Establishing a Lipid Model of Host-Pathogen Interaction Using Multimodal Mass Spectrometry Imaging in a Francisella Infection

Abstract

Host membranes are intimately involved in the immune response to any infection, including formation of lipid docking sites for proteins, organization of immune signaling complexes in lipid rafts, and maintaining a reservoir of fatty acids that contribute to acute inflammation. Francisella species maintain several host immune evasion strategies, one of which involves induction of the immunomodulatory lipid prostaglandin E2 (PGE2). The source of PGE2 is arachidonic acid (AA), a structural component of membrane phospholipids. Mass spectrometry imaging (MSI) was used to map and characterize both host- and pathogen-borne lipids using Francisella infected spleens in a murine model. Here, we identified and mapped the unique bacterial molecule, lipid A within infected mouse spleens by MALDI-MSI, confirming the in vivo structure (m/z 1665.1) in a mammalian infection. Francisella lipid A mapped primarily to the red pulp of the spleen, with signal first appearing between 24 and 36 hours post-infection, corresponding to the onset of bacteremia. Numerous changes in host lipid levels were correlated with progression of the infection. A phosphatidylinositol species, 1-stearoyl, 2-arachidonyl phosphatidylinositol (SAPI) was identified in the periphery of the splenic white pulp, suggesting a cell-specific origin. SAPI abundance peaks at 24 hours and is depleted in the timepoints preceding lethality (48 to 60 hours). In vitro reports demonstrate that SAPI is the earliest source of AA in activated macrophages. We have subsequently linked importation of SAPI into the spleen by monocytic infiltrates, which increases the total SAPI load. Additionally, accumulation of cholesterol was observed by SIMS-Imaging in the infected spleens and may be another indicator of immune infiltration. These data highlight a role for newly immigrant cells in contributing to the pool of total inflammatory lipids. Here, MSI is presented as a new approach to studying lipid-level host-pathogen interactions, facilitating targeted and untargeted discovery.