Pulmonary vascular malfunction and inflammation during Orientia tsutsugamushi infection

Scrub typhus is a potentially lethal illness caused by infection with the bacterium, Orientia tsutsugamushi. Scrub typhus remains a neglected tropical disease, despite being a tremendous burden in endemic areas. Lung injury is one of the most common pathologies that arise from severe cases of scrub typhus, but the underlying mechanisms are unclear. Development of relevant animal models allow for investigation into the cellular populations that are contributing to the observed lung pathology during O. tsutsugamushi infection. The first objective of this work was to characterize the activation of the pulmonary endothelium during infection. Although O. tsutsugamushi is considered an endotheliotropic bacterium, no research has been conducted to evaluate changes in the endothelium in vivo during infection. Prolonged activation and loss of barrier integrity in pulmonary endothelium are initial steps to the development of lung injury during infection. To this end, we analyzed the increase of expression of the activation markers and decrease of barrier proteins on pulmonary endothelial cells. Activation of endothelial cells results in the recruitment of circulating immune cells, especially neutrophils; however, excessive neutrophil recruitment can exacerbate lung injury and endothelial damage. Neutrophils were recruited in significant numbers by late infection and developed an activated phenotype. Further, depletion of neutrophils at various points of infection attenuated either weight loss or mortality of infected mice and modulated the macrophage and T cell populations in the lungs. These data suggest a pathogenic and regulatory role of neutrophils during scrub typhus. In addition to endothelial cells, O. tsutsugamushi has been shown to replicate in macrophages in vitro and in vivo. The second objective of this study was to determine the state of macrophage polarization in the lungs of mice lethally infected with O. tsutsugamushi and determine how this polarization affects bacterial growth. Macrophages were recruited in high numbers to the lung at both day 6 and day 9 of infection, and they were almost entirely polarized to an inflammatory, “M1”, phenotype. The M1 polarization was also shown to be capable of slowing bacterial growth in vitro in comparison to macrophages polarized to an “M2” phenotype. Mechanisms of macrophage activation and restriction of O. tsutsugamushi growth are unknown.