AUTHOR=Köppen Kristin , Fatykhova Diana , Holland Gudrun , Rauch Jessica , Tappe Dennis , Graff Mareike , Rydzewski Kerstin , Hocke Andreas C. , Hippenstiel Stefan , Heuner Klaus 

TITLE=Ex vivo infection model for Francisella using human lung tissue

JOURNAL=Frontiers in Cellular and Infection Microbiology

VOLUME=Volume 13 - 2023

YEAR=2023

URL=https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1224356

DOI=10.3389/fcimb.2023.1224356

ISSN=2235-2988

ABSTRACT=Tularemia is mainly caused by Francisella tularensis (Ft) subsp. tularensis (Ftt) and Ft subsp. holarctica (Fth) in humans and in more than 200 animal species including rabbits and hares. Human clinical manifestations depend on the route of infection and range from flu-like symptoms to severe pneumonia with a mortality rate up to 60% without treatment. So far, only 2D cell culture and animal models are used to study Francisella virulence, but the gained results are transferable to human infections only to a certain extent. In this study, we firstly established an ex vivo human lung tissue infection model using different Francisella strains: Fth Life Vaccine Strain (LVS), Fth LVS ∆iglC, Fth human clinical isolate A-660 and a German environmental Francisella species strain W12-1067 (F-W12). Only LVS and A-660 were able to grow within the human lung explants, whereas LVS ∆iglC and F-W12 did not replicate. Using human lung tissue, we observed a greater increase of bacterial load per explant for patient isolate A-660 compared to LVS, whereas a similar replication of both strains was observed in cell culture models with human macrophages. Alveolar macrophages were mainly infected in human lung tissue, but Fth was also sporadically detected within white blood cells as demonstrated by spectral immunofluorescence and electron microscopy. Although Fth replicated within lung tissue, an overall low induction of pro-inflammatory cytokines and chemokines was observed. A-660-infected lung explants secreted slightly less of IL-1β, MCP-1, IP-10 and IL-6 compared to Fth LVS-infected explants, suggesting a more repressed immune response for patient isolate A-660. When LVS and A-660 were used for simultaneous co-infections, only the ex vivo model reflected the less virulent phenotype of LVS, as it was outcompeted by A-660. The human ex vivo model delivers considerable advantages and is able to discriminate virulent Francisella from less-or non-virulent strains.