction and ALI. Following infection with a lethal dose of O. tsutsugamushi in mice, lung tissues had a significant increase in ICAM1, VEGFR2, and angiopoietin-2 (Ang2), as measured by flow cytometry and immunofluorescence. Several unique features were also observed. First, a progressive decrease or loss of endothelial functional markers (Ang1 and Tie2) and CD41+ platelets, even after the peak of bacterial replication (around day 6), implying continued endothelial cell stress and tissue damage. Second, a sustained neutrophil influx and activation as disease progressed, and lung-recruited neutrophils became highly activated in releasing azurophilic granules or myeloperoxidase around day 10 (prior to host death). Finally, lung-derived macrophages were highly polarized to an M1 phenotype (CD80+CD64+CD11b+Ly6G−) at day 6-day 10, with no signs of M2 activation (CD206+CD64+CD11b+Ly6G−). This study reveals specific biomarkers for vascular stress/dysfunction and uncovers a type 1-skewed, but type 2-suppressed, immune responses in the lungs of lethally-infected mice. More importantly, it furthers findings from human patients and cells, implying an immunopathogenic role in ALI development during severe scrub typhus. Understanding of leukocyte effector molecules and endothelial stress pathways triggered by bacterial vs. host factors will help control this neglected tropical disease.