Neutrophil recruitment and dysregulation in an intradermal murine model of Orientia tsutsugamushi Infection

Abstract

Abstract Scrub typhus is a life-threatening zoonosis caused by Orientia tsutsugamushi organisms that are transmitted by the larvae of trombiculid mites. However, the reason that innate immune cells fail to control local infection is unknown. By using a newly established intradermal (i.d.) infection mouse model, we analyzed myeloid cell subpopulations in the inoculation site at 4 hours post-infection (hpi) and found significant recruitment of neutrophils, langerhans cells and Ly6chi monocytes in the skin. To investigate what type of cells can be infected, we i.d. challenged mice with CFSE-labeled bacteria. We found that the majority of infected cells at 4 hpi were neutrophils and monocytes, which comprised around 90% of all the CFSE+ cells. To determine the fates of these immune cells carrying Orientia, we examined CFSE+ cells in various organs at 24 hpi. Our flow cytometry data demonstrated that langerhans cells were the only infected cell subset that migrated to draining lymph nodes (dLN) but were not found in the blood, spleen, bone marrow or lungs at this early stage. There were very few infected neutrophils in either the skin or dLN at 24 hpi, indicating that bacterial challenge may promote neutrophil death. To confirm this result, we infected bone marrow-derived neutrophils and monitored cell death for 36 h. Our data showed that Orientia infection increased neutrophil death from 12 to 36 hpi. Interestingly, infected neutrophils displayed an immunosuppressive phenotype, as evidenced by decreased TNF-a, CXCL10 and ICAM-1, but increased arginase-1, CCL2 and IL-4. Collectively, our data suggest that Orientia-triggered neutrophil dysregulation and death may contribute to bacterial dissemination and systemic infection in scrub typhus.