Virulence of representative Japanese Francisella tularensis and immunologic consequences of infection in mice

ABSTRACT Francisella tularensis, which causes tularemia, is widely distributed in the Northern hemisphere. F. tularensis strains isolated in Japan are genetically unique from non‐Japanese strains; however, their phenotypic properties have not been well studied. Thus, mice were infected with representative Japanese strains of F. tularensis and their virulence and mouse immune responses to them assessed. Of four representative Japanese strains, the Ebina, Jap and Tsuchiya strains were susceptible to H2O2 and did not grow well intracellularly. Only Yama strain grew intracellularly and was lethal to mice. Infection with Yama strain resulted in drastic increases in IFN‐γ, CD4 and CD8 double‐positive T cells and Th1 cells (CD3, CD4 and Tim3‐positive cells), and a decrease in the ratio of CD8‐positive CD4‐negative T cells in mice. C57BL/6J mice that survived infection produced IgM antibodies to LPS and IgG2c antibodies to 43, 19 and 17 kDa proteinase K‐sensitive components. These data are valuable for understanding the phenotypic properties of F. tularensis in Japan.