Francisella is a highly infectious gram-negative bacterium that causes tularemia in humans and animals. It can survive and multiply in a variety of cells, including macrophages, dendritic cells, amoebae, and arthropod-derived cells. However, the intracellular life cycle of a bacterium varies depending on the cell type. Shortly after the infection of mammalian cells, the bacterium escapes the phagosome into the cytosol, where it replicates. In contrast, in the amoebae Acanthamoeba castellanii and Hartmannella vermiformis, the bacterium replicates within the membrane-bound vacuole. In recent years, the amoeba Dictyostelium discoideum has emerged as a powerful model to study the intracellular cycle and virulence of many pathogenic bacteria. In this study, we used D. discoideum as a model for the infection and isolation of Francisella novicida-containing vacuoles (FCVs) formed after bacteria invade the amoeba. Our results showed that F. novicida localized in a vacuole after invading D. discoideum. Here, we developed a method to isolate FCV and determined its composition by proteomic analyses. Proteomic analyses revealed 689 proteins, including 13 small GTPases of the Rab family. This is the first evidence of F. novicida-containing vacuoles within amoeba, and this approach will contribute to our understanding of host–pathogen interactions and the process of pathogen vacuole formation, as vacuoles containing bacteria represent direct contact between pathogens and their hosts. Furthermore, this method can be translocated on other amoeba models.

Autophagy is a crucial mechanism in the host response to intracellular bacterial pathogens during which microorganisms may undergo direct degradation in autophagolysosomes. As a highly virulent intracellular pathogen, Francisella tularensis has developed survival strategies to escape from the phagosome, replicate in the cytosol of mononuclear cells, and avoid degradation within the double-membrane vacuole during the autophagy-mediated response. The aim of this study was to investigate the role of the ATG5 autophagy protein in the host immune response to Francisella tularensis subsp. holarctica, live vaccine strain (LVS), since ATG5 plays an important role in autophagosome formation during canonical autophagy. In vitro experiments were conducted on immortalized bone marrow macrophages subjected to starvation-induced autophagy. Transgenic mice deficient in ATG5 of cells of the myeloid lineage (monocytes/macrophages and granulocytes) were used to analyze the immunological responses after intradermal infection. Cytokine levels were analyzed using Luminex, RT-qPCR, and ELISA, while inflammatory cell infiltration in the lung was analyzed by immunohistochemistry. Our results demonstrate that induced autophagy decreased bacterial replication in vitro. However, ATG5 deficiency in myeloid cells in vivo significantly diminished levels of pro-inflammatory cytokine IFN-γ in the sera, spleen, liver, and lung during Francisella infection. The attenuated pro-inflammatory response also led to significantly reduced macrophage and T cell infiltration in the lung tissue. Our findings also reveal that neutralization of IL-1β in myeloid ATG5ΔMye mice increased susceptibility to tularemia by increasing bacterial burden in organs.