In Situ Microscopy Analysis Reveals Local Innate Immune Response Developed around Brucella Infected Cells in Resistant and Susceptible Mice

Abstract

Brucella are facultative intracellular bacteria that chronically infect humans and animals causing brucellosis. Brucella are able to invade and replicate in a broad range of cell lines in vitro, however the cells supporting bacterial growth in vivo are largely unknown. In order to identify these, we used a Brucella melitensis strain stably expressing mCherry fluorescent protein to determine the phenotype of infected cells in spleen and liver, two major sites of B. melitensis growth in mice. In both tissues, the majority of primary infected cells expressed the F4/80 myeloid marker. The peak of infection correlated with granuloma development. These structures were mainly composed of CD11b ⁺ F4/80 ⁺ MHC-II ⁺ cells expressing iNOS/NOS2 enzyme. A fraction of these cells also expressed CD11c marker and appeared similar to inflammatory dendritic cells (DCs). Analysis of genetically deficient mice revealed that differentiation of iNOS ⁺ inflammatory DC, granuloma formation and control of bacterial growth were deeply affected by the absence of MyD88, IL-12p35 and IFN-γ molecules. During chronic phase of infection in susceptible mice, we identified a particular subset of DC expressing both CD11c and CD205, serving as a reservoir for the bacteria. Taken together, our results describe the cellular nature of immune effectors involved during Brucella infection and reveal a previously unappreciated role for DC subsets, both as effectors and reservoir cells, in the pathogenesis of brucellosis.

Author Summary

Brucella are facultative intracellular bacteria chronically infecting humans and animals causing brucellosis, one of the most common zoonotic disease worldwide which can result in infertility and chronic debilitating disease. The cells supporting Brucella growth in vivo remain largely unknown. In order to identify these, we constructed a Brucella melitensis strain expressing a fluorescent protein that allowed us to characterize infected cells by microscopy of the spleen and liver from infected mice. In both tissues, the majority of primary infected cells were cells from the macrophage lineage. The peak of infection correlated with granuloma development. These structures contained the majority of bacteria and were mainly composed of cells expressing CD11b, F4/80, MHC-II, which are specific of activated monocytes/macrophages. A fraction of granuloma cells also expressed CD11c and were similar to inflammatory dendritic cells (DCs). During the chronic phase of infection in susceptible mice, we identified a particular subset of DC expressing CD205 and serving as a reservoir for the bacteria. Overall, our results describe the nature of immune cells infected by Brucella in vivo and reveal an unappreciated role for DC subsets, both as effectors and reservoir cells. These results could help develop new therapeutic strategies to control Brucella infection.